Step-down DC-to-DC converter

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.