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Control circuits and techniques are provided for use with DC-DC converters. The circuits and techniques may, in some implementations, be used with LED driver systems. In some embodiments control circuits are provided that rely on multiple different feedback paths to adjust a duty cycle of a DC-DC co

Control circuits and techniques are provided for use with DC-DC converters. The circuits and techniques may, in some implementations, be used with LED driver systems. In some embodiments control circuits are provided that rely on multiple different feedback paths to adjust a duty cycle of a DC-DC converter. In some embodiments, control circuits are provided that switch between multiple capacitors to provide as duty cycle control signal for use with a DC-DC converter.

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1. An electronic circuit for use in driving a plurality of loads coupled to a common voltage node, each load in the plurality of loads including a series-connected string of load devices, the electronic circuit comprising: a plurality of current regulators to regulate current through corresponding o

1. An electronic circuit for use in driving a plurality of loads coupled to a common voltage node, each load in the plurality of loads including a series-connected string of load devices, the electronic circuit comprising: a plurality of current regulators to regulate current through corresponding ones of the plurality of loads; andcontrol circuitry to control a DC-DC converter to generate a regulated voltage on the common voltage node, the control circuitry comprising: a duty cycle control unit to control a duty cycle of the DC-DC converter, the duty cycle control unit being responsive to a duty cycle control signal at a control input thereof that is indicative of a duty cycle to be used by the duty cycle control unit;at least one capacitor to carry a voltage to act as the duty cycle control signal for the duty cycle control unit; andat least one error amplifier to facilitate adjustment of the voltage on the at least one capacitor based on feedback, the at least one error amplifier being configured to generate an error signal to adjust the voltage on the at least one capacitor based, during first time periods, on the output voltage of the DC-DC converter and, during second time periods, on feedback from the plurality of current regulators, wherein the second time periods are different from the first time periods. 2. The electronic circuit of claim 1, wherein: the plurality of loads includes a plurality of light emitting diode (LED) channels, wherein each LED channel in the plurality of LED channels includes a series-connected string of LEDs. 3. The electronic circuit of claim 2, wherein: the at least one error amplifier is configured to generate the error signal based on a difference between a present output voltage of the DC-DC converter and a past output voltage of the DC-DC converter, during the first time periods. 4. The electronic circuit of claim 3, wherein: the at least one error amplifier is configured to generate the error signal based on a difference between a voltage across a current regulator associated with a first LED channel and an LED regulation voltage associated with the first LED channel, during the second time periods. 5. The electronic circuit of claim 4, wherein: the first time periods include “off” portions of a dimming duty cycle of the first LED channel; andthe second time periods include “on” portions of the dimming duty cycle of the first LED channel;wherein the past output voltage of the DC-DC converter is a voltage during a most recent “on” portion of the dimming duty cycle of the first LED channel. 6. The electronic circuit of claim 4, wherein: the first time periods include periods when all LED channels in the plurality of LED channels are on; andthe second time periods include periods when less than all LED channels in the plurality of LED channels are on;wherein the past output voltage of the DC-DC converter is a voltage during a most recent period when all LED channels in the plurality of LED channels were on. 7. The electronic circuit of claim 4, wherein: the first LED channel comprises a dominant LED channel, the dominant LED channel being a channel in the plurality of LED channels that requires a highest voltage on the common voltage node, wherein the dominant LED channel can change with time. 8. The electronic circuit of claim 3, wherein: the at least one error amplifier is configured to generate the error signal based on a difference between an average voltage across multiple current regulators of the plurality of current regulators and an LED regulation voltage during the second time periods. 9. The electronic circuit of claim 2, further comprising: LED dimming logic coupled to the plurality of current sinks, wherein the LED dimming logic is capable of individually controlling at least some of the plurality of current regulators to set different dimming duty cycles and different current levels for corresponding LED channels. 10. The electronic circuit of claim 9, wherein: the LED dimming logic is capable of individually controlling at least some of the plurality of current regulators to set different illumination “turn on” times for different LED channels. 11. The electronic circuit of claim 2, wherein: the electronic circuit is implemented as an integrated circuit having at least one contact for connection to an external DC-DC converter. 12. The electronic circuit of claim 2, wherein: the at least one capacitor includes at least one first capacitor to carry a first voltage to act as the duty cycle control signal for the duty cycle control unit;the electronic circuit further comprising: at least one second capacitor to carry a second voltage to act as the duty cycle control signal for the duty cycle control unit; anda switch circuit to alternately couple the first capacitor and the second capacitor to the control input of the duty cycle control unit in response to one or more control signals. 13. The electronic circuit of claim 1, wherein: the plurality of current regulators includes a plurality of current sinks. 14. A method for operating a duty cycle control unit to generate a switching signal for a DC-DC converter, the DC-DC converter to generate an output voltage to power a plurality of light emitting diode (LED) channels coupled to a common voltage node, each LED channel in the plurality of LED channels including a series-connected string of LEDs, the duty cycle control unit having an input to receive a duty cycle control signal indicative of a duty cycle to be used for the DC-DC converter, the method comprising: generating an error signal for use in adjusting a voltage level on at least one capacitor coupled to the input of the duty cycle control unit based on feedback, wherein generating the error signal includes generating the error signal based on the output voltage of the DC-DC converter during first time periods and generating the error signal based on one or more voltages across one or more current regulators associated with the plurality of LED channels during second time periods, wherein the second time periods are different from the first time periods. 15. The method of claim 14, wherein: the first time periods are “on” portions of a dimming duty cycle of a first LED channel; andthe second time periods are “off” portions of the dimming duty cycle of the first LED channel. 16. The method of claim 15, wherein: the first LED channel is a dominant LED channel, wherein the dominant LED channel is a channel in the plurality of LED channels that requires a highest voltage on the common voltage node. 17. The method of claim 14, wherein: the first time periods are periods during which all of the LED channels in the plurality of LED channels are on; andthe second time periods are periods during which less than all of the LED channels in the plurality of LED channels are on. 18. The method of claim 14, wherein: generating the error signal based on one or more voltages across one or more current sinks associated with the plurality of LED channels includes generating the error signal based on a voltage across a current sink associated with a dominant LED channel, wherein the dominant LED channel is a channel in the plurality of LED channels that requires a highest voltage on the common voltage node. 19. The method of claim 14, wherein: generating the error signal based on one or more voltages across one or more current sinks associated with the plurality of LED channels includes generating the error signal based on an average of voltages across multiple current sinks. 20. An electronic circuit for use in driving a plurality of loads coupled to a common voltage node, each load in the plurality of loads including a series-connected string of load devices, the electronic circuit comprising: control circuitry for controlling a DC-DC converter to generate a regulated voltage on the common voltage node, the control circuitry comprising: a duty cycle control unit to control a duty cycle of the DC-DC converter, the duty cycle control unit being responsive to a duty cycle control signal at a control input thereof that is indicative of a duty cycle to be used by the duty cycle control unit;a first capacitor to carry a first voltage to act as a duty cycle control signal for the duty cycle control unit;a second capacitor to carry a second voltage to act as a duty cycle control signal for the duty cycle control unit; anda switch circuit to alternately couple the first capacitor and the second capacitor to the control input of the duty cycle control unit in response to one or more control signals. 21. The electronic circuit of claim 20, wherein: the plurality of loads includes a plurality of light emitting diode (LED) channels, wherein each LED channel in the plurality of LED channels includes a series-connected string of LEDs. 22. The electronic circuit of claim 21, wherein: the switch circuit is controlled to couple the first capacitor to the control input of the duty cycle control unit during odd pulse width modulation (PWM) dimming cycles and the second capacitor to the control input of the duty cycle control unit during even PWM dimming cycles. 23. The electronic circuit of claim 22, wherein: the switch circuit is a first switch circuit; andthe control circuitry further comprises a second switch circuit to allow each of the first and second capacitors to sample a maximum voltage value of the other of the first and second capacitors when the other of the first and second capacitors is coupled to the control input of the duty cycle control unit, in response to one or more control signals. 24. The electronic circuit of claim 23, wherein: the second switch circuit includes a unity gain buffer to set a voltage of one of the first and second capacitors when the other of the first and second capacitors is coupled to the control input of the duty cycle control unit. 25. The electronic circuit of claim 21, further comprising: an error amplifier to adjust a voltage on a capacitor that is currently coupled to the control input of the duty cycle control unit using an output error signal, wherein the capacitor that is currently coupled to the control input of the duty cycle control unit is either the first capacitor or the second capacitor. 26. The electronic circuit of claim 25, wherein: the error amplifier is configured to generate the output error signal based on a difference between feedback from at least one LED channel and a reference voltage, during an “on” period of a dimming duty cycle of a dominant LED channel; andthe error amplifier is configured to generate the output error signal based on a difference between a current output of the DC-DC converter and a previous output of the DC-DC converter during an “off” period of the dimming duty cycle of the dominant LED channel;wherein the dominant LED channel is a channel that requires a highest voltage on the common voltage node. 27. The electronic circuit of claim 26, wherein: the previous output of the DC-DC converter is an output of the DC-DC converter during a most recent “on” period of the dimming duty cycle of the dominant LED channel. 28. The electronic circuit of claim 26, wherein: the dominant LED channel can vary with time; andthe electronic circuit further comprises a controller to track the identity of the dominant LED channel. 29. The electronic circuit of claim 25, wherein: the error amplifier is a first error amplifier and the switch circuit is a first switch circuit; andthe control circuitry further comprises second and third error amplifiers and a second switch circuit, the second switch circuit to alternately couple the second and third error amplifiers to a first input of the first error amplifier in response to one or more control signals, where a fixed voltage level is applied to a second input of the first error amplifier. 30. The electronic circuit of claim 29, wherein: the second switch circuit is controlled to alternately couple the second and third error amplifiers to the first input of the first error amplifier based on a dimming duty cycle of a dominant LED channel in the plurality of LED channels, wherein the dominant LED channel is a channel that requires a highest voltage on the common voltage node. 31. The electronic circuit of claim 21, further comprising: a plurality of current sinks to draw current through corresponding ones of the plurality of LED channels; andLED dimming logic coupled to the plurality of current sinks, wherein the LED dimming logic is capable of individually controlling at least some of the plurality of current sinks to set different dimming duty cycles and different current levels for corresponding LED channels. 32. The electronic circuit of claim 31, wherein: the LED dimming logic is capable of individually controlling at least some of the plurality of current sinks to set different illumination turn on times for corresponding LED channels. 33. The electronic circuit of claim 21, wherein: the electronic circuit is implemented as an integrated circuit having at least one contact for connection to an external DC-DC converter. 34. A method for operating a duty cycle control unit to generate a switching signal for a DC-DC converter, the duty cycle control unit having an input to receive a duty cycle control signal indicative of a duty cycle to be used for the DC-DC converter, the method comprising: alternately coupling at least a first capacitor and a second capacitor to the input of the duty cycle control unit, the first and second capacitors each having corresponding voltages across them that act as duty cycle control signals for the duty cycle control unit when the corresponding capacitors are coupled to the duty cycle control unit. 35. The method of claim 34, wherein: the output voltage of the DC-DC converter is used to power a plurality of light emitting diode (LED) channels coupled to a common voltage node, each LED channel in the plurality of LED channels including a series-connected string of LEDs, wherein the LED channels are driven in a series of pulse width modulation (PWM) dimming cycles; andalternately coupling includes coupling the first capacitor to the input of the duty cycle control unit during odd PWM dimming cycles and coupling the second capacitor to the input of the duty cycle control unit during even PWM dimming cycles. 36. The method of claim 35, further comprising: coupling an error signal from an error amplifier to the input of the duty cycle control unit to adjust voltages on the first and second capacitors based on feedback, wherein the error signal adjusts the voltage on the first capacitor when the first capacitor is coupled to the input of the duty cycle control unit and the error signal adjusts the voltage on the second capacitor when the second capacitor is coupled to the input of the duty cycle control unit. 37. The method of claim 35, further comprising: sampling a maximum voltage value of the first capacitor using the second capacitor when the first capacitor is coupled to the input of the duty cycle control unit; andsampling a maximum voltage value of the second capacitor using the first capacitor when the second capacitor is coupled to the input of the duty cycle control unit. 38. The method of claim 37, wherein: sampling a maximum voltage value of the first capacitor includes coupling the voltage of the first capacitor to the second capacitor during an “on” portion of a dimming duty cycle of a dominant LED channel during odd PWM dimming cycles, wherein the dominant LED channel is a channel in the plurality of LED channels that requires a highest voltage on the common voltage node. 39. The method of claim 38, wherein: coupling the voltage of the first capacitor to the second capacitor includes coupling the voltage through a unity gain buffer. 40. The method of claim 34, wherein: alternately coupling includes alternately coupling a first capacitor, a second capacitor, and a third capacitor to the input of the duty cycle control unit. 41. A control circuit for controlling a DC-DC converter to generate a regulated voltage, the control circuit comprising: a duty cycle control unit to control a duty cycle of the DC-DC converter, the duty cycle control unit being responsive to a duty cycle control signal at a control input thereof that is indicative of a duty cycle to be used by the duty cycle control unit;a first capacitor to carry a first voltage to act as a duty cycle control signal for the duty cycle control unit;a second capacitor to carry a second voltage to act as a duty cycle control signal for the duty cycle control unit; anda switch circuit to alternately couple the first capacitor and the second capacitor to the control input of the duty cycle control unit in response to one or more control signals.