Safety circuitry – Rainbow Electronics MAX3737 User Manual
Page 9
Modulation compensation from bias increases the mod-
ulation current by a user-selected proportion (K) need-
ed to maintain peak-to-peak laser power as bias
current increases with temperature. Refer to Maxim
Application Note HFAN-02.2.1 for details:
This provides a first-order approximation of the current
increase needed to maintain peak-to-peak power. Slope
efficiency decreases more rapidly as temperature
increases. The MAX3737 provides additional tempera-
ture compensation as temperature increases past a
user-defined threshold (T
TH
).
Safety Circuitry
The safety circuitry contains a disable, input (TX_DIS-
ABLE), a latched fault output (TX_FAULT), and fault
detectors (Figure 5). This circuitry monitors the opera-
tion of the laser driver and forces a shutdown if a fault
is detected (Table 1). The TX_FAULT pin should be
pulled high with a 4.7k
Ω to 10kΩ resistor to V
CC
as
required by the SFP MSA. A single-point fault can be a
short to V
CC
or GND. See Table 2 to view the circuit
response to various single-point failures. The transmit
fault condition is latched until reset by a toggle of
TX_DISABLE or V
CC
. The laser driver offers redundant
laser diode shutdown through the optional shutdown
circuitry as shown in the Typical Operating Circuit. This
shutdown transistor prevents a single-point fault at the
laser from creating an unsafe condition.
K
I
I
MOD
BIAS
=
∆
∆
MAX3737
Multirate Laser Driver with Extinction
Ratio Control
MAX3737
DATA
PATH
X1/2
X1
R
PULL
= 7.5
Ω
PC_MON
IN-
SHUTDOWN
TX_FAULT
TX_DISABLE
IN+
I
MD
1
V
CC
V
CC
INPUT BUFFER
I
MOD
ENABLE
I
MOD
OUT-
SHUTDOWN
OUT+
BIAS
R
D
I
BIAS
ENABLE
SAFETY LOGIC
AND POWER
DETECTOR
BC_MON
I
BIAS
82
MC_MON
I
MOD
268
I
BIAS
I
MD
C
MD
I
BIAS
I
APCSET
APCSET
MD
R
APCSET
V
CC
V
BG
R
TH_TEMP
TH_TEMP
MODTCOMP
MODBCOMP
APCFILT2
MODSET
APCFILT1
T > T
H
V
BG
xTC
R
MODTCOMP
R
MODSET
R
MDMON
R
BC_MON
R
MC_MON
R
MODBCOMP
x268
xK
C
APC
T
Figure 4. Functional Diagram
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