The overshoot mirror sets the bias in the input buffer
stage (Figure 2). Reducing this current slows the input
stage and reduces overshoot in the modulation signal.
At the same time, the peak-to-peak output swing of the
input buffer stage is reduced. Careful design must be
used to ensure that the buffer stage can switch the out-
put stage completely into the nonlinear region. The
input swing required to completely switch the output
stage depends on both R
OSADJ
and the modulation
current. See Allowable R
OSADJ
Range vs. Modulation
Current and Maximum Modulation Current vs. Minimum
Differential Input Signal Amplitude graphs in the Typical
Operating Characteristics. For the output stage, the
width of the linear region is a function of the desired
modulation current. Increasing the modulation current
increases the linear region. Therefore, increases in the
modulation current require larger output levels from the
first stage.
Failure to ensure that the output stage switches com-
pletely results in a loss of modulation current (and
extinction ratio). In addition, if the modulation port does
not switch completely off, the modulation current will
contribute to the bias current, and may complicate
module assembly.
Automatic Power Control
The automatic power control (APC) feature allows an
optical transmitter to maintain constant power, despite
changes in laser efficiency with temperature or age. The
APC requires the use of a monitor photodiode.
The APC circuit incorporates the laser diode, the monitor
photodiode, the pin set current mirror, a transconduc-
tance amplifier, the bias set current mirror, and the laser
fail comparator (Figure 1). Light produced by the laser
diode generates an average current in the monitor pho-
todiode. This current flows into the MAX3263’s IPIN
input. The IPINSET current mirror draws current away
from the IPIN node. When the current into the IPIN node
equals the current drawn away by IPINSET, the node
voltage is set by the V
CC
x 3/5 reference of the transcon-
ductance amplifier. When the monitor current exceeds
IPINSET, the IPIN node voltage will be forced higher. If
the monitor current decreases, the IPIN node voltage is
decreased. In either case, the voltage change is ampli-
fied by the transconductance amplifier, and results in a
feedback current at the IBIASFB node. Under normal
APC operation, IBIASFB is summed with IBIASSET, and
the laser bias level is adjusted to maintain constant out-
put power. This feedback process continues until the
monitor-diode current equals IPINSET.
If the monitor-diode current is sufficiently less than IPIN-
SET (i.e., the laser stops functioning), the voltage on the
IPIN node drops below 2.6V. This triggers the failout
comparator, which provides a TTL signal indicating laser
failure. The FAILOUT output asserts only if the monitor-
diode current is low, not in the reverse situation where
the monitor current exceeds IPINSET. FAILOUT is an
open-collector output that requires an external pull-up
resistor of 2.7kΩ to V
CC
.
The transconductance amplifier can source or sink cur-
rents up to approximately 1mA. Since the laser bias gen-
erator has a gain of approximately 40, the APC function
has a limit of approximately 40mA (up or down) from the
initial set point. To take full advantage of this adjustment
range, it may be prudent to program the laser bias cur-
rent slightly higher than required for normal operation.
However, do not exceed the I
BIASOUT
absolute maxi-
mum rating of 75mA.
To maintain APC loop stability, a 0.1µF bypass capaci-
tor may be required across the photodiode. If the APC
function is not used, disconnect the IBIASFB pin.
Enable Inputs
The MAX3263 provides complementary enable inputs
(ENB+, ENB-). The laser is disabled by reducing the ref-
erence voltage outputs (VREF1, VREF2). Only one logic
state enables laser operation (Figure 3 and Table 1).
MAX3263
Single +5V, Fully Integrated,
155Mbps Laser Diode Driver
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