Patent ID: 7615980

Claim:
A power-supply device functioning as a step-down DC-DC converter which accepts a DC input voltage at an input terminal and delivers stepped-down output voltage from an output terminal, comprising: a marginal check voltage setting circuit which inputs a reference voltage to set a desired marginal check voltage, an error amplifier which with the input of the marginal check voltage and said output voltage generates the difference between the two and outputs it as an error signal, a pulse width modulation oscillator which modulates said output from the error amplifier in pulse width, a driver circuit which generates driving signals from the pulse signals from the pulse width modulation oscillator, and a pair of power semiconductor switching devices which steps down said DC input voltage in accordance with the driving signals from the driver circuit to generate said DC output voltage, wherein said marginal check voltage setting circuit includes an upper limit marginal check voltage generating circuit which inputs a reference voltage to generate upper limit marginal check voltage, a resistor network comprising a plurality of series-connected resistors, and an array of switches including a plurality of parallel-connected switches one end of each of said switches being connected to a different node of said resistors, the other end of each of said switches being connected in common to an output terminal of said marginal check voltage setting circuit, and a desired marginal check voltage increasing or decreasing relative to the reference voltage by a certain ratio is available by closing one of said plurality of switches.