Patent ID: 7642729

Claim:
A light-emitting device driver, comprising: a light-receiving device for converting the optical output of a light-emitting device into a photocurrent; a monitor voltage generating circuit that has a monitor resistor, in which current proportional the photocurrent of the light-receiving device flows, and an on/off switch for switching the current proportional to the photocurrent and outputs a monitor voltage corresponding to the voltage drop of the monitor resistor; an error amplifying circuit that compares the monitor voltage output with a reference voltage corresponding to a set value of the optical output of the light-emitting device and outputs an error voltage corresponding to the difference; a sample-and-hold circuit in which the input terminal is connected to the output terminal of the error amplifying circuit and one of a sampling mode for sampling and outputting the error voltage from the error amplifying circuit and a hold mode for holding the output voltage, regardless of the current input voltage, can be selectively switched; a driving current generating circuit for generating a driving current with a current value corresponding to the output voltage of the sample-and-hold circuit to drive the light emission of the light-emitting device; and a mode switching circuit that monitors the monitor voltage output from the monitor voltage generating circuit after the on/off switch of the monitor voltage generating circuit is switched from the on-state to the off-state to reset the driving current, detects the arrival timing of the monitor voltage to substantially the reference voltage, and switches the sample-and-hold circuit from the hold mode to the sampling mode.