MAX3996CGP Maxim Integrated Products, MAX3996CGP Datasheet - Page 10

• Manufacturer Part Number: MAX3996CGP
• Description: Laser Driver +3.0V to +5.5V, 2.5G bps VCSEL and Laser
• Manufacturer: Maxim Integrated Products

Specifications of MAX3996CGP

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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.
The MAX3996 contains an internal power-on reset
delay to reject noise on V
plugging. Adding capacitance to the PORDLY pin can
extend the delay. The POR comparator includes hys-
teresis to improve noise rejection.
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
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.
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
Figure 7. Safety Circuitry Functional Diagram
10
TX_DISABLE
______________________________________________________________________________________
STARTUP
MODSET FAULT
V
BIAS FAULT 1
BIAS FAULT 2
CC
TC FAULT
V
BG
CC
- 1V at all times.
CC
Design Procedure
Latched Fault Output
PORDLY
DELAY
during power-on or hot-
BIAS ENABLE
MODULATOR
FAULT LATCH
Power-On Reset
ENABLE
R Q
S
Select Laser
SHDNDRV
FAULT
Resistors at the MODSET and TC pins set the ampli-
tude of the modulation current. The resistor R
the temperature-stable portion of the modulation cur-
rent, and the resistor (R
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
0.021mW/mA, which reduces to η
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
where tempco = -laser tempco, 0 < tempco <
4000ppm/°C, and 2mA < i
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
simplifies considerably.
The following equations were used to derive Figure 8 and
the equations at the beginning of this section.
R
MOD
LASER TEMPCO
i
R
MOD
TC
R
[
=
TC
1 06
ppm C
Programming Modulation Current
+
.
Tempco
=
_
250
=
77


0 19 48
/
Tempco
.
°
×
Ω
50
]
(
1 0 004
−
/
+
50
+
10
R
0 22
.
6
×
/
=
L
MOD
10
.
TC)
(
Tempco



R
η
R
6
25
TC
(
MOD
T
×
< 30mA.
sets the temperature-
η
(
−
70
+
i
70
MOD
1 15
25
MOD
250
.
°
+
70
C
/
°
−
10
C
250
η
−
Ω
))
25
= 0.018mW/mA.
−
6
25
and R
)



250
Ω


Amps
52
°
C
+
Ω
−
)
TC
250
×
MOD
10
:
Ω
6
MOD
25
sets
=
-