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2. Control of laser beam intensity (APC
control]
The CPU performs aulomalic power control
(AI'C) of the laser diode to make the laser
diode
light. This control can
?miL constant
be classified inlo two types: initial APC and
between-page APC.
a. Initial AE'C
Inilial APC is performed to increase the
intensity of the laser beam from zero to
the targel value during initial rotation.
The CPU sets Lhc FORCIBLE LASER
ON signal (/LON) Lo "I,", and at the
same time the CPU changes the APCOR
signal (coarse adjustment) and lets the
AE'COUT signal gradually rise from the
l o w v a l u e . A s lhe l a s e r currenl
increases. the laser diode (LD) begins
emitting light.
The intensity of ihe laser diode (LU)
outpuL is detected by the pholodiodr
(I'D). After amplification, the output
voltage is fed back Lo the CPU as the
AUTOMATlC POWER CONTROL INPUT
signal (APCIN).
'Ihe CPlJ continues coarse adjuslment
unlil Lhr inpul value of
signal reaches 70% of the Larget value
stored in
CPU. The CPU then does
Lhe
fine adjustment with the APCOS signal,
and ends the initial APC when the
inpul value of the APCIN signal has
become almost equal Lo the larget
value.
b. Between-page
AE'C
Between-page APC is performed afler
inilial APC, and when prinling is not
being performed while waiting for the
nexl page to proceed to the printing
area, Lo correcl variations in the
inlensity of
1
he laser printing.
Belwecn-page APC is also performed
during the unblinking period, described
IaLer. Lo prevent the laser beam from
writing on the image area of the
photosensitive drum.
The CPU sets the UNBLANKING
signal (UBL. used by IC202 and not
A P C I N
t h e
output externally) to "H" and sets the
FORCIBLE LASER ON signal (/LON) to
"L".
The CPU then changes the APCOS
signal (fine adjustment]. compares the
input value of the APCIN signal with
the target value. and optimizes the
inlensity of the laser beam.
CHAPTER 2
2-11
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