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19-3162; Rev 0; 1/04
1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control
General Description
The MAX3738 is a +3.3V laser driver designed for multirate transceiver modules with data rates from 1Gbps to 2.7Gbps. Lasers can be DC-coupled to the MAX3738 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 MAX3738 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, make the MAX3738 ideal for driving FP/DFB lasers in fiber optic modules. External resistors set the required laser current levels. The MAX3738 provides transmit disable control (TX_DISABLE), single-point fault tolerance, bias-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 MAX3738 is compliant with the SFF-8472 transmitter diagnostic and SFP MSA timing requirements. The MAX3738 is offered in a 4mm x 4mm, 24-pin thin QFN package and operates over the extended -40C to +85C temperature range.
MODBCOMP
Features
Single +3.3V Power Supply 47mA Power-Supply Current 85mA Modulation Current 100mA Bias Current Automatic Power Control (APC) Modulation Compensation On-Chip Temperature Compensation Self-Biased Inputs for AC-Coupling Ground-Referenced Current Monitors Laser Shutdown and Alarm Outputs Enable Control and Laser Safety Feature
MAX3738
Ordering Information
PART MAX3738ETG TEMP RANGE -40C to 85C PINPACKAGE 24 Thin QFN PKG CODE T2444-1
Typical Application Circuit appears at end of data sheet.
Pin Configuration
TH_TEMP
APCFILT2 20
Applications
Multirate OC-24 to OC-48 FEC Transceivers Gigabit Ethernet SFF/SFP and GBIC Transceivers 1Gbps/2Gbps Fibre Channel SFF/SFP and GBIC Transceivers
MODTCOMP 1 VCC 2 IN+ 3 IN- 4 VCC 5 TX_DISABLE 6
24
23
22
21
19 18 MD 17 VCC 16 OUT+
MAX3738
APCFILT1 15 OUT14 VCC 13 BIAS 12 GND
MODSET 9 SHUTDOWN
7 PC_MON
8 BC_MON
10 GND
APCSET
TOP VIEW
11 TX_FAULT
THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY GROUND ON THE CIRCUIT BOARD.
________________________________________________________________ Maxim Integrated Products
1
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1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3738
ABSOLUTE MAXIMUM RATINGS
Supply Voltage VCC...............................................-0.5V to +6.0V IN+, 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 +150mA Continuous Power Dissipation (TA = +85C) 24-Pin TQFN (derate 20.8mW/C above +85C) .......1805mW 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 Deterministic Jitter (Notes 6, 8) Modulation-Current Temperature Stability Modulation-Current-Setting Error Modulation Off Current Monitor-Diode Input Current Range MD Pin Voltage MD Current Monitor Ratio IMD / IPC_MON 0.85 0.93 IMOD (Note 5) 20% to 80% (Notes 6, 7) (Notes 6, 7) 2.7Gbps, 5mA IMOD 85mA 1.25Gbps, 5mA IMOD 85mA (Note 6) 15 load, TA = +25C TX_DISABLE = high 5mA IMOD 10mA 10mA IMOD 85mA 5mA IMOD 10mA 10mA < IMOD 85mA 5mA IMOD 85mA 5 65 6 0.62 18 20 175 125 1.3 40 41 600 480 20 15 0.1 85 80 mA ps % psRMS psP-P ppm/C % mA TX_DISABLE = high IBIAS / IBC_MON 68 79 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, 100mAP-P (Note 4) 47 33 60 mA dB SYMBOL CONDITIONS MIN TYP MAX UNITS
With 1pF between OUT+ and OUT-
AUTOMATIC POWER AND EXTINCTION RATIO CONTROLS IMD Average current into the MD pin 18 1500 1.4 1.15 A V mA/mA
2
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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 = -40C to +85C. Typical values are at VCC = +3.3V, IBIAS = 60mA, IMOD = 60mA, TA = +25C, unless otherwise noted.) (Notes 1, 2)
PARAMETER 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 = 45k (typ) VHI = VCC VLO = GND Sinking 1mA, open collector Sourcing 100A Sinking 100A
23
MAX3738
SYMBOL (Note 6) TA = +25C K TC TTH
CONDITIONS CAPC_FILT = 0.01F, IMD / IBIAS = 1/70
MIN
TYP 3.3 100
MAX 480 15
UNITS s ppm/C % mA/mA mA/C C
K = IMOD / IBIAS TC = IMOD / T (Note 6) (Note 6)
0 0 +10
1.5 1.0 +60
CAPC_FILT = 0.01F, IMD / IBIAS = 1/80 (Note 6) CAPC_FILT = 0.01F, IMD / IBIAS = 1/80 (Note 6) VREF Figure 5 1.14 2.0 1.3
5 600 1.39
s s V V
0.8 15 -70 VCC - 0.4 0.4 -140 0.4
V A V V V
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 2 - 1 or equivalent 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).
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1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3738
Typical Operating Characteristics
(VCC = +3.3V, CAPC = 0.01F, IBIAS = 20mA, IMOD = 30mA, TA = +25C, unless otherwise noted.)
OPTICAL EYE DIAGRAM (2.7Gbps, 27 - 1 PRBS, 2.3GHz FILTER)
MAX3738 toc01
OPTICAL EYE DIAGRAM (1.25Gbps, 27 - 1 PRBS, 940MHz FILTER)
MAX3738 toc02
ELECTRICAL EYE DIAGRAM (IMOD = 30mA, 2.7Gbps, 27 - 1 PRBS)
MAX3738 toc03
1310nm FP LASER re = 8.2dB
1310nm FP LASER re = 8.2dB
1pF BETWEEN OUT+ AND OUT-
75mV/div
54ps/div
116ps/div
52ps/div
SUPPLY CURRENT (ICC) vs. TEMPERATURE (EXCLUDES BIAS AND MODULATION CURRENTS)
MAX3738 toc04
BIAS-CURRENT MONITOR RATIO vs. TEMPERATURE
MAX3738 toc05
PHOTOCURRENT MONITOR RATIO vs. TEMPERATURE
1.15 IMD/IPC_MON (mA/mA) 1.10 1.05 1.00 0.95 0.90 0.85 0.80
MAX3738 toc06
60 55 SUPPLY CURRENT (mA) 50 45 2.97V 40 35 30 3.3V 3.63V
90 88 86 IBIAS/IBC_MON (mA/mA) 84 82 80 78 76 74 72 70
1.20
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 TEMPERATURE (C)
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 TEMPERATURE (C)
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 TEMPERATURE (C)
MODULATION CURRENT vs. RMODSET
MAX3738 toc07
PHOTODIODE CURRENT vs. RAPCSET
MAX3738 toc08
DETERMINISTIC JITTER vs. MODULATION CURRENT
45 40 35 DJ (psP-P) 30 25 20 15 10 2.7Gbps
MAX3738 toc09
90 80 70 60 IMOD (mA)
1.4 1.2 1.0 IMD (mA) 0.8 0.6 0.4 0.2 0
50
50 40 30 20 10 0 1 10 RMODSET (k) 100
5 0 0.1 1 RAPCSET (k) 10 100 0 10 20 30 40 50 60 70 80 90 IMOD (mA)
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1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3738
Typical Operating Characteristics (continued)
(VCC = +3.3V, CAPC = 0.01F, IBIAS = 20mA, IMOD = 30mA, TA = +25C, unless otherwise noted.)
RANDOM JITTER vs. MODULATION CURRENT
MAX3738 toc10
COMPENSATION (K) vs. RMODBCOMP
MAX3738 toc11
TEMPERATURE COMPENSATION vs. RTH_TEMP (RMODTCOMP = 500)
MAX3738 toc12
MAX3738 toc15
2.0 1.8 1.6 1.4 RJ (psRMS) 1.2 1.0 0.8 0.6 0.4 0.2 0 0 10 20 30 40 50 60 70 80
10
100 90 80 IMOD (mA) RTH_TEMP = 7k 70 60 50 40 RTH_TEMP = 4k RTH_TEMP = 2k RTH_TEMP = 12k
1 K (mA/mA) 0.1 0.01 0.001
30 0.01 0.1 1 10 100 -10 0 10 20 30 40 50 60 70 80 90 TEMPERATURE (C) RMODBCOMP (k)
90
IMOD (mA)
TEMPERATURE COMPENSATION vs. RTH_TEMP (RMODTCOMP = 10k)
MAX3738 toc13
HOT PLUG WITH TX_DISABLE LOW
MAX3738 toc14
TRANSMITTER ENABLE
VCC FAULT HIGH TX_DISABLE t_on = 23.8s LOW LASER OUTPUT 3.3V LOW
44 42 40 IMOD (mA) 38 36 34 32 30 RTH_TEMP = 12k RTH_TEMP = 7k RTH_TEMP = 4k RTH_TEMP = 2k
3.3V VCC 0V FAULT TX_DISABLE LOW LASER OUTPUT LOW t_init = 59.6ms
-10 0 10 20 30 40 50 60 70 80 90 100 TEMPERATURE (C)
20ms/div
10s/div
TRANSMITTER DISABLE
MAX3738 toc16
RESPONSE TO FAULT
MAX3738 toc17
FAULT RECOVERY TIME
MAX3738 toc18
VCC FAULT
3.3V LOW HIGH
VPC_MON
EXTERNALLY FORCED FAULT t_fault = 160ns
VPC_MON
EXTERNALLY FORCED FAULT
FAULT HIGH FAULT TX_DISABLE TX_DISABLE LOW LASER OUTPUT t_init = 58ms HIGH LOW LOW
TX_DISABLE LOW LASER OUTPUT
91.2ns
LASER OUTPUT
20ns/div
400ns/div
40ms/div
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1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3738
Pin Description
PIN 1 2, 5, 14, 17 3 4 6 7 8 9 10, 12 11 13 15 16 18 19 20 21 22 23 24 EP NAME MODTCOMP FUNCTION Modulation-Current Compensation from Temperature. A resistor at this pin sets the temperature coefficient of the modulation current when above the threshold temperature. Leave open for zero temperature compensation. +3.3V Supply Voltage Noninverted Data Input Inverted Data Input Transmitter Disable, TTL. Laser output is disabled when TX_DISABLE is asserted high or left unconnected. The laser output is enabled when this pin is asserted low. 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. Shutdown Driver Output. Voltage output to control an external transistor for optional shutdown circuitry. Ground Open-Collector Transmit Fault Indicator (Table 1) Laser Bias-Current Output Inverted 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 to ground 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 dominant pole 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 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. Ground. Solder the exposed pad to the circuit board ground for specified thermal and electrical performance.
VCC IN+ INTX_DISABLE PC_MON BC_MON SHUTDOWN GND TX_FAULT BIAS OUTOUT+ MD APCFILT1 APCFILT2 APCSET MODSET MODBCOMP TH_TEMP Exposed Pad
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1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3738
VOLTAGE VIN+ VINSINGLE ENDED 100mV (min) 1200mV (max)
OUT30 30 Z0 = 30 30 VCC VCC
(VIN+) - (VIN-)
DIFFERENTIAL
200mV (min) 2400mV (max)
MAX3738
OUT+
0.5pF OSCILLOSCOPE IOUT+ Z0 = 30 Z0 = 50 75 50
CURRENT IOUT+ IMOD
TIME
Figure 1. Required Input Signal and Output Polarity
Figure 2. Test Circuit for Characterization
HOST BOARD FILTER DEFINED BY SFP MSA SOURCE NOISE VOLTAGE SUPPLY C1 0.1F C2 10F L1 1H C3 0.1F
MODULE TO LASER DRIVER VCC
OPTIONAL OPTIONAL
Figure 3. Supply Filter
Detailed Description
The MAX3738 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).
High-Speed Modulation Driver
The output stage is composed of a high-speed differential pair and a programmable modulation current source. The MAX3738 is optimized for driving a 15 load. The minimum instantaneous voltage required at OUT- is 0.7V for modulation currents 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 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 0.0: Interface Maxim's Laser Driver to Laser Diode for more information. 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 form the laser cathode.
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)
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1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3738
VCC SHUTDOWN
MAX3738
INPUT BUFFER IN+ INSHUTDOWN TX_FAULT TX_DISABLE RPULL = 45k IMD 1 PC_MON x1/2 RPC_MON IBIAS 82 BC_MON RBC_MON xTC T > TTH T x268 xK IAPCSET MD IMD CMD x1 VCC IBIAS VBG APCSET RAPCSET SAFETY LOGIC AND POWER DETECTOR IMOD ENABLE IBIAS ENABLE DATA PATH OUTOUT+ IMOD BIAS VCC IBIAS RD
VBG TH_TEMP RTH_TEMP MODTCOMP RMODTCOMP MODSET RMODSET MODBCOMP APCFILT1 RMODBCOMP CAPC APCFILT2
Figure 4. Functional Diagram
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 (h) of laser over time and temperature: PAVG = IMD / MON PP-P = x IMOD 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-02.21 for details:
8
K = IMOD / IBIAS 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 MAX3738 provides additional temperature compensation as temperature increases past a user-defined threshold (TTH).
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1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3738
VCC
POR AND COUNTER 60ms DELAY IMOD ENABLE COUNTER 60ms DELAY VCC IMD 1 IBIAS ENABLE
TX_DISABLE
100ns DELAY
VREF R VCC Q RS LATCH VREF S SHUTDOWN
PC_MON RPC_MON IBIAS 82 BC_MON RBC_MON EXCESSIVE APC CURRENT SETPOINT EXCESSIVE MOD CURRENT SETPOINT
COMP
COMP CMOS
TX_FAULT
TTL OPEN COLLECTOR
Figure 5. Simplified Safety Circuit
Table 1. Typical Fault Conditions
1 2 3 4 If any of the I/O pins are shorted to GND or VCC (single-point failure; see Table 2), and the bias current or the photocurrent exceeds 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.
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1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3738
Table 2. Circuit Responses to Various Single-Point Faults
PIN TX_FAULT TX_DISABLE IN+ CIRCUIT RESPONSE TO OVERVOLTATGE 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, the laser current is disabled. In this condition, the 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. 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 affect 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 the 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, the laser current is disabled. Fault state* occurs. If the shutdown circuitry is used, the laser 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 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 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
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 failure. The transmit fault condition is latched until reset by a toggle or TX_DISABLE or VCC. The laser driver offers redundant laser diode shutdown through the optional shutdown circuitry as shown in the Typical Applications Circuit. This shutdown transistor prevents a single-point fault at the laser from creating an unsafe condition.
Safety Circuitry Current Monitors
The MAX3738 features monitors (BC_MON, PC_MON) for bias current (I BIAS) and photocurrent (I MD). The monitors are realized by mirroring a fraction of the currents and developing voltages across external resistors connected to ground. Voltages greater than V REF at PC_MON or BC_MON result in a fault state. For example, connecting a 100 resistor to ground at each monitor output gives the following relationships: VBC_MON = (IBIAS / 82) x 100 VPC_MON = IMD x 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.
10
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1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control
Table 3. Optical Power Relations
PARAMETER Average Power Extinction Ratio Optical Power of a One 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 / P0 P1 = 2PAVG x 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
The laser driver automatically adjusts the bias to maintain the constant average power. For DC-coupled laser diodes: IAVG = IBIAS + IMOD / 2
MAX3738
Programming the Modulation Current with Compensation
Determine the modulation current from the laser slope efficiency: IMOD = 2 x PAVG / x (re - 1) / (re+ + 1) The modulation current of the MAX3738 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 the I MOD vs. R MODSET graph in the Typical Operating Characteristics and select the value of RMODSET that corresponds to the required current at +25C: IMOD = IMODS + K x IBIAS + IMODT IMODS = 268 x VREF / RMODSET IMODT = TC x (T - TTH) | T > TTH IMODT = 0 | T < TTH 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 / (1000 + RMODBCOMP) 10% If I MOD must be increased from I MOD1 to I MOD2 to maintain the extinction ratio at elevated temperatures, the required compensation factor is: K = (IMOD2 - IMOD1) / (IBIAS2 - IBIAS1) A threshold for additional temperature compensation can be set with a programming resistor at the TH_TEMP pin: TTH = -70C + 1.45M / (9.2k + RTH_TEMP)C 10% The temperature coefficient of thermal compensation above T TH is set by R MODTCOMP . Leaving the MODTCOMP pin open disables additional thermal compensation: TC = 1 / (0.5 + RMODTCOMP(k)) mA/C 10%
Note: Assuming a 50% average input duty cycle and mark density.
Design Procedure
When designing a laser transmitter, the optical output is usually expressed in terms of average power and extinction ratio. Table 3 shows 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 ().
Programming the Monitor-Diode Current Set Point
The MAX3738 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 APCSET pin controls the set point for the monitor diode current. An internal current regulator establishes the APCSET current in the same manner as the MODSET pin. See the IMD vs. RAPCSET graph in the Typical Operating Characteristics and select the value of RAPCSET that corresponds to the required current at +25C. IMD = 1/2 x VREF / RACPSET
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1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3738
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 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) 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: CAPC(F) 68 / (f3dB(kHz) x ( x MON)1.1) High-frequency noise can be filtered with an additional cap, CMD, from the MD pin to ground. CMD CAPC / 4 The MAX3738 is designed so turn-on time is faster than 1ms for most laser gain values ( x MON). Choosing a smaller value of CAPC 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.
Current Compliance (IMOD > 60mA), AC-Coupled
For applications requiring modulation current greater than 60mA, headroom is insufficient from proper operation of the laser driver if the laser is DC-coupled. To avoid this problem, the MAX3738'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 (VCC). When AC-coupled, the MAX3738 modulation 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 circuits for the MAX3738 laser driver. If dice are used, replace package parasitic elements with bondwire parasitic elements.
VCC
VCC
MAX3738
16k VCC
0.7nH OUTPACKAGE
PACKAGE
0.7nH IN+ 0.11pF 5k
0.11pF
0.7nH
OUT+
VCC 5k 0.7nH IN0.11pF
0.11pF
MAX3738
24k
Figure 6. Simplified Input Structure 12
Figure 7. Simplified Output Structure
______________________________________________________________________________________
1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control
Layout Considerations
To minimize loss and crosstalk, keep the connections between the MAX3738 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. port 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.
MAX3738
Exposed-Pad (EP) Package
The exposed pad on the 24-pin TQFN provides a very low thermal resistance path for heat removal from the IC. The pad is also electrical ground on the MAX3738 and should be soldered to the circuit board ground for proper thermal and electrical performance. Refer to Maxim Application Note HFAN-08.1: Thermal Consideration for QFN and Other Exposed-Pad Packages at www.maxim-ic.com for additional information.
Laser Safety and IEC 825
Using the MAX3738 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 sup-
Chip Information
TRANSISTOR COUNT: 1884 PROCESSS: SiGe/Bipolar
Typical Application Circuit
+3.3V
OPTIONAL SHUTDOWN CIRCUITRY +3.3V
TX_DISABLE
0.1F IN+ CDR 0.1F INRMODBCOMP
SHUTDOWN
TX_FAULT
VCC
0.01F +3.3V 15 OUT10 OUT+
MODBCOMP RMODTCOMP MODTCOMP RTH_TEMP TH_TEMP
MAX3738
BIAS MD CMD APCFILT1 APCFILT2 BC_MON PC_MON MODSET APCSET FERRITE BEAD
GND
RBC_MON
CAPC
REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE.
______________________________________________________________________________________
RPC_MON
RMODSET
RAPCSET
13
1Gbps to 2.7Gbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3738
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.)
PACKAGE OUTLINE 12,16,20,24L QFN THIN, 4x4x0.8 mm
21-0139
B
1
2
PACKAGE OUTLINE 12,16,20,24L QFN THIN, 4x4x0.8 mm
21-0139
B
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.
14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
24L QFN THIN.EPS

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