Patent ID: 7408390

Claim:
A voltage detecting circuit for detecting a minute voltage in a DC to DC converter circuit having a switching circuit for converting a DC input voltage to an AC voltage, and a smoothing circuit including an inductor for smoothing and converting the AC voltage from the switching circuit to a desired DC voltage, the voltage detecting circuit comprising: a first output providing one of the DC voltage output from the smoothing circuit or the AC voltage output from the switching circuit; a second output providing the other of the DC voltage output from the smoothing circuit or the AC voltage output from the switching circuit; an operational amplifier; an output unit that selectively outputs one of the DC voltage or the AC voltage from one of the first or the second output to a non-inverted input of the operational amplifier; a capacitor chargeable with the voltage output from the output unit and connected to an inverted input of the operational amplifier; and a switch that turns on to connect an output of the operational amplifier to the inverted input of the operational amplifier or off to disconnect the output of the operational amplifier to the inverted input of the operational amplifier in accordance with the state of the output unit, wherein the operational amplifier operates as a voltage follower to charge the capacitor as the switch turns on when the first output is input to the non-inverted input of the operational amplifier, and wherein the operational amplifier operates as a comparator to compare the voltage charged in the capacitor with the voltage from the second output as the switch turns off when the second output is input to the non-inverted input of the operational amplifier.