Laser Driver Circuit - Canon GP200 Service Manual

3. OPERATIONS AND TIMING
C. Laser Driver Circuit
1. Operation
The optical output of the laser (semiconductor laser) is affected largely by changes occurring in the
ambient temperature. The GP215/GP200 uses a control mechanism which varies the laser drive current
automatically to suit the ambient temperature, thereby ensuring a specific output.
When the Copy Start key is pressed, the image processor PCB sends the bias control signal (APCON*)
and the laser drive signal (LSRD) to the laser driver PCB so as to drive the bias control circuit and the con-
stant current circuit on the laser PCB, thereby turning ON the laser beam.
The laser beam is detected by the pin photodiode (PD) built into the laser element, and the PD photo
output voltage is returned to the bias control circuit.
The photo output voltage is sample-held and compared against the reference voltage from the laser
intensity setting circuit, thereby determining the value of the laser bias current. This laser bias current is
varied continuously in conjunction with the changes in temperature to ensure a specific output. (Figure 3-
404)
A series of these operations is executed for each signal line of scanning.
Sample hold
signal (SH)
Intensity selection
signal 0 (SW0)
Intensity selection
signal 1 (SW2)
Intensity selection
signal 2 (SW2)
Bias control ON
signal (AP CON*)
Image
processor
PCB
Laser drive
signal(LSRD)
3–54
COPYRIGHT © 1996 CANON INC.
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Laser driver PCB
Laser intensity
setting circuit
VR1202 VR1205 VR1206
Bias control
Limiter
circuit
VR1204
Figure 3-403
CANON GP215/200 REV.0 JULY 1996 PRINTED IN JAPAN (IMPRIME AU JAPON)
5V
Laser element
circuit
VR1203
Current
switch
Constant current
circuit
VR1201
: Do not touch in the field.