Patent ID: 7893667

Claim:
A power supply apparatus comprising: an output transistor to generate an output voltage from an input voltage by being turned on and off; an error amplifier to amplify a voltage difference between a feedback voltage commensurate with the output voltage and a predetermined reference voltage to generate an error voltage; a PWM comparator to compare the error voltage with a predetermined slope voltage to generate a PWM signal having a duty ratio corresponding to a result of comparison; and on/off control means to turn on and off the output transistor with the PWM signal, wherein the PWM comparator is operable to keep the slope voltage shifted from a ground potential to a higher voltage by a predetermined offset voltage so that a lower limit level of the slope voltage is higher than a lower limit level of the error voltage; the PWM comparator comprising, in an input stage thereof: first and second constant current sources, each having one end thereof connected to a power supply line; a first switch element connected between the other end of the first constant current source and a ground line, the first switch element operable to be being turned on and off according to the slope voltage; and a second switch element connected between the other end of the second constant current source and the ground line, the second switch element operable to be turned on and off according to the error voltage, wherein the PWM comparator further comprises an offset resistor connected between the other end of the first constant current source and one end of the first switch element or between the other end of the first switch element and the ground line, and operable to compare voltages appearing at the other ends of the first and second current sources to generate the PWM signal.