Datasheet

IC pins. See Table 3 to view the circuit response to vari-
ous single-point failures. The shutdown condition is
latched until reset by a toggle of TX_DISABLE or V
CC.
Fault Detection
All critical nodes are monitored for safety faults, and
any node voltage that differs significantly from its
expected value results in a fault (Table 1). When a fault
condition is detected, the laser is shut down. See the
Applications Information for more information on laser
safety.
Shutdown
The laser driver offers redundant bias shutdown. The
SHDNDRV output drives an optional external transistor.
The bias and modulation drivers have separate internal
disable signals.
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
_______________________________________________________________________________________ 9
Table 2. R
TC
and R
MOD
Selection Table
PIN NAME
CIRCUIT RESPONSE TO OVERVOLTAGE OR
SHORT TO V
CC
CIRCUIT RESPONSE TO UNDERVOLTAGE OR
SHORT TO GROUND
TC Does not affect laser power. Fault state* occurs.
FAULT Does not affect laser power. Does not affect laser power.
TX_DISABLE
Modulation and bias current are disabled. Normal condition for circuit operation.
PORDLY Does not affect laser power. Modulation and bias current are disabled.
IN+, IN- Does not affect laser power. Does not affect laser power.
MON1 Fault state* occurs. Does not affect laser power.
MON2 Fault state* occurs. Does not affect laser power.
COMP
A fault is detected at either the collector or the emitter
of the internal bias transistor, and a fault state* occurs.
If the shutdown circuitry is used, bias current is shut off.
Disables bias current.
MD Disables bias current.
The APC circuit responds by increasing bias current
until a fault is detected at the emitter or collector of the
bias transistor, and then a fault* state occurs.
SHDNDRV
Does not affect laser power. If the shutdown circuitry is
used, bias current is shut off.
Does not affect laser power.
BIAS
In this condition, laser forward voltage is 0V and no
light is emitted.
Fault state* occurs. If the shutdown circuitry is used,
bias current is shut off.
OUT+, OUT-
Does not affect laser power. Does not affect laser power.
MODSET Does not affect laser power. Fault* state may occur. Fault state* occurs.
Table 3. Circuit Responses to Various Single-Point Faults
*A fault state asserts the FAULT pin, disables the modulator outputs, disables the bias output, and asserts the SHDNDRV pin.
i
MOD
= 30mA i
MOD
= 15mA i
MOD
= 5mA
TEMPCO
(ppm/°C)
R
MOD
(kΩ)R
TC
(kΩ)R
MOD
(kΩ)R
TC
(kΩ)R
MOD
(kΩ)R
TC
(kΩ)
3500 17.1 1.85 34.4 3.94 104 12.3
3000 8.04 2.19 16.3 4.64 49.5 14.4
2500 5.20 2.68 10.6 5.62 32.4 17.4
2000 3.81 3.42 7.86 7.08 24.1 21.8
1500 2.98 4.64 6.21 9.53 19.1 29.1
1000 2.44 7.08 5.12 14.4 15.9 43.8
500 2.05 14.4 4.34 29.1 13.5 87.8