A method and a circuit for controlling a switching frequency signal used in a direct current to direct current (DC to DC) converter are proposed. The DC to DC converter generates an output voltage to drive a system. The switching frequency signal is generated by inputting a modulation frequency to a pulse width modulation (PWM) circuit. The method includes steps of: 1. generating a control signal from the system to stop the pulse width modulation (PWM) circuit to generate the switching frequency signal when a signal is received by the system; and 2. restarting the pulse width modulation (PWM) circuit to generate the switching frequency signal in order to increase the output voltage to drive the system when the output voltage from the DC to DC converter is under a predetermined voltage.