[0001] This relates generally to electronic circuits, and more particularly to high speed illumination driver for time-of-flight (TOF) applications.

BACKGROUND

[0002] An emerging category of electronic devices is time-of-flight (TOF) systems. The TOF systems are applicable to accelerometers, monolithic gyroscopes, light sensors, conveyor belts, depth sensing, proximity sensing, gesture recognition and imagers. A TOF system includes a light source that emits light pulses. The light pulses are emitted towards a target, which reflects the light pulses. The target is any object of interest, such as a human, an automated component, an animal or an electronic device. A TOF sensor in the TOF system receives the reflected light pulses. The TOF sensor receives the reflected light pulses after a time of flight, which is proportional to a distance of the target from the TOF system.

[0003] The TOF sensor includes a pixel array having multiple pixels. The pixel array receives the reflected light pulses. The pixel array collects light for a predetermined amount of time after the emission of the light by the light source. Light reflected from a far away object travels a longer distance and therefore has a longer time-of-flight, whereas light reflected from a nearby object is received after short time-of-flight. Each pixel in the pixel array generates a signal, which is processed in a processor to extract information about the target. The information includes a level of ambient light and a depth of the target.

[0004] The light source in a typical TOF system is a light emitting diode (LED) or a laser module. An illumination driver is required to drive the light source. The illumination driver needs to satisfy multiple conditions to be used in a TOF system. Some of these conditions are: (a) the illumination driver is required to support high current, such as in range of 100mA or greater; (b) the illumination driver is required to operate on high modulation frequencies, such as in the range of 50MHz or greater, to achieve good depth accuracy; and (c) the illumination driver is required to have a good phase noise, such as on the order of 60-70dB, to achieve a good resolution. [0005] Existing illumination drivers does not meet one or more of these conditions. Also, the existing illumination drivers are affected by thermal noise, flicker noise and large sized circuit elements. The presence of parasitic capacitance further degrades a linear nature of the illumination drivers.

SUMMARY

[0006] In described examples, a circuit includes an amplifier and a digital to analog converter (DAC). The amplifier receives a reference voltage at an input node of the amplifier. The DAC is coupled to the amplifier through a refresh switch. The DAC includes one or more current elements. Each of the one or more current elements receives a clock. The DAC includes one or more switches corresponding to the one or more current elements. A feedback switch is coupled between: the one or more switches; and a feedback node of the amplifier. The DAC provides a feedback voltage at the feedback node of the amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 illustrates a circuit.

[0008] FIG. 2 illustrates a circuit.

[0009] FIG. 3 illustrates a circuit, according to an embodiment.

[0010] FIG. 4 is a timing diagram to illustrate operation of the circuit of FIG. 3.

[0011] FIG. 5 is a flowchart of operation of the circuit of FIG. 3.

[0012] FIG. 6 illustrates a time-of-flight (TOF) system, according to an embodiment.

DETAILED DESCRIPTION OF EXAMPLE EMB ODFMENT S

[0013] FIG. 1 illustrates a circuit 100 that includes an LED (light emitting diode) 102. The LED 102 is coupled between a primary power source VDD 104 and an illumination driver 106. The illumination driver 106 includes a first transistor 108, a bias generator 110 and multiple current elements represented as 120 A to 120N. The bias generator 110 includes a current source 112 coupled to a voltage source V. A diode connected transistor 114 is coupled to the current source 112. A drain terminal of the diode connected transistor 114 is coupled to the current source 112, and a gate terminal of the diode connected transistor 114 is coupled to the drain terminal. A bias resistor R 116 is coupled between a source terminal of the diode connected transistor 114 and a secondary power source VSS 130.

[0014] The first transistor 108 is coupled between the LED 102 and the multiple current elements. A gate terminal of the first transistor 108 receives a clock CLK 118, and a drain terminal of the first transistor 108 is coupled to the LED 102. A source terminal of the first transistor 108 is coupled to the current elements represented as 120A to 120N. Each of the current elements is similar in connection and operation. Accordingly, for brevity of the description, the connection and operation of the current element 120A is discussed here.

[0015] The current element 120A includes a second transistor 122A, an additional resistor RA 124A, and a secondary switch SA 128A coupled to the secondary power source VSS 130. A gate terminal of the second transistor 122A is coupled to the diode connected transistor 114, and a drain terminal of the second transistor 122A is coupled to the source terminal of the first transistor 108 at an output node N2. A source terminal of the second transistor 122A is coupled to the additional resistor RA 124A. The secondary switch SA 128A is coupled between the additional resistor RA 124A and the secondary power source VSS 130.

[0016] Similarly, the current element 120N is an Nth one of the current elements, where N is an integer. The current element 120N also includes a second transistor 122N, an additional resistor RA 124N, and a secondary switch SN 128N coupled to the secondary power source VSS 130. The secondary switches SA 128A to SN 128N are activated by a digital to analog converter (DAC) input. The DAC input includes multiple enables signals. In one example, when an enable signal received by the secondary switch SA 128 A is at logic high, the secondary switch SA 128A is activated or closed.

[0017] In operation of the circuit 100 (FIG. 1), the illumination driver 106 draws current from the primary power source VDD 104. The current flows through the LED 102 to the illumination driver 106. The LED 102 emits light pulses based on the current drawn by the illumination driver 106 from the primary power source VDD 104. On receiving the DAC input, a set of secondary switches in the current elements 120A to 120N is activated. Accordingly, based on the DAC input, a set of current elements with the set of secondary switches is activated which draw current from the primary power source VDD 104.

[0018] The current source 112 provides current to the diode connected transistor 114 which creates a bias voltage at a bias node Nl . The bias voltage at the bias node Nl is used for biasing the second transistors in the set of current elements. Accordingly, a current flowing through the LED 102 is proportional to the activated set of current elements. The first transistor 108 is activated when the clock CLK 118 is at logic high. [0019] When the clock CLK 118 is at logic high, a current flows through the LED 102 based on the set of current elements (from among current elements 120 A to 120N) activated in response to the DAC input. When the clock CLK 118 is at logic low, no current is drawn by the illumination driver 106, so no current flows through the LED 102. In an example, when the clock CLK 118 transitions to logic high, if the current flowing through the LED 102 is Ipeak, when the clock CLK 118 transitions to logic low, the current flowing through the LED 102 is much lower than Ipeak. Accordingly, the current flowing through the LED 102 is not steady and a distortion is introduced in the current flowing through the LED 102 during a time period when the clock CLK 118 is at logic high.

[0020] This distortion of current in the LED 102 is because of multiple factors. The current element 120A is discussed here to illustrate these factors, and the other current elements behave in a similar manner to compound the distortion of the current in the LED 102. Flicker noise of the current element 120A translates into a phase noise around a frequency of the clock CLK 118. The additional resistor RA 124A aids in reducing the flicker noise. However, achieving the flicker noise specification by increasing a resistance of the additional resistor RA 124A is not possible because of proportional increase in power loss.

[0021] Also, a size of the diode connected transistor 114 in the bias generator is less than a size of the second transistor 122A. The bias generator 110 and the current element 120A behave as a current mirror circuit. In one example, when a size of the diode connected transistor 114 is x and the size of the second transistor 122A is lOOx, a current flowing through the diode connected transistor 114 is I, and a current flowing through the second transistor 122A is lOOx. Accordingly, the size of the diode connected transistor 114 is maintained much lesser than the size of the second transistor 122A. However, any noise in the diode connected transistor 114 also gets multiplied by a same factor (100 in the above example) as a ratio of size of the diode connected transistor 114 and the second transistor 122A. Accordingly, the bias generator 110 is required to be designed in a manner to meet the flicker noise specification which proportionately increases a size of the circuit 100.

[0022] A parasitic capacitance Cp 135 is introduced between the bias node Nl and the output node N2. The parasitic capacitance Cp 135, in one case, is because of large sized circuit elements in the illumination driver 106. A switching of the multiple circuit elements (because of switching of the clock CLK 118) introduces glitches at the bias node Nl . Also, the bias node Nl is not strongly held which causes a variation in the current flowing through the LED 102. All these factors also introduce nonlinearity in the current flowing through the LED 102.

[0023] FIG. 2 illustrates a circuit 200 that includes an LED (light emitting diode) 202. The LED 202 is coupled between a primary power source VDD 204 and an illumination driver 206. The illumination driver 206 includes a first transistor 208, an amplifier 210 and multiple current elements represented as 220A to 220N. The amplifier 210 receives a reference voltage Vref 212.

[0024] The first transistor 208 is coupled between the LED 202 and the current elements. A gate terminal of the first transistor 208 receives a clock CLK 218, and a drain terminal of the first transistor 208 is coupled to the LED 202. A source terminal of the first transistor 208 is coupled to the current elements represented as 220A to 220N. Each of the current elements is similar in connection and operation. Accordingly, for brevity of the description, the connection and operation of the current element 220A is discussed here.

[0025] The current element 220A includes a second transistor 222A, an additional resistor RA 224A, and a secondary switch SA 228A coupled to a secondary power source VSS 230. A gate terminal of the second transistor 222A is coupled to the amplifier 210, and a drain terminal of the second transistor 222A is coupled to the source terminal of the first transistor 208 at an output node N2. A source terminal of the second transistor 222A is coupled to the additional resistor RA 224A. The secondary switch SA 228A is coupled between the additional resistor RA 224A and the secondary power source VSS 230.

[0026] Similarly, the current element 220N is an Nth one of the current elements, where N is an integer. The current element 220N also includes a second transistor 222N, an additional resistor RA 224N, and a secondary switch SN 228N coupled to the secondary power source VSS 230. The secondary switches SA 228A to SN 228N are activated by a digital to analog converter (DAC) input. The DAC input includes multiple enables signals. In one example, when an enable signal received by the secondary switch SA 228A is at logic high, the secondary switch SA 228A is activated or closed.

[0027] In operation of the circuit 200 (FIG. 2), the illumination driver 206 draws current from the primary power source VDD 204. The current flows through the LED 202 to the illumination driver 206. The LED 202 emits light pulses based on the current drawn by the illumination driver 206 from the primary power source VDD 204. On receiving the DAC input, a set of secondary switches in the current elements 220A to 220N is activated. Accordingly, based on the DAC input, a set of current elements with the set of secondary switches is activated which draw current from the primary power source VDD 204.

[0028] The amplifier 210 generates a bias voltage at a bias node Nl . The bias voltage at the bias node Nl is used for biasing the second transistors in the set of current elements. Accordingly, a current flowing through the LED 202 is proportional to the activated set of current elements. A feedback voltage Vf 214 is measured from a current through the additional resistors in each of the current elements. The feedback voltage Vf 214 is provided to the amplifier 210. The amplifier 210 compares the reference voltage Vref 212 and the feedback voltage Vf 214 to generate the bias voltage.

[0029] The first transistor 208 is activated when the clock CLK 218 is at logic high. When the clock CLK 218 is at logic high, a current flows through the LED 202 based on the set of current elements (from among the current elements 220A to 220N) activated in response to the DAC input. When the clock CLK 218 is at logic low, no current is drawn by the illumination driver 206, so no current flows through the LED 202. In an example, when the clock CLK 218 transitions to logic high, if the current flowing through the LED 202 is Ipeak, when the clock CLK 218 transitions to logic low, the current flowing through the LED 202 is much lower than Ipeak. Accordingly, the current flowing through the LED 202 is not steady and a distortion is introduced in the current flowing through the LED 202 during a time period when the clock CLK 218 is at logic high.

[0030] This distortion of current in the LED 202 is because of multiple factors. The current element 220A is discussed here to illustrate these factors, and the other current elements behave in a similar manner to compound the distortion of the current in the LED 202. Flicker noise of the current element 220A translates into a phase noise around a frequency of the clock CLK 218. However, this flicker noise is suppressed by a gain of the amplifier 210.

[0031] A parasitic capacitance Cp 235 is introduced between the bias node Nl and the output node N2. The parasitic capacitance Cp 235, in one case, is because of large sized circuit elements in the illumination driver 206. A bandwidth of the amplifier 210 needs to be increased to compensate transients because of the parasitic capacitance Cp 235. However, a higher bandwidth requires a higher power to pump the amplifier 210. A switching of the circuit elements (because of switching of the clock CLK 218) introduces glitches at the bias node Nl . Also, the bias node Nl is not strongly held, which causes a variation in the current flowing through the LED 202. All these factors also introduce nonlinearity in the current flowing through the LED 202.

[0032] FIG. 3 illustrates a circuit 300, according to an embodiment. The circuit 300 includes an LED (light emitting diode) 302. The LED 302 is coupled between a primary power source VDD 304 and an illumination driver 306. The illumination driver 306 includes an amplifier 310 and a digital to analog converter (DAC) 308. The amplifier 310 receives a reference voltage Vref 312 at an input node Nl of the amplifier 310. The amplifier 310 receives a feedback voltage Vf at a feedback node N2 of the amplifier 310. The DAC 308 provides the feedback voltage Vf to the amplifier 310. The DAC 308 is coupled to the amplifier 310 through a refresh switch REF 338. The DAC 308 includes one or more current elements illustrated as 320A, 320B to 320N, one or more switches illustrated as SA 340A, SB 340B to SN 340N, and a feedback switch FB 336.

[0033] Each of the current elements receives a clock CLK 318. The switches SA 340A, SB 340B to SN 340N respectively correspond to one or more current elements 320A, 320B to 320N. Accordingly, a first switch of the one or more switches is activated when a first current element of the one or more current elements is activated. The first switch corresponds to the first current element. For example, the switch SA 340A corresponds to the current element 320A, and the switch SB 340B corresponds to the current element 320B. Accordingly, the switch SA 340A is activated when the current element 320A is activated, and the switch SB 340B is activated when the current element 320B is activated. The feedback switch FB 336 is coupled between: the switches SA 340A, SB 340B to SN 340N; and the feedback node N2 of the amplifier 310.

[0034] The DAC 308 receives a DAC input. The DAC input includes one or more enable signals illustrated as DAC1, DAC2 to DACN. The enable signals DAC1, DAC2 to DACN correspond to current elements 320A, 320B to 320N. For example, the current element 320A receives the enable signal DAC1, and the current element 320B receives the enable signal DAC2. When the enable signal is at logic high, it activates the corresponding current element. For example, the current element 320A is activated when the enable signal DAC1 is at logic high.

[0035] Each of the current elements 320A, 320B to 320N is similar in connection and operation. Accordingly, for brevity of the description, the connection and operation of the current element 320A is discussed here. The current element 320A includes an AND gate 332A, a first transistor 326A, a second transistor 322A, and a resistor R 324A coupled to a secondary power source VSS 330. The AND gate 332A receives the clock CLK 318 and the enable signal DAC1 and generates a control signal.

[0036] A gate terminal of the first transistor 326A is coupled to the AND gate 332A and receives the control signal, and a drain terminal of the first transistor 326A is coupled to the LED 302. A source terminal of the first transistor 326A is coupled to the second transistor 322A. A gate terminal of the second transistor 322A is coupled to the amplifier 310 through the refresh switch REF 338, and a drain terminal of the second transistor 322A is coupled to the source terminal of the first transistor 326A. A source terminal of the second transistor 322A is coupled to the resistor R 324A at a primary node NA. One end of the resistor R 324A is coupled to the secondary power source VSS 330.

[0037] Similarly, the current element 320N is an Nth one of the current elements, where N is an integer. The current element 320N also includes an AND gate 332N, a first transistor 326N, a second transistor 322N, and a resistor R 324N coupled to the secondary power source VSS 330. The AND gate 332N receives the clock CLK 318 and the enable signal DACN and generates a control signal.

[0038] Each of the switches SA 340A, SB 340B to SN 340N is coupled between the primary node (NA, NB to NN) and the feedback switch FB 336. For example, the switch SA 340A is coupled between the primary node NA and feedback switch FB 336. The switches SA 340A, SB 340B to SN 340N are activated by the enable signals. For example, when the enable signals DAC1 and DACN are at logic high: the enable signal DAC1 activates the switch SA 340A; and, similarly, the enable signal DACN activates the switch SN 340N.

[0039] In operation of the circuit 300 (FIG. 3), the illumination driver 306 draws current from the primary power source VDD 304. The current flows through the LED 302 to the illumination driver 306. The LED 302 emits light pulses based on the current drawn by the illumination driver 306 from the primary power source VDD 304. On receiving the DAC input, a set of current elements is activated which draw current from the primary power source VDD 304.

[0040] The circuit 300 operates in a feedback mode and a refresh mode. In the feedback mode, the feedback switch FB 336 is activated, and the refresh switch REF 338 is inactivated. The clock CLK 318 is at logic high. A set of current elements (from among the current elements 320A, 320B to 320N) is activated based on the DAC input. For example, the set of current elements 320A and 320B is activated when the enable signals DAC1 and DAC2 are at logic high respectively. When both the clock CLK 318 and enable signal DAC1 are at logic high, the control signal generated by the AND gate 332 A activates the first transistor 326 A. A corresponding set of switches (from among the switches SA 340 A, SB 340B to SN 340N) is activated based on the activated set of current elements. For example, the switch SA 340A is activated when the current element 320A is activated, and the switch SB 340B is activated when the current element 320B is activated.

[0041] Accordingly, a current flowing through the LED 302 is proportional to the activated set of current elements. The feedback voltage Vf at the feedback node N2 of the amplifier 310 is measured from a current through the resistor in each of the current elements. For example, when the set of current elements 320A and 320B is activated, the feedback voltage Vf is measured from a current through the resistors R 324 A and R 324B.

[0042] In the refresh mode, the feedback switch FB 336 is inactivated, and the refresh switch REF 338 is activated. The clock CLK 318 is at logic low. When the clock CLK 318 is at logic low, no current is drawn by the illumination driver 306, so no current flows through the LED 302. The amplifier 310 generates an error voltage from the feedback voltage Vf and the reference voltage Vref 312. In one example, the error voltage is a difference in the feedback voltage Vf and the reference voltage Vref 312. In another example, the error voltage is an amplified difference in the feedback voltage Vf and the reference voltage Vref 312. In yet another example, the amplifier 310 compares the feedback voltage Vf and the reference voltage Vref 312 to generate the error voltage. The error voltage is provided to a second transistor in each of the current elements. For example, the set of current elements 320A and 320B is activated in the feedback mode. Accordingly, in the refresh mode, the second transistors 322A and 322B receives the error voltage from the amplifier 310. The error voltage is also provided to other second transistors but, because the corresponding current elements are inactivated, it did not effect on the current through the LED 302.

[0043] The error voltage changes a current through each of the current elements in a subsequent feedback mode. Accordingly, in the above example, when the set of current elements 320A and 320B is activated in a subsequent feedback mode, the error voltage changes a current through the current elements 320A and 320B. [0044] A parasitic capacitance Cp 335 introduces glitch in the error voltage. However, any glitch introduced in the error voltage changes a current through the set of current elements which is again fed back to the amplifier 310 for correction. Accordingly, a current flowing through the set of current elements is a function of the reference voltage Vref 312 and the resistor in each of the current elements. The circuit 300 provides one or more switches SA 340A, SB 340B to SN 340N in a feedback path of the amplifier 310. This reduces the area requirement of the circuit 300. In one example, the area requirement is reduced by 30% as compared to the circuit 100.

[0045] Also, a resistance of the switches SA 340A, SB 340B to SN 340N does not affect the operation of the circuit 300, because no current flows through the switches SA 340A, SB 340B to SN 340N. A bandwidth of the amplifier 310 is low as compared to the amplifier 210 of FIG. 2. Accordingly, a lower power is required to drive the amplifier 310, so a lower power is required to realize high modulation frequency in the illumination driver 306.

[0046] FIG. 4 is a timing diagram to illustrate operation of the circuit 300. In a feedback mode 402, the feedback switch FB 336 is activated, and the refresh switch REF 338 is inactivated. The clock CLK 318 is at logic high. A set of current elements (from among the current elements 320A, 320B to 320N) is activated based on the DAC input. A corresponding set of switches (from among the switches SA 340A, SB 340B to SN 340N) is activated based on the activated set of current elements. The feedback voltage Vf at the feedback node N2 of the amplifier 310 is measured through a current through the resistor in each of the current elements.

[0047] The feedback switch FB 336 is inactivated before the clock CLK 118 transitions to logic low. In a refresh mode, the feedback switch FB 336 is inactivated, and the refresh switch REF 338 is activated. The clock CLK 318 is at logic low. When the clock CLK 318 is at logic low, no current is drawn by the illumination driver 306, so no current flows through the LED 302. The amplifier 310 generates an error voltage from the feedback voltage Vf and the reference voltage Vref 312. In one example, the error voltage is a difference in the feedback voltage Vf and the reference voltage Vref 312. In another example, the error voltage is an amplified difference in the feedback voltage Vf and the reference voltage Vref 312. The error voltage is provided to a second transistor in each of the current elements. For example, the set of current elements 320A and 320B is activated in the feedback mode 402. Accordingly, in the refresh mode 404, the second transistors 322 A and 322B receives the error voltage from the amplifier 310. [0048] The error voltage changes a current through each of the current elements in a subsequent feedback mode 406. Accordingly, in the above example, when the set of current elements 320A and 320B is activated in a subsequent feedback mode 406, the error voltage changes a current through the current elements 320A and 320B.

[0049] FIG. 5 is a flowchart 500 of operation of the circuit 300. At step 502, a set of current elements of one or more current elements is activated based on a DAC input and when a clock is at logic high. In circuit 300, a set of current elements (from among the current elements 320A, 320B to 320N) is activated based on the DAC input. The DAC input includes one or more enable signals (represented as DAC1, DAC2 to DACN in FIG. 3) corresponding to the one or more current elements. For example, the set of current elements 320A and 320B is activated when the enable signals DAC1 and DAC2 are at logic high respectively.

[0050] At step 504, a corresponding set of switches of one or more switches is activated based on the activated set of current elements. For example, the switch SA 340A is activated when the current element 320A is activated, and the switch SB 340B is activated when the current element 320B is activated. The one or more enable signals activate the one or more switches. For example, the switch SA 340A is activated by the enable signal DAC1, and the switch SB 340B is activated by the enable signal DAC2. A feedback voltage is measured from a current through a resistor in each of the current elements, at step 506. For example, when the set of current elements 320A and 320B is activated, the feedback voltage Vf is measured from a current through the resistors R 324A and R 324B. Accordingly, a current flowing through the LED 302 is proportional to the activated set of current elements. A primary power source provides current to each of the current elements.

[0051] The current elements are inactivated when the clock is at logic low. The feedback voltage Vf and a reference voltage are compared to generate an error voltage. The error voltage is provided to each of the current elements. The set of current elements is activated when the clock is at logic high. The error voltage is configured to change a current through each of the current elements. Accordingly, in the above example, when the set of current elements 320A and 320B is activated, the error voltage changes a current through the current elements 320A and 320B.

[0052] FIG. 6 illustrates a time-of-flight (TOF) system 600, according to an embodiment. The TOF system 600 includes a circuit 601. The circuit 601 includes an LED (light emitting diode) 602. The LED 602 is coupled between a primary power source VDD 604 and a digital to analog converter (DAC) 608. In one example, the LED 602 is an infrared (IR) LED that transmits IR light. The circuit 601 includes an amplifier 610 coupled to the DAC 608.

[0053] The TOF system 600 includes a pixel array 612. The pixel array 612 includes one or more pixels illustrated as 614. The pixel array 612 is coupled to an analog to digital converter (ADC) 616. A processor 620 is coupled to the ADC 616. For example, the processor 620 can be a CISC-type (complex instruction set computer) CPU, RISC-type CPU (reduced instruction set computer), or a digital signal processor (DSP).

[0054] The circuit 601 is similar to the circuit 300 in connection and operation. The amplifier 610 receives a reference voltage Vref at an input node of the amplifier 610. The amplifier 610 receives a feedback voltage Vf at a feedback node of the amplifier 610. The DAC 608 provides the feedback voltage Vf to the amplifier 610. The DAC 608 is coupled to the amplifier 610 through a refresh switch. The DAC 608 includes one or more current elements, one or more switches, and a feedback switch.

[0055] The circuit 601 provides one or more switches in a feedback path of the amplifier 610. This reduces the area requirement of the circuit 601. A bandwidth of the amplifier 610 is low as compared to the amplifier 210 of FIG. 2. Accordingly, a lower power is required to drive the amplifier 610, so a lower power is required to realize high modulation frequency in the circuit 601.

[0056] The LED 602 transmit light pulses based on a current drawn by the DAC 608 from the primary power source VDD 604. The transmitted light pulses are scattered by a target 615 to generate reflected light pulses. The pixel array 612 receives the reflected light pulses. The ADC 616 converts an analog signal from each of the pixels to a digital signal. The processor 620 processes the digital signal to generate an image of the target 615.

[0057] Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.