General Description
The MAX3996 is a high-speed laser driver for small-
form-factor (SFF) fiber optic LAN transmitters. It con-
tains a bias generator, a laser modulator, and
comprehensive safety features. Automatic power con-
trol (APC) adjusts the laser bias current to maintain
average optical power, regardless of changes in tem-
perature or laser properties. The driver accommodates
common anode or differential laser configurations. The
output current range of the MAX3996 is appropriate for
VCSELs and high-efficiency edge-emitting lasers.
The MAX3996 operates up to 3.2Gbps. It can switch up
to 30mA of laser modulation current and sink up to
60mA bias current. Adjustable temperature compensa-
tion is provided to keep the optical extinction ratio with-
in specifications over the operating temperature range.
The MAX3996 accommodates various laser packages,
including low-cost TO-46 headers. Low deterministic jit-
ter (9psP-P), combined with fast edge transitions,
(65ps) provides excellent margins compared to indus-
try-standard transmitter eye masks.
This laser driver provides extensive safety features to
guarantee single-point fault tolerance. Safety features
include a transmit disable, redundant shutdown, and
laser-bias monitoring. The safety circuit detects faults
that could cause hazardous light levels and immediate-
ly disables the laser output. The MAX3996 safety cir-
cuits are compliant with SFF and small-form-factor
pluggable (SFP) multisource agreements (MSA).
The MAX3996 is available in a compact 4mm ✕4mm,
20-pin QFN package and a 20-pin thin QFN package. It
operates over a temperature range of 0°C to +70°C.
Applications
Fibre Channel Optical Transmitters
VCSEL Transmitters
Gigabit Ethernet Optical Transmitters
ATM LAN Optical Transmitters
10 Gigabit Ethernet WWDM
Features
♦9psP-P Deterministic Jitter
♦20-Pin QFN 4mm ✕4mm Package
♦3.0V to 5.5V Supply Voltage
♦Automatic Power Control
♦Integrated Safety Circuits
♦30mA Laser Modulation Current
♦Temperature Compensation of Modulation
Current
♦Compliant with SFF and SFP MSA
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
________________________________________________________________ Maxim Integrated Products 1
MAX3996
0.01µF
0.01µF
CPORDLY
RTC RMOD CCOMP
N.C.
0.01µF
0.01µF
0.01µF
L1*
25Ω
OPTIONAL SHUTDOWN
CIRCUITRY
1.8kΩ
VCC
VCC
VCC
TX_DISABLE
FAULT
IN+
IN-
PORDLY
TC MODSET MON1 MON2 COMP GND
MD
BIAS
OUT+
OUT-
SHDNDRV
*FERRITE BEAD
RSET
Typical Application Circuit
Ordering Information
19-2194; Rev 3; 5/04
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
PART TEMP
RANGE
PIN-
PACKAGE
PACKAGE
CODE
MAX3996CGP
0°C to +70°C
20 QFN G2044-3
MAX3996CTP+
0°C to +70°C 20 Thin QFN
T2044-3
Pin Configuration appears at end of data sheet.
+Denotes Lead-Free Package
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VCC = 3.0V to 5.5V, TA= 0°C to +70°C, unless otherwise noted. Typical values are at VCC = 3.3V, TC pin not connected, TA=
+25°C.) (Figure 1)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Supply Voltage at VCC...........................................-0.5V to +7.0V
Voltage at TX_DISABLE, PORDLY, MON1, COMP,
IN+, IN-, MD, BIAS, MODSET, TC..........-0.5V to (VCC + 0.5V)
Voltage between COMP and MON2 .....................................2.3V
Voltage between IN+ and IN- ..................................................5V
Voltage at OUT+, OUT-.........................(VCC - 2V) to (VCC + 2V)
Voltage between MON1 and MON2 .....................................1.5V
Voltage between BIAS and MON2...........................................4V
Current into FAULT, SHDNDRV ..........................-1mA to +25mA
Current into OUT+, OUT- ....................................................60mA
Current into BIAS ..............................................................120mA
Continuous Power Dissipation (TA= +70°C)
20-Pin QFN (derate 20mW/°C)...................................1600mW
Operating Ambient Temperature Range .............-40°C to +85°C
Operating Junction Temperature Range...........-40°C to +150°C
Storage Temperature Range.... .........................-55°C to +150°C
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
VCC = 3.3V, IMOD = 15mA 47
Supply Current
ICC
(Figure1)
(Note 1)
VCC = 5.5V, IMOD = 30mA,
RMODSET = 2.37kΩ52 75 mA
Data Input Voltage Swing VID Total differential signal (Figure 2)
200 2200
mVP-P
TX_DISABLE Input Current 0 < VPIN < VCC
-100 +100
µA
TX_DISABLE Input High Voltage
VIH 2.0 V
TX_DISABLE Input Low Voltage
VIL 0.8 V
FAULT Output High Voltage VOH IOH = -100µA, 4.7kΩ < RFAULT < 10kΩ2.4 V
FAULT Output Low Voltage VOL IOL = 1mA 0.4 V
BIAS GENERATOR
Minimum Bias Current IBIAS Current into BIAS pin 1 mA
Maximum Bias Current IBIAS Current into BIAS pin 60 mA
APC loop is closed
1.04 1.12
FAULT = high
VCC - 0.73
MD Quiescent Voltage VMD
TX_DISABLE = high
VCC - 0.73
V
Monitor Resistance RMON (Figure 4) 9.3 11
12.7
Ω
MD Input Current FAULT = low, TX_DISABLE = low -3
+0.8
+3 µA
BIAS Current During Fault
IBIAS_OFF
10 µA
APC Time Constant CCOMP = 0.1µF 35 µs
POWER-ON RESET (POR)
POR Threshold Measured at VCC
2.65
2.7 3.0 V
PORDLY = open (Note 3) 30 55 µs
POR Delay
tPORDLY
CPORDLY = 0.001µF (Note 3) 1.7 2.4 ms
POR Hysteresis 20 mV
SHUTDOWN
ISHDNDRV = 10µA, FAULT = high
VCC - 0.4
ISHDNDRV = 1mA, FAULT = low
VCC - 2.4
Voltage at SHDNDRV
ISHDNDRV = 15mA, FAULT = low 0
VCC - 1.2
V
LASER MODULATOR
Data Rate
< 3.2
Gbps
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(VCC = 3.0V to 5.5V, TA= 0°C to +70°C, unless otherwise noted. Typical values are at VCC = 3.3V, TC pin not connected, TA=
+25°C.) (Figure 1)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Minimum Modulation Current iMOD 2
mAP-P
Maximum Modulation Current iMOD RL ≤ 25Ω30 40
mAP-P
Accuracy of Modulation Current
(Part-to-Part Variation)
RMODSET = 2.37kΩ
(iMOD ≈ 30mAP-P into 25Ω)-10
+10
%
iMOD = 5mA into 25Ω, 20% to 80% (Note 3) 54
100
iMOD = 10mA into 25Ω, 20% to 80% (Note 3)
55
125
Edge Transition Time tr, tf
iMOD = 30mA into 25Ω, 20% to 80% (Note 3)
65
130
ps
iMOD = 5mA into 25Ω (Notes 2, 3) 17 35
iMOD = 10mA into 25Ω (Notes 2, 3) 14 22Deterministic Jitter
iMOD = 30mA into 25Ω (Notes 2, 3) 9 20
psP-P
Random Jitter (Note 3) 2 8
psRMS
Modulation Current During Fault
iMOD_OFF
15
200
µAP-P
Tempco = MAX, RMOD = open
4000
Modulation Current Tempco Tempco = MIN, RTC = open 50
ppm/°C
Input Resistance RIN Differential 85
115
Ω
Output Resistance ROUT Single ended; outputs to VCC 42 50 58 Ω
Input Common-Mode Voltage
VCC - 0.3
V
SAFETY FEATURES (See Typical Operating Characteristics)
MODSET and TC Pin
Fault Threshold
200
mV
BIAS Pin Fault Threshold A fault will be triggered if VBIAS is less than
this voltage
300 400
mV
Excessive Bias Current Fault A fault will be triggered if VMON2 exceeds
this voltage
400 440
mV
TX Disable Time t_off Time from rising edge of TX_DISABLE to
IBIAS
= IBIAS _OFF and i
M OD = i
M OD_OFF ( Note 3)
0.06
5µs
TX Disable Negate Time t_on Time from falling edge of TX_DISABLE to
IBIA S
and i
M OD
at 95% of stead y state ( N ote 3) 37
500
µs
Reset Initialization Time t_init
Fr om p ow er ON or neg ation of FAU LT usi ng
TX _D IS ABLE . Ti me to set FAULT = l ow, i
M OD =
95% of stead y state and IBIAS
= 95% of steady
state ( N ote 3)
23
200
ms
Fault Assert Time t_fault Time from fault to FAULT = high, CFAULT
< 20pF, RFAULT = 4.7kΩ (Note 3) 14 50 µs
TX_DISABLE Reset
t_reset
Time TX_DISABLE must be held high to
reset FAULT (Note 3)
0.01
1µs
Note 1: Supply current excludes bias and modulation currents.
Note 2: Deterministic jitter is the peak-to-peak deviation from the ideal time crossings measured with a K28.5 bit pattern
00111110101100000101.
Note 3: AC characteristics guaranteed by design and characterization.
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
4 _______________________________________________________________________________________
Typical Operating Characteristics
(VCC = 3.3V, TA= +25°C, unless otherwise noted.)
120mV/
div
64ps/div
ELECTRICAL EYE DIAGRAM
(iMOD = 30mA, 27 - 1 PRBS, 2.5Gbps)
MAX3996 toc01
25Ω LOAD
120mV/
div
52ps/div
ELECTRICAL EYE DIAGRAM
(iMOD = 30mA, 27 - 1 PRBS, 3.2Gbps)
MAX3996 toc02
25Ω LOAD
57ps/div
OPTICAL EYE DIAGRAM
(iMOD = 5mA, 850nm VCSEL, 27 - 1 PRBS,
2.5Gbps, 1870MHz FILTER)
MAX3996 toc03
57ps/div
MAX3996 toc04
OPTICAL EYE DIAGRAM
(iMOD = 15mA, 1310nm LASER, 27 - 1 PRBS,
2.5Gbps, 1870MHz FILTER)
20
40
30
60
50
70
80
TRANSITION TIME
vs. MODULATION CURRENT
MAX3996 toc05
iMOD (mA)
TRANSITION TIME (ps)
5152010 25 30 35
FALL TIME
RISE TIME
0
10
5
20
15
25
30
DETERMINISTIC JITTER
vs. MODULATION CURRENT
MAX3996 toc06
iMOD (mA)
DETERMINISTIC JITTER (psP-P)
5152010 25 30 35
TOTAL DJ
PWD
30
35
40
45
50
55
60
65
70
03015 45 60 75
SUPPLY CURRENT vs.
TEMPERATURE (iMOD = 15mA)
MAX3996 toc07
AMBIENT TEMPERATURE (°C)
SUPPLY CURRENT (mA)
EXCLUDES IBIAS, iMOD
25Ω LOAD
10µ
100µ
10m
1m
100m
1
POR DELAY vs. CPORDLY
MAX3996 toc08
CPORDLY (F)
POR DELAY (s)
10p 1n100p 10n 100n
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
_______________________________________________________________________________________ 5
LASER
OUPUT
TX_DISABLE
VCC
FAULT
10.0ms/div
HOT PLUG WITH
TX_DISABLE LOW
MAX3996 toc09
3.3V
t_init = 23mS
0V
LOW
LOW
Typical Operating Characteristics (continued)
(VCC = 3.3V, TA= +25°C, unless otherwise noted.)
LASER
OUPUT
TX_DISABLE
VCC
FAULT
10.0ms/div
STARTUP WITH SLOW
RAMPING SUPPLY
MAX3996 toc10
0V
LOW
LOW
3.3V
LASER
OUPUT
TX_DISABLE
VCC
FAULT
20.0µs/div
TRANSMITTER ENABLE
MAX3996 toc11
LOW
LOW
HIGH
3.3V
t_on = 37µs
LASER
OUPUT
TX_DISABLE
VCC
FAULT
20.0ns/div
TRANSMITTER DISABLE
MAX3996 toc12
LOW
LOW
HIGH
3.3V
t_off = 60ns
ELECTRICAL
OUPUT
FAULT
VMON2
IBIAS
10.0µs/div
RESPONSE TO FAULT
MAX3996 toc13
ON
OFF
LOW
HIGH
t_fault = 14µs
EXTERNALLY
FORCED FAULT
LASER
OUPUT
TX_DISABLE
VTC
FAULT
10.0µs/div
FAULT RECOVERY TIME
MAX3996 toc14
EXTERNAL
FAULT REMOVED
LASER
OUPUT
TX_DISABLE
VTC
FAULT
1.00ms/div
FREQUENT ASSERTION OF
TX_DISABLE
MAX3996 toc15
EXTERNALLY
FORCED FAULT
OV
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
6 _______________________________________________________________________________________
Pin Description
PIN NAME FUNCTION
1TC
Temperature Compensation Set. The resistor at TC programs the temperature-increasing component
of the laser-modulation current.
2 FAULT Fault Indicator. See Table 1.
3, 9 GND Ground
4
TX_DISABLE
Transmit Disable. Laser output is disabled when TX_DISABLE is high or left unconnected. The laser
output is enabled when this pin is asserted low.
5 PORDLY Power-On Reset Delay. A capacitor connected between PORDLY and GND can be used to extend the
delay for the power-on reset circuit. See the Design Procedure section.
6, 16, 19
VCC Supply Voltage
7 IN+ Noninverting Data Input
8 IN- Inverting Data Input
10 MON1 Attaches to the emitter of the bias driving transistor through a 10Ω resistor. See the Design Procedure
section.
11 MON2 This pin attaches to the emitter of the bias driving transistor. See the Design Procedure section.
12 COMP A capacitor connected from this pin to ground sets the dominant pole of the APC loop. See the Design
Procedure section.
13 MD Monitor Diode Connection. MD is used for automatic power control.
14 SHDNDRV Shutdown Driver Output. Provides a redundant laser shutdown.
15 BIAS Laser Bias Current Output
17 OUT+ Positive Modulation-Current Output. Current flows from this pin when input data is high.
18 OUT- Negative Modulation-Current Output. Current flows to this pin when input data is high.
20 MODSET A resistor connected from this pin to ground sets the desired modulation current.
EP
Exposed Pad
Ground. This must be soldered to the circuit board ground for proper thermal and electrical
performance. See the Layout Considerations section.
Detailed Description
The MAX3996 contains a bias generator with automatic
power control and smooth start, a laser modulator, a
power-on reset (POR) circuit, and safety circuitry
(Figure 3).
Bias Generator
Figure 4 shows the bias generator circuitry that con-
tains a power-control amplifier, smooth-start circuitry,
and two bias-fault sensors. The power-control amplifier
combined with an internal NPN transistor provides DC
laser current to bias the laser in a light-emitting state.
The APC circuitry adjusts the laser bias current to main-
tain average power over temperature and changing
laser properties. The smooth-start circuitry prevents
current spikes to the laser during power-up or enable,
ensuring compliance with safety requirements and
extending the life of the laser.
The MD input is connected to the anode of a monitor
diode, which is used to sense laser power. The BIAS
output is connected to the cathode of the laser through
an inductor or ferrite bead. The power-control amplifier
drives a transistor to control the laser’s bias current. In
a fault condition (Table 1), the base of the bias-driving
transistor is pulled low to ensure that bias current is
turned off.
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
_______________________________________________________________________________________ 7
MAX3996
MODULATION CURRENT
GENERATOR
VCC
3.0V TO 5.5V
ICC
IN+
IN-
0.01µF
0.01µF
VID RIN
ROUT ROUT
VCC
OUT-
OUT+
0.01µF
25Ω
0.01µF
25Ω
iMOD
iOUT
FERRITE
BEAD*
RMOD
MODSETTC
*MURATA
BLM11HA102SG
Figure 1. Output Load for AC Specification
iMOD
CURRENT
VID = VIN+ - VIN-
VIN-
VIN+
VOLTS
TIME
100mVP-P MIN
1100mVP-P MAX
200mVP-P MIN
2200mVP-P MAX
SINGLE-ENDED SIGNAL
DIFFERENTIAL SIGNAL
Figure 2. Required Input Signal and Modulation-Current Polarity
MAX3996
SAFETY
CIRCUITRY BIAS
GENERATOR
WITH SMOOTH
START
MODULATION CURRENT
GENERATOR
MODULATION
ENABLE
MODULATION
FAULT
100Ω
IN+
IN-
INPUT BUFFER LASER
MODULATION
50Ω50Ω
VCC
OUT-
OUT+
TC MODSET
POR CIRCUIT
BIAS ENABLE
BIAS
MD
COMP
MON1
MON2
PORDLY
TX_DISABLE
VCC FAULT SHDNDRV
Figure 3. Laser Driver Functional Diagram
MAX3996
RMON (11Ω)
400mV
400mV
BIAS
DISABLE
1.1V
SMOOTH START
POWER-CONTROL
AMPLIFIER
BIAS
FAULT 1
BIAS
FAULT 2
MD
BIAS
MON2
MON1
COMP
Figure 4. Bias Circuitry
MAX3996
Smooth-Start
During startup, the laser does not emit light, and the
APC loop is not closed. The smooth-start circuit pulls
the MD pin to approximately 2.5V during the POR delay
and while TX_DISABLE is high. This causes the power-
control amplifier to shut off the bias transistor. When
POR delay is over and TX_DISABLE is low, the MD pin
is released and pulled to GND by RSET because there
is no laser power and thus no monitor diode current.
The output voltage of the power-control amplifier then
begins to increase. A capacitor attached to COMP
(CCOMP) slows the slew rate and allows a controlled
increase in bias current (Figure 11). Maxim recom-
mends CCOMP = 0.1µF.
Modulation Circuitry
The modulation circuitry consists of an input buffer, a
current mirror, and a high-speed current switch (Figure
5). The modulator drives up to 30mA of modulation cur-
rent into a 25Ωload.
Many of the modulator performance specifications
depend on total modulator current. To ensure good driver
performance, the voltage at either OUT+ or OUT- must
not be less than VCC - 1V.
The amplitude of the modulation current is set with resis-
tors at the MODSET and temperature coefficient (TC)
pins. The resistor at MODSET (RMOD) programs the
temperature-stable portion of the modulation current,
and the resistor at TC (RTC) programs the temperature-
increasing portion of the modulation current. Figure 6
shows modulation current as a function of temperature
for two extremes: RTC is open (the modulation current
has zero temperature coefficient), and RMOD is open
(the modulation temperature coefficient is 4000ppm/°C).
Intermediate temperature coefficient values of the mod-
ulation current can be obtained as described in the
Design Procedure section. Table 2 is the RTC and RMOD
selection table.
Safety Circuitry
The safety circuitry contains a disable input, a fault
latch, and fault detectors (Figure 7). This circuitry moni-
tors the operation of the laser driver and forces a shut-
down if a single-point fault is detected. A single-point
fault can be a short to VCC or GND, or between any two
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
8 _______________________________________________________________________________________
PIN FAULT CONDITION
MON2 VMON2 > 400mV
BIAS VBIAS < 400mV
TC, MODSET VMODSET or VTC < 200mV
Table 1. Typical Fault Conditions
MAX3996
100Ω
IN+
IN-
INPUT BUFFER
50Ω50Ω
VCC
OUT+
OUT-
1.2V REFERENCE
0ppm/°C
MODSET
FAULT
200mV
MODSET
RMOD
1.2V REFERENCE
4000ppm/°C
TC FAULT
200mV
TC
RTC
Σ
CURRENT AMPLIFIER
96X
ENABLE
MODULATION
CURRENT
GENERATOR
CURRENT
SWITCH
Figure 5. Modulation Circuitry
0.6
0.8
0.7
1.0
0.9
1.2
1.1
1.3
020304010 50 60 70 80 90 100 110
JUNCTION TEMPERATURE (°C)
iMOD/(iMOD AT +52°C)
RTC ≥ 1.9kΩ
RMOD = OPEN
TEMPCO = 4000ppm/°C
RTC = OPEN
TEMPCO = 50ppm/°C
Figure 6. Modulation Current vs. Temperature for Maximum
and Minimum Temperature Coefficient
IC pins. See Table 3 to view the circuit response to vari-
ous single-point failures. The shutdown condition is
latched until reset by a toggle of TX_DISABLE or VCC.
Fault Detection
All critical nodes are monitored for safety faults, and
any node voltage that differs significantly from its
expected value results in a fault (Table 1). When a fault
condition is detected, the laser is shut down. See the
Applications Information for more information on laser
safety.
Shutdown
The laser driver offers redundant bias shutdown. The
SHDNDRV output drives an optional external transistor.
The bias and modulation drivers have separate internal
disable signals.
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
_______________________________________________________________________________________ 9
Table 2. RTC and RMOD Selection Table
PIN NAME CIRCUIT RESPONSE TO OVERVOLTAGE OR
SHORT TO VCC
CIRCUIT RESPONSE TO UNDERVOLTAGE OR
SHORT TO GROUND
TC Does not affect laser power. Fault state* occurs.
FAULT Does not affect laser power. Does not affect laser power.
TX_DISABLE
Modulation and bias current are disabled. Normal condition for circuit operation.
PORDLY Does not affect laser power. Modulation and bias current are disabled.
IN+, IN- Does not affect laser power. Does not affect laser power.
MON1 Fault state* occurs. Does not affect laser power.
MON2 Fault state* occurs. Does not affect laser power.
COMP
A fault is detected at either the collector or the emitter
of the internal bias transistor, and a fault state* occurs.
If the shutdown circuitry is used, bias current is shut off.
Disables bias current.
MD Disables bias current.
The APC circuit responds by increasing bias current
until a fault is detected at the emitter or collector of the
bias transistor, and then a fault* state occurs.
SHDNDRV Does not affect laser power. If the shutdown circuitry is
used, bias current is shut off. 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,
bias current is shut off.
OUT+, OUT-
Does not affect laser power. Does not affect laser power.
MODSET Does not affect laser power. Fault* state may occur. Fault state* occurs.
Table 3. Circuit Responses to Various Single-Point Faults
*A fault state asserts the FAULT pin, disables the modulator outputs, disables the bias output, and asserts the SHDNDRV pin.
iMOD = 30mA iMOD = 15mA iMOD = 5mA
TEMPCO
(ppm/°C) RMOD (kΩ)R
TC (kΩ)R
MOD (kΩ)R
TC (kΩ)R
MOD (kΩ)R
TC (kΩ)
3500 17.1 1.85 34.4 3.94 104 12.3
3000 8.04 2.19 16.3 4.64 49.5 14.4
2500 5.20 2.68 10.6 5.62 32.4 17.4
2000 3.81 3.42 7.86 7.08 24.1 21.8
1500 2.98 4.64 6.21 9.53 19.1 29.1
1000 2.44 7.08 5.12 14.4 15.9 43.8
500 2.05 14.4 4.34 29.1 13.5 87.8
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 VCC 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 VCC - 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 RMOD sets
the temperature-stable portion of the modulation cur-
rent, and the resistor (RTC) 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 η25 =
0.021mW/mA, which reduces to η70 = 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 RMOD and RTC:
where tempco = -laser tempco, 0 < tempco <
4000ppm/°C, and 2mA < iMOD < 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 iMOD-
simplifies considerably.
The following equations were used to derive Figure 8 and
the equations at the beginning of this section.
iRR
RT C Amps
MOD L MOD
TC
=×++Ω
+
+Ω
+°
−
77 50
50
115
250
106
250 1 0 004 25
.
.(.( ))
R
Tempco i
RTempco 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
VCC PORDLY
SHDNDRV
FAULT
TX_DISABLE
VBG
Figure 7. Safety Circuitry Functional Diagram
Determine Modulator Configuration
The MAX3996 can be used in several configurations.
For modulation currents less than 20mA, Maxim recom-
mends the configuration shown in the Typical
Application Circuit. Outputs greater than 20mA could
cause the voltage at the modulator output to be less
than VCC - 1V, which might degrade laser output. For
large currents, Maxim recommends the configuration in
Figure 9. A differential configuration is in Figure 10.
Designing the Bias Filter and
Output Pullup Beads
To reduce deterministic jitter, add a ferrite bead induc-
tor (L1) between the BIAS pin and the cathode of the
laser. Select L1 to have an impedance >100Ωbetween
f = 10MHz and f = 2GHz, and a DC resistance < 3Ω;
Maxim recommends the Murata BLM11HA102SG.
These inductors are also desirable for connecting the
OUT+ and OUT- pins to VCC.
Programming Laser Power and
Bias Fault Threshold
The IC is designed to drive a common anode laser with
a photodiode. A servo-control loop is formed by the
internal NPN bias-driving transistor, the laser diode, the
monitor diode (RSET), and the power-control amplifier
(Figure 11). The voltage at MD is stabilized to 1.1V. The
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
______________________________________________________________________________________ 11
1000
11 100 1000
10
RMOD (kΩ)
RTC (kΩ)
10
500ppm
1000ppm
1500ppm 2000ppm
3000ppm
2500ppm
3500ppm
5mA
10mA
25mA
20mA
30mA
15mA
RL = 25Ω
Figure 8. RTC vs. RMOD for Various Conditions
MAX3996
0.01µF
0.01µF
CPORDLY
RTC RMOD CCOMP
N.C.
0.01µF
0.01µF
L1*
25Ω
OPTIONAL SHUTDOWN
CIRCUITRY
1.8kΩ
VCC
VCC
VCC
TX_DISABLE
FAULT
IN+
IN-
PORDLY
TC MODSET MON1 MON2 COMP GND MD
BIAS
OUT+
OUT-
SHDNDRV
*FERRITE BEAD
RSET
VCC
L2*
VCC
L3*
0.01µF
Figure 9. Large Modulation Current
MAX3996
0.01µF
0.01µF
CPORDLY
RTC
RMOD
CCOMP
N.C.
0.01µF
0.01µF
L1*
VCC
VCC
VCC
TX_DISABLE
FAULT
IN+
IN-
PORDLY
TC MODSET MON1 MON2 COMP GND MD
BIAS
OUT-
OUT+
SHDNDRV
*FERRITE BEAD
RSET
L2*
Figure 10. Differential Configuration
MAX3996
monitor photodiode current is set by ID= VMD/RSET.
Determine the desired monitor current (ID), and then
select RSET = 1.1V/ID.
A bias stabilizing capacitor (CCOMP) must be connect-
ed between the COMP pin and ground to obtain the
desired APC loop time constant. This improves power-
supply and ground noise rejection. A capacitance of
0.1µF usually is sufficient to obtain time constants of up
to 35µs.
The degeneration resistance between MON2 and
ground determines the bias current that causes a fault
and affects the APC time constant. Select RMON (the
total resistance between MON2 and ground) =
400mV/(maximum bias current). A degeneration resis-
tance of 10Ωcan be obtained by grounding MON1.
Increasing RMON increases the APC time constant.
The discrete components for use with the common
anode with photodiode configuration are:
RSET = 1.1V/ID
CCOMP = 0.1µF (typ)
L1 = ferrite bead, see the Bias Filter section
RMON = 400mV/(maximum bias current)
Programming POR Delay
A capacitor can be added to PORDLY to increase the
delay when powering up the part. The delay will be
approximately:
See the Typical Operating Characteristics section.
Designing the Laser-Compensation
Filter Network
Laser package inductance causes the laser impedance
to increase at high frequencies, leading to ringing,
overshoot, and degradation of the laser output. A laser-
compensation filter network can be used to reduce the
laser impedance at high frequencies, thereby reducing
output ringing and overshoot.
tConds
PORDLY
=
×−
14 10 6
.
sec
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
12 ______________________________________________________________________________________
MAX3996
VCC
ID
RSET
MONITOR
DIODE 1.1V
POWER-CONTROL
AMPLIFIER
VCC
SHUTDOWN
CIRCUIT
OPTIONAL
SHUTDOWN
CIRCUITRY
SHDNDRV
LASER
L1*
11Ω
SMOOTH
START
BIAS
DISABLE
MD
BIAS
MON2
MON1
IBIAS
COMP
CCOMP
0.1µF
*FERRITE BEAD
Figure 11. APC Loop
The compensation components (RFand CF) are most
easily determined by experimentation. For interfacing
with edge-emitting lasers, refer to application note
HFAN-2.0, Interfacing Maxim Laser Drivers with Laser
Diodes. Begin with RF= 50Ωand CF= 2pF. Increase
CFuntil the desired transmitter response is obtained
(Figure 12).
Using External Shutdown
To achieve single-point fault tolerance, Maxim recom-
mends an external shutdown transistor (Figure 11). In
the event of a fault, SHDNDRV asserts high, placing the
shutdown transistor in cutoff mode and thereby shutting
off the bias current.
Applications Information
Laser Safety and IEC825
The International Electrotechnical Commission (IEC)
determines standards for hazardous light emissions
from fiber optic transmitters. IEC 825 defines the maxi-
mum light output for various hazard levels. The
MAX3996 provides features that facilitate compliance
with IEC825. A common safety precaution is single-
point fault tolerance, whereby one unplanned short,
open, or resistive connection does not cause excess
light output. When this laser driver is used, as shown in
the Typical Application Circuit, the circuits respond to
faults as listed in Table 3. Using this laser driver alone
does not ensure that a transmitter design is compliant
with IEC825. The entire transmitter circuit and compo-
nent selections must be considered. Customers must
determine the level of fault tolerance required by their
applications, recognizing that Maxim products are not
designed or authorized for use as components in sys-
tems intended for surgical implant into the body, for
applications intended to support or sustain life, or for
any other application where the failure of a Maxim
product could create a situation where personal injury
or death may occur.
Layout Considerations
The MAX3996 is a high-frequency product whose per-
formance largely depends upon the circuit board layout.
Use a multilayer circuit board with a dedicated ground
plane. Use short laser-package leads placed close to
the modulator outputs. Power supplies must be capaci-
tively bypassed to the ground plane, with surface-mount
capacitors placed near the power-supply pins.
The dominant pole of the APC circuit normally is at
COMP. To prevent a second pole in the APC that can
lead to oscillations, ensure that parasitic capacitance at
MD is minimized (10pF).
Common Questions
Laser output is ringing or contains overshoot. Induc-
tive laser packaging often causes this. Try reducing the
length of the laser leads. Modify the filter components to
reduce the driver’s output edge speed (see the Design
Procedure section). Extreme ringing can be caused by
low voltage at the OUT± pins. This might indicate that
pullup beads or a lower modulation current are needed.
Low-frequency oscillation on the laser output. This
is more prevalent at low temperatures. The APC might
be oscillating. Try increasing the value of CCOMP or
add additional degeneration by placing some resis-
tance from MON1 to GND. Ensure that the parasitic
capacitance at the MD node is kept very small (<10pF).
The APC is not needed. Connect BIAS to VCC, leave
MD open, and connect MON2 and COMP to ground.
The modulator is not needed. Leave TC and MODSET
open. Connect IN+ to VCC, IN- to ground through
750Ω, and leave OUT+ and OUT- open.
Interface Models
Figures 13–17 show typical models for the inputs and
outputs of the MAX3996, including package parasitics.
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
______________________________________________________________________________________ 13
TIME
POWER
UNCOMPENSATED
CORRECTLY COMPENSATED
OVERCOMPENSATED
Figure 12. Laser Compensation
MAX3996 4kΩ
FAULT
NOTE: THE FAULT PIN IS AN OPEN-COLLECTOR OUTPUT
Figure 13. FAULT Output
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
14 ______________________________________________________________________________________
MAX3996
550Ω60Ω
10kΩ
VCC
SHDNDRV
Figure 14. SHDNDRV Output
MAX3996
PACKAGE
OUT-
1.1nH
0.15pF
1pF
50Ω50Ω
VCC VCC
1pF
OUT+
PACKAGE
1.1nH
0.15pF
Figure 15. Modulator Outputs
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
______________________________________________________________________________________ 15
MAX3996
PACKAGE
IN+
1.1nH
0.15pF 1pF
VCC
VCC
IN-
1.1nH
0.15pF 1pF
VCC
100Ω
Figure 16. Data Inputs
VCC
VCC
VCC
MAX3996
BIAS
MON2
MON1
11Ω
Figure 17. BIAS Output
20 QFN (4mm x 4mm)
TOP VIEW
1
2
3
4
5
678910
11
12
13
14
15
1617181920
TC
FAULT
GND
VCC
VCC
VCC
IN+
IN-
GND
MON1
TX_DISABLE
PORDLY MON2
COMP
MD
SHDNDRV
BIAS
MODSET
OUT-
OUT+
MAX3996
EXPOSED PAD IS CONNECTED TO GND
20 THIN QFN (4mm x 4mm)
TOP VIEW
1
2
3
4
5
678910
11
12
13
14
15
1617181920
TC
FAULT
GND
VCC
VCC
VCC
IN+
IN-
GND
MON1
TX_DISABLE
PORDLY MON2
COMP
MD
SHDNDRV
BIAS
MODSET
OUT-
OUT+
MAX3996
EXPOSED PAD IS CONNECTED TO GND
Pin Configurations
Chip Information
TRANSISTOR COUNT: 1061
PROCESS: SILICON BIPOLAR
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
PART PACKAGE
TYPE
PACKAGE
CODE
MAX3996CGP 20 QFN
4mm x 4mm x 0.9mm G2044-3
MAX3996CTP+20 Thin QFN
4mm x 4mm x 0.8mm T2044-3
Package Information
For the latest package outline information, go to
www.maxim-ic.com/packages.
