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19-3118; Rev 0; 12/03
3.2Gbps SFP VCSEL Driver with Diagnostic Monitors
General Description
The MAX3740A is a high-speed VCSEL driver for smallform-factor (SFF) and small-form-factor pluggable (SFP) fiber optic LAN transmitters. It contains a bias generator, a laser modulator, and comprehensive safety features. The automatic power control (APC) adjusts the laser bias current to maintain average optical power over changes in temperature and laser properties. The driver accommodates common cathode and differential configurations. The MAX3740A operates up to 3.2Gbps. It can switch up to 15mA of laser modulation current and source up to 15mA of bias current. Adjustable temperature compensation is provided to keep the optical extinction ratio within specifications over the operating temperature range. The MAX3740A interfaces with the Dallas DS1858 to meet SFF-8472 timing and diagnostic requirements. The MAX3740A accommodates various VCSEL packages, including low-cost TO-46 headers. The MAX3740A safety circuit detects faults that could cause hazardous light levels and disables the VCSEL output. The safety circuits are compliant with SFF and SFP multisource agreements (MSA). The MAX3740A is available in a compact 4mm 4mm, 24-pin thin QFN package and operates over the -40C to +85C temperature range. 2mA to 15mA Modulation Current 1mA to 15mA Bias Current Optional Peaking Current to Improve VCSEL Edge Speed Supports Common Cathode and Differential Configuration Automatic Power Control Safety Circuits Compliant with SFF and SFP MSAs 4mm 4mm 24-Pin Thin QFN Package
Features
Supports all SFF-8472 Digital Diagnostics
MAX3740A
Ordering Information
PART MAX3740AETG TEMP RANGE -40C to +85C PIN-PACKAGE 24 Thin QFN (4mm x 4mm)
Applications
Multirate (1Gbps to 3.2Gbps) SFP/SFF Modules Gigabit Ethernet Optical Transmitters Fibre Channel Optical Transmitters Infiniband Optical Transmitters
Typical Application Circuit
+3.3V 4.7k
VCC FAULT PWRMON MODSET
TX_DISABLE SQUELCH
RMODSET
0.1F IN+ IN0.1F TC1 RTC TC2
MAX3740A
REF COMP 0.047F MD BIAS L1* 0.01F OUT+ CF OUT0.01F 50 RPWRSET
BIASSET RBIASSET GND PEAKSET BIASMON
RF
RPEAKSET
RBIASMON
OPTIONAL COMPONENT
*FERRITE BEAD
________________________________________________________________ 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.
3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740A
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (VCC) ..............................................-0.5V to 6.0V Voltage at TX_DISABLE, IN+, IN-, FAULT, SQUELCH TC1, TC2, MODSET, PEAKSET, BIASSET, BIAS, BIASMON, COMP, MD, REF, PWRMON ...............................................-0.5V to (VCC + 0.5V) Voltage at OUT+, OUT- .........................(VCC - 2V) to (VCC + 2V) Current into FAULT ............................................ -1mA to +25mA Current into OUT+, OUT- ....................................................60mA Continuous Power Dissipation (TA = +85C) 24-Lead Thin QFN (derate 20.8mW/C above +85C).................................1354mW Operating Temperature Range ...........................-40C to +85C Storage Temperature Range .............................-55C to +150C Lead Temperature (soldering, 10s) .................................+300C
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, TC1 and TC2 are shorted, PEAKSET open, TA = +25C, unless otherwise noted.)
PARAMETER SYMBOL CONDITIONS SQUELCH set low, IMOD = 2mAP-P TX_DISABLE set low, peaking is not used IMOD = 15mAP-P (Note 1) Additional current when peaking is used (Note 2) Additional current when SQUELCH is high ICC-SHDN FAULT OUTPUT Output High Voltage Output Low Voltage TX_DISABLE INPUT Input Impedance Input High Voltage Input Low Voltage Power-Down Time SQUELCH Squelch Threshold Squelch Hysteresis Time to Squelch Data Time to Resume from Squelch BIAS GENERATOR (Note 4) Bias Current Accuracy of Programmed Bias Current IBIAS BIAS Minimum Maximum 5mA IBIAS 15mA 1mA IBIAS 5mA 15 -8 -12 +8 +12 1 mA % (Note 3) (Note 3) 25 10 0.02 0.02 5.00 5.00 85 mVP-P mVP-P s s VIH VIL The time for ICC to reach ICC-SHDN when TX_DISABLE transitions high 50 4.7 2.0 0.8 10.0 k V V s VOH VOL RLOAD = 10k to 2.97V RLOAD = 4.7k to 3.63V 2.4 0.4 V V Total current when TX_DISABLE is high MIN TYP 32 55 15 5 3.9 68 mA 20 10 5 MAX UNITS
Supply Current
ICC
2
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3.2Gbps SFP VCSEL Driver with Diagnostic Monitors
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +2.97V to +3.63V, TA = -40C to +85C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA = +25C, unless otherwise noted.)
PARAMETER Bias Current During Fault BIASMON Gain BIASMON Stability AUTOMATIC POWER CONTROL (APC) MD Nominal Voltage Voltage at REF MD Voltage During Fault MD Input Current APC Time Constant PWRMON Nominal Gain LASER MODULATOR (Note 7) Data Input Voltage Swing Output Resistance Modulation Current Minimum Peaking Current Range Maximum Peaking Current Range Peaking Current Duration Tolerance of Programmed Modulation Current Minimum Programmable Temperature Coefficient Maximum Programmable Temperature Coefficient Modulation Transition Time Deterministic Jitter Random Jitter Laser Modulation During Fault or while Squelch is Active Input Resistance Input Bias Voltage VIN tR, tF DJ RJ IMOD_OFF Differential resistance 85 Temperature range 0C to +70C 5mA IMOD 15mA, 20% to 80% (Note 6) 5mA IMOD 15mA, 3.2Gbps (Notes 6, 8) (Note 6) TC1 is shorted to TC2 -10 0 +5000 65 12 1.3 15 100 VCC 0.3 95 20 4 50 115 IMOD VID Minimum Maximum Single-ended resistance at OUT+ Single-ended resistance at OUTMinimum Maximum 15 0.2 2 80 +10 2200 80 72 105 100 2 250 mVP-P mAP-P mA mA ps % ppm/C ppm/C ps psP-P psRMS AP-P V Normal operation (FAULT = low) CCOMP = 0.047F (Note 6) VPWRMON / (VREF - VMD) -2 5 1.85 VMD VREF APC loop is closed 1 1.2 VREF 0.2 1.8 0 0.7 20 2.15 2.45 +2 2 2.2 V V V A s V/V SYMBOL IBIAS_OFF CONDITIONS Current out of the BIAS pin 1mA < IBIAS < 3mA 3mA IBIAS 15mA (Notes 5,6) 0.0875 0.085 -10 MIN TYP 1.5 0.105 0.105 MAX 10 0.1375 0.125 +10 UNITS A mA/mA %
MAX3740A
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3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740A
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +2.97V to +3.63V, TA = -40C to +85C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA = +25C, unless otherwise noted.)
PARAMETER High-Current Fault Threshold VBIAS Fault Threshold Power-Monitor Fault Threshold TX Disable Time SYMBOL VBMTH VBTH VPMTH t_OFF CONDITIONS VBIASMON > VBMTH causes a fault VBIAS referenced to VCC VPWRMON > VPMTH causes a fault Time from rising edge of TX_DISABLE to IBIAS = IBIAS_OFF and IMOD = IMOD_OFF (Note 6) Time from rising edge of TX_DISABLE to IBIAS and IMOD at 99% of steady state (Note 6) Time to set VFAULT = low after power-on or after rising edge of TX_DISABLE (Note 6) Time after power-on to transmitter-on with TX_DISABLE low (Note 6) Time from fault occurrence to VFAULT = high; CFAULT < 20pF, RFAULT = 4.7k (Note 6) Time from fault to IBIAS = IBIAS_OFF and IMOD = IMOD_OFF (Note 6) Time TX_DISABLE must be held high to reset FAULT (Note 6) MIN 0.7 -0.250 0.7 TYP 0.8 -0.2 0.8 1.8 MAX 0.9 -0.150 0.9 5 UNITS V V V s
SAFETY FEATURES (see the Typical Operating Characteristics section)
TX Disable Negate Time
t_ON
55
500
s
Fault Reset Time Power-On Time
t_INIT1 t_INIT2
60 60
200 200
ms ms
Fault Assert Time
t_FAULT
1.4
50
s
Fault Delay Time TX_DISABLE Reset
t_FLTDLY t_RESET
1
5 1
s s
Note 1: Supply current measurements exclude IBIAS from the total current. Note 2: Tested with RPEAK = 1.18k. Note 3: Measured by applying a pattern that contains 20s of K28.5, followed by 5s of zeros, then 20s of K28.5, followed by 5s of ones. Data rate is equal to 2.5Gbps, with inputs filtered using 1.8GHz Bessel filters. Note 4: VBIAS < VCC - 0.7V. Note 5: Variation of bias monitor gain for any single part over the range of VCC, temperature, 3mA < IBIAS < 15mA. Note 6: Guaranteed by design and characterization. Note 7: Measured electrically with a 50 load AC-coupled to OUT+. Note 8: Deterministic jitter is the peak-to-peak deviation from the ideal time crossings measured with a K28.5 bit pattern at 3.2Gbps
4
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3.2Gbps SFP VCSEL Driver with Diagnostic Monitors
Typical Operating Characteristics
(VCC = +3.3V, RTC = 0, PEAKSET open, measured electrically with a 50 load AC-coupled to OUT+, TA = +25C, unless otherwise noted.)
ELECTRICAL EYE WITH MAX PEAKING ELECTRICAL EYE
MAX3740A toc01 MAX3740A toc03
MAX3740A
ELECTRICAL EYE WITH PEAKING
MAX3740A toc02
3.2Gbps, K28.5, 10mA MODULATION, PEAKING OFF
3.2Gbps, K28.5, 10mA MODULATION, RPEAKSET = 2.4k
3.2Gbps, K28.5, 10mA MODULATION, RPEAKSET = 500
73mV/div 73mV/div 73mV/div
50ps/div 50ps/div 50ps/div
OPTICAL EYE
MAX3740A toc04
OPTICAL EYE
MAX3740A toc05
IBIASMON vs. BIAS CURRENT
1.6 1.4 IBIASMON (mA) 1.2 1.0 0.8 0.6 0.4
MAX3740A toc06
1.8
ER = 8.2dB, 2.125Gbps, K28.5, 850nm VCSEL, WITH 2.3GHz O-TO-E CONVERTER
ER = 8.2dB, 2.5Gbps, K28.5, 850nm VCSEL SONET MASK WITH +20% MARGIN
EMCORE SC-TOSA-8585-3420 VCSEL 68ps/div
EMCORE SC-TOSA-8585-3420 VCSEL 58ps/div
0.2 0 0 4 8 BIAS CURRENT (mA) 12 16
DETERMINISTIC JITTER vs. MODULATION CURRENT
MAX3740A toc07
RANDOM JITTER vs. MODULATION CURRENT
MAX3740A toc08
TRANSITION TIME vs. MODULATION CURRENT
MAX3740A toc09
40 35 DETERMINISTIC JITTER (psP-P) 30 25 20 15 10 5 0 0 5 IMOD (mAP-P) 10
7 6 RANDOM JITTER (psRMS) 5 4 3 2 1 0
100 90 TRANSITION TIME (ps) 80 70 60 50 40 FALL RISE
15
0
5 IMOD (mAP-P)
10
15
2
4
6
8
10
12
14
16
IMOD (mAP-P)
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3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740A
Typical Operating Characteristics (continued)
(VCC = +3.3V, RTC = 0, PEAKSET open, measured electrically with a 50 load AC-coupled to OUT+, TA = +25C, unless otherwise noted.)
BIAS CURRENT vs. RBIASSET
MAX3740A toc10
MODULATION CURRENT vs. RMODSET
MEASURED WITH A 50 ELECTRICAL LOAD MODULATION CURRENT (AP-P)
MAX3740A toc11
MONITOR DIODE CURRENT vs. RPWRSET
MAX3740A toc12
100m
100m
10m
MONITOR DIODE CURRENT (A)
1m
BIAS CURRENT (A)
10m
10m
100
1m
10
100 1k 10k RBIASSET () 100k
1m 100 1k RMODSET () 10k
1 100 1k RPWRSET () 10k
SUPPLY CURRENT vs. TEMPERATURE
MAX3740A toc13
INPUT RETURN LOSS
MAX3740A toc14
OUTPUT RETURN LOSS
-2 -4 -6 S22 (dB) -8 -10 -12 -14 -16 SINGLE-ENDED MEASUREMENT
MAX3740A toc15 MAX3740A toc18
80 70 SUPPLY CURRENT (mA) 60 50 40 30 20 10 -40 -15 10 35 60 IMOD = 2mA IMOD = 15mA
0 -5 -10 S11 (dB) -15 -20 -25 -30 -35 DIFFERENTIAL MEASUREMENT
0
85
-40 100M
1G FREQUENCY (Hz)
10G
-18 100M
1G FREQUENCY (Hz)
10G
TEMPERATURE (C)
MODULATION CURRENT vs. TEMPERATURE
MAX3740A toc16
MODULATION CURRENT TEMPCO vs. RTC
REFERENCED TO +25C
MAX3740A toc17
MONITOR DIODE CURRENT vs. TEMPERATURE
300 275 MONITOR DIODE CURRENT (A) 250 225 200 175 150 125 100 -40 -15 10 35 60 85
11 MODULATION CURRENT (mAP-P) 10
RTC = 100 RTC = 1k
5500 4500 TEMPCO (ppm/C) 3500 2500 1500 500 -500
RMOD = 1.35k
9 8 7 6 5 4 0 10 20 30 40 50 60 70 80 90 TEMPERATURE (C) RTC = 5k RTC = 10k RTC = 60k RTC = 100k RTC = 500k
100
1k
10k RTC ()
100k
1M
TEMPERATURE (C)
6
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3.2Gbps SFP VCSEL Driver with Diagnostic Monitors
Typical Operating Characteristics (continued)
(VCC = +3.3V, RTC = 0, PEAKSET open, measured electrically with a 50 load AC-coupled to OUT+, TA = +25C, unless otherwise noted.)
HOT PLUG WITH TX_DISABLE LOW
MAX3740A toc19
MAX3740A
STARTUP WITH SLOW RAMPING SUPPLY
MAX3740A toc20
TX_DISABLE NEGATE TIME
MAX3740A toc21
3.3V VCC OV LOW VCC OV FAULT LOW FAULT LOW 3.3V VCC
3.3V
FAULT
TX_DISABLE LOW LASER OUTPUT
t_INIT = 60ms
TX_DISABLE LOW LASER OUTPUT
t_INIT = 62ms
TX_DISABLE
HIGH
t_ON = 54s
LOW
LASER OUTPUT 20ms/div 20s/div
20ms/div
TRANSMITTER DISABLE
MAX3740A toc22
RESPONSE TO FAULT
MAX3740A toc23
3.3V VCC t_OFF = 1.86s FAULT LOW
EXTERNALLY FORCED VPWRMON FAULT FAULT t_FAULT = 245ns LOW HIGH
TX_DISABLE
LOW
HIGH
TX_DISABLE
LOW
LASER OUTPUT 1s/div
LASER OUTPUT 200ns/div
FAULT RECOVERY TIME
MAX3740A toc24
FREQUENT ASSERTION OF TX_DISABLE
MAX3740A toc25
VPWRMON
EXTERNAL FAULT REMOVED
VPWRMON
EXTERNALLY FORCED FAULT
FAULT
HIGH HIGH
FAULT LOW
TX_DISABLE
LOW
LOW t_INIT = 54s
TX_DISABLE
LASER OUTPUT 40s/div
LASER OUTPUT 200s/div
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7
3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740A
Pin Description
PIN 1, 10, 13 2 3 4 5 6 7, 16, 20 8 9 11 12 14 15 17 18 19 NAME GND TX_DISABLE IN+ INFAULT SQUELCH VCC TC1 TC2 MODSET PEAKSET OUTOUT+ BIASSET BIAS BIASMON Ground Transmit Disable. Driver output is disabled when TX_DISABLE is high or left unconnected. The driver output is enabled when the pin is asserted low. Noninverted Data Input Inverted Data Input Fault Indicator. Open-drain output with ESD protection. FAULT is asserted high during a fault condition. Squelch Enable. Squelch is enabled when the pin is set high. Squelch is disabled when the pin is set low or left open. +3.3V Supply Voltage Temperature Compensation Set Pin 1. A resistor placed between TC1 and TC2 (RTC) programs the temperature coefficient of the modulation current. Temperature Compensation Set Pin 2. A resistor placed between TC1 and TC2 (RTC) programs the temperature coefficient of the modulation current. Modulation Set. A resistor connected from MODSET to ground (RMODSET) sets the desired modulation current amplitude. Peaking Current Set. A resistor connected between PEAKSET and ground (RPEAKSET) programs the peaking current amplitude. To disable peaking, leave PEAKSET open. Inverted Modulation-Current Output Noninverted Modulation-Current Output Bias Current Set. When a closed-loop configuration is used, connect a 1.7k resistor between ground and BIASSET to set the maximum bias current. When an open configuration is used, connect a resistor between BIASSET and ground (RBIASSET) to program the VCSEL bias current. Bias Current Output Bias Current Monitor. The output of BIASMON is a sourced current proportional to the bias current. A resistor connected between BIASMON and ground (RBIASMON) can be used to form a groundreferenced bias monitor. Compensation Pin. A capacitor between COMP and MD compensates the APC. A typical value of 0.047F is recommended. For open-loop configuration, short the COMP pin to GND to deactivate the APC. Monitor Diode Connection Reference Pin. Reference monitor used for APC. A resistor between REF and MD (RPWRSET) sets the photo monitor current when the APC loop is closed. Average Power Monitor. The pin is used to monitor the transmit optical power. For open-loop configuration, connect PWRMON to GND. Ground. Must be soldered to the circuit board ground for proper thermal and electrical performance. See the Layout Considerations section. FUNCTION
21 22 23 24 EP
COMP MD REF PWRMON Exposed Pad
8
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3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740A
PWRMON REF RPWRSET MD SMOOTHSTART 1.6V (2VBE) 1.2V IBIAS 9 RBIASMON 200 1.8V 2X MAX3740A POWERCONTROL AMPLIFIER IBIAS 34 BIAS GENERATOR BIASMON CURRENT AMPLIFIER ENABLE BIAS FERRITE BEAD
IPD
COMP CCOMP
BIASSET RBIASSET
Figure 1. Bias Generator
Detailed Description
The MAX3740A contains a bias generator with automatic power control (APC), safety circuit, and a laser modulator with optional peaking compensation.
The BIASMON output provides a current proportional to the laser bias current given by: IBIASMON = IBIAS / 9 When APC is not used (no monitor diode, open-loop configuration) connect the COMP and PWRMON pins to GND. In this mode, the bias current is set by the resistor RBIASSET. When a closed-loop configuration is used, connect a 1.7k resistor between ground and BIASSET to set the maximum bias current.
Bias Generator
Figure 1 shows the bias generator circuitry that contains a power-control amplifier and smooth-start circuitry. An internal PNP transistor provides DC laser current to bias the laser in a light-emitting state. The APC circuitry adjusts the laser-bias current to maintain average power over temperature and changing laser properties. The smooth-start circuitry prevents current spikes to the laser during power-up or enable, ensuring compliance with safety requirements and extending the life of the laser. The MD input is connected to the cathode of a monitor diode, which is used to sense laser power. The BIAS output is connected to the anode of the laser through an inductor or ferrite bead. The power-control amplifier drives a current amplifier to control the laser's bias current. During a fault condition, the bias current is disabled. The PWRMON output provides a voltage proportional to average laser power given by: VPWRMON = 2 IPD RPWRSET
Safety Circuit
The safety circuit contains an input disable (TX_DISABLE), a latched fault output (FAULT), and fault detectors (Figure 2). This circuit monitors the operation of the laser driver and forces a shutdown (disables laser) if a fault is detected (Table 1). Table 2 contains the circuit's response to various single-point failures. The transmit fault condition is latched until reset by a toggle of TX_DISABLE or VCC. The FAULT pin should be pulled high with a 4.7k to 10k resistor.
Table 1. Fault Conditions
PIN BIAS BIASMON PWRMON FAULT CONDITION VBIAS > VCC - 0.2V VBIASMON > 0.8V VPWRMON > 0.8V
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9
3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740A
Table 2. Circuit Response to Various Single-Point Faults (Closed-Loop APC Configuration)
PIN NAME FAULT TX_DISABLE IN+ INSQUELCH TC1 TC2 MODSET PEAKSET OUT+ OUTBIASSET BIAS BIASMON COMP CIRCUIT RESPONSE TO VCC SHORT Does not affect laser power. Modulation and bias current are disabled. Does not affect laser power. Does not affect laser power. Does not affect laser power. Does not affect laser power. The laser modulation is increased, but average power is not affected. Modulation current is disabled. Does not affect laser power. Modulation current is disabled. Does not affect laser power. Laser bias is disabled. Fault state* occurs. Note that VCSEL emissions may continue; care must be taken to prevent this condition. Fault state* occurs. The bias current is reduced, and the average power of the laser output is reduced. IBIAS increases to the value determined by RBIASSET; if the bias-monitor fault threshold is exceeded, a fault is signaled. IBIAS increases to the value determined by RBIASSET; if the bias-monitor fault threshold is exceeded, a fault is signaled. Fault state* occurs. CIRCUIT RESPONSE TO GND SHORT Does not affect laser power. Normal condition for circuit operation. Does not affect laser power. Does not affect laser power. Does not affect laser power. Does not affect laser power. Modulation current is disabled. The laser modulation is increased, but average power is not affected. Does not affect laser power. Modulation current is disabled. Does not affect laser power. Fault state* occurs. Disables VCSEL. Does not affect laser power. IBIAS increases to the value determined by RBIASSET; if the bias monitor fault threshold is exceeded, a fault is signaled. The bias current is reduced, and the average power of the laser output is reduced.
MD
REF PWRMON
The bias current is reduced, and the average power of the laser output is reduced. Does not affect laser power.
*A fault state asserts the FAULT pin, disables the modulator output, and disables the bias output.
Modulation Circuit
The modulation circuitry consists of an input buffer, a current mirror, and a high-speed current switch (Figure 3). The modulator drives up to 15mA of modulation into a 50 VCSEL load. The amplitude of the modulation current is set with resistors at MODSET and temperature coefficient (TC1, TC2) pins. The resistor at MODSET (RMODSET) programs the temperature-stable portion of the modulation current, and the resistor between TC1 and TC2 (RTC) programs the temperature coefficient of the modulation current. For appropriate RTC and RMODSET values, see the Typical Operating Characteristics section.
Design Procedure
Select Laser
Select a communications-grade laser with a rise time of 260ps or better for 1.25Gbps, or 130ps or better for 2.5Gbps applications. Use a high-efficiency laser that requires low modulation current and generates a lowvoltage swing. Trim the leads to reduce laser package inductance. The typical package leads have inductance of 25nH per inch (1nH/mm). This inductance causes a large voltage swing across the laser. A compensation filter network can also be used to reduce ringing, edge speed, and voltage swing (see the Designing the Compensation Filter Network section).
10
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3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740A
TX_DISABLE
BIAS VBIAS FAULT VCC - 0.2V FAULT OUTPUT FAULT
BIASMON HIGH-CURRENT FAULT R 0.8V S R-S LATCH HIGH-POWER FAULT Q ENABLE
PWRMON
0.8V TX_DISABLE
POR SAFETY CIRCUIT
MAX3740A
Figure 2. Safety Circuit
VCC
MAX3740A
INPUT BUFFER IN+ SIGNAL DETECT
ROUT-
ROUT+ OUT+
CURRENT SWITCH
OUT100 PEAKING CONTROL PEAKSET MODULATION CURRENT GENERATOR
INSQUELCH
ENABLE
RPEAKSET
CURRENT AMPLIFIER 30x
TEMPERATURE COMPENSATION 1V
200
TC1 RTC
TC2
MODSET RMODSET
Figure 3. Modulation Circuit ______________________________________________________________________________________ 11
3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740A
Programming Modulation Current
The modulation current output of the MAX3740A is controlled by a resistor (R MODSET ) placed between MODSET and ground. The RMODSET resistor controls the amount of current being sourced to the VCSEL. The modulation current is given by the following: ROUT+ IMOD = (IMODSET ) x 30 x ROUT+ + RLOAD ROUT+ 1 IMOD = x 30 x R 200 + RMODSET OUT+ + RLOAD 0.021mW/mA at +25C, which reduces to 0.018mW/mA at +85C. The temperature coefficient is given by the following: Laser tempco = (SE85 - SE25 ) x 1E6 SE25 x (85 - 25) = -2380ppm / C
[
]
From the Typical Operating Characteristics, the value of RTC, which offsets the tempco of the laser, is 9k. If modulation temperature compensation is not desired, short TC1 and TC2.
Programming the APC Loop
It is important to note that the modulation current being sourced by the MAX3740A is affected by the load impedance of the VCSEL. The Modulation Current vs. RMODSET graph in the Typical Operating Characteristics shows the current into a 50 electrical load. Program the average optical power by adjusting R PWRSET . To select the resistance, determine the desired monitor current to be maintained over temperature and lifetime. See the Monitor Diode Current vs. RPWRSET graph in the Typical Operating Characteristics section, and select the value of RPWRSET that corresponds to the required current.
Programming Bias Current
The bias current output of the MAX3740A is controlled by a resistor (RBIASSET) placed between BIASSET and ground. In open-loop operation the RBIASSET controls the bias current level of the VCSEL. In closed-loop operation the RBIASSET controls the maximum bias current provided by the APC. The bias current is given by the following: IBIAS = (IBIASSET ) x 34 1.2 IBIAS = x 34 200 + RBIASSET The Bias Current vs. RBIASSET graph is also shown in the Typical Operating Characteristics.
Input Termination Requirements
The MAX3740A data inputs are SFP MSA compatible. On-chip 100 differential input impedance is provided for optimal termination (Figure 4). Because of the on-chip biasing network, the MAX3740A inputs self-bias to the proper operating point to accommodate AC-coupling.
VCC
MAX3740A PACKAGE IN+ 1nH 0.5pF 50 VCC
16k
Photodiode Selection
To ensure stable operation of the APC circuit, the time constant of the MD node should be shorter than the APC time constant. (tAPC = 5s if CAPC = 0.047F). 5s t MD , RMD x CMD = 250ns 20 20
t APC
VCC 50 IN1nH 0.5pF 24k
For typical IPD = 400A, RPWRSET = 500, select a photodiode with capacitance less than 500pF.
Programming Modulation-Current Tempco
Compute the required modulation tempco from the slope efficiency of the laser at TA = +25C and at a higher temperature. Then select the value of RTC from the Typical Operating Characteristics. For example, suppose a laser has a slope efficiency (SE) of
12
Figure 4. Simplified Input Structure
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3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740A
VCC UNCOMPENSATED
ROUT-
ROUT+
PACKAGE POWER 1nH 0.5pF 1nH 0.5pF OUT+ OUTCORRECTLY COMPENSATED
OVERCOMPENSATED
TIME MAX3740A
Figure 7. Laser Compensation
Figure 5. Simplified Output Structure
VCC
The compensation components (RF and CF) are most easily determined by experimentation. Begin with RF = 50 and CF = 1pF. Increase CF until the desired transmitter response is obtained (Figure 7). Refer to Application Note HFAN-2-0: Interfacing Maxim Laser Drives with Laser Diodes for more information.
MAX3740A
FAULT
Exposed-Pad (EP) Package
The exposed pad on the 24-pin thin QFN provides a very low thermal resistance path for heat removal from the IC. The pad is also electrical ground on the MAX3740A and must 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 for additional information.
Figure 6. Fault Circuit Interface
Applications Information
Interface Models
Figures 4 and 5 show simplified input and output circuits for the MAX3740A laser driver. Figure 6 shows the fault circuit interface.
Laser Safety and IEC 825
The International Electrotechnical Commission (IEC) determines standards for hazardous light emissions from fiber optic transmitters. IEC 825 defines the maximum light output for various hazard levels. The MAX3740A provides features that facilitate compliance with IEC 825. A common safety precaution is single-point fault tolerance, whereby one unplanned short, open, or resistive connection does not cause excess light output. Using this laser driver alone does not ensure that a transmitter design is compliant with IEC 825. The entire transmitter circuit and component selections must be considered. Customers must determine the level of fault tolerance required by their applications, 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
Layout Considerations
To minimize inductance, keep the connections between the MAX3740A output pins and laser diode as short as possible. Use good high-frequency layout techniques and multilayer boards with uninterrupted ground planes to minimize EMI and crosstalk.
Designing the Compensation Filter Network
Laser package inductance causes the laser impedance to increase at high frequencies, leading to ringing, overshoot, and degradation of the laser output. A laser compensation filter network can be used to reduce the laser impedance at high frequencies, thereby reducing output ringing and overshoot.
______________________________________________________________________________________
3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740A
Functional Diagram
FAULT COMP MD REF PWRMON BIASMON
TX_DISABLE
SAFETY CIRCUITRY ENABLE VCC MAX3740A
BIAS GENERATOR WITH APC
BIAS BIASSET
LASER MODULATOR
SQUELCH IN+ 100 INMODULATION CURRENT GENERATOR SIGNAL DETECT PEAKING CONTROL
OUTOUT+
ENABLE
TC1
TC2
MODSET
PEAKSET
Chip Information
PWRMON REF
Pin Configuration
COMP
VCC
TOP VIEW
24
23
22
21
20
19 18 17 16
BIASMON
MD
TRANSISTOR COUNT: 3806 PROCESS: SiGe BIPOLAR
GND TX_DISABLE
1 2 3 4 5 6 10 11 12
BIAS BIASSET VCC OUT+ OUTGND
Package Information
For the latest package outline information, go to www.maxim-ic.com/packages.
PART MAX3740AETG PACKAGE TYPE 24 Thin QFN (4mm x 4mm x 0.8mm) PACKAGE CODE T2444-1
IN+ INFAULT SQUELCH
MAX3740A
15 14 13
7
8 TC1
VCC
TC2
9
GND
MODSET
24 THIN QFN (4mm x 4mm)
*EXPOSED PAD IS CONNECTED TO GND
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) 2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
PEAKSET

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