Fixed frequency ripple regulator

1. A ripple regulator for providing a pulse width modulation (PWM) signal for regulating an output voltage of a power converter switching stage, the regulator comprising:

a ripple circuit for providing a ripple voltage;

a comparison circuit for comparing the ripple voltage to an output of an error amplifier; and

a PWM circuit producing the PWM signal and receiving an output of the comparison circuit and an external clock signal input, the external clock signal input determining a first edge of the PWM signal and the output of said comparison circuit determining a second edge of the PWM signal;

wherein the PWM signal maintains a fixed frequency in response to a load transient.

2. The ripple regulator of claim 1 , wherein the error amplifier receives a feedback voltage from the converter output voltage and a reference voltage.

3. The ripple regulator of claim 1 , wherein the ripple circuit comprises:

a capacitor; and

first and second amplifiers for converting first and second voltages into currents that charge and discharge the capacitor, the first and second amplifiers having respective output terminals.

4. The ripple regulator of claim 3 , wherein the ripple circuit further comprises a first transistor coupled between the output terminals of the first and second amplifiers, the first transistor being controlled by the PWM signal provided by the PWM circuit.

5. The ripple regulator of claim 4 , wherein the amplifiers are transconductance amplifiers.

6. The ripple regulator of claim 3 , wherein the PWM circuit comprises a flip-flop circuit for generating the PWM signal and the output of the error amplifier indicates whether the output voltage should be regulated to increase or decrease.

7. The ripple regulator of claim 6 , wherein the PWM circuit is set by an edge of the external clock signal input and reset when the ripple voltage exceeds the output signal of the error amplifier.

8. The ripple regulator of claim 3 , wherein a duty cycle of the PWM signal is changed by the falling negative edge of the PWM signal and the fixed frequency in response to the load transient is substantially equal to a steady-state frequency of the PWM signal.

9. The ripple regulator of claim 1 , wherein the power converter switching stage includes high and low power transistors connected in series at a switching node.

10. The ripple regulator of claim 9 , wherein the power converter switch stage comprises a buck regulator including an inductor coupled between the switching node and an output capacitor.