19-3387; Rev 0; 8/04 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors 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 Ordering Information PART TEMP RANGE -40C to +85C 24 Thin QFN (4mm x 4mm) MAX3795ETG+ -40C to +85C 24 Thin QFN (4mm x 4mm) +Denotes lead-free package. Pin Configuration REF MD COMP VCC BIASMON 23 22 21 20 19 Fibre-Channel Optical Transmitters PWRMON TOP VIEW Gigabit Ethernet Optical Transmitters 24 Applications GND 1 18 TX_DISABLE 2 17 BIASSET IN+ 3 16 VCC IN- 4 15 OUT+ FAULT 5 14 OUT- SQUELCH 6 13 GND 9 10 TC2 GND 12 8 TC1 11 7 MAX3795 VCC Multirate (1Gbps to 4.25Gbps) SFP/SFF Modules PIN-PACKAGE MAX3795ETG MODSET 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. Supports All SFF-8472 Digital Diagnostics PEAKSET 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. Features BIAS THIN QFN (4mm x 4mm) EXPOSED PAD IS CONNECTED TO GND ________________________________________________________________ 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 MAX3795 General Description MAX3795 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors 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 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 35 71 81 mA Additional current when peaking is used, RPEAK = 1.18k ICC-SHDN TYP 15 Additional current when SQUELCH is high 5 Total current when TX_DISABLE is high 7 10 FAULT OUTPUT Output High Voltage VOH RLOAD = 10k to 2.97V Output Low Voltage VOL RLOAD = 4.7k to 3.63V 2.4 V 0.4 V 10.0 k TX_DISABLE INPUT Input Impedance RPULL 4.7 Input High Voltage VIH 2.0 Input Low Voltage VIL V 0.8 The time for ICC to reach ICC-SHDN when TX_DISABLE transitions high Power-Down Time 8 50 V s SQUELCH Squelch Threshold 25 Squelch Hysteresis 85 6 mVP-P mVP-P Time to Squelch Data (Note 3) 0.02 5.00 s Time to Resume from Squelch (Note 3) 0.02 5.00 s BIAS GENERATOR Maximum Bias Pin Voltage Bias Current Accuracy of Programmed Bias Current 2 VBIAS-MAX IBIAS BIAS Referenced to VCC -0.65 Minimum Maximum V 1 15 5mA IBIAS 15mA -8 +8 1mA IBIAS 5mA -12 +12 _______________________________________________________________________________________ mA % 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors (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 Bias Current During Fault IBIAS_OFF BIASMON Gain GBIASMON IBIASMON / IBIAS BIASMON Stability MIN Current out of the BIAS pin TYP MAX UNITS 1.5 10 A 1mA < IBIAS < 3mA 0.0875 0.105 0.1375 3mA IBIAS < 15mA 0.085 0.105 0.125 (Notes 2, 4) -10 mA/mA +10 % V AUTOMATIC POWER CONTROL (APC) MD Nominal Voltage VMD Voltage at REF VREF APC loop is closed 1 VREF 0.2 2 1.2 1.8 2.2 MD Voltage During Fault 0 MD Input Current Normal operation (FAULT = low) APC Time Constant CCOMP = 0.047F, IPD / ILASER = 0.02 PWRMON Nominal Gain VPWRMON / (VREF - VMD) -2 0.7 1.85 2.15 V V +2 A 2.45 V/V 90 s LASER MODULATOR (Load is 50 AC-Coupled to OUT+) Differential Input Voltage VID Input Common-Mode Voltage VCM Minimum 0.25 Maximum 2.4 1.75 S11 f < 4GHz 12.7 dB SDD11 f < 4GHz 11 dB IMOD Laser Modulation During Fault or Squelch Active IMOD_OFF Current into OUT+ RLOAD 50 Minimum Maximum 115 RIN Single-Ended Input Return Loss Modulation Current 100 V Differential Input Resistance Differential Input Return Loss 85 VP-P 2 15 DC tested 15 mA 50 AP-P +10 % Tolerance of Programmed Modulation Current TC1 is shorted to TC2 Minimum Peaking Current RPEAKSET = 10k 0.2 mA Maximum Peaking Current RPEAKSET = 1k 2 mA -10 Peaking Current Duration 75 ROUT Output Resistance Single-ended resistance 42 Minimum Programmable Temperature Coefficient Maximum Programmable Temperature Coefficient Modulation Transition Time (Note 2) Temperature range 0C to +70C tR 50 load, no peaking, 5mA IMOD 15mA tF 50 load, no peaking, 5mA IMOD 15mA 50 ps 58 0 ppm/C +5000 ppm/C -40C to +85C 49 +100C 58 -40C to +85C 56 +100C 64 72 79 ps _______________________________________________________________________________________ 3 MAX3795 ELECTRICAL CHARACTERISTICS (continued) MAX3795 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 SYMBOL Deterministic Jitter DJ Random Jitter RJ CONDITIONS MIN 5mA IMOD 15mA, 4.25Gbps, K28.5 (Notes 2, 5) +100C -40C to +85C TYP MAX 8.4 15.6 12.7 APC closed loop 0.5 APC open loop (Note 2) 0.5 0.9 UNITS psP-P psRMS SAFETY FEATURES (see the Typical Operating Characteristics) 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 2) 1.8 5 s TX Disable Negate Time t_ON Time from rising edge of TX_DISABLE to IBIAS and IMOD at 99% of steady state (Note 2) 55 500 s Fault Reset Time t_INIT1 Time to set VFAULT = low after power-on or after rising edge of TX_DISABLE (Note 2) 60 200 ms Power-On Time t_INIT2 Time after power-on to transmitter-on with TX_DISABLE low (Note 2) 60 200 ms Fault Assert Time t_FAULT Time from fault occurrence to VFAULT = high; CFAULT < 20pF, RFAULT = 4.7k (Note 2) 1.4 50 s Fault Delay Time t_FLTDLY Time from fault to IBIAS = IBIAS_OFF and IMOD = IMOD_OFF; measured with a continuously occurring fault (Note 2) 1 5 s TX_DISABLE Reset t_RESET Time TX_DISABLE must be held high to reset FAULT (Note 2) 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 ELECTRICAL EYE DIAGRAM OPTICAL EYE DIAGRAM ELECTRICAL EYE DIAGRAM MAX3795 toc01 MAX3795 toc03 MAX3795 toc02 4.25Gbps, K28.5, 10mA MODULATION, PEAKING OFF 1Gbps, K28.5, -3dBm, 850nm VCSEL ADVANCED OPTICAL COMPONENTS, 1 HFE4191-541 1Gbps, K28.5, 10mA MODULATION, RPEAKSET = 1.4k 75mV/div 75mV/div 2 3 40ps/div 135ps/div 152ps/div IBIASMON vs. BIAS CURRENT OPTICAL EYE DIAGRAM MAX3795 toc04 MAX3795 toc05 1.8 3.125Gbps, K28.5, -7dBm, 850nm VCSEL, ADVANCED OPTICAL COMPONENTS HFE4191-541 1.6 1.4 IBIASMON (mA) 4.25Gbps, K28.5, -7dBm, 850nm VCSEL, ADVANCED OPTICAL COMPONENTS HFE4191-541 MAX3795 toc06 OPTICAL EYE DIAGRAM 1.2 1.0 0.8 0.6 0.4 0.2 0 34ps/div 0 50ps/div 4 8 16 12 BIAS CURRENT (mA) 20 15 10 2.5 2.0 1.5 1.0 0 0 0 2 4 6 8 10 12 14 16 18 20 MODULATION CURRENT (mAP-P) FALL TIME 50 40 RISE TIME 30 20 0.5 5 MEASURED FROM 20% 60 TRANSITION TIME (ps) 25 IBIAS = 5mA 3.0 RANDOM JITTER (psRMS) 30 70 MAX3795 toc08 35 DETERMINISTIC JITTER (psP-P) 3.5 MAX3795 toc07 40 TRANSITION TIME vs. MODULATION CURRENT RANDOM JITTER vs. MODULATION CURRENT MAX3795 toc09 DETERMINISTIC JITTER vs. MODULATION CURRENT 10 0 2 4 6 8 10 12 14 16 18 20 MODULATION CURRENT (mAP-P) 0 2 4 6 8 10 12 14 MODULATION CURRENT (mA) _______________________________________________________________________________________ 5 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.) Typical Operating Characteristics (continued) (VCC = +3.3V, RTC = 0, PEAKSET open, measured electrically with a 50 load AC-coupled to OUT+, TA = +25C, unless otherwise noted.) BIAS CURRENT vs. RBIASSET 100 10 100 0.1 1 RBIASSET () 10 90 IMOD = 15mA OUTPUT RETURN LOSS DIFFERENTIAL MEASUREMENT AT IN -5 0 MAX3795 toc14 MAX3795 toc13 0 -5 -15 S22 (dB) S11 (dB) 60 -15 -20 IMOD = 2mA -20 -25 -30 -25 40 SINGLE-ENDED MEASUREMENT -10 -10 70 10 RPWRSET () INPUT RETURN LOSS SUPPLY CURRENT vs. TEMPERATURE 50 1 0 100 RMODSET (k) 100 80 10 1 1 1 MAX3795 toc12 1m MAX3795 toc15 1 10 10m MONITOR DIODE CURRENT (A) 10 MAX3740A toc11 MAX3795 toc10 MODULATION CURRENT (mAP-P) -35 30 -30 20 -35 -40 -15 10 35 60 -40 -45 100 85 10G 1G TEMPERATURE (C) 1G FREQUENCY (Hz) MODULATION CURRENT vs. TEMPERATURE MODULATION CURRENT TEMPCO vs. RTC 10 RTC = 1k 9 RTC = 5k 8 RTC = 10k 7 RTC = 60k RTC = 100k 6 RTC = 500k 5 REFERENCED TO +25C 4500 TEMPCO (ppm/C) RMODSET = 1.8k RTC = 100 5500 MAX3795 toc16 11 MODULATION CURRENT (mAP-P) 100 FREQUENCY (Hz) MAX3795 toc17 BIAS CURRENT (mA) 100 0.1 3500 2500 1500 500 4 -500 0 10 20 30 40 50 60 TEMPERATURE (C) 6 MONITOR DIODE CURRENT vs. RPWRSET MODULATION CURRENT vs. RMODSET 100 SUPPLY CURRENT (mA) MAX3795 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors 70 80 90 100 1k 10k 100k 1M RTC () _______________________________________________________________________________________ 10G 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors MONITOR DIODE CURRENT vs. TEMPERATURE MAX3795 toc20 MAX3795 toc19 MAX3795 toc18 275 MONITOR DIODE CURRENT (A) STARTUP WITH SLOW RAMPING SUPPLY HOT PLUG WITH TX_DISABLE LOW 300 3.3V 3.3V VCC 250 VCC 0V 0V FAULT FAULT 225 LOW LOW 200 TX_DISABLE 175 LOW 150 LOW LASER OUTPUT LASER OUTPUT 125 t_INIT = 62ms TX_DISABLE t_INIT = 60ms 100 -40 -15 10 35 60 85 20ms/div 20ms/div TEMPERATURE (C) TX_DISABLE NEGATE TIME TRANSMITTER DISABLE MAX3795 toc21 3.3V 3.3V VCC t_OFF = 2.2s FAULT HIGH TX_DISABLE t_ON = 131s HIGH LOW LOW HIGH TX_DISABLE LOW LOW LASER OUTPUT LASER OUTPUT LASER OUTPUT 40s/div 1s/div FAULT RECOVERY TIME 4s/div FREQUENT ASSERTION OF TX_DISABLE MAX3795 toc24 EXTERNAL FAULT REMOVED VPWRMON FAULT t_FAULT = 2.16s FAULT LOW LOW TX_DISABLE MAX3795 toc23 EXTERNALLY FORCED VPWRMON FAULT VCC FAULT RESPONSE TO FAULT MAX3795 toc22 MAX3795 toc25 VPWRMON EXTERNALLY FORCED FAULT FAULT HIGH LOW HIGH TX_DISABLE LOW LOW t_INIT = 54s LASER OUTPUT 40s/div TX_DISABLE LASER OUTPUT 200s/div _______________________________________________________________________________________ 7 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.) 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors MAX3795 Pin Description 8 PIN NAME FUNCTION 1, 10, 13 GND 2 TX_DISABLE 3 IN+ Noninverted Data Input 4 IN- Inverted Data Input 5 FAULT 6 SQUELCH 7, 16, 20 VCC +3.3V Supply Voltage 8 TC1 Temperature Compensation Set Pin 1. A resistor placed between TC1 and TC2 (RTC) programs the temperature coefficient of the laser modulation current. 9 TC2 Temperature Compensation Set Pin 2. A resistor placed between TC1 and TC2 (RTC) programs the temperature coefficient of the laser modulation current. 11 MODSET Modulation Set. A resistor connected from MODSET to ground (RMODSET) programs the desired modulation current amplitude. 12 PEAKSET Peaking Current Set. A resistor connected between PEAKSET and ground (RPEAKSET) programs the peaking current amplitude. To disable peaking, leave PEAKSET open. Ground Transmit Disable. Driver output is disabled when TX_DISABLE is high or left unconnected. The driver output is enabled when the pin is asserted low. Fault Indicator. Open-drain output with ESD protection. FAULT is asserted high during a fault condition. Squelch Enable. Squelch is enabled when the pin is set high. Squelch is disabled when the pin is set low or left open. 14 OUT- Inverted Modulation Current Output 15 OUT+ Noninverted Modulation Current Output 17 BIASSET 18 BIAS 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 0.047F is recommended. For open-loop configuration, short the COMP pin to GND to deactivate the APC circuit. 22 MD Monitor Diode Connection 23 REF Reference Pin. Reference monitor used for APC. A resistor between REF and MD (RPWRSET) programs the photomonitor current when the APC loop is closed. 24 PWRMON EP Exposed Pad Bias-Current Set. When a closed-loop configuration is used, connect a 1.7k 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 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. _______________________________________________________________________________________ 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors MAX3795 PWRMON REF 1.8V (2VBE + 0.2) RPWRSET CURRENT AMPLIFIER 2X MAX3795 POWERCONTROL AMPLIFIER MD ENABLE IBIAS 34 BIAS BIAS GENERATOR FERRITE BEAD SMOOTHSTART IPD BIASMON 1.6V (2VBE) IBIAS 9 1V RBIASMON 200 COMP BIASSET RBIASSET CCOMP 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 FAULT CONDITION BIAS VBIAS > VCC - 0.2V BIASMON VBIASMON > 0.8V PWRMON VPWRMON > 0.8V _______________________________________________________________________________________ 9 MAX3795 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors Table 2. Circuit Response to Various Single-Point Faults (Closed-Loop APC Configuration) PIN NAME FAULT CIRCUIT RESPONSE TO VCC SHORT CIRCUIT RESPONSE TO GND SHORT CIRCUIT RESPONSE TO OPEN Does not affect laser power. Does not affect laser power. Does not affect laser power. Modulation and bias current are disabled. Normal condition for circuit operation. Modulation and bias current are disabled. IN+ Does not affect laser power. Does not affect laser power. Does not affect laser power. IN- Does not affect laser power. Does not affect laser power. Does not affect laser power. SQUELCH Does not affect laser power. Does not affect laser power. Does not affect laser power. TC1 Does not affect laser power. Does not affect laser power. The laser modulation is decreased, but average power is not affected. TC2 The laser modulation is increased, but average power is not affected. Modulation current is disabled. The laser modulation is decreased, but average power is not affected. MODSET Modulation current is disabled. The laser modulation is increased, but average power is not affected. The laser modulation is decreased, but average power is not affected. PEAKSET Does not affect laser power. Does not affect laser power. Does not affect laser power. OUT+ Modulation current is disabled. Modulation current is disabled. Modulation current is disabled. OUT- Does not affect laser power. Does not affect laser power. Does not affect laser power. Laser bias is disabled. Fault state* occurs. Laser bias is disabled. Fault state* occurs. Note that VCSEL emissions may continue. Care must be taken to prevent this condition. This disables the VCSEL. This disables the VCSEL. Fault state* occurs. Does not affect laser power. Fault state* occurs. 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 biasmonitor fault threshold is exceeded, a fault is signaled. APC loop will be unstable. If the bias-monitor fault threshold is exceeded, a fault is signaled. MD 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. IBIAS increases to the value determined by RBIASSET. If the bias monitor fault threshold is exceeded, a fault is signaled. REF 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. The bias current is reduced, and the average power of the laser output is reduced. Fault state* occurs. Does not affect laser power. Does not affect laser power. TX_DISABLE BIASSET BIAS BIASMON PWRMON *A fault state asserts the FAULT pin, disables the modulator output, and disables the bias output. Modulation Circuit The modulation circuitry consists of an input buffer, a current mirror, and a high-speed current switch (Figure 3). The modulator drives up to 15mA of modulation into a 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 FAULT OUTPUT VCC - 0.2V FAULT BIASMON HIGH-CURRENT FAULT R 0.8V Q ENABLE S R-S LATCH PWRMON HIGH-POWER FAULT 0.8V POR TX_DISABLE SAFETY CIRCUIT MAX3795 Figure 2. Safety Circuit VCC MAX3795 ROUT ROUT INPUT BUFFER OUT+ CURRENT SWITCH IN+ OUTSIGNAL DETECT 100 IN- PEAKING CONTROL PEAKSET SQUELCH ENABLE CURRENT AMPLIFIER 40x MODULATION CURRENT GENERATOR RPEAKSET TEMPERATURE COMPENSATION 1V 200 TC2 TC1 MODSET RMODSET RTC Figure 3. Modulation Circuit ______________________________________________________________________________________ 11 MAX3795 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors current. For appropriate RTC and RMODSET values, see the Typical Operating Characteristics. 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 200 + RMODSET ROUT + 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. 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 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+ 1k VCC 1nH 0.5pF 50 VCC 50 IN- 1nH 0.5pF Figure 4. Simplified Input Structure 12 ______________________________________________________________________________________ 15pF 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors Interface Models 50W PACKAGE 50W 1nH OUT- 0.5pF 1nH Figures 4 and 5 show simplified input and output circuits for the MAX3795 laser driver. Figure 6 shows the fault circuit interface. Layout Considerations OUT+ 0.5pF 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 VCC Laser Safety and IEC 825 MAX3795 FAULT 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 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. IPD 1 x ILASER 2 x x C APC x 50 Input Termination Requirements 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. 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. ______________________________________________________________________________________ 13 MAX3795 Applications Information VCC 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors MAX3795 Functional Diagram COMP FAULT MD REF BIAS BIAS GENERATOR WITH APC SAFETY CIRCUITRY TX_DISABLE BIASMON PWRMON BIASSET ENABLE VCC LASER MODULATOR MAX3795 ROUT ROUT SQUELCH OUTOUT+ IN+ SIGNAL DETECT 100 PEAKING CONTROL INMODULATION CURRENT GENERATOR ENABLE TC1 TC2 MODSET Package Information Chip Information TRANSISTOR COUNT: 3806 PROCESS: SiGe BIPOLAR For the latest package outline information, go to www.maxim-ic.com/packages. PART 14 PEAKSET PACKAGE TYPE PACKAGE CODE MAX3795ETG 24 Thin QFN (4mm x 4mm x 0.8mm) T2444-1 MAX3795ETG+ 24 Thin QFN (4mm x 4mm x 0.8mm) T2444-1 ______________________________________________________________________________________ 1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors +3.3V 4.7k VCC FAULT PWRMON MODSET TX_DISABLE RMODSET SQUELCH MAX3795 0.1F REF IN+ RPWRSET COMP 0.047F IN- MD TC1 BIAS 0.1F RTC L1* 0.01F TC2 L1* 0.01F OUT+ CF BIASSET RBIASSET CF OUTGND PEAKSET BIASMON RPEAKSET 0.01F 50 RF 0.01F 56 RF 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. MAX3795 Typical Application Circuit