Datasheet

Determine Modulator Configuration
The MAX3996 can be used in several configurations.
For modulation currents less than 20mA, Maxim recom-
mends the configuration shown in the Typical
Application Circuit. Outputs greater than 20mA could
cause the voltage at the modulator output to be less
than V
CC
- 1V, which might degrade laser output. For
large currents, Maxim recommends the configuration in
Figure 9. A differential configuration is in Figure 10.
Designing the Bias Filter and
Output Pullup Beads
To reduce deterministic jitter, add a ferrite bead induc-
tor (L1) between the BIAS pin and the cathode of the
laser. Select L1 to have an impedance >100Ω between
f = 10MHz and f = 2GHz, and a DC resistance < 3Ω;
Maxim recommends the Murata BLM11HA102SG.
These inductors are also desirable for connecting the
OUT+ and OUT- pins to V
CC
.
Programming Laser Power and
Bias Fault Threshold
The IC is designed to drive a common anode laser with
a photodiode. A servo-control loop is formed by the
internal NPN bias-driving transistor, the laser diode, the
monitor diode (R
SET
), and the power-control amplifier
(Figure 11). The voltage at MD is stabilized to 1.1V. The
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
______________________________________________________________________________________ 11
1000
1
1 100 1000
10
R
MOD
(kΩ)
R
TC
(kΩ)
10
500ppm
1000ppm
1500ppm
2000ppm
3000ppm
2500ppm
3500ppm
5mA
10mA
25mA
20mA
30mA
15mA
R
L
= 25
Ω
Figure 8. R
TC
vs. R
MOD
for Various Conditions
MAX3996
0.01µF
0.01µF
C
PORDLY
R
TC
R
MOD
C
COMP
N.C.
0.01µF
0.01µF
L1*
25Ω
OPTIONAL SHUTDOWN
CIRCUITRY
1.8kΩ
V
CC
V
CC
V
CC
TX_DISABLE
FAULT
IN+
IN-
PORDLY
TC MODSET MON1 MON2 COMP GND
MD
BIAS
OUT+
OUT-
SHDNDRV
*FERRITE BEAD
R
SET
V
CC
L2*
V
CC
L3*
0.01µF
Figure 9. Large Modulation Current
MAX3996
0.01µF
0.01µF
C
PORDLY
R
TC
R
MOD
C
COMP
N.C.
0.01µF
0.01µF
L1*
V
CC
V
CC
V
CC
TX_DISABLE
FAULT
IN+
IN-
PORDLY
TC MODSET MON1 MON2 COMP GND
MD
BIAS
OUT-
OUT+
SHDNDRV
*FERRITE BEAD
R
SET
L2*
Figure 10. Differential Configuration