Patent ID: 7812580

Claim:
A control circuit of a power supply apparatus in which a step down switching regulator and a linear regulator can be switched, the control circuit comprising: an output stage, including a switching transistor and a synchronous rectifier transistor connected in series between an input terminal and grounding, which outputs a voltage at a connection point of the two transistors, as a switching voltage, to a switching regulator output circuit; a pulse width modulator, which generates a pulse width modulation signal with which a duty ratio is controlled so that output voltage of the switching regulator output circuit approaches a predetermined reference voltage; a compulsory OFF circuit, which monitors the switching voltage, and switches OFF the synchronous rectifier transistor when the switching voltage exceeds a first threshold voltage; a minimum ON time setting circuit, which receives the pulse width modulation signal, limits the duty ratio of the pulse width modulation signal so that ON time of the switching transistor is longer than a predetermined minimum value; a driver circuit, which generates a first and a second gate voltage to be applied to a gate terminal of the switching transistor and the synchronous rectifier transistor, based on an output signal of the minimum ON time setting circuit; and a selector circuit, which monitors an error voltage between the output voltage and the reference voltage, and when a predetermined state continues for a predetermined determination period, puts the linear regulator in an operating state, and puts the step down switching regulator in a halt state.