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19-2818; Rev 1; 8/03
KIT ATION EVALU ABLE AVAIL
Multirate Laser Driver with Extinction Ratio Control
General Description Features
o Single +3.3V Power Supply o 47mA Power-Supply Current o 85mA Modulation Current o 100mA Bias Current o Automatic Power Control (APC) o Modulation Compensation o On-Chip Temperature Compensation o Self-Biased Inputs for AC-Coupling o Ground-Referenced Current Monitors o Laser Safety, Shutdown, and Alarm Outputs
MAX3737
The MAX3737 is a +3.3V laser driver designed for multirate transceiver modules with data rates from 155Mbps to 2.7Gbps. Lasers can be DC-coupled to the MAX3737 for reduced component count and ease of multirate operation. Laser extinction ratio control (ERC) combines the features of automatic power control (APC), modulation compensation, and built-in thermal compensation. The APC loop maintains constant average optical power. Modulation compensation increases the modulation current in proportion to the bias current. These control loops combined with thermal compensation maintain a constant optical extinction ratio over temperature and lifetime. The MAX3737 accepts differential data input signals. The wide 5mA to 60mA (up to 85mA AC-coupled) modulation current range and up to 100mA bias current range makes the MAX3737 ideal for driving FP/DFB lasers in fiber-optic modules. External resistors set the required laser current levels. The MAX3737 provides transmit disable control (TX_DISABLE), single-point fault tolerance, bias-current monitoring, modulation-current monitoring, and photocurrent monitoring. The device also offers a latched failure output (TX_FAULT) to indicate faults, such as when the APC loop is no longer able to maintain the average optical power at the required level. The MAX3737 is compliant with the SFF8472 transmitter diagnostic and SFP MSA timing requirements. The MAX3737 is offered in a 5mm x 5mm 32-pin thin QFN and QFN package and operates over the -40C to +85C extended temperature range.
Ordering Information
PART MAX3737ETJ MAX3737EGJ TEMP RANGE -40C to +85C -40C to +85C PIN-PACKAGE 32 Thin QFN 32 QFN
Pin Configurations
31 TH_TEMP 27 APCFILT2 26 APCFILT1 29 MODSET 28 APCSET
TOP VIEW
30 MODBCOMP
32 MODTCOMP
25 VMD 24 23 22 21
GND TX_DISABLE
VCC
1 2 3 4 5 6 7 8 10 11 12 13 14 15 16 9
MD
VCC
Applications
Multirate OC-3 to OC-48 FEC Transceivers Gigabit Ethernet SFF/SFP and GBIC Transceivers 1Gbps/2Gbps Fibre Channel SFF/SFP and GBIC Transceivers
OUT+ OUT+ OUTOUTVCC
IN+ INVCC
MAX3737EGJ
20 19 18 17
PC_MON BC_MON
BIAS
GND
VCC
MC_MON
TX_FAULT
Functional Diagram and Typical Application Circuit appear at end of data sheet.
5mm x 5mm QFN
*THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY GROUND TO ACHIEVE SPECIFIED PERFORMANCE.
Pin Configurations continued at end of data sheet. ________________________________________________________________ Maxim Integrated Products 1
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.
SHUTDOWN
GND
GND
VBS
Multirate Laser Driver with Extinction Ratio Control MAX3737
ABSOLUTE MAXIMUM RATINGS
Supply Voltage VCC...............................................-0.5V to +6.0V IN+, IN-, TX_DISABLE, TX_FAULT, SHUTDOWN, MC_MON, BC_MON, PC_MON, VBS, VMD, APCFILT1, APCFILT2, MD, TH_TEMP, MODTCOMP, MODBCOMP, MODSET, and APCSET Voltage .......................................-0.5V to VCC + 0.5V OUT+, OUT-, BIAS Current.............................-20mA to +150mA Continuous Power Dissipation (TA = +85C) 32-Pin QFN (derate 21.2mW/C above +85C) ...............1.3W Operating Junction Temperature Range ...........-55C to +150C Storage Temperature Range .............................-55C to +150C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VCC = +2.97V to +3.63V, TA = -40C to +85C. Typical values are at VCC = +3.3V, IBIAS = 60mA, IMOD = 60mA, TA = +25C, unless otherwise noted.) (Notes 1, 2)
PARAMETER POWER SUPPLY Supply Current Power-Supply Noise Rejection I/O SPECIFICATIONS Differential Input Swing Common-Mode Input LASER BIAS Bias-Current Setting Range Bias Off Current Bias-Current Monitor Ratio LASER MODULATION Modulation-Current Setting Range Output Edge Speed Output Overshoot/Undershoot Random Jitter IMOD (Note 5) 20% to 80% (Notes 6, 7) (Note 7) (Notes 6, 7) 2.7Gbps 1.25Gbps Deterministic Jitter (Notes 6, 8) 622Mbps 155Mbps Modulation-Current Temperature Stability (Note 6) 5mA IMOD 10mA 10mA < IMOD 85mA 5mA IMOD 10mA 10mA < IMOD 85mA 5mA IMOD 10mA 10mA < IMOD 85mA 5mA IMOD 10mA 10mA < IMOD 85mA 5mA IMOD 10mA 10mA < IMOD 85mA 5 71 52 6 0.65 25.6 16 32 15 39 21 65 46 150 1.3 40 40 41 41 46 46 100 70 480 ppm/C psP-P 85 80 80 mA ps % ps TX_DISABLE = high IBIAS/IBC_MON 68 82 1 100 0.1 95 mA mA mA/mA VID VCM DC-coupled, Figure 1 0.2 1.7 2.4 VCC VID/4 VP-P V ICC PSNR (Note 3) f 1MHz, 100mVP-P (Notes 4, 6) 47 33 60 mA dB SYMBOL CONDITIONS MIN TYP MAX UNITS
2
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Multirate Laser Driver with Extinction Ratio Control
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +2.97V to +3.63V, TA = -40C to +85C. Typical values are at VCC = +3.3V, IBIAS = 60mA, IMOD = 60mA, TA = +25C, unless otherwise noted.) (Notes 1, 2)
PARAMETER Modulation-Current Setting Error Modulation Off Current Modulation-Current Monitor Ratio EXTINCTION RATIO CONTROLS Monitor-Diode Input Current Range MD Pin Voltage MD-Current Monitor Ratio APC Loop Time Constant APC Setting Stability APC Setting Accuracy IMOD Compensation Setting Range by Bias IMOD Compensation Setting Range by Temperature Threshold Setting Range for Temperature Compensation LASER SAFETY AND CONTROL Bias and Modulation Turn-Off Delay Bias and Modulation Turn-On Delay Threshold Voltage at Monitor Pins INTERFACE SIGNALS TX_DISABLE Input High TX_DISABLE Input Low TX_DISABLE Input Current TX_FAULT Output Low Shutdown Output High Shutdown Output Low VHI VLO RPULL = 7.5k VHI = VCC VLO = GND Sinking 1mA, open collector Sourcing 100A Sinking 100A VCC 0.4 0.4 -450 2.0 0.8 15 -800 0.4 V V A V V V VREF CAPC_FILT = 0.01F, IMD/IBIAS = 1/80 (Note 6) CAPC_FILT = 0.01F, IMD/IBIAS = 1/80 (Note 6) Figure 5 1.14 1.3 5 600 1.39 s s V K TC TTH TA = +25C K = IMOD/IBIAS TC = IMOD/ (Note 6) (Note 6) 0 0 10 IMD/IPC_MON CAPC_FILT = 0.01F, IMD/IBIAS = 1/70 0.85 1.0 3.3 100 480 15 1.5 1.0 60 IMD Average current into the MD pin 18 1500 1.4 1.15 A V mA/mA s ppm/C % mA/mA mA/C C SYMBOL 15 load, TA = +25C TX_DISABLE = high IMOD/IMC_MON 223 262 CONDITIONS 5mA IMOD 10mA 10mA < IMOD 85mA MIN TYP MAX 20 15 0.1 302 UNITS % mA mA/mA
MAX3737
Note 1: Note 2: Note 3: Note 4: Note 5: Note 6: Note 7: Note 8:
AC characterization is performed using the circuit in Figure 2 using a PRBS 223 - 1 or equivalent test pattern. Specifications at -40C are guaranteed by design and characterization. Excluding IBIAS and IMOD. Input data is AC-coupled. TX_FAULT open, SHUTDOWN open. Power-supply noise rejection (PSNR) = 20log10(Vnoise (on VCC)/VOUT). VOUT is the voltage across the 15 load when IN+ is high. The minimum required voltage at the OUT+ and OUT- pins is +0.75V. Guaranteed by design and characterization. Tested with 00001111 pattern at 2.7Gbps. DJ includes pulse-width distortion (PWD). _______________________________________________________________________________________ 3
Multirate Laser Driver with Extinction Ratio Control MAX3737
Typical Operating Characteristics
(VCC = +3.3V, CAPC = 0.01F, IBIAS = 20mA, IMOD = 30mA, TA = +25C, unless otherwise noted.)
OPTICAL EYE DIAGRAM (2.7Gbps, 27 - 1PRBS, 2.3GHz FILTER)
MAX3737 toc01
OPTICAL EYE DIAGRAM (1.25Gbps, 27 - 1PRBS, 940MHz FILTER)
1310nm FP LASER Er = 8.2dB
MAX3737 toc02 MAX3737 toc04
1310nm FP LASER Er = 8.2dB
54ps/div
116ps/div
OPTICAL EYE DIAGRAM (155Mbps, 27 - 1PRBS, 117MHz FILTER, CAPC = 0.1F)
MAX3737 toc03
ELECTRICAL EYE DIAGRAM (IMOD = 30mA, 2.7Gbps, 27 - 1PRBS)
75mV/div
920ps/div
52ps/div
SUPPLY CURRENT (ICC) vs. TEMPERATURE (EXCLUDES BIAS AND MODULATION CURRENTS)
MAX3737 toc05
BIAS-CURRENT MONITOR GAIN vs. TEMPERATURE
MAX3737 toc06
PHOTO-CURRENT MONITOR GAIN vs. TEMPERATURE
1.15 IMD/IPC_MON (mA/mA) 1.10 1.05 1.00 0.95 0.90 0.85 0.80
MAX3737 toc07
65 60 SUPPLY CURRENT (mA) 55 VCC = 3.63V 50 45 40 35 -40 -20 0 20 40 60 80 TEMPERATURE (C) VCC = 2.97V VCC = 3.3V IMOD = 60mA IBIAS = 60mA
90 88 86 IBIAS/IBC_MON (mA/mA) 84 82 80 78 76 74 72 70 -40 -15 10 35 60
1.20
85
-40
-15
10
35
60
85
TEMPERATURE (C)
TEMPERATURE (C)
4
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Multirate Laser Driver with Extinction Ratio Control
Typical Operating Characteristics (continued)
(VCC = +3.3V, CAPC = 0.01F, IBIAS = 20mA, IMOD = 30mA, TA = +25C, unless otherwise noted.)
MODULATION-CURRENT MONITOR GAIN vs. TEMPERATURE
290 280 IMOD/IMC_MON (mA/mA) 270 IMOD (mA) 260 250 240 230 220 210 200 -40 -15 10 35 60 85 TEMPERATURE (C)
MAX3737 toc08
MAX3737
MODULATION CURRENT vs. RMODSET
MAX3737 toc09
PHOTODIODE CURRENT vs. RAPCSET
1.4 1.2 IMD (mA) 1.0 0.8 0.6 0.4 0.2 0
MAX3737 toc10
300
90 80 70 60 50 40 30 20 10 0 1 10 RMODSET (k)
1.6
100
0.1
1 RAPCSET (k)
10
100
DETERMINISTIC JITTER vs. MODULATION CURRENT
MAX3737 toc11
RANDOM JITTER vs. MODULATION CURRENT
MAX3737 toc12
50 45 40 35
2.0
1.5 RJ (psRMS)
DJ (psP-P)
30 25 20 15 10 5 0 0 10 20 30 40 50 60 70 80 90 IMOD (mA)
1.0
0.5
0 0 20 40 60 80 100 IMOD (mA)
COMPENSATION (K) vs. RMODBCOMP
MAX3737 toc13
TEMPERATURE COMPENSATION vs. RTH_TEMP (RMODTCOMP = 500)
RTH_TEMP = 12k 90 80 RTH_TEMP = 7k RTH_TEMP = 4k RTH_TEMP = 2k 50 40
MAX3737 toc14
10
100
1 K (mA/mA) IMOD (mA) 0.1 0.01 0 0.1 1 RMODBCOMP (k) 10 100 70 60
30 -20 0 20 40 60 80 100 TEMPERATURE (C)
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5
Multirate Laser Driver with Extinction Ratio Control MAX3737
Typical Operating Characteristics (continued)
(VCC = +3.3V, CAPC = 0.01F, IBIAS = 20mA, IMOD = 30mA, TA = +25C, unless otherwise noted.)
TEMPERATURE COMPENSATION vs. RTH_TEMP (RMODTCOMP = 10k)
44 42 40 IMOD (mA) 38 RTH_TEMP = 2k 36 34 32 30 -20 0 20 40 60 80 100 TEMPERATURE (C) LASER OUTPUT 20ms/div RTH_TEMP = 12k RTH_TEMP = 7k RTH_TEMP = 4k FAULT TX_DISABLE LOW t_init = 60ms
MAX3737 toc15
HOT PLUG WITH TX_DISABLE LOW
MAX3737 toc16
3.3V VCC 0V
LOW
TRANSMITTER ENABLE
MAX3737 toc17
TRANSMITTER DISABLE
MAX3737 toc18
VCC
3.3V
3.3V VCC
FAULT
LOW t_on = 75s LOW
FAULT
LOW
TX_DISABLE
HIGH
TX_DISABLE LASER OUTPUT
LOW
t_off = 134ns HIGH
LASER OUTPUT 20s/div
40ns/div
RESPONSE TO FAULT
MAX3737 toc19
FAULT RECOVERY TIME
MAX3737 toc20
VPC_MON
EXTERNALLY FORCED FAULT t_fault = 0.9s
VPC_MON EXTERNAL FAULT REMOVED FAULT HIGH TX_DISABLE LOW LASER OUTPUT HIGH HIGH LOW
t_init = 68ms
FAULT
LOW LOW
LOW
TX_DISABLE
LASER OUTPUT
1s/div
100ms/div
6
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Multirate Laser Driver with Extinction Ratio Control
Pin Description
PIN 1, 10, 15, 16 2 3, 6, 11, 18, 23 4 5 7 8 9 12 13 14 17 19, 20 21, 22 24 25 26 27 28 29 30 31 NAME GND TX_DISABLE VCC IN+ INPC_MON BC_MON MC_MON TX_FAULT SHUTDOWN VBS BIAS OUTOUT+ MD VMD APCFILT1 APCFILT2 APCSET MODSET MODBCOMP TH_TEMP Ground Transmitter Disable, TTL. Laser output is disabled when TX_DISABLE is asserted high or left unconnected. The laser ouput is enabled when this pin is asserted low. +3.3V Supply Voltage Noninverted Data Input Inverted Data Input Photodiode-Current Monitor Output. Current out of this pin develops a ground-referenced voltage across 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 an external resistor that is proportional to the bias current. Modulation-Current Monitor Output. Current out of this pin develops a ground-referenced voltage across an external resistor that is proportional to the modulation current amplitude. Open-Collector Transmit Fault Indicator (Table 1) Shutdown Driver Output. Voltage output to control an external transistor for optional shutdown circuitry. Bias Voltage Sense. Isolated tap (3k 15%) on the bias output reduces component count when a precision bias sense resistor is used. Laser Bias-Current Output Inverted Modulation-Current Output (Connect Pins 19 and 20 Together). IMOD flows into this pin when input data is low. Noninverted Modulation-Current Output (Connect Pins 21 and 22 Together). 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 to ground is required to filter the high-speed AC monitor photocurrent. Monitor Photodiode Voltage Sense. Isolated tap (3k 15%) on the MD input reduces component count when a precision photodiode current-sense resistor is used. Connect a capcitor (CAPC) between pin 26 (APCFILT1) and pin 27 (APCFILT2) to set the dominant pole of the APC feedback loop. (See Pin 26.) 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 the modulation 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, above which compensation is added to the modulation current. FUNCTION
MAX3737
32
Modulation-Current Compensation from Temperature. A resistor at this pin sets the temperature MODTCOMP coefficient of the modulation current when above the threshold temperature. Leave open for zero temperature compensation. Exposed Pad Ground. Solder the exposed pad to the circuit board ground for specified thermal and electrical performance.
EP
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7
Multirate Laser Driver with Extinction Ratio Control MAX3737
VOLTAGE VIN+ VINSINGLE ENDED 100mV (MIN) 1200mV (MAX)
VCC 30 MAX3737 OUTZ0 = 30 30 0.5pF OUT+ OUT+ IOUT+ Z0 = 30 75 Z0 = 50 50 VCC 30
(VIN+) - (VIN-)
DIFFERENTIAL 200mVP-P (MIN), 2400mV (MAX)
OUT-
CURRENT IOUT+ IMOD
TIME
OSCILLOSCOPE
Figure 1. Required Input Signal and Output Polarity
Figure 2. Test Circuit for Characterization
HOST BOARD FILTER DEFINED BY SFP MSA L1 1H MODULE TO LASER DRIVER VCC OPTIONAL C3 0.1F OPTIONAL
SOURCE NOISE VOLTAGE SUPPLY
C1 0.1F
C2 10F
Figure 3. Supply Filter
Detailed Description
The MAX3737 laser driver consists of three main parts: a high-speed modulation driver, biasing block with ERC, and safety circuitry. The circuit design is optimized for high-speed, low-voltage (+3.3V) operation (Figure 4).
At data rates of 2.7Gbps, any capacitive load at the cathode of a laser diode degrades optical output performance. Because the BIAS output is directly connected to the laser cathode, minimize the parasitic capacitance associated with the pin by using an inductor to isolate the BIAS pin parasitics from the laser cathode.
High-Speed Modulation Driver
The output stage is composed of a high-speed differential pair and a programmable modulation current source. The MAX3737 is optimized for driving a 15 load. The minimum instantaneous voltage required at OUT+ is 0.7V for modulation current up to 60mA and 0.75V for currents from 60mA to 85mA. Operation above 60mA can be accomplished by AC-coupling or with sufficient voltage at the laser to meet the driver output voltage requirement. To interface with the laser diode, a damping resistor (RD) is required. The combined resistance due to the series damping resistor and the equivalent series resistance (ESR) of the laser diode should equal 15. To further damp aberrations caused by laser diode parasitic inductance, an RC shunt network may be necessary. Refer to Maxim Application Note HFAN 02.0: Interfacing Maxim's Laser Drivers to Laser Diodes for more information.
8
Extinction Ratio Control
The extinction ratio (r e ) is the laser on-state power divided by the off-state power. Extinction ratio remains constant if peak-to-peak and average power are held constant: re = (2PAVG + PP-P) / (2PAVG - PP-P) Average power is regulated using APC, which keeps constant current from a photodiode coupled to the laser. Peak-to-peak power is maintained by compensating the modulation current for reduced slope efficiency () of the laser over time and temperature: PAVG = IMD MON
PP-P = x IMOD
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Multirate Laser Driver with Extinction Ratio Control MAX3737
VCC
SHUTDOWN
MAX3737
IN+ ININPUT BUFFER DATA PATH IMOD OUTOUT+ RD
SHUTDOWN TX_FAULT TX_DISABLE RPULL = 7.5k IMD 1 PC_MON RMDMON BC_MON RBC_MON MC_MON RMC_MON IMOD 268
IMOD ENABLE SAFETY LOGIC AND POWER IBIAS ENABLE DETECTOR
BIAS VCC IBIAS
VCC
IBIAS VBG xTC x268 xK APCSET RAPCSET
IBIAS 82 T > TH T
X1/2 IAPCSET MD IMD X1 CMD
VBG TH_TEMP RTH_TEMP MODTCOMP RMODTCOMP MODSET RMODSET MODBCOMP RMODBCOMP APCFILT1 CAPC APCFILT2
Figure 4. Functional Diagram
Modulation compensation from bias increases the modulation current by a user-selected proportion (K) needed to maintain peak-to-peak laser power as bias current increases with temperature. Refer to Maxim Application Note HFAN-02.2.1 for details: K= IMOD IBIAS
Safety Circuitry
The safety circuitry contains a disable, input (TX_DISABLE), a latched fault output (TX_FAULT), and fault detectors (Figure 5). This circuitry monitors the operation of the laser driver and forces a shutdown if a fault is detected (Table 1). The TX_FAULT pin should be pulled high with a 4.7k to 10k resistor to VCC as required by the SFP MSA. A single-point fault can be a short to VCC or GND. See Table 2 to view the circuit response to various single-point failures. The transmit fault condition is latched until reset by a toggle of TX_DISABLE or VCC. The laser driver offers redundant laser diode shutdown through the optional shutdown circuitry as shown in the Typical Operating Circuit. This shutdown transistor prevents a single-point fault at the laser from creating an unsafe condition.
This provides a first-order approximation of the current increase needed to maintain peak-to-peak power. Slope efficiency decreases more rapidly as temperature increases. The MAX3737 provides additional temperature compensation as temperature increases past a user-defined threshold (TTH).
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9
Multirate Laser Driver with Extinction Ratio Control MAX3737
Table 1. Typical Fault Conditions
1 2 3 4 If any of the I/O pins is shorted to GND or VCC (single-point failure; see Table 2), and the bias current or the photocurrent exceed the programmed threshold. End-of-life (EOL) condition of the laser diode. The bias current and/or the photocurrent exceed the programmed threshold. Laser cathode is grounded and photocurrent exceeds the programming threshold. No feedback for the APC loop (broken interconnection, defective monitor photodiode), and the bias current exceeds the programmed threshold.
Table 2. Circuit Responses to Various Single-Point Faults
PIN TX_FAULT TX_DISABLE IN+ CIRCUIT RESPONSE TO OVERVOLTAGE OR SHORT TO VCC Does not affect laser power. Modulation and bias currents are disabled. The optical average power increases and a fault occurs if VPC_MON exceeds the threshold. The APC loop responds by decreasing the bias current. The optical average power decreases and the APC loop responds by increasing the bias current. A fault state occurs if VBC_MON exceeds the threshold voltage. This disables bias current. A fault state occurs. Does not affect laser power. If the shutdown circuitry is used, laser current is disabled. In this condition, laser forward voltage is 0V and no light is emitted. The APC circuit responds by increasing the bias current until a fault is detected, then a fault state* occurs. Does not affect laser power. Fault state* occurs. Fault state* occurs. Fault state* occurs. IBIAS increases until VBC_MON exceeds the threshold voltage. IBIAS increases until VBC_MON exceeds the threshold voltage. Does not affect laser power. Does not affect laser power. CIRCUIT RESPONSE TO UNDERVOLTAGE OR SHORT TO GROUND Does not effect laser power. Normal condition for circuit operation. The optical average power decreases and the APC loop responds by increasing the bias current. A fault state occurs if VBC_MON exceeds the threshold voltage. The optical average power increases and a fault occurs if VPC_MON exceeds the threshold. The APC loop responds by decreasing the bias current. The APC circuit responds by increasing bias current until a fault is detected, then a fault* state occurs. Does not affect laser power. Fault state* occurs. If the shutdown circuitry is used, laser current is disabled. Fault state* occurs. If the shutdown circuitry is used, laser current is disabled. Does not affect laser power. Does not affect laser power. Does not affect laser power. Does not affect laser power. IBIAS increases until VBC_MON exceeds the threshold voltage. IBIAS increases until VBC_MON exceeds the threshold voltage. Fault state* occurs. Fault state* occurs.
IN-
MD SHUTDOWN BIAS OUT+ OUTPC_MON BC_MON MC_MON APCFILT1 APCFILT2 MODSET APCSET
*A fault state asserts the TX_FAULT pin, disables the modulation and bias currents, and asserts the SHUTDOWN pin.
Safety Circuitry Current Monitors The MAX3737 features monitors (MC_MON, BC_MON, PC_MON) for modulation current (IMOD), bias current (IBIAS), and photocurrent (IMD). The monitors are realized
10
by mirroring a fraction of the currents and developing voltages across external resistors connected to ground. Voltages greater than VREF at MC_MON, PC_MON, or BC_MON result in a fault state. For example, connecting a
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Multirate Laser Driver with Extinction Ratio Control
100 resistor to ground at each monitor output gives the following relationships: VMC_MON = (IMOD / 268) 100 VBC_MON = (IBIAS / 82) 100 VPC_MON = IMD 100 External sense resistors can be used for high-accuracy measurement of bias and photodiode currents. On-chip isolation resistors are included to reduce the number of components needed to implement this function. The APCSET pin controls the set point for the monitordiode current. An internal current regulator establishes the APCSET current in the same manner as the MODSET pin. See the I MD vs. R APCSET graph in the Typical Operating Characteristics and select the value of RAPCSET that corresponds to the required current at +25C: IMD = 1 VREF x 2 RAPCSET
MAX3737
Design Procedure
When designing a laser transmitter, the optical output is usually expressed in terms of average power and extinction ratio. Table 3 gives relationships that are helpful in converting between the optical average power and the modulation current. These relationships are valid if the mark density and duty cycle of the optical waveform are 50%. For a desired laser average optical power (PAVG) and optical extinction ratio (re), the required bias and modulation currents can be calculated using the equations in Table 3. Proper setting of these currents requires knowledge of the laser to monitor transfer (MON) and slope efficiency ().
The laser driver automatically adjusts the bias to maintain the constant average power. For DC-coupled laser diodes: I IAVG = IBIAS + MOD 2
Programming the Modulation Current with Compensation
Determine the modulation current from the laser slope efficiency: IMOD = 2 x PAVG r -1 xe re + 1
Programming the Monitor Diode Current Set Point
The MAX3737 operates in APC mode at all times. The bias current is automatically set so average laser power is determined by the APCSET resistor: PAVG = IMD MON
The modulation current of the MAX3737 consists of a static modulation current (IMODS), a current proportional to IBIAS, and a current proportional to temperature. The portion of IMOD set by MODSET is established by an internal current regulator, which maintains the reference voltage of VREF across the external programming resistor. See to the IMOD vs. RMODSET graph in the Typical Operating Characteristics and select the value of RMODSET that corresponds to the required current at +25C:
Table 3. Optical Power Relations
PARAMETER Average power Extinction ratio Optical power of a 1 Optical power of a zero Optical amplitude Laser slope efficiency Modulation current Threshold current Bias current (AC-coupled) Laser to monitor transfer SYMBOL PAVG re P1 P0 PP-P IMOD ITH IBIAS MON RELATION PAVG = (P0 + P1) / 2 r e = P1 / P 0 P1 = 2PAVG re / (re + 1) P0 = 2PAVG / (re + 1) PP-P = P1 - P0 = PP-P / IMOD IMOD = PP-P / P0 at I ITH IBIAS ITH + IMOD / 2 IMD / PAVG
Note: Assuming a 50% average input duty cycle and mark density. ______________________________________________________________________________________ 11
Multirate Laser Driver with Extinction Ratio Control MAX3737
IMOD = IMODS + K x IBIAS + IMODT IMODS = 268 x VREF RMODSET | T TTH
Current Compliance (IMOD > 60mA), AC-Coupled
For applications requiring modulation current greater than 60mA, headroom is insufficient for proper operation of the laser driver if the laser is DC-coupled. To avoid this problem, the MAX3737's modulation output can be AC-coupled to the cathode of a laser diode. An external pullup inductor is necessary to DC-bias the modulation output at VCC. Such a configuration isolates laser forward voltage from the output circuitry and allows the output at OUT+ to swing above and below the supply voltage (V CC ). When AC-coupled, the MAX3737 modulation current can be programmed up to 85mA. Refer to Maxim Application Note HFAN 02.0: Interfacing Maxim's Laser Drivers to Laser Diodes for more information on AC-coupling laser drivers to laser diodes. For compliance: VOUT+ = VCC IMOD 2 x RD + RL
IMODT = TC x (T - TTH ) | T > TTH IMODT = 0
An external resistor at the MODBCOMP pin sets current proportional to IBIAS. Open circuiting the MODBCOMP pin can turn off the interaction between IBIAS and IMOD: K= 1700 10% 1000 + RMODBCOMP
If I MOD must be increased from I MOD1 to I MOD2 to maintain the extinction ratio at elevated temperature, the required compensation factor is: -I I K = MOD2 MOD1 IBIAS2 - IBIAS1 A threshold for additional temperature compensation can be set with a programming resistor at the TH_TEMP pin: TTH = - 70C + 1.45M C 10% 9.2k + RTH _ TEMP
(
)
0.75V
Determine CAPC
The APC loop filter capacitor CAPC must be selected to balance the requirements for fast turn-on and minimal interaction with low frequencies in the data pattern. The low-frequency cutoff is: C APC (F) 68 x ( x MON )1.1 f3DB (kHz)
The temperature coefficient of thermal compensation above TTH is set by RMODTCOMP. Leaving the MODTCOMP pin open disables additional thermal compensation: TC = mA 10% 0.5 + RMODTCOMP (k) C 1
High-frequency noise can be filtered with an additional cap CMD from the MD pin to ground: CMD CAPC 4
Current Compliance (IMOD 60mA), DC-Coupled
The minimum voltage at the OUT+ and OUT- pins is 0.7V. For: VDIODE--Diode bias point voltage (1.2V typ) RL--Diode bias point resistance (5 typ) RD--Series matching resistor (20 typ) For compliance:
VOUT+ = VCC - VDIODE - IMOD x RD + RL - IBIAS x RL 0.7V
The MAX3737 is designed so that turn-on time is faster than 1ms for most laser gain values ( MON ). Choosing a smaller value of C APC reduces turn-on time. Careful balance between turn-on time and low-frequency cutoff may be needed at low data rates for some values of laser gain.
Interface Models
Figures 6 and 7 show simplified input and output circuits for the MAX3737 laser driver. If dice are used, replace package parasitic elements with bondwire parasitic elements.
(
)
12
______________________________________________________________________________________
Multirate Laser Driver with Extinction Ratio Control MAX3737
VCC POR AND COUNTER 60ms DELAY
MAX3737 IMOD ENABLE 100ns DELAY IBIAS ENABLE
TX_DISABLE
COUNTER 60ms DELAY VCC IMD 1 VCC IBIAS 82 VCC IMOD 268 VREF S COMP R VREF Q
PC_MON RPC_MON
SHUTDOWN COMP RS LATCH CMOS TX_FAULT TTL OPEN COLLECTOR
BC_MON RBC_MON
MC_MON RMC_MON
VREF COMP EXCESSIVE MODULATION CURRENT
Figure 5. Simplified Safety Circuit
Layout Considerations
VCC
MAX3737
PACKAGE 0.83nH 0.11pF VCC
16k
To minimize loss and crosstalk, keep the connections between the MAX3737 output and the laser diode as short as possible. Use good high-frequency layout techniques and multilayer boards with uninterrupted ground plane to minimize EMI and crosstalk. Circuit boards should be made using low-loss dielectrics. Use controlled-impedance lines for data inputs, as well as the module output.
5k
Laser Safety and IEC 825
Using the MAX3737 laser driver alone does not ensure that a transmitter design is IEC 825 compliant. The entire transmitter circuit and component selections must be considered. Each customer must determine the level of fault tolerance required by their application, recognizing that Maxim products are not designed or authorized for use as components in systems intended for surgical implant into the body, for applications intended to support or sustain life, or for any other application where the failure of a Maxim product could create a situation where personal injury or death may occur.
13
VCC 5k 0.83nH 0.11pF 24k
Figure 6. Simplified Input Structure
______________________________________________________________________________________
Multirate Laser Driver with Extinction Ratio Control MAX3737
Exposed-Pad (EP) Package
The exposed-pad on the 32-pin QFN provides a very low thermal resistance path for heat removal from the IC. The pad is also electrical ground on the MAX3737 and should be soldered to the circuit board ground for proper thermal and electrical performance. Refer to Maxim Application Note HFAN-08.1: Thermal Considerations for QFN and Other Exposed Pad Packages at www.maximic.com for additional information.
Pin Configurations (continued)
31 TH_TEMP 27 APCFILT2 26 APCFILT1 29 MODSET 28 APCSET
TOP VIEW
30 MODBCOMP
32 MODTCOMP
25 VMD 24 23 22 21
GND TX_DISABLE
VCC
1 2 3 4 5 6 7 8 10 11 12 13 14 15 16 9
MD
VCC
OUT+ OUT+ OUTOUTVCC
VCC
IN+ INPACKAGE
MAX3737ETJ
20 19 18 17
VCC
PC_MON
0.82nH OUT0.11pF
BC_MON
BIAS
GND
VCC
MC_MON
0.82nH OUT+ 0.11pF
5mm x 5mm THIN QFN
*THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY GROUND TO ACHIEVE SPECIFIED PERFORMANCE.
MAX3737
Chip Information
TRANSISTOR COUNT: 2727 PROCESS: SiGe/Bipolar
Figure 7. Simplified Output Structure
14
______________________________________________________________________________________
SHUTDOWN
TX_FAULT
GND
GND
VBS
Multirate Laser Driver with Extinction Ratio Control
Typical Operating Circuit
+3.3V OPTIONAL SHUTDOWN CIRCUITRY
MAX3737
+3.3V
VCC
TX_DISABLE
SHUTDOWN
TX_FAULT
+3.3V 15 OUT10 OUT+
0.01F
0.1F IN+ CDR 0.1F INRMODBCOMP MODBCOMP RMODTCOMP MODTCOMP RTH_TEMP TH_TEMP
BIAS
FERRITE BEAD
MAX3737
MD CMD
APCFILT1
APCFILT2
MC_MON
BC_MON
MODSET
GND
APCSET
RMODSET
RAPCSET
CAPC
RMC_MON
RBC_MON
RPC_MON
REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE
______________________________________________________________________________________
PC_MON
15
Multirate Laser Driver with Extinction Ratio Control MAX3737
Package 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.)
32L QFN.EPS
16
______________________________________________________________________________________
Multirate Laser Driver with Extinction Ratio Control
Package Information (continued)
(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.)
QFN THIN.EPS
MAX3737
0.15 C A
D2
C L
D
b D2/2
0.10 M C A B
PIN # 1 I.D.
D/2
0.15 C B
k
PIN # 1 I.D. 0.35x45
E/2 E2/2 E (NE-1) X e
C L
E2
k L
DETAIL A
e (ND-1) X e
C L
C L
L
L
e 0.10 C A 0.08 C
e
C
A1 A3
PROPRIETARY INFORMATION TITLE:
PACKAGE OUTLINE 16, 20, 28, 32L, QFN THIN, 5x5x0.8 mm
APPROVAL DOCUMENT CONTROL NO. REV.
21-0140
C
1 2
COMMON DIMENSIONS
EXPOSED PAD VARIATIONS
NOTES: 1. DIMENSIONING & TOLERANCING CONFORM TO ASME Y14.5M-1994. 2. ALL DIMENSIONS ARE IN MILLIMETERS. ANGLES ARE IN DEGREES. 3. N IS THE TOTAL NUMBER OF TERMINALS. 4. THE TERMINAL #1 IDENTIFIER AND TERMINAL NUMBERING CONVENTION SHALL CONFORM TO JESD 95-1 SPP-012. DETAILS OF TERMINAL #1 IDENTIFIER ARE OPTIONAL, BUT MUST BE LOCATED WITHIN THE ZONE INDICATED. THE TERMINAL #1 IDENTIFIER MAY BE EITHER A MOLD OR MARKED FEATURE. 5. DIMENSION b APPLIES TO METALLIZED TERMINAL AND IS MEASURED BETWEEN 0.25 mm AND 0.30 mm FROM TERMINAL TIP. 6. ND AND NE REFER TO THE NUMBER OF TERMINALS ON EACH D AND E SIDE RESPECTIVELY. 7. DEPOPULATION IS POSSIBLE IN A SYMMETRICAL FASHION. 8. COPLANARITY APPLIES TO THE EXPOSED HEAT SINK SLUG AS WELL AS THE TERMINALS. 9. DRAWING CONFORMS TO JEDEC MO220. 10. WARPAGE SHALL NOT EXCEED 0.10 mm.
PROPRIETARY INFORMATION TITLE:
PACKAGE OUTLINE 16, 20, 28, 32L, QFN THIN, 5x5x0.8 mm
APPROVAL DOCUMENT CONTROL NO. REV.
21-0140
C
2 2
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 17 (c) 2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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