System and method for PFM/PWM mode transition within a multi-phase buck converter

1. A multiphase voltage regulator, comprising:

a plurality of DC/DC voltage regulators, each of the DC/DC voltage regulators associated with a phase of the multiphase voltage regulator, each of the plurality of DC/DC voltage regulators, comprising:

a first switching transistor connected between an input voltage node and a phase node responsive to switching control signals;

a second switching transistor connected between the phase node and a ground node responsive to the switching control signals;

an inductor connected between the phase node and an output voltage node;

control logic for generating the switching control signals responsive to a pulse control signal;

PFM/PWM transition logic for generating the pulse control signal, wherein the pulse control signal transitions between a PWM signal and a PFM signal responsive to an error voltage, a feedback voltage from the output voltage node and an inductor current through the inductor; and

an error amplifier for generating the error voltage responsive to the feedback voltage and a reference voltage, wherein the output of each error amplifier in each of the plurality of phases are connected to each other.

2. The multiphase voltage regulator of claim 1 , wherein the PFM/PWM transition logic further comprises:

first control logic for generating each of the PWM signal and the PFM signal and selecting one of the PWM signal and the PFM signal responsive to a mode select signal; and

mode selection logic for generating the mode select signal responsive to the error voltage, the feedback voltage and the inductor current.

3. The multiphase voltage regulator of claim 2 , wherein the first control logic further comprises:

a PWM comparator for generating the PWM signal responsive to a comparison of the error voltage with a sum of the inductor current and a slope current;

a PFM comparator for generating the PFM signal responsive to a comparison of the inductor current with a peak value;

a multiplexer connected to receive the PWM signal and the PFM signal and for outputting one of the PWM signal and the PFM signal as the pulse control signal responsive to the mode select signal.

4. The multiphase voltage regulator of claim 2 , wherein the mode selection logic further comprises:

a zero crossing detector for detecting when the inductor current falls below zero and generating a zero detect signal responsive thereto;

a hysteresis comparator for generating a hysteresis signal controlling when the pulse control signal transitions from the PFM signal to the PWM signal;

a transition comparator for determining when to switch between the PFM signal and the PWM signal responsive to a comparison of the error voltage and a transition threshold level; and

control logic for generating the mode select signal responsive to the output of the zero detect signal, the hysteresis signal and the output of the transition comparator.

5. The multiphase voltage regulator of claim 4 , wherein the control logic generates the mode select signal to transition from the PFM signal to the PWM signal responsive to the error voltage exceeding the transition threshold level.

6. The multiphase voltage regulator of claim 5 , wherein the transition from the PFM signal to the PWM signal is further responsive to a feedback voltage falling below a second reference voltage.

7. The multiphase voltage regulator of claim 4 , wherein the control logic generates the mode select signal to transition from the PWM signal to the PFM signal responsive to the inductor current falling below zero.

8. A multiphase voltage regulator, comprising:

a plurality of DC/DC voltage regulators, each of the DC/DC voltage regulators associated with a phase of the multiphase voltage regulator, each of the plurality of DC/DC voltage regulators, comprising:

a first switching transistor connected between an input voltage node and a phase node responsive to a switching control signals;

a second switching transistor connected between the phase node and a ground node responsive to the switching control signals;

an inductor connected between the phase node and an output voltage node;

control logic for generating the switching control signals responsive to a pulse control signal;

a PWM comparator for generating a PWM signal responsive to a comparison of an error voltage with a sum of an inductor current through the inductor and a slope current;

a PFM comparator for generating a PFM signal responsive to a comparison of the inductor current with a peak value;

a multiplexer connected to receive the PWM signal and the PFM signal and for outputting one of the PWM signal and the PFM signal as a pulse control signal responsive to a mode select signal;

a zero crossing detector for detecting when the inductor current falls below zero and generating a zero detect signal responsive thereto;

a hysteresis comparator for generating a hysteresis signal controlling when the pulse control signal transitions from the PFM signal to the PWM signal;

a transition comparator for determining when to switch between the PFM signal and the PWM signal responsive to a comparison of the error voltage and a transition threshold level;

control logic for generating the mode select signal responsive to the output of the zero detect signal, the hysteresis signal and the output of the transition comparator; and

an error amplifier for generating the error voltage responsive to the feedback voltage and a reference voltage, wherein the output of each error amplifier in each of the plurality of phases are connected to each other.

9. The multiphase voltage regulator of claim 8 , wherein the control logic generates the mode select signal to transition from the PFM signal to the PWM signal responsive to the error voltage exceeding the transition threshold level.

10. The multiphase voltage regulator of claim 9 , wherein the transition from the PFM signal to the PWM signal is further responsive to a feedback voltage falling below a second reference voltage.

11. The multiphase voltage regulator of claim 8 , wherein the control logic generates the mode select signal to transition from the PWM signal to the PFM signal responsive to the inductor current falling below zero.

12. A method for operating a multiphase voltage regulator, comprising the steps of:

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

generating the switching control signals responsive to a pulse control signal, wherein the pulse control signal comprises either a PWM signal or a PFM signal;

transitioning between the PWM signal and the PFM signal responsive to an error voltage, a feedback voltage from the output voltage node and an inductor current through an inductor;

generating the error voltage responsive to the feedback voltage and a reference voltage.

13. The method of claim 12 further including the step of connecting an error voltage generated in each phase of the multiphase regulator to a same node.

14. The method of claim 12 , wherein the step of transitioning further comprises the steps of:

generating each of the PWM signal and the PFM signal;

generating a mode select signal responsive to the error voltage, the feedback voltage and the inductor current; and

selecting one of the PWM signal and the PFM signal responsive to the mode select signal.

15. The method of claim 14 , wherein the step of generating each of the PWM signal and the PFM signal further comprises the steps of:

generating the PWM signal responsive to a comparison of the error voltage with a sum of the inductor current and a slope current;

generating the PFM signal responsive to a comparison of the inductor current with a peak value; and

multiplexing between the PWM signal and the PFM signal as the pulse control signal responsive to the mode select signal.

16. The method of claim 14 , wherein the step of generating the mode select signal further comprises the steps of:

detecting when the inductor current falls below zero;

generating a zero detect signal responsive to the inductor current falling below zero;

generating a hysteresis signal controlling when the pulse control signal transitions from the PFM signal to the PWM signal;

determining when to switch between the PFM signal and the PWM signal responsive to a comparison of the error voltage and a transition threshold level; and

generating the mode select signal responsive to the output of the zero detect signal, the hysteresis signal and results of the comparison between the error voltage and the transition threshold level.

17. The method of claim 16 , wherein the step of generating the mode select signal further comprises the step of transitioning from the PFM signal to the PWM signal responsive to the error voltage exceeding the transition threshold level.

18. The method of claim 17 , wherein the step of generating the mode select signal further comprises the step of transitioning from the PFM signal to the PWM signal responsive to a feedback voltage falling below a second reference voltage.

19. The method of claim 16 , wherein the step of generating the mode select signal further comprises the step of transitioning from the PWM signal to the PFM signal responsive to the inductor current falling below zero for a predetermined period of time.