Datasheet

MAX3735/MAX3735A
2.7Gbps, Low-Power SFP Laser Drivers
12 ______________________________________________________________________________________
Pattern-Dependent Jitter
To minimize the pattern-dependent jitter associated
with the APC loop time constant, and to guarantee loop
stability, connect a capacitor between APCFILT1 and
APCFILT2 (see the Applications Information section for
more information about choosing C
APC
values). A
capacitor attached to the photodiode anode (C
MD
) is
also recommended to filter transient currents that origi-
nate from the photodiode. To maintain stability and
proper phase margin associated with the two poles
created by C
APC
and C
MD
, C
APC
should be 20x
greater than C
MD
for the MAX3735. C
APC
should be 4x
to 20x greater than C
MD
for the MAX3735A.
Table 2. Circuit Responses to Various Single-Point Faults
PIN NAME
CIRCUIT RESPONSE TO OVERVOLTAGE
OR SHORT TO V
CC
CIRCUIT RESPONSE TO UNDERVOLTAGE
OR SHORT TO GROUND
TX_FAULT Does not affect laser power. Does not affect laser power.
TX_DISABLE Modulation and bias currents are disabled. Normal condition for circuit operation.
IN+
The optical average power increases and a fault occurs
if V
PC_MON
exceeds the threshold. The APC loop
responds by decreasing the bias current.
The optical average power decreases and the APC
loop responds by increasing the bias current. A fault
state occurs if V
BC_MON
exceeds the threshold voltage.
IN-
The optical average power decreases and the APC
loop responds by increasing the bias current. A fault
state occurs if V
BC_MON
exceeds the threshold voltage.
The optical average power increases and a fault occurs
if V
PC_MON
exceeds the threshold. The APC loop
responds by decreasing the bias current.
MD Disables bias current. A fault state occurs.
The APC circuit responds by increasing bias current
until a fault is detected, then a fault state* occurs.
SHUTDOWN
Does not affect laser power. If the shutdown circuitry is
used, laser current is disabled and a fault state* occurs.
Does not affect laser power.
BIAS
In this condition, laser forward voltage is 0V and no light
is emitted.
Fault state* occurs. If the shutdown circuitry is used, the
laser current is disabled.
OUT+
The APC circuit responds by increasing the bias current
until a fault is detected, then a fault state* occurs.
Fault state* occurs. If the shutdown circuitry is used,
laser current is disabled.
OUT- Does not affect laser power. Does not affect laser power.
PC_MON Fault state* occurs. Does not affect laser power.
BC_MON Fault state* occurs. Does not affect laser power.
APCFILT1
IBIAS increases until V
BC_MON
exceeds the threshold
voltage.
IBIAS increases until V
BC_MON
exceeds the threshold
voltage.
APCFILT2
IBIAS increases until V
BC_MON
exceeds the threshold
voltage.
IBIAS increases until V
BC_MON
exceeds the threshold
voltage.
MODSET Does not affect laser power. Fault state* occurs.
APCSET Does not affect laser power. Fault state* occurs.
*A fault state asserts the TX_FAULT pin, disables the modulation and bias currents, and asserts the SHUTDOWN pin.
Table 3. Optical Power Definitions
PARAMETER SYMBOL RELATION
Average Power P
AVG
P
AVG
= (P
0
+ P
1
) / 2
Extinction Ratio r
e
r
e
= P
1
/ P
0
Optical Power High P
1
P
1
= 2P
AVG
x r
e
/ (r
e
+ 1)
Optical Power Low P
0
P
0
= 2P
AVG
/ (r
e
+ 1)
Optical Amplitude P
P-P
P
P-P
= P
1
- P
0
Laser Slope
Efficiency
ηη = P
P-P
/ I
MOD
Modulation Current I
MOD
I
MOD
= P
P-P
/ η