Datasheet

MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
10 ______________________________________________________________________________________
Laser-Biasing and APC
To maintain constant average optical power, the
MAX3735/MAX3735A incorporate an APC loop to com-
pensate for the changes in laser threshold current over
temperature and lifetime. A back-facet photodiode
mounted in the laser package is used to convert the
optical power into a photocurrent. The APC loop
adjusts the laser bias current so that the monitor cur-
rent is matched to a reference current set by R
APCSET
.
The time constant of the APC loop is determined by an
external capacitor (C
APC
). For possible C
APC
values,
see the Applications Information section.
Safety Circuitry
The safety circuitry contains an input disable
(TX_DISABLE), a latched fault output (TX_FAULT), and
fault detectors (Figure 5). This circuitry monitors the
operation of the laser driver and forces a shutdown if a
fault is detected (Table 1). 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 (see the Typical Applications Circuit). The
TX_FAULT pin should be pulled high with a 4.7kΩ to
10kΩ resistor to V
CC
as required by the SFP MSA.
Safety Circuitry Current Monitors
The MAX3735/MAX3735A feature monitors (BC_MON,
PC_MON) for bias current (I
BIAS
) and photo current
(I
MD
). The monitors are realized by mirroring a fraction
of the currents and developing voltages across external
resistors connected to ground. Voltages greater than
1.38V at PC_MON or BC_MON result in a fault state.
For example, connecting a 100Ω resistor to ground on
each monitor output gives the following relationships:
V
BC_MON
= (I
BIAS
/ 76) x 100Ω
V
PC_MON
= I
MD
x 100Ω
V
CC
V
CC
I
APCSET
C
APC
I
BIAS
I
MOD
R
D
I
MD
R
APCSET
x38
V
CC
R
PC_MON
PC_MON
100Ω
IN-
IN+
I
MOD
ENABLE
I
BIAS
ENABLE
INPUT BUFFER
SHUTDOWN
DATA
PATH
X270
SAFETY LOGIC
AND
POWER
DETECTOR
TX_FAULT
C
MD
TX_DISABLE
APCFILT1 APCFILT2
SHUTDOWN
MODSET
BC_MON
x1
76
I
BIAS
1
I
MD
APCSET
MD
MAX3735
MAX3735A
V
CC
V
BG
(4.7kΩ
TO 10kΩ)
R
BC_MON
V
CC
V
BG
R
MODSET
V
CC
OUT-
OUT+
BIAS
V
CC
15Ω
Figure 4. Functional Diagram