Patent ID: 7548050

Claim:
In a switching power supply device, comprising: an inductor, a capacitor disposed between an output side and a ground potential of said inductor, a first switch element for supplying a current from an input voltage to an input side of said inductor, a second switch element which comes on when said first switch element is off to set the input side of said inductor to a prescribed potential, and a control circuit which creates a first control signal to be supplied to said first and second switch elements so as to set the output voltage obtained from the output side of said inductor to a desired level, a method performed by the control circuit comprising the steps of: creating, by a voltage detecting circuit, a detection signal by which the arrival of the voltage on said input side of said inductor at a first voltage corresponding to said input voltage is detected, validating the operation of said voltage detecting circuit when a load circuit supplied with said output voltage is in a light load state, and generating a second control signal which, when said second switch element is turned off, turns on the first switch element with the detection signal of said voltage detecting circuit, and invalidating the detection signal of said voltage detecting circuit when said load circuit is in a heavy load state and, after said second switch element is turned off, turning on said first switch element.