Compensation offset adjustment scheme for fast reference voltage transitioning

1. A pulse width modulation (PWM) control circuit for a voltage regulator, comprising:

a compensation network having a first input sensing an output voltage, a second input receiving a reference voltage, and an output providing a compensation voltage;

a ramp generator which provides a ramp voltage;

an offset adjust circuit which adjusts a selected one of said ramp voltage and said compensation voltage based on said reference voltage; and

a comparator circuit which compares said compensation voltage with said ramp voltage and which provides a PWM signal for controlling said output voltage.

2. The PWM control circuit of claim 1 , wherein said offset adjust circuit generates an offset voltage based on said reference voltage and a gain G of the voltage regulator.

3. The PWM control circuit of claim 2 , wherein said offset adjust circuit multiplies said reference voltage by 1−1/G to generate said offset voltage.

4. The PWM control circuit of claim 2 , wherein said offset adjust circuit subtracts said offset voltage from said compensation voltage to provide an adjusted compensation voltage to said comparator circuit.

5. The PWM control circuit of claim 2 , wherein said offset adjust circuit subtracts said offset voltage from said ramp voltage to provide an adjusted ramp voltage to said comparator circuit.

6. The PWM control circuit of claim 1 , wherein:

said compensation network comprises an error amplifier having first and second inputs and an output, wherein said first input receives said reference voltage;

wherein said compensation network further comprises a capacitor coupled between said second input and said output of said error amplifier; and

wherein said offset adjust circuit minimizes voltage change across said capacitor in response to a voltage change of said reference voltage.

7. The PWM control circuit of claim 1 , wherein said offset adjust circuit controls said ramp generator to adjust a slew rate of said ramp signal in response to a change of said reference voltage.

8. The PWM control circuit of claim 7 , wherein said ramp generator and said offset adjust circuit comprise:

a voltage-controlled current source coupled between ground and a node developing said ramp voltage and providing a current as a function of a reference voltage to said node;

a capacitor coupled between ground and said node; and

a switch circuit coupled between ground and said node that enables said voltage-controlled current source to charge said capacitor between minimum and maximum voltage levels at a rate based on a clock signal.

9. A DC-DC voltage converter, comprising:

a gate drive circuit receiving a pulse width modulation (PWM) signal and providing first and second gate drive signals;

a first switch having current terminals coupled in series between an input voltage and a phase node and having a control terminal receiving said first gate drive signal;

a second switch having current terminals coupled in series between said phase node and ground and having a control terminal receiving said second gate drive signal;

an output inductor having a first end coupled to said phase node and a second end providing an output voltage; and

a PWM control circuit, comprising:

a compensation network having a first input sensing said output voltage, a second input receiving a reference voltage, and an output providing a compensation voltage;

a ramp generator which provides a ramp voltage;

an offset adjust circuit which adjusts a selected one of said ramp voltage and said compensation voltage based on said reference voltage; and

a comparator circuit which compares said compensation voltage with said ramp voltage and which provides said PWM signal.

10. The DC-DC voltage converter of claim 9 , wherein said offset adjust circuit generates an offset voltage based on said reference voltage and a gain of the voltage regulator.

11. The DC-DC voltage converter of claim 10 , wherein said offset adjust circuit subtracts said offset voltage from said compensation voltage to provide an adjusted compensation voltage to said comparator circuit.

12. The DC-DC voltage converter of claim 10 , wherein said offset adjust circuit subtracts said offset voltage from said ramp voltage to provide an adjusted ramp voltage to said comparator circuit.

13. The DC-DC voltage converter of claim 9 , wherein said offset adjust circuit controls said ramp generator to adjust a slew rate of said ramp signal in response to a change of said reference voltage.

14. A method of compensating for a transition of a reference voltage of a voltage regulator, comprising:

generating a compensation voltage based on the reference voltage and an output voltage of the voltage regulator;

providing a ramp voltage;

adjusting either one of the compensation voltage and the ramp voltage in response to a transition of the reference voltage; and

comparing the ramp voltage with the compensation voltage to generate a pulse width modulation (PWM) signal for controlling the output voltage.

15. The method of claim 14 , wherein said adjusting either one of the compensation voltage and the ramp voltage comprises combining an offset voltage that is based on the reference voltage and a gain G of the voltage regulator.

16. The method of claim 15 , wherein said combining an offset voltage comprises multiplying the reference voltage by 1−1/G.

17. The method of claim 15 , wherein said adjusting either one of the compensation voltage and the ramp voltage comprises subtracting the offset voltage from the compensation voltage to provide an adjusted compensation voltage and wherein said comparing comprises comparing the ramp voltage with the adjusted compensation voltage.

18. The method of claim 15 , wherein said adjusting either one of the compensation voltage and the ramp voltage comprises subtracting the offset voltage from the ramp voltage to provide an adjusted ramp voltage and wherein said comparing comprises comparing the compensation voltage with the adjusted ramp voltage.

19. The method of claim 14 , wherein said adjusting either one of the compensation voltage and the ramp voltage in response to a transition of the reference voltage comprises minimizing a difference between the compensation voltage and the reference voltage.

20. The method of claim 14 , wherein said adjusting either one of the compensation voltage and the ramp voltage in response to a transition of the reference voltage comprises adjusting a slew rate of the ramp signal.