Canon LBP1760P Service Manual page 62

CHAPTER 2
2. Automatic power control of laser diode
The laser driver IC conducts the automatic power control (APC) of the laser diode so that the
laser diode emits a beam of constant intensity.
When the FORCED LASER ON signal (/LON) becomes "L," or the VIDEO DATA ENABLE sig-
nal (/ENBL) and /VDO signal become "L," the laser diode emits light according to the standard
voltage of laser driver PCB. The photo diode in the laser diode detects the beam intensity and
feeds back the corresponding current value to the control amplification circuit. It is then com-
pared with the current value set by the power adjustment circuit, and the intensity of the cur-
rent to the laser diode is controlled.
The APC of this printer does not go through the CPU like the conventional APC does. Instead,
it uses the APC method that constantly the beam intensity detection and control for each dot
while the laser driver is emitting laser.
While in the conventional method, the beam intensity was measured by emitting laser for a
certain period of time between pages or during unblanking, this method controls the laser out-
put, by detecting the intensity of the laser which is actually forming an image. Therefore, the
APC of this printer has higher reliability of control over the conventional APC.
3. Horizontal synchronization control
The CPU generates an unblanking signal inside the IC501 based on the BD INPUT signal (/BDI)
sent from the BD PCB of the laser/scanner unit.
IC501 sets the FORCED LASER ON signal (/LON) to "L" during unblanking period. The laser
driver turns the laser diode ON when the /LON signal is "L."
The laser beam turned on by the /LON signal is reflected at the small fixed BD mirror locat-
ed in the light path where the laser beam scanning starts, and is input to the BD PCB in the
laser/scanner unit.
The BD PCB detects the laser beam, generates a BD INPUT signal (/BDI), and sends it to the
CPU. The CPU generates the HORIZONTAL SYNC signal (/BD) according to the /BDI signal and
sends it to the video controller.
4. Laser diode emission control
Based on the paper size data, the CPU generates the VIDEO DATA ENABLE signal (/ENBL) and
sends it to the logic circuit of the laser driver IC. The /VDO signal is masked by the /ENBL sig-
nal in the logic circuit, which controls the emission period of the laser diode.
The paper size data is supplied to the CPU by the cassette paper size detection switches for
cassette pick-up or by the paper size specification command from the video controller for multi-
purpose tray pick-up.
The CPU generates the IMAGE TOP ERASE AND BOTTOM ERASE signal (TOPE) and the
HORIZONTAL IMAGE MASK signal (MSK) according to the paper size data. (TOPE and MSK are
used inside the CPU and are not output externally.)
To inhibit the laser beam from writing on non-image area during the non-unblanking period,
the CPU sets the VIDEO DATA ENABLE signal (/ENBL) to "L" only when the TOPE and MSK sig-
nals are "L." The laser driver turns the laser diode ON when the /ENBL signal and /VDO sig-
nals are "L."
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