A power supply includes a first switch and a second switch coupled in series between an input voltage terminal to which an input voltage is applied and a reference voltage terminal to which a reference voltage lower than the input voltage is applied, an inductor disposed between a junction coupling the first and second switches and an output terminal from which an output voltage is output, and a controller controlling the first and second switches to be alternately switched at a given switching cycle depending on an error of the output voltage with respect to a target voltage, wherein the controller changes the switching cycle from a first cycle to a second cycle longer than the first cycle, depending on a voltage at the junction when the second switch is in a turned-on state.