System and method for controlling switching power supply

1. A switching power supply comprising:

a signal generator receiving a first signal, generating a second signal using the first signal, and providing one of the first signal and the second signal as a selected signal based on a first frequency of the first signal, the first signal having a first ripple amplitude, the second signal having a second ripple amplitude less than the first ripple amplitude;

a controller generating a modulation signal in response to the selected signal; and

a power converter converting an input signal into an output signal in response to the modulation signal.

2. The switching power supply of claim 1 , wherein the first frequency of the first signal is higher than a second frequency of the second signal, and

wherein the signal generator provides the second signal as the selected signal in response to the first signal having the first frequency that is equal to or greater than a threshold frequency.

3. The switching power supply of claim 1 , wherein the signal generator provides the first signal as the selected signal in response to the first signal having the first frequency that is lower than a threshold frequency.

4. The switching power supply of claim 1 , wherein the signal generator includes a frequency detector comparing the first frequency of the first signal to a threshold frequency and generating a transition signal according to the comparison result.

5. The switching power supply of claim 4 , wherein the comparison result is a first comparison result, and

wherein the frequency detector includes:

a low-pass filter converting the first signal into a filtered signal;

an offset circuit causing the filtered signal to have a DC value that is lower than a DC value of the first signal by a predetermined offset value;

a first comparator comparing the first signal and the filtered signal to output a second comparison result;

a first one-shot pulse generator generating a pulse signal in response to the second comparison result; and

a frequency determining circuit generating the transition signal in response to the pulse signal.

6. The switching power supply of claim 5 , wherein the frequency determining circuit includes:

a second one-shot pulse generator generating a first control pulse signal in response to the pulse signal;

a third one-shot pulse generator generating a second control pulse signal in response to a reference pulse signal;

a capacitor having a first end and a second end, the second end being connected to a ground;

a first switching device causing a first current to flow into the capacitor to charge the capacitor in response to the first control pulse signal;

a second switching device causing a second current to flow from the capacitor to discharge the capacitor in response to the second control pulse signal; and

a second comparator comparing an intermediate voltage at the first end of the capacitor to a threshold voltage to output the transition signal.

7. The switching power supply of claim 4 , wherein the signal generator further includes:

a low-pass filter converting the first signal into the second signal;

a first switching device transmitting the second signal as the selected signal in response to the transition signal having a first logic value; and

a second switching device transmitting the first signal as the selected signal in response to the transition signal having a second logic value.

8. The switching power supply of claim 7 , wherein the controller includes:

a first comparator comparing the first signal to a threshold signal and outputting a set signal;

a second comparator comparing the selected signal to a ramp signal and outputting a reset signal; and

a flip-flop generating the modulation signal in response to the set signal and the reset signal.

9. The switching power supply of claim 4 , wherein the selected signal is a reset signal, wherein the controller includes:

a first on-time controller generating a first reset control signal in response to the first signal; and

a second on-time controller generating a second reset control signal, an on-time of the second reset control signal being substantially constant, and

wherein the signal generator further includes:

a first logic gate providing a first logic output signal as the reset signal in response to the first reset control signal and the transition signal having a first logic value; and

a second logic gate providing a second logic output signal as the reset signal in response to the second reset control signal and the transition signal having a second logic value.

10. The switching power supply of claim 4 , wherein the controller includes a threshold generator generating a threshold signal, and

wherein the signal generator further includes:

a first switching device transmitting the threshold signal as the selected signal in response to the transition signal having a first logic value; and

a second switching device transmitting the first signal as the selected signal in response to the transition signal having a second logic value.

11. The switching power supply of claim 1 , wherein the signal generator includes a frequency detector comparing the first frequency of the first signal to a threshold frequency and generating first and second resistance control signals in response to the comparison result.

12. The switching power supply of claim 11 , wherein the signal generator further includes:

a low-pass filter converting the first signal into the second signal;

a first variable resistor transmitting a portion of the first signal as a first component of the selected signal in response to the first resistance control signal; and

a second variable resistor transmitting a portion of the second signal as a second component of the selected signal in response to the second resistance control signal.

13. The switching power supply of claim 12 , wherein the first variable resistor has a first resistance value and a second variable resistor has a second resistance value, and

wherein a ratio of the first resistance value over the second resistance value increases in response to the first signal having the first frequency that is equal to or greater than the threshold frequency.

14. The switching power supply of claim 11 , wherein the controller includes a threshold generator generating a threshold signal, and

wherein the signal generator further includes:

a first variable resistor transmitting a portion of the first signal as a first component of the selected signal in response to the first resistance control signal; and

a second variable resistor transmitting a portion of the threshold signal as a second component of the selected signal in response to the second resistance control signal.

15. A method for controlling a switching power supply, the method comprising:

generating a second signal based on a first signal, the first signal having a first ripple amplitude, the second signal having a second ripple amplitude less than the first ripple amplitude;

providing one of the first signal and the second signal as a selected signal based on a first frequency of the first signal;

generating a modulation signal in response to the selected signal; and

converting an input signal to an output signal in response to the modulation signal.

16. The method of claim 15 , wherein the first frequency of the first signal is higher than a second frequency of the second signal, the method further comprising:

providing the second signal as the selected signal in response to the first signal having the first frequency that is equal to or greater than a threshold frequency.

17. The method of claim 15 , further comprising:

comparing the first frequency of the first signal to a threshold frequency; and

generating a transition signal in response to the comparison result.

18. The method of claim 17 , wherein the comparison result is a first comparison result, and

wherein comparing the first frequency of the first signal to the threshold frequency includes:

converting the first signal into a filtered signal;

causing the filtered signal to have a DC value that is lower than a DC value of the first signal by a predetermined offset value;

comparing the first signal and the filtered signal to output a second comparison result; and

generating a pulse signal in response to the second comparison result.

19. The method of claim 18 , wherein generating the transition signal includes:

generating a first control pulse signal in response to the pulse signal;

generating a second control pulse signal in response to a reference pulse signal;

causing a first current to flow into a capacitor to charge the capacitor in response to the first control pulse signal;

causing a second current to flow from the capacitor to discharge the capacitor in response to the second control pulse signal; and

comparing an intermediate voltage at an end of the capacitor to a threshold voltage to output the transition signal.

20. A switching power supply comprising:

a signal generator providing a selected signal according to a first signal and a second signal, the first signal having a first ripple amplitude, the second signal having a second ripple amplitude less than the first ripple amplitude;

a controller generating a modulation signal in response to the selected signal; and

a power converter converting an input signal into an output signal in response to the modulation signal,

wherein the signal generator includes a frequency detector comparing a first frequency of the first signal to a threshold frequency and generating a transition signal according to the comparison result.