19-3118; Rev 3; 1/10 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors 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 -40°C to +85°C temperature range. Features ♦ Supports all SFF-8472 Digital Diagnostics ♦ 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 Ordering Information PART TEMP RANGE PIN-PACKAGE MAX3740AETG -40°C to +85°C 24 Thin QFN-EP* MAX3740AETG+ -40°C to +85°C 24 Thin QFN-EP* +Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad. Applications Typical Application Circuit Multirate (1Gbps to 3.2Gbps) SFP/SFF Modules +3.3V Gigabit Ethernet Optical Transmitters 4.7kΩ† VCC Fibre Channel Optical Transmitters FAULT PWRMON MODSET Infiniband Optical Transmitters TX_DISABLE RMODSET SQUELCH MAX3740A 0.1μF REF IN+ RPWRSET COMP 0.047μF IN- MD TC1 BIAS 0.1μF RTC† L1* 0.01μF TC2 OUT+ CF† BIASSET RBIASSET OUTGND PEAKSET BIASMON RPEAKSET† 0.01μF 50Ω RF† RBIASMON †OPTIONAL COMPONENT *FERRITE BEAD ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX3740A General Description MAX3740A 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors 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 = +85°C) 24-Lead Thin QFN (derate 20.8mW/°C above +85°C).................................1354mW Operating Temperature Range ...........................-40°C to +85°C Storage Temperature Range .............................-55°C to +150°C Lead Temperature (soldering, 10s) .................................+300°C 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 = -40°C to +85°C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA = +25°C, unless otherwise noted.) PARAMETER Supply Current SYMBOL ICC ICC-SHDN CONDITIONS MIN SQUELCH set low, IMOD = 2mAP-P TX_DISABLE set low, peaking is not used IMOD = 15mAP-P (Note 1) TYP MAX UNITS 32 55 68 Additional current when peaking is used (Note 2) 15 20 Additional current when SQUELCH is high 5 10 3.9 5 mA Total current when TX_DISABLE is high FAULT OUTPUT Output High Voltage VOH RLOAD = 10kΩ to 2.97V Output Low Voltage VOL RLOAD = 4.7kΩ to 3.63V 2.4 V 0.4 V 10.0 kΩ TX_DISABLE INPUT Input Impedance 4.7 Input High Voltage VIH Input Low Voltage VIL 2.0 The time for ICC to reach ICC-SHDN when TX_DISABLE transitions high Power-Down Time V 0.8 50 V µs SQUELCH Squelch Threshold 25 Squelch Hysteresis 10 85 mVP-P mVP-P Time to Squelch Data (Note 3) 0.02 5.00 µs Time to Resume from Squelch (Note 3) 0.02 5.00 µs BIAS GENERATOR (Note 4) Bias Current Accuracy of Programmed Bias Current 2 IBIAS ΔBIAS Minimum Maximum 1 15 5mA ≤ IBIAS ≤ 15mA -8 +8 1mA ≤ IBIAS ≤ 5mA -12 +12 _______________________________________________________________________________________ mA % 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors (VCC = +2.97V to +3.63V, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA = +25°C, unless otherwise noted.) PARAMETER Bias Current During Fault SYMBOL IBIAS_OFF BIASMON Gain BIASMON Stability CONDITIONS MIN Current out of the BIAS pin TYP MAX UNITS 1.5 10 µA 1mA < IBIAS < 3mA 0.0875 0.105 0.1375 3mA ≤ IBIAS ≤ 15mA 0.085 0.105 0.125 (Notes 5,6) -10 mA/mA +10 % AUTOMATIC POWER CONTROL (APC) MD Nominal Voltage VMD Voltage at REF VREF APC loop is closed 1 VREF 0.16 2 V 1.2 1.8 2.2 V +2 µA MD Voltage During Fault 0 MD Input Current Normal operation (FAULT = low) -2 0.7 APC Time Constant CCOMP = 0.047µF (Note 6) 5 20 PWRMON Nominal Gain VPWRMON / (VREF - VMD) 1.85 2.15 V µs 2.45 V/V LASER MODULATOR (Note 7) Data Input Voltage Swing VID Output Resistance Modulation Current IMOD Minimum Maximum 250 2200 Single-ended resistance at OUT+ 80 105 Single-ended resistance at OUT- 72 100 Minimum Maximum 2 15 mVP-P Ω mAP-P Minimum Peaking Current Range 0.2 mA Maximum Peaking Current Range 2 mA Peaking Current Duration 80 ps Tolerance of Programmed Modulation Current TC1 is shorted to TC2 -10 Minimum Programmable Temperature Coefficient Maximum Programmable Temperature Coefficient Temperature range 0°C to +70°C +10 % 0 ppm/°C +5000 ppm/°C t R , tF 5mA ≤ IMOD ≤ 15mA, 20% to 80% (Note 6) 65 95 ps Deterministic Jitter DJ 5mA ≤ IMOD ≤ 15mA, 3.2Gbps (Notes 6, 8) 12 20 psP-P Random Jitter RJ (Note 6) 1.3 4 psRMS 15 50 µAP-P 100 115 Ω Modulation Transition Time Laser Modulation During Fault or while Squelch is Active IMOD_OFF Input Resistance Input Bias Voltage Differential resistance VIN 85 VCC 0.3 V _______________________________________________________________________________________ 3 MAX3740A ELECTRICAL CHARACTERISTICS (continued) MAX3740A 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors ELECTRICAL CHARACTERISTICS (continued) (VCC = +2.97V to +3.63V, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA = +25°C, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS SAFETY FEATURES (see the Typical Operating Characteristics section) High-Current Fault Threshold VBIAS Fault Threshold VBMTH VBTH VBIASMON > VBMTH causes a fault VBIAS referenced to VCC 0.7 0.8 0.9 V -0.250 -0.2 -0.150 V 0.7 0.8 0.9 V Power-Monitor Fault Threshold VPMTH VPWRMON > VPMTH causes a fault TX Disable Time t_OFF Time from rising edge of TX_DISABLE to IBIAS = IBIAS_OFF and IMOD = IMOD_OFF (Note 6) 1.8 5 µs TX Disable Negate Time t_ON Time from rising edge of TX_DISABLE to IBIAS and IMOD at 99% of steady state (Note 6) 55 500 µs Fault Reset Time t_INIT1 Time to set VFAULT = low after power-on or after rising edge of TX_DISABLE (Note 6) 60 200 ms Power-On Time t_INIT2 Time after power-on to transmitter-on with TX_DISABLE low (Note 6) 60 200 ms Fault Assert Time t_FAULT Time from fault occurrence to VFAULT = high; CFAULT < 20pF, RFAULT = 4.7kΩ (Note 6) 1.4 50 µs Fault Delay Time t_FLTDLY Time from fault to IBIAS = IBIAS_OFF and IMOD = IMOD_OFF (Note 6) 1 5 µs TX_DISABLE Reset t_RESET Time TX_DISABLE must be held high to reset FAULT (Note 6) 1 µs Note 1: Supply current measurements exclude IBIAS from the total current. Note 2: Tested with RPEAK = 1.18kΩ. Note 3: Measured by applying a pattern that contains 20µs of K28.5, followed by 5µs of zeros, then 20µs of K28.5, followed by 5µs 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 50Ω 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 (00111110101100000101). 4 _______________________________________________________________________________________ 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors ELECTRICAL EYE ELECTRICAL EYE WITH MAX PEAKING ELECTRICAL EYE WITH PEAKING MAX3740A toc01 MAX3740A toc03 MAX3740A toc02 3.2Gbps, K28.5, 10mA MODULATION, PEAKING OFF 3.2Gbps, K28.5, 10mA MODULATION, RPEAKSET = 500Ω 3.2Gbps, K28.5, 10mA MODULATION, RPEAKSET = 2.4kΩ 73mV/div 73mV/div 73mV/div 50ps/div 50ps/div 50ps/div OPTICAL EYE MAX3740A toc04 IBIASMON vs. BIAS CURRENT MAX3740A toc05 ER = 8.2dB, 2.5Gbps, K28.5, 850nm VCSEL SONET MASK WITH +20% MARGIN 1.6 1.4 IBIASMON (mA) ER = 8.2dB, 2.125Gbps, K28.5, 850nm VCSEL, WITH 2.3GHz O-TO-E CONVERTER 1.8 MAX3740A toc06 OPTICAL EYE 1.2 1.0 0.8 0.6 0.4 EMCORE SC-TOSA-8585-3420 VCSEL 0.2 EMCORE SC-TOSA-8585-3420 VCSEL 0 68ps/div 0 58ps/div 4 8 16 12 BIAS CURRENT (mA) DETERMINISTIC JITTER vs. MODULATION CURRENT 25 20 15 10 5 4 3 2 0 0 0 5 10 IMOD (mAP-P) 15 MAX3740A toc09 80 RISE 70 FALL 60 50 1 5 90 TRANSITION TIME (ps) 6 RANDOM JITTER (psRMS) 30 100 MAX3740A toc08 35 DETERMINISTIC JITTER (psP-P) 7 MAX3740A toc07 40 TRANSITION TIME vs. MODULATION CURRENT RANDOM JITTER vs. MODULATION CURRENT 40 0 5 10 IMOD (mAP-P) 15 2 4 6 8 10 12 14 16 IMOD (mAP-P) _______________________________________________________________________________________ 5 MAX3740A Typical Operating Characteristics (VCC = +3.3V, RTC = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (VCC = +3.3V, RTC = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, TA = +25°C, unless otherwise noted.) 100μ 10m 1m 100μ 1m 10k 100k 100 1k RBIASSET (Ω) SUPPLY CURRENT vs. TEMPERATURE OUTPUT RETURN LOSS INPUT RETURN LOSS DIFFERENTIAL MEASUREMENT -5 0 -4 40 -6 -15 S22 (dB) S11 (dB) 50 -20 -25 30 IMOD = 2mA 10 -15 10 35 85 60 -16 -40 100M -18 100M 10G 1G MODULATION CURRENT vs. TEMPERATURE MODULATION CURRENT TEMPCO vs. RTC MONITOR DIODE CURRENT vs. TEMPERATURE 9 RTC = 5kΩ RTC = 10kΩ RTC = 60kΩ RTC = 100kΩ 6 RTC = 500kΩ 5 4500 3500 2500 1500 500 10 20 30 40 50 60 TEMPERATURE (°C) 70 80 90 275 250 225 200 175 150 125 100 -500 4 300 MAX3740A toc18 RTC = 1kΩ REFERENCED TO +25°C MONITOR DIODE CURRENT (μA) 10 7 5500 TEMPCO (ppm/°C) RMOD = 1.35kΩ MAX3740A toc17 FREQUENCY (Hz) RTC = 100Ω 0 10G 1G FREQUENCY (Hz) 11 8 -14 -35 TEMPERATURE (°C) MAX3740A toc16 -40 -8 -10 -12 -30 20 SINGLE-ENDED MEASUREMENT -2 -10 60 10k RPWRSET (Ω) 0 MAX3740A toc13 IMOD = 15mA 1k 100 RMODSET (Ω) 80 70 10μ 1μ 10k MAX3740A toc14 1k SUPPLY CURRENT (mA) 10m MAX3740A toc12 MEASURED WITH A 50Ω ELECTRICAL LOAD MAX3740A toc15 1m MAX3740A toc11 MAX3740A toc10 10m 100m MODULATION CURRENT (AP-P) BIAS CURRENT (A) 100m 6 MONITOR DIODE CURRENT vs. RPWRSET MODULATION CURRENT vs. RMODSET MONITOR DIODE CURRENT (A) BIAS CURRENT vs. RBIASSET MODULATION CURRENT (mAP-P) MAX3740A 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors 100 1k 10k RTC (Ω) 100k 1M -40 -15 10 35 TEMPERATURE (°C) _______________________________________________________________________________________ 60 85 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors TX_DISABLE NEGATE TIME STARTUP WITH SLOW RAMPING SUPPLY HOT PLUG WITH TX_DISABLE LOW MAX3740A toc21 MAX3740A toc20 MAX3740A toc19 3.3V 3.3V VCC FAULT 3.3V VCC OV VCC OV FAULT LOW TX_DISABLE LOW t_INIT = 62ms TX_DISABLE t_INIT = 60ms FAULT LOW TX_DISABLE LOW HIGH t_ON = 54μs 20μs/div 20ms/div 20ms/div LOW LASER OUTPUT LASER OUTPUT LASER OUTPUT LOW RESPONSE TO FAULT TRANSMITTER DISABLE MAX3740A toc22 3.3V MAX3740A toc23 EXTERNALLY FORCED VPWRMON FAULT VCC t_OFF = 1.86μs FAULT FAULT HIGH LOW TX_DISABLE TX_DISABLE t_FAULT = 245ns LOW LOW LOW HIGH LASER OUTPUT LASER OUTPUT 200ns/div 1μs/div FREQUENT ASSERTION OF TX_DISABLE FAULT RECOVERY TIME MAX3740A toc25 MAX3740A toc24 EXTERNAL FAULT REMOVED VPWRMON FAULT VPWRMON EXTERNALLY FORCED FAULT FAULT HIGH LOW HIGH TX_DISABLE LOW LOW t_INIT = 54μs LASER OUTPUT 40μs/div TX_DISABLE LASER OUTPUT 200μs/div _______________________________________________________________________________________ 7 MAX3740A Typical Operating Characteristics (continued) (VCC = +3.3V, RTC = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, TA = +25°C, unless otherwise noted.) 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740A Pin Description 8 PIN NAME FUNCTION 1, 10, 13 GND 2 TX_DISABLE 3 IN+ Noninverted Data Input 4 IN- Inverted Data Input 5 FAULT 6 SQUELCH 7, 16, 20 VCC +3.3V Supply Voltage 8 TC1 Temperature Compensation Set Pin 1. A resistor placed between TC1 and TC2 (RTC) programs the temperature coefficient of the modulation current. 9 TC2 Temperature Compensation Set Pin 2. A resistor placed between TC1 and TC2 (RTC) programs the temperature coefficient of the modulation current. 11 MODSET Modulation Set. A resistor connected from MODSET to ground (RMODSET) sets the desired modulation current amplitude. 12 PEAKSET Peaking Current Set. A resistor connected between PEAKSET and ground (RPEAKSET) programs the peaking current amplitude. To disable peaking, leave PEAKSET open. 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. 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. 14 OUT- Inverted Modulation-Current Output 15 OUT+ Noninverted Modulation-Current Output 17 BIASSET 18 BIAS 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 19 BIASMON 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. 21 COMP Compensation Pin. A capacitor between COMP and MD compensates the APC. A typical value of 0.047μF is recommended. For open-loop configuration, short the COMP pin to GND to deactivate the APC. 22 MD Monitor Diode Connection 23 REF 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. 24 PWRMON — EP Average Power Monitor. The pin is used to monitor the transmit optical power. For open-loop configuration, connect PWRMON to GND. Exposed Pad. Ground. Must be soldered to the circuit board ground for proper thermal and electrical performance. See the Layout Considerations section. _______________________________________________________________________________________ 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740A PWRMON REF 1.8V RPWRSET CURRENT AMPLIFIER 2X MAX3740A POWERCONTROL AMPLIFIER MD ENABLE IBIAS 34 BIAS BIAS GENERATOR FERRITE BEAD SMOOTHSTART IPD BIASMON 1.6V (2VBE) IBIAS 9 1.2V RBIASMON 200Ω COMP BIASSET RBIASSET CCOMP 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. 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 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 1.7kΩ resistor between ground and BIASSET to set the maximum bias current. 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 4.7kΩ to 10kΩ resistor. Table 1. Fault Conditions PIN FAULT CONDITION BIAS VBIAS > VCC - 0.2V BIASMON VBIASMON > 0.8V PWRMON VPWRMON > 0.8V _______________________________________________________________________________________ 9 MAX3740A 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors Table 2. Circuit Response to Various Single-Point Faults (Closed-Loop APC Configuration) PIN NAME FAULT TX_DISABLE CIRCUIT RESPONSE TO VCC SHORT CIRCUIT RESPONSE TO GND SHORT Does not affect laser power. Does not affect laser power. Modulation and bias current are disabled. Normal condition for circuit operation. IN+ Does not affect laser power. Does not affect laser power. Does not affect laser power. IN- Does not affect laser power. SQUELCH Does not affect laser power. Does not affect laser power. TC1 Does not affect laser power. Does not affect laser power. TC2 The laser modulation is increased, but average power is not affected. Modulation current is disabled. Modulation current is disabled. The laser modulation is increased, but average power is not affected. MODSET PEAKSET Does not affect laser power. Does not affect laser power. OUT+ Modulation current is disabled. Modulation current is disabled. OUT- Does not affect laser power. Does not affect laser power. Laser bias is disabled. Fault state* occurs. Fault state* occurs. Note that VCSEL emissions may continue; care must be taken to prevent this condition. Disables VCSEL. Fault state* occurs. Does not affect laser power. COMP 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. MD 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. REF 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. Fault state* occurs. Does not affect laser power. BIASSET BIAS BIASMON PWRMON *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 50Ω 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. 10 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). ______________________________________________________________________________________ 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740A TX_DISABLE BIAS VBIAS FAULT FAULT OUTPUT VCC - 0.2V FAULT BIASMON HIGH-CURRENT FAULT R 0.8V Q ENABLE S R-S LATCH PWRMON HIGH-POWER FAULT 0.8V POR TX_DISABLE SAFETY CIRCUIT MAX3740A Figure 2. Safety Circuit VCC MAX3740A ROUT+ ROUT- INPUT BUFFER OUT+ CURRENT SWITCH IN+ OUTSIGNAL DETECT 100Ω IN- PEAKING CONTROL PEAKSET SQUELCH ENABLE CURRENT AMPLIFIER 30x MODULATION CURRENT GENERATOR RPEAKSET TEMPERATURE COMPENSATION 1V 200Ω TC2 TC1 MODSET RMODSET RTC Figure 3. Modulation Circuit ______________________________________________________________________________________ 11 MAX3740A 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors 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 ) × 30 × ⎢ ⎥ R + R LOAD ⎦ ⎣ OUT+ ⎡⎛ ⎤ ⎡ ⎞ ⎤ ROUT+ 1 IMOD = ⎢⎜ × 30⎥ × ⎢ ⎥ ⎟ ⎢⎣⎝ 200 + RMODSET ⎠ ⎥⎦ ⎣ ROUT+ + RLOAD ⎦ [ ] 0.021mW/mA at +25°C, which reduces to 0.018mW/mA at +85°C. The temperature coefficient is given by the following: (SE85 − SE25 ) × 1E6 SE25 × (85 − 25) = −2380ppm / °C Laser tempco = From the Typical Operating Characteristics, the value of RTC, which offsets the tempco of the laser, is 9kΩ. 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 50Ω electrical load. 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: 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. Input Termination Requirements The MAX3740A data inputs are SFP MSA compatible. On-chip 100Ω 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. IBIAS = (IBIASSET ) × 34 ⎛ ⎞ 1.2 IBIAS = ⎜ ⎟ × 34 ⎝ 200 + RBIASSET ⎠ VCC The Bias Current vs. RBIASSET graph is also shown in the Typical Operating Characteristics. MAX3740A 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 = 5µs if CAPC = 0.047µF). PACKAGE IN+ 16kΩ VCC 1nH 0.5pF 50Ω 5μs t MD ≤ , RMD × CMD ≤ = 250ns 20 20 t APC VCC 50Ω For typical IPD = 400µA, RPWRSET = 500Ω, 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 = +25°C 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 ___________________________________________________ IN- 1nH 0.5pF 24kΩ Figure 4. Simplified Input Structure 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors UNCOMPENSATED PACKAGE ROUT+ 1nH OUT- 0.5pF 1nH OUT+ CORRECTLY COMPENSATED POWER ROUT- MAX3740A VCC OVERCOMPENSATED 0.5pF TIME MAX3740A Figure 7. Laser Compensation The compensation components (RF and CF) are most easily determined by experimentation. Begin with RF = 50Ω 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. Figure 5. Simplified Output Structure VCC MAX3740A Exposed-Pad (EP) Package FAULT Figure 6. Fault Circuit Interface Applications Information 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. Interface Models Laser Safety and IEC 825 Figures 4 and 5 show simplified input and output circuits for the MAX3740A laser driver. Figure 6 shows the fault circuit interface. 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. 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. ______________________________________________________________________________________ 13 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740A Functional Diagram COMP FAULT MD REF BIAS BIAS GENERATOR WITH APC SAFETY CIRCUITRY TX_DISABLE BIASMON PWRMON BIASSET ENABLE VCC LASER MODULATOR MAX3740A SQUELCH OUTOUT+ IN+ SIGNAL DETECT 100Ω PEAKING CONTROL INMODULATION CURRENT GENERATOR TC2 PEAKSET MODSET Pin Configuration REF MD COMP VCC BIASMON 22 21 20 19 BIAS 17 BIASSET IN+ 3 16 VCC 15 OUT+ 14 OUT- 13 GND *EP 12 6 PEAKSET SQUELCH 11 5 10 4 9 INFAULT MAX3740A TC2 21-0139 18 2 8 T2444-4 DOCUMENT NO. 1 7 24 TQFN-EP (4mm x 4mm x 0.75mm) PACKAGE CODE GND TX_DISABLE VCC PACKAGE TYPE PWRMON Package Information For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. 23 TOP VIEW TC1 TRANSISTOR COUNT: 3806 PROCESS: SiGe BIPOLAR 24 Chip Information GND TC1 MODSET ENABLE THIN QFN (4mm x 4mm) *EXPOSED PAD IS CONNECTED TO GND 14 ______________________________________________________________________________________ 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors REVISION NUMBER REVISION DATE 0 12/03 1 2 3 DESCRIPTION PAGES CHANGED Initial release. — 6/04 Added a lead-free package to the Ordering Information table. 1 In the Electrical Characteristics table, modified the MD Nominal Voltage parameter of VREF - 0.2V (typ) to VREF - 0.16V (typ). 3 5/06 Modified Figure 1 to clarify the meaning of the arrow labeled IPD. 9 Updated the Package Information section to correct the package code. 14 1/10 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 ____________________ 15 © 2010 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc. MAX3740A Revision History