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ISL58125
Applications Information
APC System Overview
As the laser heats up, or ages, its output power declines
relative to the applied current, so some form of power
control is required. The laser is optically coupled to a
photo-diode, so that the laser’s optical output can be
LOW. This can be used to detect fault conditions. Since
both channel’s overcurrent detectors are ORed together,
ERRB going low could indicate a fault in either or both
output channels. Note that the ISL58125 does not shut
itself down when an overcurrent condition is detected.
The device controller is expected to take any required
action. The maximum current is governed by Equation 3:
I TRIP = ------------------------------------------ Amp
measured. Laser optical output power is controlled by
comparing the externally applied control voltage with the
1.05V
( R OVR × 0.012 )
(EQ. 3)
voltage produced by the IV-amplifier which converts the
photo-diode’s output current into a voltage. Since the
calibrated gain is stored as a digital number in a register,
the ISL58125 exhibits none of the time-dependent droop
that is seen in most printers' laser diode drivers. This is
of particular importance during high dot/inch graphics
modes where the line may be slowed down very
significantly to allow 2400 dots per inch or even more.
Fixed-Threshold Laser Bias Control
.
100
80
60
40
20
When a laser is driven from below threshold to well
above threshold, it exhibits a few cycles of a damped
oscillation. The amplitude of this oscillation is minimized
when the laser is kept above threshold. The “fixed” bias
mode is set by asserting a logic Low on the SLPEN pin. To
0
1k
2k 3k 4k
R OVR ( Ω )
FIGURE 2. R OVR vs I TRIP CURRENT
5k
set the laser bias threshold currents, I BIAS , connect
external resistors from RBIAS pins to GND. Figure 1
shows value of R BIAS corresponding to desired bias
current.
100
10
Controlling the Sampling
The switching levels are sampled independently. This can
be done during the “off-paper” period.
During calibration mode, the internal servo control will
bring the laser diode output power level to match the
voltage control level set by VC voltage.
Typical Application
Upon the printer being powered up, the lasers should be
calibrated. This would establish nominal light power
outputs, typically a few milliwatts at the laser regardless
of the ambient temperature and also any laser aging.
1
0.1 1
R BIAS (k Ω )
FIGURE 1. R BIAS vs BIAS CURRENT
10
Once everything is ready for printing, the paper is in
position and the mirror-motor is phase-locked then the
print line(s) can be written. Before, or after, the beam is
over the photo-sensitive drum, each laser can be re-
calibrated. This continual re-calibration will compensate
I SW = I SW Gain × ? ---------------- – --------------- ?
(EQ. 1)
? R R ?
(EQ. 2)
I BIAS = BiasChannelGain × ------------------------------------
Scaling External Resistors
R SET is used to scale the switching output current. Switching
output current, I SW , is the function of VC and R SET
VC 2
DAC SET
Where I SW Gain = ~17, R DAC = 400 Ω .
R BIAS sets bias threshold current. Figure 1 exhibits the
relationship between I BIAS and R BIAS . The bias current
is set as Equation 2:
InternalVref
R BIAS
Where BiasChannelGain = ~40, InternalVref = 1.0V.
R OVR resistor defines the maximum current that can
flow in either laser before the ERRB pin puts out a logical
8
for any temperature drift of the laser, especially at the
initial warming up period.
Since the calibrated gain is stored as a digital number in
a register, the ISL58125 exhibits no time-dependent
droop. With no droop to degrade performance the only
limitation now is the lasers' own temperature change
along the line. This in turn can be compensated for to
some extent by adding a data-dependent compensation
signal to the analog VCx input pin. It may be found that
in fast draft modes for example, that the laser
temperature change is sufficiently small that many lines
can be written before the laser(s) need to be re-
calibrated. If the printed page has a low enough duty
cycle, no re-calibration may be needed at all.
The ISL58125 has analog voltage inputs to allow the
laser power level to be adjusted during the line. Typically
this would be driven with a PWM, low bandwidth signal to
FN6625.1
July 22, 2013
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