Datasheet

MAX3996
monitor photodiode current is set by I
D
= V
MD
/R
SET
.
Determine the desired monitor current (I
D
), and then
select R
SET
= 1.1V/I
D
.
A bias stabilizing capacitor (C
COMP
) 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 R
MON
(the
total resistance between MON2 and ground) =
400mV/(maximum bias current). A degeneration resis-
tance of 10Ω can be obtained by grounding MON1.
Increasing R
MON
increases the APC time constant.
The discrete components for use with the common
anode with photodiode configuration are:
R
SET
= 1.1V/I
D
C
COMP
= 0.1µF (typ)
L1 = ferrite bead, see the Bias Filter section
R
MON
= 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:
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.
t
C
onds
PORDLY
=
×
−
14 10
6
.
sec
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
12 ______________________________________________________________________________________
MAX3996
V
CC
I
D
R
SET
MONITOR
DIODE
1.1V
POWER-CONTROL
AMPLIFIER
V
CC
SHUTDOWN
CIRCUIT
OPTIONAL
SHUTDOWN
CIRCUITRY
SHDNDRV
LASER
L1*
11Ω
SMOOTH
START
BIAS
DISABLE
MD
BIAS
MON2
MON1
I
BIAS
COMP
C
COMP
0.1µF
*FERRITE BEAD
Figure 11. APC Loop