19-2529; Rev 1; 4/03 KIT ATION EVALU E L B AVAILA 2.7Gbps, Low-Power SFP Laser Drivers Features ♦ SFP Reference Design Available ♦ Fully Compliant with SFP and SFF-8472 MSAs ♦ Programmable Modulation Current from 10mA to 60mA (DC-Coupled) ♦ Programmable Modulation Current from 10mA to 85mA (AC-Coupled) ♦ Programmable Bias Current from 1mA to 100mA ♦ Edge Transition Times <51ps ♦ 27mA (typ) Power-Supply Current ♦ Multirate 155Mbps to 2.7Gbps Operation ♦ Automatic Average Power Control ♦ On-Chip Pullup Resistor for TX_DISABLE ♦ 24-Pin 4mm × 4mm QFN package Ordering Information PART Applications TEMP RANGE PIN-PACKAGE MAX3735E/D -40°C to +85°C Dice* MAX3735ETG -40°C to +85°C 24 Thin QFN-EP** MAX3735EGG -40°C to +85°C 24 QFN-EP** MAX3735AETG -40°C to +85°C 24 Thin QFN-EP** *Dice are designed to operate from -40°C to +85°C, but are tested and guaranteed only at TA = +25°C. **EP = Exposed pad. Gigabit Ethernet SFP/SFF Transceiver Modules 1G/2G Fibre Channel SFP/SFF Transceiver Modules Multirate OC3 to OC48-FEC SFP/SFF Transceiver Modules Pin Configuration appears at end of data sheet. Typical Application Circuit +3.3V OPTIONAL SHUTDOWN CIRCUITRY 0.1µF SHUTDOWN VCC TX_FAULT PC_MON APCFILT2 CAPC +3.3V 0.01µF 10Ω OUT+ BIAS BC_MON APCSET MODSET DS1858/DS1859 H0 CONTROLLER IC H1 15Ω OUT- OUT+ MAX3735 MAX3735A IN- APCFILT1 IN+ SERDES GND 0.1µF TX_DISABLE +3.3V FERRITE BEAD MD CMD RBC_MON MON1 M0N2 M0N3 +3.3V RPC_MON REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE ________________________________________________________________ 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 MAX3735/MAX3735A General Description The MAX3735/MAX3735A are +3.3V laser drivers for SFP/SFF applications from 155Mbps up to 2.7Gbps. The devices accept differential input data and provide bias and modulation currents for driving a laser. DCcoupling to the laser allows for multirate applications and reduces the number of external components. The MAX3735/MAX3735A are fully compliant with the SFP MSA timing and the SFF-8472 transmit diagnostic requirements. An automatic power-control (APC) feedback loop is incorporated to maintain a constant average optical power over temperature and lifetime. The wide modulation current range of 10mA to 60mA (up to 85mA AC-coupled) and bias current of 1mA to 100mA make this product ideal for driving FP/DFB laser diodes in fiber-optic modules. The resistor range for the laser current settings is optimized to interface with the DS1858 SFP controller IC. The MAX3735/MAX3735A provide transmit-disable control, a single-point latched transmit-failure monitor output, photocurrent monitoring, and bias-current monitoring to indicate when the APC loop is unable to maintain the average optical power. The MAX3735A also features improved multirate operation. The MAX3735/MAX3735A come in package and die form, and operate over the extended temperature range of -40°C to +85°C. MAX3735/MAX3735A 2.7Gbps, Low-Power SFP Laser Drivers ABSOLUTE MAXIMUM RATINGS Supply Voltage, VCC..............................................-0.5V to +6.0V Current into BIAS, OUT+, OUT- ......................-20mA to +150mA Current into MD.....................................................-5mA to +5mA Voltage at IN+, IN-, TX_DISABLE, TX_FAULT, SHUTDOWN ...........................................-0.5V to (VCC + 0.5V) Voltage at BIAS, PC_MON, BC_MON, MODSET, APCSET .................................-0.5V to (VCC + 0.5V) Voltage at OUT+, OUT-.............................+0.5V to (VCC + 1.5V) Voltage at APCFILT1, APCFILT2 ..............................-0.5V to +3V Continuous Power Dissipation (TA = +85°C ) 24-Lead Thin QFN (derate 20.8mW/°C above +85°C).............................................................1354mW 24-Lead QFN (derate 20.8mW/°C above +85°C).............................................................1354mW Operating Ambient Temperature Range (TA)......-40°C to +85°C Storage Ambient Temperature Range...............-55°C to +150°C Die Attach Temperature...................................................+400°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 at VCC = +3.3V, IBIAS = 20mA, IMOD = 30mA, TA = +25°C, unless otherwise noted.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS 27 50 mA 2400 mVP-P POWER SUPPLY Power-Supply Current ICC Excludes the laser bias and modulation currents (Note 2) VID VID = (VIN+) - (VIN-), Figure 1 I/O SPECIFICATIONS Differential Input Voltage 200 0.6 × VCC Common-Mode Input Voltage Differential Input Resistance TX_DISABLE Input Pullup Resistance RPULL 100 115 Ω 4.7 7.4 10.0 kΩ VHIGH = VCC TX_DISABLE Input Current V 85 15 VLOW = GND, VCC = 3.3V, RPULL = 7.4kΩ TX_DISABLE Input High Voltage VIH TX_DISABLE Input Low Voltage VIL TX_FAULT Output High Voltage VOH IOH = 100µA sourcing (Note 3) TX_FAULT Output Low Voltage VOL IOL = 1mA sinking (Note 3) SHUTDOWN Output High Voltage VOH IOH = 100µA sourcing SHUTDOWN Output Low Voltage VOL IOL = 100µA sinking IBIAS Current into BIAS pin -450 2 µA V 0.8 V 0.4 V 2.4 V VCC - 0.4 V 0.4 V 100 mA Current into BIAS pin during TX_FAULT or TX_DISABLE 100 µA During SFP module hot plugging (Notes 4, 5, 11) 10 % BIAS GENERATOR Bias On-Current Range Bias Off-Current IBIASOFF Bias Overshoot Bias-Current Monitor Gain Bias-Current Monitor Gain Stability 2 IBC_MON 1 External resistor to GND defines the voltage gain, IBIAS = 1mA, RBC_MON = 69.28kΩ 10.0 12 13.5 IBIAS = 100mA, RBC_MON = 693.25Ω 11.5 13 13.5 1mA ≤ IBIAS ≤ 100mA (Notes 4, 6) MAX3735 -8 +8 MAX3735A -6 +6 _______________________________________________________________________________________ mA/A % 2.7Gbps, Low-Power SFP Laser Drivers (VCC = +2.97V to +3.63V, TA = -40°C to +85°C. Typical values at VCC = +3.3V, IBIAS = 20mA, IMOD = 30mA, TA = +25°C, unless otherwise noted.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS AUTOMATIC POWER-CONTROL LOOP 18µA ≤ IMD ≤ 1500µA MD Reverse Bias Voltage MD Average Current Range IMD IPC_MON MD-Current Monitor Gain Stability External resistor to GND defines the voltage gain; IMD = 18µA, RPC_MON = 50kΩ V 18 1500 IBIAS = 1mA (MAX3735) -880 +880 IBIAS = 1mA (MAX3735A) -110 +110 IBIAS = 100mA -650 +650 -16 +16 APC Closed Loop 1mA ≤ IBIAS ≤ 100mA (Note 8) Average Power Setting Accuracy MD-Current Monitor Gain Average current into MD pin APC closed loop (Notes 4, 7) Average Power-Setting Stability 1.6 MAX3735 0.8 MAX3735A 0.9 1 ppm/°C % 1.23 1.1 1 µA IMD = 1.5mA, RPC_MON = 600Ω 0.95 18µA ≤ IMD ≤ 1500µA (Notes 4, 6) MAX3735 -10 +10 MAX3735A -4 +4 Current into OUT+ pin; RL ≤ 15Ω, VOUT+, VOUT- ≥ 0.6V (DC-coupled) 10 60 Current into OUT+ pin; RL ≤ 15Ω_, VOUT+, VOUT- ≥ 2.0V (AC-coupled) 10 85 A/A 1.05 % LASER MODULATOR Modulation On-Current Range Modulation Off-Current IMOD IMODOFF mA Current into OUT+ pin during TX_FAULT or TX_DISABLE 100 µA Modulation-Current Stability (Note 4) IMOD = 10mA -480 +480 IMOD = 60mA -255 +255 Modulation-Current Absolute Accuracy 10mA ≤ IMOD ≤ 60mA (Note 8) -15 +15 % ppm/°C Modulation-Current Rise Time tR 20% to 80%, 10mA ≤ IMOD ≤ 60mA (Note 4) 42 65 ps Modulation-Current Fall Time tF 20% to 80%, 10mA ≤ IMOD ≤ 60mA (Note 4) 50 80 ps 10mA ≤ IMOD ≤ 60mA at 2.67Gbps (Notes 4, 9, 10) 18 38 Deterministic Jitter Random Jitter At 1.25Gbps (K28.5 pattern) RJ 11.5 At 622Mbps (Note 9) 18 At 155Mbps (Note 9) 40 10mA ≤ IMOD ≤ 60mA (Note 4) 0.7 ps 1.0 psRMS _______________________________________________________________________________________ 3 MAX3735/MAX3735A ELECTRICAL CHARACTERISTICS (continued) MAX3735/MAX3735A 2.7Gbps, Low-Power SFP Laser Drivers ELECTRICAL CHARACTERISTICS (continued) (VCC = +2.97V to +3.63V, TA = -40°C to +85°C. Typical values at VCC = +3.3V, IBIAS = 20mA, IMOD = 30mA, TA= +25°C, unless otherwise noted.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS SAFETY FEATURES Excessive Bias-Current Comparator Threshold Range TX_FAULT always occurs for VBC_MON ≥ 1.38V, TX_FAULT never occurs for VBC_MON ≤ 1.22V 1.22 1.30 1.39 V Excessive MD-Current Comparator Threshold Range TX_FAULT always occurs for VPC_MON ≥ 1.38V, TX_FAULT never occurs for VPC_MON ≤ 1.22V 1.22 1.30 1.39 V 0.14 5 µs 1 ms 600 µs 60 200 ms SFP TIMING REQUIREMENTS TX_DISABLE Assert Time TX_DISABLE Negate Time TX_DISABLE Negate Time During FAULT Recovery t_off Time from rising edge of TX_DISABLE to IBIAS = IBIASOFF and IMOD = IMODOFF (Note 4) t_on Time from falling edge of TX_DISABLE to IBIAS and IMOD at 95% of steady state when TX_FAULT = 0 before reset CAPC = 2.7nF, MAX3735 (Note 4) MAX3735A (Note 11) Time from falling edge of TX_DISABLE to t_onFAULT IBIAS and IMOD at 95% of steady state when TX_FAULT = 1 before reset (Note 4) TX_FAULT Reset Time or PowerOn Time t_init From power-on or negation of TX_FAULT using TX_DISABLE (Note 4) 60 200 ms TX_FAULT Assert Time t_fault Time from fault to TX_FAULT on, CFAULT ≤ 20pF, RFAULT = 4.7kΩ (Note 4) 3.3 50 µs 5 µs TX_DISABLE to Reset Time TX_DISABLE must be held high to reset TX_FAULT (Note 4) Specifications at -40°C are guaranteed by design and characterization. Dice are tested at TA = +25°C only. Maximum value is specified at IMOD = 60mA, IBIAS = 100mA. TX_FAULT is an open-collector output and must be pulled up with a 4.7kΩ to 10kΩ resistor. Guaranteed by design and characterization. VCC turn-on time must be ≤ 0.8s, DC-coupled interface. Gain stability is defined by the digital diagnostic document (SFF-8472, rev. 9.0) over temperature and supply variation. Assuming that the laser diode to photodiode transfer function does not change with temperature. Accuracy refers to part-to-part variation. Deterministic jitter is measured using a 223 - 1 PRBS or equivalent pattern. Broadband noise is filtered through the network as shown in Figure 3. One capacitor, C < 0.47µF, and one 0603 ferrite bead or inductor can be added (optional). This supply voltage filtering reduces the hotplugging inrush current. The supply noise must be < 100mVP-P up to 2MHz. Note 11: CAPC values chosen as shown in Table 4 (MAX3735A). Note 1: Note 2: Note 3: Note 4: Note 5: Note 6: Note 7: Note 8: Note 9: Note 10: 4 _______________________________________________________________________________________ 2.7Gbps, Low-Power SFP Laser Drivers MAX3735/MAX3735A VCC VCC VOLTAGE VOUT+ (100mV min, 1200mV max) VOUT- 30Ω 30Ω MAX3735 MAX3735A OUT- (VIN+) - (VIN-) (200mVP-P min, 2400mVP-P max) 30Ω 0.5pF OUT+ CURRENT IOUT+ OSCILLOSCOPE IOUT+ OUT+ IMOD 75Ω 50Ω TIME Figure 1. Required Input Signal and Output Polarity Figure 2. Output Termination for Characterization HOST BOARD MODULE FILTER DEFINED BY SFP MSA L1 1µH SOURCE NOISE VOLTAGE SUPPLY C1 0.1µF C2 10µF OPTIONAL C3 0.1µF TO LASER DRIVER VCC OPTIONAL Figure 3. Supply Filter _______________________________________________________________________________________ 5 Typical Operating Characteristics (VCC = +3.3V, CAPC = 0.01µF, IBIAS = 20mA, and IMOD = 30mA, TA = +25°C, unless otherwise noted.) OPTICAL EYE OPTICAL EYE MAX3735 toc02 MAX3735 toc01 ER = 8.2dB, 2.7Gbps, 2.3GHz FILTER 27 - 1 PRBS, 1310nm FP LASER ER = 8.2dB, 1.25Gbps, 900MHz FILTER K28.5 PATTERN, 1310nm FP LASER 54ps/div 115ps/div OPTICAL EYE ELECTRICAL EYE MAX3735 toc03 MAX3735 toc04 2.7Gbps, 27 - 1 PRBS, 30mA MODULATION ER = 12dB, 155Mbps, 117MHz FILTER 27 - 1 PRBS, 1310nm FP LASER 85mV/div 919ps/div 58ps/div SUPPLY CURRENT vs. TEMPERATURE BIAS-CURRENT MONITOR GAIN vs. TEMPERATURE 40 MAX3735 toc06 18 GAIN (mA/A) 55 25 16 14 12 10 10 -40 -15 10 35 TEMPERATURE (°C) 6 20 MAX3735 toc05 EXCLUDES IBIAS AND IMOD 70 SUPPLY CURRENT (mA) MAX3735/MAX3735A 2.7Gbps, Low-Power SFP Laser Drivers 60 85 -40 -15 10 35 60 85 TEMPERATURE (°C) _______________________________________________________________________________________ 2.7Gbps, Low-Power SFP Laser Drivers MODULATION CURRENT vs. RMODSET 2.5 90 80 1.4 1.5 1.0 1.2 60 IMD (mA) IMOD (mA) 50 40 0.5 0 10 35 60 0.8 20 0.4 10 0.2 0 -15 0 85 10 1 100 1 0 10 100 TEMPERATURE (°C) RMODSET (kΩ) RAPCSET (kΩ) EDGE TRANSITION TIME vs. MODULATION CURRENT DETERMINISTIC JITTER vs. MODULATION CURRENT RANDOM JITTER vs. MODULATION CURRENT 70 50 40 DJ (psP-P) 60 FALL TIME 50 40 30 DJ (INCLUDING PWD) 20 RISE TIME 2.5 2.0 1.5 1.0 0.5 10 30 3.0 MAX3735 toc12 60 MAX3735 toc10 80 RANDOM JITTER (psRMS) -40 1.0 0.6 30 MAX3735 toc11 GAIN (A/A) 1.6 70 2.0 EDGE TRANSITION TIME (ps) MONITOR DIODE CURRENT vs. RAPCSET 1.8 MAX3735 toc08 100 MAX3735 toc07 3.0 MAX3735 toc09 PHOTOCURRENT MONITOR GAIN vs. TEMPERATURE PWD 0 20 10 20 30 40 50 60 0 10 20 30 40 HOT PLUG WITH TX_DISABLE LOW LASER OUTPUT 20 30 40 3.3V 60 50 IMOD (mA) TRANSMITTER ENABLE MAX3735 toc14 3.3V TX_DISABLE 10 STARTUP WITH SLOW RAMPING SUPPLY MAX3735 toc13 FAULT 60 IMOD (mA) IMOD (mA) VCC 50 MAX3735 toc15 VCC 3.3V 0V VCC LOW FAULT LOW TX_DISABLE t_init = 60ms 20ms/div 0V LOW FAULT LOW TX_DISABLE HIGH t_on = 44µs LOW LASER OUTPUT LOW LASER OUTPUT 20ms/div 12µs/div _______________________________________________________________________________________ 7 MAX3735/MAX3735A Typical Operating Characteristics (continued) (VCC = +3.3V, CAPC = 0.01µF, IBIAS = 20mA, and IMOD = 30mA, TA = +25°C, unless otherwise noted.) MAX3735/MAX3735A 2.7Gbps, Low-Power SFP Laser Drivers Typical Operating Characteristics (continued) (VCC = +3.3V, CAPC = 0.01µF, IBIAS = 20mA, and IMOD = 30mA, TA = +25°C, unless otherwise noted.) RESPONSE TO FAULT TRANSMITTER DISABLE MAX3735 toc17 MAX3735 toc16 VCC 3.3V VPC_MON EXTERNALLY FORCED FAULT t_fault = 0.9µs FAULT LOW TX_DISABLE LOW FAULT LOW HIGH HIGH TX_DISABLE LOW t_off = 134ns LASER OUTPUT LASER OUTPUT 1µs/div 40ns/div FAULT RECOVERY TIME FREQUENT ASSERTION OF TX_DISABLE MAX3735 toc18 VPC_MON VPC_MON EXTERNAL FAULT REMOVED HIGH EXTERNALLY FORCED FAULT FAULT LOW TX_DISABLE HIGH LOW LASER OUTPUT FAULT TX_DISABLE LOW t_init = 60ms 100ms/div 8 MAX3735 toc19 LASER OUTPUT 4µs/div _______________________________________________________________________________________ 2.7Gbps, Low-Power SFP Laser Drivers PIN NAME 1, 4, 8, 14, 18 VCC FUNCTION 2 IN+ Noninverted Data Input 3 IN- Inverted Data Input 5 PC_MON 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. 6 BC_MON 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. 7, 12, 22 GND 9 SHUTDOWN 10 TX_FAULT 11 MODSET 13 BIAS Laser Bias Current Output 15, 16 OUT+ Noninverted Modulation Current Output. Connect pins 15 and 16 externally to minimize parasitic inductance of the package. IMOD flows into this pin when input data is high. 17 OUT- Inverted Modulation Current Output. IMOD flows into this pin when input data is low. 19 MD Monitor Diode 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. 20 APCFILT1 Connect a capacitor (CAPC) between pin 20 (APCFILT1) and pin 21 (APCFILT2) to set the dominant pole of the APC feedback loop. 21 APCFILT2 See APCFILT1 23 APCSET 24 TX_DISABLE EP Exposed Pad +3.3V Supply Voltage Ground Shutdown Driver Output. Voltage output to control an external transistor for optional shutdown circuitry. Open-Collector Transmit Fault Indicator (Table 1). A resistor connected from this pad to ground sets the desired modulation current. A resistor connected from this pin to ground sets the desired average optical power. 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. Ground. Must be soldered to the circuit board ground for proper thermal and electrical performance (see the Exposed Pad Package section). Detailed Description The MAX3735/MAX3735A laser drivers consist of three parts: a high-speed modulation driver, a laser-biasing block with automatic power control (APC), and safety circuitry (Figure 4). The circuit design is optimized for high-speed and low-voltage (+3.3V) operation. High-Speed Modulation Driver The output stage are composed of a high-speed differential pair and a programmable modulation current source. The MAX3735/MAX3735A are optimized for driving a 15Ω load; the minimum instantaneous voltage required at OUT+ is 0.6V. Modulation current swings up to 60mA are possible when the laser diode is DC-coupled to the driver and up to 85mA when the laser diode is AC-coupled to the driver. To interface with the laser diode, a damping resistor (RD) is required for impedance matching. The combined resistance of the series damping resistor and the equivalent series resistance of the laser diode should equal 15Ω. To reduce optical output aberrations and duty-cycle distortion caused by laser diode parasitic inductance, an RC shunt network might be necessary. Refer to Maxim Application Note HFAN 02.0: Interfacing Maxim’s Laser Drivers to Laser Diodes for more information. At data rates of 2.7Gbps, any capacitive load at the cathode of a laser diode degrades optical output performance. Because the BIAS output is directly connected to the laser cathode, minimize the parasitic capacitance associated with the pin by using an inductor to isolate the BIAS pin parasitics from the laser cathode. _______________________________________________________________________________________ 9 MAX3735/MAX3735A Pin Description MAX3735/MAX3735A 2.7Gbps, Low-Power SFP Laser Drivers VCC SHUTDOWN VCC MAX3735 MAX3735A INPUT BUFFER IN+ 15Ω OUT- DATA PATH 100Ω IN- OUT+ VCC IMOD VCC BIAS IMD IBIAS VCC 1 PC_MON IBIAS ENABLE IMOD ENABLE RD VBG APCSET x38 IAPCSET RPC_MON VCC RAPCSET X270 SAFETY LOGIC AND POWER DETECTOR IBIAS 76 VBG BC_MON MD IMD CMD VCC x1 (4.7kΩ TO 10kΩ) RBC_MON MODSET RMODSET APCFILT1 TX_FAULT TX_DISABLE APCFILT2 SHUTDOWN CAPC Figure 4. Functional Diagram Laser-Biasing and APC To maintain constant average optical power, the MAX3735/MAX3735A incorporate an APC loop to compensate for the changes in laser threshold current over temperature and lifetime. A back-facet photodiode mounted in the laser package is used to convert the optical power into a photocurrent. The APC loop adjusts the laser bias current so that the monitor current is matched to a reference current set by RAPCSET. The time constant of the APC loop is determined by an external capacitor (CAPC). For possible CAPC values, see the Applications Information section. Safety Circuitry The safety circuitry contains an input disable (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). A single-point fault can be a short to VCC or GND. See Table 2 to view the circuit 10 response to various single-point failures. The transmit fault condition is latched until reset by a toggle of TX_DISABLE or VCC. The laser driver offers redundant laser diode shutdown through the optional shutdown circuitry (see the Typical Applications Circuit). The TX_FAULT pin should be pulled high with a 4.7kΩ to 10kΩ resistor to VCC as required by the SFP MSA. Safety Circuitry Current Monitors The MAX3735/MAX3735A feature monitors (BC_MON, PC_MON) for bias current (IBIAS) and photo current (IMD). The monitors are realized by mirroring a fraction of the currents and developing voltages across external resistors connected to ground. Voltages greater than 1.38V at PC_MON or BC_MON result in a fault state. For example, connecting a 100Ω resistor to ground on each monitor output gives the following relationships: VBC_MON = (IBIAS / 76) x 100Ω VPC_MON = IMD x 100Ω ______________________________________________________________________________________ 2.7Gbps, Low-Power SFP Laser Drivers POR AND COUNTER FOR t_init MAX3735/MAX3735A VCC MAX3735 MAX3735A IMOD ENABLE TX_DISABLE 100ns DELAY COUNTER FOR t_onfault IBIAS ENABLE VCC IMD R VBG PC_MON RS LATCH COMP RPC_MON Q 1 VCC SHUTDOWN S IBIAS BC_MON RBC_MON VBG CMOS 76 MODSET SHORTCIRCUIT DETECTOR TX_FAULT COMP TTL OPEN COLLECTOR Figure 5. Safety Circuitry Table 1. Typical Fault Conditions 1. If any of the I/O pins is shorted to GND or VCC (singlepoint failure, see Table 2), and the bias current or the photocurrent exceed the programmed threshold. 2. End-of-life (EOL) condition of the laser diode. The bias current and/or the photocurrent exceed the programmed threshold. 3. Laser cathode is grounded and the photocurrent exceeds the programmed thresholds. 4. No feedback for the APC loop (broken interconnection, defective monitor photodiode), and the bias current exceeds the programmed threshold. Design Procedure When designing a laser transmitter, the optical output usually is expressed in terms of average power and extinction ratio. Table 3 shows relationships 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%. Programming the Modulation Current For a given laser power (PAVG), slope efficiency (η), and extinction ratio (re), the modulation current can be calculated using Table 3. See the Modulation Current vs. RMODSET graph in the Typical Operating Characteristics section, and select the value of RMODSET that corresponds to the required current at +25°C. Programming the APC Loop Program the average optical power by adjusting -RAPCSET. To select the resistance, determine the desired monitor current to be maintained over temperature and lifetime. See the Monitor Diode Current vs. RAPCSET graph in the Typical Operating Characteristics section, and select the value of RAPCSET that corresponds to the required current. Interfacing with Laser Diodes To minimize optical output aberrations caused by signal reflections at the electrical interface to the laser diode, a series-damping resistor (R D ) is required (Figure 4). Additionally, the MAX3735/MAX3735A outputs are optimized for a 15Ω load. Therefore, the series combination of RD and RL, where RL represents the laser-diode resistance, should equal 15Ω. Typical values for RD are 8Ω to 13Ω. For best performance, place a bypass capacitor (0.01µF typ) as close as possible to the anode of the laser diode. An RC shunt network between the laser cathode and ground minimizes optical output aberrations. Starting values for most coaxial lasers are RCOMP = 50Ω in series with CCOMP = 8pF. Adjust these values experimentally until the optical output waveform is optimized. Refer to Maxim Application Note: HFAN 02.0: Interfacing Maxim’s Laser Drivers to Laser Diodes for more information. ______________________________________________________________________________________ 11 MAX3735/MAX3735A 2.7Gbps, Low-Power SFP Laser Drivers Table 2. Circuit Responses to Various Single-Point Faults TX_FAULT CIRCUIT RESPONSE TO UNDERVOLTAGE OR SHORT TO GROUND CIRCUIT RESPONSE TO OVERVOLTAGE OR SHORT TO V CC PIN NAME Does not affect laser power. Does not affect laser power. Modulation and bias currents are disabled. Normal condition for circuit operation. IN+ 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. IN- 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. MD Disables bias current. A fault state occurs. The APC circuit responds by increasing bias current until a fault is detected, then a fault state* occurs. SHUTDOWN Does not affect laser power. If the shutdown circuitry is used, laser current is disabled and a fault state* occurs. Does not affect laser power. BIAS In this condition, laser forward voltage is 0V and no light is emitted. Fault state* occurs. If the shutdown circuitry is used, the laser current is disabled. OUT+ The APC circuit responds by increasing the bias current until a fault is detected, then a fault state* occurs. Fault state* occurs. If the shutdown circuitry is used, laser current is disabled. OUT- Does not affect laser power. Does not affect laser power. Fault state* occurs. Does not affect laser power. TX_DISABLE PC_MON BC_MON Fault state* occurs. Does not affect laser power. APCFILT1 IBIAS increases until VBC_MON exceeds the threshold voltage. IBIAS increases until VBC_MON exceeds the threshold voltage. APCFILT2 IBIAS increases until VBC_MON exceeds the threshold voltage. IBIAS increases until VBC_MON exceeds the threshold voltage. MODSET Does not affect laser power. Fault state* occurs. 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. Table 3. Optical Power Definitions PARAMETER SYMBOL RELATION Average Power PAVG Extinction Ratio re r e = P1 / P0 Optical Power High P1 P1 = 2PAVG x re / (re + 1) Optical Power Low P0 P0 = 2PAVG / (re + 1) Optical Amplitude PP-P Laser Slope Efficiency Modulation Current 12 PAVG = (P0 + P1) / 2 PP-P = P1 - P0 η η = PP-P / IMOD IMOD IMOD = PP-P / η Pattern-Dependent Jitter To minimize the pattern-dependent jitter associated with the APC loop time constant, and to guarantee loop stability, connect a capacitor between APCFILT1 and APCFILT2 (see the Applications Information section for more information about choosing C APC values). A capacitor attached to the photodiode anode (CMD) is also recommended to filter transient currents that originate from the photodiode. To maintain stability and proper phase margin associated with the two poles created by C APC and C MD , C APC should be 20x greater than CMD for the MAX3735. CAPC should be 4x to 20x greater than CMD for the MAX3735A. ______________________________________________________________________________________ 2.7Gbps, Low-Power SFP Laser Drivers Determine RAPCSET The desired monitor diode current is estimated by IMD = PAVG x ρMON = 200µA. The Monitor Diode vs. RAPCSET graph in the Typical Operating Characteristics section shows that RAPCSET should be 3kΩ. The value can also be estimated using the equation below: IMD = 1.23 / (2 × RAPCSET) Optional Shutdown Output Circuitry The SHUTDOWN control output features extended eye safety when the laser cathode is grounded. An external transistor is required to implement this circuit (Figure 4). In the event of a fault, SHUTDOWN asserts high, placing the optional shutdown transistor in cutoff mode and thereby shutting off the laser current. Applications Information An example of how to set up the MAX3735/MAX3735A follows: Select a communications-grade laser for 2.488Gbps. Assume that the laser output average power is PAVG = 0dBm, the operating temperature is -40°C to +85°C, minimum extinction ratio is 6.6 (8.2dB), and the laser diode has the following characteristics: Wavelength: λ = 1.3µm Threshold current: ITH = 22mA at +25°C Threshold temperature coefficient: βTH = 1.3% / °C Laser-to-monitor transfer: ρMON = 0.2A/W Laser slope efficiency: η = 0.05mW/mA at +25°C Determine RMODSET To achieve a minimum extinction ratio (re) of 6.6 over temperature and lifetime, calculate the required extinction ratio at +25°C. Assuming the results of the calculation are re = 20 at +25°C, the peak-to-peak optical power P P-P = 1.81mW, according to Table 3. The required modulation current is 1.81mW / (0.05mW/mA) = 36.2mA. The Modulation Current vs. RMODSET graph in the Typical Operating Characteristics section shows that RMODSET should be 9.5kΩ. The value can also be estimated using the equation below: IMOD = 1.23 / (0.0037 × RMODSET) Determine CAPC In order to meet SFP timing requirements and minimize pattern-dependent jitter, the CAPC capacitor value is determined by the laser-to-monitor transfer and other variables. The following equations and table can be used to choose the CAPC values for the MAX3735 and MAX3735A, respectively. The equations and table assume a DC-coupled laser. Refer to Maxim Application Note HFDN 23.0: Choosing the APC Loop VCC VCC MAX3735 MAX3735A 16kΩ VCC PACKAGE IN+ MAX3735 MAX3735A 0.97nH PACKAGE 0.81nH OUT- 0.11pF 0.11pF 50Ω 0.99nH OUT+ 0.11pF VCC 50Ω IN- 0.97nH 0.99nH 0.11pF OUT+ 24kΩ 0.11pF K = 0.3 Figure 6. Simplified Input Structure Figure 7. Simplified Output Structure ______________________________________________________________________________________ 13 MAX3735/MAX3735A Input Termination Requirements The MAX3735/MAX3735A data inputs are SFP MSA compliant. On-chip 100Ω differential input impedance is provided for optimal termination (Figure 6). Because of the on-chip biasing network, the MAX3735/MAX3735A inputs self-bias to the proper operating point to accommodate AC-coupling. MAX3735/MAX3735A 2.7Gbps, Low-Power SFP Laser Drivers Capacitors Used with MAX3735/MAX3735A SFP Module Designs for more information on choosing CAPC for DCand AC-coupled laser interfaces. MAX3735 Use the following equation to find the CAPC value when using the MAX3735: CAPC = 4.04 × 10-9 × t_on × η × ρMON (29.3 + 20.6 ITH 0.22 ITH2) × (1947 + 833 IMOD - 7.78 IMOD2 + 0.103 IMOD3) where units are: CAPC in µF, ITH, and IMOD in mA and tON in µs. CMD can then be chosen as approximately 20x smaller than CAPC for the MAX3735. MAX3735A Use Table 4 to choose CAPC when using the MAX3735A. CAPC should be chosen according to the highest gain of the lasers (generally at cold temperature). CAPC selection assumes a 34% reduction in the gain of the lasers at +85°C from the cold (-40°C) values. Table 4. MAX3735A CAPC Selection LASER GAIN (A/A) CAPC (µF) 0.005 0.039 0.007 0.047 0.010 0.068 0.020 0.100 0.030 0.120 0.040 0.120 where Gain = IMD/(IBIAS - ITH + 0.5 x IMOD) for DC-coupled lasers. CMD can then be chosen as approximately 4x to 20x smaller than CAPC for the MAX3735A Using the MAX3735/MAX3735A with Digital Potentiometers For more information on using the MAX3735/MAX3735A with the Dallas DS1858/DS1859 SFP controller, refer to Maxim Application Note HFAN 2.3.3: Optimizing the Resolution of Laser Driver Setting Using Linear Digital Potentiometers for more information. Modulation Currents Exceeding 60mA For applications requiring a modulation current greater than 60mA, headroom is insufficient for proper operation of the laser driver if the laser is DC-coupled. To avoid this problem, the MAX3735/MAX3735A’s modulation output can be AC-coupled to the cathode of a laser diode. An external pullup inductor is necessary to DCbias the modulation output at VCC. Such a configuration 14 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 MAX3735/MAX3735A modulation current can be programmed from 10mA to 85mA. Refer to Maxim Application Note HFAN 02.0: Interfacing Maxim’s Laser Drivers to Laser Diodes for more information on ACcoupling laser drivers to laser diodes. Interface Models Figures 6 and 7 show simplified input and output circuits for the MAX3735/MAX3735A laser driver. If dice are used, replace package parasitic elements with bondwire parasitic elements. Wire Bonding Die The MAX3735 uses gold metalization with a thickness of 5µm (typ). Maxim characterized this circuit with gold wire ball bonding (1-mil diameter wire). Die-pad size is 94 mil (2388µm) square, and die thickness is 15 mil (381µm). Refer to Maxim Application Note HFAN-08.0.1: Understanding Bonding Coordinates and Physical Die Size for additional information. Layout Considerations To minimize inductance, keep the connections between the MAX3735 output pins and laser diode as close as possible. Optimize the laser diode performance by placing a bypass capacitor as close as possible to the laser anode. Use good high-frequency layout techniques and multiple-layer boards with uninterrupted ground planes to minimize EMI and crosstalk. Exposed-Pad Package The exposed pad on the 24-pin QFN provides a very low thermal resistance path for heat removal from the IC. The pad is also electrical ground on the MAX3735/ MAX3735A 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. Laser Safety and IEC 825 Using the MAX3735/MAX3735A 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. Each user must determine the level of fault tolerance required by the application, recognizing that Maxim products are neither designed nor 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 in which the failure of a Maxim product could create a situation where personal injury or death may occur. ______________________________________________________________________________________ 2.7Gbps, Low-Power SFP Laser Drivers 0.079" (2.007mm) GND 7 VCC 6 IN+ 5 IN- 4 VCC 3 PC_MON 2 BC_MON 1 TX_DISABLE APCSET 8 9 GND 10 APCFILT2 APCFILT1 11 12 MD GND 13 14 15 VCC 16 OUT- 17 OUT0.060" (1.524mm) 29 28 27 GND GND VCC 26 25 24 23 22 GND GND 18 OUT+ 19 OUT+ 20 VCC 21 BIAS INDEX PAD SHUTDOWN TX_FAULT MODSET ______________________________________________________________________________________ 15 MAX3735/MAX3735A MAX3735 Chip Topography Bonding Coordinates Pin Configuration X Y TX_DISABLE APCSET GND APCFILT2 APCFILT1 MD Table 5. MAX3735 Bondpad Locations 47 47 24 23 22 21 20 19 COORDINATES PAD NAME 1* BC_MON 2 PC_MON 47 229 3 VCC 47 514 4 IN- 47 5 IN+ 47 6 VCC 47 1063 7 GND 242 1149 8 TX_DISABLE 452 TOP VIEW VCC 1 18 VCC 696 IN+ 2 17 OUT- 878 IN- 3 16 OUT+ 15 OUT+ MAX3735 MAX3735A 14 VCC BC_MON 6 13 BIAS 9 APCSET 636 1149 10 GND 819 1149 11 APCFILT2 1008 1149 12 APCFILT1 1193 1149 13 MD 1383 1149 14 GND 1567 1149 15 VCC 1758 1032 16** OUT- 1758 888 17** OUT- 1758 742 18** OUT+ 1758 579 19** OUT+ 1758 433 20 VCC 1758 289 21 BIAS 1758 93 22 GND 1578 -64 23 GND 1401 -64 24 MODSET 1205 -64 25 TX_FAULT 1016 -64 26 SHUTDOWN 818 -64 27 VCC 623 -64 28 GND 435 -64 29 GND 245 -64 7 8 9 10 11 12 GND 5 MODSET PC_MON TX_FAULT 1149 SHUTDOWN 4 VCC VCC GND MAX3735/MAX3735A 2.7Gbps, Low-Power SFP Laser Drivers Thin QFN* (4mm x 4mm) *THE EXPOSED PAD MUST BE CONNECTED TO CIRCUIT BOARD GROUND FOR PROPER THERMAL AND ELECTRICAL PERFORMANCE. Chip Information TRANSISTOR COUNT: 327 SUBSTRATE CONNECTED TO GND DIE SIZE: 60 mils x 79 mils PROCESS: SiGe Bipolar *Index pad. Orient the die with this pad in the lower-left corner. **Bond out both pairs of OUT- and OUT+ to minimize series inductance. 16 ______________________________________________________________________________________ 2.7Gbps, Low-Power SFP Laser Drivers 12,16,20, 24L QFN.EPS PACKAGE OUTLINE 12,16,20,24L QFN, 4x4x0.90 MM 21-0106 E 1 2 PACKAGE OUTLINE 12,16,20,24L QFN, 4x4x0.90 MM 21-0106 E 2 2 ______________________________________________________________________________________ 17 MAX3735/MAX3735A Package Information (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.) Package 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.) 24L QFN THIN.EPS MAX3735/MAX3735A 2.7Gbps, Low-Power SFP Laser Drivers PACKAGE OUTLINE 12,16,20,24L QFN THIN, 4x4x0.8 mm 21-0139 A PACKAGE OUTLINE 12,16,20,24L QFN THIN, 4x4x0.8 mm 21-0139 A 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. 18 ____________________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.