MAX3738资料
19-3162; Rev 0; 1/04
1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
General Description
FeaturesThe MAX3738 is a +3.3V laser driver designed for mul-tirate transceiver modules with data rates from 1Gbps♦Single +3.3V Power Supplyto 2.7Gbps. Lasers can be DC-coupled to the♦47mA Power-Supply CurrentMAX3738 for reduced component count and ease ofmultirate operation.
♦85mA Modulation CurrentLaser extinction ratio control (ERC) combines the features♦100mA Bias Current
of automatic power control (APC), modulation compensa-♦Automatic Power Control (APC)tion, and built-in thermal compensation. The APC loopmaintains constant average optical power. Modulation♦Modulation Compensation
compensation increases the modulation current in pro-♦On-Chip Temperature Compensationportion to the bias current. These control loops, com-bined with thermal compensation, maintain a constant♦Self-Biased Inputs for AC-Couplingoptical extinction ratio over temperature and lifetime.
♦Ground-Referenced Current MonitorsThe MAX3738 accepts differential data input signals.The wide 5mA to 60mA (up to 85mA AC-coupled) mod-♦Laser Shutdown and Alarm Outputsulation current range and up to 100mA bias current♦Enable Control and Laser Safety Feature
range, make the MAX3738 ideal for driving FP/DFBlasers in fiber optic modules. External resistors set theOrdering Informationrequired laser current levels. The MAX3738 providestransmit disable control (TX_DISABLE), single-pointPARTTEMPPIN-PKGfault tolerance, bias-current monitoring, and photocur-RANGEPACKAGECODErent monitoring. The device also offers a latched failureMAX3738ETG-40°C to 85°C24 Thin QFNT2444-1output (TX_FAULT) to indicate faults, such as when theAPC loop is no longer able to maintain the averageoptical power at the required level. The MAX3738 isTypical Application Circuit appears at end of data sheet.
compliant with the SFF-8472 transmitter diagnostic andSFP MSA timing requirements.
The MAX3738 is offered in a 4mm x 4mm, 24-pin thinQFN package and operates over the extended -40°C to+85°C temperature range.
Pin Configuration
PTOP VIEWPMMOT21CETApplicationsEBSELTLTSIITDDFF_OOCCCHMMPPPTAAAMultirate OC-24 to OC-48 FEC Transceivers242322212019Gigabit Ethernet SFF/SFP and GBICMODTCOMP118MDTransceivers
VCC217VCC1Gbps/2Gbps Fibre Channel SFF/SFP and GBICIN+316OUT+Transceivers
IN-4MAX373815OUT-VCC514VCCTX_DISABLE613BIAS789101112NNNDDOONLTNMMWOGUG__DFACCT_PBUXHTSTHE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLYGROUND ON THE CIRCUIT BOARD.________________________________________________________________Maxim Integrated Products1
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.
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1Gbps to 2.7Gbps SFF/SFP Laser Driver withExtinction Ratio ControlMAX3738ABSOLUTE MAXIMUM RATINGS
Supply Voltage VCC...............................................-0.5V to +6.0VIN+, IN-, TX_DISABLE, TX_FAULT, SHUTDOWN,BC_MON, PC_MON, APCFILT1, APCFILT2, MD, TH_TEMP, MODTCOMP, MODBCOMP,
MODSET, and APCSET Voltage.............-0.5V to (VCC+ 0.5V)
OUT+, OUT-, BIAS Current.............................-20mA to +150mAContinuous Power Dissipation (TA= +85°C)
24-Pin TQFN (derate 20.8mW/°C above +85°C).......1805mWOperating Junction Temperature Range...........-55°C to +150°CStorage 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 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.
ELECTRICAL CHARACTERISTICS
(VCC= +2.97V to +3.63V, TA= -40°C to +85°C. Typical values are at VCC= +3.3V, IBIAS= 60mA, IMOD= 60mA, TA= +25°C, unlessotherwise noted.) (Notes 1, 2)
PARAMETERPOWER SUPPLYSupply CurrentPower-Supply Noise RejectionI/O SPECIFICATIONSDifferential Input SwingCommon-Mode InputLASER BIASBias-Current-Setting RangeBias Off CurrentBias-Current Monitor RatioLASER MODULATIONModulation Current-SettingRangeOutput Edge SpeedOutput Overshoot/UndershootRandom JitterDeterministic Jitter (Notes 6, 8)Modulation-Current TemperatureStabilityModulation-Current-Setting ErrorModulation Off CurrentMonitor-Diode Input CurrentRangeMD Pin VoltageMD Current Monitor RatioIMD / IPC_MON0.850.93IMOD(Note 5)20% to 80%(Notes 6, 7)(Notes 6, 7)2.7Gbps, 5mA ≤ IMOD ≤ 85mA1.25Gbps, 5mA ≤ IMOD ≤ 85mA(Note 6)15Ω load,TA = +25°CTX_DISABLE = high5mA ≤ IMOD ≤ 10mA10mA ≤ IMOD ≤ 85mA5mA ≤ IMOD ≤ 10mA10mA < IMOD ≤ 85mA5mA ≤ IMOD ≤ 85mA565±60.621820±175±1251.34041±600±480±20±150.18580mAps%psRMSpsP-Pppm/°C%mATX_DISABLE = highIBIAS / IBC_MON687911000.195mAmAmA/mAVIDVCMDC-coupled, Figure 10.21.72.4VCC -VID / 4VP-PVICCPSNR(Note 3)f ≤ 1MHz, 100mAP-P (Note 4)473360mAdBSYMBOLCONDITIONSMINTYPMAXUNITSWith 1pF between OUT+ and OUT-AUTOMATIC POWER AND EXTINCTION RATIO CONTROLSIMDAverage current into the MD pin1815001.41.15µAVmA/mA2_______________________________________________________________________________________
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1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
ELECTRICAL CHARACTERISTICS (continued)
(VCC= +2.97V to +3.63V, TA= -40°C to +85°C. Typical values are at VCC= +3.3V, IBIAS= 60mA, IMOD= 60mA, Totherwise noted.) (Notes 1, 2)
A= +25°C, unlessPARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSAPC Loop Time ConstantCAPC_FILT = 0.01µF, ∆IMD / ∆IBIAS = 1/703.3µsAPC Setting Stability(Note 6)±100±480ppm/°CAPC Setting AccuracyTA = +25°C±15%IMOD Compensation-SettingRange by BiasKK = ∆IMOD / ∆IBIAS01.5mA/mAIMOD Compensation-SettingRange by TemperatureTCTC = ∆IMOD / ∆T (Note 6)01.0mA/°CThreshold-Setting Range forTemperature CompensationTTH(Note 6)+10+60°CLASER SAFETY AND CONTROLBias and Modulation Turn-OffCAPC_FILT = 0.01µF, ∆IMD / ∆IBIAS = 1/80Delay(Note 6)5µsBias and Modulation Turn-OnCAPC_FILT = 0.01µF, ∆IMD / ∆IBIAS = 1/80Delay(Note 6)600µsThreshold Voltage at Monitor PinsVREFFigure 51.141.31.39VINTERFACE SIGNALSTX_DISABLE Input HighVHI2.0VTX_DISABLE Input LowVLORPULL = 45kΩ (typ)0.8VTX_DISABLE Input CurrentVHI = VCC15VLO = GND-70-140µATX_FAULT Output LowSinking 1mA, open collector0.4VShutdown Output HighSourcing 100µAVCC - 0.4VShutdown Output LowSinking 100µA0.4VNote 1:AC characterization is performed using the circuit in Figure 2 using a PRBS 223- 1 or equivalent pattern.Note 2:Specifications at -40°C are guaranteed by design and characterization.
Note 3:Excluding IBIASand IMOD. Input data is AC-coupled. TX_FAULT open, SHUTDOWN open.
Note 4:Power-supply noise rejection (PSNR) = 20log10(Vnoise (on VCC) / ∆VOUT). VOUTis the voltage across the 15Ωload when IN+is high.
Note 5:The minimum required voltage at the OUT+ and OUT- pins is +0.75V.Note 6:Guaranteed by design and characterization.Note 7:Tested with 00001111 pattern at 2.7Gbps.Note 8:
DJ includes pulse-width distortion (PWD).
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1Gbps to 2.7Gbps SFF/SFP Laser Driver withExtinction Ratio ControlMAX3738Typical Operating Characteristics
(VCC= +3.3V, CAPC= 0.01µF, IBIAS= 20mA, IMOD= 30mA, TA= +25°C, unless otherwise noted.)
OPTICAL EYE DIAGRAM
(2.7Gbps, 27 - 1 PRBS, 2.3GHz FILTER)
MAX3738 toc01OPTICAL EYE DIAGRAM
(1.25Gbps, 27 - 1 PRBS, 940MHz FILTER)
MAX3738 toc02ELECTRICAL EYE DIAGRAM
(IMOD = 30mA, 2.7Gbps, 27 - 1 PRBS)
1pF BETWEEN OUT+AND OUT-MAX3738 toc031310nm FP LASERre = 8.2dB1310nm FP LASERre = 8.2dB75mV/div
54ps/div116ps/div52ps/div
SUPPLY CURRENT (ICC) vs. TEMPERATURE(EXCLUDES BIAS AND MODULATION CURRENTS)
MAX3738 toc04BIAS-CURRENT MONITOR RATIO
vs. TEMPERATURE
MAX3738 toc05PHOTOCURRENT MONITOR RATIO
vs. TEMPERATURE
1.15IMD/IPC_MON (mA/mA)1.101.051.000.950.900.850.80
MAX3738 toc066055SUPPLY CURRENT (mA)5045
2.97V403530
3.3V3.63V908886IBIAS/IBC_MON (mA/mA)8482807876747270
1.20
-40-30-20-100102030405060708090
TEMPERATURE (°C)
-40-30-20-100102030405060708090
TEMPERATURE (°C)
-40-30-20-100102030405060708090
TEMPERATURE (°C)
MODULATION CURRENT vs. RMODSET
MAX3738 toc07PHOTODIODE CURRENT vs. RAPCSET
MAX3738 toc08DETERMINISTIC JITTER vs. MODULATION CURRENT
454035DJ (psP-P)302520151050
2.7GbpsMAX3738 toc0990807060IMOD (mA)1.41.21.0IMD (mA)0.80.60.40.20
50
504030201001
10RMODSET (kΩ)
100
0.11
RAPCSET (kΩ)
101000102030405060708090
IMOD (mA)
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1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
Typical Operating Characteristics (continued)(VCC= +3.3V, CAPC= 0.01µF, IBIAS= 20mA, IMOD= 30mA, TA= +25°C, unless otherwise noted.)
RANDOM JITTER
TEMPERATURE COMPENSATION vs.vs. MODULATION CURRENT
COMPENSATION (K) vs. RMODBCOMP
RTH_TEMP (RMODTCOMP = 500Ω)
2.0010111100
21ccoocott1.8 t88 8333777333XXAA90MMRTH_TEMP = 12kΩX1.6AM1.4180)RS)TH_TEMP = 7kΩM1.2)AARm/m70s(p D(1.0A mOJ( R0.8KMI60RTH_TEMP = 4kΩ0.60.150RTH_TEMP = 2kΩ0.40.240
00.01
30
0
10
20
30
40
50
60
70
80
90
0.001
0.01
0.1
1
10
100
-100
102030405060708090
IMOD (mA)
RMODBCOMP (kΩ)
TEMPERATURE (°C)
TEMPERATURE COMPENSATION vs.RTH_TEMP (RMODTCOMP = 10kΩ)
HOT PLUG WITH TX_DISABLE LOW
TRANSMITTER ENABLE
44MAX3738 toc14MAX3738 toc1531cot 833.3VVCC427R3X3.3VTH_TEMP = 12kΩAMVCC
t_init = 59.6msLOW40RTH_TEMP = 7kΩ0VFAULT
)AΩHIGHm38RTH_TEMP = 4k( DFAULTLOWORTH_TEMP = 2kΩTX_DISABLE
TX_DISABLE
t_on = 23.8µsMI36LOWLOW34LASERLASER32OUTPUT
OUTPUT
30
-100102030405060708090100
20ms/div10µs/div
TEMPERATURE (°C)
TRANSMITTER DISABLE
RESPONSE TO FAULT
MAX3738 toc16MAX3738 toc17FAULT RECOVERY TIME
MAX3738 toc18VCC3.3VVPC_MON
EXTERNALLYVPC_MON
LOWFORCED FAULTEXTERNALLYFORCED FAULTFAULT
t_fault = 160nsHIGHFAULT
HIGHTX_DISABLE
91.2nsFAULTt_init = 58msLOWLOWTX_DISABLE
TX_DISABLE
HIGHLASERLASEROUTPUT
LOWLOWOUTPUT
LASEROUTPUT
20ns/div400ns/div
40ms/div
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1Gbps to 2.7Gbps SFF/SFP Laser Driver withExtinction Ratio ControlMAX3738Pin DescriptionPIN12, 5, 14,1734678910, 121113151618192021222324EPNAMEMODTCOMPFUNCTIONModulation-Current Compensation from Temperature. A resistor at this pin sets the temperaturecoefficient of the modulation current when above the threshold temperature. Leave open for zerotemperature compensation.+3.3V Supply VoltageNoninverted Data InputInverted Data InputTransmitter Disable, TTL. Laser output is disabled when TX_DISABLE is asserted high or leftunconnected. The laser output is enabled when this pin is asserted low.Photodiode-Current Monitor Output. Current out of this pin develops a ground-referenced voltageacross an external resistor that is proportional to the monitor diode current.Bias-Current Monitor Output. Current out of this pin develops a ground-referenced voltage across anexternal resistor that is proportional to the bias current.Shutdown Driver Output. Voltage output to control an external transistor for optional shutdowncircuitry.GroundOpen-Collector Transmit Fault Indicator (Table 1)Laser Bias-Current OutputInverted Modulation-Current Output. IMOD flows into this pin when input data is low.Noninverted Modulation-Current Output. IMOD flows into this pin when input data is high.Monitor Photodiode Input. Connect this pin to the anode of a monitor photodiode. A capacitor toground is required to filter the high-speed AC monitor photocurrent.Connect a capacitor (CAPC) between pin 19 (APCFILT1) and pin 20 (APCFILT2) to set the dominantpole of the APC feedback loop.(See pin 19)A resistor connected from this pin to ground sets the desired average optical power.A resistor connected from this pin to ground sets the desired constant portion of themodulation current.Modulation-Current Compensation from Bias. Couples the bias current to the modulation current.Mirrors IBIAS through an external resistor. Leave open for zero-coupling.Threshold for Temperature Compensation. A resistor at this pin programs the temperature abovewhich compensation is added to the modulation current.Ground. Solder the exposed pad to the circuit board ground for specified thermal and electricalperformance.VCCIN+IN-TX_DISABLEPC_MONBC_MONSHUTDOWNGNDTX_FAULTBIASOUT-OUT+MDAPCFILT1APCFILT2APCSETMODSETMODBCOMPTH_TEMPExposed Pad6_______________________________________________________________________________________
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1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
VOLTAGEVCCVCCVIN+SINGLE ENDED100mV (min)VIN-1200mV (max)30Ω30ΩOUT-Z0 = 30Ω(VIN+) - (VIN-)DIFFERENTIAL200mV (min)30Ω0.5pF2400mV (max)MAX3738IOSCILLOSCOPEOUT+CURRENTOUT+Z0 = 30ΩZ0 = 50ΩIOUT+I75Ω50ΩMODTIMEFigure 1. Required Input Signal and Output PolarityFigure 2. Test Circuit for Characterization
HOST BOARDMODULEFILTER DEFINED BY SFP MSASOURCEL1TO LASERNOISE1µHOPTIONALDRIVER VCCVOLTAGEC1C2C3SUPPLY0.1µF10µF0.1µFOPTIONALFigure 3. SupplyFilter
Detailed Descriptionthe laser diode should equal 15Ω. To further dampThe MAX3738 laser driver consists of three main parts:aberrations caused by laser diode parasitic induc-a high-speed modulation driver, biasing block withtance, an RC shunt network may be necessary. Refer toERC, and safety circuitry. The circuit design is opti-Maxim Application Note HFAN 0.0:Interface Maxim’smized for high-speed, low-voltage (+3.3V) operationLaser Driver to Laser Diode for more information.(Figure4).
At data rates of 2.7Gbps, any capacitive load at theHigh-Speed Modulation Driver
cathode of a laser diode degrades optical output perfor-The output stage is composed of a high-speed differ-mance. Because the BIAS output is directly connectedential pair and a programmable modulation currentto the laser cathode, minimize the parasitic capacitancesource. The MAX3738 is optimized for driving a 15Ωassociated with the pin by using an inductor to isolateload. The minimum instantaneous voltage required atthe BIAS pin parasitics form the laser cathode.
OUT- is 0.7V for modulation currents up to 60mA andExtinction Ratio Control
0.75V for currents from 60mA to 85mA. OperationThe extinction ratio (re) is the laser on-state powerabove 60mA can be accomplished by AC-coupling ordivided by the off-state power. Extinction ratio remainswith sufficient voltage at the laser to meet the driverconstant if peak-to-peak and average power are heldoutput voltage requirement.
constant:
To interface with the laser diode, a damping resistorre = (2PAVG+ PP-P) / (2PAVG- PP-P)
(RD) is required. The combined resistance dampingresistor and the equivalent series resistance (ESR) of
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1Gbps to 2.7Gbps SFF/SFP Laser Driver withExtinction Ratio ControlMAX3738VCCSHUTDOWNMAX3738INPUT BUFFERIN+IN-SHUTDOWNTX_FAULTTX_DISABLERPULL = 45kΩIMD1PC_MONRPC_MONIBIAS82BC_MONRBC_MONxTCT > TTHTx268xKIAPCSETMDIMDCMDx1VCCIBIASx1/2VBGAPCSETRAPCSETSAFETY LOGICANDPOWER DETECTORIMODENABLEIBIASENABLEDATAPATHIMODBIASVCCIBIASOUT-OUT+RDVBGTH_TEMPRTH_TEMPMODTCOMPRMODTCOMPMODSETRMODSETMODBCOMPAPCFILT1RMODBCOMPCAPCAPCFILT2Figure 4. Functional Diagram
Average power is regulated using APC, which keepsconstant current from a photodiode coupled to thelaser. Peak-to-peak power is maintained by compen-sating the modulation current for reduced slope effi-ciency (h) of laser over time and temperature:
PAVG= IMD / ρMONPP-P= ηx IMOD
Modulation compensation from bias increases the mod-ulation current by a user-selected proportion (K) need-ed to maintain peak-to-peak laser power as biascurrent increases with temperature. Refer to MaximApplication Note HFAN-02.21for details:
8
K = ∆IMOD/ ∆IBIAS
This provides a first-order approximation of the currentincrease needed to maintain peak-to-peak power.Slope efficiency decreases more rapidly as tempera-ture increases. The MAX3738 provides additional tem-perature compensation as temperature increases pasta user-defined threshold (TTH).
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1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
VCCPOR AND COUNTER60ms DELAYTX_DISABLEIMODENABLECOUNTER100ns DELAYI60ms DELAYBIASENABLEVCCIMD1VREFPC_MONRQRPC_MONVCCCOMPRSIBIASLATCH82VREFBC_MONSSHUTDOWNRBC_MONCOMPCMOSEXCESSIVEAPC CURRENTSETPOINTTX_FAULTEXCESSIVEMOD CURRENTTTLSETPOINTOPEN COLLECTORFigure 5. Simplified Safety Circuit
Table 1. Typical Fault Conditions
1If any of the I/O pins are shorted to GND or VCC (single-point failure; see Table 2), and the bias current or the photocurrentexceeds the programmed threshold.2End-of-life (EOL) condition of the laser diode. The bias current and/or the photocurrent exceed the programmed threshold.3Laser cathode is grounded and photocurrent exceeds the programming threshold.4No feedback for the APC loop (broken interconnection, defective monitor photodiode), and the bias current exceeds theprogrammed threshold._______________________________________________________________________________________9
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1Gbps to 2.7Gbps SFF/SFP Laser Driver withExtinction Ratio ControlMAX3738Table 2. Circuit Responses to Various Single-Point Faults
PINTX_FAULTTX_DISABLEIN+CIRCUIT RESPONSE TO OVERVOLTATGE ORSHORT TO VCCDoes not affect laser power.Modulation and bias currents are disabled.The optical average power increases, and a fault occursif VPC_MON exceeds the threshold. The APC loopresponds by decreasing the bias current.The optical average power decreases and the APC loopresponds by increasing the bias current. A fault stateoccurs if VBC_MON exceeds the threshold voltage.This disables bias current. A fault state occurs.Does not affect laser power. If the shutdown circuitry isused, the laser current is disabled.In this condition, the laser forward voltage is 0V and nolight is emitted.The APC circuit responds by increasing the bias currentuntil a fault is detected; then a fault state* occurs.Does not affect laser power.Fault state* occurs.Fault state* occurs.IBIAS increases until VBC_MON exceeds the thresholdvoltage.IBIAS increases until VBC_MON exceeds the thresholdvoltage.Does not affect laser power.Does not affect laser power.CIRCUIT RESPONSE TO UNDERVOLTAGE ORSHORT TO GROUNDDoes not affect laser power.Normal condition for circuit operation.The optical average power decreases, and the APC loopresponds by increasing the bias current. A fault stateoccurs if VBC_MON exceeds the threshold voltage.The optical average power increases and a fault occursif VPC_MON exceeds the threshold. The APC loopresponds by decreasing the bias current.The APC circuit responds by increasing the bias currentuntil a fault is detected; then a fault* state occurs.Does not affect laser power.Fault state* occurs. If the shutdown circuitry is used, thelaser current is disabled.Fault state* occurs. If the shutdown circuitry is used, thelaser current is disabled.Does not affect laser power.Does not affect laser power.Does not affect laser power.IBIAS increases until VBC_MON exceeds the thresholdvoltage.IBIAS increases until VBC_MON exceeds the thresholdvoltage.Fault state* occurs.Fault state* occurs.IN-MDSHUTDOWNBIASOUT+OUT-PC_MONBC_MONAPCFILT1APCFILT2MODSETAPCSET*A fault state asserts the TX_FAULT pin, disables the modulation and bias currents, and asserts the SHUTDOWN pin.
Safety Circuitry
The safety circuitry contains a disable input (TX_DISABLE), a latched fault output (TX_FAULT), andfault detectors (Figure5). This circuitry monitors theoperation of the laser driver and forces a shutdown if afault is detected (Table1). The TX_FAULT pin shouldbe pulled high with a 4.7kΩto 10kΩresistor to VCCasrequired by the SFP MSA. A single-point fault can be ashort to VCCor GND. See Table2 to view the circuitresponse to various single-point failure. The transmitfault condition is latched until reset by a toggle orTX_DISABLE or VCC. The laser driver offers redundantlaser diode shutdown through the optional shutdowncircuitry as shown in the Typical Applications Circuit.This shutdown transistor prevents a single-point fault atthe laser from creating an unsafe condition.
Safety Circuitry Current Monitors
The MAX3738 features monitors (BC_MON, PC_MON)for bias current (IBIAS) and photocurrent (IMD). Themonitors are realized by mirroring a fraction of the cur-rents and developing voltages across external resistorsconnected to ground. Voltages greater than VREFatPC_MON or BC_MON result in a fault state. For exam-ple, connecting a 100Ωresistor to ground at each mon-itor output gives the following relationships:
VBC_MON= (IBIAS/ 82) x 100Ω
VPC_MON= IMDx 100Ω
External sense resistors can be used for high-accuracymeasurement of bias and photodiode currents. On-chipisolation resistors are included to reduce the number ofcomponents needed to implement this function.
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1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
Table 3. Optical Power RelationsThe laser driver automatically adjusts the bias to main-tain the constant average power. For DC-coupledPARAMETERSYMBOLRELATIONlaser diodes:
Average PowerPAVGPAVG = (P0 + P1) / 2IAVG= IBIAS+ IMOD/ 2
Extinction Ratiorere = P1 / P0Optical Power of a OneP1P1 = 2PAVG x re / (re + 1)Programming the Modulation Current with
Optical Power of a ZeroPCompensation
0P0 = 2PAVG / (re + 1)Determine the modulation current from the laser slopeOptical AmplitudePP-PPP-P = P1 - P0efficiency:
Laser Slope Efficiencyηη = PP-P / IMODIMOD= 2 x PAVG/ ηx (re- 1) / (re+ + 1)
Modulation CurrentIMODIMOD = PP-P / ηThe modulation current of the MAX3738 consists of aThreshold CurrentITHP0 at I ≥ ITHstatic modulation current (IBias Current, and a current proportional to temperature.MODS), a current proportion-al to I(AC-Coupled)IBIASIBIAS ≥ ITH + IMOD / 2The portion of IBIASan internal current regulator, which maintains the refer-MODset by MODSET is established byLaser to MonitorTransferρMONIMD / PAVGence voltage of VREFacross the external programmingresistor. See the IMODvs. RMODSETgraph in theNote:Assuming a 50% average input duty cycle and mark Typical Operating Characteristicsand select the valuedensity.
of RMODSET that corresponds to the required current at +25°C:
Design Procedure
IMOD= IMODS+ K x IBIAS+ IMODT
When designing a laser transmitter, the optical output isIusually expressed in terms of average power andMODS= 268 x VREF/ RMODSETextinction ratio. Table3 shows relationships that areIMODT= TC x (T - TTH) | T > TTHhelpful in converting between the optical averageIMODT= 0 | T BIAS. Open circuiting the MODBCOMPpin can turn off the interaction between IBIASand IMOD: For a desired laser average optical power (PAVG) andK = 1700 / (1000 + Roptical extinction ratio (rMODBCOMP) ±10% lation currents can be calculated using the equations ine), the required bias and modu-If IMODmust be increased from IMOD1to IMOD2toTable3. Proper setting of these currents requiresmaintain the extinction ratio at elevated temperatures,knowledge of the laser to monitor transfer (ρthe required compensation factor is: η). MON) andslope efficiency (K = (IMOD2- IMOD1) / (IBIAS2- IBIAS1)Programming the Monitor-Diode Current A threshold for additional temperature compensationSet Point can be set with a programming resistor at theThe MAX3738 operates in APC mode at all times. TheTH_TEMP pin: bias current is automatically set so average laser powerTTH= -70°C + 1.45MΩ/ (9.2kΩ+ RTH_TEMP)°C ±10%is determined by the APCSET resistor: The temperature coefficient of thermal compensationPAVG= IMD / ρMON above TTHis set by RMODTCOMP. Leaving the The APCSET pin controls the set point for the monitorMODTCOMP pin open disables additional thermaldiode current. An internal current regulator establishescompensation: the APCSET current in the same manner as the TC = 1 / (0.5 + RMODTCOMP(kΩ)) mA/°C ±10% MODSET pin. See the IMDvs. RAPCSETgraph in theTypical Operating Characteristicsand select the valueof RAPCSETthat corresponds to the required currentat +25°C. IMD= 1/2 x VREF/ RACPSET ______________________________________________________________________________________11 MAX3738元器件交易网www.cecb2b.com 1Gbps to 2.7Gbps SFF/SFP Laser Driver withExtinction Ratio ControlMAX3738Current Compliance (IMOD≤60mA), DC-Coupled The minimum voltage at the OUT+ and OUT- pinsis 0.7V.For: VDIODE= Diode bias point voltage (1.2V typ)RL= Diode bias point resistance (5Ωtyp)RD= Series matching resistor (20Ωtyp)For compliance: VOUT+= VCC- VDIODE- IMODx (RD+ RL) - IBIASx RL≥0.7V Maxim’s Laser Drivers to Laser Diodes for more informa-tion on AC-coupling laser drivers to laser diodes.For compliance: VOUT+= VCC- IMOD / 2 x (RD+ RL) ≥0.75VThe APC loop filter capacitor (CAPC) must be selectedto balance the requirements for fast turn-on and mini-mal interaction with low frequencies in the data pattern.The low-frequency cutoff is: CAPC(µF) ≅68 / (f3dB(kHz) x (ηx ρMON)1.1)High-frequency noise can be filtered with an additionalcap, CMD, from the MD pin to ground. CMD≅CAPC / 4The MAX3738 is designed so turn-on time is faster than1ms for most laser gain values (ηx ρMON). Choosing asmaller value of CAPCreduces turn-on time. Carefulbalance between turn-on time and low-frequency cutoffmay be needed at low data rates for some values oflaser gain. Determine CAPC Current Compliance (IMOD> 60mA), AC-Coupled For applications requiring modulation current greaterthan 60mA, headroom is insufficient from proper opera-tion of the laser driver if the laser is DC-coupled. Toavoid this problem, the MAX3738’s modulation outputcan be AC-coupled to the cathode of a laser diode. Anexternal pullup inductor is necessary to DC-bias themodulation output at VCC. Such a configuration isolateslaser forward voltage from the output circuitry and allowsthe output at OUT+ to swing above and below the sup-ply voltage (VCC). When AC-coupled, the MAX3738modulation current can be programmed up to 85mA.Refer to Maxim Application Note HFAN 02.0:Interfacing Interface Models Figures 6 and 7 show simplified input and output cir-cuits for the MAX3738 laser driver. If dice are used,replace package parasitic elements with bondwire par-asitic elements. VCCVCCMAX3738PACKAGE16kΩVCC0.7nHOUT-PACKAGE0.7nHIN+0.11pF5kΩ0.11pF0.7nHOUT+VCC5kΩ0.7nHIN-0.11pF0.11pFMAX373824kΩFigure 6. Simplified Input Structure12 Figure 7. Simplified Output Structure ______________________________________________________________________________________ 元器件交易网www.cecb2b.com 1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control Layout Considerations port or sustain life, or for any other application where theTo minimize loss and crosstalk, keep the connectionsfailure of a Maxim product could create a situationbetween the MAX3738 output and the laser diode aswhere personal injury or death may occur. short as possible. Use good high-frequency layouttechniques and multilayer boards with uninterruptedExposed-Pad (EP) Package ground plane to minimize EMI and crosstalk. CircuitThe exposed pad on the 24-pin TQFN provides a veryboards should be made using low-loss dielectrics. Uselow thermal resistance path for heat removal from the IC.controlled-impedance lines for data inputs, as well asThe pad is also electrical ground on the MAX3738 andthe module output. should be soldered to the circuit board ground for properthermal and electrical performance. Refer to MaximLaser Safety and IEC 825 Application Note HFAN-08.1:Thermal Consideration Using the MAX3738 laser driver alone does not ensurefor QFN and Other Exposed-Pad Packages at that a transmitter design is IEC 825 compliant. Thewww.maxim-ic.com for additional information. entire transmitter circuit and component selections mustbe considered. Each customer must determine the levelChip Information of fault tolerance required by their application, recogniz-TRANSISTOR COUNT: 1884ing that Maxim products are not designed or authorizedfor use as components in systems intended for surgicalPROCESSS: SiGe/Bipolar implant into the body, for applications intended to sup- Typical Application Circuit +3.3VOPTIONAL SHUTDOWNCIRCUITRY+3.3VELLTCNBUCAVWO0.1µFSFADID_T_XUTIN+XHµFTS15Ω+3.3V0.01CDR0.1µFOUT-IN-10ΩOUT+RMODBCOMPMODBCOMPMAX3738BIASRMODTCOMPFERRITE BEADMODTCOMPRTH_TEMPMDTH_TEMPCMDT12NNETSELTLTOOIIDDSFFMMNOCCC__GMPPPCCAAABPTNNETSEOODSCCAPCMMOP__MACCRRBPRRREPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE.______________________________________________________________________________________13MAX3738元器件交易网www.cecb2b.com 1Gbps to 2.7Gbps SFF/SFP Laser Driver withExtinction Ratio ControlMAX3738Package Information (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,go to www.maxim-ic.com/packages.)PACKAGE OUTLINE12,16,20,24L QFN THIN, 4x4x0.8 mm21-0139B12PACKAGE OUTLINE12,16,20,24L QFN THIN, 4x4x0.8 mm21-0139B22Maxim 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. 14____________________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. 24L QFN THIN.EPS 因篇幅问题不能全部显示，请点此查看更多更全内容