Laser driver circuit

Abstract

A driver circuit applies electric current to a semiconductor laser for emission of light suitable for use in a laser printer wherein a photosensitive medium is exposed by the light for imprinting an image of marks or pixels upon the medium. The current is the sum of bias current plus signal current, the latter being a sequence of pulses providing image data. The circuit includes a bypass around the laser, and the signal current is pulsed by using a switching circuit which applies a steady input current alternately to the laser and the bypass under control of a digital data signal. The switching circuit is an array of emitter-coupled logic elements operating in parallel for reduced inductance and higher switching speed. The steady input current is provided by a switching current regulator wherein a power transistor, driven by a pulse width modulator, alternately connects and disconnects current from a DC power supply in conjunction with a smoothing filter comprising an inductor and a capacitor. A current sensing resistor connects in series with the parallel circuit of laser and bypass to output a sense signal representing the amplitude of the steady input current. The sensor signal drives the pulse width modulator via a loop compensation element to establish an average value of laser current. An additional ripple current source, responsive to a high frequency component of the sensor signal, is added to the input current to counteract any ripple from the regulator.