Method and apparatus to compensate for supply voltage variations in a PWM-based voltage regulator

1. A controller for a switching voltage regulator that receives a supply voltage and generates an output voltage for a load, the controller comprising:

an input terminal configured for receiving a feedback signal indicative of an output condition for the switching voltage regulator;

an error amplifier configured for generating an error signal based on a comparison of the feedback signal to a reference signal, wherein the reference signal indicates a desired output condition for the switching voltage regulator;

a feed-forward circuit configured to receive the error signal from the error amplifier at a first input terminal that is directly coupled to the error amplifier, and a sensed signal indicative of a supply voltage level at a second input terminal that is directly coupled to the supply voltage, wherein the feed-forward circuit generates at an output an adjusted error signal that has a substantially proportional relationship to the error signal and wherein the adjusted error signal output by the feed-forward circuit increases when the sensed signal indicative of the supply voltage level decreases and decreases when the sensed signal indicative of the supply voltage level increases such that the adjusted error signal is adjusted in relation to said received error signal with a substantially inverse relationship to the sensed supply voltage level; and

an oscillator configured to generate a periodic ramp voltage whose frequency does not vary based on the adjusted error signal; and

a pulse-width modulation comparator configured to receive the adjusted error signal from the output of the inverse feed-forward circuit at a first input terminal, and to receive the periodic ramp voltage from the oscillator at a second input terminal that is directly coupled to the oscillator, wherein the pulse-width modulation comparator is configured to generate a pulse-width modulated output signal based on the adjusted error signal and the periodic ramp voltage, wherein a product of the supply voltage level and duty-cycle of the pulse-width modulated output signal is substantially constant for a given reference signal.

2. The controller of claim 1 , wherein the switching voltage regulator comprises at least one semiconductor switch and the pulse-width modulated output signal controls the semiconductor switch to maintain a substantially constant output voltage for the switching voltage regulator in response to transients in the supply voltage.

3. The controller of claim 1 , wherein the feed-forward circuit comprises:

a voltage-controlled current source configured to receive the sensed signal indicative of the supply voltage level and to generate an offset current that tracks transients in the supply voltage, wherein the voltage-controlled current source comprises a first resistor and a current mirror circuit; and

a summing resistor with a first terminal directly coupled to an output of the error amplifier and a second terminal coupled to an output of the voltage-controlled current source, wherein the summing resistor conducts the offset current and the adjusted error signal is provided at the second terminal of the summing resistor.

4. The controller of claim 1 , wherein the feed-forward circuit comprises a plurality of transistors arranged in a translinear configuration to conduct at least a first current signal, a second current signal and a third current signal, wherein the first current signal is substantially proportional to a product of the second current signal and the third current signal.

5. The controller of claim 4 , wherein the first current signal is derived from the error signal, the second current signal is derived from the sensed signal indicative of the supply voltage level, and the third current signal is used to generate the adjusted error signal.

6. The controller of claim 1 , wherein the feed-forward circuit comprises a translinear circuit configured to generate an output current signal based on a first current signal and a second current signal, wherein the output current signal is proportional to the first current signal and inversely proportional to the second current signal, the first current signal corresponds to the error signal generated by the error amplifier, and the second current signal corresponds to the sensed signal indicative of the supply voltage level.

7. The controller of claim 6 , wherein the feed-forward circuit further comprises a first voltage-to-current converter configured to generate the first current signal from the error signal and a second voltage-to-current converter configured to generate the second current signal from the sensed signal.

8. The controller of claim 1 , wherein the adjusted error signal reflects a change in the supply voltage level within a half periodic cycle of the periodic ramp voltage.

9. The controller of claim 1 , wherein the switching voltage regulator is a DC-to-DC power converter with an output voltage level that varies with the duty cycle of the pulse-width modulated output signal, and the feedback signal to the error amplifier indicates the output voltage level of the DC-to-DC power converter.

10. The controller of claim 1 , wherein the switching voltage regulator is an inverter with an output voltage amplitude that varies with the duty cycle of the pulse-width modulated output signal, and the feedback signal to the error amplifier indicates current conducted by a load.

11. The controller of claim 10 , wherein the inverter controls power in a backlight system, the load comprises at least one cold cathode fluorescent lamp, and the reference signal to the error amplifier determines a brightness level of the cold cathode fluorescent lamp.

12. A method to compensate for variations in a supply voltage level in a voltage regulator, the method comprising:

generating an error signal based on a difference between a feedback signal and a reference signal;

receiving the error signal directly from the error amplifier at a first input terminal of a feed-forward circuit;

sensing a supply voltage level at a second input terminal of the feed-forward circuit that is directly coupled to the supply voltage;

generating an adjusted error signal with the feed-forward circuit based on the received error signal and the sensed supply voltage level, wherein the generated adjusted error signal is proportional to the error signal wherein the adjusted error signal increases when the sensed supply voltage level decreases and decreases when the sensed supply voltage increases such that the adjusted error signal is adjusted in relation to the error signal in a manner that is inversely proportional to the supply voltage level and such that the adjusted error signal is adjusted in relation to said received error signal with a substantially inverse relationship to the sensed supply voltage level;

generating with an oscillator a periodic ramp voltage whose frequency does not vary based on the adjusted error signal; and

generating a pulse-width modulated output signal, using a comparator, based on a comparison of the adjusted error signal to the periodic ramp signal, the periodic ramp signal being received by the comparator directly from the oscillator, wherein a product of the sensed supply voltage level and duty-cycle of the pulse-width modulated output signal is substantially constant for a given reference signal.

13. The method of claim 12 , wherein generating the adjusted error signal comprises generating an offset signal that tracks the supply voltage level and combining the offset signal with the error signal to generate the adjusted error signal.

14. The method of claim 12 , wherein generating the adjusted error signal comprises:

generating a first current signal that tracks the error signal;

generating a second current signal that tracks the supply voltage level; and

providing the first current signal and the second current signal to a translinear circuit to generate a third current signal that is proportional to the first current signal and inversely proportional to the second current signal, wherein the third current signal is used to generate the adjusted error signal.

15. The method of claim 12 , further comprising driving a semiconductor switch with the pulse-width modulated output signal to generate an output voltage for the voltage regulator.

16. The method of claim 12 , wherein the voltage regulator is an inverter that controls power to a lamp, the feedback signal indicates a lamp current, and the reference signal indicates a desired brightness level for the lamp.

17. The method of claim 12 , wherein the voltage regulator is a DC-to-DC voltage regulator, the feedback signal indicates an output voltage level, and the reference signal indicates a desired output voltage level.

18. A PWM controller comprising:

means for generating an error signal based on a feedback signal and a reference signal;

means for generating an adjusted error signal by receiving the error signal directly from the means for generating an error signal at a first input terminal of the means for generating the adjusted error signal and by receiving a supply voltage level at a second input terminal of the means for generating the adjusted error signal that is directly coupled to the supply voltage, wherein generating the adjusted error signal is based on the error signal and a supply voltage level, wherein the adjusted error signal is proportional to the error signal and wherein the adjusted error signal increases when the supply voltage level decreases and decreases when the supply voltage such that the adjusted error signal is adjusted, by the means for generating an adjusted error signal, in relation to said received error signal with a substantially inverse relationship to the sensed supply voltage level;

means for generating a periodic ramp voltage whose frequency does not vary based on the adjusted error signal; and

means for generating a pulse-width modulated output signal with a duty cycle that varies with the adjusted error signal, wherein the means for generating the pulse-width modulated output signal is configured to receive the periodic ramp voltage directly from the means for generating the periodic ramp voltage.

19. The PWM controller of claim 18 , wherein the means for generating the adjusted error signal comprises means for generating an offset signal that tracks changes in the supply voltage level and means for combining the offset signal with the error signal.

20. The PWM controller of claim 18 , wherein the means for generating the adjusted error signal comprises means for generating a first current signal that tracks the error signal, means for generating a second current signal that tracks the supply voltage level, and means for generating a third current signal that is proportional to the first current signal and inversely proportional to the second current signal.