19-2679; Rev 2; 7/03 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors Features ♦ Supports all SFF-8472 Digital Diagnostics The MAX3740 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 MAX3740 interfaces with the Dallas DS1858 to meet SFF-8472 timing and diagnostic requirements. The MAX3740 accommodates various VCSEL packages, including low-cost TO-46 headers. The MAX3740 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). ♦ Automatic Power Control ♦ 2mA to 15mA Modulation Current ♦ 1mA to 15mA Bias Current ♦ Optional Peaking Current to Improve VCSEL Edge Speed ♦ Supports Common Cathode and Differential Configuration ♦ Safety Circuits Compliant with SFF and SFP MSAs ♦ 4mm ✕ 4mm 24-Pin Thin QFN Package Ordering Information PART TEMP RANGE PIN-PACKAGE MAX3740ETG -40°C to +85°C 24 Thin QFN (4mm ✕ 4mm) The MAX3740 is available in a compact 4mm ✕ 4mm, 24-pin thin QFN package and operates over the -40°C to +85°C temperature range. Applications Typical Application Circuit Multirate (1Gbps to 3.2Gbps) SFP/SFF Modules +3.3V Gigabit Ethernet Optical Transmitters FAULT Fibre Channel Optical Transmitters VCC Infiniband Optical Transmitters 4.7kΩ PWRMON SQUELCH MON2 MODSET H0 REF MAX3740 0.1µF H1 MON1 DS1858 L0 L1 BIASMON COMP IN+ OUT1 IN1 FAULT TX_DISABLE RBIASMON 0.047µF IN- MD BIAS 0.1µF TC1 RTC L1* † 0.01µF OUT+ TC2 BIASSET GND RBIASSET PEAKSET RPEAKSET† CF† OUT0.01µF 50Ω RF† †OPTIONAL COMPONENT *FERRITE BEAD ________________________________________________________________ Maxim Integrated Products 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. 1 MAX3740 General Description The MAX3740 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. MAX3740 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 CONDITIONS MIN SQUELCH set low, IMOD = 2mAP-P TX_DISABLE set low, peaking is not used IMOD = 15mAP-P (Note 1) MAX UNITS 32 55 67 15 20 5 10 3.9 5 mA Additional current when peaking is used (Note 2) Additional current when SQUELCH is high ICC-SHDN TYP 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 Current into FAULT pin with VCC = 0V and VFAULT = 3.3V Output Leakage V 0.5 0.4 V 40 µA 10.0 kΩ 0.8 V 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 50 µs SQUELCH Squelch Threshold 25 Squelch Hysteresis 85 10 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 ∆BIAS 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 CONDITIONS IBIAS < 3mA BIASMON Nominal Gain MIN TYP MAX UNITS 1.5 10 µA 0.0925 0.105 0.1375 0.085 0.105 0.125 1 VREF 0.2 2 1.2 1.8 2.2 Current out of the BIAS pin 3mA ≤ IBIAS ≤ 15mA mA/mA AUTOMATIC POWER CONTROL (APC) MD Nominal Voltage VMD Voltage at REF VREF APC loop is closed MD Voltage During Fault 0 MD Input Current Normal operation (FAULT = low) -2 0.7 APC Time Constant CCOMP = 0.047µF (Note 5) 5 10 PWRMON Nominal Gain VPWRMON / (VREF - VMD) 1.9 2.15 V V V +2 µA 2.4 V/V µs LASER MODULATOR (Note 6) 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 tR, tF 5mA ≤ IMOD ≤ 15mA, 20% to 80% (Note 5) 65 95 ps Deterministic Jitter DJ 5mA ≤ IMOD ≤ 15mA, 3.2Gbps (Notes 5, 7) 12 20 psP-P Random Jitter RJ (Note 5) 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 MAX3740 ELECTRICAL CHARACTERISTICS (continued) MAX3740 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 5) 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 5) 55 500 µs Fault Reset Time t_INIT1 Time to set VFAULT = low after power-on or after rising edge of TX_DISABLE (Note 5) 60 200 ms Power-On Time t_INIT2 Time after power-on to transmitter-on with TX_DISABLE low (Note 5) 60 200 ms Fault Assert Time t_FAULT Time from fault occurrence to VFAULT = high; CFAULT < 20pF, RFAULT = 4.7kΩ (Note 5) 1.4 50 µs Fault Delay Time t_FLTDLY Time from fault to IBIAS = IBIAS_OFF and IMOD = IMOD_OFF (Note 5) 1 5 µs TX_DISABLE Reset t_RESET Time TX_DISABLE must be held high to reset FAULT (Note 5) 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: Guaranteed by design and characterization. Note 6: Measured electrically with a 50Ω load AC-coupled to OUT+. Note 7: 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 PEAKING MAX3740 toc01 ELECTRICAL EYE WITH MAX PEAKING MAX3740 toc02 3.2Gbps, K28.5, 10mA MODULATION, PEAKING OFF MAX3740 toc03 3.2Gbps, K28.5, 10mA MODULATION, RPEAKSET = 2.4kΩ 73mV/div 3.2Gbps, K28.5, 10mA MODULATION, RPEAKSET = 500Ω 73mV/div 73mV/div 50ps/div 50ps/div 50ps/div OPTICAL EYE IBIASMON vs. BIAS CURRENT MAX3740 toc05 MAX3740 toc04 1.8 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 MAX3740 toc06 OPTICAL EYE 1.2 1.0 0.8 0.6 0.4 0.2 EMCORE SC-TOSA-8585-3420 VCSEL EMCORE SC-TOSA-8585-3420 VCSEL 0 0 58ps/div 68ps/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 MAX3740 toc09 80 RISE 70 FALL 60 50 1 5 90 TRANSITION TIME (ps) 6 RANDOM JITTER (psRMS) 30 100 MAX3740 toc08 35 DETERMINISTIC JITTER (psP-P) 7 MAX3740 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 MAX3740 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.) 10 8 6 4 12 10 8 6 4 2 2 0 0 20 30 40 2 4 6 8 1000 30 10 35 85 60 -4 -20 -8 -10 -12 -14 -35 -16 -40 100M -18 100M 10G 1G 10G 1G FREQUENCY (Hz) FREQUENCY (Hz) 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 4 30 40 50 60 TEMPERATURE (°C) 70 80 90 275 250 225 200 175 150 125 100 -500 20 300 MAX3740 toc18 RTC = 1kΩ REFERENCED TO +25°C MONITOR DIODE CURRENT (µA) 10 TEMPCO (ppm/°C) RMOD = 1.35kΩ 7 5500 MAX3740 toc16 RTC = 100Ω 10 -6 -15 TEMPERATURE (°C) 11 10 8 SINGLE-ENDED MEASUREMENT -2 -30 IMOD = 2mA 6 OUTPUT RETURN LOSS -25 0 4 0 S22 (dB) 40 8 2 RPWRSET (kΩ) DIFFERENTIAL MEASUREMENT -5 S11 (dB) 50 10 400 0 -10 -15 600 INPUT RETURN LOSS 60 -40 800 10 0 MAX3740 toc13 IMOD = 15mA 20 MAX3740 toc12 1200 0 0 SUPPLY CURRENT vs. TEMPERATURE SUPPLY CURRENT (mA) 1400 RMODSET (kΩ) 80 6 1600 200 RBIASSET (kΩ) 70 1800 MAX3740 toc14 10 MAX3740 toc11 14 2000 MAX3740 toc15 12 16 MAX3740 toc17 BIAS CURRENT (mA) 14 18 MODULATION CURRENT (mAP-P) MAX3740 toc10 16 0 MONITOR DIODE CURRENT vs. RPWRSET MODULATION CURRENT vs. RMODSET MONITOR DIODE CURRENT (µA) BIAS CURRENT vs. RBIASSET MODULATION CURRENT (mAP-P) MAX3740 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 HOT PLUG WITH TX_DISABLE LOW TX_DISABLE NEGATE TIME STARTUP WITH SLOW RAMPING SUPPLY MAX3740 toc19 MAX3740 toc21 MAX3740 toc20 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 LASER OUTPUT HIGH RESPONSE TO FAULT MAX3740 toc22 3.3V MAX3740 toc23 EXTERNALLY FORCED VPWRMON FAULT VCC t_OFF = 1.86µs TX_DISABLE FAULT LOW LOW HIGH LASER OUTPUT TX_DISABLE t_FAULT = 245ns LOW HIGH LOW LASER OUTPUT 1µs/div 200ns/div FAULT RECOVERY TIME FREQUENT ASSERTION OF TX_DISABLE MAX3740 toc24 EXTERNAL FAULT REMOVED VPWRMON FAULT LOW 20µs/div 20ms/div TRANSMITTER DISABLE FAULT t_ON = 54µs LASER OUTPUT LASER OUTPUT 20ms/div LOW MAX3740 toc25 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 MAX3740 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 MAX3740 Pin Description 8 PIN NAME 1, 10, 13 GND FUNCTION 2 TX_DISABLE 3 IN+ Noninverted Data Input 4 IN- Inverted Data Input 5 FAULT Fault Indicator. Open-drain output. FAULT is asserted high during a fault condition. Note: This pin does not have ESD protection. 6 SQUELCH Squelch Enable. Squelch is enabled when the pin is set high. Squelch is disabled when the pin is set low or left open. 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. 14 OUT- Inverted Modulation-Current Output 15 OUT+ Noninverted Modulation-Current Output 17 BIASSET 18 BIAS 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 Exposed Pad Bias Current Set. When a closed-loop configuration is used, connect a 1.7kΩ 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 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. _______________________________________________________________________________________ 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740 PWRMON REF 1.8V RPWRSET CURRENT AMPLIFIER 2X MAX3740 POWERCONTROL AMPLIFIER MD ENABLE IBIAS 40 BIAS BIAS GENERATOR FERRITE BEAD SMOOTHSTART IPD BIASMON 1.6V (2VBE) IBIAS 9 0.8V RBIASMON 200Ω COMP BIASSET RBIASSET CCOMP Figure 1. Bias Generator Detailed Description The MAX3740 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 MAX3740 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. IN- Does not affect laser power. 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. MODSET Modulation current is disabled. The laser modulation is increased, but average power is not affected. 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. BIASSET BIAS BIASMON 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. 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 MAX3740 FAULT TX_DISABLE BIAS VBIAS FAULT VCC - 0.2V OPENDRAIN NMOS BIASMON HIGH-CURRENT FAULT R Q ENABLE 0.8V S PWRMON HIGH-POWER FAULT R-S LATCH 0.8V MAX3740 POR TX_DISABLE SAFETY CIRCUIT Figure 2. Safety Circuit VCC MAX3740 OUT+ OUT- CURRENT SWITCH INPUT BUFFER IN+ ROUT+ ROUT- SIGNAL DETECT 100Ω PEAKING CONTROL IN- PEAKSET SQUELCH ENABLE CURRENT AMPLIFIER 30x MODULATION CURRENT GENERATION RPEAKSET TEMPERATURE COMPENSATION 1V TC1 TC2 RTC MODSET RMODSET Figure 3. Modulation Circuit ______________________________________________________________________________________ 11 MAX3740 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors Programming Modulation Current See the Modulation Current vs. RMODSET graph in the Typical Operating Characteristics, and select the value of RMODSET that corresponds to the required current at +25°C. 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 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 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 MAX3740 data inputs are SFP MSA compatible. Onchip 100Ω differential input impedance is provided for optimal termination (Figure 4). Because of the on-chip biasing network, the MAX3740 inputs self-bias to the proper operating point to accommodate AC-coupling. VCC VCC MAX3740 PACKAGE IN+ 16kΩ ROUT- ROUT+ PACKAGE VCC 1nH OUT- 1nH 0.5pF 0.5pF 50Ω 1nH 0.5pF VCC 50Ω IN- 1nH 0.5pF MAX3740 24kΩ Figure 4. Simplified Input Structure 12 Figure 5. Simplified Output Structure ______________________________________________________________________________________ OUT+ 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740 Applications Information UNCOMPENSATED Interface Models Figures 4 and 5 show simplified input and output circuits for the MAX3740 laser driver. To minimize inductance, keep the connections between the MAX3740 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. CORRECTLY COMPENSATED POWER Layout Considerations OVERCOMPENSATED 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. 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 6). Refer to Application Note HFAN-2-0: Interfacing Maxim Laser Drives with Laser Diodes for more information. 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 MAX3740 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. TIME Figure 6. Laser Compensation 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. ESD Protection The FAULT pin of the MAX3740 does not include ESD protection. If this pin is connected to the DS1858, protection is not needed. Protection can be provided with external diodes as shown in Figure 7. 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 MAX3740 provides features that facilitate compliance with IEC 825. A common safety precaution is singlepoint 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 VCC MAX3740 FAULT PHILLIPS BAV99 Figure 7. External Diode Protection ______________________________________________________________________________________ 13 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors MAX3740 Functional Diagram COMP FAULT MD REF BIAS BIAS GENERATOR WITH APC SAFETY CIRCUITRY TX_DISABLE BIASMON PWRMON BIASSET ENABLE VCC LASER MODULATOR MAX3740 SQUELCH OUTOUT+ IN+ SIGNAL DETECT 100Ω PEAKING CONTROL INMODULATION CURRENT GENERATOR PEAKSET MODSET Pin Configuration PWRMON REF MD COMP VCC BIASMON 22 21 20 19 3 16 VCC IN- 4 15 OUT+ FAULT 5 14 OUT- SQUELCH 6 13 GND 12 MAX3740 11 T2444-1 IN+ MODSET 24 Thin QFN (4mm ✕ 4mm ✕ 0.8mm) BIASSET PEAKSET MAX3740ETG PACKAGE CODE BIAS 17 9 PACKAGE TYPE 18 2 8 PART 1 7 For the latest package outline information, go to www.maxim-ic.com/packages. GND TX_DISABLE VCC Package Information 23 TOP VIEW TC1 TRANSISTOR COUNT: 3806 PROCESS: SiGe BIPOLAR 24 Chip Information 10 TC2 TC2 TC1 GND ENABLE 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 © 2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.