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MAX3667 Datasheet(PDF) 6 Page - Maxim Integrated Products

Part No. MAX3667
Description  3.3V, 622Mbps SDH/SONET Laser Driver with Automatic Power Control
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
Logo MAXIM - Maxim Integrated Products

MAX3667 Datasheet(HTML) 6 Page - Maxim Integrated Products

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_______________Detailed Description
Low-voltage operation of laser diodes and optical
transmitters produces stringent headroom conditions
for laser drivers. Fast changes in modulation current
produce large inductive voltage spikes, creating device
saturation problems. Therefore, for +3.3V operation, the
MAX3667’s modulation current should be AC coupled
to the cathode of a laser diode. The recommended DC
blocking capacitor value is 1µF. A simplified block dia-
gram of the modulation driver is shown in Figure 1.
The IMOD pin is internally biased through a 31
Ω pull-up
resistor. This design decouples the headroom associat-
ed with the modulation driver from the forward voltage
drop of the laser diode, allowing the circuit to tolerate
greater di/dt voltage transients. The design of the
MAX3667 assumes a maximum DC forward-voltage
drop of 1.6V across the laser diode. Bias current is DC
coupled to the laser diode separately at the IBIAS out-
put. In most applications, some small amount of resis-
tance should be added in series with the DC blocking
capacitor to help damp out the aberrations created by
parasitic elements.
Automatic Power Control
The automatic power control (APC) feature allows an
optical transmitter to maintain constant power, despite
changes in laser efficiency due to temperature and
aging. The APC loop requires the use of a PIN monitor
photodiode, which generates a current proportional to
the laser diode output power. A scaled version of the
current flowing into the MD pin is compared to a scaled
version of the current flowing out of the APCSET pin.
When these currents are of equal value, the inputs of
the operational transconductance amplifier (OTA) are
balanced, and COMP is forced to approximately 1V.
When the average value of the monitor diode current
exceeds the value established by the APCSET current,
the COMP voltage is forced lower. If the average value
of the monitor diode current is less than the value
established by the APCSET current, the COMP node
voltage is forced higher. The output of the OTA (the
APC pin), when connected directly to BIASSET (closed-
loop condition), is used as an error signal to adjust the
bias current flowing into BIASSET. The maximum OTA
output current is approximately ±250µA.
+3.3V, 622Mbps SDH/SONET Laser Driver
with Automatic Power Control
6
_______________________________________________________________________________________
MAX3667
INPUT
(MODULATION)
31
VCC
VCC
VCC
INPUT
(BIAS)
LASER
DIODE
RFILT
22
RDAMP
4.7
0.01
µF
1
µF
MODMON
IBIAS
IMOD
QMOD
IQMOD
BIASMON
Figure 1. Simplified Modulation Driver Block Diagram


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