Patent ID: 7307406

Claim:
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, wherein said control circuit: includes a voltage detecting circuit which creates 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, validates the operation of said voltage detecting circuit when a load circuit supplied with said output voltage is in a light load state, and generates 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 invalidates 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, turns on said first switch element.