Patent ID: 7567069

Claim:
A step-up power supply device, comprising: a step-up converter including an inductor and switch element which are connected in series and a rectifier and smoother which rectify and smooth a voltage across the switch element, the step-up converter being designed such that an input voltage to the inductor is boosted by an on/off operation of the switch element and an output voltage is output from the smoother; a startup circuit which controls the on/off operation of the switch element when the output voltage is smaller than a reference voltage; and a control circuit which operates in substitution for the startup circuit when the output voltage is higher than the reference voltage to control the on/off operation of the switch element such that the output voltage reaches a target value, wherein the startup circuit includes an oscillation circuit for generating a pulse signal which has a predetermined on/off ratio, a current comparison circuit for comparing a current of the switch element with a predetermined restriction value, and a drive circuit for generating a drive signal based on which the switch element is turned on/off, the drive signal being generated from the pulse signal to turn on the switch element, and the drive signal being generated from the output of the current comparison circuit or the pulse signal to turn off the switch element.