Frequency lock loop for constant switching frequency of DC-DC converters
A frequency lock loop for a constant switching frequency of DC-DC converter, wherein the frequency lock loop includes a modulation circuit to generate a modulation signal in response to an input signal of the DC-DC converter and a frequency signal. Wherein a timer of the DC-DC converter generates a timing signal in response to the input signal, and wherein the frequency signal is a function of the timing signal.
1. A switching power supply controller comprising:
a PWM timing circuit configured to generate a sample signal and a reset signal, wherein a leading edge of the sample signal occurs earlier than a leading edge of the reset signal;
a frequency detection capacitor configured to be charged by a current source, wherein a voltage across the frequency detection capacitor is reset by the reset signal;
a sample and hold circuit configured to detect a peak voltage across the frequency detection capacitor;
a comparator configured to receive a ramp signal, a threshold voltage signal, the peak voltage and a predetermined reference, wherein a difference between the peak voltage and the predetermined reference is configured to adjust a comparison result between the ramp signal and the threshold voltage signal; and
a timer configured to generate a timing signal based on the comparison result.
2. The switching power supply controller of claim 1 , wherein:
the sample signal is configured to control a sample switch, and wherein the sample switch is connected between a common node of the current source and the frequency detection capacitor, and the sample and hold circuit.
3. The switching power supply controller of claim 1 , further comprising:
a filter coupled to an output of the sample and hold circuit, wherein the peak voltage is tapped at an output of the filter.
4. The switching power supply controller of claim 3 , wherein:
the PWM timing circuit, the sample and hold circuit, the current source, the frequency detection capacitor, the filter and the comparator form a frequency feedback loop operatively associated with the timer.
5. The switching power supply controller of claim 1 , wherein:
the timer is a constant on/off time generator.
6. The switching power supply controller of claim 1 , wherein:
the ramp signal is generator by a ramp generator.
7. The switching power supply controller of claim 6 , wherein:
the ramp generator comprises a resistor, a first switch, a ramp capacitor and a second switch, and wherein:
the resistor, the first switch and the ramp capacitor area connected in series between an input voltage bus and ground; and
the second switch is connected in parallel with the ramp capacitor.
8. The switching power supply controller of claim 7 , wherein:
the ramp signal is tapped at a common node of the first switch and the ramp capacitor.
9. The switching power supply controller of claim 1 , wherein:
the comparator comprises a first input, a second input, a third input, a fourth input and an output, and wherein:
the first input is configured to receive the peak voltage;
the second input is configured to receive the predetermined reference;
the third input is configured to receive the ramp signal;
the fourth input is configured to receive the threshold voltage signal; and
the output is configured to generate the comparison result fed into the timer.
10. The switching power supply controller of claim 1 , wherein:
a falling edge of the sample signal is aligned with the leading edge of the reset signal.
11. A method of removing variable switching frequencies from a direct current to direct current (DC-DC) converter utilizing a constant on/off time generator, the method comprising:
charging a frequency detection capacitor by a current source, wherein a voltage across the frequency detection capacitor is reset by a reset signal;
in response to a sample signal, sampling a peak voltage across the frequency detection capacitor, wherein a leading edge of the sample signal occurs earlier than a leading edge of the reset signal;
comparing the peak voltage with a predetermined reference;
converting a voltage difference between the peak voltage with the predetermined reference into a current difference; and
adjusting a comparison result between a ramp signal and a threshold voltage signal based on the current difference.
12. The method of claim 11 , further comprising:
feeding the comparison result into a timer.
13. The method of claim 12 , wherein:
the timer is the constant on/off time generator.
14. The method of claim 11 , wherein:
the peak voltage is proportional to a switching frequency of a pulse-width modulation signal.
15. The method of claim 14 , further comprising:
in response to an operating condition where the switching frequency is higher than a target frequency, configuring the voltage difference between peak voltage with the predetermined reference to increase an on-time of the pulse-width modulation signal so as to lower the switching frequency.
16. The method of claim 14 , further comprising:
in response to an operating condition where the switching frequency is lower than a target frequency, configuring the voltage difference between peak voltage with the predetermined reference to reduce an on-time of the pulse-width modulation signal so as to increase the switching frequency.