Power supply device capable of stably supplying output voltage with increased responsiveness

A power supply device comprising:

• a synchronous rectifier step-down converter to alternately turn on and off a switch and a synchronous rectifier switch;
• a reference voltage generator to generate a predetermined reference voltage;
• an error amplifier to output an error voltage such that an output voltage of the step-down converter approaches the predetermined reference voltage;
• a pulse-width signal generator to generate a pulse-width signal controlling timing of turning on and off the switch and the synchronous rectifier switch based on the error voltage;
• a driver circuit to control turning on and off the switch and the synchronous rectifier switch based on the signal generated by the pulse-width signal generator;
• an inductor connected in series with output of the step-down converter;
• a bypass switch to short-circuit an output terminal of the step-down converter to an input terminal of the step-down converter;
• and
• a mode controller to control turning on and off the bypass switch,
• wherein, while the bypass switch is turned on, output of the error amplifier having a reference voltage value that is set to a predetermined divided voltage ratio of the output voltage is inverted.