CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U. S. Provisional Patent Application

No. 62/463, 159, filed February 24, 2017, U.S. Provisional Patent Application No. 62/562,008, filed

September 22, 2017, and U.S. Provisional Patent Application No. 62/580,671, filed

November 2, 2017, the entire disclosures of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] Light-emitting diode (LED) light sources (e.g., LED light engines) are replacing conventional incandescent, fluorescent, and halogen lamps as a primary form of lighting devices. LED light sources may comprise a plurality of light-emitting diodes mounted on a single structure and provided in a suitable housing. LED light sources may be more efficient and provide longer operational lives as compared to incandescent, fluorescent, and halogen lamps. An LED driver control device (e.g., an LED driver) may be coupled between a power source, such as an

alternating-current (AC) power source or a direct-current (DC) power source, and an LED light source for regulating the power supplied to the LED light source. For example, the LED driver may regulate the voltage provided to the LED light source, the current supplied to the LED light source, or both the current and voltage.

[0003] Different control techniques may be employed to drive LED light sources including, for example, a current load control technique and a voltage load control technique. An LED light source driven by the current load control technique may be characterized by a rated current (e.g., approximately 350 milliamps) to which the magnitude (e.g., peak or average magnitude) of the current through the LED light source may be regulated to ensure that the LED light source is illuminated to the appropriate intensity and/or color. An LED light source driven by the voltage load control technique may be characterized by a rated voltage (e.g., approximately 15 volts) to which the voltage across the LED light source may be regulated to ensure proper operation of the LED light source. If an LED light source rated for the voltage load control technique includes multiple parallel strings of LEDs, a current balance regulation element may be used to ensure that the parallel strings have the same impedance so that the same current is drawn in each of the parallel strings.

[0004] The light output of an LED light source may be dimmed. Methods for dimming an

LED light source may include, for example, a pulse-width modulation (PWM) technique and a constant current reduction (CCR) technique. In pulse-width modulation dimming, a pulsed signal with a varying duty cycle may be supplied to the LED light source. For example, if the LED light source is being controlled using a current load control technique, the peak current supplied to the LED light source may be kept constant during an on-time of the duty cycle of the pulsed signal. The duty cycle of the pulsed signal may be varied, however, to vary the average current supplied to the LED light source, thereby changing the intensity of the light output of the LED light source. As another example, if the LED light source is being controlled using a voltage load control technique, the voltage supplied to the LED light source may be kept constant during the on-time of the duty cycle of the pulsed signal. The duty cycle of the load voltage may be varied, however, to adjust the intensity of the light output. Constant current reduction dimming may be used if an LED light source is being controlled using the current load control technique. In constant current reduction dimming, current may be continuously provided to the LED light source. The DC magnitude of the current provided to the LED light source, however, may be varied to adjust the intensity of the light output.

[0005] Examples of LED drivers are described in U.S. Patent No. 8,492,987, issued

July 23, 2013, entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE; U.S. Patent No. 9,655, 177, issued May 16, 2017, entitled FORWARD CONVERTER HAVING A PRIMARY-SIDE CURRENT SENSE CIRCUIT; and U. S. Patent No. 9,247,608, issued January 26, 2016, entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE; the entire disclosures of which are hereby incorporated by reference. SUMMARY OF THE INVENTION

[0006] As described herein, a load control device for controlling the intensity of a lighting load may be configured to turn on the lighting load to obtain a fast turn-on time that may be substantially consistent across different lighting loads that have different load voltages. The load control device may comprise a power converter circuit that may be configured to receive a first voltage and to produce a second voltage across a capacitor, and a control circuit that is operatively coupled to the power converter circuit for controlling the power converter circuit to generate the second voltage across the capacitor. The control circuit may be configured to determine a learned voltage (e.g., a learned capacitor voltage and/or a learned load voltage) from the magnitude of the second voltage of the capacitor. For example, the control circuit may measure the magnitude of the second voltage of the capacitor and/or store the measured voltage as the learned voltage. The control circuit may determine an operating parameter for the power converter circuit as a function of the learned voltage. The control circuit may be configured to control the power converter circuit according to the operating parameter while the capacitor is charging until the magnitude of the second voltage reaches a threshold.

[0007] In an example, the load control device may also include a load regulation circuit configured to receive the second voltage (e.g., a bus voltage) and to control the magnitude of a load current conducted through the lighting load. The control circuit may be operatively coupled to the load regulation circuit for controlling the magnitude of the load current to control the intensity of the lighting load. The control circuit may determine an on-time for controlling a semiconductor switch of the power converter circuit as a function of the learned voltage, and control the semiconductor switch conductive using the pre-load on-time to charge the capacitor until the magnitude of the second voltage reaches the threshold.

[0008] In another example, the power converter circuit may operate as a load regulation circuit to control the magnitude of the load current conducted through the lighting load. The control circuit may be operatively coupled to the power converter circuit for controlling the magnitude of the load current to control the intensity of the lighting load. The control circuit may set a magnitude of a target-current control signal for controlling the power converter circuit based on the learned voltage to charge the capacitor until the magnitude of the second voltage reaches the threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Fig. 1 is a simplified block diagram of an example light-emitting diode (LED) driver for controlling the intensity of an LED light source.

[0010] Fig. 2 is a simplified flowchart of an example turn-on procedure for controlling a power converter circuit of an LED driver.

[0011] Fig. 3 is a simplified schematic diagram of an example LED driver showing a power converter circuit and an LED drive circuit.

[0012] Fig. 4 is a plot showing an example relationship between a learned load voltage and an on-time for controlling a field-effect transistor of the power converter circuit of Fig. 3.

[0013] Figs. 5A and 5B show example waveforms of a bus voltage of the LED driver of

Fig. 3 turning on two different LED light sources.

[0014] Fig. 6 is a simplified flowchart of another example turn-on procedure for controlling a power converter circuit of an LED driver.

[0015] Fig. 7 is a simplified block diagram of another example LED driver.

[0016] Fig. 8 is a simplified schematic diagram of another example LED driver.

[0017] Fig. 9A is an example plot of a relationship between an operating frequency and a target current of the LED driver of Fig. 8.

[0018] Fig. 9B is an example plot of a relationship between a magnitude of a target-current control signal and the target current of the LED driver of Fig. 8. [0019] Figs. 10A and 10B show example waveforms illustrating the operation of the LED driver of Fig. 8.

[0020] Fig. 1 1 A shows example waveforms illustrating the operation of the LED driver of

Fig. 8 when the LED driver is learning a load voltage.

[0021] Fig. 1 IB shows example waveforms illustrating the operation of the LED driver of

Fig. 8 when the LED driver is turning on an LED light source using the learned load voltage.

[0022] Fig. 12 is a simplified flowchart of another example turn-on procedure for controlling a power converter circuit of an LED driver.

DETAILED DESCRIPTION OF THE INVENTION

[0023] Fig. 1 is a simplified block diagram of a light-emitting diode (LED) driver 100 for controlling the intensity of an LED light source 102 (e.g., an LED light engine). The LED light source 102 is shown as a plurality of LEDs connected in series but may comprise a single LED or a plurality of LEDs connected in parallel or a suitable combination thereof, depending on the particular lighting system. In addition, the LED light source 102 may alternatively comprise one or more organic light-emitting diodes (OLEDs). The LED driver 100 may be adapted to work with a plurality of different LED light sources, which may be rated to operate using different load control techniques, different dimming techniques, and different magnitudes of load current and voltage.

[0024] The LED driver 100 may comprise a hot terminal H and a neutral terminal N for receiving an alternating-current (AC) voltage VAC from an AC power source (not shown). The LED driver 100 may comprise a radio-frequency (RFI) filter and rectifier circuit 1 10, which may receive the AC voltage VAC. The RFI filter and rectifier circuit 1 10 may operate to minimize the noise provided on the AC power source and to generate a rectified voltage VRECT. The LED driver 100 may comprise a power converter circuit 120, e.g., a buck-boost flyback converter, which may receive the rectified voltage VRECT and generate a variable direct-current (DC) bus voltage VBUS across a capacitor (e.g., a storage capacitor, such as a bus capacitor CBUS). The power converter circuit 120 may alternatively comprise any suitable power converter circuit for generating an appropriate bus voltage, such as, for example, a boost converter, a buck converter, a single-ended primary-inductance converter (SEPIC), a Cuk converter, or other suitable power converter circuit. The power converter circuit 120 may also provide electrical isolation between the AC power source and the LED light source 102, and/or operate as a power factor correction (PFC) circuit to adjust the power factor of the LED driver 100 towards a power factor of one.

[0025] The LED driver 100 may comprise a load regulation circuit, e.g., an LED drive circuit 130, which may receive the bus voltage VBUS and control the amount of power delivered to the LED light source 102 so as to control the intensity of the LED light source. The LED drive circuit 130 may comprise a controllable-impedance circuit, such as a linear regulator, as will be described in greater detail below. To control the amount of power delivered to the LED light source 102, the LED drive circuit 130 may be configured to control the magnitude of a load current ILOAD through the LED light source 102 and/or the magnitude of a load voltage VLOAD across the LED light source.

[0026] The LED driver 100 may include a control circuit 140 for controlling the operation of the power converter circuit 120 and/or the LED drive circuit 130. The control circuit 140 may comprise, for example, a controller or any other suitable processing device, such as, for example, a microcontroller, a programmable logic device (PLD), a microprocessor, an application specific integrated circuit (ASIC), or a field-programmable gate array (FPGA). The control circuit 140 may be configured to control the LED drive circuit 130 to turn the LED light source 102 on and off and to adjust (e.g., dim) a present intensity LPRES of the LED light source 102 towards a target

intensity LTRGT, which may range across a dimming range of the LED light source, e.g., between a low-end intensity LLE (e.g., approximately 0. 1 - 10%) and a high-end intensity LHE (e.g.,

approximately 100%). The control circuit 140 may be configured to control the LED drive circuit 130 to control the load voltage VLOAD across the LED light source 102 and/or the load current ILOAD through the LED light source to control the amount of power delivered to the LED light source (e.g., depending upon a mode of operation of the LED driver as will be described in greater detail below). In an example, when power is first applied to the LED driver 100, the control circuit 140 may be configured to execute a startup routine before executing a turn-on routine (e.g., a turn-on procedure) to illuminate the LED light source 102.

[0027] The control circuit 140 may be configured to control the magnitude of the load current ILOAD through the LED light source 102 or the load voltage VLOAD across the LED light source using two different modes of operation: a current load control mode (e.g., for using the current load control technique) and a voltage load control mode (e.g., for using the voltage load control technique). The control circuit 140 may be configured to adjust the magnitude to which the LED drive circuit 130 controls the load current ILOAD through the LED light source 102 in the current load control mode, or the magnitude to which the LED drive circuit 130 controls the load voltage VLOAD across the LED light source in the voltage load control mode. When operating in the current load control mode, the control circuit 140 may be configured to control the intensity of the LED light source 102 using two different dimming modes: a PWM dimming mode (e.g., for using the PWM dimming technique) and a CCR dimming mode (e.g., for using the CCR dimming technique). When operating in the voltage load control mode, the LED driver 100 may be configured to adjust the amount of power delivered to the LED light source 102 using the PWM dimming technique.

[0028] The control circuit 140 may be coupled to a memory 150 for storing operational characteristics of the LED driver 100 (e.g., the target intensity LTRGT, the low-end intensity LLE, the high-end intensity LHE, etc.). The memory 150 may be implemented as an external integrated circuit (IC) or as an internal circuit of the control circuit 140. The LED driver 100 may also comprise a communication circuit 160, which may be coupled to, for example, a wired

communication link or a wireless communication link, such as a radio-frequency (RF)

communication link or an infrared (IR) communication link. The control circuit 140 may be configured to determine the target intensity LTRGT of the LED light source 102 or the operational characteristics stored in the memory 150 in response to digital messages received via the communication circuit 160. The control circuit 140 may be configured to execute the turn-on routine, e.g., in response to receiving a command to turn on the LED light source 102. [0029] The LED driver 100 may further comprise a power supply 170, which may receive the rectified voltage VRECT and generate a plurality of direct-current (DC) supply voltages for powering the circuitry of the LED driver. Specifically, the power supply 170 may generate a first non-isolated supply voltage Vcci (e.g., approximately 14 volts) for powering the control circuitry of the power converter circuit 120, a second isolated supply voltage Vcc2 (e.g., approximately 9 volts) for powering the control circuitry of the LED drive circuit 130, and a third non-isolated supply voltage Vcc3 (e.g., approximately 5 volts) for powering the control circuit 140.

[0030] As previously mentioned, the control circuit 140 may manage the operation of the power converter circuit 120 and/or the LED drive circuit 130 to control the intensity of the LED light source 102. The control circuit 140 may receive from the power converter circuit 120 a bus voltage feedback signal VBUS-FB, which may be representative of the magnitude of the bus voltage VBUS. The control circuit 140 may provide a bus voltage control signal VBUS-CNTL to the power converter circuit 120 for controlling the magnitude of the bus voltage VBUS towards a target bus voltage VBUS-TRGT (e.g., from approximately 8 volts to 60 volts). When operating in the current load control mode, the LED drive circuit 130 may control a peak magnitude IPK of the load current ILOAD conducted through the LED light source 102 between a minimum load

current ILOAD-MIN and a maximum load current ILOAD-MAX in response to a peak current control signal VIPK provided by the control circuit 140.

[0031] The control circuit 140 may receive a load current feedback signal VILOAD, which may be representative of an average magnitude IAVE of the load current ILOAD flowing through the LED light source 102. The control circuit 140 may also receive a regulator voltage feedback signal VREG-FB that may be representative of the magnitude of a regulator voltage VREG (e.g., a controllable-impedance voltage) across the linear regulator of the LED drive circuit 130 as will be described in greater detail below. The difference between the magnitudes of the bus voltage VBUS and the regulator feedback voltage VREG-FB may be representative of the magnitude of the load voltage VLOAD across the LED light source 102.

[0032] The control circuit 140 may be configured to control the LED drive circuit 130 to control the amount of power delivered to the LED light source 102 using the two different modes of operation (e.g., the current load control mode and the voltage load control mode). During the current load control mode, the LED drive circuit 130 may regulate the peak magnitude IPK of the load current ILOAD through the LED light source 102 in response to the load current feedback

signal VILOAD (e.g., using closed loop control). The target load current ITRGT may be stored in the memory 150 and may be programmed to be any specific magnitude depending upon the LED light source 102.

[0033] To control the intensity of the LED light source 102 during the current load control mode, the control circuit 140 may control the LED drive circuit 130 to adjust the amount of power delivered to the LED light source 102 using the PWM dimming technique and/or the CCR dimming technique. Using the PWM dimming technique, the control circuit 140 may control the peak magnitude IPK of the load current ILOAD through the LED light source 102 to the target load current ITRGT and pulse-width modulate the load current ILOAD to dim the LED light source 102 and achieve the target load current ITRGT. Specifically, the LED drive circuit 130 may control a duty cycle DC ILOAD of the load current ILOAD in response to a duty cycle DCDIM of a dimming control signal VDIM provided by the control circuit 140. The intensity of the LED light source 102 may be dependent upon the duty cycle DCILOAD of the pulse-width modulated load current ILOAD. Using the CCR dimming technique, the control circuit 140 may not pulse-width modulate the load

current ILOAD, but instead may adjust the magnitude of the target load current ITRGT SO as to adjust the average magnitude IAVE of the load current ILOAD through the LED light source 102 (which may be equal to the peak magnitude IPK of the load current ILOAD in the CCR dimming mode).

[0034] During the voltage load control mode, the LED drive circuit 130 may regulate the magnitude (e.g., the DC voltage) of the load voltage VLOAD across the LED light source 102 to a target load voltage VTRGT. The target load voltage VTRGT may be stored in the memory 150 and may be programmed to be any specific magnitude depending upon the LED light source 102. The control circuit 140 may be configured to dim the LED light source 102 using only the PWM dimming technique during the voltage load control mode. Specifically, the control circuit 140 may adjust a duty cycle DCVLOAD of the load voltage VLOAD to dim the LED light source 102. An example of a configuration procedure for the LED driver 100 is described in greater detail in U. S. Patent No. 8,492,988, issued July 23, 2013, entitled CONFIGURABLE LOAD CONTROL DEVICE FOR LIGHT-EMITTING DIODE LIGHT SOURCES, the entire disclosure of which is hereby incorporated by reference.

[0035] The control circuit 140 may be configured to determine or learn (e.g., measure or receive an indication of) one or more operational characteristics of the LED light source 102 (e.g., learned load characteristics). For example, when the control circuit 140 is operating in the current control mode, the control circuit may be configured to determine the magnitude of a voltage representative of the magnitude of the load voltage VLOAD. During the current control mode, the magnitude of the load voltage VLOAD generated across the LED light source 102 may be dependent upon the magnitude of the load current ILOAD (e.g., the target load current ITRGT to which the control circuit 140 is regulating the load current ILOAD) as well as the internal circuitry of the LED light source. The control circuit 140 may be configured to determine (e.g., measure) the magnitude of the voltage that is representative of the magnitude of the load voltage VLOAD (e.g., when the target intensity LTRGT is at the high-end intensity LHE) and/or to store the measurement in the memory 150 as a learned load voltage VLEARNED.

[0036] Since the control circuit 140 may operate to minimize the regulator voltage VREG across the linear regulator of the LED drive circuit 130 (e.g., to approximately 0.4-0.6 V), the magnitude of the bus voltage VBUS may be approximately equal to the load voltage VLOAD and thus representative of the magnitude of the load voltage VLOAD. The control circuit 140 may be configured to determine (e.g. , measure) the magnitude of the bus voltage VBUS stored in the bus capacitor CBUS using the bus voltage feedback signal VBUS-FB from the power converter circuit 120 and/or store the measurement in the memory 150 as the learned load voltage VLEARNED (e.g., a learned capacitor voltage). The control circuit 140 may be configured to calculate the magnitude of the load voltage VLOAD by subtracting the magnitude of a regulator voltage VREG (e.g., as determined from the regulator voltage feedback signal VREG-FB) from the magnitude of the bus voltage VBUS and use the calculated value as the learned load voltage VLEARNED. The control circuit 140 may include a load voltage measurement circuit (not shown) coupled across the LED light source 102 for directly measuring the magnitude of the load voltage VLOAD, which may be stored as the learned load voltage VLEARNED. Additionally or alternatively, the control circuit 140 may be configured to determine (e.g., measure) the magnitude of a voltage that is representative of the magnitude of the load voltage VLOAD at the low-end intensity LLE and/or store the measurement in the memory 150 as the learned load voltage VLEARNED.

[0037] The control circuit 140 may be configured to control the power converter circuit 120 and/or the LED drive circuit 130 using the learned load voltage VLEARNED. For example, the control circuit 140 may be configured to control the power converter circuit 120 in response to the learned load voltage VLEARNED when turning on the LED light source 102. The control circuit 140 may be configured to control the rate at which the bus capacitor CBUS charges in response to the learn load voltage VLEARNED to ensure that the bus voltage VBUS quickly increases to the appropriate level and the LED light source 102 is illuminated as soon as possible. For example, in response to receiving a command to turn on the LED light source 102 and/or in response to power being applied to the LED driver 100 to turn on the LED light source, the control circuit 140 may control the power converter circuit 120 using open loop control in response to the magnitude of the learned load

voltage VLEARNED until the magnitude of the bus voltage VBUS reaches or exceeds a charging threshold VTH-CH. The charging threshold VTH-CH may be, for example, a function of the learned load voltage VLEARNED. For example, the control circuit 140 may be configured to determine an operating parameter (e.g., a pre-load parameter) as a function of the learned load voltage VLEARNED and use the operating parameter to control the power converter circuit 120 using open loop control (e.g., as will be described in greater detail below). In addition, the charging threshold VTH-CH may be a fixed threshold (e.g., a predetermined threshold). After the magnitude of the bus voltage VBUS reaches or exceeds a bus voltage threshold VTH-BUS, the control circuit 140 may then begin to control the power converter circuit 120 using closed loop control in response to the magnitude of the bus voltage feedback signal VBUS-FB to regulate the magnitude of the bus voltage VBUS towards the target bus voltage VBUS-TRGT.

[0038] Fig. 2 is a simplified flowchart of an example turn-on procedure 200 for controlling a power converter circuit of an LED driver (e.g., the power converter circuit 120 of the LED driver 100). For example, the turn-on procedure 200 may be executed by a control circuit (e.g., the control circuit 140) at step 210 in response to receiving a command to turn on the LED light source 102 and/or in response to power being applied to the LED driver to turn on the LED light source. The control circuit may retrieve a learned load characteristic (e.g., the learned load voltage VLEARNED) from the memory 150 at step 212 and may determine the value of the charging threshold VTH-CH as a function of the learned load voltage VLEARNED at step 214. The control circuit may determine an operating parameter (e.g., a pre-load on-time) for the power converter circuit as a function of the learned load voltage VLEARNED at step 216 and control the power converter circuit using the operating parameter at step 218. While the magnitude of a capacitor voltage (e.g., the bus voltage VBUS) is less than the charging threshold VTH-CH at step 220, the control circuit may continue to control the power converter circuit using the operating parameter at step 218. When the magnitude of the capacitor voltage is greater than or equal to the charging threshold VTH-CH at step 220, the control circuit may begin to control the power converter circuit using closed loop control at step 222 to regulate the magnitude of the capacitor voltage towards a target capacitor voltage (e.g., the target bus voltage VBUS-TRGT), before the turn-on procedure 200 exits.

[0039] Fig. 3 is a simplified schematic diagram of a load control device, e.g., an LED driver 300 (such as the LED driver 100 of Fig. 1) for controlling the intensity of an LED light source 302. The LED driver 300 may comprise a flyback converter circuit 320 (e.g., the power converter circuit 120), an LED drive circuit 330 (e.g., the LED drive circuit 130), and a control circuit 340 (e.g., the control circuit 140). The flyback converter circuit 320 may comprise a flyback transformer 310 having a primary winding coupled in series with a flyback switching transistor, e.g., a field-effect transistor (FET) Q312, or other suitable semiconductor switch. The secondary winding of the flyback transformer 310 may be coupled to a bus capacitor CBUS via a diode D314. The power converter circuit 320 may comprise a voltage divider comprising two resistors R316, R318 coupled across the bus capacitor CBUS for generating a bus voltage feedback signal VBUS-FB.

[0040] The control circuit 340 may generate a bus voltage control signal VBUS-CNTL for controlling a flyback controller 322 of the flyback converter circuit 320. The flyback controller 322 may receive the bus voltage control signal VBUS-CNTL from the control circuit 140 via a filter circuit 324 (e.g., a resistor-capacitor filter) and an optocoupler circuit 326, which may provide electrical isolation between the power converter circuit 320 and the control circuit 340. The flyback controller 322 may also receive a control signal representative of the current through the FET Q312 from a feedback resistor R328, which may be coupled in series with the FET. The flyback controller 322 may render the FET Q312 conductive and non-conductive to selectively conduct current through the flyback transformer 310 to thus generate the bus voltage VBUS. For example, the flyback controller 322 may be configured to adjust an on-time toN of the FET Q312 (e.g., the time that the FET Q312 conductive during each operating cycle of the power converter circuit 320) in response to the bus voltage control signal VBUS-CNTL to control the magnitude of the bus

voltage VBUS.

[0041] The LED drive circuit 330 may comprise a linear regulator (e.g., a controllable- impedance circuit) including a power semiconductor switch, e.g., a regulation field-effect transistor (FET) Q332, coupled in series with the LED light source 302 for conducting a load current ILOAD through the LED light source. The control circuit 340 may generate a peak current control signal VIPK that may be coupled to the gate of the regulation FET Q332 through a filter circuit 334, an amplifier circuit 336, and a gate resistor R338. The control circuit 340 may be configured to control a duty cycle DCIPK of the peak current control signal VIPK to control a peak magnitude IPK of the load current ILOAD conducted through the LED light source 302 to a target load current ITRGT.

[0042] The LED drive circuit 330 may comprise a load current feedback circuit 342 coupled in series with the regulation FET Q332 and a regulator voltage feedback circuit 344 coupled in parallel with the regulation FET Q332. The load current feedback circuit 342 may generate a load current feedback signal VILOAD, which may be provided to the control circuit 340 and may be representative of an average magnitude IAVE of the load current ILOAD. The regulator voltage feedback circuit 344 may generate the regulator voltage feedback signal VREG-FB, which may also be provided to the control circuit 340 and may be representative of the regulator voltage VREG generated across the series combination of the regulation FET Q332 and the load current feedback circuit 342. Other examples of feedback circuits for the LED drive circuit 330 are described in greater detail in U. S. Patent No. 8,466,628, issued June 18, 2013, entitled CLOSED-LOOP LOAD CONTROL CIRCUIT HAVING A WIDE OUTPUT RANGE, the entire disclosure of which is hereby incorporated by reference.

[0043] When operating in the current load control mode, the control circuit 340 may control the regulation FET Q332 to operate in the linear region, such that the peak magnitude IPK of the load current ILOAD may be dependent upon the DC magnitude of the gate voltage at the gate of the regulation transistor Q332. In other words, the regulation FET Q332 may provide a

controllable-impedance in series with the LED light source 302. If the magnitude of the regulator voltage VREG drops too low, the regulation FET Q332 may be driven into the saturation region, such that the regulation FET Q332 may become fully conductive and the control circuit 340 may no longer be able to control the peak magnitude IPK of the load current ILOAD. Therefore, the control circuit 340 may adjust the magnitude of the bus voltage VBUS to prevent the magnitude of the regulator voltage VREG from dropping below a minimum regulator voltage threshold VREG-MIN (e.g., approximately 0.4 volts). In addition, the control circuit 340 may also be configured to adjust the magnitude of the bus voltage VBUS to control the magnitude of the regulator voltage VREG to be less than a maximum regulator voltage threshold VREG-MAX (e.g., approximately 0.6 volts) to prevent the power dissipated in regulation FET Q332 from becoming too large, thus increasing the total efficiency of the LED driver 300.

[0044] When operating in the voltage load control mode, the control circuit 340 may be configured to drive the regulation FET Q332 into the saturation region, such that the magnitude of the load voltage VLOAD may be approximately equal to the magnitude of the bus voltage VBUS (minus the small voltage drops due to the on-state drain-source resistance RDS-ON of the FET

regulation Q332 and the resistance of the feedback resistor R344).

[0045] The LED drive circuit 330 may also comprise a dimming FET Q350, which may be coupled between the gate of the regulation FET Q332 and circuit common. The dimming control signal VDIM from the control circuit 340 may be provided to the gate of the dimming FET Q350. When the dimming FET Q350 is rendered conductive, the regulation FET Q332 may be rendered non-conductive, and when the dimming FET Q250 is rendered non-conductive, the regulation FET Q332 may be rendered conductive. While using the PWM dimming technique during the current load control mode, the control circuit 340 may adjust the duty cycle DCDIM of the dimming control signal VDIM to thus control when the regulation FET conducts the load current ILOAD and thus the intensity of the LED light source 302. For example, the control circuit 340 may generate the dimming control signal VDIM using a constant PWM frequency foiM (e.g., approximately 500 Hz).

[0046] When using the PWM dimming technique in the current load control mode, the control circuit 340 may be configured to control the peak magnitude IPK of the load current ILOAD in response to the load current feedback signal VILOAD to maintain the average magnitude IAVE of the load current ILOAD constant (e.g., at the target lamp current ITRGT). When using the CCR dimming technique during the current load control mode, the control circuit 340 may maintain the duty cycle DCDIM of the dimming control signal VDIM at a high-end dimming duty cycle DCHE (e.g., approximately 0%, such that the FET Q332 may always be conductive) and may adjust the target load current ITRGT (via the duty cycle DCIPK of the peak current control signal VIPK) to control the intensity of the LED light source 302.

[0047] When operating in the current load control mode, the control circuit 340 may be configured to determine or learn (e.g., measure or receive an indication of) a voltage that is representative of the magnitude of the load voltage VLOAD generated across the LED light source 302 and/or store the learned magnitude in a memory (e.g., the memory 150) as a learned load

voltage VLEARNED. For example, when the control circuit is controlling the intensity of the LED light source 302 to the high-end intensity LHE, the control circuit 340 may determine (e.g., measure) the magnitude of the bus voltage VBUS using the bus voltage feedback signal VBUS-FB from the flyback converter circuit 320 and/or store the measurement in the memory as the learned load

voltage VLEARNED. The control circuit 340 may be configured to determine (e.g., measure) the magnitude of the bus voltage VBUS, for example, during the startup routine (e.g., when power is first applied to the LED driver 300) and/or after the startup routine (e.g., when the magnitude of the bus voltage VBUS is at a steady state condition).

[0048] The control circuit 340 may control the flyback converter circuit 320 in response to the learned load voltage VLEARNED when turning on the LED light source 302 to control the rate at which the bus capacitor CBUS charges to ensure that the LED light source 302 is illuminated quickly after receiving a command to turn on the LED light source 302 and/or after power is applied to the LED driver 300 to turn on the LED light source. When a command to turn on the LED light source 302 has been received and/or power has been applied to the LED driver 300 to turn on the LED light source, the control circuit 340 may be configured to control the flyback converter circuit 320 using open loop control in response to the magnitude of the learned load

voltage VLEARNED until the magnitude of the bus voltage VBUS reaches or exceeds a bus voltage threshold VTH-BUS (e.g., the charging threshold VTH-CH). The control circuit 340 may be

configured to retrieve the learned load voltage VLEARNED from the memory and may determine the value of the bus voltage threshold VTH-BUS as a function of the learned load voltage VLEARNED (e.g., VTH-BUS = η ¹ VLEARNED), where η is a constant that may be, for example, approximately 0.85.

[0049] The control circuit 340 may also be configured to determine an operating parameter for the flyback converter circuit 320 as a function of the learned load voltage VLEARNED. For example, the operating parameter for the power converter circuit 320 may be the on-time toN with which to control the FET Q312 while the bus capacitor CBUS is charging, which may be referred to a "pre-load" on-time ΪΟΝ-PRE. Fig. 4 is a plot showing an example relationship 400 between the learned load voltage VLEARNED and the pre-load on-time ΪΟΝ-PRE for the FET Q312. As shown in Fig. 4, the relationship 400 may be, for example, a linear relationship. The relationship 400 may be stored in the memory, e.g., as an equation or as a table. The relationship 400 may range from a minimum pre-load on-time ΪΟΝ-ΜΙΝ (e.g., approximately 159.6 μβεϋ) at a minimum learned load

voltage VLEARNED-MIN (e.g., approximately 15 volts) to a maximum pre-load on-time ΪΟΝ-ΜΑΧ (e.g., approximately 169.9 μβεϋ) at a maximum learned load voltage VLEARNED-MAX (e.g., approximately 38 volts). The values for the pre-load on-time ΪΟΝ-PRE of the relationship 400 may be chosen such that a turn-on delay time period for LED driver 300 may be approximately the same for different LED light sources that have different resulting load voltages.

[0050] The control circuit 340 may be configured to control the on-time toN for the

FET Q312 to the pre-load on-time ΪΟΝ-PRE using open loop control while the bus capacitor CBUS is charging and the magnitude of the bus voltage VBUS is less than the bus voltage threshold VTH-BUS. When the magnitude of the bus voltage VBUS reaches or exceeds the bus voltage threshold VTH-BUS, the control circuit 340 may then begin to control the flyback converter circuit 320 using closed loop control (e.g., by adjusting the on-time toN of the FET Q312 in response to the magnitude of the bus voltage feedback signal VBUS-FB) to regulate the magnitude of the bus voltage VBUS towards the target bus voltage VBUS-TRGT.

[0051] Figs. 5 A and 5B show example waveforms of bus voltages VBUSI, VBUS2 of an LED driver while the LED driver (e.g., the LED driver 300) is turning on two different LED light sources. For example, Fig. 5A shows a waveform 500 of the first bus voltage VBUSI when turning on an LED light source that has a first learned load voltage VLEDI of approximately 38 volts and Fig. 5B shows a waveform 510 of the second bus voltage VBUS2 when turning on an LED light source that has a second learned load voltage VLED2 of approximately 15 volts. Figs. 5 A and 5B both show the magnitude of the bus voltage VBUS with respect to time when power is first applied to the LED driver to turn on the LED light sources.

[0052] After power is applied at time to, a control circuit (e.g., the control circuit 340) of the

LED driver may execute a startup routine (e.g., a boot mode) until time ti when the control circuit begins controlling a power converter circuit (e.g., the flyback converter circuit 320) to charge a bus capacitor across which the bus voltages VBUSI, VBUS2 are produced. The magnitude of the first bus voltage VBUSI may exceed a charging threshold, e.g., a bus voltage threshold VTH-BUS, which may be function of the first learned load voltage VLEDI (e.g., VTH-BUS = 0.85 ^■ VLEDI), at time t ₂ as shown in Fig. 5A. While the bus capacitor is charging for a first charging time period TCHARGEI between times ti and t ₂ , the control circuit may control the power converter circuit using open loop control with an operating parameter determined as a function of the first learned load voltage VLEDI (e.g., as described above). After the first bus voltage VBUSI exceeds the bus voltage threshold VTH-BUS at time t2, the control circuit may control the power converter circuit using closed loop control to regulate the magnitude of the first bus voltage VBUSI towards a target bus voltage. The control circuit may control the power converter circuit using closed loop control for a control loop delay period TDELAY from time t ₂ until the LED light source turns on at time t ₃ . Thus, the LED light source may turn on after a turn-on delay time period TTURN-ONI from when the control circuit begins controlling the power converter circuit at time ti. [0053] Similarly, as shown in Fig. 5B, the magnitude of the second bus voltage VBUS2 may exceed the bus voltage threshold VTH-BUS at time U. The value of the bus voltage threshold VTH-BUS may be lower in Fig. 5B than in Fig. 5A since the second learned load voltage VLED2 is lower than the first learned load voltage VLEDI. The control circuit may control the power converter circuit with an operating parameter using open loop control for a second charging time period TCHARGE2 between times ti and t ₄ . Since the control circuit may determine the operating parameter as a function of the first and second learned load voltage VLEDI, VLED2, the values of the first and second charging time periods TCHARGEI, TCHARGE2 may be approximately equal. After the magnitude of the second bus voltage VBUS2 exceeds the bus voltage threshold VTH-BUS at time U, the control circuit may control the power converter circuit using closed loop control for the control loop delay period TDELAY from time U until the LED light source turns on at time ts, such that the LED light source may turn on after a turn-on delay time period TTURN-ON2. Since the control loop delay period TDELAY may be a constant parameter (e.g., independent of the learned load voltages VLEDI, VLED2), the first and second turn-on delay time periods TTURN-ONI, TTURN-ON2 may be approximately equal.

[0054] Fig. 6 is a simplified flowchart of an example turn-on procedure 600 for controlling a power converter circuit of an LED driver (e.g., the flyback converter circuit 320 of the LED driver 300). For example, the turn-on procedure 600 may be executed by a control circuit (e.g., the control circuit 340) at step 610 in response to receiving a command to turn on a LED light source and/or in response to power being applied to the LED driver to turn on the LED light source. The control circuit may retrieve a learned load characteristic (e.g., the learned load voltage VLEARNED) from memory at step 612 and may determine the value of a charging threshold (e.g., the bus voltage threshold VTH-BUS) as a function of the learned load voltage VLEARNED (e.g., VTH-BUS = η ¹ VLEARNED, where η may be 0.85) at step 614. The control circuit may determine an operating parameter for the power converter circuit (e.g., the pre-load on-time ΪΟΝ-PRE) as a function of the learned load voltage VLEARNED at step 616 and control the power converter circuit using the pre-load

on-time ΪΟΝ-PRE at step 618. While the magnitude of the bus voltage VBUS is less than the bus voltage threshold VTH-BUS at step 620, the control circuit may continue to control the power converter circuit using the pre-load on-time ΪΟΝ-PRE at step 618. When the magnitude of the bus voltage VBUS is greater than or equal to the bus voltage threshold VTH-BUS at step 620, the control circuit may begin to control the power converter circuit using closed loop control at step 622 to regulate the magnitude of the bus voltage VBUS towards a target bus voltage, before the turn-on procedure 600 exits.

[0055] Fig. 7 is a simplified block diagram of a load control device, such as a LED driver 700 for controlling the intensity of an LED light source 702 (e.g., an LED light engine). The LED light source 702 is shown as a plurality of LEDs connected in series but may comprise a single LED or a plurality of LEDs connected in parallel or a suitable combination thereof, depending on the particular lighting system. In addition, the LED light source 702 may alternatively comprise one or more organic light-emitting diodes (OLEDs). The LED driver 700 may be adapted to work with a plurality of different LED light sources, which may be rated at different magnitudes of load current and voltage.

[0056] The LED driver 700 may comprise a hot terminal H and a neutral terminal N for receiving an AC voltage VAC from an AC power source (not shown). The LED driver 700 may comprise an RFI filter and rectifier circuit 710, which may receive the AC voltage VAC. The RFI filter and rectifier circuit 710 may operate to minimize the noise provided on the AC power source and to generate a rectified voltage VRECT. The LED driver 700 may comprise a power

converter circuit 720 (e.g., a first power converter circuit) and a load regulation circuit, e.g., an LED drive circuit 730 (e.g., a second power converter circuit). The power converter circuit 720 may receive the rectified voltage VRECT and generate a variable DC bus voltage VBUS across a bus capacitor CBUS. The power converter circuit 720 may comprise any suitable power converter circuit for generating an appropriate bus voltage, such as, for example, a boost converter, a buck converter, a buck-boost converter, a flyback converter, a single-ended primary-inductance converter (SEPIC), a Cuk converter, or other suitable power converter circuit. The power converter circuit 720 may also provide electrical isolation between the AC power source and the LED light source 702, and operate as a PFC circuit to adjust the power factor of the LED driver 100 towards a power factor of one.

[0057] The LED drive circuit 730 may receive the bus voltage VBUS and control the amount of power delivered to the LED light source 702 so as to control the intensity of the LED light source. For example, the LED drive circuit 730 may comprise a buck converter, as will be described in greater detail below. To control the amount of power delivered to the LED light source 702, the LED drive circuit 730 may be configured to control an average magnitude of a load current ILOAD conducted through the LED light source 702.

[0058] The LED driver 700 may include a control circuit 740 for controlling the operation of the power converter circuit 720 and the LED drive circuit 730. The control circuit 740 may comprise, for example, a controller or any other suitable processing device, such as, for example, a microcontroller, a programmable logic device (PLD), a microprocessor, an application specific integrated circuit (ASIC), or a field-programmable gate array (FPGA). The control circuit 740 may be configured to control the LED drive circuit 730 to control the average magnitude of the load current ILOAD conducted through the LED light source to control the amount of power delivered to the LED light source. The control circuit 740 may be configured to control the LED drive circuit 730 to turn the LED light source 702 on and off and to adjust (e.g., dim) a present

intensity LPRES of the LED light source 702 towards a target intensity LTRGT, which may range across a dimming range of the LED light source, e.g., between a low-end intensity LLE (e.g., approximately 0.1% - 1.0%) and a high-end intensity LHE (e.g., approximately 100%).

[0059] The control circuit 740 may be configured to fade (e.g., gradually adjust over a period of time) the target intensity LTRGT (and thus the present intensity LPRES) of the LED light source 702. The control circuit 740 may be configured to fade the LED light source 702 from off to on by slowly increasing the present intensity LPRES of the LED light source from a minimum fading

intensity LFADE-MIN, which may be less than the low-end intensity LLE (e.g., such as approximately 0.02%), to the target intensity LTRGT. The control circuit 740 may be configured to fade the LED light source 702 from on to off by slowly decreasing the present intensity LPRES of the LED light source from an initial intensity greater than or equal to the low-end intensity LLE to the minimum fading intensity LFADE-MIN at which point the control circuit 740 may turn off the LED light source.

[0060] The control circuit 740 may be coupled to a memory 712 configured to store operational characteristics of the LED driver 700 (e.g., the target intensity LTRGT, the low-end intensity LLE, the high-end intensity LHE, etc.). The memory 712 may be implemented as an external integrated circuit (IC) or as an internal circuit of the control circuit 740. The LED driver 700 may also comprise a communication circuit 714, which may be coupled to, for example, a wired communication link or a wireless communication link, such as a radio-frequency (RF) communication link or an infrared (IR) communication link. The control circuit 740 may be configured to determine the target intensity LTRGT of the LED light source 702 or the operational characteristics stored in the memory 712 in response to digital messages received via the

communication circuit 714. In response to receiving a command to turn on the LED light source 702, the control circuit 740 may be configured to execute the turn-on routine. The LED driver 700 may further comprise a power supply 716, which may receive the rectified voltage VRECT and generate a direct-current (DC) supply voltage Vcc (e.g., approximately 5 volts) for powering the low-voltage circuitry of the LED driver. In addition, the power supply 716 may generate one or more additional supply voltages, for example, for powering control circuitry of the power converter circuit 720 and/or the LED drive circuit 730.

[0061] The control circuit 740 may comprise a digital control circuit, such as a

processor 742, which may be, for example, a microprocessor, a programmable logic device (PLD), a microcontroller, an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or other suitable processing device or controller. The control circuit 740 may also comprise an analog control loop circuit 750. The processor 742 and the analog control loop circuit 750 may operate together to control the LED driver circuit 730 to adjust the average magnitude of the load current ILOAD towards a target current ITRGT. The target current ITRGT may be dependent upon the target intensity LTRGT (e.g., a function of the target intensity LTRGT). The processor 742 may generate a target-current control signal VI-TRGT, which may have a DC magnitude or a duty cycle that may indicate the target current ITRGT. The processor 742 may control the DC magnitude or the duty cycle of the target-current control signal VI-TRGT based on the target intensity LTRGT of the LED light source 702.

[0062] The control circuit 740 may also comprise a latch circuit 760 that may generate a drive signal VDR for controlling the operation of the LED drive circuit 730 (e.g., for rendering a switching transistor of the LED drive circuit 730 conductive and non-conductive to regulate the average magnitude of the load current ILOAD towards the target current ITRGT). The processor 742 may generate a frequency control signal VFREQ that may set an operating frequency fop of the LED drive circuit 730. In response to the frequency control signal VFREQ, the latch circuit 760 may control the drive signal VDR to render the switching transistor of the LED drive circuit 730 conductive to start a cycle of the LED drive circuit, at which time the LED drive circuit may begin to conduct an inductor current II conducted through an inductor (not shown) of the LED drive circuit 730. The analog control loop circuit 750 may generate a peak current threshold VTH-PK, which may be used by the latch circuit 760 to render the switching transistor of the LED drive circuit 730 non-conductive in response to the magnitude of the inductor current II.

[0063] The LED driver 700 may comprise an amplifier circuit 770, which may receive a current feedback signal VI-FB from the LED drive circuit 730. The amplifier circuit 770 may amplify the current feedback signal VI-FB to generate an instantaneous current feedback signal VI-INST, which may indicate an instantaneous magnitude of the inductor current II flowing through the inductor of the LED drive circuit 730.

[0064] The LED driver 700 may further comprise a filter circuit 780, such as a boxcar filter circuit. The filter circuit 780 may receive the instantaneous current feedback signal VI-INST and generate a filtered feedback signal, e.g., an average current feedback signal VI-AVE, which may indicate an average magnitude of the inductor current II flowing through the inductor of the LED drive circuit 730 (e.g., over a specific time window). The processor 742 may generate a filter control signal VFILTER (e.g., a filter control signal) for controlling the operation of the filter circuit 780, e.g., to control when the filter circuit 780 filters the instantaneous current feedback signal VI-INST. For example, the processor 742 may control the filter control signal VFILTER to allow the filter circuit 780 to filter the instantaneous current feedback signal VI-INST over a filter window period TFILTER during each cycle of the LED drive circuit 730. The processor 742 may control the filter control signal VFILTER in a manner that is synchronous with the frequency control signal VFREQ, e.g., to start a cycle of the LED drive circuit 730 at the beginning of the filter window period TFILTER. For example, the filter window period TFILTER may have the same length during each cycle of the LED drive circuit 730 independent of the frequency of the frequency control signal VFREQ. The magnitude of the average current feedback signal VI-AVE may indicate the average magnitude of the inductor current II during the filter window period TFILTER (e.g., while the filter circuit 780 is filtering the instantaneous current feedback signal VI-INST).

[0065] The analog control loop circuit 750 of the control circuit 740 may receive the average current feedback signal VI-AVE and the latch circuit 760 may receive the instantaneous current feedback signal VI-INST. The analog control loop circuit 750 may adjust the magnitude of the peak current threshold VTH-PK in response to the target-current control signal VI-TRGT and the average current feedback signal VI-AVE. The latch circuit 760 may control the drive signal VDR to render the switching transistor of the LED drive circuit 730 conductive in response to the frequency control signal VFREQ (e.g., at the beginning of a cycle of the LED drive circuit 730). The latch circuit 760 may control the drive signal VDR to render the switching transistor non-conductive in response to the peak current threshold VTH-PK and the instantaneous current feedback signal VI-INST. After rendering the switching transistor of the LED drive circuit 730 non-conductive, the latch circuit 760 may remain in a latched state and maintain the switching transistor non-conductive until the beginning of the next cycle of the LED drive circuit 730.

[0066] The control circuit 740 may be configured to determine or learn (e.g., measure or receive an indication of) one or more operational characteristics of the LED light source 702 (e.g., learned load characteristics). For example, the control circuit 740 may be configured to determine a voltage representative of the magnitude of the load voltage VLOAD. The magnitude of the load voltage VLOAD generated across the LED light source 702 may be dependent upon the magnitude of the load current ILOAD (e.g., the target load current ITRGT to which the control circuit 740 is regulating the load current ILOAD) as well as the internal circuitry of the LED light source. The control circuit 740 may be configured to determine (e.g., measure) the magnitude of the load voltage VLOAD and/or store the measurement in the memory 712 as a learned load voltage VLEARNED. The control circuit 740 may be configured to determine (e.g., measure) the magnitude of the load voltage VLOAD using a load voltage feedback signal VV-LOAD received from the LED drive circuit 730. For example, the LED drive circuit 730 may comprise a resistive divider circuit (not shown) coupled across the LED light source 702 for generating the load voltage feedback signal VV-LOAD as a scaled load voltage. The load voltage feedback signal VV-LOAD may be received by an analog-to-digital converter (ADC) of the processor 742 for learning the magnitude of the load voltage VLOAD.

[0067] The control circuit 740 may be configured to determine (e.g., measure) the magnitude of the load voltage VLOAD when the target intensity LTRGT is at or near the low-end intensity LLE. For example, the control circuit 740 may be configured to determine (e.g., measure) the magnitude of the load voltage VLOAD while the control circuit 740 is fading the LED light source 702 from on to off, for example, while the average magnitude of the load current ILOAD is within a measurement window that may range from a maximum learning threshold ILEARN-MAX to a minimum learning

threshold ILEARN-MIN. The maximum learning threshold ILEARN-MAX and the minimum learning threshold ILEARN-MIN may be functions of a rated (or maximum) current IRATED of the LED light source 702, for example, 0.0020 TRATED and 0.0002 TRATED, respectively.

[0068] The control circuit 740 may be configured to control the LED drive circuit 730 using the learned load voltage VLEARNED. For example, the control circuit 740 may be configured to control the LED drive circuit 730 in response to the learned load voltage VLEARNED when turning on the LED light source 702. The control circuit 740 may be configured to charge (e.g., "pre-charge") an output capacitor (not shown) of the LED drive circuit 730 prior to attempting to turn on the LED light source 702. In response to receiving a command to turn on the LED light source 702 and/or in response to power being applied to the LED driver 700 to turn on the LED light source, the control circuit 740 may pre-charge the output capacitor until the magnitude of the load voltage VLOAD reaches or exceeds a pre-charge voltage threshold VTH-PC, which may be, for example, a function of the learned load voltage VLEARNED (e.g., as will be described in greater detail below). The pre-charging of the output capacitor may allow the LED driver 700 to turn-on the LED light source 702 quickly and consistently, e.g., when fading on to the low-end intensity LLE.

[0069] The control circuit 740 may be configured to determine an operating parameter (e.g., a pre-load parameter) as a function of the learned load voltage VLEARNED and use the operating parameter to control the LED drive circuit 730 to pre-charge the output capacitor of the LED drive circuit 730 prior to turning the LED light source 702 on (e.g., as will be described in greater detail below). For example, the control circuit 740 may be configured to determine the DC magnitude or the duty cycle of the target-current control signal VI-TRGT to use while pre-charging the output capacitor of the LED drive circuit 730 as a function of the learned load voltage VLEARNED. In addition, the processor 742 may generate a start-up control signal VSTART-UP for controlling the analog control loop circuit 750 while pre-charging the output capacitor of the LED drive circuit 730 to maintain the output of the analog control loop circuit 750 at a predetermined voltage.

[0070] After the magnitude of the load voltage VLOAD reaches or exceeds the pre-charge voltage threshold VTH-PC, the processor 742 may control the start-up control signal VSTART-UP to allow the analog control loop circuit 750 to control the LED drive circuit 730 using closed loop control in response to the current feedback signal VI-FB to regulate the magnitude of the load current ILOAD towards the target current ITRGT.

[0071] Fig. 8 is a simplified schematic diagram of a load control device, e.g., an LED driver 800 (such as the LED driver 700 of Fig. 1) for controlling the intensity of an LED light source 802. The LED driver 800 may comprise a bus capacitor CBUS for storing a bus voltage VBUS, which may be generated by a first power converter circuit (e.g., the power converter circuit 720 of the LED driver 700). The LED driver 800 may comprise a second power converter circuit, e.g., an LED drive circuit 830, which may be configured to control the magnitude of a load current ILOAD conducted through the LED light source 802. The LED driver 800 may further comprise a control circuit 840, which may be a hybrid analog-digital control circuit (e.g., the control circuit 740 of the LED driver 700). The control circuit 840 may comprise a processor 842, a low-pass filter circuit 844, an analog control loop circuit (e.g., which may include an integrator circuit 850), and a latch circuit 860. The latch circuit 860 may generate a drive signal VDR, which may be provided to the LED driver circuit 830. The LED driver 800 may further comprise an amplifier circuit 870 and a filter circuit 880 (e.g., a boxcar filter circuit) for generating an instantaneous current feedback signal VI-INST and an average current feedback signal VI-AVE, respectively.

[0072] As shown in FIG. 8, the LED drive circuit 830 may comprise a buck converter. The

LED drive circuit 830 may comprise a switching transistor, e.g., a field-effect transistor (FET) Q832, which may be controlled in response to the drive signal VDR to control the magnitude of the load current ILOAD. The LED drive circuit 830 may also comprise an inductor L834, a switching diode D835, an output capacitor C836, and a feedback resistor R838. The drive signal VDR may be coupled to a gate of the FET Q832 through a gate drive circuit 839. When the FET Q832 is conductive, the inductor L834 may conduct an inductor current II from the bus capacitor CBUS through the parallel combination of the output capacitor C836 and the LED light source 802. When the FET Q832 is non-conductive, the inductor L834 may conduct the inductor current II through the switching diode D835 and the parallel combination of the output capacitor C836, and the LED light source 802. The LED light source 802 may conduct the average component of the inductor current II and the output capacitor C836 may conduct the transient component of the inductor current II. The average magnitude of the load current ILOAD may be approximately equal to the average magnitude of the inductor current II.

[0073] The current feedback signal VI-FB may be generated across the feedback resistor R838 of the LED drive circuit 830 and may be proportional to the magnitude of the inductor current II. The current feedback signal VI-FB may be received by the amplifier circuit 870. The amplifier circuit 870 may comprise an operational amplifier U872 and may be configured as a non-inverting amplifier circuit. The operational amplifier U872 may have a non-inverting input that may receive the current feedback signal VI-FB. The amplifier circuit 870 may also comprise a resistor R874 coupled between an inverting input of the operational amplifier U872 and circuit common, and a resistor R876 coupled between the inverting input and an output of the operational amplifier U872. The amplifier circuit 870 may be configured to generate the instantaneous current feedback signal VI-INST, which may be an amplified version of the current feedback signal VI-FB and may indicate the instantaneous magnitude of the inductor current II.

[0074] The filter circuit 880 may filter the instantaneous current feedback signal VI-INST to generate the average current feedback signal VI-AVE, which may indicate the average magnitude of the inductor current II. The filter circuit 880 may comprise a controllable switching circuit 882 and a low-pass filter circuit (e.g., a third-order low-pass filter circuit) that includes resistors R884, R886, R888 and capacitors C885, C887, C889. The processor 842 may generate a filter control signal VFILTER for rendering the controllable switching circuit 882 conductive and non-conductive. When the controllable switching circuit 882 is conductive, the filter circuit 880 may be configured to filter the instantaneous current feedback signal VI-INST to generate the average current feedback signal VI-AVE. When the controllable switching circuit 882 is non-conductive, the capacitors C885, C887, C889 of the filter circuit 880 may maintain the magnitude of the average current feedback signal VI-AVE at a value that indicates the average magnitude of the inductor current II during the period of time when the controllable switching circuit 882 was previously conductive.

[0075] The processor 842 may generate a pulse-width modulated (PWM) signal VPWM, which may be received by the low-pass filter circuit 844 of the control circuit 840. The low-pass filter circuit 844 may be configured to generate a target-current control signal VI-TRGT, which may have a DC magnitude that indicates the target current ITRGT. For example, the low-pass filter circuit 844 may comprise a resistor-capacitor (RC) circuit having a resistor R846 and a

capacitor C848. The processor 842 may be configured to control the duty cycle of the pulse-width modulated signal VPWM to adjust the magnitude of the target-current control signal VI-TRGT.

[0076] The average current feedback signal VI-AVE generated by the filter circuit 880 and the target-current control signal VI-TRGT generated by the low-pass filter circuit 844 may be received by the integrator circuit 850. The integrator circuit 850 may comprise an operational amplifier U852 having a non-inverting input coupled to the target-current control signal VI-TRGT and an inverting input coupled to the average current feedback signal VI-AVE via a resistor R854. The integrator circuit 850 may comprise a capacitor C856 coupled between the inverting input and an output of the operational amplifier U852, such that the integrator circuit 850 may be configured to integrate the error between the average current feedback signal VI-AVE and the target-current control

signal VI-TRGT. The integrator circuit 850 may generate a peak current threshold VTH-PK having a DC magnitude that may increase or decrease by amounts dependent upon the error between the magnitude of the target-current control signal VI-TRGT and the average current feedback signal VI-AVE. The integrator circuit 850 may comprise a controllable switching circuit 858 coupled in parallel with the capacitor C856. The controllable switching circuit 858 may be rendered conductive and non-conductive in response to a startup control signal VSTART-UP received from the processor 842 during a startup routine (e.g., as will be described in greater detail below). [0077] The latch circuit 860 may receive the peak current threshold VTH-PK generated by the integrator circuit 850 and the instantaneous current feedback signal VI-INST generated by the amplifier circuit 870. The latch circuit 860 may comprise a comparator U862 configured to compare the magnitude of the instantaneous current feedback signal VI-INST to the magnitude of the peak current threshold Vra. The comparator U862 may generate a latch control signal VLATCH at an output. When the magnitude of the instantaneous current feedback signal VI-INST is less than the magnitude of the peak current threshold Vra, the comparator U862 may drive the latch control signal VLATCH at the output high (e.g., towards the supply voltage Vcc). When the magnitude of the instantaneous current feedback signal VI-INST exceeds the magnitude of the peak current

threshold VTH-PK, the comparator U862 may drive the latch control signal VLATCH at the output low (e.g., towards circuit common).

[0078] The processor 842 may generate a frequency control signal VFREQ that may set an operating frequency fop of the LED drive circuit 830. The latch circuit 860 may comprise a PWM control circuit 866, which may receive the latch control signal VLATCH from the comparator U262 and the frequency control signal VFREQ from the processor 842. The PWM control circuit 866 may generate the drive signal VDR, which may be received by the gate drive circuit 839 of the LED drive circuit 830. When the frequency control signal VFREQ is driven high at the beginning of a cycle of the LED driver circuit 830, the PWM control circuit 866 may drive the magnitude of the drive signal VDR high, which may render the FET Q832 of the LED drive circuit 830 conductive. When the magnitude of the instantaneous current feedback signal VI-INST exceeds the magnitude of the peak current threshold signal Vra, the comparator U862 may drive the latch control signal VLATCH low, which may cause the PWM control circuit 866 to drive the magnitude of the drive signal VDR low. The PWM control circuit 866 may maintain the magnitude of the drive signal VDR low until the processor 842 drives the magnitude of the frequency control signal VFREQ high once again at the end of the present cycle and the beginning of the next cycle of the LED drive circuit 830.

[0079] The processor 842 may control the frequency of the frequency control signal VFREQ and the duty cycle of the pulse-width modulated control signal VPWM (and thus the magnitude of the target-current control signal VI-TRGT) in dependence upon the target current ITRGT of the LED light source 802 using open loop control. Fig. 9 A is an example plot of a relationship 900 between the frequency of the frequency control signal VFREQ (e.g., the operating frequency fop of the LED drive circuit 830) and the target current ITRGT. Fig. 9B is an example plot of a relationship 910 between the magnitude of the target-current control signal VI-TRGT and the target current ITRGT. For example, the target current ITRGT may range between a high-end current IHE (e.g., approximately 150 raA) at the high-end intensity LHE and a low-end current ILE (e.g., approximately 150 μΑ) at the low-end intensity LLE.

[0080] The processor 842 may operate in first and second modes of operation depending upon whether the target current ITRGT is less than or greater than approximately a transition current ITRAN (e.g., approximately 16.8 raA). Near the low-end intensity LLE (e.g., when the target current ITRGT is less than approximately the transition current ITRAN), the processor 842 may operate in the first operating mode during which the processor 842 may adjust the frequency of the frequency control signal VFREQ between a minimum operating frequency fiviiN and a maximum operating frequency fiviAx (e.g., linearly) with respect to the target current ITRGT while holding the magnitude of the target-current control signal VI-TRGT constant (e.g., at a minimum voltage VMIN). Near the high-end intensity LHE (e.g., when the target current ITRGT is greater than or equal to approximately the transition current ITRAN), the processor 842 may operate in the second operating mode during which the processor 842 may adjust the magnitude of the target-current control signal VI-TRGT between the minimum voltage VMIN and a maximum voltage VMAX (e.g., linearly) with respect to the target current ITRGT while holding the frequency control signal VFREQ constant (e.g., at the maximum operating frequency fviAx). For example, the maximum operating

frequency fiviAx may be approximately 140 kHz and the minimum operating frequency fviiN may be approximately 1250 Hz. For example, the maximum voltage VMAX may be approximately 3.3 V and the minimum voltage VMIN may be approximately 44 mV.

[0081] Figs. 10A and 10B show example waveforms illustrating the operation of the LED driver 800 shown in Fig. 8. Fig. 10A shows example waveforms illustrating the operation of the LED driver 800 when the target current ITRGT is less than the transition current ITRAN. The processor 842 may generate the frequency control signal VFREQ to set the operating frequency fop of the LED drive circuit 830. For example, an operating period TOP of the LED drive circuit 830 may be equal to the period of the frequency control signal VFREQ. The processor 842 may set the operating frequency fop (and thus the operating period TOP) in dependence upon the target current ITRGT (e.g., as shown in Fig. 9 A). The processor 842 may generate the frequency control signal VFREQ to have a predetermined on-time TFREQ-ON, which may have the same length each cycle of the LED drive circuit 830 (e.g., independent of the frequency of the frequency control signal VFREQ or the target current ITRGT).

[0082] The processor 842 may generate the filter control signal VFILTER in a synchronous manner with respect to the frequency control signal VFREQ. For example, the processor 842 may drive both the filter control signal VFILTER and the frequency control signal VFREQ high at the same time to start a cycle of the LED drive circuit 830 (e.g., at time ti in Fig. 10A). At time ti, the PWM control circuit 866 of the latch circuit 860 may drive the magnitude of the drive signal VDR high (e.g., towards the supply voltage Vcc) causing the FET Q832 of the LED drive circuit 830 to be rendered conductive. At this time, the inductor L834 of the LED drive circuit 830 may begin to conduct the inductor current II. When the instantaneous current feedback signal VI-INST (which may be proportional to the magnitude of the inductor current II) exceeds the magnitude of the peak current threshold signal Vra, the PWM control circuit 866 may drive the magnitude of the drive voltage VDR low (e.g., towards circuit common) as shown at time t ₂ of Fig. 10A, which may cause the FET Q832 of the LED drive circuit 830 to be rendered non-conductive. The drive signal VDR may be characterized by an on-time TON and a period that may be equal to the operating period TOP as shown in Fig. 10A. The PWM control circuit 866 may render the FET Q832 conductive for the length of the on-time TON of the drive signal VDR during each operating cycle of the LED drive circuit 830. The inductor current II may have a peak magnitude IPK as shown in Fig. 10A. The magnitude of the inductor current II may begin to decrease at time t ₂ until the magnitude of the inductor current II drops to zero amps at time t ₃ .

[0083] The processor 842 may drive the frequency control signal VFREQ low at the end of the predetermined on -time TFREQ-ON (e.g., at time U in Fig. 10A). The processor 842 may drive the filter control signal VFILTER low at the end of a filter window period TFILTER (e.g., at time ts in Fig. 10A). The processor 842 may drive both the filter control signal VFILTER and the frequency control signal VFREQ high to start another cycle of the LED drive circuit 830 at the end of the operating period TOP (e.g., at time te in Fig. 10A).

[0084] When the target current ITRGT is less than the transition current ITRAN, the

processor 842 may hold the magnitude of the target-current control signal VI-TRGT constant at the minimum voltage VMIN, and linearly adjust the frequency of the frequency control signal VFREQ between the minimum frequency fviiN and the maximum frequency fviAx as a function of the target current ITRGT (e.g., as shown in Figs. 9A and 9B). The filter circuit 880 may be configured to filter the instantaneous current feedback signal VI-INST during the filter window period TFILTER each cycle of the LED drive circuit 830. When the target current ITRGT is less than the transition current ITRAN, the filter control signal VFILTER may be a periodic signal characterized by the operating

frequency fop. The processor 842 may maintain the length of the filter window period TFILTER of the filter control signal VFILTER constant from one cycle of the LED driver circuit 830 to the next cycle independent of the frequency of the frequency control signal VFREQ. A duty cycle of the filter control signal VFILTER may vary as the frequency of the frequency control signal VFREQ is adjusted.

[0085] Since the target-current control signal VI-TRGT and the filter window period TFILTER are held constant, the on-time TON of the drive signal VDR may be approximately the same each cycle of the LED drive circuit 830 even though the frequency of the drive signal VDR (e.g., the operating period TOP) may vary in dependence upon the target current ITRGT. AS a result, the peak and average magnitudes of the inductor current II during the filter window period TFILTER may be approximately the same from one cycle to the next of the LED drive circuit 830 independent of the target current ITRGT when the target current ITRGT is less than the transition current ITRAN. The length of the filter window period TFILTER may be sized to ensure that the inductor current II drops to zero amps before the end of the filter window period TFILTER when the target current ITRGT is less than the transition current ITRAN. When the target current is less than the transition current ITRAN, the LED drive circuit 830 may be configured to operate in a discontinuous mode of operation.

[0086] Fig. 10B shows example waveforms illustrating the operation of the LED driver 800 when the target current ITRGT is greater than the transition current ITRAN. When the target current ITRGT is greater than the transition current ITRAN, the processor 842 may linearly adjust the magnitude of the target-current control signal VI-TRGT between the minimum voltage VMIN and the maximum voltage VMAX as a function of the target current ITRGT (e.g., as shown in Figs. 9A and 9B). In addition, the processor 842 may hold the frequency of the frequency control signal VFREQ constant at the maximum operating frequency fviAx (e.g., causing the operating period TOP to be held constant at a minimum operating period TMIN). When the target current ITRGT is greater than the transition current ITRAN, the processor 842 may control the duty cycle of the filter control signal VFILTER to a maximum filter duty cycle (e.g., 100%). For example, the operating period TOP may be equal to the length of the filter window period TFILTER when the target current ITRGT is greater than the transition current ITRAN. AS a result, the processor 842 may drive the filter control signal VFILTER high at all times (e.g., the filter control signal VFILTER is a constant signal) while the target current ITRGT is greater than the transition current ITRAN as shown in Fig. 10B. The average current feedback signal VI-AVE may indicate the average magnitude of the inductor current II when the target current ITRGT is greater than the transition current ITRAN. Additionally or alternatively, the processor 842 may drive the filter control signal VFILTER high approximately all of the time (e.g., almost all of the time), for example at substantially large duty cycle (e.g., approximately 90% or greater).

[0087] Because the processor 842 varies the magnitude of the target-current control signal VI-TRGT as a function of the target current ITRGT, the length of the on-time TON of the drive signal VDR may vary as a function of the target current ITRGT even though the frequency of the drive signal VDR (e.g., the operating period TOP) is held constant. As the target current ITRGT increases, the peak current IPK of the inductor current may increase to a point at which the LED drive circuit 830 may begin to operate in a continuous mode of operation. Since the minimum operating period TMIN (e.g., the operating period TOP when the target current ITRGT is greater than the transition

current ITRAN) may be equal to the length of the filter window time period TFILTER, the processor 842 may be configured to smoothly transition the LED driver 800 between the first operating mode when the target current ITRGT is less than the transition current ITRAN and the second operating mode when the target current ITRGT is greater than the transition current ITRAN. [0088] The length of the predetermined on-time TFREQ-ON of the frequency control signal TFREQ is less than the length of the operating period TOP when the target current ITRGT is greater than the transition current ITRAN. The processor 842 may drive the frequency control signal TFREQ low (e.g., at time ti in Fig. 10B) and then high (e.g., at time ts) at the end of each cycle of the LED drive circuit 830. This causes the PWM control circuit 866 of the latch circuit 860 to stop maintaining the magnitude of the drive signal VDR low, and to drive the magnitude of the drive signal VDR high again when the frequency control signal TFREQ is driven high to begin the next cycle of the LED drive circuit 830 (e.g., at time ts).

[0089] The processor 842 of the control circuit 840 may be configured to determine or learn

(e.g., measure or receive an indication of) the magnitude of the load voltage VLOAD (e.g., developed across the capacitor C836) and/or store the measurement in memory (e.g., the memory 712) as a learned load voltage VLEARNED (e.g., a learned capacitor voltage). The magnitude of the load voltage VLOAD generated across the LED light source 802 may be dependent upon the magnitude of the load current ILOAD (e.g., the target load current ITRGT to which the control circuit 840 is regulating the load current ILOAD) as well as the internal circuitry of the LED light source. The processor 842 may be configured to receive a load voltage feedback signal from the LED drive circuit 830 (e.g., the load voltage feedback signal VV-LOAD of the LED driver 100), which may be a scaled version of the load voltage VLOAD generated by a resistive divider circuit (not shown) of the LED drive circuit 830. The processor 842 may sample the load voltage feedback signal using an analog-to-digital converter (ADC) to measure the magnitude of the load voltage VLOAD.

[0090] Fig. 11 A shows example waveforms illustrating the operation of the LED driver 800 when the processor 842 is learning the load voltage VLOAD. The processor 842 may be configured to determine (e.g., measure) the magnitude of the load voltage VLOAD while the processor 842 is fading the LED light source 802 from on to off. As shown in Fig. 11 A, when fading the LED light source 802 from on to off, the processor 842 may begin to decrease the average magnitude of the load current ILOAD from an initial current IINIT at time to, at which time the magnitude of the load voltage VLOAD may also begin to decrease, e.g., from an initial voltage VINIT. The processor 842 may be configured to determine (e.g., measure) the magnitude of the load voltage VLOAD while the average magnitude of the load current ILOAD is within a measurement window that may range from a maximum learning threshold ILEARN-MAX and a minimum learning threshold ILEARN-MIN (e.g., between times twiN-START and twiN-END as shown in Fig. 1 1 A). The maximum learning threshold ILEARN-MAX and the minimum learning threshold ILEARN-MIN may be functions of a rated (or maximum) current IRATED of the LED light source 802, for example, 0.0020 TRATED and 0.0002 TRATED, respectively. The processor 842 may be configured to periodically sample the load voltage feedback signal during the measurement window, and to process the plurality of samples to determine the learned load voltage VLEARNED. For example, the processor 842 may be configured to process the plurality of samples of the load voltage feedback signal by calculating an average or median value of the plurality of samples or filtering the samples using a digital low-pass filter.

[0091] The processor 842 may be configured to measure the load voltage VLOAD and determine the learned load voltage VLEARNED when (e.g., each time that) the processor 842 turns the LED light source 802 off (e.g., fades the LED light source off). The processor 842 may be configured to overwrite the learned load voltage VLEARNED stored in the memory with the learned load voltage VLEARNED determined the last time that the processor 842 turned off the LED light source 802. In addition, the processor 842 may be configured to process the learned load

voltages VLEARNED from multiple turn-off events (e.g., calculate the average or median value of the multiple learned load voltages) before overwriting the learned load voltage VLEARNED stored in the memory.

[0092] The processor 842 may be configured to control the LED drive circuit 830 using the learned load voltage VLEARNED, for example, when turning on the LED light source 802. Fig. 1 IB shows example waveforms illustrating the operation of the LED driver 800 when the processor 842 is fading on the LED light source 802 (e.g., fading on to a target intensity LTRGT that corresponds to a target current ITRGT). In response to receiving a command to turn on the LED light source 802 and/or in response to power being applied to the LED driver 800 to turn on the LED light source, the processor 842 may be configured to pre-charge the output capacitor C836 of the LED drive circuit 830 during a pre-charge period TPRE-CHARGE prior to attempting to turn on the LED light source 802. During the pre-charge period TPRE-CHARGE, the processor 842 may be configured to control the duty cycle of the pulse-width modulated signal VPWM (and thus the DC magnitude of the target-current control signal VI-TRGT) as a function of the learned load voltage VLEARNED to cause the output capacitor C836 to charge faster than normal (e.g., faster than if the processor 242 controlled the DC magnitude of the target-current control signal VI-TRGT in response to the target current ITRGT as shown in Fig. 9B). The faster rate at which the output capacitor C836 charges during the pre-charge period TPRE-CHARGE may allow the processor 842 to turn-on the LED light source 802 quickly and consistently, e.g., when fading the LED light source on to the low-end intensity LLE.

[0093] The control circuit 840 may be configured to pre-charge the output capacitor C836 of the LED drive circuit 830 until the magnitude of the load voltage VLOAD reaches or exceeds a pre-charge voltage threshold VTH-PC. The pre-charge voltage threshold VTH-PC may be determined, for example, as a function of the learned load voltage VLEARNED (e.g., VTH-PC = a ^■ VLEARNED, where a is a constant that may be, for example, approximately 0.90). Since the magnitude of the load voltage VLOAD may be greater when the LED light source 802 is cold than when the LED light source 802 is warm, the constant a may be sized to be less than one to ensure that the LED drive circuit 830 does not overshoot the learned load voltage VLEARNED when pre-charging the output capacitor C836. Additionally or alternatively, the pre-charge voltage threshold VTH-PC may be determined, for example, using a different function of the learned load voltage VLEARNED (e.g., VTH-PC = VLEARNED - β, where β is a constant that may be, for example, approximately one volt). Additionally or alternatively, the pre-charge voltage threshold VTH-PC may be a fixed threshold (e.g., a predetermined threshold). The processor 842 may be configured to cease pre-charging the output capacitor C836 if the magnitude of the load voltage VLOAD does not exceed the pre-charge voltage threshold VTH-PC within a timeout period. The processor 842 may be configured to select the value of the duty cycle of the pulse-width modulated signal VPWM based on the learned load

voltage VLEARNED such that pre-charge period TPRE-CHARGE for the LED driver 800 may be approximately the same for different LED light sources that have different resulting load voltages.

[0094] The processor 842 may control the start-up control signal VSTART-UP to render the controllable switching circuit 858 of the integrator circuit 850 conductive during the pre-charge period TPRE-CHARGE. After the magnitude of the load voltage VLOAD reaches or exceeds the pre-charge voltage threshold VTH-PC, the processor 842 may control the start-up control signal VSTART-UP to render the controllable switching circuit 858 of the integrator circuit 850 non-conductive. This may allow the integrator circuit 50 and the latch circuit 860 to control the LED drive circuit 830 using closed loop control in response to the current feedback signal VI-FB to regulate the magnitude of the load current ILOAD towards the target current ITRGT.

[0095] Fig. 12 is a simplified flowchart of an example turn-on procedure 1200 for controlling a power converter circuit of an LED driver (e.g., the LED drive circuit 830 of the LED driver 800). For example, the turn-on procedure 1200 may be executed by a control circuit (e.g., the control circuit 840) at step 1210 in response to receiving a command to turn on a LED light source and/or in response to power being applied to the LED driver to turn on the LED light source. The control circuit may retrieve the learned load voltage VLEARNED from memory at step 1212 and may set the value of the pre-charge threshold VTH-PC based on the learned load voltage VLEARNED at step 1214. At 1216, the control circuit may set the duty cycle of the pulse-width modulated signal VPWM based on the learned load voltage VLEARNED. The control circuit may set the frequency of the frequency control signal VFREQ to be equal to the maximum operating frequency fiviAx at 1218, and may drive the startup control signal VSTART-UP high to render the controllable switching circuit 858 conductive at 1220. When the magnitude of the bus voltage VBUS is greater than or equal to the pre-charge threshold VTH-PC at step 1222, the control circuit may drive the startup control signal VSTART-UP low to render the controllable switching circuit 858 non-conductive at 1224 and set the target current ITRGT based on the target intensity LTRGT at 1226. The control circuit may begin to fade on the LED light source at 1228 by starting at the minimum fading intensity LFADE-MIN and slowly increasing the present intensity LPRES of the LED light source towards the target intensity LTRGT, before the turn-on procedure 1200 exits.