Patent ID: 7138786

Claim:
A power supply driver circuit constituting a switched-mode power supply equipment controlling, by pulse-width modulation (PWM), switching of a power semiconductor device passing a current in an inductor, and outputting a voltage converted from an input voltage, comprising: a current sensing semiconductor device in which drain terminals of the current sensing semiconductor device and said power semiconductor device, or collector terminals thereof, are coupled together, the same drive voltage being applied to control terminals thereof; a current sensing resistor coupled to the source terminal or emitter terminal of said current sensing semiconductor device; an operational amplifier circuit having a first input terminal connected to a first node which is a connection node connecting said current sensing resistor and said current sensing semiconductor device and a second terminal supplied with a voltage of the same potential as a source voltage or emitter voltage of said power semiconductor device; and a current sensing circuit which has a feedback loop formed so that a pair of input terminals of said operational amplifier circuit have the same potential, and outputs a signal emanating in said current sensing resistor as a current sensing signal; wherein a route passing the current in the current sensing circuit is ensured regardless of a switching state to make an undefined potential well defined.