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19-3387; Rev 0; 8/04 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors General Description The MAX3795 is a high-speed VCSEL driver for smallform-factor (SFF) and small-form-factor pluggable (SFP) fiber optic 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 MAX3795 operates up to 4.25Gbps. 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 MAX3795 interfaces with the Dallas DS1856/DS1859 to meet SFF-8472 timing and diagnostic requirements. The MAX3795 accommodates various VCSEL packages, including low-cost TO-46 headers. The MAX3795 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 (MSAs). The MAX3795 is available in a compact 4mm 4mm, 24-pin thin QFN package and operates over the -40C to +85C temperature range. The MAX3795 is pin-forpin compatible with the MAX3740A and is available in lead-free packages. 3.3V 10% Single Supply 2mA to 15mA Modulation Current 1mA to 15mA Bias Current 52ps Transition Time 8.4ps Deterministic Jitter Optional Peaking Current to Improve VCSEL Edge Speed Supports Common-Cathode and Differential Configuration Safety Circuits Compliant with SFF and SFP MSAs Pin Compatible to MAX3740A Features Supports All SFF-8472 Digital Diagnostics MAX3795 Ordering Information PART MAX3795ETG MAX3795ETG+ TEMP RANGE PIN-PACKAGE -40C to +85C 24 Thin QFN (4mm x 4mm) -40C to +85C 24 Thin QFN (4mm x 4mm) +Denotes lead-free package. Applications PWRMON Pin Configuration COMP REF VCC Multirate (1Gbps to 4.25Gbps) SFP/SFF Modules Gigabit Ethernet Optical Transmitters Fibre-Channel Optical Transmitters TOP VIEW 24 23 22 21 20 19 18 17 16 BIASMON MD GND TX_DISABLE IN+ INFAULT SQUELCH 1 2 3 4 5 6 10 11 12 BIAS BIASSET VCC OUT+ OUTGND MAX3795 15 14 13 7 8 9 THIN QFN (4mm x 4mm) EXPOSED PAD IS CONNECTED TO GND ________________________________________________________________ Maxim Integrated Products PEAKSET MODSET VCC GND TC1 TC2 1 For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com. 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors MAX3795 ABSOLUTE MAXIMUM RATINGS Supply Voltage (VCC) ............................................-0.5V to +4.0V 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 + 1V) Current into FAULT ............................................ -1mA to +25mA Current into OUT+, OUT- ....................................................60mA Continuous Power Dissipation (TA = +85C) 24-Pin Thin QFN (derate 20.8mW/C above +85C).................................1354mW Storage Temperature Range .............................-55C to +150C Lead Temperature (soldering, 10s) .................................+300C Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VCC = +2.97V to +3.63V, TA = -40C to +85C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA = +25C, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS SQUELCH set low, IMOD = 2mAP-P TX_DISABLE set low, peaking is not used IMOD = 15mAP-P (Note 1) Additional current when peaking is used, RPEAK = 1.18k Additional current when SQUELCH is high ICC-SHDN FAULT OUTPUT Output High Voltage Output Low Voltage TX_DISABLE INPUT Input Impedance Input High Voltage Input Low Voltage Power-Down Time SQUELCH Squelch Threshold Squelch Hysteresis Time to Squelch Data Time to Resume from Squelch BIAS GENERATOR Maximum Bias Pin Voltage Bias Current Accuracy of Programmed Bias Current VBIAS-MAX IBIAS BIAS Referenced to VCC Minimum Maximum 5mA IBIAS 15mA 1mA IBIAS 5mA 15 -8 -12 +8 +12 -0.65 1 V mA % (Note 3) (Note 3) 25 6 0.02 0.02 5.00 5.00 85 mVP-P mVP-P s s RPULL VIH VIL The time for ICC to reach ICC-SHDN when TX_DISABLE transitions high 50 4.7 2.0 0.8 8 10.0 k V V s VOH VOL RLOAD = 10k to 2.97V RLOAD = 4.7k to 3.63V 2.4 0.4 V V Total current when TX_DISABLE is high MIN TYP 35 71 15 5 7 10 81 mA MAX UNITS Supply Current ICC 2 _______________________________________________________________________________________ 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors ELECTRICAL CHARACTERISTICS (continued) (VCC = +2.97V to +3.63V, TA = -40C to +85C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA = +25C, unless otherwise noted.) PARAMETER Bias Current During Fault BIASMON Gain BIASMON Stability AUTOMATIC POWER CONTROL (APC) MD Nominal Voltage Voltage at REF MD Voltage During Fault MD Input Current APC Time Constant PWRMON Nominal Gain Normal operation (FAULT = low) CCOMP = 0.047F, IPD / ILASER = 0.02 VPWRMON / (VREF - VMD) Minimum Maximum 2.4 1.75 85 f < 4GHz f < 4GHz Current into OUT+ RLOAD 50 DC tested TC1 is shorted to TC2 RPEAKSET = 10k RPEAKSET = 1k ROUT Single-ended resistance 42 -10 0.2 2 75 50 0 Temperature range 0C to +70C tR tF 50 load, no peaking, 5mA IMOD 15mA 50 load, no peaking, 5mA IMOD 15mA -40C to +85C +100C -40C to +85C +100C +5000 49 58 56 64 79 72 ps 58 Minimum Maximum 15 15 50 +10 100 12.7 11 2 115 1.85 -2 VMD VREF APC loop is closed 1 1.2 VREF 0.2 1.8 0 0.7 90 2.15 2.45 0.25 +2 2 2.2 V V V A s V/V SYMBOL IBIAS_OFF CONDITIONS Current out of the BIAS pin 1mA < IBIAS < 3mA 3mA IBIAS < 15mA 0.0875 0.085 -10 MIN TYP 1.5 0.105 0.105 MAX 10 0.1375 0.125 +10 UNITS A mA/mA % MAX3795 GBIASMON IBIASMON / IBIAS (Notes 2, 4) LASER MODULATOR (Load is 50 AC-Coupled to OUT+) Differential Input Voltage Input Common-Mode Voltage Differential Input Resistance Single-Ended Input Return Loss Differential Input Return Loss Modulation Current Laser Modulation During Fault or Squelch Active Tolerance of Programmed Modulation Current Minimum Peaking Current Maximum Peaking Current Peaking Current Duration Output Resistance Minimum Programmable Temperature Coefficient Maximum Programmable Temperature Coefficient VID VCM RIN S11 SDD11 IMOD IMOD_OFF VP-P V dB dB mA AP-P % mA mA ps ppm/C ppm/C Modulation Transition Time (Note 2) _______________________________________________________________________________________ 3 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors MAX3795 ELECTRICAL CHARACTERISTICS (continued) (VCC = +2.97V to +3.63V, TA = -40C to +85C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA = +25C, unless otherwise noted.) PARAMETER Deterministic Jitter Random Jitter SYMBOL DJ RJ CONDITIONS 5mA IMOD 15mA, 4.25Gbps, K28.5 (Notes 2, 5) +100C APC closed loop APC open loop (Note 2) VBIASMON > VBMTH causes a fault VBIAS referenced to VCC VPWRMON > VPMTH causes a fault Time from rising edge of TX_DISABLE to IBIAS = IBIAS_OFF and IMOD = IMOD_OFF (Note 2) Time from rising edge of TX_DISABLE to IBIAS and IMOD at 99% of steady state (Note 2) Time to set VFAULT = low after power-on or after rising edge of TX_DISABLE (Note 2) Time after power-on to transmitter-on with TX_DISABLE low (Note 2) Time from fault occurrence to VFAULT = high; CFAULT < 20pF, RFAULT = 4.7k (Note 2) Time from fault to IBIAS = IBIAS_OFF and IMOD = IMOD_OFF; measured with a continuously occurring fault (Note 2) Time TX_DISABLE must be held high to reset FAULT (Note 2) 0.7 -0.250 0.7 -40C to +85C MIN TYP 8.4 12.7 0.5 0.5 0.8 -0.2 0.8 1.8 0.9 0.9 -0.150 0.9 5 MAX 15.6 UNITS psP-P psRMS SAFETY FEATURES (see the Typical Operating Characteristics) High-Current Fault Threshold VBIAS Fault Threshold Power-Monitor Fault Threshold TX Disable Time VBMTH VBTH VPMTH t_OFF V V V s TX Disable Negate Time t_ON 55 500 s Fault Reset Time Power-On Time t_INIT1 t_INIT2 60 60 200 200 ms ms Fault Assert Time t_FAULT 1.4 50 s Fault Delay Time t_FLTDLY 1 5 s TX_DISABLE Reset t_RESET 1 s Note 1: Supply current measurements exclude IBIAS from the total current. Note 2: AC characteristics guaranteed by design and characterization. Note 3: Measured by applying a pattern that contains 20s of K28.5, followed by 5s of zeros, then 20s of K28.5, followed by 5s of ones. Data rate is equal to 2.5Gbps, with inputs filtered using 1.8GHz Bessel filters. Note 4: Variation of bias monitor gain for any single part over the range of VCC, temperature, 3mA < IBIAS < 15mA. Note 5: Deterministic jitter measured at 4.25Gbps with a K28.5 pattern (00111110101100000101). 4 _______________________________________________________________________________________ 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors MAX3795 Typical Operating Characteristics (VCC = +3.3V, RTC = 0, PEAKSET open, measured electrically with a 50 load AC-coupled to OUT+, TA = +25C, unless otherwise noted.) ELECTRICAL EYE DIAGRAM MAX3795 toc01 ELECTRICAL EYE DIAGRAM MAX3795 toc02 OPTICAL EYE DIAGRAM MAX3795 toc03 4.25Gbps, K28.5, 10mA MODULATION, PEAKING OFF 1Gbps, K28.5, 10mA MODULATION, RPEAKSET = 1.4k 1Gbps, K28.5, -3dBm, 850nm VCSEL ADVANCED OPTICAL COMPONENTS, 1 HFE4191-541 75mV/div 75mV/div 2 3 40ps/div 152ps/div 135ps/div OPTICAL EYE DIAGRAM MAX3795 toc04 OPTICAL EYE DIAGRAM MAX3795 toc05 IBIASMON vs. BIAS CURRENT 1.6 1.4 IBIASMON (mA) 1.2 1.0 0.8 0.6 0.4 0.2 0 MAX3795 toc06 4.25Gbps, K28.5, -7dBm, 850nm VCSEL, ADVANCED OPTICAL COMPONENTS HFE4191-541 3.125Gbps, K28.5, -7dBm, 850nm VCSEL, ADVANCED OPTICAL COMPONENTS HFE4191-541 1.8 34ps/div 50ps/div 0 4 8 BIAS CURRENT (mA) 12 16 DETERMINISTIC JITTER vs. MODULATION CURRENT MAX3795 toc07 RANDOM JITTER vs. MODULATION CURRENT MAX3795 toc08 TRANSITION TIME vs. MODULATION CURRENT MEASURED FROM 20% FALL TIME MAX3795 toc09 40 35 DETERMINISTIC JITTER (psP-P) 30 25 20 15 10 5 0 0 2 4 6 8 3.5 3.0 RANDOM JITTER (psRMS) 2.5 2.0 1.5 1.0 0.5 0 IBIAS = 5mA 70 60 TRANSITION TIME (ps) 50 40 30 20 10 RISE TIME 10 12 14 16 18 20 0 2 4 6 8 10 12 14 16 18 20 0 2 4 6 8 10 12 14 MODULATION CURRENT (mAP-P) MODULATION CURRENT (mAP-P) MODULATION CURRENT (mA) _______________________________________________________________________________________ 5 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors MAX3795 Typical Operating Characteristics (continued) (VCC = +3.3V, RTC = 0, PEAKSET open, measured electrically with a 50 load AC-coupled to OUT+, TA = +25C, unless otherwise noted.) MONITOR DIODE CURRENT vs. RPWRSET MAX3740A toc11 MAX3795 toc12 BIAS CURRENT vs. RBIASSET MAX3795 toc10 MODULATION CURRENT vs. RMODSET 100 MODULATION CURRENT (mAP-P) 10m 100 MONITOR DIODE CURRENT (A) 1m BIAS CURRENT (mA) 10 10 100 1 10 0.1 1 10 RBIASSET () 100 1 0.1 1 RMODSET (k) 10 100 1 0 1 RPWRSET () 10 SUPPLY CURRENT vs. TEMPERATURE MAX3795 toc13 INPUT RETURN LOSS MAX3795 toc14 OUTPUT RETURN LOSS -5 -10 -15 S22 (dB) -20 -25 -30 -35 SINGLE-ENDED MEASUREMENT MAX3795 toc15 100 90 SUPPLY CURRENT (mA) 80 70 60 50 40 30 20 -40 -15 10 35 60 IMOD = 2mA IMOD = 15mA 0 -5 -10 S11 (dB) -15 -20 -25 -30 -35 DIFFERENTIAL MEASUREMENT AT IN 0 -40 -45 100 1G FREQUENCY (Hz) 10G 100 1G FREQUENCY (Hz) 10G 85 TEMPERATURE (C) MODULATION CURRENT vs. TEMPERATURE MAX3795 toc16 MODULATION CURRENT TEMPCO vs. RTC REFERENCED TO +25C MAX3795 toc17 11 10 MODULATION CURRENT (mAP-P) RTC = 100 RTC = 1k RMODSET = 1.8k 5500 4500 TEMPCO (ppm/C) 3500 2500 1500 500 -500 9 8 7 6 5 4 0 10 20 30 40 50 60 70 80 90 TEMPERATURE (C) RTC = 5k RTC = 10k RTC = 60k RTC = 100k RTC = 500k 100 1k 10k RTC () 100k 1M 6 _______________________________________________________________________________________ 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors MAX3795 Typical Operating Characteristics (continued) (VCC = +3.3V, RTC = 0, PEAKSET open, measured electrically with a 50 load AC-coupled to OUT+, TA = +25C, unless otherwise noted.) MONITOR DIODE CURRENT vs. TEMPERATURE MAX3795 toc18 HOT PLUG WITH TX_DISABLE LOW MAX3795 toc19 STARTUP WITH SLOW RAMPING SUPPLY MAX3795 toc20 300 275 MONITOR DIODE CURRENT (A) 250 225 200 175 150 125 100 -40 -15 10 35 60 3.3V VCC 0V VCC 0V FAULT LOW LOW TX_DISABLE LOW LASER OUTPUT 20ms/div 3.3V FAULT TX_DISABLE LOW LASER OUTPUT 85 t_INIT = 62ms t_INIT = 60ms 20ms/div TEMPERATURE (C) TX_DISABLE NEGATE TIME MAX3795 toc21 TRANSMITTER DISABLE MAX3795 toc22 RESPONSE TO FAULT MAX3795 toc23 3.3V VCC VCC 3.3V t_OFF = 2.2s EXTERNALLY FORCED VPWRMON FAULT FAULT t_FAULT = 2.16s LOW TX_DISABLE HIGH FAULT LOW TX_DISABLE HIGH t_ON = 131s LASER OUTPUT 40s/div LOW FAULT LOW TX_DISABLE LOW HIGH LOW LASER OUTPUT 1s/div LASER OUTPUT 4s/div FAULT RECOVERY TIME MAX3795 toc24 FREQUENT ASSERTION OF TX_DISABLE MAX3795 toc25 VPWRMON EXTERNAL FAULT REMOVED VPWRMON EXTERNALLY FORCED FAULT FAULT HIGH HIGH FAULT LOW TX_DISABLE LOW LOW t_INIT = 54s TX_DISABLE LASER OUTPUT 40s/div LASER OUTPUT 200s/div _______________________________________________________________________________________ 7 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors MAX3795 Pin Description PIN 1, 10, 13 2 3 4 5 6 7, 16, 20 8 9 11 12 14 15 17 18 19 NAME GND TX_DISABLE IN+ INFAULT SQUELCH VCC TC1 TC2 MODSET PEAKSET OUTOUT+ BIASSET BIAS BIASMON Ground Transmit Disable. Driver output is disabled when TX_DISABLE is high or left unconnected. The driver output is enabled when the pin is asserted low. Noninverted Data Input Inverted Data Input Fault Indicator. Open-drain output with ESD protection. FAULT is asserted high during a fault condition. Squelch Enable. Squelch is enabled when the pin is set high. Squelch is disabled when the pin is set low or left open. +3.3V Supply Voltage Temperature Compensation Set Pin 1. A resistor placed between TC1 and TC2 (RTC) programs the temperature coefficient of the laser modulation current. Temperature Compensation Set Pin 2. A resistor placed between TC1 and TC2 (RTC) programs the temperature coefficient of the laser modulation current. Modulation Set. A resistor connected from MODSET to ground (RMODSET) programs the desired modulation current amplitude. Peaking Current Set. A resistor connected between PEAKSET and ground (RPEAKSET) programs the peaking current amplitude. To disable peaking, leave PEAKSET open. Inverted Modulation Current Output Noninverted Modulation Current Output Bias-Current Set. When a closed-loop configuration is used, connect a 1.7k resistor between ground and BIASSET to program the maximum bias current. When an open configuration is used, connect a resistor between BIASSET and ground (RBIASSET) to program the VCSEL bias current. Bias-Current Output Bias-Current Monitor. The output of BIASMON is a sourced current proportional to the bias current. A resistor connected between BIASMON and ground (RBIASMON) can be used to form a groundreferenced bias monitor. Compensation Pin. A capacitor between COMP and MD compensates the APC. A typical value of 0.047F is recommended. For open-loop configuration, short the COMP pin to GND to deactivate the APC circuit. Monitor Diode Connection Reference Pin. Reference monitor used for APC. A resistor between REF and MD (RPWRSET) programs the photomonitor current when the APC loop is closed. Average Power Monitor. The pin is used to monitor the transmit optical power. For open-loop configuration, connect PWRMON to GND. Ground. Must be soldered to the circuit board ground for proper thermal and electrical performance. See the Layout Considerations section. FUNCTION 21 22 23 24 EP COMP MD REF PWRMON Exposed Pad 8 _______________________________________________________________________________________ 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors MAX3795 PWRMON REF RPWRSET MD SMOOTHSTART 1.6V (2VBE) 1V IBIAS 9 RBIASMON 200 1.8V (2VBE + 0.2) 2X MAX3795 POWERCONTROL AMPLIFIER IBIAS 34 BIAS GENERATOR BIASMON CURRENT AMPLIFIER ENABLE BIAS FERRITE BEAD IPD COMP CCOMP BIASSET RBIASSET Figure 1. Bias Generator Detailed Description The MAX3795 contains a bias generator with APC, safety circuit, and a laser modulator with optional peaking compensation (see the Functional Diagram). 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 x IPD x RPWRSET where VPWRMON = 0.4V (typ) The BIASMON output provides a current proportional to the laser bias current given by: IBIASMON = IBIAS x GBIASMON When APC is not used (no monitor diode), connect the COMP and PWRMON pins to GND. In this mode, bias current is set by the resistor (RBIASSET) between the BIASSET pin and GND. When a closed-loop configuration is used, connect a 1.7k 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 4.7k to 10k resistor. Table 1. Fault Conditions PIN BIAS BIASMON PWRMON FAULT CONDITION VBIAS > VCC - 0.2V VBIASMON > 0.8V VPWRMON > 0.8V _______________________________________________________________________________________ 9 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors MAX3795 Table 2. Circuit Response to Various Single-Point Faults (Closed-Loop APC Configuration) PIN NAME FAULT TX_DISABLE IN+ INSQUELCH TC1 TC2 MODSET PEAKSET OUT+ OUTBIASSET BIAS BIASMON CIRCUIT RESPONSE TO VCC SHORT Does not affect laser power. Modulation and bias current are disabled. Does not affect laser power. Does not affect laser power. Does not affect laser power. Does not affect laser power. The laser modulation is increased, but average power is not affected. Modulation current is disabled. Does not affect laser power. Modulation current is disabled. Does not affect laser power. Laser bias is disabled. Fault state* occurs. Note that VCSEL emissions may continue. Care must be taken to prevent this condition. Fault state* occurs. The bias current is reduced, and the average power of the laser output is reduced. IBIAS increases to the value determined by RBIASSET. If the biasmonitor fault threshold is exceeded, a fault is signaled. IBIAS increases to the value determined by RBIASSET. If the biasmonitor fault threshold is exceeded, a fault is signaled. Fault state* occurs. CIRCUIT RESPONSE TO GND SHORT Does not affect laser power. Normal condition for circuit operation. Does not affect laser power. Does not affect laser power. Does not affect laser power. Does not affect laser power. Modulation current is disabled. The laser modulation is increased, but average power is not affected. Does not affect laser power. Modulation current is disabled. Does not affect laser power. Fault state* occurs. This disables the VCSEL. Does not affect laser power. IBIAS increases to the value determined by RBIASSET. If the biasmonitor fault threshold is exceeded, a fault is signaled. The bias current is reduced, and the average power of the laser output is reduced. CIRCUIT RESPONSE TO OPEN Does not affect laser power. Modulation and bias current are disabled. Does not affect laser power. Does not affect laser power. Does not affect laser power. The laser modulation is decreased, but average power is not affected. The laser modulation is decreased, but average power is not affected. The laser modulation is decreased, but average power is not affected. Does not affect laser power. Modulation current is disabled. Does not affect laser power. Laser bias is disabled. This disables the VCSEL. Fault state* occurs. APC loop will be unstable. If the bias-monitor fault threshold is exceeded, a fault is signaled. IBIAS increases to the value determined by RBIASSET. If the bias monitor fault threshold is exceeded, a fault is signaled. The bias current is reduced, and the average power of the laser output is reduced. Does not affect laser power. COMP MD REF The bias current is reduced, and the average power of the laser output is reduced. 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 50 VCSEL load. 10 The amplitude of the modulation current is set with resistors at MODSET and temperature coefficient (TC1, TC2) pins. The resistor at MODSET (R MODSET) programs the temperature-stable portion of the modulation current, and the resistor between TC1 and TC2 (RTC) programs the temperature coefficient of the modulation ______________________________________________________________________________________ 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors MAX3795 TX_DISABLE BIAS VBIAS FAULT VCC - 0.2V FAULT OUTPUT FAULT BIASMON HIGH-CURRENT FAULT R 0.8V S R-S LATCH HIGH-POWER FAULT Q ENABLE PWRMON 0.8V TX_DISABLE POR SAFETY CIRCUIT MAX3795 Figure 2. Safety Circuit VCC MAX3795 ROUT INPUT BUFFER IN+ SIGNAL DETECT ROUT OUT+ CURRENT SWITCH OUT100 PEAKING CONTROL PEAKSET MODULATION CURRENT GENERATOR INSQUELCH ENABLE RPEAKSET CURRENT AMPLIFIER 40x TEMPERATURE COMPENSATION 1V 200 TC1 RTC TC2 MODSET RMODSET Figure 3. Modulation Circuit ______________________________________________________________________________________ 11 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors MAX3795 current. For appropriate RTC and RMODSET values, see the Typical Operating Characteristics. Programming Modulation-Current Tempco Compute the required modulation tempco from the slope efficiency of the laser at TA = +25C and at a higher temperature. Then select the value of RTC from the Typical Operating Characteristics. For example, suppose a laser has a slope efficiency (SE) of 0.021mW/mA at +25C, which reduces to 0.018mW/mA at +85C. The temperature coefficient is given by the following: Laser tempco = (SE85 - SE25 ) SE25 x (85 - 25) = -2380ppm / C x 106 Design Procedure Select Laser Select a communications-grade laser with a rise time of 90ps or better for 4.25Gbps applications. Use a highefficiency laser that requires low modulation current and generates a low-voltage 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. Programming Modulation Current A resistor (RMODSET) placed between the MODSET pin and ground controls the modulation current out of the MAX3795 to the VCSEL. The modulation current is given by the following: ROUT 1 IMOD = x 40 x R 200 + RMODSET OUT + RLOAD It is important to note that the load impedance of the VCSEL affects the modulation current being sourced by the MAX3795. The Modulation Current vs. RMODSET graph in the Typical Operating Characteristics shows the current into a 50 load. Capacitance at the MODSET pin should be 20pF. From the Typical Operating Characteristics, the value of RTC, which offsets the tempco of the laser, is 9k. If modulation temperature compensation is not desired, short TC1 and TC2. Programming the APC Loop 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, and select the value of RPWRSET that corresponds to the required current. VCC Programming Bias Current The bias current output of the MAX3795 is controlled by a resistor (RBIASSET) placed between the BIASSET pin and ground. In open-loop operation, BIASSET controls the bias current level of the VCSEL. In closed-loop operation (APC); the RBIASSET controls the maximum allowed bias current. The open-loop bias current is given by the following: 1.2 IBIAS = x 34 200 + RBIASSET The Bias Current vs. R BIASSET graph in the Typical Operating Characteristics shows the current into a 50 load. Capacitance at the BIASSET pin should be 20pF. MAX3795 PACKAGE IN+ 1nH 0.5pF 50 VCC 50 IN1nH VCC 1k 0.5pF 15pF Figure 4. Simplified Input Structure 12 ______________________________________________________________________________________ 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors VCC Applications Information Interface Models PACKAGE 1nH 0.5pF 1nH 0.5pF OUT+ OUT- MAX3795 50W 50W Figures 4 and 5 show simplified input and output circuits for the MAX3795 laser driver. Figure 6 shows the fault circuit interface. Layout Considerations To minimize inductance, keep the connections between the MAX3795 output pins and laser diode as short as possible. Use multilayer boards with uninterrupted ground planes to minimize EMI and crosstalk. Exposed-Pad (EP) Package MAX3795 Figure 5. Simplified Output Structure 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 MAX3795 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. VCC Laser Safety and IEC 825 FAULT MAX3795 Figure 6. Fault Circuit Interface IPD = VREF - VMD 0.2V RPWRSET RPWRSET The low frequency cutoff of a transmitter using APC is given by: f3dB IPD 1 x ILASER 2 x x C APC x 50 Input Termination Requirements The MAX3795 data inputs are SFP MSA compatible. Onchip, 100 differential input impedance is provided for optimal termination (Figure 4). Because of the on-chip biasing network, the MAX3795 inputs self-bias to the proper operating point to accommodate AC-coupling. 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 MAX3795 provides features that facilitate compliance with IEC 825. A common safety precaution is single-point fault tolerance, whereby one unplanned short, open, or resistive connection does not cause excess light output. Using this laser driver alone does not ensure that a transmitter design is compliant with IEC 825. The entire transmitter circuit and component selections must be considered. Customers must determine the level of fault tolerance required by their applications, recognizing that Maxim products are not designed or authorized for use as components in systems intended for surgical implant into the body, for applications intended to support or sustain life, or for any other application where the failure of a Maxim product could create a situation where personal injury or death may occur. ______________________________________________________________________________________ 13 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors MAX3795 Functional Diagram FAULT COMP MD REF PWRMON BIASMON TX_DISABLE SAFETY CIRCUITRY ENABLE VCC MAX3795 ROUT ROUT BIAS GENERATOR WITH APC BIAS BIASSET LASER MODULATOR SQUELCH IN+ 100 INMODULATION CURRENT GENERATOR SIGNAL DETECT PEAKING CONTROL OUTOUT+ ENABLE TC1 TC2 MODSET PEAKSET Chip Information TRANSISTOR COUNT: 3806 PROCESS: SiGe BIPOLAR Package Information For the latest package outline information, go to www.maxim-ic.com/packages. PART MAX3795ETG MAX3795ETG+ PACKAGE TYPE 24 Thin QFN (4mm x 4mm x 0.8mm) 24 Thin QFN (4mm x 4mm x 0.8mm) PACKAGE CODE T2444-1 T2444-1 14 ______________________________________________________________________________________ 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors Typical Application Circuit +3.3V 4.7k MAX3795 VCC FAULT PWRMON MODSET TX_DISABLE SQUELCH RMODSET 0.1F IN+ IN0.1F TC1 RTC TC2 MAX3795 REF COMP 0.047F MD BIAS L1* 0.01F OUT+ CF OUT0.01F 50 0.01F 56 0.01F CF RF L1* RPWRSET BIASSET RBIASSET GND PEAKSET BIASMON RF RPEAKSET RBIASMON L2* OPTIONAL COMPONENT SINGLE-ENDED DRIVE DIFFERENTIAL DRIVE *FERRITE BEAD 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 (c) 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. |
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