System and method for preventing controller induced pulse skipping at low duty cycle operations

1. An apparatus, comprising:

a voltage regulator for generating a regulated output voltage responsive to an input voltage and drive control signals;

an error amplifier for generating an error voltage signal responsive to the regulated output voltage and a reference voltage;

a variable gain filter for amplifying the error voltage signal by a gain substantially proportional with an inverse of the reference voltage;

a PWM modulator for generating a PWM control signal responsive to the error voltage signal, a ramp voltage and the inverse of the reference voltage;

control circuitry within the PWM modulator for maintaining the error voltage signal applied to the PWM modulator at substantially a same DC voltage level over the reference voltage operating range and maintaining the error voltage signal above a minimum value of the ramp voltage; and

driver circuitry for generating the drive control signals responsive to the PWM control signal.

2. The apparatus of claim 1 , wherein the variable gain filter is a high pass filter that is responsive to a variable gain control signal.

3. The apparatus of claim 1 , wherein the variable gain filter is a high pass filter that maintains the error voltage signal at a same DC voltage level.

4. The apparatus of claim 1 , wherein the PWM modulator further comprises:

a latch circuit for generating the PWM control signal responsive to a pulse output and a first control signal;

a first comparator for generating a first output responsive to the ramp voltage and the error voltage signal;

a pulse generator circuit for generating the pulse output responsive to the first output; and

a second comparator for generating the first control signal responsive to a comparison of a combination of the error voltage signal and a current sense signal with a second ramp signal.

5. The apparatus of claim 4 , wherein the control circuitry further includes:

a multiplier for multiplying an inverse of the reference voltage by a nominal up ramp slew rate signal; and

a current source for generating the second ramp signal at a node associated with the current source responsive to a balance current and an output of the multiplier.

6. The apparatus of claim 1 further including a current sensor for generating a current sense signal that is applied to the regulated output voltage.

7. A method for generating a PWM control signal, comprising:

generating an error voltage signal responsive to a regulated output voltage and a reference voltage;

comparing a ramp voltage with the error voltage signal and generating a pulse signal responsive thereto;

modifying the error voltage signal responsive to a current sense signal;

generating a first voltage responsive to an inverse of the reference voltage and a nominal upramp slew rate signal;

comparing the modified error voltage signal with the first voltage to generate a first control signal; and

generating the PWM control signal responsive to the pulse signal and the first control signal.

8. The method of claim 7 , wherein the step of generating the first voltage further includes:

multiplying the nominal upramp slew rate signal with the inverse of the reference voltage to generate a second control signal; and

controlling a current source responsive to the second control signal and a current control signal to generate the first voltage.

9. The method of claim 7 further including:

applying the PWM control signal to a voltage regulator;

generating drive control signals responsive to the PWM control signal;

generating a regulated output voltage responsive to an input voltage and the drive control signals; and

generating the error voltage signal responsive to the regulated output voltage and the reference voltage.

10. The method of claim 9 further including amplifying the error voltage signal by a gain substantially proportional with an inverse of the reference voltage.

11. The method of claim 10 , wherein the amplifying further comprises amplifying the error voltage signal responsive to a variable gain control signal.

12. The method of claim 9 further including:

generating the current sense signal responsive to a current through an inductor of the voltage regulator; and

applying the current sense signal to the regulated output voltage.

13. The method of claim 7 , further including maintaining the error voltage signal at a same DC voltage level over an operating range of the reference voltage.

14. The method of claim 7 further including:

generating the current sense signal responsive to a current through an inductor of the voltage regulator; and

applying the current sense signal to the regulated output voltage.

15. A PWM controller for generating a PWM control signal for a voltage regulator, comprising:

a first input for receiving an error voltage signal, wherein the error voltage signal is generated in response to a regulated output voltage and a reference voltage;

a second input for receiving a ramp voltage signal;

a third input for receiving an inverse of the reference voltage;

an output for providing the PWM control signal;

modulator circuitry for generating the PWM control signal responsive to the error voltage signal, the ramp voltage signal and the inverse of the reference voltage, wherein the modulator circuitry includes a circuit for generating the PWM control signal responsive to a pulse output and a first control signal, a first comparator for generating a first output responsive to the ramp voltage signal and the error voltage signal, a pulse generator circuit for generating the pulse output responsive to the first output, and a second comparator for generating the first control signal responsive to a comparison of a combination of the error voltage signal and a current sense signal with a second ramp signal; and

control circuitry for maintaining the error voltage signal applied to the modulator circuitry at substantially a same DC voltage level over the reference voltage operating range and maintaining the error voltage signal above a minimum value of the ramp voltage signal.

16. The PWM controller of claim 15 wherein the control circuitry further comprises:

a multiplier for multiplying the inverse of the reference voltage by a nominal up ramp slew rate signal; and

a current source for generating the second ramp signal at a node associated with the current source responsive to a balance current and an output of the multiplier.

17. The PWM controller of claim 15 wherein the modulator circuitry reduces the gain of the PWM modulator by a factor of the inverse of the reference voltage.

18. A system, comprising:

a voltage regulator for generating a regulated output voltage at an output responsive to an input voltage and drive control signals;

an error amplifier for generating an error voltage signal responsive to the regulated output voltage and a reference voltage;

a variable gain filter for amplifying the error voltage signal by a gain substantially proportional with an inverse of the reference voltage;

a PWM modulator for generating a PWM control signal responsive to the error voltage signal, a ramp voltage and the inverse of the reference voltage, control circuitry within the PWM modulator for maintaining the error voltage signal applied to the PWM modulator at substantially a same DC voltage level over the reference voltage operating range and maintaining the error voltage signal above a minimum value of the ramp voltage;

driver circuitry for generating the drive control signals responsive to the PWM control signal; and

a load coupled to the output of the voltage regulator.

19. The system of claim 18 , wherein the load is selected from a group consisting of a processor, a memory, an input device, an output device and a storage device.