Patent ID: 7679942

Claim:
A step-down DC-to-DC converter comprising: an input terminal; an output terminal; a switching circuit connected between the input terminal and the output terminal and configured to reduce an input voltage down to a predetermined voltage and to output the predetermined voltage from the output terminal, the switching circuit including: a switching transistor configured to perform a switching operation, and an inductor, wherein the switching transistor is connected between the input terminal and the inductor and the inductor is connected between the switching transistor and the output terminal; a rectifying transistor including a P-type transistor and connected in series between the switching transistor and a common ground, wherein a gate of the rectifying transistor is connected to and controlled by a voltage at a connection point that is between the switching transistor and the rectifying transistor; a synchronous rectifying transistor connected in parallel to the rectifying transistor and configured to receive an external control signal through a control electrode thereof so as to perform a switching operation in a manner complementary with the switching operation of the switching transistor; and a drive control circuit configured to cause the synchronous rectifying transistor to turn on when the external control signal represents an instruction of a synchronous rectifying mode and to cause the synchronous rectifying transistor to turn off into a cutoff state when the external control signal represents an instruction of a non-synchronous-rectifying mode.