Patent ID: 8237369

Claim:
A light source driving circuit, comprising: an operation amplifier, comprising a first input terminal, a second input terminal and an output terminal, wherein the first input terminal couples to a predetermined voltage; a transistor, comprising a first terminal, a second terminal and a control terminal, wherein the first terminal couples to a light source, and the control terminal is directly connected to the output terminal of the operation amplifier; a switch unit, comprising a first signal terminal receiving a first voltage which is constant and is greater than the predetermined voltage, a second signal terminal coupled to the second input terminal of the operation amplifier, a third signal terminal coupled to the second terminal of the transistor, and an enabling terminal receiving a driving voltage, wherein the switch unit transmits a voltage of the third signal terminal to the second signal terminal when the driving voltage is enabled so that the operation amplifier is operated as a voltage follower to turn on the transistor, and the switch unit transmits a voltage of the first signal terminal to the second signal terminal when the driving voltage is disabled so that the operation amplifier is operated as an open loop voltage comparator and outputs a negative saturation voltage to directly turn off the transistor; and a first resistor, coupled between the second terminal of the transistor and a second voltage.