MAX3996CGP Maxim Integrated Products, MAX3996CGP Datasheet - Page 8

• Manufacturer Part Number: MAX3996CGP
• Description: Laser Driver +3.0V to +5.5V, 2.5G bps VCSEL and Laser
• Manufacturer: Maxim Integrated Products

Specifications of MAX3996CGP

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
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
increase in bias current (Figure 11). Maxim recom-
mends C
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
The amplitude of the modulation current is set with resis-
tors at the MODSET and temperature coefficient (TC)
pins. The resistor at MODSET (R
temperature-stable portion of the modulation current,
and the resistor at TC (R
increasing portion of the modulation current. Figure 6
shows modulation current as a function of temperature
for two extremes: R
has zero temperature coefficient), and R
(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
selection table.
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
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
Table 1. Typical Fault Conditions
8
MON2
BIAS
TC, MODSET
COMP
_______________________________________________________________________________________
PIN
) slows the slew rate and allows a controlled
COMP
= 0.1µF.
CC
TC
V
V
V
- 1V.
MON2
BIAS
MODSET
is open (the modulation current
CC
TC
< 400mV
> 400mV
or GND, or between any two
) programs the temperature-
FAULT CONDITION
or V
Modulation Circuitry
TC
MOD
< 200mV
Safety Circuitry
SET
Smooth-Start
) programs the
because there
TC
MOD
and R
is open
MOD
Figure 5. Modulation Circuitry
Figure 6. Modulation Current vs. Temperature for Maximum
and Minimum Temperature Coefficient
IN+
IN-
100Ω
TC
R
TC
MAX3996
INPUT BUFFER
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
200mV
0
Σ
10
1.2V REFERENCE
4000ppm/°C
20 30 40
ENABLE
JUNCTION TEMPERATURE (°C)
R
R
TEMPCO = 4000ppm/°C
TC FAULT
TC
MOD
≥
CURRENT
SWITCH
= OPEN
1.9kΩ
50 60 70 80 90 100 110
50Ω
CURRENT AMPLIFIER
MODSET
R
TEMPCO = 50ppm/°C
R
TC
MOD
= OPEN
V
96X
CC
200mV
50Ω
MODULATION
GENERATOR
1.2V REFERENCE
CURRENT
0ppm/°C
MODSET
FAULT
OUT+
OUT-