Datasheet

MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
10 ______________________________________________________________________________________
PIN NAME CIRCUIT RESPONSE TO V
CC
SHORT CIRCUIT RESPONSE TO GND SHORT
FAULT Does not affect laser power. Does not affect laser power.
TX_DISABLE Modulation and bias current are disabled. Normal condition for circuit operation.
IN+ Does not affect laser power. Does not affect laser power.
IN- Does not affect laser power. Does not affect laser power.
SQUELCH Does not affect laser power. Does not affect laser power.
TC1 Does not affect laser power. Does not affect laser power.
TC2
The laser modulation is increased, but average power
is not affected.
Modulation current is disabled.
MODSET Modulation current is disabled.
The laser modulation is increased, but average power
is not affected.
PEAKSET Does not affect laser power. Does not affect laser power.
OUT+ Modulation current is disabled. Modulation current is disabled.
OUT- Does not affect laser power. Does not affect laser power.
BIASSET Laser bias is disabled. Fault state* occurs.
BIAS
Fault state* occurs. Note that VCSEL emissions may
continue; care must be taken to prevent this condition.
Disables VCSEL.
BIASMON Fault state* occurs. Does not affect laser power.
COMP
The bias current is reduced, and the average power of
the laser output is reduced.
I
BIAS
increases to the value determined by R
BIASSET
; if
the bias monitor fault threshold is exceeded, a fault is
signaled.
MD
I
BIAS
increases to the value determined by R
BIASSET
; if
the bias-monitor fault threshold is exceeded, a fault is
signaled.
The bias current is reduced, and the average power of
the laser output is reduced.
REF
I
BIAS
increases to the value determined by R
BIASSET
; if
the bias-monitor fault threshold is exceeded, a fault is
signaled.
The bias current is reduced, and the average power of
the laser output is reduced.
PWRMON Fault state* occurs. Does not affect laser power.
Table 2. Circuit Response to Various Single-Point Faults (Closed-Loop APC Configuration)
* A fault state asserts the FAULT pin, disables the modulator output, and disables the bias output.
Modulation Circuit
The modulation circuitry consists of an input buffer, a
current mirror, and a high-speed current switch (Figure
3). The modulator drives up to 15mA of modulation into
a 50Ω VCSEL load.
The amplitude of the modulation current is set with
resistors at MODSET and temperature coefficient (TC1,
TC2) pins. The resistor at MODSET (R
MODSET
) pro-
grams the temperature-stable portion of the modulation
current, and the resistor between TC1 and TC2 (R
TC
)
programs the temperature coefficient of the modulation
current. For appropriate R
TC
and R
MODSET
values, see
the
Typical Operating Characteristics section.
Design Procedure
Select Laser
Select a communications-grade laser with a rise time of
260ps or better for 1.25Gbps, or 130ps or better for
2.5Gbps applications. Use a high-efficiency laser that
requires low modulation current and generates a low-
voltage swing. Trim the leads to reduce laser package
inductance. The typical package leads have induc-
tance of 25nH per inch (1nH/mm). This inductance
causes a large voltage swing across the laser. A com-
pensation filter network can also be used to reduce
ringing, edge speed, and voltage swing (see the
Designing the Compensation Filter Network section).