Unit - Canon LBP-3260 Service Manual

CHAPTER 2
it exceeds 3.805V.
2. Automatic power control of laser diode
The laser driver IC controls 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 ENABLE signal
(/ENBL) and /VDOUT signal become "L," the laser diode emits light according to the standard
voltage of the 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
compared with the current value set in the power adjustment circuit, and the intensity of the
current to the laser diode is controlled.
The printer uses the APC method that conducts the beam intensity detection and control for
each dot while the laser driver is emitting laser.
intensity was measured by emitting laser for a certain period of time between pages or during
unblinking, this method controls the laser output, 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 IC201 based on the BD input signal
(/BDI) sent from the BD PCB of the laser/scanner unit.
IC201 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 a BEAM DETECTION signal (/BD) according to the /BDI signal and
sends it to the video controller PCB.
4. Laser diode emission control
Based on the paper size data, the CPU generates a VIDEO ENABLE signal (/LENBL) and sends
it to the logic circuit of the laser driver IC. The /VDOUT signal is masked by the /LENBL 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 switches for cassette
pick-up and by the paper size specification command from the video controller PCB for multi-
purpose tray pick-up.
The CPU generates the IMAGE TOP ERASE AND BOTTOM ERASE signal (TOPE) and HORI-
ZONTAL IMAGE MASK signal (MSK) according to the paper size data. (TOPE and MSK are used
inside the CPU and are not output externally.)
To prevent the laser beam from writing on non-image area during the non-unblanking peri-
od, the CPU sets the VIDEO ENABLE signal (/LENBL) to "L" only when the TOPE and MSK sig-
nals are "L." The laser driver turns the laser diode ON when the /LENBL signal and /VDOUT
signal are "L."
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While in the conventional method, the beam