A power supply apparatus switchably operates an LDO regulator and a DC/DC converter connected in parallel so as to control a battery voltage Vbat from a power supply at a constant output voltage VO. A detected voltage Vs at a shared output terminal shared by the LDO regulator and the DC/DC converter is input to a feedback input terminal and fed back to the LDO regulator and the DC/DC converter. In order to suppress an undershoot occurring when switching from the LDO regulator to the DC/DC converter, a timing signal generating unit supplies a capability switching timing signal to a pre-drive circuit and switches the control capability of the DC/DC converter from a low capability mode at startup to a high capability mode.