MAXIM MAX3996CTP+

19-2194; Rev 3; 5/04
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
The MAX3996 is a high-speed laser driver for smallform-factor (SFF) fiber optic LAN transmitters. It contains a bias generator, a laser modulator, and
comprehensive safety features. Automatic power control (APC) adjusts the laser bias current to maintain
average optical power, regardless of changes in temperature 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 compensation is provided to keep the optical extinction ratio within specifications over the operating temperature range.
The MAX3996 accommodates various laser packages,
including low-cost TO-46 headers. Low deterministic jitter (9ps P-P ), combined with fast edge transitions,
(65ps) provides excellent margins compared to industry-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 immediately disables the laser output. The MAX3996 safety circuits are compliant with SFF and small-form-factor
pluggable (SFP) multisource agreements (MSA).
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
Ordering Information
PINPACKAGE
TEMP
RANGE
MAX3996CGP
0°C to +70°C
20 QFN
G2044-3
MAX3996CTP+
0°C to +70°C
20 Thin QFN
T2044-3
+Denotes Lead-Free Package
Typical Application Circuit
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.
VCC
OPTIONAL SHUTDOWN
CIRCUITRY
Applications
VCC
1.8kΩ
Fibre Channel Optical Transmitters
0.01µF
VCC
TX_DISABLE
VCSEL Transmitters
SHDNDRV
0.01µF
Gigabit Ethernet Optical Transmitters
FAULT
OUT-
ATM LAN Optical Transmitters
10 Gigabit Ethernet WWDM
PACKAGE
CODE
PART
0.01µF
0.01µF
IN+
OUT+
MAX3996
L1*
0.01µF
25Ω
INBIAS
Pin Configuration appears at end of data sheet.
PORDLY
MD
TC
MODSET MON1 MON2 COMP
GND
CPORDLY
RTC
RMOD
N.C.
CCOMP
RSET
*FERRITE BEAD
________________________________________________________________ Maxim Integrated Products
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.
1
MAX3996
General Description
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
ABSOLUTE MAXIMUM RATINGS
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
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.
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)
PARAMETER
Supply Current
Data Input Voltage Swing
SYMBOL
ICC
(Figure1)
(Note 1)
VID
TX_DISABLE Input Current
TX_DISABLE Input High Voltage
TX_DISABLE Input Low Voltage
CONDITIONS
MIN
TYP
VCC = 3.3V, IMOD = 15mA
47
VCC = 5.5V, IMOD = 30mA,
RMODSET = 2.37kΩ
52
MAX
75
UNITS
mA
Total differential signal (Figure 2)
200
2200
mVP-P
0 < VPIN < VCC
-100
+100
µA
VIH
2.0
V
VIL
0.8
FAULT Output High Voltage
VOH
IOH = -100µA, 4.7kΩ < RFAULT < 10kΩ
FAULT Output Low Voltage
VOL
IOL = 1mA
Minimum Bias Current
IBIAS
Current into BIAS pin
Maximum Bias Current
IBIAS
Current into BIAS pin
2.4
V
V
0.4
V
1
mA
BIAS GENERATOR
APC loop is closed
MD Quiescent Voltage
VMD
60
FAULT = high
RMON
MD Input Current
BIAS Current During Fault
1.12
VCC - 0.73
TX_DISABLE = high
Monitor Resistance
mA
1.04
VCC - 0.73
(Figure 4)
9.3
11
12.7
Ω
FAULT = low, TX_DISABLE = low
-3
+0.8
+3
µA
10
µA
IBIAS_OFF
APC Time Constant
V
CCOMP = 0.1µF
35
µs
POWER-ON RESET (POR)
POR Threshold
POR Delay
Measured at VCC
tPORDLY
2.65
2.7
PORDLY = open (Note 3)
30
55
µs
CPORDLY = 0.001µF (Note 3)
1.7
2.4
ms
20
mV
POR Hysteresis
3.0
V
SHUTDOWN
ISHDNDRV = 10µA, FAULT = high
Voltage at SHDNDRV
VCC - 0.4
ISHDNDRV = 1mA, FAULT = low
ISHDNDRV = 15mA, FAULT = low
VCC - 2.4
0
V
VCC - 1.2
LASER MODULATOR
Data Rate
2
< 3.2
_______________________________________________________________________________________
Gbps
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
(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
Minimum Modulation Current
iMOD
Maximum Modulation Current
iMOD
Accuracy of Modulation Current
(Part-to-Part Variation)
Edge Transition Time
tr, tf
Deterministic Jitter
Random Jitter
CONDITIONS
MIN
TYP
RL ≤ 25Ω
30
40
RMODSET = 2.37kΩ
(iMOD ≈ 30mAP-P into 25Ω)
-10
Input Resistance
mAP-P
+10
54
100
125
iMOD = 30mA into 25Ω, 20% to 80% (Note 3)
65
130
iMOD = 5mA into 25Ω (Notes 2, 3)
17
35
iMOD = 10mA into 25Ω (Notes 2, 3)
14
22
iMOD = 30mA into 25Ω (Notes 2, 3)
9
20
85
Single ended; outputs to VCC
42
Input Common-Mode Voltage
ps
psP-P
2
8
psRMS
200
µAP-P
ppm/°C
50
Differential
%
15
4000
Tempco = MIN, RTC = open
ROUT
mAP-P
55
Tempco = MAX, RMOD = open
RIN
Output Resistance
2
iMOD = 10mA into 25Ω, 20% to 80% (Note 3)
iMOD_OFF
Modulation Current Tempco
UNITS
iMOD = 5mA into 25Ω, 20% to 80% (Note 3)
(Note 3)
Modulation Current During Fault
MAX
50
115
Ω
58
Ω
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
Excessive Bias Current Fault
A fault will be triggered if VMON2 exceeds
this voltage
400
440
mV
300
400
mV
TX Disable Time
t_off
Time from rising edge of TX_DISABLE to
IBIAS = IBIAS_OFF and iMOD = iMOD_OFF (Note 3)
0.06
5
µs
TX Disable Negate Time
t_on
Time from falling edge of TX_DISABLE to
IBIAS and iMOD at 95% of steady state (Note 3)
37
500
µs
Reset Initialization Time
t_init
From power ON or negation of FAULT using
TX_DISABLE. Time to set FAULT = low, iMOD =
95% of steady state and IBIAS = 95% of steady
state (Note 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
1
µs
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.
_______________________________________________________________________________________
3
MAX3996
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics
(VCC = 3.3V, TA = +25°C, unless otherwise noted.)
25Ω LOAD
120mV/
div
MAX3996 toc03
MAX3996 toc02
25Ω LOAD
OPTICAL EYE DIAGRAM
(iMOD = 5mA, 850nm VCSEL, 27 - 1 PRBS,
2.5Gbps, 1870MHz FILTER)
ELECTRICAL EYE DIAGRAM
(iMOD = 30mA, 27 - 1 PRBS, 3.2Gbps)
MAX3996 toc01
ELECTRICAL EYE DIAGRAM
(iMOD = 30mA, 27 - 1 PRBS, 2.5Gbps)
120mV/
div
64ps/div
57ps/div
52ps/div
OPTICAL EYE DIAGRAM
(iMOD = 15mA, 1310nm LASER, 27 - 1 PRBS,
2.5Gbps, 1870MHz FILTER)
TRANSITION TIME
vs. MODULATION CURRENT
DETERMINISTIC JITTER
vs. MODULATION CURRENT
TRANSITION TIME (ps)
FALL TIME
60
RISE TIME
50
40
25
20
15
TOTAL DJ
10
PWD
5
20
57ps/div
MAX3996 toc06
70
30
DETERMINISTIC JITTER (psP-P)
MAX3996 toc05
MAX3996 toc04
80
30
0
5
10
15
20
25
30
35
5
10
15
iMOD (mA)
1
MAX3996 toc08
EXCLUDES IBIAS, iMOD
25Ω LOAD
65
25
POR DELAY vs. CPORDLY
MAX3996 toc07
70
20
iMOD (mA)
SUPPLY CURRENT vs.
TEMPERATURE (iMOD = 15mA)
100m
60
POR DELAY (s)
SUPPLY CURRENT (mA)
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
55
50
45
40
10m
1m
100µ
35
10µ
30
0
15
30
45
60
AMBIENT TEMPERATURE (°C)
4
75
10p
100p
1n
10n
CPORDLY (F)
_______________________________________________________________________________________
100n
30
35
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
STARTUP WITH SLOW
RAMPING SUPPLY
LOW
FAULT
MAX3996 toc11
0V
VCC
3.3V
LOW
FAULT
LOW
TX_DISABLE
LOW
HIGH
t_init = 23mS
LOW
TX_DISABLE
TX_DISABLE
LASER
OUPUT
LASER
OUPUT
RESPONSE TO FAULT
t_off = 60ns
LOW
FAULT RECOVERY TIME
EXTERNALLY
FORCED FAULT
ON
FAULT
20.0µs/div
MAX3996 toc13
TRANSMITTER DISABLE
MAX3996 toc12
10.0ms/div
VMON2
LOW
LASER
OUPUT
10.0ms/div
3.3V
VCC
t_on = 37µs
t_fault = 14µs
MAX3996 toc14
FAULT
3.3V
VCC
VCC
TRANSMITTER ENABLE
MAX3996 toc10
3.3V
0V
MAX3996 toc09
HOT PLUG WITH
TX_DISABLE LOW
EXTERNAL
FAULT REMOVED
VTC
FAULT
IBIAS
OFF
HIGH
HIGH
LOW
TX_DISABLE
FAULT
LASER
OUPUT
TX_DISABLE
LOW
ELECTRICAL
OUPUT
20.0ns/div
LASER
OUPUT
10.0µs/div
10.0µs/div
VTC
MAX3996 toc15
FREQUENT ASSERTION OF
TX_DISABLE
EXTERNALLY
FORCED FAULT
OV
FAULT
TX_DISABLE
LASER
OUPUT
1.00ms/div
_______________________________________________________________________________________
5
MAX3996
Typical Operating Characteristics (continued)
(VCC = 3.3V, TA = +25°C, unless otherwise noted.)
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
MAX3996
Pin Description
PIN
NAME
1
TC
FUNCTION
Temperature Compensation Set. The resistor at TC programs the temperature-increasing component
of the laser-modulation current.
2
FAULT
3, 9
GND
4
TX_DISABLE
5
PORDLY
6, 16, 19
VCC
7
IN+
Noninverting Data Input
8
IN-
Inverting Data Input
10
MON1
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
14
SHDNDRV
15
BIAS
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
EP
Exposed Pad
6
Fault Indicator. See Table 1.
Ground
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.
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.
Supply Voltage
Attaches to the emitter of the bias driving transistor through a 10Ω resistor. See the Design Procedure
section.
Monitor Diode Connection. MD is used for automatic power control.
Shutdown Driver Output. Provides a redundant laser shutdown.
Laser Bias Current Output
A resistor connected from this pin to ground sets the desired modulation current.
Ground. This must be soldered to the circuit board ground for proper thermal and electrical
performance. See the Layout Considerations section.
_______________________________________________________________________________________
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
MAX3996
3.0V TO 5.5V
VOLTS
VIN+
ICC
VCC
iOUT
ROUT
100mVP-P MIN
1100mVP-P MAX
VIN-
VCC
MAX3996
SINGLE-ENDED SIGNAL
DIFFERENTIAL SIGNAL
FERRITE
BEAD*
ROUT
200mVP-P MIN
2200mVP-P MAX
VID = VIN+ - VINOUTOUT+
0.01µF
IN+
0.01µF
VID
CURRENT
iMOD
iMOD
0.01µF
RIN
0.01µF
IN-
25Ω
25Ω
MODULATION CURRENT
GENERATOR
TC
TIME
Figure 2. Required Input Signal and Modulation-Current Polarity
MODSET
Bias Generator
RMOD
*MURATA
BLM11HA102SG
Figure 1. Output Load for AC Specification
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).
VCC
FAULT
Figure 4 shows the bias generator circuitry that contains 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 maintain 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.
SHDNDRV
SAFETY
CIRCUITRY
POR CIRCUIT
TX_DISABLE
BIAS ENABLE
BIAS
GENERATOR
WITH SMOOTH
START
MD
COMP
MON1
MON2
MON2
POWER-CONTROL
AMPLIFIER
BIAS
FAULT 2
MAX3996
50Ω
BIAS
MD
VCC
MAX3996
BIAS
FAULT 1
1.1V
BIAS
PORDLY
400mV
SMOOTH START
BIAS
DISABLE
RMON (11Ω)
400mV
MON1
50Ω
COMP
IN+
INPUT BUFFER
OUTOUT+
LASER
MODULATION
Figure 4. Bias Circuitry
100Ω
INMODULATION
ENABLE
MODULATION
FAULT
MODULATION CURRENT
GENERATOR
TC
MODSET
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.
Figure 3. Laser Driver Functional Diagram
_______________________________________________________________________________________
7
Table 1. Typical Fault Conditions
PIN
VCC
FAULT CONDITION
MON2
VMON2 > 400mV
BIAS
VBIAS < 400mV
TC, MODSET
VMODSET or VTC < 200mV
50Ω
OUT+
100Ω
IN-
ENABLE
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 resistors at the MODSET and temperature coefficient (TC)
pins. The resistor at MODSET (R MOD) programs the
temperature-stable portion of the modulation current,
and the resistor at TC (RTC) programs the temperatureincreasing 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 modulation 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 monitors the operation of the laser driver and forces a shutdown if a single-point fault is detected. A single-point
fault can be a short to VCC or GND, or between any two
CURRENT AMPLIFIER
96X
MODULATION
CURRENT
GENERATOR
Σ
1.2V REFERENCE
4000ppm/°C
200mV
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 current into a 25Ω load.
OUT-
CURRENT
SWITCH
INPUT BUFFER
IN+
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 powercontrol 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 recommends CCOMP = 0.1µF.
8
50Ω
MAX3996
1.2V REFERENCE
0ppm/°C
200mV
TC FAULT
TC
MODSET
FAULT
MODSET
RTC
RMOD
Figure 5. Modulation Circuitry
1.3
1.2
iMOD/(iMOD AT +52°C)
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
RTC ≥ 1.9kΩ
RMOD = OPEN
TEMPCO = 4000ppm/°C
1.1
1.0
0.9
RTC = OPEN
TEMPCO = 50ppm/°C
0.8
0.7
0.6
0 10 20 30 40 50 60 70 80 90 100 110
JUNCTION TEMPERATURE (°C)
Figure 6. Modulation Current vs. Temperature for Maximum
and Minimum Temperature Coefficient
_______________________________________________________________________________________
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
MAX3996
Table 2. RTC and RMOD Selection Table
iMOD = 30mA
iMOD = 15mA
iMOD = 5mA
TEMPCO
(ppm/°C)
RMOD (kΩ)
RTC (kΩ)
RMOD (kΩ)
RTC (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
RMOD (kΩ)
RTC (kΩ)
Table 3. Circuit Responses to Various Single-Point Faults
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.
Modulation and bias current are disabled.
Normal condition for circuit operation.
TX_DISABLE
PORDLY
Does not affect laser power.
Modulation and bias current are disabled.
IN+, IN-
Does not affect laser power.
Does not affect laser power.
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.
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.
MON1
MD
SHDNDRV
BIAS
OUT+, OUTMODSET
Does not affect laser power. If the shutdown circuitry is
used, bias current is shut off.
In this condition, laser forward voltage is 0V and no
light is emitted.
Does not affect laser power.
Fault state* occurs. If the shutdown circuitry is used,
bias current is shut off.
Does not affect laser power.
Does not affect laser power. Fault* state may occur.
Fault state* occurs.
Does not affect laser power.
*A fault state asserts the FAULT pin, disables the modulator outputs, disables the bias output, and asserts the SHDNDRV pin.
IC pins. See Table 3 to view the circuit response to various single-point failures. The shutdown condition is
latched until reset by a toggle of TX_DISABLE or VCC.
Applications Information for more information on laser
safety.
Fault Detection
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.
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
Shutdown
_______________________________________________________________________________________
9
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
Programming Modulation Current
VCC
PORDLY
STARTUP
DELAY
VBG
SHDNDRV
BIAS ENABLE
TX_DISABLE
MODULATOR
ENABLE
FAULT LATCH
FAULT
R Q
BIAS FAULT 1
BIAS FAULT 2
TC FAULT
MODSET FAULT
S
Resistors at the MODSET and TC pins set the amplitude of the modulation current. The resistor RMOD sets
the temperature-stable portion of the modulation current, and the resistor (R TC) sets the temperatureincreasing portion of the modulation current. To
determine the appropriate temperature coefficient from
the slope efficiency (η) of the laser, use the following
equation:
η70 − η25
LASER _ TEMPCO =
× 106
η
70
−
25
°
C
°
C
(
)
25
[ppm / °C]
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:
Figure 7. Safety Circuitry Functional Diagram
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.
R TC =
RMOD =
0.22
Tempco / 106 × iMOD
−
250Ω
Tempco / 106 (R TC + 250Ω) 52
 0.19 − 48 × Tempco / 106 


−
250Ω
Power-On Reset
The MAX3996 contains an internal power-on reset
delay to reject noise on VCC during power-on or hotplugging. Adding capacitance to the PORDLY pin can
extend the delay. The POR comparator includes hysteresis 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 OUTmust 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.
10
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 modulation currents. If no temperature compensation is
desired, leave TC open, and the equation for iMODsimplifies considerably.
The following equations were used to derive Figure 8 and
the equations at the beginning of this section.

1.15
+

R
 MOD + 250Ω

1.06
(1 + 0.004(T − 25°C))Amps
RTC + 250Ω

iMOD = 77 ×
50
50 + RL
______________________________________________________________________________________
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
1000
500ppm
1000ppm
2000ppm
1500ppm
RTC (kΩ)
2500ppm
3000ppm
3500ppm
Designing the Bias Filter and
Output Pullup Beads
10
5mA
To reduce deterministic jitter, add a ferrite bead inductor (L1) between the BIAS pin and the cathode of the
laser. Select L1 to have an impedance >100Ω between
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.
10mA
15mA
20mA
25mA
30mA
RL = 25Ω
1
1
100
10
1000
RMOD (kΩ)
Figure 8. RTC vs. RMOD for Various Conditions
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
VCC
VCC
OPTIONAL SHUTDOWN
CIRCUITRY
VCC
VCC
L2*
1.8kΩ
VCC
VCC
TX_DISABLE
SHDNDRV
TX_DISABLE
L2*
FAULT
VCC
0.01µF
0.01µF
0.01µF
IN+
MAX3996
L3*
IN+
0.01µF
MAX3996
0.01µF
0.01µF
OUT+
0.01µF
OUT-
FAULT
SHDNDRV
VCC
0.01µF
L1*
OUT-
0.01µF
OUT+
L1*
IN-
IN25Ω
BIAS
PORDLY
PORDLY
TC
MODSET MON1 MON2 COMP GND
TC MODSET MON1 MON2 COMP GND
BIAS
MD
CPORDLY
RTC
CPORDLY
RTC
N.C.
N.C.
RMOD
CCOMP
*FERRITE BEAD
MD
RSET
RSET
CCOMP
RMOD
*FERRITE BEAD
Figure 9. Large Modulation Current
Figure 10. Differential Configuration
______________________________________________________________________________________
11
MAX3996
Determine Modulator Configuration
The MAX3996 can be used in several configurations.
For modulation currents less than 20mA, Maxim recommends 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.
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
VCC
OPTIONAL
SHUTDOWN
CIRCUITRY
SHDNDRV
VCC
MAX3996
SMOOTH
START
MONITOR
DIODE
LASER
SHUTDOWN
CIRCUIT
L1*
1.1V
BIAS
IBIAS
MD
MON2
POWER-CONTROL
AMPLIFIER
RSET
ID
*FERRITE BEAD
11Ω
BIAS
DISABLE
CCOMP
0.1µF
MON1
COMP
Figure 11. APC Loop
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 connected between the COMP pin and ground to obtain the
desired APC loop time constant. This improves powersupply 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 resistance of 10Ω can 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:
12
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:
t=
CPORDLY
1.4 × 10 −6
sec onds
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 lasercompensation filter network can be used to reduce the
laser impedance at high frequencies, thereby reducing
output ringing and overshoot.
______________________________________________________________________________________
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
UNCOMPENSATED
POWER
CORRECTLY COMPENSATED
product could create a situation where personal injury
or death may occur.
Layout Considerations
The MAX3996 is a high-frequency product whose performance 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 capacitively 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).
OVERCOMPENSATED
TIME
Figure 12. Laser Compensation
Using External Shutdown
To achieve single-point fault tolerance, Maxim recommends 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 maximum light output for various hazard levels. The
MAX3996 provides features that facilitate compliance
with IEC825. A common safety precaution is singlepoint 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 component 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 systems 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
Common Questions
Laser output is ringing or contains overshoot. Inductive 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 resistance 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 V CC , 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
4kΩ
FAULT
NOTE: THE FAULT PIN IS AN OPEN-COLLECTOR OUTPUT
Figure 13. FAULT Output
______________________________________________________________________________________
13
MAX3996
The compensation components (RF and 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 = 50Ω and CF = 2pF. Increase
CF until the desired transmitter response is obtained
(Figure 12).
MAX3996
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
VCC
MAX3996
10kΩ
550Ω
60Ω
SHDNDRV
Figure 14. SHDNDRV Output
VCC
VCC
PACKAGE
PACKAGE
50Ω
1.1nH
50Ω
OUT-
OUT+
0.15pF
1.1nH
0.15pF
1pF
1pF
MAX3996
Figure 15. Modulator Outputs
14
______________________________________________________________________________________
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
MAX3996
VCC
VCC
MAX3996
PACKAGE
MAX3996
BIAS
VCC
1.1nH
IN+
VCC
0.15pF
1pF
MON2
100Ω
VCC
1.1nH
VCC
11Ω
INMON1
0.15pF
1pF
Figure 16. Data Inputs
Figure 17. BIAS Output
Pin Configurations
VCC
OUT-
OUT+
VCC
MODSET
VCC
OUT-
OUT+
VCC
TOP VIEW
MODSET
20
19
18
17
16
20
19
18
17
16
FAULT
2
14
SHDNDRV
GND
3
13
MD
TX_DISABLE
4
12
PORDLY
5
11
6
7
8
9
10
VCC
IN+
IN-
GND
MON1
MAX3996
TC
1
15
BIAS
FAULT
2
14
SHDNDRV
GND
3
13
MD
COMP
TX_DISABLE
4
12
COMP
MON2
PORDLY
5
11
MON2
MAX3996
6
7
8
9
10
MON1
BIAS
GND
15
IN-
1
IN+
TC
VCC
TOP VIEW
20 QFN (4mm x 4mm)
20 THIN QFN (4mm x 4mm)
EXPOSED PAD IS CONNECTED TO GND
EXPOSED PAD IS CONNECTED TO GND
Chip Information
TRANSISTOR COUNT: 1061
PROCESS: SILICON BIPOLAR
______________________________________________________________________________________
15
3.0V to 5.5V, 2.5Gbps VCSEL
and Laser Driver
MAX3996
Package Information
For the latest package outline information, go to
www.maxim-ic.com/packages.
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
PART
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implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
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Printed USA
is a registered trademark of Maxim Integrated Products.