MAX3869 Datasheet, PDF (7/16 Pages) Maxim Integrated Products – +3.3V, 2.5Gbps SDH/SONET Laser Driver with Current Monitors and APC

+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Current Monitors and APC
Pin Description (continued)
PIN
NAME
FUNCTION
20
OUT-
Negative Modulation-Current Output. I_MOD flows through this pad when input data is low.
22
GND4
Ground for Output Circuitry
23
GND3
Ground for APC
24
MD
Monitor Diode Input. Connect this pad to a monitor photodiode anode. A capacitor to ground
is required to filter high-speed AC monitor photocurrent.
25
V_CC3
Power Supply for APC
26
CAPC
A capacitor connected from this pad to ground controls the dominant pole of the APC feed-
back loop (C_APC = 0.1µF).
27
GND2
Ground for Internal Reference
28
N.C.
No Connection. Leave unconnected.
29
APCSET
A resistor connected from this pad to ground sets the desired average optical power.
Connect 100kΩ from this pad to ground if APC is not used.
30
MODSET A resistor connected from this pad to ground sets the desired modulation current.
31
BIASMAX
A resistor connected from this pad to ground sets the maximum bias current. The APC
function can subtract from this maximum value, but cannot add to it.
32
V_CC2
Power Supply for Internal Reference
_______________Detailed Description
The MAX3869 laser driver consists of two main parts: a
high-speed modulation driver and a laser-biasing block
with automatic power control (APC). The circuit design
is optimized for both high-speed and low-voltage
(+3.3V) operation. To minimize the pattern-dependent
jitter of the input signal at speeds as high as 2.5Gbps,
the device accepts a differential PECL clock signal for
data retiming. When LATCH is high, the input data is
synchronized by the clock signal. When LATCH is low,
the input data is directly applied to the output stage.
The output stage is composed of a high-speed differential
pair and a programmable modulation current source.
Since the modulation output drives a maximum current
of 60mA into the laser with an edge speed of 100ps,
large transient voltage spikes can be generated (due to
the parasitic inductance). These transients and the
laser forward voltage leave insufficient headroom for
the proper operation of the laser driver if the modulation
output is DC-coupled to the laser diode. To solve this
problem, the MAX3869’s modulation output is designed
to be AC-coupled to the cathode of a laser diode. An
external pull-up inductor is necessary to DC-bias the
modulation output at V_CC. Such a configuration isolates
laser forward voltage from the output circuitry and
allows the output at OUT+ to swing above and below
the supply voltage V_CC. A simplified functional diagram
is shown in Figure 4.
The MAX3869 modulation output is optimized for driv-
ing a 25Ω load; the minimum required voltage at OUT+
is 2.0V. Modulation current swings of 80mA are possi-
ble, but due to minimum power-supply and jitter
requirements at 2.5Gbps, the specified maximum mod-
ulation current is limited to 60mA. To interface with the
laser diode, a damping resistor (R_D) is required for
impedance matching. An RC shunt network may also
be necessary to compensate for the laser-diode para-
sitic inductance, thereby improving the optical output
aberrations and duty-cycle distortion.
At the data rate of 2.5Gbps, any capacitive load at the
cathode of a laser diode will degrade the optical output
performance. Since the BIAS output is directly connect-
ed to the laser cathode, minimize the parasitic capaci-
tance associated with this pin by using an inductor to
isolate the BIAS pin from the laser cathode.
Automatic Power Control
To maintain constant average optical power, the
MAX3869 incorporates an APC loop to compensate for
the changes in laser threshold current over temperature
and lifetime. A back-facet photodiode mounted in the
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