Datasheet

MAX3996
Latched Fault Output
An open-collector FAULT output is provided with the
MAX3996. This output is latched until the power is
switched off, then on, or until TX_DISABLE is switched
to HIGH and then LOW.
Power-On Reset
The MAX3996 contains an internal power-on reset
delay to reject noise on V
CC
during power-on or hot-
plugging. Adding capacitance to the PORDLY pin can
extend the delay. The POR comparator includes hys-
teresis to improve noise rejection.
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. To meet the MAX3996’s AC
specifications, the voltage at both OUT+ and OUT-
must remain above V
CC
- 1V at all times.
Use a high-efficiency laser that requires low modulation
current and generates a low voltage swing. Trimming
the leads can reduce laser package inductance.
Typical package leads have inductance of 25nH per
inch (1nH/mm); this inductance causes a large voltage
swing across the laser. A compensation filter network
also can be used to reduce ringing, edge speed, and
voltage swing.
Programming Modulation Current
Resistors at the MODSET and TC pins set the ampli-
tude of the modulation current. The resistor R
MOD
sets
the temperature-stable portion of the modulation cur-
rent, and the resistor (R
TC)
sets the temperature-
increasing portion of the modulation current. To
determine the appropriate temperature coefficient from
the slope efficiency (η) of the laser, use the following
equation:
For example, if a laser has a slope efficiency η
25
=
0.021mW/mA, which reduces to η
70
= 0.018mW/mA.
Using the above equation will produce a laser tempco
of -3175ppm/°C.
To obtain the desired modulation current and tempco
for the device, the following equations can be used to
determine the required values of R
MOD
and R
TC
:
where tempco = -laser tempco, 0 < tempco <
4000ppm/°C, and 2mA < i
MOD
< 30mA.
Figure 8 shows a family of curves derived from these
equations. The straight diagonal lines depict constant
tempcos. The curved lines represent constant modula-
tion currents. If no temperature compensation is
desired, leave TC open, and the equation for i
MOD
-
simplifies considerably.
The following equations were used to derive Figure 8 and
the equations at the beginning of this section.
i
RR
R
T C Amps
MOD
L MOD
TC
=×
++Ω
+



+Ω
+°



−
77
50
50
115
250
106
250
1 0 004 25
.
.
(.( ))
R
Tempco i
R
Tempco R
Tempco
TC
MOD
MOD
TC
=
×
Ω
=
+Ω
()
×




Ω
−
−
−
022
10
250
10 250 52
019 48 10
250
6
6
6
.
/
/
./
LASER TEMPCO
ppm C
CC
_
[/]°
=
°°
()
×
−
−
ηη
η
70 25
25
6
70 25
10
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
10 ______________________________________________________________________________________
RQ
S
FAULT LATCH
BIAS FAULT 1
BIAS FAULT 2
TC FAULT
MODSET FAULT
BIAS ENABLE
MODULATOR
ENABLE
DELAY
STARTUP
V
CC
PORDLY
SHDNDRV
FAULT
TX_DISABLE
V
BG
Figure 7. Safety Circuitry Functional Diagram