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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3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver

________________________________________________________________ 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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3.0V to 5.5V, 2.5Gbps VCSELand 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 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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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)

MAX

3996

toc0

125Ω LOAD

120mV/div

52ps/div

ELECTRICAL EYE DIAGRAM (iMOD = 30mA, 27 - 1 PRBS, 3.2Gbps)

MAX

3996

toc0

2

25Ω LOAD

57ps/div

OPTICAL EYE DIAGRAM (iMOD = 5mA, 850nm VCSEL, 27 - 1 PRBS,

2.5Gbps, 1870MHz FILTER)

MAX

3996

toc0

3

57ps/div

MAX

3996

toc0

4

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

3996

toc0

5

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

toc0

6

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

toc0

7

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

toc0

8

CPORDLY (F)

POR

DELA

Y (s

)

10p 1n100p 10n 100n

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_______________________________________________________________________________________ 5

LASEROUPUT

TX_DISABLE

VCC

FAULT

10.0ms/div

HOT PLUG WITHTX_DISABLE LOW

MAX

3996

toc0

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

toc1

0

0V

LOW

LOW

3.3V

LASEROUPUT

TX_DISABLE

VCC

FAULT

20.0µs/div

TRANSMITTER ENABLE

MAX

3996

toc1

1

LOW

LOW

HIGH

3.3V

t_on = 37µs

LASEROUPUT

TX_DISABLE

VCC

FAULT

20.0ns/div

TRANSMITTER DISABLE

MAX

3996

toc1

2

LOW

LOW

HIGH

3.3Vt_off = 60ns

ELECTRICALOUPUT

FAULT

VMON2

IBIAS

10.0µs/div

RESPONSE TO FAULT

MAX

3996

toc1

3

ON

OFF

LOW

HIGH

t_fault = 14µs

EXTERNALLYFORCED FAULT

LASEROUPUT

TX_DISABLE

VTC

FAULT

10.0µs/div

FAULT RECOVERY TIME

MAX

3996

toc1

4

EXTERNALFAULT REMOVED

LASEROUPUT

TX_DISABLE

VTC

FAULT

1.00ms/div

FREQUENT ASSERTION OFTX_DISABLE

MAX

3996

toc1

5

EXTERNALLYFORCED FAULT

OV

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6 _______________________________________________________________________________________

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


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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_______________________________________________________________________________________ 9

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

10


10 ______________________________________________________________________________________

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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______________________________________________________________________________________ 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


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.

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