An error amplifier generates an error signal VERR that corresponds to the difference between a feedback signal VFB and a predetermined reference voltage VREF. A first oscillator generates a first cyclic signal VOSC1 having a sloping segment and a first frequency. A second oscillator generates a second cyclic signal VOSC2 having a sloping segment and a second frequency that is lower than the first frequency. A first pulse modulator generates a first pulse signal having a pulse width that corresponds to the error signal VERR, and clamps its pulse width such that it does not become smaller than a first minimum pulse width. A second pulse modulator generates a second pulse signal having a pulse width that corresponds to the error signal VERR. A synthesizing unit combines the first pulse signal and the second pulse signal so as to generate a driving pulse signal.