3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
OPTIONAL
VCC
SHUTDOWN
CIRCUITRY
SHDNDRV
VCC
MONITOR
DIODE
SMOOTH
START
1.1V
MAX3996
MD
RSET
ID
POWER-CONTROL
AMPLIFIER
SHUTDOWN
CIRCUIT
BIAS
DISABLE
IBIAS
11Ω
LASER
L1*
BIAS
MON2
MON1
*FERRITE BEAD
Figure 11. APC Loop
monitor photodiode current is set by ID = VMD/RSET.
Determine the desired monitor current (ID), and then
select RSET = 1.1V/ID.
A bias stabilizing capacitor (CCOMP) must be connect-
ed between the COMP pin and ground to obtain the
desired APC loop time constant. This improves power-
supply and ground noise rejection. A capacitance of
0.1µF usually is sufficient to obtain time constants of up
to 35µs.
The degeneration resistance between MON2 and
ground determines the bias current that causes a fault
and affects the APC time constant. Select RMON (the
total resistance between MON2 and ground) =
400mV/(maximum bias current). A degeneration resis-
tance of 10Ω can be obtained by grounding MON1.
Increasing RMON increases the APC time constant.
The discrete components for use with the common
anode with photodiode configuration are:
CCOMP
0.1µF
COMP
RSET = 1.1V/ID
CCOMP = 0.1µF (typ)
L1 = ferrite bead, see the Bias Filter section
RMON = 400mV/(maximum bias current)
Programming POR Delay
A capacitor can be added to PORDLY to increase the
delay when powering up the part. The delay will be
approximately:
t = CPORDLY seconds
1.4 × 10−6
See the Typical Operating Characteristics section.
Designing the Laser-Compensation
Filter Network
Laser package inductance causes the laser impedance
to increase at high frequencies, leading to ringing,
overshoot, and degradation of the laser output. A laser-
compensation filter network can be used to reduce the
laser impedance at high frequencies, thereby reducing
output ringing and overshoot.
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