Smooth transition of a power supply from a first mode, such as a pulse-frequency-modulation (PFM) mode, to a second mode, such as a pulse-width-modulation (PWM) mode

1. A power-supply controller, comprising:

a generator configured to provide a switching signal that causes a power supply to generate a regulated output signal;

an adjuster configured to impart a condition to the power supply while the power supply is operating in a first mode, the condition being approximately equal to a condition that the power supply would have if the power supply were operating in a second mode; and

a transitioner configured to cause the power supply to transition from the first mode to the second mode such that a width of a last pulse of the switching signal before the transition is different from a width of a first pulse of the switching signal after the transition.

2. The power-supply controller of claim 1 wherein while the power supply is operating in the first mode, the generator is configured to generate the switching signal having a frequency that depends on a load being powered by the power supply.

3. The power-supply controller of claim 1 wherein while the power supply is operating in the second mode, the generator is configured to generate the switching signal having a fixed frequency.

4. The power-supply controller of claim 1 wherein the condition includes a level of a control signal.

5. The power-supply controller of claim 1 wherein the condition includes a voltage level of a control signal.

6. The power-supply controller of claim 1 wherein:

the condition includes a level of a control signal; and

the adjuster is configured to adjust the level of the control signal while the power supply is operating in the first mode.

7. The power-supply controller of claim 1 wherein:

while the power supply is operating in the first mode, the generator is configured to generate the switching signal having a first frequency that depends on a load being powered by the power supply;

while the power supply is operating in the second mode, the generator is configured to generate the switching signal having a fixed second frequency; and

the adjuster is configured to impart the condition to the power supply by adjusting a control signal in response to a value that is related to a ratio of the second frequency to a maximum value of the first frequency.

8. The power-supply controller of claim 1 wherein:

while the power supply is operating in the first mode, the generator is configured to generate the switching signal having a first frequency that depends on a load being powered by the power supply;

while the power supply is operating in the second mode, the generator is configured to generate the switching signal having a fixed second frequency; and

the transitioner is configured to cause the power supply to transition from the first mode to the second mode when a level of charge that the power supply delivers to a load during a time period while the switching signal has the first frequency is approximately equal to, or is less than, a level of energy that the power supply would deliver to the load during the same time period while the switching signal is at the fixed second frequency.

9. The power-supply controller of claim 1 wherein the transitioner is configured to cause the power supply to transition from the first mode to the second mode when a level of charge that the power supply delivers to a load during a time period in the first mode is approximately equal to, or is less than, a level of charge that the power supply would deliver to the load during the same time period in the second mode.

10. The power-supply controller of claim 1 wherein:

while the power supply is operating in the first mode, the generator is configured to generate the switching signal having a frequency that depends on a load being powered by the power supply; and

the transitioner is configured to cause the power supply to transition from the first mode to the second mode in response to the frequency having a relationship to a frequency threshold.

11. A power supply, comprising:

an input node configured to receive an input signal;

an output node configured to provide a regulated output signal in response to the input signal;

a generator configured to generate a switching signal in response to a control signal during a first operating mode;

an adjuster configured to cause a value of the control signal during a second operating mode to equal approximately a value that the control signal would have in the first operating mode; and

a transitioner configured to transition from the second operating mode to the first operating mode such that a width of a pulse of the switching signal immediately before the transition is different from a width of a pulse of the switching signal immediately after the transition.

12. The power supply of claim 11 wherein:

the input signal includes an input voltage; and

the output signal includes a regulated output voltage.

13. The power supply of claim 11 wherein the first mode includes a pulse-width-modulation mode.

14. The power supply of claim 11 wherein the second mode includes a pulse-frequency-modulation mode.

15. The power supply of claim 11 wherein the adjuster is configured to set the control signal in response to a feedback signal.

16. The power supply of claim 11 wherein the generator is configured to generate the switching signal independently of the control signal during the second operating mode.

17. The power supply of claim 11 wherein the adjuster is configured to cause the value of the control signal during the second operating mode to equal approximately the value that the control signal would have during the first operating mode by adjusting the control signal in response to a value that is related to a ratio of a frequency of the switching signal during the first mode to a frequency of the switching signal during the second mode.

18. The power supply of claim 11 wherein the transitioner is configured to transition from the second mode to the first mode in response to a frequency of the switching signal during the second mode being greater than or equal to a threshold.

19. The power supply of claim 11 wherein the generator and adjuster are disposed on a same integrated circuit.

20. The power supply of claim 11 , further comprising:

an inductor coupled to the input node and to the output node; and

wherein during the second mode the generator is configured to generate the switching signal such that during each period of the switching signal, a charging current flows through the inductor until the charging current is greater than or equal to a current threshold.

21. The power supply of claim 11 , further comprising:

an inductor coupled to the input node and to the output node; and

wherein the transitioner is configured to transition from the second mode to the first mode while there is approximately no current flowing through the inductor.

22. A system, comprising:

a power supply including

an input node configured to receive an input signal,

an output node configured to provide a regulated output signal,

a generator configured to generate a switching signal in response to a control signal during a first operating mode, and

an adjuster configured to cause a value of the control signal during a second operating mode to equal approximately a value that the control signal would have during the first operating mode; and

a transitioner configured to transition the power supply from the second operating mode to the first operating mode such that a width of a pulse of the switching signal immediately before the transition is different from a width of a pulse of the switching signal immediately after the transition; and

a load coupled to the output node.

23. The system of claim 22 wherein the power supply includes a boost converter.

24. The system of claim 22 wherein the load includes an integrated circuit.

25. A method, comprising:

generating a regulated output signal using a first feedback signal and independently of a first control signal;

generating the first control signal

while generating the regulated output signal independently of the first control signal, and

in response to a combination of the first feedback signal and a ration of a frequency of the switching signal during a first operating mode and a frequency of the switching signal during a second operating mode, and

during the first operating mode, generating a switching signal in response to the regulated output signal and the first feedback signal, and independently of the first control signal.

26. The method of claim 25 , further comprising:

wherein the regulated output signal includes a regulated output voltage;

wherein the first feedback signal includes a signal that is related to a current flowing through an inductor; and

the first control signal is related to a load that is coupled to the regulated output voltage.

27. The method of claim 25 wherein generating the first control signal includes generating a voltage across a capacitor.

28. The method of claim 25 wherein generating the first control signal includes generating the first control signal further in response to a second control signal.

29. The method of claim 25 wherein generating the first control signal includes generating the first control signal further in response to a combination of a ramp signal and the ratio.