Datasheet
Detailed Description
The MAX3996 contains a bias generator with automatic
power control and smooth start, a laser modulator, a
power-on reset (POR) circuit, and safety circuitry
(Figure 3).
Bias Generator
Figure 4 shows the bias generator circuitry that con-
tains a power-control amplifier, smooth-start circuitry,
and two bias-fault sensors. The power-control amplifier
combined with an internal NPN transistor provides DC
laser current to bias the laser in a light-emitting state.
The APC circuitry adjusts the laser bias current to main-
tain average power over temperature and changing
laser properties. The smooth-start circuitry prevents
current spikes to the laser during power-up or enable,
ensuring compliance with safety requirements and
extending the life of the laser.
The MD input is connected to the anode of a monitor
diode, which is used to sense laser power. The BIAS
output is connected to the cathode of the laser through
an inductor or ferrite bead. The power-control amplifier
drives a transistor to control the laser’s bias current. In
a fault condition (Table 1), the base of the bias-driving
transistor is pulled low to ensure that bias current is
turned off.
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
_______________________________________________________________________________________ 7
MAX3996
MODULATION CURRENT
GENERATOR
V
CC
3.0V TO 5.5V
I
CC
IN+
IN-
0.01µF
0.01µF
V
ID
R
IN
R
OUT
R
OUT
V
CC
OUT-
OUT+
0.01µF
25Ω
0.01µF
25Ω
i
MOD
i
OUT
FERRITE
BEAD*
R
MOD
MODSETTC
*MURATA
BLM11HA102SG
Figure 1. Output Load for AC Specification
i
MOD
CURRENT
V
ID
= V
IN+
- V
IN-
V
IN-
V
IN+
VOLTS
TIME
100mV
P-P
MIN
1100mV
P-P
MAX
200mV
P-P
MIN
2200mV
P-P
MAX
SINGLE-ENDED SIGNAL
DIFFERENTIAL SIGNAL
Figure 2. Required Input Signal and Modulation-Current Polarity
MAX3996
SAFETY
CIRCUITRY
BIAS
GENERATOR
WITH SMOOTH
START
MODULATION CURRENT
GENERATOR
MODULATION
ENABLE
MODULATION
FAULT
100Ω
IN+
IN-
INPUT BUFFER
LASER
MODULATION
50Ω 50Ω
V
CC
OUT-
OUT+
TC MODSET
POR CIRCUIT
BIAS ENABLE
BIAS
MD
COMP
MON1
MON2
PORDLY
TX_DISABLE
V
CC FAULT SHDNDRV
Figure 3. Laser Driver Functional Diagram
MAX3996
R
MON
(11Ω)
400mV
400mV
BIAS
DISABLE
1.1V
SMOOTH START
POWER-CONTROL
AMPLIFIER
BIAS
FAULT 1
BIAS
FAULT 2
MD
BIAS
MON2
MON1
COMP
Figure 4. Bias Circuitry