19-0323; Rev 4; 8/97 Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver ________________________Applications Laser Diode Transmitters 531Mbps and 1062Mbps Fibre Channel ____________________________Features ♦ Rise Times Less than 250ps ♦ Differential PECL Inputs ♦ Single +5V Supply ♦ Automatic Power Control ♦ Temperature-Compensated Reference Voltage ♦ Complementary Enable Inputs ______________Ordering Information PART MAX3261CCJ MAX3261ECJ MAX3261E/D __________Typical Operating Circuit +5V +5V VCCA VCCA VCCA GNDA OUT+ GNDA OUTGNDA PECL INPUTS VINGND A GND B 32 31 30 29 28 27 26 25 TO OPEN FIBER CONTROL IBIASOUT IMODSET IBIASSET IBIASFB OSADJ VREF1 VCCB ENB+ PHOTODIODE MAX3261 LASER IPIN IBIASOUT ENBSLWSTRT +5V OUT- ZO = 25Ω MICROSTRIP VREF1 FAILOUT VREF2 IBIASFB 2.7k IBIASSET IMODSET OSADJ IPINSET GNDB VIN+ GNDB VINGNDB VCCB VCCB ENB- 9 10 11 12 13 14 15 16 MAX3261 24 23 22 21 20 19 18 17 OUT+ VIN+ ENB+ 1 2 3 4 5 6 7 8 +5V VCCA VCCB __________________Pin Configuration GNDA GNDA IPIN SLWSTRT GNDB VREF2 IPINSET FAILOUT PIN-PACKAGE 32 TQFP 32 TQFP Dice* *Dice are designed to operate over a -40°C to +140°C junction temperature (Tj) range. Tested and guaranteed at Tj = +25°C. 622Mbps SDH/SONET Gigabit Ethernet TOP VIEW TEMP. RANGE 0°C to +70°C -40°C to +85°C -40°C to +85°C TQFP ________________________________________________________________ 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. MAX3261 _______________General Description The MAX3261 is a complete, easy-to-program, single +5V-powered, 1.25Gbps laser diode driver with complementary enable inputs and automatic power control (APC). The MAX3261 accepts differential PECL inputs and provides complementary output currents. A temperature-stabilized reference voltage is provided to simplify laser current programming. This allows modulation current to be programmed up to 30mA and bias current to be programmed up to 60mA with two external resistors. Complementary enable inputs allow the MAX3261 to interface with open-fiber-control architecture—a feature not found in other 1.25Gbps laser diode drivers. An APC circuit is provided to maintain constant laser power in transmitters that use a monitor photodiode. Only two external components are required to implement the APC function. The MAX3261’s fully integrated feature set includes a TTL-compatible laser failure indicator and a programmable slow-start circuit to prevent laser damage. The slow-start is preset to 50ns and can be extended by adding an external capacitor. MAX3261 Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver ABSOLUTE MAXIMUM RATINGS Terminal Voltage (with respect to GND) Supply Voltages (VCCA, VCCB) ...............................-0.3V to 6V VIN+, VIN-, FAILOUT...............................................0V to VCC_ OUT+, OUT-, IBIASOUT .......................................1.5V to VCC_ ENB+, ENB- .......................VCC_ or 5.5V, whichever is smaller Differential Input Voltage (| VIN+ - VIN- |) ...........................3.8V Input Current IBIASOUT ............................................................0mA to 75mA OUT+, OUT- ........................................................0mA to 40mA IBIASSET ........................................................0mA to 1.875mA IMODSET...............................................................0mA to 2mA IPIN, IPINSET, OSADJ...........................................0mA to 2mA FAILOUT..............................................................0mA to 10mA IBIASFB................................................................-2mA to 2mA Output Current VREF1, VREF2.....................................................0mA to 20mA SLWSTRT ..............................................................0mA to 5mA Continuous Power Dissipation (TA = +70°C) TQFP (derate 10.2mW/°C above +70°C)......................816mW Operating Temperature Ranges MAX3261CCJ ......................................................0°C to +70°C MAX3261ECJ ...................................................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-55°C to +175°C Processing Temperature (die) .........................................+400°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. DC ELECTRICAL CHARACTERISTICS (VCC = VCCA = VCCB = +4.75V to +5.25V, TA = TMIN to TMAX, unless otherwise noted. Typical values are at VCC = +5V and TA = +25°C.) (Note 1) PARAMETER Range of Programmable Laser Bias Current Reference Voltage SYMBOL CONDITIONS TYP IBIAS VREF Available Reference Current IREF Supply Current IVCC PECL Input High VIH PECL Input Low VIL TTL Input High VIH TA = +25°C 3.15 3.3 MAX UNITS 60 mA 3.45 12 50 VCC - 1.165 VCC - 1.475 VIL VOH Loaded with 2.7kΩpull-up resistor to VCC VOL Loaded with 2.7kΩpull-up resistor to VCC mA V 2 FAILOUT Output High V mA (Note 2) TTL Input Low FAILOUT Output Low MIN V V 0.8 4.5 V V 0.5 V Note 1: Dice are tested at TA = +25°C. Note 2: IVCC = IVCCA + IVCCB, IBIAS = 60mA, IMOD = 30mA, and IPIN = 140µA. AC ELECTRICAL CHARACTERISTICS (VCC = VCCA = VCCB = +4.75V to +5.25V, RLOAD (at OUT+ and OUT-) = 25Ωconnected to VCC, TA = -40°C to +85°C, unless otherwise noted. Typical values are at VCC = +5V and TA = +25°C.) (Note 3) PARAMETER Range of Programmable Modulation Current Modulation-Current Rise and Fall Time SYMBOL IMOD tR, tF Aberrations, Rising and Falling Edge Modulation-Current PulseWidth Distortion CONDITIONS Minimum differential input swing is 1100mVp-p (Note 4) IBIAS = 25mA, IMOD = 12mA, 4ns unit interval; measured from 10% to 90%. IMOD = 12mA, TA = +25°C PWD MIN TYP MAX3261E/D ±10 MAX3261ECJ ±15 IBIAS = 25mA, IMOD = 12mA, 4ns unit interval Note 3: AC characteristics are guaranteed by design and characterization. Note 4: An 1100mVp-p differential is equivalent to complementary 550mVp-p signals on VIN+ and VIN-. 2 _______________________________________________________________________________________ MAX UNITS 30 mA 250 ps % 80 ps Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver EYE DIAGRAM (622Mbps, LOAD AT OUT- = 1300nm LASER WITH 467MHz BESSEL FILTER)* -35mV 40.23ns 200ps/div -250mV 38.14ns MAX3261-5 50mV/div 58mV/div -66.2mV 38.15ns 39.91ns RBIASSET vs. BIAS CURRENT 7 37.78ns RMODSET vs. MODULATION CURRENT 12 MAX3261-07 8 40.15ns 200ps/div DIFFERENTIAL INPUT SWING = 1100 mVp-p 10 117ps/div 38.95ns RPINSET vs. MONITOR CURRENT MAX3261-08 117ps/div 39.3ns 513.7mV -35mV 38.74ns 117ps/div MAX3261CCJ EYE DIAGRAM (1062Mbps, LOAD AT OUT- = 1300nm LASER WITH 800MHz BESSEL FILTER)* 30mV/div MAX3261-4 265mV -50mV 6 1,000,000 100,000 5 4 3 8 RPINSET (Ω) RMODSET (kΩ) RBIASSET (kΩ) 38.13ns MAX3261CCJ EYE DIAGRAM (622Mbps, LOAD AT OUT- = 1300nm LASER WITH 467MHz BESSEL FILTER)* EYE DIAGRAM (1062Mbps, LOAD AT OUT- = 1300nm LASER WITH 800MHz BESSEL FILTER)* 450mV 40.14ns 200ps/div MAX3261-09 38.23ns MAX3261-6 -250mV 250.3mV 50mV/div 30mV/div 50mV/div MAX3261-2 265mV MAX3261-1 250.3mV EYE DIAGRAM (1062Mbps, LOAD = 25Ω, NOT FILTERED) MAX3261-3 EYE DIAGRAM (622Mbps, LOAD = 25Ω, NOT FILTERED) 6 10,000 4 2 1000 2 1 0 0 0 20 40 IBIAS (mA) 60 100 0 5 10 15 20 25 MODULATION CURRENT (mAp-p) 30 0 500 1000 MONITOR CURRENT (µA) * LASER = EPITAXX EDL 1300RFC TO-STYLE HEADER _______________________________________________________________________________________ 3 MAX3261 __________________________________________Typical Operating Characteristics (MAX3261E/D, load at OUT+ and OUT- = 25Ω, VCC = VCCA = VCCB = +5V, TA = +25°C, unless otherwise noted.) ____________________________Typical Operating Characteristics (continued) (MAX3261E/D, LOAD at OUT+ = OUT- = 25Ω, VCC = VCCA = VCCB = +5V, TA = +25°C, unless otherwise noted.) % CHANGE (w.r.t. +25°C) 6 4 2 0 -2 -4 -6 50 2 1 0 48 -1 46 44 42 40 38 36 -8 -2 -10 20 60 40 34 0 80 20 10 30 40 50 60 70 80 MAX3261-13 10 8 6 ALLOWABLE RANGE 2 0 5 10 15 20 25 MODULATION CURRENT (mAp-p) 60 40 MAXIMUM MODULATION CURRENT vs. MINIMUM DIFFERENTIAL INPUT SIGNAL AMPLITUDE 30 MAXIMUM MODULATION CURRENT (mAp-p) ALLOWABLE ROSADJ (kΩ) 12 0 20 TEMPERATURE (°C) ALLOWABLE ROSADJ vs. MODULATION CURRENT 4 0 TEMPERATURE (°C) TEMPERATURE (°C) 40 MAX3261-14 0 4 MAX3261-12 APC DISABLED SUPPLY CURRENT (mA) 8 MAX3261-11 3 MAX3261-10 10 SUPPLY CURRENT vs. TEMPERATURE PERCENT CHANGE IN BIAS CURRENT vs. TEMPERATURE PERCENT CHANGE IN MODULATION CURRENT vs. TEMPERATURE % CHANGE (w.r.t. +25°C) MAX3261 Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver RMODSET = 1.2kΩ ROSADJ = 2kΩ 35 30 25 20 15 10 5 0 0 400 800 1200 1600 MINIMUM DIFFERENTIAL INPUT SIGNAL AMPLITUDE (mVp-p) _______________________________________________________________________________________ 2000 80 Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver PIN 1, 2, 25, 27, 29 3 NAME 4 SLWSTRT GNDA IPIN FUNCTION Ground for Bias and Modulation Current Drivers Monitor Photodiode Current Input. Connect IPIN to photodiode’s anode. Slow-Start Capacitor Input. Connect capacitor to ground or leave unconnected to set start-up time, tSTARTUP = 25.4kΩ (CSLWSTRT + 2pF). 5, 9, 11, 13 6 GNDB Ground for Voltage Reference and Automatic Power-Control Circuitry VREF2 7 IPINSET Temperature-Compensated Reference Output. VREF2 is internally connected to VREF1. Monitor Photodiode Programming Input. Connect IPINSET to VREF1 or VREF2 through a resistor to set the monitor current when using automatic power control (see Typical Operating Characteristics). 8 FAILOUT 10 VIN+ Noninverting PECL Data Input 12 VIN- 14, 15, 18 VCCB 16 ENB- Inverting PECL Data Input +5V Supply Voltage for Voltage Reference and Automatic Power-Control Circuitry. Connect VCCB to the same potential as VCCA, but provide separate bypassing for VCCA and VCCB. Inverting Enable TTL Input. Output currents are enabled only when ENB+ is high and ENB- is low. 17 ENB+ Noninverting Enable TTL Input. Output currents are enabled only when ENB+ is high and ENB- is low. 19 VREF1 20 OSADJ 21 IBIASFB 22 IBIASSET 23 IMODSET 24 IBIASOUT Temperature-Compensated Reference Output. VREF1 is internally connected to VREF2. Overshoot-Adjust Input. Connect to internal voltage reference through a resistor to adjust the overshoot of the modulation output signal (see Typical Operating Characteristics). Bias-Feedback Current Output. Output from automatic power-control circuit. Connect to IBIASSET when using APC. Laser Bias Current-Programming Input. Connect to internal voltage reference through a resistor to set bias current (see Typical Operating Characteristics). IIBIASOUT = 40 x (IIBIASSET + IIBIASFB). Laser Modulation Current-Programming Input. Connect to internal voltage reference through a resistor to set modulation current (see Typical Operating Characteristics). IMOD = 20 x IIMODSET. Laser Bias Current Output. Connect to laser cathode through an R-L compensation network (see the Bias Network Compensation section). Modulation Output. When VIN+ is high and VIN- is low, OUT- sinks IMOD. 26 OUT- 28 OUT+ 30, 31, 32 VCCA Failout Output. Active-low, open-collector TTL output indicates if automatic power-control loop is out of regulation due to insufficient monitor-diode current (when VIPIN is below the 2.6V threshold). Connect FAILOUT to VCC_ through a 2.7kΩ pull-up resistor. Modulation Output. When VIN+ is low and VIN- is high, OUT+ sinks IMOD. +5V Supply Voltage for Bias and Modulation Current Drivers. Connect VCCA to the same potential as VCCB, but provide separate bypassing for VCCA and VCCB. _______________________________________________________________________________________ 5 MAX3261 ______________________________________________________________Pin Description _______________Detailed Description The MAX3261 laser driver has three main sections: a reference generator with temperature compensation, a laser bias block with automatic power control, and a high-speed modulation driver. The reference generator provides temperature-compensated biasing and a voltage-reference output. The voltage reference is used to program the current levels of the high-speed modulation driver, laser diode, and PIN (p+, intrinsic, n-) monitor diode. The laser bias block sets the bias current in the laser diode and maintains it above the threshold current. A current-controlled current source (current mirror) programs the bias, with IBIASSET as the input. The mirror’s gain is approximately 40. Keep the output voltage of the bias stage above 2.2V to prevent saturation. The modulation driver consists of a high-speed input buffer and a common-emitter differential output stage. The modulation current mirror sets the laser modulation current in the output stage. This current is switched between the OUT+ and OUT- ports of the laser driver. The modulation VCC MAX3261 OUT+ VIN+ LASER VINOUTPHOTODIODE VCCA 20 x IIMODSET VCCB IBIASOUT GNDA 40 x IIBIASSET GNDB BIAS COMPENSATION +2.6V FAILOUT ENB+ ENB- COMPARATOR MAIN BIAS GENERATOR IPIN +3V TRANSCONDUCTANCE AMPLIFIER SLWSTRT 1 X IIPINSET IPINSET IBIASFB IBIASSET RBIASSET IMODSET RMODSET IOSADJ VREF1 OR VREF2 BANDGAP REFERENCE ROSADJ MAX3261 Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver RPINSET Figure 1. Functional Diagram 6 _______________________________________________________________________________________ LOOPSTABILITY CAPACITOR 1000pF Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver MAX3261 OUTPUTS VCC MAX3261 280Ω 280Ω ENB+ INPUTS DATA OUT (LOAD = 1300nm LASER AT OUT-) 9Ω 9Ω 2µs/div 400Ω 2(IIOSADJ) 2(IIOSADJ) IIMODSET Figure 3. Enable/Disable Operation Automatic Power Control INPUT BUFFER OUTPUT STAGE Figure 2. MAX3261 Modulation Driver (Simplified) current mirror has a gain of approximately 20. Keep the voltages at OUT+ and OUT- above 2.2V to prevent saturation. The overshoot mirror sets the bias in the input buffer stage (Figure 2). Reducing this current slows the input stage and reduces overshoot in the modulation signal. At the same time, the peak-to-peak output swing of the input buffer stage is reduced. Careful design must be used to ensure that the buffer stage can switch the output stage completely. The input swing required to completely switch the output stage depends on both R OSADJ and the modulation current. See Allowable ROSADJ Range vs. Modulation Current and Modulation Current vs. Differential Input Signal graphs in the Typical Operating Characteristics. Failure to ensure that the output stage switches completely results in a loss of modulation current (and extinction ratio). In addition, if the modulation port does not switch completely off, the modulation current will contribute to the bias current, and may complicate module assembly. The automatic power control (APC) feature allows an optical transmitter to maintain constant power, despite changes in laser efficiency with temperature or age. The APC feature requires the use of a monitor photodiode. The APC circuit incorporates the laser diode, the monitor photodiode, the PIN set current mirror, a transconductance amplifier, the bias set current mirror, and the laser fail comparator (Figure 1). Light produced by the laser diode generates an average current in the monitor photodiode. This current flows into the MAX3261’s IPIN input. The PIN set current mirror draws current away from the IPIN node. When the current into the IPIN node equals the current drawn away by IPINSET, the node voltage is set by the 3/5 x VCC reference of the transconductance amplifier. When the monitor current exceeds IPINSET, the IPIN node voltage will be forced higher. If the monitor current decreases, the IPIN node voltage is decreased. In either case, the voltage change is amplified by the transconductance amplifier, and results in a feedback current at the IBIASFB node. Under normal APC operation, IBIASFB is summed with IBIASSET, and the laser bias level is adjusted to maintain constant output power. This feedback process continues until the monitor-diode current equals IPINSET. If the monitor-diode current is sufficiently less than IPINSET (i.e., the laser stops functioning), the voltage on _______________________________________________________________________________________ 7 MAX3261 Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver the IPIN node will drop below 2.6V. This will trigger the failout comparator, which provides a TTL signal indicating laser failure. The FAILOUT output asserts only if the monitor-diode current is low, not in the reverse situation where the monitor current exceeds IPINSET. FAILOUT is an open-collector output that requires an external pull-up resistor of 2.7kΩto VCC. The transconductance amplifier can source or sink currents up to approximately 1mA. Since the laser bias generator has a gain of approximately 40, the APC function has a limit of approximately 40mA (up or down) from the initial set point. To take full advantage of this adjustment range, it may be prudent to program the laser bias current slightly higher than required for normal operation. However, do not exceed the IBIASOUT absolute maximum rating of 75mA. To maintain APC loop stability, a 1000pF bypass capacitor may be required across the photodiode. If the APC function is not used, leave IBIASFB unconnected. Enable Inputs The MAX3261 provides complementary enable inputs (ENB+, ENB-) for interfacing with open-fiber-control architecture. The laser is disabled by reducing the reference voltage outputs (VREF1, VREF2). Only one logic state will enable laser operation (Table 1). With a 1000pF stability capacitor, the MAX3261 modulation and bias can be enabled and disabled within 5µs (Figure 3). This timing satisfies the requirements of the Open Fiber Control system used in Fibre Channel networks. Temperature Considerations The MAX3261 output currents are programmed by current mirrors. These mirrors each have a 2VBE temperature coefficient. The reference voltage (VREF) is adjusted 2VBE so these changes largely cancel, resulting in output currents that are very stable with respect to temperature (see Typical Operating Characteristics). Wire Bonding Die For reliable operation, the MAX3261 has gold metallization. Make connections to the die with gold wire only, using ball bonding techniques. Wedge bonding is not recommended. Pad size is 4mils. Table 1. MAX3261 Truth Table ENB0 0 1 1 8 ENB+ 0 1 0 1 OUTPUT CURRENTS DISABLED ENABLED DISABLED DISABLED __________________Design Procedure Interfacing Suggestions Use high-frequency design techniques for the board layout of the MAX3261 laser driver. High-speed interfaces often require fixed-impedance transmission lines (Figure 5). Adding some damping resistance in series with the laser raises the load impedance, making the transmission line more realizable, and it also helps reduce power consumption (see the section Reducing Power Consumption). Minimize any series inductance to the laser, and place a bypass capacitor as close to the laser’s anode as possible. Power connections labeled VCCA are used to supply the laser modulation and laser bias circuits. VCCB connections supply the bias-generator and automatic-power control circuits. For optimum operation, isolate these supplies from each other by independent bypass filtering. VCCA, VCCB, GNDA, and GNDB all have multiple pins. Connect all pins to optimize the MAX3261’s highfrequency performance. Ground connections between signal lines (VIN+, VIN-, OUT+, OUT-) improve the quality of the signal path by reducing the impedance of the interconnect. Multiple connections, in general, reduce inductance in the signal path and improve the highspeed signal quality. GND pins should be tied to the ground plane with short runs and multiple vias. Avoid ground loops, since they are a source of high-frequency interference. The MAX3261 data inputs accept PECL input signals, which require 50Ωtermination to (VCC - 2V). Figure 4 shows alternative termination techniques. When a termination voltage is not available, use the Theveninequivalent termination. When interfacing with a non-PECL signal source, use one of the other alternative termination methods shown in Figure 4. Bias Network Compensation When driving the laser diode with transmission lines, it is important to maintain a constant load impedance in order to minimize aberrations due to reflections. The inductive nature of laser packages will cause the laser impedance to increase with frequency, and the parasitic capacitance of the laser driver bias output (IBIASOUT) has some loading effects at high frequency. Of these two effects, the loading due to the laser lead inductance dominates. Impedance variation must be compensated for high-frequency operation. One possible approach is to use a shunt R-C network in parallel with the laser diode to compensate for the laser impedance (Figures 5 and 6). Add an R-L circuit in series with the bias output to compensate for the IBIASOUT capacitance (Figures 5 and 7). _______________________________________________________________________________________ Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver MAX3261 5V PECL SIGNAL SOURCE 5V 82Ω 82Ω VIN+ 120Ω a) THEVENIN-EQUIVALENT TERMINATION MAX3261 VIN120Ω NON-PECL SIGNAL SOURCE VIN+ 5V 50Ω 50Ω 680Ω 50Ω 1.8k b) DIFFERENTIAL NON-PECL TERMINATION MAX3261 50Ω VIN- NON-PECL SIGNAL SOURCE VIN+ 50Ω 180Ω 68Ω 5V c) SINGLE-ENDED NON-PECL TERMINATION MAX3261 5V 680Ω VIN1.8k 5V ECL SIGNAL SOURCE OV 1.3k 1.3k VIN+ 50Ω d) ECL TERMINATION 3.6k MAX3261 -2V THIS SYMBOL REPRESENTS A TRANSMISSION LINE WITH CHARACTERISTIC IMPEDANCE Zo = 50Ω. VIN50Ω 3.6k -2V Figure 4. Alternative PECL Data-Input Terminations _______________________________________________________________________________________ 9 MAX3261 Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver Reducing Power Consumption The laser driver typically consumes 40mA of current for internal functions. Typical load currents, such as 12mA of modulation current and 20mA of bias current, bring the total current requirement to 72mA. If this were dissipated entirely in the laser driver, it would generate 360mW of heat. Fortunately, a substantial portion of this power is dissipated across the laser diode. A typical laser diode will drop approximately 1.6V when forward biased. This leaves 3.4V at the MAX3261’s OUT- terminal. It is safe to reduce the output terminal voltage even further with a series damping resistor. Terminal voltage levels down to 2.2V can be used without degrading the laser driver’s high-frequency performance. Power dissipation can be further reduced by adding a series resistor on the laser driver’s OUT+ side. Select the series resistor so the OUT+ terminal voltage does not drop below 2.2V with the maximum modulation current. __________Applications Information Programming the MAX3261 Laser Driver Programming the MAX3261 is best explained by an example. Assume the following laser diode characteristics: Wavelength λ 780nm Threshold Current ITH 20mA at +25°C (+0.35mA/°C temperature variation) Monitor Responsivity ρmon 0.1A/W (monitor current / average optical power into the fiber) Modulation Efficiency η 0.1mW/mA (worst case) Now assume the communications system has the following requirements: Average Power PAVE 0dBm (1mW) Extinction Ratio Er 6dB (Er = 4) Temperature Range Tr 0°C to +70°C 1) Determine the value of IPINSET: The desired monitor-diode current is (PAVE)(ρmon) = (1mW)(0.1A/W) = 100µA. The R PINSET vs. Monitor Current graph in the Typical Operating Characteristics shows that RPINSET should be 18kΩ. 2) Determine RMODSET: equations results in P1 = (2 x PAVE x Er) / (Er + 1) and P0 = (2 x PAVE) / (Er + 1). In this example, P1 = 1.6mW and P0 = 0.4mW. The optical modulation is 1.2mW. The modulation current required to produce this output is 1.2mW / η = (1.2mW) / (0.1mA/mW) = 12mA. The Typical Operating Characteristics show that RMODSET = 3.9kΩ yields the desired modulation current. 3) Determine the value of ROSADJ: Using the Allowable ROSADJ vs. Modulation Current graph in the Typical Operating Characteristics, a 5.6kΩ resistor is chosen for 12mA of modulation current. The maximum ROSADJ values given in the graph minimize aberrations in the waveform and ensure that the driver stage operates fully limited. 4) Determine the value of RBIASSET: The automatic power control circuit can adjust the bias current 40mA from the initial setpoint. This feature makes the laser driver circuit reasonably insensitive to variations of laser threshold from lot to lot. The bias setting can be determined using one of two methods: a) Set the bias at the laser threshold. b) Set the bias at the midpoint of the highest and lowest expected threshold values. Method A is straightforward. In the second method, it is assumed that the laser threshold will increase with age. The lowest threshold current occurs at 0°C, when the laser is new. The highest threshold current occurs at +70°C, at the end of the product’s life. Assume the laser is near the end of life when its threshold reaches two-times its original value. Lowest Bias Current: ITH + ∆ITH = 20mA + (0.35mA/°C)(-25°C) = 11.25mA Highest Bias Current: 2 x ITH + ∆ITH = 40mA + (0.35mA/°C)(+45°C) = 55.8mA In this case, set the initial bias value to 34mA (which is the midpoint of the two extremes). The adjustment range of the MAX3261 maintains the average laser power at either extreme. The Typical Operating Characteristics show that RBIASSET = 1.8kΩ delivers the required bias current. The average power is defined as (P1 + P0) / 2, where P1 is the average amplitude of a transmitted “one” and P0 is the average amplitude of a transmitted “zero.” The extinction ratio is P1/P0. Combining these 10 ______________________________________________________________________________________ Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver EYE DIAGRAM WITH R-C COMPENSATION (622Mbps, LOAD AT OUT- = 1300nm LASER WITH 467MHz BESSEL FILTER)* 25Ω 100pF 0.01µF SERIES R-L 47µH IBIASOUT 265mV MAX3261-FG06 LASER PHOTODIODE IPIN SHUNT RC 200Ω 18Ω OUT- 0.01µF AS CLOSE TO THE LASER ANODE AS POSSIBLE 30mV/div 1000pF MAX3261 MAX3261 +5V 25Ω OUT+ AS CLOSE TO THE LASER CATHODE AS POSSIBLE ZO = 25Ω -35mV MICROSTRIP 200ps/div *EPITAXX EDL 1300 RFC, TO-STYLE HEADER Figure 5. Typical Laser Interface with Bias Compensation Figure 6. Eye Diagram with R-C Compensation (LOAD at OUT- = 1300nm Laser) Laser Safety and IEC 825 EYE DIAGRAM WITH R-C AND R-L COMPENSATION (622Mbps, LOAD AT OUT- = 1300nm LASER WITH 467MHz BESSEL FILTER)* 30mV/div MAX3261-FG07 265mV Using the MAX3261 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. -35mV 200ps/div *EPITAXX EDL 1300 RFC, TO-STYLE HEADER Figure 7. Eye Diagram with R-C and R-L Compensation (LOAD at OUT- = 1300nm Laser) ______________________________________________________________________________________ 11 GNDA OUT- OUT- GNDA OUT+ OUT+ GNDA V CC A V CC A V CC A ____________________________________________________________Chip Topography GNDA IBIASOUT GNDA IMODSET IPIN IBIASSET SLWSTRT IBIASFB ENB- 0.080" (2.032mm) V CC B N.C. V CC B ENB+ N.C. V CC B FAILOUT GNDB V CC B IPINSET VIN- VREF2 VIN- VREF1 GNDB GNDB VIN+ OSADJ VIN+ GNDB GNDB MAX3261 Single +5V, Fully Integrated, 1.25Gbps Laser Diode Driver 0.080" (2.032mm) TRANSISTOR COUNT: 197 SUBSTRATE CONNECTED TO GNDA AND GNDB 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, 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 for 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. 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. 12 ____________________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.