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19-3161; Rev 1; 7/04
KIT ATION EVALU ABLE AVAIL
155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control
General Description 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
MAX3646
The MAX3646 is a +3.3V laser driver designed for multirate transceiver modules with data rates from 155Mbps to 622Mbps. Lasers can be DC-coupled to the MAX3646 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 MAX3646 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 MAX3646 ideal for driving FP/DFB lasers in fiber optic modules. External resistors set the required laser current levels. The MAX3646 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 MAX3646 is compliant with the SFF-8472 transmitter diagnostic and SFP MSA timing requirements. The MAX3646 is offered in a 4mm x 4mm, 24-pin thin QFN package and operates over the extended -40C to +85C temperature range.
Ordering Information
PART MAX3646ETG MAX3646ETG+ TEMP RANGE -40C to +85C -40C to +85C PINPACKAGE 24 Thin QFN 24 Thin QFN PKG CODE T2444-1 T2444-1
+Denotes lead-free package.
Pin Configuration
TH_TEMP APCFILT2 20 APCFILT1 19 18 MD 17 VCC 16 OUT+ 15 OUT14 VCC 13 BIAS 7 PC_MON 8 BC_MON 9 SHUTDOWN 10 GND 11 TX_FAULT 12 GND TOP VIEW MODBCOMP
MODSET 22
Applications
Multirate OC-3 to OC-12 FEC Transceivers 125Mbps Ethernet SFP, GBIC, and 1 x 9 Transceivers
MODTCOMP 1 VCC 2 IN+ 3 IN- 4 VCC 5 TX_DISABLE 6
24
23
21
MAX3646
THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY GROUND ON THE CIRCUIT BOARD.
Typical Application Circuit appears at end of data sheet.
________________________________________________________________ Maxim Integrated Products
APCSET
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.
155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3646
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 QFN (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) 622Mbps, 5mA IMOD 85mA 155Mbps, 5mA IMOD 85mA (Note 6) 15 load, TA = +25C 5mA IMOD 10mA 10mA < IMOD 85mA 5mA IMOD 10mA 10mA < IMOD 85mA 5mA IMOD 85mA 5 100 6 1.1 24 45 175 125 2.5 46 100 600 480 20 15 0.1 85 200 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
(Note 7) (with 2pF between OUT+ and OUT-)
TX_DISABLE = high
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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155Mbps to 622Mbps 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 (typical) VHI = VCC VLO = GND Sinking 1mA, open collector Sourcing 100A Sinking 100A
23
MAX3646
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 622Mbps. DJ includes pulse-width distortion (PWD).
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155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3646
Typical Operating Characteristics
(VCC = +3.3V, CAPC = 0.01F, IBIAS = 20mA, IMOD = 30mA, TA = +25C, unless otherwise noted.)
OPTICAL EYE DIAGRAM (622.08Mbps, 27 - 1 PRBS, 467MHz FILTER)
MAX3646 toc01
OPTICAL EYE DIAGRAM (155Mbps, 27 - 1 PRBS, 117MHz FILTER)
MAX3646 toc02
ELECTRICAL EYE DIAGRAM (IMOD = 30mA, 622.08MHz, 27 - 1 PRBS)
MAX3646 toc03
1310nm FP LASER re = 8.2dB
1310nm FP LASER re = 8.2dB
2pF BETWEEN OUT+ AND OUT-
75mV/div
270ps/div
1ns/div
320ps/div
SUPPLY CURRENT (ICC) vs. TEMPERATURE (EXCLUDES BIAS AND MODULATION CURRENTS)
MAX3646 toc04
BIAS-CURRENT MONITOR RATIO vs. TEMPERATURE
MAX3646 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
MAX3646 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
MAX3646 toc07
PHOTODIODE CURRENT vs. RAPCSET
MAX3646 toc08
DETERMINISTIC JITTER vs. MODULATION CURRENT
155mbps 90 80 DJ (psP-P) 70 60 50
MAX3646 toc09
90 80 70 60 IMOD (mA)
1.4 1.2 1.0 IMD (mA) 0.8 0.6 0.4 0.2 0
100
50 40 30 20 10 0 1 10 RMODSET (k) 100
40 30 20 0.1 1 RAPCSET (k) 10 100 0 10 20 30 40 50 60 70 80 90 IMOD (mA)
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155Mbps to 622Mbps SFF/SFP 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.)
MAX3646
RANDOM JITTER vs. MODULATION CURRENT
1.8 1.6 1.4 RJ (psRMS) K (mA/mA) 1.2 1.0 0.8 0.6 0.4 0.2 0 0 10 20 30 40 50 60 70 80 90 IMOD (mA)
MAX3646 toc10
COMPENSATION (K) vs. RMODBCOMP
MAX3646 toc11
TEMPERATURE COMPENSATION vs. RTH_TEMP (RMODTCOMP = 500)
MAX3646 toc12
MAX3646 toc15
2.0
10
100 90 80 IMOD (mA) RTH_TEMP = 7k 70 60 50 40 RTH_TEMP = 4k RTH_TEMP = 2k RTH_TEMP = 12k
1
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)
TEMPERATURE COMPENSATION vs. RTH_TEMP (RMODTCOMP = 10k)
MAX3646 toc13
HOT PLUG WITH TX_DISABLE LOW
MAX3646 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
MAX37646 toc16
RESPONSE TO FAULT
MAX3646 toc17
FAULT RECOVERY TIME
MAX3646 toc18
VCC FAULT
VPC_MON 3.3V LOW
EXTERNALLY FORCED FAULT t_fault = 160ns
VPC_MON
EXTERNALLY FORCED FAULT
HIGH TX_DISABLE LOW LASER OUTPUT 91.2ns FAULT TX_DISABLE LASER OUTPUT
FAULT HIGH TX_DISABLE LOW LASER OUTPUT t_init = 58ms HIGH LOW LOW
20ns/div
400ns/div
40ms/div
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155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3646
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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155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3646
VOLTAGE VIN+ VINSINGLE ENDED 100mV (min) 1200mV (max)
30 OUTVCC VCC 30 Z0 = 30 30
(VIN+) - (VIN-)
DIFFERENTIAL
200mV (min) 2400mV (max)
MAX3646
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 MAX3646 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 MAX3646 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. 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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155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3646
VCC SHUTDOWN
MAX3646
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 MAX3646 provides additional temperature compensation as temperature increases past a user-defined threshold (TTH).
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155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3646
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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155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3646
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. 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.
IN-
MD SHUTDOWN BIAS OUT+ OUTPC_MON BC_MON APCFILT1 APCFILT2 MODSET
APCSET Does not affect laser power. Fault state* occurs. *A fault state asserts the TX_FAULT pin, disables the modulation and bias currents, and asserts the SHUTDOWN pin.
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155Mbps to 622Mbps 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) SYMBOL PAVG re P1 P0 PP-P IMOD ITH IBIAS RELATION PAVG = (P0 + P1) / 2 r e = P1 / P 0 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
isolation resistors are included to reduce the number of components needed to implement this function.
MAX3646
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 ().
Laser to monitor IMD / PAVG MON transfer Note: Assuming a 50% average input duty cycle and mark density.
Programming the Monitor-Diode Current Set Point
The MAX3646 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 The laser driver automatically adjusts the bias to maintain the constant average power. For DC-coupled laser diodes: IAVG = IBIAS + IMOD / 2
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 Operation Circuit. This shutdown transistor prevents a single-point fault at the laser from creating an unsafe condition.
Safety Circuitry Current Monitors
The MAX3646 features monitors (BC_MON, PC_MON) for bias current (IBIAS) and photocurrent (IMD). The monitors are realized by mirroring a fraction of the currents and developing voltages across external resistors connected to ground. Voltages greater than VREF 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
Programming the Modulation Current with Compensation
Determine the modulation current form the laser slope efficiency: IMOD = 2 x PAVG / x (re - 1)/(re+ + 1) The modulation current of the MAX3646 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
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11
155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3646
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%
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
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 MAX3646'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 MAX3646 modulation current can be programmed up to 85mA. Refer to Maxim Application Note HFAN 02.0: Interfacing
VCC
VCC
MAX3646
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
MAX3646
24k
Figure 6. Simplified Input Structure 12
Figure 7. Simplified Output Structure
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155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control
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 balance between turn-on time and low-frequency cutoff may be needed at low data rates for some values of laser gain.
MAX3646
Interface Models
Figures 6 and 7 show simplified input and output circuits for the MAX3646 laser driver. If dice are used, replace package parasitic elements with bondwire parasitic elements.
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 MAX3646 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
Layout Considerations
To minimize loss and crosstalk, keep the connections between the MAX3646 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.
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
MAX3646
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
155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control MAX3646
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.)
PACKAGEOUTLINE 12,16,20,24LTHINQFN,4x4x0.8mm
21-0139
C
1
2
PACKAGEOUTLINE 12,16,20,24LTHINQFN,4x4x0.8mm
21-0139
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.
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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