Patent ID: 7554305

Claim:
A step-down circuit, comprising: an N channel type output transistor of which voltage at a control end thereof is controlled so as to step down a power supply voltage inputted from an input end thereof to a desired voltage and output the step-down voltage from an output end thereof; a charge pump circuit, connected to the control end of said output transistor, for raising the voltage of said control end; a discharge circuit for discharging the electric charge at the control end of said output transistor, wherein the step-down voltage outputted from said output transistor decreases; an oscillator for supplying a clock signal with constant frequency to said charge pump circuit; and a comparator for comparing a divided voltage divided from the step-down voltage outputted from the output end of said output transistor and a reference voltage; wherein, when the step-down voltage outputted from said output transistor is higher than the desired voltage, said discharge circuit discharges the electric charge at the control end of said output transistor based on the comparison result of said comparator without stopping the operation of said charge pump circuit; and when the step-down voltage outputted from said output transistor is equal to or lower than the desired voltage, said charge pump circuit raises the voltage at the control end of said output transistor in response to the clock signal and the operation of said discharge circuit is stopped.