Patent ID: 8897039

Claim:
A pulse frequency modulation (PFM) controller for controlling a switching mode power supply, the controller comprising: an output terminal for providing a control signal to turn on and off a current in the power supply to regulate an output of the power supply; a first input terminal for receiving a feedback signal related to the output of the power supply, the feedback signal exhibiting a ringing waveform when the current in the power supply is turned off; a second input terminal for receiving a voltage signal related to the current in the power supply; and a control circuit configured to provide the control signal in response to the feedback signal, the control signal being adapted to turn on the current when the feedback signal is substantially at a valley of the ringing waveform of the feedback signal; wherein the control circuit comprises: a driver circuit coupled to the output terminal for providing the control signal, the driver circuit having a first state and a second state, wherein in the first state the driver circuit provides the control signal for turning on the current in the power supply, and in the second state the driver circuit provides the control signal for turning off the current in the power supply; a first circuit coupled to the first input terminal for receiving the feedback signal, the first circuit being configured for providing a first signal in response to the feedback signal; a second circuit coupled to the first circuit and the driver circuit, the second circuit being configured to provide a second signal to set the driver circuit to the first state in response to the first signal and when the feedback signal is substantially at a valley of its ringing waveform; and a third circuit for providing a third signal for setting the driver circuit to the second state; wherein the second circuit includes: a comparator for comparing the feedback signal with a first reference signal, the ringing waveform of the feedback signal being characterized by a first oscillation frequency; and an oscillator for providing an oscillation signal in response to an output signal of the comparator, the oscillation signal being characterized by a 50% duty cycle and a second oscillation frequency that is approximately twice the first oscillation frequency.