MAX3869 Datasheet, PDF (8/16 Pages) Maxim Integrated Products – +3.3V, 2.5Gbps SDH/SONET Laser Driver with Current Monitors and APC

+3.3V, 2.5Gbps SDH/SONET Laser Driver
with Current Monitors and APC
V_CC
LATCH
L_P2
MAX3869
DATA
CLK
DQ
0
MUX
1
ENABLE
BIASMON
MOBMON
165x
I_BIAS
37
I_MOD
29
MODSET
R_MODSET
BIASMAX
R_BIASMAX
L_P1
OUT+
I_MOD
OUT-
I_{BIAS BIAS}
40x
5x
MD
FAILURE
DETECTOR
CAPC
FAIL
C_APC
I_MD
APCSET
R_APCSET
R_P
R_D
C_D
25Ω
V_CC
1000pF
Figure 4. Functional Diagram
laser package is used to convert the optical power into
a photocurrent. The APC loop adjusts the laser bias
current so that the monitor current is matched to a ref-
erence current set by R_APCSET. The time constant of
the APC loop is determined by an external capacitor
(C_APC). To eliminate the pattern-dependent jitter asso-
ciated with the APC loop-time constant, and to guaran-
tee loop stability, the recommended value for C_APC is
0.1µF.
When the APC loop is functioning, the maximum allow-
able bias current is set by an external resistor, R_BIASMAX.
An APC failure flag (FAIL) is set low when the bias current
can no longer be adjusted to achieve the desired aver-
age optical power. To filter out the APC loop noise, use
an external capacitor at APCFILT with a recommended
value of 0.1µF.
APC closed-loop operation requires the user to set three
currents with external resistors connected between
ground and BIASMAX, MODSET, and APCSET. Detailed
guidelines for these resistor settings are described in
the Design Procedure section.
Open-Loop Operation
If necessary, the MAX3869 is fully operational without
APC. In this case, the laser current is directly set by two
external resistors connected from ground to BIASMAX
and MODSET. See the Design Procedure section for
more details on open-loop operation.
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