Datasheet
MAX3996
Smooth-Start
During startup, the laser does not emit light, and the
APC loop is not closed. The smooth-start circuit pulls
the MD pin to approximately 2.5V during the POR delay
and while TX_DISABLE is high. This causes the power-
control amplifier to shut off the bias transistor. When
POR delay is over and TX_DISABLE is low, the MD pin
is released and pulled to GND by R
SET
because there
is no laser power and thus no monitor diode current.
The output voltage of the power-control amplifier then
begins to increase. A capacitor attached to COMP
(C
COMP
) slows the slew rate and allows a controlled
increase in bias current (Figure 11). Maxim recom-
mends C
COMP
= 0.1µF.
Modulation Circuitry
The modulation circuitry consists of an input buffer, a
current mirror, and a high-speed current switch (Figure
5). The modulator drives up to 30mA of modulation cur-
rent into a 25Ω load.
Many of the modulator performance specifications
depend on total modulator current. To ensure good driver
performance, the voltage at either OUT+ or OUT- must
not be less than V
CC
- 1V.
The amplitude of the modulation current is set with resis-
tors at the MODSET and temperature coefficient (TC)
pins. The resistor at MODSET (R
MOD
) programs the
temperature-stable portion of the modulation current,
and the resistor at TC (R
TC
) programs the temperature-
increasing portion of the modulation current. Figure 6
shows modulation current as a function of temperature
for two extremes: R
TC
is open (the modulation current
has zero temperature coefficient), and R
MOD
is open
(the modulation temperature coefficient is 4000ppm/°C).
Intermediate temperature coefficient values of the mod-
ulation current can be obtained as described in the
Design Procedure section. Table 2 is the R
TC
and R
MOD
selection table.
Safety Circuitry
The safety circuitry contains a disable input, a fault
latch, and fault detectors (Figure 7). This circuitry moni-
tors the operation of the laser driver and forces a shut-
down if a single-point fault is detected. A single-point
fault can be a short to V
CC
or GND, or between any two
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
8 _______________________________________________________________________________________
PIN FAULT CONDITION
MON2 V
MON2
> 400mV
BIAS V
BIAS
< 400mV
TC, MODSET V
MODSET
or V
TC
< 200mV
Table 1. Typical Fault Conditions
MAX3996
100Ω
IN+
IN-
INPUT BUFFER
50Ω 50Ω
V
CC
OUT+
OUT-
1.2V REFERENCE
0ppm/°C
MODSET
FAULT
200mV
MODSET
R
MOD
1.2V REFERENCE
4000ppm/°C
TC FAULT
200mV
TC
R
TC
Σ
CURRENT AMPLIFIER
96X
ENABLE
MODULATION
CURRENT
GENERATOR
CURRENT
SWITCH
Figure 5. Modulation Circuitry
0.6
0.8
0.7
1.0
0.9
1.2
1.1
1.3
020304010 50 60 70 80 90 100 110
JUNCTION TEMPERATURE (
°
C)
i
MOD
/(i
MOD
AT +52°C)
R
TC
≥
1.9k
Ω
R
MOD
= OPEN
TEMPCO = 4000ppm/
°
C
R
TC
= OPEN
TEMPCO = 50ppm/
°
C
Figure 6. Modulation Current vs. Temperature for Maximum
and Minimum Temperature Coefficient