MAX3867 Ver la hoja de datos (PDF) - Maxim Integrated
Número de pieza
componentes Descripción
Fabricante
MAX3867 Datasheet PDF : 12 Pages
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+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Automatic Power Control
PIN
25
29
30
35
34, 36, 40
37
38
44
46
47
48
NAME
BIAS
OUT+
OUT-
MD
GND3
VCC3
CAPC
APCSET
MODSET
BIASMAX
VCC2
Pin Description (continued)
FUNCTION
Laser Bias Current Output
Positive Modulation-Current Output. IMOD flows through this pad when input data is high.
Negative Modulation-Current Output. IMOD flows through this pad when input data is low.
Monitor Diode Input. Connect this pad to a monitor photodiode anode. A capacitor to ground
is required to filter high-speed AC monitor photocurrent.
Ground for APC
Power Supply for APC
A capacitor connected from this pad to ground controls the dominant pole of the APC feed-
back loop. (CAPC = 0.1µF)
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.
A resistor connected from this pad to ground sets the desired modulation current.
A resistor connected from this pad to ground sets the maximum bias current. The APC
function can subtract from this maximum value, but can not add to it.
Power Supply for internal reference
_______________Detailed Description
The MAX3867 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 MAX3867’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 VCC. 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 VCC. A simplified functional diagram
is shown in Figure 4.
The MAX3867 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 (RD) is required for
impedance matching. An RC shunt network is also nec-
essary to compensate for the laser-diode parasitic
inductance, thereby improving the optical output aber-
rations 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 connected
to the laser cathode, minimize the parasitic capacitance
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
MAX3867 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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