Patent ID: 7586296

Claim:
A power supply apparatus comprising: a stepdown switching power supply circuit including a first switching element connected to an internal power supply line of said power supply apparatus at one end thereof, and a smoothing circuit operating to smooth a voltage at the other end of said switching element, said stepdown switching power supply circuit outputting, from said smoothing circuit, a voltage lower than a voltage applied to said internal power supply line by on/off driving said first switching element; a filter circuit located in the upstream of said stepdown switching power supply circuit, said filter circuit including an inductor connected in series between said internal power supply line and an external power supply line of said power supply apparatus, said external power supply line being applied with an external voltage of an external voltage source; a stepup switching power supply circuit including the inductor connected in series between said internal power supply line and an external power supply line, and a second switching element connected in series between an end of said inductor connected to said internal power supply line and a ground line to which a low-voltage side terminal of said external voltage source is connected, said stepup switching power supply circuit generating, at said one end of said inductor, a voltage higher than said external voltage by on/off driving said second switching element; and a mode switching circuit for switching an operation mode of said power supply apparatus between a stepdown mode where said second switching element is kept at an off state, and said first switching element is on/off driven so that a voltage lower than said external voltage is outputted from said stepdown switching power supply circuit, and a stepup mode where said second switching element is on/off driven, and said first switching element is kept at an on state so that a voltage higher than said external voltage is outputted from said stepdown switching power supply circuit, and wherein said mode switching circuit is configured to switch said operation mode from said stepdown mode to said stepup mode upon detecting that said external voltage falls below a predetermined value, and to switch said operation mode from said stepup mode to said stepdown mode upon detecting that said external voltage exceeds said predetermined value.