19-1249; Rev 0b; 10/97 KIT ATION EVALU E L B AVAILA 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown ____________________________Features ♦ 60mA Modulation Current ♦ 80mA Bias Current ♦ 200ps Edge Speed ♦ Modulation-Current Temperature Compensation ♦ Automatic Power Control ♦ Laser-Fail Indicator with Latched Shutdown ♦ Smooth Laser Start-Up Ordering Information PART MAX3766EEP MAX3766E/D TEMP. RANGE PIN-PACKAGE -40°C to +85°C -40°C to +85°C 20 QSOP Dice* *Dice are designed to operate over this range, but are tested and guaranteed at TA = +25°C only. Contact factory for availability. ________________________Applications 622Mbps ATM Transmitters 1.25Gbps Fiber Optic LAN Transmitters Pin Configuration 1.25Gbps Ethernet Transmitters TOP VIEW BIASMAX 1 20 REF1 TC 2 19 POWERSET REF2 3 Typical Application Circuits appear at end of data sheet. 18 MD 17 GNDOUT MOD 4 GND 5 MAX3766 16 BIAS IN- 6 15 OUT+ IN+ 7 14 OUT- GND 8 13 VCCOUT VCC 9 12 FAIL ENABLE 10 11 SAFETY QSOP ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 408-737-7600 ext. 3468. MAX3766 ________________General Description The MAX3766 is a complete, easy-to-program laser driver for fiber optic LAN transmitters, optimized for operation at 622Mbps. It includes a laser modulator, automatic power control (APC), and a failure indicator with latched shutdown. Laser modulation current can be programmed up to 60mA at 622Mbps. A programmable modulation temperature coefficient can be used to keep the transmitted extinction ratio nearly constant over a wide temperature range. The modulator operates at data rates up to 1.25Gbps at reduced modulation current. APC circuitry uses feedback from the laser’s monitor photodiode to adjust the laser bias current, producing constant output power regardless of laser temperature or age. The MAX3766 supports laser bias currents up to 80mA. The MAX3766 provides extensive laser safety features, including a failure indicator with latched shutdown and a smooth start-up bias generator. These features help ensure that the transmitter output does not reach hazardous levels. The MAX3766 is available in a compact 20-pin QSOP and dice. MAX3766 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown ABSOLUTE MAXIMUM RATINGS Supply Voltage, VCC, VCCOUT .................................-0.5V to 7.0V Voltage at IN+, IN-, ENABLE, SAFETY, FAIL ...........................................-0.5V to (VCC + 0.5V) Voltage at MOD, BIASMAX, POWERSET, TC ..........-0.5V to 4.0V Current out of REF1, REF2 .................................-0.1mA to 10mA Current into OUT+, OUT- ....................................-5mA to 100mA Current into BIAS.................................................-5mA to 130mA Current into MD .......................................................-5mA to 5mA Current into FAIL ...................................................-5mA to 30mA Current into SAFETY..............................................-5mA to 10mA Continuous Power Dissipation (TA = +85°C) QSOP (derate 9.1mW/°C above +85°C) .......................590mW Operating Junction Temperature Range ...........-40°C to +150°C Processing Temperature (dice) .......................................+400°C Storage Temperature Range .............................-55°C to +150°C Lead Temperature (soldering, 10sec) .............................+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 PARAMETER CONDITIONS MIN TYP MAX UNITS 4.5 5.0 5.5 V RECOMMENDED OPERATING CONDITIONS Supply Voltage, VCC Ambient Operating Temperature (Note 1) -40 25 85 °C Differential Input Signal Amplitude VIN+ - VIN-, common-mode input = VCC - 1.3V, Figure 1 500 1000 1800 mV Input Common-Mode Voltage Referenced to VCC -1.4 -1.3 -1.19 V Enable Input High 2.0 V Enable Input Low 0.8 Voltage at OUT+, OUT- VCC - 2.5 Voltage at BIAS FAIL Load V VCC - 2.5 All DC testing uses 5.1kΩ load Data Rate Data Duty Cycle While using APC Laser to PIN Coupling Not tested V V 2.7 to 20 kΩ DC to 1.25 Gbps 50 % 0.001 to 0.1 mA/mA DC PARAMETERS Supply Current ICC (Note 2) 21 FAIL Output High R F AIL = 5.1kΩ 4.3 FAIL Output Low IMD > 15µA, R F AIL = 5.1kΩ Bias-Current Range (Note 3) 25 32 0.33 0.44 V 80 mA 10 µA 60 mA 200 µA 10 µA V 0.5 Bias Current when Driver is Disabled or Shut Down 0.1 Modulation-Current Programmable Range Input data high (Note 3) Modulation Current Input data low (Note 2) 2 1 Modulation Current when Driver is Disabled or Shut Down Minimum Modulation-Current Temperature Compensation RTC = 0Ω Maximum Modulation-Current Temperature Compensation RTC = open 2 mA -50 ppm/°C 5600 ppm/°C _______________________________________________________________________________________ 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown PARAMETER CONDITIONS Monitor-Diode Current Programmable Range MIN TYP 15 2.1 MAX UNITS 2000 µA 2.3 V Monitor-Diode Bias Voltage VCC - VMD 1.5 Upper MD Voltage for Failure Referenced to VCC -1.2 Lower MD Voltage for Failure Referenced to VCC Range of MD for No Failure Width of operating window, centered at nominal VMD REF1 Voltage for Failure Referenced to nominal VREF1 0.5 REF1 Reference Voltage TA = +25°C, VREF1 2.8 3.1 3.4 V REF2 Reference Voltage TA = +25°C, VREF2 2.1 2.4 2.7 V IMOD = 60mA 210 400 IMOD = 30mA 160 300 IMOD = 10mA 125 250 IMOD = 30mA 10 IMOD = 60mA 5 IMOD = 30mA 20 80 IMOD = 10mA 80 120 RMS, TA = +25°C, VCC = +5V, IMOD = 30mA 2 3 MAX3766 ELECTRICAL CHARACTERISTICS (continued) V -2.8 300 V mV V AC PARAMETERS (Notes 4, 5, and 6) Output Edge Speed (20% to 80%) Output Aberrations Pulse-Width Distortion Random Jitter ps % 80 ps ps Note 1: Dice are tested at room temperature only (TA = +25°C). Note 2: VCC = +5.5V, RBIASMAX = 887Ω, RMOD = 887Ω, RPOWERSET = 287Ω, RTC = 0Ω, VBIAS = VOUT+ = VOUT- = 3.0V. Supply current excludes IBIAS, IOUT+, IOUT-, and IFAIL. Note 3: Total output current must be reduced at high temperatures with packaged product to maintain maximum junction temperature of Tj = +150°C. See the Design Procedure section. Note 4: All AC parameters are measured with a 25Ω load. IMOD is the AC current amplitude at either OUT pin. The AC voltage at OUT is greater than VCC - 2.5V. Note 5: Pulse-width distortion is measured at the 50% crossing point. Data input is a 155MHz square wave, with tR ≈ 300ps. Note 6: AC specifications are guaranteed by design and characterization. _______________________________________________________________________________________ 3 Typical Operating Characteristics (Typical Operating Characteristics are measured on the MAX3766 evaluation kit, VCC = +5.0V, TA = +25°C, unless otherwise noted.) DIE MODULATION CURRENT vs. RMOD (TJ = +25°C) 30 20 4000 2000 10 0 100 LASER MODULATION CURRENT (mAp-p) MAX3766-02 MAX3766-01 6000 TEMPCO (ppm/°C) 0 TEMPCO = 3000ppm/°C TEMPCO = 0ppm/°C TEMPCO = 5600ppm/°C 10 1 0 20 40 60 80 100 1 10 100 AMBIENT TEMPERATURE (°C) 1M 1 100 10 TEMPCO = 5600ppm/°C 60 RTC = 330Ω RTC = 1kΩ RTC = 3.3kΩ 40 RTC = 10kΩ 30 200 TA = +85°C TA = +25°C 150 100 TA = -40°C 50 PULSE-WIDTH DISTORTION (ps) 0 20 10 100 1k 10k -40 100k -20 0 20 40 60 80 EYE DIAGRAM (622Mbps, 1300nm LASER, 470MHz FILTER) EYE DIAGRAM (1.244Gbps, 25Ω LOAD, IMOD = 60mA) 125µW/div 31 2 - 1 PRBS 60mA/div 20 30 40 50 60 MONITOR CURRENT vs. RPOWERSET 10 MD CURRENT (mA) 31 10 MODULATION CURRENT (mAp-p) MAX3766-08 AMBIENT TEMPERATURE (°C) MAX3766-07 RMOD (Ω) 2 - 1 PRBS 0 100 MAX3766-09 1 100k 250 RTC = 100kΩ 1 10k MODULATION EDGE SPEED AND PWD vs. AMPLITUDE RTC = 100Ω 50 1k RMOD (Ω) 70 MODULATION CURRENT (mAp-p) TEMPCO = 3000ppm/°C 10 100k MODULATION CURRENT vs. TEMPERATURE MAX3766-04 TEMPCO = 0ppm/°C 10k RTC (Ω) MODULATION CURRENT vs. RMOD (20 QSOP, TA = +25°C) 100 1k 20% TO 80% EDGE SPEED (ps) -20 MAX3766-05 -40 MAX3766-06 SUPPLY CURRENT (mA) 40 MODULATION CURRENT TEMPCO vs. RTC MAX3766-03 SUPPLY CURRENT vs. TEMPERATURE (EXCLUDES OUTPUT CURRENTS AND IFAIL) LASER MODULATION CURRENT (mAp-p) MAX3766 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown 1 0.1 0.01 161ps/div 81ps/div 0.1 1 10 RPOWERSET (kΩ) 4 _______________________________________________________________________________________ 100 1000 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown BIAS CURRENT vs. RBIASMAX (NO APC, OPEN-LOOP CONFIGURATION) DDJ (ps) IMD (mA) 0.1 NOMINAL = 2µA 100 1k 10k 100k 13 2 PRBS PATTERN 72 CONSECUTIVE ZEROS 120 100 0.001 10 180 140 0.01 -40 -20 0 20 40 60 80 -40 100 -20 0 20 40 60 AMBIENT TEMPERATURE (°C) AMBIENT TEMPERATURE (°C) UNSUCCESSFUL STARTUP SUCCESSFUL STARTUP SMOOTH STARTUP VMD VENABLE VMD OPTICAL OUTPUT WITH DATA ON FAIL FAIL VSAFETY (τ = 30µs) 100µW/ div OPTICAL OUTPUT WITH DATA OFF VSAFETY (τ = 1500µs) DATA OUT (AC COUPLED) DATA OUT (AC COUPLED) 50µs/div 10µs/div 5µs/div REFERENCE VOLTAGE vs. TEMPERATURE ABRUPT SHUTDOWN 3.20 VREF1 3.00 VOLTAGE (V) OPTICAL OUTPUT WITH DATA OFF MAX3766-17 3.40 OPTICAL OUTPUT WITH DATA ON 100µW/ div 100 MAX3766-15 VENABLE 80 MAX3766-14 RBIASMAX (Ω) MAX3766-16 1 223- 1 PRBS PATTERN 200 160 NOMINAL = 20µA 1 MAX3766-12 220 NOMINAL = 200µA MAX3766-13 IBIAS (mA) 240 NOMINAL = 1mA 1 10 260 MAX3766-11 10 MAX3766-10 100 DATA-DEPENDENT JITTER vs. TEMPERATURE (CMD = 0.1µF) MONITOR CURRENT vs. TEMPERATURE 2.80 2.60 2.40 VREF2 RTC = OPEN 2.20 2.00 5µs/div -45 -25 -5 20 40 60 80 100 AMBIENT TEMPERATURE (°C) _______________________________________________________________________________________ 5 MAX3766 Typical Operating Characteristics (continued) (Typical Operating Characteristics are measured on the MAX3766 evaluation kit, VCC = +5.0V, TA = +25°C, unless otherwise noted.) 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown MAX3766 Pin Description 6 PIN NAME FUNCTION 1 BIASMAX The current into BIASMAX sets the maximum laser bias current. Connecting BIASMAX directly to REF1 allows the largest possible bias current. 2 TC The resistance (RTC) between TC and REF1 programs the temperature coefficient of REF2. Connecting TC directly to REF1 produces the minimum tempco. Leaving TC unconnected produces the maximum tempco. 3 REF2 REF2 is the reference voltage used to program the modulation current. The tempco of REF2 is programmed by RTC. 4 MOD The current into MOD programs the laser modulation current. Connect MOD to REF2 with a resistor or potentiometer. 5, 8 GND Ground. All grounds must be connected. 6 IN- Inverting Data Input 7 IN+ Noninverting Data Input 9 VCC Positive Supply Voltage. All VCC pins must be connected. 10 ENABLE ENABLE is a TTL-compatible input. When low or open, this pin disables the output modulation and bias current. 11 SAFETY A capacitor to ground at SAFETY determines the turn-on delay for the safety circuits. If SAFETY is grounded or TTL low, internal safety shutdown features are disabled. A TTL high at SAFETY enables the internal safety shutdown features. 12 FAIL The FAIL output asserts low if the voltage at MD is above or below nominal. FAIL also asserts if REF1 is inadvertently tied to the positive supply. FAIL has TTL-compatible output voltage levels. 13 VCCOUT 14 OUT- Inverting Modulation-Current Output 15 OUT+ Noninverting Modulation-Current Output 16 BIAS Connection for the DC Laser Bias Current 17 GNDOUT 18 MD 19 POWERSET 20 REF1 Supply Voltage for the Output Current Drivers Ground for the Output Current Drivers Input for the laser monitor photodiode current. The current into POWERSET programs the average optical output power when automatic power control is used. REF1 is a voltage reference used to program laser bias current and average power. _______________________________________________________________________________________ 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown VOLTS VIN+ DIFFERENTIAL INPUT 250mV MIN 900mV MAX VINVIN+ SINGLE-ENDED INPUT 500mV MIN 1800mV MAX VIN- Laser Modulation-Current Driver RESULTING SIGNAL VIN+ - VIN500mV MIN 1800mV MAX IOUT+ The laser modulation-current driver consists of a current mirror and an emitter coupled pair. The mirror has a gain of +30mA/mA. Modulation-current amplitude is programmed with external resistor RMOD connected from REF2 to MOD. RMOD can be estimated as follows: IMOD TIME Figure 1. Required Input Signal and Output Polarity _______________Detailed Description Figure 2 is a functional block diagram of the MAX3766 laser driver. The major functional blocks are the reference generator, PECL input buffer, laser-bias circuit, modulation-current driver, automatic power control (APC), failure detection, and safety circuit. Reference Generator The MAX3766 provides adjustments for maximum laser-bias current, laser modulation current, and average laser power. To program these adjustments, simply use the currents obtained by inserting a resistor in series with integrated voltage references REF1 and REF2. The temperature coefficient (tempco) of REF1 compensates for the tempco of the bias, modulation, and APC current mirrors. Therefore, a programming current derived from REF1 is constant with temperature. REF2 provides a positive tempco, which can be applied to the modulation current. A positive modulation-current tempco will compensate for the thermal characteristics of typical laser diodes. The modulationcurrent tempco is programmed by an external resistor (RTC), which is connected from REF1 to TC. RTC and an internal 2kΩ resistor form a weighted sum of the temperature-compensated reference (REF1) and the temperature-increasing reference, which is buffered and output at REF2. REF1 and REF2 are stable with no bypass capacitance. Bypass filtering REF1 or REF2 is not required. ( ) − 520Ω 1.55V 30 RMOD = IMOD with RTC = 0Ω. The MAX3766 AC output drives up to 60mA of laser current. Pulse-width distortion and overshoot are lowest between 30mA and 60mA. However, output edge speed increases at lower currents. When the output current is between 2mA and 60mA, the edge speed is suitable for communications up to 622Mbps. Edge speeds below 30mA are suitable for communications up to 1.25Gbps (see Typical Operating Characteristics). The modulation-current tempco can be programmed with an external resistor R TC , as described in the Reference Generator section. An internal 520Ω resistor is included to limit the maximum modulation current if MOD is connected directly to REF2. If the MAX3766 is shut down or disabled, the modulation programming current is shunted to ground. Any remaining modulation current is switched to OUT-. For optimum performance, the voltage at OUT+ and OUT- must always exceed VCC - 2.5V. Laser Bias Circuit The laser bias circuit is a current mirror with a gain of +40mA/mA. Redundant controls disable the bias current during a shutdown or disable event: the programming current is switched off, and any remaining bias output current is switched away from the laser. Ensure that the voltage at BIAS always remains above VCC - 2.5V. If the bias circuit is not used, connect BIAS to VCC. _______________________________________________________________________________________ 7 MAX3766 PECL Input Buffer The differential PECL input signals are connected to the high-speed PECL input buffer at IN+ and IN-. The input impedance at IN+ and IN- is greater than 100kΩ, and the input bias current is less than 10µA. The MAX3766’s data inputs are not self-biasing. The common-mode input should be set by the external PECL termination circuitry. To obtain good AC performance, inputs should always be greater than 2.2V and less than VCC. MAX3766 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown RPOWERSET RMOD 5.1k RBIASMAX RTC VCC FAIL VCC VCC MOD REF2 TC REF1 BANDGAP 520Ω BIASMAX POWERSET 520Ω 300Ω CMD 2k TEMP SAFETY ENABLE IBIASMAX V TEMP APC SAFETY START-UP CIRCUIT FAILURE MD POWERSET MIRROR 1X V REFERENCE GENERATOR MONITOR PHOTODIODE IAPC MONITORDIODE AMPLIFIER FAILURE DETECTION VCC SHDN VCC - 2V LASER ILBP AUTOMATIC POWER CONTROL MAX3766 BIAS VCC BIAS MIRROR 40X SHDN IN+ LASER BIAS CIRCUIT INPECL INPUT BUFFER MODULATION MIRROR 30X OUT+ OUTMODULATIONCURRENT DRIVER GNDOUT VCC VCCOUT Figure 2. Functional Diagram The available laser bias current is programmed by connecting external resistor R BIASMAX from REF1 to BIASMAX. The BIASMAX programming current is adjusted by the APC circuit and amplified by the laser bias circuit. An internal 520Ω resistor between BIASMAX and the mirror input at internal node APC limits the maximum laser bias current when BIASMAX is connected directly to REF1. BIASMAX can be directly connected to REF1 in space-constrained designs, causing the maximum 8 programming current (about 2.5mA) to flow into BIASMAX. Selecting a BIASMAX resistor saves power and limits the transmitter’s maximum light output. RBIASMAX can be estimated as follows: ( ) 1.55V 40 RBIASMAX = I BIASMAX − 520Ω This equation applies to maximum bias currents above 10mA. _______________________________________________________________________________________ 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown RPOWERSET = 1.55V I MOD − 300Ω The monitor-diode amplifier senses the current from the monitor photodiode at MD, provides gain, and adjusts the laser bias programming current (ILBP). The monitordiode amplifier forces the monitor-diode current to equal the current programmed at POWERSET. The monitor-diode amplifier can reduce the laser bias programming current, but cannot increase it. Therefore, the APC circuit can adjust laser bias current between 0 and the setting determined by RBIASMAX. When the APC feedback loop is closed, the voltage at MD is approximately 2V below VCC. If the loop cannot close due to excess or insufficient photocurrent, a failure is detected by the failure-detection circuit. Internal circuitry prevents the voltage at MD from dropping below VCC - 3.2V. The stability and time constant of the APC feedback loop is determined by an external compensation capacitor (CMD) of at least 0.1µF. Connect the compensation capacitor from V CC to MD, as shown in Typical Application Circuits, to ensure a smooth startup at power-on or transmitter enable. the reference voltage would rise, the current at POWERSET would increase, and the APC loop would attempt to add laser current beyond the intended value. Either failure condition causes the FAIL output to assert TTL low. The FAIL output buffer is an open-collector output and is designed to operate with a 5.1kΩ external pull-up resistor. Safety/Start-Up Circuit The safety circuit includes the digital logic needed to provide a latched internal shutdown signal (SHDN) for disabling the laser if a failure condition exists. The MAX3766 produces less than 20µA of total laser current when disabled by safety features or by the ENABLE input. Figure 4 is a simplified schematic of the safety circuit. If ENABLE is low or open, the laser bias and modulation outputs are disabled by SHDN, regardless of the state of the safety logic. The TTL-compatible ENABLE input is internally pulled low with a 100kΩ resistor. There are two useful safety configurations: failure indication and latched shutdown. Failure-Indication Configuration Select the failure-indication configuration by connecting SAFETY to ground. In this configuration, a failure condition is reported at FAIL, but does not cause a latched shutdown. This configuration requires no additional circuitry for start-up. VMD VCC 2V If a monitor diode is not available, the APC feature can be disabled by connecting RPOWERSET to GND and leaving MD unconnected. 200mV 200mV FAILURE (INTERNAL) Failure Detection Figure 3 shows a simplified schematic of the failuredetection circuit. The failure-detection circuit senses two conditions. First, if the APC control loop cannot control the monitor current due to laser undercurrent, overcurrent, or a fault condition, a window comparator detects that V MD is above or below V CC - 2V and asserts the failure signal. Second, if REF1 is shorted to the positive supply (or any another voltage above the normal operating level), a comparator detects this condition and asserts the failure signal. If left undetected, 0.5V VREF1 VBANDGAP Figure 3. Failure-Detection Circuit (Simplified) _______________________________________________________________________________________ 9 MAX3766 Automatic Power Control Transmitters employing a laser with monitor photodiode can use the APC circuit to maintain constant power, regardless of laser threshold changes due to temperature and aging. The APC circuit consists of the POWERSET current mirror and the monitor diode amplifier. The POWERSET current mirror provides an accurate method of programming the back facet monitor photodiode current, which is assumed to be proportional to laser output power. An external resistor from REF1 to POWERSET programs the current in the unity-gain current mirror. RPOWERSET can be estimated as follows: MAX3766 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown OPEN COLLECTOR FAILURE (INTERNAL) FAIL SIMPLIFIED OPEN-COLLECTOR OUTPUT CIRCUIT RESET DOMINATE RS FLIP-FLOP S VCC OUT Q 200k SAFETY R OPEN COLLECTOR CSAFETY SHDN (INTERNAL) IN ENABLE 100k Figure 4. Simplified Safety Circuit Schematic Latched Shutdown Configuration This configuration is shown in the Typical Application Circuits (configured for best performance), and can be selected by connecting a capacitor (C SAFETY) to ground at SAFETY. In this configuration, the transmitter is shut down when a failure is detected. It can be restarted only by a power-on cycle or a toggle of the ENABLE input. During start-up, FAIL is asserted until laser power reaches the programmed level. The safety circuit must be disabled at power-on or at transmitter enable, providing enough time for the APC circuit to reach the programmed laser power level. In space-constrained designs, CSAFETY can be selected to provide a shutdown delay. When power is initially applied, or when the ENABLE signal is toggled from a logic 0 to a logic 1, the voltage at SAFETY is low, and rises with a time constant set by CSAFETY and an internal 200kΩ pull-up resistor. The SAFETY signal is inverted and resets the input of a reset-dominant RS flip-flop. The internal signal FAILURE from the failure-detection circuit is connected to the set input of the flip-flop. After SAFETY has gone high (allowing time for the APC feedback loop to settle) and if internal signal FAILURE is low, the flip-flop output is low, and the bias and modulation outputs are allowed to remain on. Refer to Figure 5 for a timing diagram of start-up in the latched shutdown configuration. 10 VCC VCC ON (OR ENABLE SWITCHED TO ON STATE) tON LASER BIAS AND MODULATION CURRENT FAIL OUTPUT SAFETY OUTPUT CURRENTS ENABLED AFTER A FIXED DELAY tAPC tSAFETY FAIL DEASSERTS WHEN THE APC LOOP SETTLES SAFETY FEATURES START CHECKING THE FAILURE SIGNAL AFTER A TIME SET BY A CAPACITOR ON THE SAFETY INPUT. AFTER THIS TIME, THE LASER DRIVER IS DISABLED IF A FAILURE OCCURS. Figure 5. Start-Up Sequence Timing The duration of tSAFETY must be about 10 times tAPC for a successful start-up. After start-up, the transmitter operates normally until a failure is detected, causing the output currents to be shut down. The laser-current outputs remain off until the failure condition is eliminated and the ENABLE input is toggled, or until the power is cycled. A potential problem with this transmitterenable method is that a slow-rising power supply may not enable the transmitter. ______________________________________________________________________________________ 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown Select Laser VCC Select a communications-grade laser with a rise time of 0.5ns or better for 622Mbps applications. The voltage swing at the OUT+ pin affects the output waveform, and is largely determined by the laser resistance, inductance, and modulation current. To obtain the MAX3766’s AC specifications, the output voltage at OUT+ must remain above VCC - 2.5V at all times. An approximation for the minimum voltage at OUT+ is given by the following equation (Table 1): ENABLE MAX809MEUR-T MAX3766* SAFETY RESET CSAFETY *IN LATCHED SHUTDOWN CONFIGURATION ( VOUT(MIN) = VCC(MIN) - VLASER - IMOD Figure 6. Reset Pulse Generator If PC board space is not a constraint, Maxim recommends enabling the transmitter with a reset-pulse generator, such as the MAX809, which generates a reset signal after VCC reaches 4.5V (Figure 6). This method ensures that the transmitter starts correctly, even if the supply ramps very slowly. ) L RL + RD + t r Select a laser that meets the output voltage criteria. A high-efficiency laser requires low modulation current and generates low voltage swing at OUT+. Laser package inductance can be reduced by trimming leads. Typical package leads have inductance of 25nH per inch (1nH/mm). A compensation filter network can also be used to reduce ringing, edge speed, and voltage swing. Table 1. Output Voltage Approximation VARIABLE DESCRIPTION TYPICAL VALUE VOUT(MIN) Approximation for the lowest voltage at the OUT+ pin 2.2V VCC(MIN) Minimum power supply 4.5V Laser forward voltage at operating power 1.3V VLASER RL Laser dynamic resistance 3Ω IOUT Laser modulation current 30mA Any damping resistance or line termination in series with the laser (but not in series with BIAS) 10Ω L Total series inductance of laser, laser package, and board traces to the MAX3766 6nH tr 20% to 80% rise time of the laser modulation current, filtered by a compensation network RD 300ps (20% to 80%) ______________________________________________________________________________________ 11 MAX3766 __________________Design Procedure VCC OR ENABLE MAX3766 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown Set Modulation-Current Tempco Compute the required modulation tempco from the slope efficiency of the laser at TA = +25°C and at a hot temperature. Then select the value of RTC from the Typical Operating Characteristics. For example, suppose a laser has a slope efficiency (SE) of 0.021mW/mA at +25°C, which reduces to 0.018mW/mA at +85°C. The temperature coefficient is given by the following: (SE85 ) − SE25 SE25 • 85 − 25 = −2380ppm / °C Laser tempco = ( ) • 106 From the Typical Operating Characteristics, the value for RTC, which offsets the tempco of the laser, is 3kΩ. If modulation temperature compensation is not desired, connect TC directly to REF1. Set Modulation Current The modulation-current amplitude can be programmed with a fixed resistor or adjusted with a potentiometer. A small internal resistance is provided to prevent damage if the potentiometer is adjusted to the end of its range. The value of RMOD can be selected from the Typical Operating Characteristics. Example: A transmitter requires average power of -8dBm (160µW), with an extinction ratio of 15. The optical signal output is 280µW (see Optical Power Relations). If the slope efficiency is 0.021mW/mA at +25°C, then the required modulation current is 0.280mW / 0.021mW/mA = 13.3mA. From the Typical Operating Characteristics, the value of RMOD is selected to be 3kΩ. Set Average Laser Power and Maximum Bias Current When APC is used, the average power control is programmed by RPOWERSET, which is typically a potentiometer. The value of RPOWERSET can be estimated from the Typical Operating Characteristics. Example: Suppose a transmitter’s output power will be adjusted to -8dBm (160µW) average power during manufacturing. The coupling efficiency from laser to monitor photodiode varies from 0.4A/W to 0.8A/W for the selected laser, causing monitor current to vary between 64µA and 128µA. From the Typical Operating Characteristics , R POWERSET should be adjustable between 12kΩ and 24kΩ. 12 Select RBIASMAX to provide sufficient current for a hot laser at its end of life. For example, if the expected laser threshold at +85°C and end of life is 40mA, then from the Typical Operating Characteristics, RBIASMAX should be 1kΩ or less. If APC is not used, the laser bias current is programmed by R BIASMAX . Select R BIASMAX from the Typical Operating Characteristics. Set APC Time Constant Capacitor CMD determines the APC time constant, and must be large enough not to cause data-dependent jitter. For 622Mbps SONET/ATM applications, Maxim recommends selecting CMD ≥ 0.1µF. Select CSAFETY When using the latched shutdown configuration, determine the minimum value of CSAFETY from the Typical Operating Characteristics. Calculate CSAFETY as follows: CSAFETY = CMD 20kΩ • IMD For example: If CMD is 0.1µF and typical monitor current (IMD) is 100µA, then the value of CSAFETY should be 50nF or larger. This ensures that tSAFETY is at least 10 times the tAPC. Design Bias Filter To reduce data-dependent jitter, add a filter at BIAS (see Typical Operating Circuit). Maxim recommends a 1µH inductor or ferrite bead with a self-resonance frequency of 200MHz or more. Design Laser-Compensation Filter Network Laser package lead inductance causes the laser impedance to increase at high frequencies, which leads to ringing, overshoot, and degradation of the output eye. A laser-compensation filter network can be used to reduce the output load seen by the MAX3766 at high frequencies, thereby reducing output ringing and overshoot. The compensation components (RCOMP and CCOMP) are most easily determined by experimentation. Begin with a no-compensation network, and observe the ring frequency (fn) of the laser and laser driver (Figure 7). Begin with RCOMP = 25Ω and CCOMP = 1/(2πfn RCOMP). Increase C COMP until the desired transmitter eye is obtained. ______________________________________________________________________________________ 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown MAX3766 UNCOMPENSATED OPTICAL POWER P1 CORRECTLY COMPENSATED PAVE OVERCOMPENSATED P0 500ps/div TIME Figure 7. Example Laser Compensation Calculate Power Consumption The MAX3766’s junction temperature must be kept below +150°C at all times. Calculate total power dissipated on the MAX3766 by laser power as follows: Power = VCC (ICC + IBIAS + IMOD) - (IMOD / 2 + IBIAS) VLASER. where IBIAS is the maximum bias current allowed by RBIASMAX, IMOD is the AC modulation current, VLASER is the typical laser forward voltage. Junction temperature = power (Watts) • 110 (°C/W). __________Applications Information Optical Power Relations Many MAX3766 specifications relate to output current amplitude. When working with fiber optic transmitters, the output is normally expressed in terms of average optical power and extinction ratio (Figure 8). Table 2 lists relations that are helpful in converting optical power to output signal amplitude when designing with the MAX3766. The relations are true if the average duty cycle of the input data is 50%. Input Terminations The MAX3766’s data inputs must be biased externally. Refer to Figure 9 for common input terminations. Laser Safety and IEC 825 The International Electrotechnical Commission (IEC) determines standards for hazardous light emissions from fiber optic transmitters. Specification IEC 825 defines the maximum light output for various hazard levels. The MAX3766 provides features that aid compliance with IEC 825. Figure 8. Optical Power Relations Table 2. Optical Power Definitions PARAMETER SYMBOL RELATION PAVE = (P0 + P1) / 2 Average Power PAVE Extinction Ratio re re = P1 / P0 Optical Power of a “1” P1 P1 = 2PAVE Optical Power of a “0” P0 P0 = 2PAVE / re + 1 Signal Amplitude PINPUT re re + 1 ( ) PINPUT = P1 - P0 = 2PAVE re - 1 re + 1 A common safety requirement is single-point fault tolerance, whereby one unplanned short, open, or resistive connection does not cause excess light output. When the MAX3766 is used in the latched shutdown configuration, as shown in Typical Application Circuits, the circuit responds as shown in Table 3. Using the MAX3766 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 customer must determine the level of fault tolerance required by their application, 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 MAX3766 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown Table 3. MAX3766 Fault Response in Latched Shutdown Configuration PIN CIRCUIT RESPONSE TO UNDERVOLTAGE OR SHORT TO GROUND Depending upon the setting of RPOWERSET, there is either no effect, or a latched shutdown. Bias current reduction causes a low laser output, resulting in a latched shutdown. High voltage on REF1 causes a failure and latched shutdown. Modulation and bias currents are reduced or off; no hazard exists. Modulation current is increased. Either the APC circuit will reduce power levels, or an overcurrent will be detected at MD, causing a failure signal and latched shutdown. Modulation current is reduced; no hazard exists. ENABLE Normal condition for circuit operation. Modulation and bias currents are shut down. IN+, IN- Forces output to either constant 1 or 0. APC maintains the power level at the programmed level. Forces output to either constant 1 or 0. APC maintains the power level at the programmed level. SAFETY Normal condition for circuit operation. Safety shutdown features are disabled, but a hazard is not created. No effect on circuit. No effect on circuit. Voltage increase at these pins will turn off the laser. High laser output asserts FAIL. A complete short will destroy the laser, eliminating the hazard. A resistive short may cause a hazard. External circuitry combined with the FAIL signal may be used to protect against a resistive short (Figure 10). Normal condition for circuit operation. Forces output to be logic 1. APC maintains the power level at the programmed level. MD Voltage increase at MD causes a failure and output current shutdown. Voltage decrease at MD causes a failure and output current shutdown. POWERSET Laser output increases, but is limited by the setting of RBIASMAX. Laser output decreases. BIASMAX REF1 REF2, TC, MOD FAIL OUT+, BIAS OUT- 14 CIRCUIT RESPONSE TO OVERVOLTAGE OR SHORT TO VCC ______________________________________________________________________________________ 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown MAX3766 VCC VCC MAX3766 PECL OUTPUT IN 5.1k 50Ω VCC - 2V VCC VCC 68Ω RF OR NONPECL OUTPUT IN FAIL LASER RESET 5.1k MAX3766 OUT+ 100k FAIL 180Ω MAX3766 VCC TTL OR CMOS OUTPUT R2 2.87k R1 10kΩ MAX3766 Figure 10. External Laser Shutdown Circuit IN R3 11.8k SINGLE-ENDED TERMINATION IS SHOWN. THE OTHER INPUT SHOULD BE TERMINATED SIMILARLY, OR CONNECTED TO VCC - 1.3V. Figure 9. Input Terminations Layout Considerations The MAX3766 is a high-frequency product. The performance of the circuit is largely dependent upon the layout of the circuit board. Use a multilayer circuit board with a dedicated ground plane. Use short laser package leads placed close to OUT+ and OUT- to keep output inductance low. Power supplies should be capacitively bypassed to the ground plane with surface-mount capacitors placed near the power-supply pins. Solutions to Common Problems 1) Laser output is ringing and contains overshoot. This is often caused by inductive laser packaging. Try reducing the lead length of the laser pins. Modify the compensation network to reduce the driver’s output edge speed (see Design Procedure). This problem can also occur if the voltage at OUT+, OUT-, or BIAS is below VCC - 2.5V. Test this by increasing the supply voltage, or reducing the modulation current. 2) Low-frequency oscillation on the bias-current output. Ensure CMD ≥ 0.1µF. 3) Modulation driver is not needed. If only the bias-current driver and safety circuits are needed, connect IN+ to VCC, and leave IN- unconnected. Connect OUT+ and OUT- to the supply. Leave MOD, TC, and REF2 unconnected. 4) APC is not needed. If only the high-speed modulation driver is used, connect BIAS to VCC, and leave POWERSET, MD, FAIL, and BIASMAX unconnected. Connect SAFETY to ground. 5) Laser edge switching speed is low. Refer to the Design Bias Filter section. It may be necessary to select LBIAS with a higher self-resonating frequency. Wire Bonding Die The MAX3766 uses bondpads with gold metalization. Make connections to the die with gold wire only, using ball bonding techniques. Wedge bonding is not recommended. Pad size is 4 mils (0.1mm) square. Die thickness is typically 15 mils (0.38mm). Interface Models Figure 11 shows typical models for the inputs and outputs of the MAX3766, including package parasitics. If dice are used, replace the package parasitic elements with bondwire parasitic elements. ______________________________________________________________________________________ 15 MAX3766 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown INPUT OUTPUT OUT+ PACKAGE OUTPACKAGE 0.2pF 0.2pF 0.2pF 250Ω 1.5nH 1.5nH Q1 1.5nH 1.5nH 0.4pF VCC VCC 1.5nH VCC 1pF VCC 2pF 1pF 250Ω Q2 VIN0.2pF PACKAGE VCC VCC VIN+ 0.2pF BIAS 0.4pF VCC - 2.5 Q3 Q5 Q4 VCC - 2.5 VCC - 2.5 IBIAS IMOD Q1, Q2 INPUT BIAS CURRENT ≅ 1µA Q1, Q2 INPUT RESISTANCE ≅ 1MΩ Q3, Q4 OUTPUT RESISTANCE ≅ 100kΩ Q5 OUTPUT RESISTANCE ≅ 100kΩ Figure 11. Interface Models 16 ______________________________________________________________________________________ 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown VCC FAILURE-INDICATION CONFIGURATION LASER 0.01µF VCC CMD ENABLE OUT- VCCOUT VCC MD OUT+ IN+ IN- BIAS VCC BIASMAX MAX3766 RFAIL 5.1k RPOWERSET POWERSET FAIL SAFETY REF1 TC MOD REF2 GNDOUT GND RMOD CONFIGURED FOR MINIMUM COMPONENT COUNT. VCC LATCHED SHUTDOWN CONFIGURATION LASER 0.01µF CMD RD VCC RCOMP ROUT- LBIAS CCOMP VCCOUT VCC ENABLE OUT- MD IN+ IN- OUT+ BIAS RBIASMAX VCC BIASMAX RFAIL 5.1k MAX3766 RPOWERSET POWERSET FAIL SAFETY REF1 RTC TC MOD REF2 GNDOUT GND CSAFETY (OPTIONAL—SEE TEXT) RMOD CONFIGURED FOR BEST PERFORMANCE. RTC SETS THE TEMPERATURE COEFFICIENT OF THE MODULATION CURRENT. ______________________________________________________________________________________ 17 MAX3766 Typical Application Circuits MAX3766 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown ___________________Chip Topography IN+ IN- GND MOD GND REF2 TC VCC BIASMAX ENABLE REF1 0.056" (1.422mm) SAFETY POWERSET FAIL MD VCCOUT OUT- OUT+ BIAS GNDOUT 0.045" (1.143mm) TRANSISTOR COUNT: 725 SUBSTRATE CONNECTED TO GND AND GNDOUT. 18 ______________________________________________________________________________________ 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown QSOP.EPS ______________________________________________________________________________________ 19 MAX3766 ________________________________________________________Package Information MAX3766 622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown NOTES Maxim makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does Maxim assume any liability arising out of the application or use of any product or circuit and specifically disclaims any and all liability, including without limitation consequential or incidental damages. “Typical” parameters can and do vary in different applications. All operating parameters, including “typicals” must be validated for each customer application by customer’s technical experts. Maxim products are not designed, intended or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the Maxim product could create a situation where personal injury or death may occur. 20 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 1997 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.