Background

At present, conventional lighting applications are more and more replaced by LED illumination systems. In general, an LED based lighting application comprises an LED fixture comprising one or more LEDs and an LED driver for powering the LED fixture. Such an LED driver in general comprises a power converter such as a switched mode power supply (e.g. a Buck or Boost converter) and a control unit for controlling the power converter and/or the LED fixture.

In general, LED based illumination can provide several advantages over conventional lighting applications, such as incandescent lamps or the like, as it enables an increased functionality. In particular, in an LED based lighting application, an increased functionality (with respect to e.g. color or color temperature, intensity) can be realized by an accurate control of the current supplied to the LED or LEDs of the LED fixture. By application of duty- cycle or amplitude control of the current supplied to the LED or LEDs of an LED fixture, illumination parameters such as color or intensity can be changed.

To control the LED or LEDs output, a control unit (e.g. comprising a microcontroller or the like) is used to control the current as provided to the LEDs by controlling the power converter (e.g. controlling the amplitude of the current) or controlling the duty-cycle at which the current is applied (e.g. by opening and closing a switch provided in parallel to an LED). With respect to the latter, an adjustment of the signal that is outputted by the control unit to a suitable control signal for operating such a switch, may be required. In particular, such an adjustment should ensure the proper operation of the switch, irrespective of the voltage the LED or LEDs are working at. In particular, when a number of LEDs of the LED fixture are connected in series and are each provided with a parallel switch, the voltage at which the LEDs are operating may differ from LED to LED. The operating voltage may further vary over time. In such case, a voltage level shifting circuit is often applied. Known solutions to shift a control signal for an LED that is outputted by a control unit to the appropriate level for controlling the switch may however result in an important dissipation or may affect the accuracy at which the current can be controlled.

With respect to the latter, it is worth noting that an accurate control of the shape of a current pulse provided to an LED becomes more and more important for obtaining a required intensity or color. In particular at comparatively low intensity levels which are realized by applying comparatively short pulses, an accurate pulse control is required to e.g. obtain a desired dimming resolution.

In view of the above, it is an object of the present invention to provide a control unit for an LED fixture at least mitigating one of the drawbacks mentioned.

Summary of the invention:

According to a first aspect of the invention, there is provided a control circuit for an LED fixture comprising one or more LEDs or LED groups, the control circuit comprising

- a control unit comprising a set point terminal for receiving a set point signal representing a desired illumination of the LED fixture;

a voltage level shifting circuit, the voltage level shifting circuit comprising

an input terminal for receiving an input signal representing a desired operating state of an LED or LED group of the LED fixture;

- an output terminal for providing a control signal for controlling a switch associated with the LED or LED group;

the voltage level shifting circuit being configured to convert a voltage level of the input signal to a voltage level of the control signal;

whereby the voltage level shifting circuit further comprises a discharge path for a parasitic capacitance of the switch different from a charge path for the parasitic capacitance.

In accordance with an aspect of the present invention, there is provided a control circuit for controlling an LED fixture. In accordance with the present invention, an LED fixture is considered to comprise at least one LED. An LED fixture can comprise a plurality of LEDs, e.g. of different colour. The LEDs of an LED fixture can, in general, be arranged in series or in parallel. In general, an LED fixture is powered from an LED driver, the LED driver typically comprising a power converter such as a Buck or Boost converter for providing a current to the LED fixture and a control circuit. The present invention provides in such a control circuit, whereby the control circuit comprises a control unit comprising a set point terminal for receiving a set point signal representing a desired illumination of the LED fixture. Such a control unit may take the form of a microprocessor or controller or the like. In accordance with the present invention, the control unit of the control circuit comprises a set point terminal for receiving a signal representing a desired illumination of the LED fixture. Such signal, also referred to as a set point signal may e.g. be provided by a user interface. The signal may e.g. be a digital or analogue signal representing a desired illumination intensity, i.e. a dimming level, and/or a desired colour. In an embodiment, the control circuit according to the present invention may also control one or more electronic switches associated with the LED or LEDs of the LED fixture. In case the LEDs of an LED fixture are series connected, the same current as supplied by the power converter is supplied to all LEDs, enabling an accurate control of the illumination of each LED compared to the other LEDs, thus e.g.

enabling an accurate colour control. Assuming e.g. the LED fixture to comprise three series connected LEDs and each LED being provided with an electronic switch (e.g. a FET or MOSFET) connected in parallel to the LED. By controlling the operating state of the switches, the current as provided by the power converter can be directed through the LED or through the switch thus enabling the LED to provide illumination at a specific duty-cycle, thus controlling the intensity as perceived by an observer. In order to control the switches associated with the LEDs of the LED fixture, a control signal at an appropriate voltage level needs to be made available at the switch (e.g. at a gate of the FET or MOSFET). In case an LED fixture comprises a series connection of LEDs, the required voltage for controlling a particular switch may vary (depending on the operating state of the other switches) and may be comparatively high compared to a conventional output level as found at an output of a microcontroller (e.g. 5 V DC). As such, a circuit is often provided for ensuring an appropriate conversion between a voltage level as available at an output terminal of a control unit, e.g. the control unit as applied in the control circuit according to the present invention, and a required voltage for controlling a switch associated with an LED. Note that, in accordance with the present invention, the control unit is configured to provide an input signal representing a desired operating state of an LED of the LED fixture. Such a signal may thus also represent a desired operating state of a switch associated with the LED and/or a command signal for controlling the operating state of such a switch. However, as mentioned, when such a signal is outputted by the control unit, the signal level may be insufficient to control the switch associated with the LED.

As such, the control circuit according to the present invention further comprises a voltage level shifting circuit. In accordance with the present invention, a voltage level shifting circuit comprise an input terminal for receiving an input signal representing a desired operating state of an LED of the LED fixture, i.e. the signal as outputted by the control unit of the control circuit according to the present invention. As such, the input terminal of the voltage level shifting circuit may e.g. be connected to an output of the control unit as discussed, the input signal typically being a low voltage DC signal. In accordance with the present invention, the voltage level shifting circuit further comprise an output terminal for providing a control signal for controlling a switch associated with the LED, whereby the voltage level shifting circuit is configured to convert a voltage level of the input signal to a voltage level of the control signal.

As an example, such a voltage level shifting circuit can comprise a voltage divider (e.g. a resistive voltage divider) connected to the input terminal and controlling an operating state of a first transistor connecting a further voltage divider to a voltage supply, the further voltage divider providing the control signal at the output terminal.

Such a known arrangement can be applied to provide a control signal to a gate of an electronic switch such as an FET at an appropriate voltage thus controlling the FET to conduct or not.

In accordance with the first aspect of the invention, the voltage level shifting circuit of the control circuit for the LED fixture is further provided with a discharge path for discharging a parasitic capacitance of the switch, the discharge path being different from a charge path for the parasitic capacitance. By applying such a discharge path, a more accurate current control (and thus intensity control) of the LED fixture can be obtained, as will be explained in more detail below. The application of a specific discharge path different from a current path that enables the charging of the parasitic capacitance enables an optimisation of the voltage level shifter substantially without having to consider any current shape issues. This can be understood as follows: when a voltage divider is applied to provide an output signal at an appropriate voltage level, it may be advantageous to apply resistors having a comparatively high resistance for the voltage divider in order to reduce dissipation in the control circuit. The application of a comparatively high resistance may however result in a comparatively slow discharging of a parasitic capacitance of the electronic switch. As a result, the instance at which the electronic switch ceases to conduct would be uncertain thus affecting the controllability of the duty cycle at which the LED is operated. This may, in particular at comparatively low intensities, adversely affect the resolution that can be applied to the intensity. Since a specific colour set point is often obtained by operating a plurality of LEDs at a different duty cycle, the resolution which can be obtained with respect to colour is equally affected. In accordance with the invention, a separate discharge path is provided which enables the discharging of a parasitic capacitance.

In an embodiment, the discharge path is dimensioned to enable a comparatively fast discharge of the parasitic capacitance enabling the switch to stop conducting comparatively fast. As such, the steepness at which the current through the LED increases can be comparatively high enabling the application of an accurate duty cycle.

The control circuit can be considered an interface between a control unit or controller

(providing e.g. a digital output signal representing a desired operating state of an LED) and a switch or switch assembly that actually controls the current through the LED.

In an embodiment, the input signal as applied to the control circuit is a pulsed input signal. By using such a pulsed input signal (which can be either a signal to open or to close the switch), the slope of the current passing through the LED (either ascending or descending) can be controlled, e.g. made less steep under algorithmic control. In an embodiment, the control circuit comprises a chargeable capacitance which can be applied as a supply voltage for the voltage level shifting circuit. Such a capacitance can be charged by a voltage difference between the supply voltage (which is e.g. used to supply a power converter) and the forward voltage of a subset of the LEDs of the LED fixture. The capacitance is arranged in such manner that the voltage available at a terminal of the capacitance increases when the forward voltage of the subset of LEDs increases. As such, when the capacitance has been charged, a comparatively high voltage source will be available for controlling a switch associated with an LED which is operated at a

comparatively high voltage.

According to a second aspect of the present invention, there is provided a control circuit for an LED fixture which enables the control of two LEDs or two LED groups from a single tri-state input signal. As such, there is provided a control circuit for an LED fixture comprising a first and a second LED group, each LED group comprising at least one LED, the control circuit comprising:

- an input terminal for receiving a tri-state output signal from a control unit;

- a first output terminal for providing a first control signal for controlling a first switch associated with the first LED group;

- a second output terminal for providing a second control signal for controlling a second switch associated with the second LED group;

the control circuit further being arranged to transform each state of the tri-state output signal to a distinct pair of control signals comprising the first control signal and the second control signal, so as to obtain three distinct operating states of the LED fixture.

The control circuit according to the second aspect of the invention can thus be applied as an interface between a control unit (e.g. a microcontroller or controller configured to receive a signal representative of a desired illumination of the LED fixture, processing the signal and outputting a tri-state output signal), the control circuit having an input terminal for, in use, connecting to a single tri-state output of the control unit and a first and second output terminal for providing a first and second control signal controlling a first and second switch associated with a first and second LED group of the LED fixture.

Within the meaning of the present invention, a tri-state signal is a signal having three possible states, e.g. a low state, whereby the signal has a low voltage level, a high state, whereby the signal has a high voltage level and a high impedance state, whereby the signal is perceived as a high impedance.

The control circuit according to the second aspect of the invention enables the three possible output states of a tri-state output terminal to be mapped to three operating states of an LED fixture comprising two LEDs or LED groups. A conventional control of two LEDs would require two output terminals at the control unit, each controlling an LED current, more specifically a switch associated with each LED. As such, the use of the control circuit according to the present invention enable the control of two LED groups from a single output terminal of a control unit. This may represent an important cost saving with respect to the design of the control unit or it opens the possibility to increase the functionality of the control unit (e.g. a control unit having two output terminals) since only one output terminal is required to control two LEDs or LED groups of an LED fixture.

The control circuit according to the second aspect of the invention may advantageously be applied to control an LED fixture comprising LEDs or LED groups of two different colours thereby enabling a colour set point to be realised ranging from the colour of the first LED or LEDs to the colour of the second LED or LEDs. An important implementation would be to control an LED fixture for providing an essentially white light ranging from warm white to cold white.

As such, the present invention further provides an LED driver for an LED based lighting application, the LED driver comprising a control unit according to the second aspect of the invention and a power supply (e.g. a switched mode power supply such as a Buck or Boost converter or a hysteretic or a linear regulator) for supplying a current to an LED fixture comprising an LED in use providing a warm white light and an LED in use providing a cold white light. Brief description of the drawings:

Figure 1 depicts an LED based lighting application comprising a control circuit according to the invention applying a voltage level shifting for adapting to the varying voltages along the serial connected LED chain.

Figure 2 depicts a typical prior art voltage level shifter.

Figure 3 depicts a solution for generating a voltage larger than the supply voltage needed for controlling the parallel FET at the top of the serial LED chain.

Figure 4 depicts an embodiment of a control circuit comprising a voltage level shifter according to the invention.

Figure 5 depicts an LED based lighting application comprising a control circuit according to the second aspect of the invention.

Figure 1 schematically depicts an LED based lighting system comprising a current source 101 (e.g. a power converter such as a Buck or Boost converter) which can be supplied from an external supply voltage 100, e.g. a 12 V DC supply. In the arrangement as shown, the current source 101 provides a current Is to an LED fixture 103 comprising a series connection of three LED groups 103.1 , 103.2 and 103.3. To switch these LED groups ON or OFF, each LED group has been provided with a FET 102 (in general, a switch) across them. When this FET is in its conductive state, substantially no current is conducted by the associated LED or LED group. When the FET is in its non-conductive state, the entire current Is flows through the associated LED group. The FETs 102 are controlled by a control circuit comprising a control unit 106 (e.g. a microcontroller) and a plurality of voltage level shifting circuits 107. In the embodiment as shown, the control unit 106 has a set point terminal 106.1 configured to receive a set point representing a desired illumination of the LED fixture. The control unit 106 processing the set point signal as received and outputs one or more signals 1 10 representing a desired operating state of the switch 102. In the arrangement as shown, the signal 110 as outputted by the control unit is used as an input signal to a voltage level shifting circuit 107, that is configured to convert a voltage level of the signal 1 10 as received from the control unit 106 to an appropriate level for controlling the FET 102. As such, the voltage level shifting circuit 107 can bridge the voltage difference between the control unit 106 (or controller) and the FETs 102. To illustrate this, one can assume the control unit 106 to provide an output signal 1 10 of e.g. 0 to 5 V. In case the switches associated with LED groups 103.2 and 103.3 are open and a nominal current is provided to the LED groups, the voltage at node 1 1 1 between LED groups 103.1 and 103.2 can e.g. be 10 V. In such a situation, the voltage level of the output signal 106 would be insufficient to control the FET 102 associated with LED group 103.1. The voltage level shifting circuit 107 (which is explained in more detail below) ensures that the output signal 1 10 as provided by the control unit to the voltage level shifting circuit at a comparatively low level results in the appropriate operating state of the FET 102, irrespective of the voltage level the FET (or associated LED or LED group) is operating at, i.e. the voltage level at node 1 1 1.

The control unit 106 can either control the LEDs without feedback, or can, as shown in Figure 1 , use f.e. one or more light sensors 105 to catch the light of one or more LEDs and provide a feedback signal representing the captured illumination to enable the control unit to control the LED fixture to provide the desired brightness.

Figure 2 schematically shows a straight-forward prior-art voltage level shifting circuit (also referred to as a voltage level shifter) as can be applied to ensure that a control unit output signal can result in an appropriate operation of a switch associated with an LED group. In Figure 2, terminal or node 200 represents an output port of a control unit or controller, at which port an output signal can be made available.

Assuming the port 200 to operate in a first state representing a high internal impedance and a second state representing a low internal impedance, that is port 200 is substantially at ground level, the operation of the prior art voltage level shifter can be understood as follows: In the first state, in which controller port 200 is tri-stated (having a high internal impedance), T1 will be in its non-conducting mode. The gate of T2 will then eventually be at substantially the same potential as the source of T2 due to resistor R4. T2 will be in its non-conductive state and the associated LED will radiate light due to the current Is provided to the LED. When the controller port 200 is subsequently brought to the second state, i.e. a low impedance state at substantially ground level, then T1 will conduct, due to the reduced base voltage obtained via voltage divided R1/R2, and will pull the gate of T2 towards Vsupply 203 via resistors R3 and R4. As a result, the gate to source voltage of T2, whereby the potential at the source of T2 201 depends on the operating state of the lower part of the LED array, can increase and, at a certain gate-source voltage, T2 will enter its conducting state. The lower part of the LED array can e.g. comprise one or more LEDs or LED groups connected between node 201 and ground 205. As a result, the current Is will substantially bypass the LED. The LED will therefore not radiate any light. In the voltage level shifter as shown, the R3/R4 voltage divider is applied to shift the voltage level at the collector of T1 to a level as required at the gate of T2 while the source of T2 can, in principle, be at any voltage between ground and Vsupply 203, depending on an operating state of any further LEDs or LED groups that are connected at node 201.

Due to the presence of R3 and R4 and the parasitic capacitance C2 present across the gate/source connections of T2, the gate to source voltage of T2 will not lower instantly but will decay gradually when T1 is switched off again. This may cause the LED to radiate light for some time after the command to do so has been removed. Such a delay may cause an error in the amount of light radiated by the LED fixture, causing f.e. deviations from the desired characteristic of the light, e.g. color and/or intensity.

Reversely, when switching T1 on again, also a delay is caused because C2 needs to be charged via R3/R4 first before the voltage across the gate and source of T2 will reach the threshold to switch from its non-conducting to its conducting state, causing similar deviations in the desired characteristic of the light. In the circuit as schematically depicted in Figure 2, the delay to switch ON and the delay to switch OFF cannot be independently set by dimensioning the components in the circuit. Furthermore, because of the active charging of the parasitic capacitance C2 but inactive discharging the delays are typically different causing a resulting difference in the desired characteristic of the light.

As such, the following disadvantages can be associated with this type of circuit:

Not possible to independently set the steepness of the current slope when switching ON from an OFF state;

The LED-ON slope is typically slow w.r.t. the LED OFF slope.

· FETs, which are typically used as switches, can typically withstand up to 20V gate to source voltage. Exceeding that limit may damage the FET. As such, R3/R4 may need to be dimensioned to avoid such overvoltage. This limits the freedom of choosing their values for other characteristics to be optimized.

The slopes will be chosen to be not as steep as possible to avoid EMI. This too limits the dimensioning freedom.

· R3 and R4 will dissipate. As efficiency of lighting applications in general becomes more and more important, such dissipation should be avoided as much as possible.

In accordance with the first aspect of the invention, a control circuit for an LED fixture is provided which, at least partly, mitigates one or more of the disadvantages mentioned.

In Figure 3, a voltage level shifting circuit of a control circuit according to an embodiment of the present invention is schematically depicted. The working principle of the voltage level shifting circuit is as follows:

In Figure 3, terminal 400 represents an input terminal of the voltage level shifting circuit , at which terminal an output signal of a control unit (not shown) can be received, the output signal representing a desired operating state of the LED or LED group or a switch

associated with the LED or LED group. Assuming the circuit to operate in a first state whereby the terminal 400 is tri-stated (or at a high enough potential, near or equal to

Vsupply) to leave T1 in the non-conducting state. In this respect, tri-stated refers to the third possible output state, i.e. a high impedance state.

When T1 is not conducting, the gate of T2 will have the same potential as the source of T2 in the static situation, due to a.o. R3. In such a state, T2 is not conducting and the supply current Is flows through the LED, which is therefore radiating light and thus is in its ON-state. In order to switch the LED off, the control unit output port 400 can be brought to a low voltage level (e.g. ground level), causing T1 to conduct, and thus pulling T1's collector voltage to a high level (i.e. near the supply voltage 403). This will pull up the gate of T2 via R4, which will start conducting when its gate-source voltage is sufficiently high, said potential being dependent on further LEDs or LED groups that can be present at node 401 and their operating state. When T2 is conducting, substantially all current will flow through T2, the LED will thus be in an OFF state. As FETs usually have a limited gate-source voltage, a Zener diode Z2 can be provided to protect T2.

Due to the parasitic capacitance of mainly T2, indicated by C2, and the Zener diode Z2, the slope of the current through the LED will be limited. By proper dimensioning of the circuit (mainly the value of R4), the steepness of the slope can be chosen.

In an embodiment, the control unit providing the output signal at the output port 400, can provide a pulsed output signal instead of a continuous output signal. By using such a pulsed output signal (i.e. comprising a sequence of pulses) on terminal 400 and by proper dimensioning of R1 , R2, C1 and the circuit between T1 and T2, the slope at which the current through an LED or LED group increases or decreases can be adjusted, made less steep depending on the pulsed signal applied.

In order to switch the LED back to its ON-state, the controller can again tri-state its port 400, causing T1 to conduct no longer, thus no longer pulling high the voltage at its collector terminal, which will then be pulled towards the potential on T2's source terminal, eventually causing T2 to enter its non-conducting state and thus to switch ON the LED.

Due to T2's parasitic gate-source capacitance C2 as well as the Zener's capacitance, the potential at T2's gate will not drop immediately. Said capacitance must first discharge via a route. The available routes are firstly via R4, secondly via the Zener diode Z2 and thirdly via the circuit around T3. As R4 is preferably selected to optimize the LED OFF slope, R4 will have a too high value to reach the desired steepness of the LED ON slope. Also the Zener will allow only a marginal current, as it is below its Zener voltage. This leaves the circuit around T3 to increase the steepness of the LED ON slope. In accordance with the present invention, T3 and associated circuit provide an alternative discharge path for the parasitic capacitance enabling an independent dimensioning of the discharging.

As soon as T1 stops conducting, the parasitic capacitance across T2's gate-source will mainly discharge via D2, T3's emitter-base diode and R3. This delivers independent freedom for dimensioning the discharge curve through choosing the value of R3. The parasitic capacitance C2 can discharge via R3 at least as long as T2's Vgs (gate to source voltage) is larger than 2 diode forward voltages (the forward voltage over D2 and the

Emitter-Collector voltage of T3). Typical values for Vgs are between 2.5 to 4 Volts. 2 diode forward voltages will amount to 0.9 to 1.4 Volts. Below 1.4 Volts, T2 will no longer conduct, so that during an important part of the Vgs curve, R3 will be dominant. Because of the current through the emitter-base diode, T3 will start conducting until an equilibrium has settled between the current through R3 and the current through R5 or until C2 has been substantially exhausted. The conducting T3 and a low valued R5 provide an even faster discharge path for C2, as the emitter-collector voltage will be low in the conducting state of T3.

With respect to the prior art circuit, the level-shifter according to the invention will deliver a fast LED ON current slope and an independent control over the LED OFF and LED ON current slopes. By applying a pulsed output signal at port 400 the steepness of both current slopes can be controlled, i.e. made less steep.

The control circuit according to the invention thus enables the discharging of a parasitic capacitance of a FET, in general, an electronic switch, to be substantially independently controlled, both when the switch is opened from a closed state or when the switch is closed from an open state. In accordance with the invention, the charging and discharging of the parasitic capacitance in both situations substantially takes place along different paths. The circuit as schematically depicted in Figure 3 provides an alternative discharge path compared to the discharging via the voltage divider R3/R4 as shown in Figure 2. As a consequence, the voltage divider R4/R6 of the circuit as shown in Figure 3 need not be dimensioned in order to obtain a certain current slope but can be dimensioned to achieve other characteristics, such as an optimization with respect to dissipation.

Unlike the characteristics of the prior art level shifter, in the control circuit according to the invention, a dedicated path for charging the parasitic capacitance and a, different, dedicated path for discharging the parasitic capacitance are provided. In an embodiment, both paths can be dimensioned such that the LED OFF and LED ON current slopes are substantially equal.

When a controlled current slope is applied, i.e. by providing a pulsed output signal, one or more of the following advantages can be realized:

less EMI (by controlling the current slope, the frequency content of the current pulse, which may be high, can be controlled thus enabling the reduction of frequency components which may cause EMI)

less cross-talk between the LED-groups (less abrupt changes allow the HW more time adjust; Also this allows the SW more time to adjust brightness in the other channels in case brightness and/or color feedback is used)

less sensitive to tolerances of circuit components as adjustments can be made to the pulsed output signal to realize a certain current slope.

more predictable surfaces under the current pulses through the LED and thus more predictable brightness / color at the total light-fixture system level.

By applying a control circuit according to the invention, a sub-optimal compromise is avoided due to the substantially independent control of the LED OFF slope from the LED ON slope. In Figure 4, a further improvement to a control circuit for an LED fixture is schematically depicted. This improvement may be applied in a control circuit as e.g. shown in Figure 3 but may equally be applied in known voltage level shifters as e.g. shown in Figure 2. The improvement relates to providing a control signal to a switch operating in parallel to an LED that is part of a series connected array of LEDs and which is connected at an end of the array that is connected to the power source, e.g. current source. As such, this particular LED or LED group (also referred to as the upper LED) may operate at a comparatively high voltage when the other LEDs of the array, which are thus connected between node 301 and ground 310, are emitting light. Controlling the current through this LED may present a particular problem, for which a solution is presented in Figure 4. In Figure 4, a control circuit similar to the control circuit of Figure 2 is shown, for controlling an LED that is connected nearest the current source 306. In case this control circuit would be connected to the supply voltage 303 (see Figure 2 schematically showing the control circuit being connected to the supply voltage 203), the following problem may occur: For this control circuit to properly operate when all LEDs of the array are operated near 100%, it must submit a voltage to the gate 307 of its associated T2 FET which is higher than Vsupply (303). Figure 4

schematically shows how such a voltage can be generated in the circuit itself. As shown in Figure 4, this is done using the components Z1 and C1. When the voltage at node 301 is comparatively low, compared to Vsupply, C1 is charged to a voltage that, when added to the voltage at T2's source-pin (301) will typically be sufficiently higher than Vsupply and enable switching of the topmost T2 even when 301 is at or near Vsupply (303).

The working principle is as follows. Most of the time less than 100% intensity is required. This means that there will be times when 1 or more of the T2 FETs in the serial LED array below the upper LED (i.e. between 301 and ground 310) will conduct. This means approximately 4 Volts (1 LED forward voltage) less of voltage accumulation across the rest of the array. So 301 will at most be Vsupply minus approximately 4 Volts. The topmost level shifter can operate under these circumstances. Note that in this situation, C1 is charged via Z1 to approximately 3.4 Volts ( 4 Volts between 303 and 301 minus 1 diode forward voltage across Zener diode Z1). When all FETs in the remaining part of the array would be conducting, the voltage across C1 could rise to approximately 23V. Should all FETs then be switched from conducting to non-conducting, then 301 would rise to at least 20 Volts, thus threatening the functioning of the control circuit of the upper LED. However, as there is from approximately 3 to 23Volts across C1 , the voltage at 304 will rise from 23 to 43 Volts, which is sufficient to switch T2 at all times. As such, the control circuit controlling the upper LED is not supplied from the supply voltage 303 but from the voltage at node 304 which will be, due to the charging of C1 , sufficient to control the gate of T2, irrespective of the voltage at node 301.

When for prolonged times all FETs in the lower part of the chain would be non-conducting, then the voltage at 301 will always be at its maximum and C1 will not charge as much as is desired. In such cases, a 100% requested intensity could be mapped onto 99% total duty cycle in switching the FETs, thus forcing the FETs in the lower part of the chain to enter their conducting state for 1 % during each control period. C1 will then be charged in a guaranteed manner.

In accordance with a second aspect of the invention, a control circuit is provided which enables two LEDs or LED groups to be controlled from a single tri-state output port. In accordance with the present invention, tri-state output is used to denote that an output of a port (e.g. an output port of a microprocessor) can have three distinct states, i.e. a low voltage state (e.g. 0 V), a high voltage state (e.g. 5 V) and a high impedance state.

An embodiment of the control circuit according to the second aspect of the invention is schematically depicted in Figure 5. In Figure 5, a tri-state output signal 500 of a control unit is provided at an input terminal of the control circuit. The control circuit further provides two control signals at the gates of T2 and T5 for controlling switches T2 and T5 thus controlling the current through the LEDs associated with these switches. Both LEDs are series connected and are supplied from a current source 51 1 , providing a current 512, Isource. Note that, instead of each switch T2 and T5 controlling a single LED, each switch may e.g. control an LED group as well.

The operation of the circuit can be understood as follows:

When the output 500 is in a low voltage state (indicated by L), the voltage at the base of T3 will be low and thus, T3 will not conduct. As a result, T1 will also remain open, as will T2. As a result, the LED associated with T2 will be in the ON state (i.e. the current 512 will flow through the LED). When the output 500 is in a low voltage state, T4 will be in an open state, thus causing T5 to conduct. As a result, the LED associated with T5 will substantially be short-circuited (by T5) and will thus be in an OFF state.

When the output 500 is in a high voltage state (e.g. 5V) (indicated by H), the voltage at the base of T3 will be sufficient to operate T3 in a conductive state, thus enabling T1 and T2 to conduct as well. As a result, the LED associated with T2 will substantially be short- circuited and will thus be in an OFF state. When the output 500 is high, T4 will conduct and, as a result, T5 will be in an open state. As a result, the LED associated with T5 will be in the ON state (i.e. the current 512 will flow through the LED).

When the output 500 is in a high impedance state (indicated by Z), the voltage at the base of T3 will be low and thus, T3 will not conduct. As a result, T1 will also remain open, as will T2. As a result, the LED associated with T2 will be in the ON state (i.e. the current 512 will flow through the LED). At the same time, when the output 500 is in a high impedance state, T4 will conduct and, as a result, T5 will be in an open state. As a result, the LED associated with T5 will be in the ON state (i.e. the current 512 will flow through the LED).

Summarizing, depending on the state of output 500, the LEDs can be operated as indicated in the following table:

Table 1 :

Port Upper LED Lower LED

L ON OFF Z ON ON H OFF ON The circuit as described in Figure 5 thus enables to control two LEDs from a single control port or terminal. Using the circuit, one can control the LEDs such that only one LED is ON or that both LEDs are ON. In order to turn both LEDs OFF, one can consider turning of the current source 51 1. Assuming both LEDs to be identical, the above circuit can be used for dimming purposes between 50% and 100%. In case both LEDs have a different rating (e.g. a 1W LED combined with a 3W LED), the above circuit would enable dimming between 25% and 100%.

Alternatively, one could control the LEDs (or LED groups) such that one LED is ON or that both LEDs are OFF. This would, in the example of the 1 W LED and the 3W LED enable a dimming between 0% and 75%.

In general, in case a first LED group has a rating A and a second LED group has a rating B, B > A, and assuming that a 100% intensity corresponds to both LED groups emitting light; then one can either use the tri-state signal to control the LED fixture between an intensity of 0% and 100*B/(A+B)% or to control the LED fixture to generate and intensity between 100*A/(A+B)% and 100%. In the former case, the three operating states for the LED fixture are:

First LED group on and second LED group off

- Second LED group on and first LED group off

First and second LED groups off

In the latter case, the three operating states for the LED fixture are:

- First LED group on and second LED group off

- Second LED group on and first LED group off

First and second LED groups on.

By modulating the tri-state control signal at a comparatively high frequency, one can adjust the intensity to a desired value in the indicated range.

The above circuit may also be applied for controlling LEDs having a different colour, e.g. white at different colour temperature. As such, the control circuit as described can enable an LED fixture comprising a warm white LED and a cold white LED to output white light at different colour temperatures and different intensities by a single signal provided to the control circuit.

In order to obtain three distinct operating conditions for two LEDs from a single tri-state signal, the control circuit as shown can be considered to comprise two sub-circuits. These two sub-circuits (indicated by dotted lines 520 and 525) are designed in such manner that they transform the tri-state signal to an ON/OFF state of the associated LED (the upper LED being associated with circuit 520 and T2, the lower LED being associated with circuit 525 and T5). As such, each sub-circuit ensures that any of the possible states of the tri-state signal corresponds to either an ON state or an OFF state of the associated LED. As an example, sub-circuit 520 results in an ON state of the associated LED when the output 500 is either L or Z whereas sub-circuit 525 results in an ON state of the associated LED when the output 500 is either H or Z. The OFF state of the associated LED is thus only obtained in a single one of three possible states of the output 500. In the circuit as shown, the OFF state of an LED is obtained by different states of the output signal. The OFF state of the upper LED requires an H state of the output signal whereas an OFF state of the lower LED requires an L state. As such, three distinct states for the LED fixture as a whole can be realised, each state being characterised by an operating state of the upper LED and an operating state of the lower LED. The tri-state output signal provided at an input terminal of the control circuit is thus mapped to three distinct operating states of the LED fixture.

Generalising this, a control circuit can be obtained that converts n tri-state signals to

3 ⁿ operating states of an LED fixture. In case n=2, 9 distinct operating states can thus be obtained which could be applied for controlling an LED fixture comprising 3 LEDs or LED groups.

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention.

The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language, not excluding other elements or steps). Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.

A single processor or other unit may fulfil the functions of several items recited in the claims.