3.0V to 5.5V, 2.5Gbps VCSEL and Laser DriverMAX3996 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver 6...
Transcript of 3.0V to 5.5V, 2.5Gbps VCSEL and Laser DriverMAX3996 3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver 6...
General DescriptionThe 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, andcomprehensive safety features. Automatic power con-trol (APC) adjusts the laser bias current to maintainaverage optical power, regardless of changes in tem-perature or laser properties. The driver accommodatescommon anode or differential laser configurations. Theoutput current range of the MAX3996 is appropriate forVCSELs and high-efficiency edge-emitting lasers.
The MAX3996 operates up to 3.2Gbps. It can switch upto 30mA of laser modulation current and sink up to60mA 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 toguarantee single-point fault tolerance. Safety featuresinclude a transmit disable, redundant shutdown, andlaser-bias monitoring. The safety circuit detects faultsthat could cause hazardous light levels and immediate-ly disables the laser output. The MAX3996 safety cir-cuits are compliant with SFF and small-form-factorpluggable (SFP) multisource agreements (MSA).
The MAX3996 is available in a compact 4mm 4mm,20-pin QFN package and a 20-pin thin QFN package. Itoperates over a temperature range of 0°C to +70°C.
ApplicationsFibre 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 ModulationCurrent
Compliant with SFF and SFP MSA
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________________________________________________________________ Maxim Integrated Products 1
MAX39960.01µF
0.01µF
CPORDLY
RTC RMOD CCOMPN.C.
0.01µF
0.01µF
0.01µF
L1*25Ω
OPTIONAL SHUTDOWNCIRCUITRY
1.8kΩ
VCC
VCC
VCCTX_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.
PARTTEMP
RANGEPIN-PACKAGE
PACKAGECODE
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
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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 functionaloperation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure toabsolute maximum rating conditions for extended periods may affect device reliability.
Supply Voltage at VCC...........................................-0.5V to +7.0VVoltage at TX_DISABLE, PORDLY, MON1, COMP,
IN+, IN-, MD, BIAS, MODSET, TC..........-0.5V to (VCC + 0.5V) Voltage between COMP and MON2 .....................................2.3VVoltage between IN+ and IN- ..................................................5VVoltage at OUT+, OUT-.........................(VCC - 2V) to (VCC + 2V)Voltage between MON1 and MON2 .....................................1.5VVoltage between BIAS and MON2...........................................4V
Current into FAULT, SHDNDRV ..........................-1mA to +25mACurrent into OUT+, OUT- ....................................................60mACurrent into BIAS ..............................................................120mAContinuous Power Dissipation (TA = +70°C)
20-Pin QFN (derate 20mW/°C)...................................1600mWOperating Ambient Temperature Range .............-40°C to +85°COperating Junction Temperature Range. ..........-40°C to +150°CStorage Temperature Range.... .........................-55°C to +150°C
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
VCC = 3.3V, IMOD = 15mA 47
Supply CurrentICC
(Figure1)(Note 1)
VCC = 5.5V, IMOD = 30mA,RMODSET = 2.37kΩ
52 75mA
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 < 10kΩ 2.4 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 - 0.73MD Quiescent Voltage VMD
TX_DISABLE = high VCC - 0.73
V
Monitor Resistance RMON (Figure 4) 9.3 11 12.7 ΩMD 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 µsPOR 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 - 2.4Voltage at SHDNDRV
ISHDNDRV = 15mA, FAULT = low 0 VCC - 1.2
V
LASER MODULATOR
Data Rate < 3.2 Gbps
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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 ≤ 25Ω 30 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 125Edge Transition Time tr, tfiMOD = 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 4000Modulation Current Tempco
Tempco = MIN, RTC = open 50ppm/°C
Input Resistance RIN Differential 85 115 ΩOutput Resistance ROUT Single ended; outputs to VCC 42 50 58 ΩInput Common-Mode Voltage VCC - 0.3 V
SAFETY FEATURES (See Typical Operating Characteristics)
MODSET and TC PinFault Threshold
200 mV
BIAS Pin Fault ThresholdA fault will be triggered if VBIAS is less thanthis voltage
300 400 mV
Excessive Bias Current FaultA fault will be triggered if VMON2 exceedsthis voltage
400 440 mV
TX Disable Time t_offTime from rising edge of TX_DISABLE toIBIAS = IBIAS _OFF and i M OD = i M OD_OFF ( Note 3)
0.06 5 µs
TX Disable Negate Time t_onTime from falling edge of TX_DISABLE toIBIA 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 steadystate ( N ote 3)
23 200 ms
Fault Assert Time t_faultTime from fault to FAULT = high, CFAULT< 20pF, RFAULT = 4.7kΩ (Note 3)
14 50 µs
TX_DISABLE Reset t_resetTime TX_DISABLE must be held high toreset 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.
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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)
MAX
3996
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125Ω LOAD
120mV/div
52ps/div
ELECTRICAL EYE DIAGRAM (iMOD = 30mA, 27 - 1 PRBS, 3.2Gbps)
MAX
3996
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25Ω LOAD
57ps/div
OPTICAL EYE DIAGRAM (iMOD = 5mA, 850nm VCSEL, 27 - 1 PRBS,
2.5Gbps, 1870MHz FILTER)
MAX
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57ps/div
MAX
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OPTICAL EYE DIAGRAM (iMOD = 15mA, 1310nm LASER, 27 - 1 PRBS,
2.5Gbps, 1870MHz FILTER)
20
40
30
60
50
70
80
TRANSITION TIMEvs. MODULATION CURRENT
MAX
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iMOD (mA)
TRAN
SITI
ON T
IME
(ps)
5 15 2010 25 30 35
FALL TIME
RISE TIME
0
10
5
20
15
25
30
DETERMINISTIC JITTERvs. MODULATION CURRENT
MAX
3996
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iMOD (mA)
DETE
RMIN
ISTI
C JIT
TER
(ps P
-P)
5 15 2010 25 30 35
TOTAL DJ
PWD
30
35
40
45
50
55
60
65
70
0 3015 45 60 75
SUPPLY CURRENT vs.TEMPERATURE (iMOD = 15mA)
MAX
3996
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AMBIENT TEMPERATURE (°C)
SUPP
LY C
URRE
NT (m
A)
EXCLUDES IBIAS, iMOD25Ω LOAD
10µ
100µ
10m
1m
100m
1POR DELAY vs. CPORDLY
MAX
3996
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CPORDLY (F)
POR
DELA
Y (s
)
10p 1n100p 10n 100n
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LASEROUPUT
TX_DISABLE
VCC
FAULT
10.0ms/div
HOT PLUG WITHTX_DISABLE LOW
MAX
3996
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93.3V
t_init = 23mS
0V
LOW
LOW
Typical Operating Characteristics (continued)(VCC = 3.3V, TA = +25°C, unless otherwise noted.)
LASEROUPUT
TX_DISABLE
VCC
FAULT
10.0ms/div
STARTUP WITH SLOWRAMPING SUPPLY
MAX
3996
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0
0V
LOW
LOW
3.3V
LASEROUPUT
TX_DISABLE
VCC
FAULT
20.0µs/div
TRANSMITTER ENABLE
MAX
3996
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LOW
LOW
HIGH
3.3V
t_on = 37µs
LASEROUPUT
TX_DISABLE
VCC
FAULT
20.0ns/div
TRANSMITTER DISABLE
MAX
3996
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2
LOW
LOW
HIGH
3.3Vt_off = 60ns
ELECTRICALOUPUT
FAULT
VMON2
IBIAS
10.0µs/div
RESPONSE TO FAULT
MAX
3996
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ON
OFF
LOW
HIGH
t_fault = 14µs
EXTERNALLYFORCED FAULT
LASEROUPUT
TX_DISABLE
VTC
FAULT
10.0µs/div
FAULT RECOVERY TIME
MAX
3996
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EXTERNALFAULT REMOVED
LASEROUPUT
TX_DISABLE
VTC
FAULT
1.00ms/div
FREQUENT ASSERTION OFTX_DISABLE
MAX
3996
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EXTERNALLYFORCED FAULT
OV
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Pin Description
PIN NAME FUNCTION
1 TCTemperature Compensation Set. The resistor at TC programs the temperature-increasing componentof the laser-modulation current.
2 FAULT Fault Indicator. See Table 1.
3, 9 GND Ground
4 TX_DISABLETransmit Disable. Laser output is disabled when TX_DISABLE is high or left unconnected. The laseroutput is enabled when this pin is asserted low.
5 PORDLYPower-On Reset Delay. A capacitor connected between PORDLY and GND can be used to extend thedelay 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 MON1Attaches to the emitter of the bias driving transistor through a 10Ω resistor. See the Design Proceduresection.
11 MON2 This pin attaches to the emitter of the bias driving transistor. See the Design Procedure section.
12 COMPA capacitor connected from this pin to ground sets the dominant pole of the APC loop. See the DesignProcedure 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 PadGround. This must be soldered to the circuit board ground for proper thermal and electricalperformance. See the Layout Considerations section.
Detailed DescriptionThe MAX3996 contains a bias generator with automaticpower control and smooth start, a laser modulator, apower-on reset (POR) circuit, and safety circuitry(Figure 3).
Bias GeneratorFigure 4 shows the bias generator circuitry that con-tains a power-control amplifier, smooth-start circuitry,and two bias-fault sensors. The power-control amplifiercombined with an internal NPN transistor provides DClaser 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 changinglaser properties. The smooth-start circuitry preventscurrent spikes to the laser during power-up or enable,ensuring compliance with safety requirements andextending the life of the laser.
The MD input is connected to the anode of a monitordiode, which is used to sense laser power. The BIASoutput is connected to the cathode of the laser throughan inductor or ferrite bead. The power-control amplifierdrives a transistor to control the laser’s bias current. Ina fault condition (Table 1), the base of the bias-drivingtransistor is pulled low to ensure that bias current isturned off.
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MAX3996
MODULATION CURRENTGENERATOR
VCC
3.0V TO 5.5V
ICC
IN+
IN-
0.01µF
0.01µFVID RIN
ROUT ROUT
VCC
OUT-
OUT+
0.01µF
25Ω
0.01µF
25Ω
iMOD
iOUT
FERRITEBEAD*
RMOD
MODSETTC
*MURATABLM11HA102SG
Figure 1. Output Load for AC Specification
iMOD
CURRENT
VID = VIN+ - VIN-
VIN-
VIN+
VOLTS
TIME
100mVP-P MIN1100mVP-P MAX
200mVP-P MIN2200mVP-P MAX
SINGLE-ENDED SIGNAL
DIFFERENTIAL SIGNAL
Figure 2. Required Input Signal and Modulation-Current Polarity
MAX3996
SAFETYCIRCUITRY BIAS
GENERATORWITH SMOOTH
START
MODULATION CURRENTGENERATOR
MODULATIONENABLE
MODULATIONFAULT
100Ω
IN+
IN-
INPUT BUFFER LASERMODULATION
50Ω 50Ω
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
400mVBIAS
DISABLE
1.1V
SMOOTH START
POWER-CONTROLAMPLIFIER
BIASFAULT 1
BIASFAULT 2
MD
BIAS
MON2
MON1
COMP
Figure 4. Bias Circuitry
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Smooth-StartDuring startup, the laser does not emit light, and theAPC loop is not closed. The smooth-start circuit pullsthe MD pin to approximately 2.5V during the POR delayand while TX_DISABLE is high. This causes the power-control amplifier to shut off the bias transistor. WhenPOR delay is over and TX_DISABLE is low, the MD pinis released and pulled to GND by RSET because thereis no laser power and thus no monitor diode current.The output voltage of the power-control amplifier thenbegins to increase. A capacitor attached to COMP(CCOMP) slows the slew rate and allows a controlledincrease in bias current (Figure 11). Maxim recom-mends CCOMP = 0.1µF.
Modulation CircuitryThe modulation circuitry consists of an input buffer, acurrent mirror, and a high-speed current switch (Figure5). The modulator drives up to 30mA of modulation cur-rent into a 25Ω load.
Many of the modulator performance specificationsdepend on total modulator current. To ensure good driverperformance, the voltage at either OUT+ or OUT- mustnot 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 thetemperature-stable portion of the modulation current,and the resistor at TC (RTC) programs the temperature-increasing portion of the modulation current. Figure 6shows modulation current as a function of temperaturefor two extremes: RTC is open (the modulation currenthas 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 theDesign Procedure section. Table 2 is the RTC and RMODselection table.
Safety CircuitryThe safety circuitry contains a disable input, a faultlatch, 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-pointfault can be a short to VCC or GND, or between any two
3.0V to 5.5V, 2.5Gbps VCSELand 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
50Ω 50Ω
VCC
OUT+
OUT-
1.2V REFERENCE0ppm/°C
MODSETFAULT
200mV
MODSET
RMOD
1.2V REFERENCE4000ppm/°C
TC FAULT200mV
TC
RTC
Σ
CURRENT AMPLIFIER96XENABLE
MODULATIONCURRENT
GENERATOR
CURRENTSWITCH
Figure 5. Modulation Circuitry
0.6
0.8
0.7
1.0
0.9
1.2
1.1
1.3
0 20 30 4010 50 60 70 80 90 100 110JUNCTION TEMPERATURE (°C)
i MOD
/(iM
OD A
T +5
2°C)
RTC ≥ 1.9kΩRMOD = OPENTEMPCO = 4000ppm/°C
RTC = OPENTEMPCO = 50ppm/°C
Figure 6. Modulation Current vs. Temperature for Maximumand Minimum Temperature Coefficient
IC pins. See Table 3 to view the circuit response to vari-ous single-point failures. The shutdown condition islatched until reset by a toggle of TX_DISABLE or VCC.
Fault DetectionAll critical nodes are monitored for safety faults, andany node voltage that differs significantly from itsexpected value results in a fault (Table 1). When a faultcondition is detected, the laser is shut down. See the
Applications Information for more information on lasersafety.
ShutdownThe laser driver offers redundant bias shutdown. TheSHDNDRV output drives an optional external transistor.The bias and modulation drivers have separate internaldisable signals.
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Table 2. RTC and RMOD Selection Table
PIN NAMECIRCUIT RESPONSE TO OVERVOLTAGE OR
SHORT TO VCC
CIRCUIT RESPONSE TO UNDERVOLTAGE ORSHORT 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.
COMPA fault is detected at either the collector or the emitterof 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 currentuntil a fault is detected at the emitter or collector of thebias transistor, and then a fault* state occurs.
SHDNDRVDoes not affect laser power. If the shutdown circuitry isused, bias current is shut off.
Does not affect laser power.
BIASIn this condition, laser forward voltage is 0V and nolight 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 = 5mATEMPCO(ppm/°C) RMOD (kΩ) RTC (kΩ) 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
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Latched Fault OutputAn open-collector FAULT output is provided with theMAX3996. This output is latched until the power isswitched off, then on, or until TX_DISABLE is switchedto HIGH and then LOW.
Power-On ResetThe MAX3996 contains an internal power-on resetdelay to reject noise on VCC during power-on or hot-plugging. Adding capacitance to the PORDLY pin canextend the delay. The POR comparator includes hys-teresis to improve noise rejection.
Design ProcedureSelect Laser
Select a communications-grade laser with a rise time of260ps or better for 1.25Gbps or 130ps or better for2.5Gbps applications. To meet the MAX3996’s ACspecifications, the voltage at both OUT+ and OUT-must remain above VCC - 1V at all times.
Use a high-efficiency laser that requires low modulationcurrent and generates a low voltage swing. Trimmingthe leads can reduce laser package inductance.Typical package leads have inductance of 25nH perinch (1nH/mm); this inductance causes a large voltageswing across the laser. A compensation filter networkalso can be used to reduce ringing, edge speed, andvoltage swing.
Programming Modulation CurrentResistors at the MODSET and TC pins set the ampli-tude of the modulation current. The resistor RMOD setsthe temperature-stable portion of the modulation cur-rent, and the resistor (RTC) sets the temperature-increasing portion of the modulation current. Todetermine the appropriate temperature coefficient fromthe slope efficiency (η) of the laser, use the followingequation:
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 tempcoof -3175ppm/°C.
To obtain the desired modulation current and tempcofor the device, the following equations can be used todetermine 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 theseequations. The straight diagonal lines depict constanttempcos. The curved lines represent constant modula-tion currents. If no temperature compensation isdesired, leave TC open, and the equation for iMOD-simplifies considerably.
The following equations were used to derive Figure 8 andthe equations at the beginning of this section.
iR R
RT C Amps
MODL MOD
TC
= ×+ + Ω
+
+ Ω+ °
−
7750
501 15
250
1 06250
1 0 004 25
.
.( . ( ))
RTempco i
RTempco R
Tempco
TCMOD
MODTC
=×
Ω
=+ Ω( )
×
Ω
−
−
−
0 22
10250
10 250 52
0 19 48 10250
6
6
6
.
/
/
. /
LASER TEMPCOppm C C C
_[ / ]°
=° °( ) ×
−
−
η ηη
70 25
25
670 25
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3.0V to 5.5V, 2.5Gbps VCSELand Laser Driver
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R Q
S
FAULT LATCH
BIAS FAULT 1BIAS FAULT 2
TC FAULTMODSET FAULT
BIAS ENABLE
MODULATORENABLE
DELAY
STARTUP
VCC PORDLY
SHDNDRV
FAULT
TX_DISABLE
VBG
Figure 7. Safety Circuitry Functional Diagram
Determine Modulator ConfigurationThe MAX3996 can be used in several configurations.For modulation currents less than 20mA, Maxim recom-mends the configuration shown in the TypicalApplication Circuit. Outputs greater than 20mA couldcause the voltage at the modulator output to be lessthan VCC - 1V, which might degrade laser output. Forlarge currents, Maxim recommends the configuration inFigure 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 thelaser. Select L1 to have an impedance >100Ω betweenf = 10MHz and f = 2GHz, and a DC resistance < 3Ω;Maxim recommends the Murata BLM11HA102SG.These inductors are also desirable for connecting theOUT+ and OUT- pins to VCC.
Programming Laser Power and Bias Fault Threshold
The IC is designed to drive a common anode laser witha photodiode. A servo-control loop is formed by theinternal NPN bias-driving transistor, the laser diode, themonitor diode (RSET), and the power-control amplifier(Figure 11). The voltage at MD is stabilized to 1.1V. The
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3.0V to 5.5V, 2.5Gbps VCSELand Laser Driver
______________________________________________________________________________________ 11
1000
11 100 1000
10
RMOD (kΩ)
R TC
(kΩ
)
10
500ppm
1000ppm
1500ppm 2000ppm
3000ppm2500ppm
3500ppm
5mA
10mA
25mA20mA
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 SHUTDOWNCIRCUITRY
1.8kΩ
VCC
VCC
VCCTX_DISABLE
FAULT
IN+
IN-
PORDLY
TC MODSET MON1 MON2 COMP GNDMD
BIAS
OUT+
OUT-
SHDNDRV
*FERRITE BEAD
RSET
VCC
L2*
VCC
L3*
0.01µF
Figure 9. Large Modulation Current
MAX39960.01µF
0.01µF
CPORDLYRTC
RMOD
CCOMPN.C.
0.01µF
0.01µF
L1*
VCC
VCC
VCCTX_DISABLE
FAULT
IN+
IN-
PORDLY
TC MODSET MON1 MON2 COMP GNDMD
BIAS
OUT-
OUT+
SHDNDRV
*FERRITE BEAD
RSET
L2*
Figure 10. Differential Configuration
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monitor photodiode current is set by ID = VMD/RSET.Determine the desired monitor current (ID), and thenselect RSET = 1.1V/ID.
A bias stabilizing capacitor (CCOMP) must be connect-ed between the COMP pin and ground to obtain thedesired APC loop time constant. This improves power-supply and ground noise rejection. A capacitance of0.1µF usually is sufficient to obtain time constants of upto 35µs.
The degeneration resistance between MON2 andground determines the bias current that causes a faultand affects the APC time constant. Select RMON (thetotal resistance between MON2 and ground) =400mV/(maximum bias current). A degeneration resis-tance of 10Ω can be obtained by grounding MON1.Increasing RMON increases the APC time constant.
The discrete components for use with the commonanode with photodiode configuration are:
RSET = 1.1V/IDCCOMP = 0.1µF (typ)
L1 = ferrite bead, see the Bias Filter section
RMON = 400mV/(maximum bias current)
Programming POR DelayA capacitor can be added to PORDLY to increase thedelay when powering up the part. The delay will beapproximately:
See the Typical Operating Characteristics section.
Designing the Laser-Compensation Filter Network
Laser package inductance causes the laser impedanceto increase at high frequencies, leading to ringing,overshoot, and degradation of the laser output. A laser-compensation filter network can be used to reduce thelaser impedance at high frequencies, thereby reducingoutput ringing and overshoot.
tC
ondsPORDLY=× −1 4 10 6.
sec
3.0V to 5.5V, 2.5Gbps VCSELand Laser Driver
12 ______________________________________________________________________________________
MAX3996VCC
IDRSET
MONITORDIODE
1.1V
POWER-CONTROLAMPLIFIER
VCC
SHUTDOWNCIRCUIT
OPTIONALSHUTDOWNCIRCUITRY
SHDNDRV
LASER
L1*
11Ω
SMOOTHSTART
BIASDISABLE
MD
BIAS
MON2
MON1
IBIAS
COMPCCOMP0.1µF
*FERRITE BEAD
Figure 11. APC Loop
The compensation components (RF and CF) are mosteasily determined by experimentation. For interfacingwith edge-emitting lasers, refer to application noteHFAN-2.0, Interfacing Maxim Laser Drivers with LaserDiodes. Begin with RF = 50Ω and CF = 2pF. IncreaseCF until the desired transmitter response is obtained(Figure 12).
Using External ShutdownTo achieve single-point fault tolerance, Maxim recom-mends an external shutdown transistor (Figure 11). Inthe event of a fault, SHDNDRV asserts high, placing theshutdown transistor in cutoff mode and thereby shuttingoff the bias current.
Applications InformationLaser Safety and IEC825
The International Electrotechnical Commission (IEC)determines standards for hazardous light emissionsfrom fiber optic transmitters. IEC 825 defines the maxi-mum light output for various hazard levels. TheMAX3996 provides features that facilitate compliancewith IEC825. A common safety precaution is single-point fault tolerance, whereby one unplanned short,open, or resistive connection does not cause excesslight output. When this laser driver is used, as shown inthe Typical Application Circuit, the circuits respond tofaults as listed in Table 3. Using this laser driver alonedoes not ensure that a transmitter design is compliantwith IEC825. The entire transmitter circuit and compo-nent selections must be considered. Customers mustdetermine the level of fault tolerance required by theirapplications, recognizing that Maxim products are notdesigned or authorized for use as components in sys-tems intended for surgical implant into the body, forapplications intended to support or sustain life, or forany other application where the failure of a Maxim
product could create a situation where personal injuryor death may occur.
Layout ConsiderationsThe MAX3996 is a high-frequency product whose per-formance largely depends upon the circuit board layout.Use a multilayer circuit board with a dedicated groundplane. Use short laser-package leads placed close tothe modulator outputs. Power supplies must be capaci-tively bypassed to the ground plane, with surface-mountcapacitors placed near the power-supply pins.
The dominant pole of the APC circuit normally is atCOMP. To prevent a second pole in the APC that canlead to oscillations, ensure that parasitic capacitance atMD is minimized (10pF).
Common QuestionsLaser output is ringing or contains overshoot. Induc-tive laser packaging often causes this. Try reducing thelength of the laser leads. Modify the filter components toreduce the driver’s output edge speed (see the DesignProcedure section). Extreme ringing can be caused bylow voltage at the OUT± pins. This might indicate thatpullup beads or a lower modulation current are needed.
Low-frequency oscillation on the laser output. Thisis more prevalent at low temperatures. The APC mightbe oscillating. Try increasing the value of CCOMP oradd additional degeneration by placing some resis-tance from MON1 to GND. Ensure that the parasiticcapacitance at the MD node is kept very small (<10pF).
The APC is not needed. Connect BIAS to VCC, leaveMD open, and connect MON2 and COMP to ground.
The modulator is not needed. Leave TC and MODSETopen. Connect IN+ to VCC, IN- to ground through750Ω, and leave OUT+ and OUT- open.
Interface ModelsFigures 13–17 show typical models for the inputs andoutputs of the MAX3996, including package parasitics.
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______________________________________________________________________________________ 13
TIME
POW
ER
UNCOMPENSATED
CORRECTLY COMPENSATED
OVERCOMPENSATED
Figure 12. Laser Compensation
MAX39964kΩ
FAULT
NOTE: THE FAULT PIN IS AN OPEN-COLLECTOR OUTPUT
Figure 13. FAULT Output
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14 ______________________________________________________________________________________
MAX3996
550Ω 60Ω
10kΩ
VCC
SHDNDRV
Figure 14. SHDNDRV Output
MAX3996
PACKAGE
OUT-1.1nH
0.15pF1pF
50Ω 50Ω
VCC VCC
1pF
OUT+
PACKAGE
1.1nH
0.15pF
Figure 15. Modulator Outputs
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______________________________________________________________________________________ 15
MAX3996PACKAGE
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
6 7 8 9 10
11
12
13
14
15
1617181920
TC
FAULT
GND
V CC
V CC
V CC
IN+
IN-
GND
MON
1
TX_DISABLE
PORDLY MON2
COMP
MD
SHDNDRV
BIAS
MOD
SET
OUT-
OUT+
MAX3996
EXPOSED PAD IS CONNECTED TO GND
20 THIN QFN (4mm x 4mm)
TOP VIEW
1
2
3
4
5
6 7 8 9 10
11
12
13
14
15
1617181920
TC
FAULT
GND
V CC
V CC
V CC
IN+
IN-
GND
MON
1
TX_DISABLE
PORDLY MON2
COMP
MD
SHDNDRV
BIAS
MOD
SET
OUT-
OUT+
MAX3996
EXPOSED PAD IS CONNECTED TO GND
Pin Configurations
Chip InformationTRANSISTOR COUNT: 1061
PROCESS: SILICON BIPOLAR
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Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses areimplied. 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.
PARTPACKAGETYPE
PACKAGECODE
MAX3996CGP20 QFN4mm x 4mm x 0.9mm
G2044-3
MAX3996CTP+ 20 Thin QFN4mm x 4mm x 0.8mm
T2044-3
Package InformationFor the latest package outline information, go towww.maxim-ic.com/packages.