19-2294; Rev 0; 1/02 2.7Gbps, +3.3V DC-Coupled Laser Driver ANUAL N KIT M IO T A U EVAL BLE AVAILA Features ♦ Single +3.3V Power Supply ♦ 35mA Supply Current ♦ Programmable Bias Current from 1mA to 100mA ♦ Programmable Modulation Current from 5mA to 60mA (Up to 80mA AC-Coupled) ♦ Bias Current and Modulation Current Monitors ♦ 70ps Rise/Fall Time ♦ Automatic Average Power Control with Failure Monitor ♦ Complies with ANSI, ITU, and Bellcore SDH/SONET Specifications ♦ Laser Current-Enable Control Ordering Information PART Applications SDH/SONET Transmission Systems MPLS Transmitter Systems TEMP RANGE PIN-PACKAGE MAX3850EGJ -40°C to +85°C 32 QFN MAX3850E/D -40°C to +85°C Dice* *Dice are designed to operate over this range, but are tested and guaranteed at TA = +25°C only. Contact factory for availability. Typical Application Circuits are continued at the end of the data sheet. Add/Drop Multiplexers Digital Cross-Connects Section Regenerators Pin Configuration appears at the end of the data sheet. Typical Application Circuits 3.3V 0.01µF LD FAIL LATCH ENABLE 3.3V DATA+ VCC MAX3890 DATA- OUT+ SERIALIZER WITH CLOCK GEN CLOCK+ 16Ω OUT- 100Ω 11Ω 50Ω MAX3850 8.0pF 100Ω 0.1µF 0.1µF TYPICAL APPLICATION CIRCUIT WITH DC-COUPLED INPUTS BIASMON MODMON CAPC APCFILT GND BIAS MD APCSET MODSET BIASMAX CLOCK- 392Ω 1000pF 392Ω 3.3V Covered by U.S. Patent numbers 5,802,089 and 5,883,910 ________________________________________________________________ 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 MAX3850 General Description The MAX3850 is a +3.3V DC-coupled laser driver for SDH/SONET applications up to 2.7Gbps. The device accepts differential data and clock inputs and provides bias and modulation currents for driving a laser. If a clock signal is available, a synchronizing input latch can be used to reduce jitter. An automatic power-control (APC) feedback loop is incorporated to maintain a constant average optical power over temperature and lifetime. The wide modulation current range of 5mA to 60mA (up to 80mA AC-coupled) and bias current of 1mA to 100mA are easy to program, making this product ideal for SDH/SONET applications. The MAX3850 also provides laser current-enable control, two current monitors that are directly proportional to the laser bias and modulation currents, and a failure-monitor output to indicate when the APC loop is unable to maintain the average optical power. Designed to be DC-coupled to the laser with a supply voltage of only 3.3V, the MAX3850 greatly simplifies interface requirements. The MAX3850 is available in a small 32-pin QFN package as well as dice. MAX3850 2.7Gbps, +3.3V DC-Coupled Laser Driver ABSOLUTE MAXIMUM RATINGS Supply Voltage, VCC..............................................-0.5V to +4.0V Current into BIAS ............................................-20mA to +150mA Current into OUT+, OUT- ................................-20mA to +100mA Current into MD.....................................................-5mA to +5mA Voltage at DATA+, DATA-, CLK+, CLK-, ENABLE, LATCH, FAIL, BIASMON, MODMON, CAPC, MODSET, BIASMAX, APCSET................-0.5V to (VCC + 0.5V) Voltage at APCFILT ...............................................-0.5V to +3.0V Voltage at OUT+, OUT-.............................................0.4V to 4.8V Voltage at BIAS............................................1.0V to (VCC + 0.5V) Continuous Power Dissipation (TA = +85°C) 32-Pin QFN (derate 21.2mW/°C above +85°C) ........13.84mW Storage Temperature Range .............................-65°C to +165°C Operating Junction Temperature Range ...........-55°C to +150°C Processing Temperature (die) .........................................+400°C Lead Temperature (soldering,10s) ..................................+300°C Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. DC ELECTRICAL CHARACTERISTICS (VCC = +3.0V to +3.6V, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, IMOD = 30mA, IBIAS = 60mA, TA = +25°C, unless otherwise noted.) (Note 1) PARAMETER SYMBOL CONDITIONS Supply Voltage VCC Supply Current ICC (Note 2) IBIAS (Note 3) Bias-Current Range Bias Off-Current IBIAS-OFF Bias-Current Stability Differential Input Voltage Range Common-Mode Input Voltage VICM MAX 3.3 3.6 V 35 65 mA 100 mA 100 µA 1 IBIAS = 100mA -480 12 480 IBIAS = 1mA -1000 456 1000 (Figure 1) 200 VCC 1.49 LVPECL compatible TTL Input High Voltage ENABLE, LATCH TTL Input Low Voltage ENABLE, LATCH TTL Output High Voltage FAIL sourcing 50µA TTL Output Low Voltage Sinking 100µA VCC 1.32 ppm/°C mVP-P VCC VID/4 V V 0.8 V 2.4 VCC 0.3 VCC V 0 0.025 0.4 V 1.5 IMD UNITS 1600 2.0 Monitor-Diode Reverse Bias Voltage Monitor-Diode DC Current Range TYP 3.0 ENABLE = low (Note 4) APC open loop VID MIN V 18 1000 IMD = 1mA -480 95 480 IMD = 18µA -1000 295 1000 Monitor-Diode Set-Point Stability (Note 6) Monitor-Diode Bias Absolute Accuracy (Note 5) -15 µA ppm/°C +15 % BIASMON to IBIAS Gain ABIAS IBIAS/IBIASMON 36 41 46 A/A MODMON to IMOD Gain AMOD IMOD/IMODMON 25 30 35 A/A Modulation-Current Range IMOD 2 VOUT+, VOUT- = 0.6V (DC-coupled) 5 60 VOUT+, VOUT- = 2.0V (AC-coupled) 5 80 _______________________________________________________________________________________ mA 2.7Gbps, +3.3V DC-Coupled Laser Driver (VCC = +3.0V to +3.6V, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, IMOD = 30mA, IBIAS = 60mA, TA = +25°C, unless otherwise noted.) (Note 1) PARAMETER Modulation Off-Current SYMBOL CONDITIONS MIN TYP IMOD-OFF ENABLE = low (Note 4) Modulation-Current Stability MAX UNITS 100 µA IMOD = 60mA -480 66 480 IMOD = 5mA -1000 110 1000 ppm/°C AC ELECTRICAL CHARACTERISTICS (VCC = +3.0V to +3.6V, IMOD = 5mA to 60mA, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, IMOD = 30mA, TA = +25°C.) (Note 7) PARAMETER SYMBOL CONDITIONS MIN TYP Maximum Data Rate MAX UNITS 2.7 Gbps Input Latch Setup Time tSU LATCH = high (Figure 3) 90 ps Input Latch Hold Time tH LATCH = high (Figure 3) 60 ps Output Rise Time tR 20% to 80% edge speeds (Note 8) 66 110 ps Output Fall Time tF 20% to 80% edge speeds (Note 8) 70 100 ps 30mA ≤ IMOD ≤ 60 (Note 8) ±2 IMOD = 5mA (Note 8) ±12 Output Overshoot IMOD Enable/Startup Delay 270 ns APC open loop, CAPC and CAPCFILT = 0 370 ns RJOUT (Note 8) 0.66 1.4 psRMS TJOUT LATCH = high, 27- 1 PRBS with 80 inserted 0s and 80 inserted 1s 8.6 50 psP-P IBIAS Typical Startup Delay Output Random Jitter Output Deterministic Jitter % Dice are tested at TA = +25°C only. Specifications at -40°C are guaranteed by design and characterization. Tested at RMODSET = 2.61kΩ, RBIASMAX = 1.96kΩ, excluding IBIAS and IMOD. Voltage on BIAS pin is (VCC - 1.5V). The bias and modulation currents will be switched off if any of the current set pins are grounded. Accuracy refers to part-to-part variation. Assuming the laser-to-monitor diode transfer function does not change with temperature. Guaranteed by design and characterization. Note 7: AC characteristics are guaranteed by design and characterization using the characterization circuit of Figure 2. Note 8: Measured with repeating 0000 1111 pattern, LATCH = high. Note 1: Note 2: Note 3: Note 4: Note 5: Note 6: _______________________________________________________________________________________ 3 MAX3850 DC ELECTRICAL CHARACTERISTICS (continued) MAX3850 2.7Gbps, +3.3V DC-Coupled Laser Driver DATA+ 100mV MIN DATA- 800mV MAX 200mVP-P MIN (DATA+) - (DATA-) 1600mVP-P MAX IOUT+ IMOD Figure 1. Required Input Signal and Output Polarity VCC tCLK VCC CLK MAX3850 30Ω 30Ω tSU 1.0pF Z0 = 30Ω OUT- DATA 30Ω 0.5pF IOUT+ OUT+ tH OSCILLOSCOPE Z0 = 30Ω Figure 3. Setup/Hold Time Definition BIAS 15Ω 75Ω 50Ω VCC Figure 2. Output Termination for Characterization 4 _______________________________________________________________________________________ 2.7Gbps, +3.3V DC-Coupled Laser Driver OPTICAL EYE DIAGRAM (2.7Gbps, 130mm FP LASER 1.87Gbps FILTER, 32-QFN) ELECTRICAL EYE DIAGRAM (IMOD = 25mA, 213 - 1 + 80 CID, 32 QFN) MAX3850 toc01 STARTUP DELAY (ms) MAX3850 toc02 100 MAX3850 toc03 BIAS CURRENT ENABLE STARTUP DELAY vs. CAPC 10 1.0 0.1 MITSUBISHI ML725C8F LASER DIODE 100p 1000p 0.01µ 0.1µ 58ps/div 1.0µ CAPC (F) ELECTRICAL EYE DIAGRAM AC-COUPLED (IMOD = 80mA, 213 - 1 + 80 CID, 32 QFN) RANDOM JITTER vs. IMOD 1.8 1.6 RANDOM JITTER (psms) MAX3850 toc04 MAX3850 toc06 2.0 MAX3850 toc05 ELECTRICAL EYE DIAGRAM (IMOD = 60mA, 213 - 1 + 80 CID, 32 QFN) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 58ps/div 58ps/div 10 20 30 40 60 50 IMOD (mA) IBIASMAX vs. RBIASMAX 10 5 0 10 20 30 40 IMOD (mA) 50 60 60 50 IMOD (mA) 15 IMOD vs. RMODSET 70 MAX3850 toc08 MAX3850 toc07 20 IBIASMAX (mA) TOTAL JITTER (psP-P) 25 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 MAX3850 toc09 DETERMINISTIC JITTER vs. IMOD 30 40 30 20 10 0 1k 10k RBIASMAX (Ω) 100k 1k 10k 100k RMODSET (Ω) _______________________________________________________________________________________ 5 MAX3850 Typical Operating Characteristics (DC-coupled output, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (DC-coupled output, TA = +25°C, unless otherwise noted.) 60 SUPPLY CURRENT (mA) 1.2 1.0 0.8 0.6 0.4 50 40 30 VCC = 3.0V VCC = 3.3V VCC = 3.6V 20 0.2 0 1k 10k 100k -15 10 35 25 -15 -40 PULSE-WIDTH DISTORTION vs. IMOD MAX3850 toc14 18 MAX3850 toc13 IBIAS = 100mA, IMOD = 50mA 10 16 14 IBIAS = 10mA, IMOD = 10mA PWD (ps) 12 20 15 10 8 6 10 4 5 2 0 -15 10 35 TEMPERATURE (°C) 6 VCC = 3.0V 0 -40 60 85 10 35 TEMPERATURE (°C) TEMPERATURE (°C) 30 GAIN (IMOD/IMODMON) 30 85 60 MODULATION-CURRENT MONITOR GAIN vs. TEMPERATURE 25 IBIAS = 10mA, IMOD = 10mA 35 15 -40 RAPCSET (Ω) 35 40 20 10 100 IBIAS = 100mA, IMOD = 50mA 45 GAIN (IBIAS /IBIASMON) 1.4 50 MAX3850 toc11 70 MAX3850 toc10 1.6 MAX3850 toc12 BIAS-CURRENT MONITOR GAIN vs. TEMPERATURE SUPPLY CURRENT vs. TEMPERATURE (EXCLUDES IBIAS, IMOD, 15Ω LOAD) IMD vs. RAPCSET IMD (mA) MAX3850 2.7Gbps, +3.3V DC-Coupled Laser Driver 20 30 40 50 60 IMOD (mA) _______________________________________________________________________________________ 60 85 2.7Gbps, +3.3V DC-Coupled Laser Driver PIN NAME 1, 4, 7 VCC1 2 DATA+ Positive Data Input 3 DATA- Negative Data Input 5 CLK+ Positive Clock Input. Connect to VCC or leave unconnected if latch function is not used. 6 CLK- 8 LATCH 9 ENABLE 10 GND1 FUNCTION Power Supply for Digital Circuits Negative Clock Input. Connect to VCC or leave unconnected if latch function is not used. TTL/CMOS Latch Input. High for latched data, low for direct data. Internal 100kΩ pullup resistor to VCC. TTL/CMOS Enable Input. High for normal operation, low to disable laser bias and modulation current. Internal 100kΩ pullup resistor to VCC. Ground for Digital Curcuits 11 BIASMON Bias Current Monitor. Current into this pin is proportional to the laser bias current. 12 MODMON Modulation Current Monitor. Current into this pin is proportional to the laser modulation current. 13 FAIL 14 APCFILT 15 GND4 Ground for Output Curcuitry 16, 18, 21 VCC4 Power Supply for Output Circuitry 17 BIAS Laser Bias Current Output 19 OUT+ Positive Modulation Current Output. IMOD flows into this pad when input data is high. 20 OUT- Negative Modulation Current Output. IMOD flows into this pad when input data is low. 22 GND4 Ground for Modulation Current Source 23 GND3 Ground for APC Circuitry 24 MD 25 VCC3 Power Supply for APC 26 CAPC A capacitor connected from this pad to ground controls the dominant pole for the APC feedback loop (CAPC = 0.1µF). 27 GND2 28 N.C. 29 APCSET A resistor connected from this pad to ground sets the desired average optical power. Connect a 100kΩ resistor from this pad to ground if APC is not used. 30 MODSET A resistor connected from this pad to ground sets the desired modulation current. 31 BIASMAX A resistor connected from this pad to ground sets the maximum bias current. The APC function can subtract from this maximum value but cannot add to it. 32 VCC2 TTL/CMOS Failure Output. Indicates APC failure when low. Connect a capacitor (CAPCFILT = 0.1µF) from this pad to ground to filter the APC noise. Monitor Diode Input. Connect this pin to a monitor photodiode anode. A capacitor to ground is required to filter high-speed AC monitor photocurrent. Ground for Internal Reference No Connection. Leave unconnected. Power Supply for Internal Reference _______________________________________________________________________________________ 7 MAX3850 Pin Description MAX3850 2.7Gbps, +3.3V DC-Coupled Laser Driver LATCH VCC MAX3850 RD OUT+ 0 IMOD MUX D DATA VCC 1 Q RCOMP OUTCCOMP CLK ENABLE IBIAS BIASMON IBIAS 41 162X BIAS 40x 5x MD IMD MODMON 1000pF IMOD 30 FAILURE DETECTION MODSET CAPC BIASMAX RMODSET GND4 RBIASMAX FAIL APCSET RAPCSET CAPC Figure 4. Functional Diagram Detailed Description The MAX3850 laser driver has two main parts: a highspeed modulation driver and a laser-biasing block with automatic power control (APC). The circuit design is optimized for high-speed, low-voltage (3.3V), directcoupled operation. To reject pattern-dependent jitter of the input signal at speeds as high as 2.7Gbps, the device accepts a differential clock signal for data retiming. When LATCH is high, the input data is synchronized by the clock signal. When LATCH is low, the input data is directly applied to the output stage. The output stage has a high-speed differential pair and a programmable modulation current source. The MAX3850 modulation output is optimized for driving a 15Ω load; the minimum required voltage at OUT+ is 0.6V. Modulation current swings up to 80mA are possible when the laser diode is AC-coupled to the driver (refer to Maxim Application Note HFAN 2.0). To 8 interface with the laser diode, a damping resistor (RD) is required for impedance matching. The combined resistance due to the series damping resistor and the equivalent series resistance of the laser diode should equal 15Ω. To reduce optical output aberrations and duty-cycle distortion caused by laser diode parasitic inductance, an RC shunt network might be necessary. At data rates of 2.7Gbps, any capacitive load at the cathode of a laser diode degrades optical output performance. Because the BIAS output is directly connected to the laser cathode, minimize the parasitic capacitance associated with the pin by using an inductor to isolate the BIAS pin from the laser cathode. Automatic Power Control To maintain constant average optical power, the MAX3850 incorporates an APC loop to compensate for the changes in laser threshold current over temperature and lifetime. A back-facet photodiode mounted in the _______________________________________________________________________________________ 2.7Gbps, +3.3V DC-Coupled Laser Driver Open-Loop Operation If necessary, the MAX3850 is fully operational without APC. To operate the MAX3850 open loop, connect a 100kΩ resistor from RAPCSET to GND and leave MD unconnected. In this case, the laser current is directly set by two external resistors connected from ground to BIASMAX and MODSET. Optional Data Input Latch If LATCH is high, the input data is retimed by the rising edge of CLK+. If LATCH is low, the input data is directly connected to the output stage. When not using the LATCH function, connect CLK+ and CLK- to VCC or leave unconnected. Enable Control The MAX3850 incorporates a laser-driver enable function. When ENABLE is low, the bias and modulation currents are off. For open-loop operation, the typical enable time is 370ns, and the typical disable time is 20ns. For closed-loop operation, the bias current is controlled by the APC loop, and the enable time will be affected by the APC loop time constant. With CAPC = 0.1µF, typical closed-loop enable time is 10ms, and typical closed-loop disable time is 40ns. For more information, see the Bias Current Enable Time Typical Operating Characteristics. laser modulation current, I MOD /30. BIASMON and MODMON should be connected through a pullup resistor to VCC. Choose a pullup resistor value that ensures a voltage at BIASMON greater than VCC - 1.5V and a voltage at MODMON greater than VCC - 2.0V. These pins should be connected VCC if not used. Slow-Start For laser safety reasons, the MAX3850 incorporates a slow-start circuit that provides a delay of 370ns for enabling a laser diode. APC Failure Monitor The MAX3850 provides an APC failure monitor (TTL/CMOS) to indicate an APC loop-tracking failure. FAIL is set low when the APC cannot adjust the bias current to maintain the desired monitor current. For example, the laser diode requires more bias current (to maintain a constant optical output) than maximum bias current set by RBIASMAX. The bias current is limited and FAIL will be asserted. In an alternate example, assume that a circuit failure causes the cathode of the laser diode to be shorted to GND, thereby causing an uncontrolled high optical output. In this case, the APC loop cannot decrease the user current, and FAIL will be asserted. Short-Circuit Protection The MAX3850 provides short-circuit protection for the modulation, bias, and monitor current sources. If BIASMAX, MODSET, or APCSET is shorted to ground, bias and modulation output will be turned off. Design Procedure When designing a laser transmitter, the optical output usually is expressed in terms of average power and extinction ratio. Table 1 shows the relationships helpful in converting between the optical average power and the modulation current. These relationships are valid if the mark density and duty cycle of the optical waveform are 50%. Programming the Modulation Current For a given laser power (PAVG), slope efficiency (η), and extinction ratio (re), the modulation current can be calculated using Table 1. See the IMOD vs. RMODSET graph in the Typical Operating Characteristics, and select the value of RMODSET that corresponds to the required current at +25°C. Current Monitors Programming the Bias Current The MAX3850 features bias and modulation-current monitor outputs. The BIASMON output sinks a current equal to 1/41 of the laser bias current, IBIAS/41. The MODMON output sinks a current equal to 1/30 of the When the MAX3850 is used in open-loop operation, the RBIASMAX resistor determines the bias current. To select this resistor, determine the required bias current at +25°C. See the IBIASMAX vs. RBIASMAX graph in the Typical _______________________________________________________________________________________ 9 MAX3850 laser package converts the optical power into a photocurrent. The APC loop adjusts the laser bias current so that the monitor current is matched to a reference current set by RAPCSET. The time constant of the APC loop is determined by an external capacitor (CAPC). To eliminate the pattern-dependent jitter associated with the APC loop time constant, and to guarantee loop stability, the recommended value for CAPC is 0.1µF. When the APC loop is functioning, an external resistor (RBIASMAX) sets the maximum allowable bias current. An APC failure flag (FAIL) is set low when the bias current cannot be adjusted to achieve the desired average optical power. To filter APC loop noise, use an external capacitor at APCFILT with a recommended value of 0.1µF. APC closed-loop operation requires that the user set three currents with external resistors connected between ground and BIASMAX, MODSET, and APCSET. Detailed guidelines for these resistor settings are described in the Design Procedure section. MAX3850 2.7Gbps, +3.3V DC-Coupled Laser Driver Operating Characteristics, and select the value of RBIASMAX that corresponds to the required current at +25°C. When using the MAX3850 in closed-loop operation, the RBIASMAX resistor sets the maximum bias current available to the laser diode over temperature and lifetime. The APC loop can subtract from this maximum value but cannot add to it. See the IBIASMAX vs. RBIASMAX graph in the Typical Operating Characteristics and select the value of RBIASMAX that corresponds to the end-of-life bias current at +85°C. Programming the APC Loop When using the MAX3850’s APC feature, program the average optical power by adjusting the APCSET resistor. To select this resistor, determine the desired monitor current to be maintained over temperature and lifetime. See the IMD vs. RAPCSET graph in the Typical Operating Characteristics and select the value of RAPCSET that corresponds to the required current. Interfacing with Laser Diodes To minimize optical output aberrations caused by signal reflections at the electrical interface to the laser diode, a series damping resistor (R D ) is required (Figure 4). Additionally, the MAX3850 outputs are optimized for a 15Ω load. Therefore, the series combination of RD and RL (where RL represents the laser-diode resistance) should equal 15Ω. Typical values for RD are 8Ω to 13Ω. For best performance, place a bypass capacitor (0.01µF typ) as close as possible to the anode of the laser diode. An RC shunt network between the laser cathode and ground minimizes optical output aberrations. Starting values for most coaxial lasers are RCOMP = 50Ω in series with CCOMP = 8.0pF. Adjust these values experimentally until the optical output waveform is optimized. (Refer to Maxim Application Note HFAN 3.0, Interfacing Maxim’s Laser Drivers with Laser Diodes.) Pattern-Dependent Jitter When transmitting NRZ data with long strings of consecutive identical digits (CIDs), low-frequency droop can occur and contribute to pattern-dependent jitter (PDJ). To minimize PDJ, carefully select the APC loop capacitor (CAPC), which dominates the APC loop time constant. To filter out noise effects and guarantee loop stability, the recommended value for CAPC is 0.1µF. Refer to Maxim Application Note HFAN11, Choosing AC-Coupling Capacitors, for more information. Input Termination Requirement The MAX3850 data and clock inputs are internally biased. Although the data and clock inputs are com10 Table 1. Optical Power Definition PARAMETER SYMBOL Average Power PAVG RELATION PAVG = (P0 + P1) / 2 Extinction Ratio re re = P1 / P0 Optical Power High P1 P1 = 2PAVG ✕ re / (re + 1) Optical Power Low P0 P0 = 2PAVG / (re + 1) Optical Amplitude PP-P Laser Slope Efficiency Modulation Current PP-P = P1 - P0 η η = PP-P / IMOD IMOD IMOD = PP-P / η patible with LVPECL signals, it is not necessary to drive the MAX3850 with a standard LVPECL signal. While DC-coupled, as long as the specified common-mode voltage and differential voltage swings are met, the MAX3850 will operate properly. Because of the on-chip biasing network, the MAX3850 data and clock inputs also will self-bias to the proper operating point to accommodate AC-coupling. Calculating Power Consumption The junction temperature of the MAX3850 dice must be kept below +150°C at all times. Approximate the total power dissipation of the MAX3850 using the following equation: P = VCC ✕ ICC + (VCC - Vf) ✕ (IBIAS + IMOD) where I BIAS is the maximum bias current set by RBIASMAX, IMOD is the modulation current, and Vf is the typical laser forward voltage. Junction Temperature = P(W) x 47(°C/W). Applications Information An example of how to set up the MAX3850: Select Laser Select a communication-grade laser for 2.488Gbps or higher data-rate applications. Assume the laser output average power is PAVG = 0dBm, the operating temperature is -40°C to +85°C, and the laser diode has the following characteristics: Wavelength: λ = 1.3µm, Threshold Current: ITH = 22mA at +25°C, Threshold Temperature Coefficient: β TH = 1.3%/°C, Laser-toMonitor Transfer: ρ MON = 0.2A/W, Laser Slope Efficiency: η = 0.05mW/mA at +25°C. Determine RAPCSET The desired monitor diode current is estimated by IMD = PAVG x ρMON = 200µA. The IMD vs. RAPCSET graph in the Typical Operating Characteristics shows RAPCSET at 6.2kΩ. ______________________________________________________________________________________ 2.7Gbps, +3.3V DC-Coupled Laser Driver PAD NAME 1 COORDINATES PAD NAME 1523 25 46 1334 VCC1 46 1213 4 DATA+ 46 5 DATA- 6 7 8 VCC1 X Y GND2 46 2 GND1 3 MAX3850 Table 2. Bondpad Locations COORDINATES X Y BIAS 1709 46 26 N.C. 1861 241 27 VCC4 1861 373 1091 28 N.C. 1861 494 46 970 29 OUT+ 1861 616 VCC1 46 848 30 OUT- 1861 737 GND1 46 727 31 N.C. 1861 859 46 605 32 VCC4 1861 980 1102 9 CLK+ 46 484 33 GND4 1861 10 CLK- 46 362 34 GND3 1861 1223 11 VCC1 46 241 35 MD 1861 1356 *12 GND1 46 46 36 GND3 1709 1523 13 LATCH 205 46 37 VCC3 1577 1523 14 ENABLE 351 46 38 CAPC 1456 1523 15 GND1 484 46 39 N.C. 1334 1523 16 GND1 605 46 40 GND3 1213 1523 17 BIASMON 727 46 41 N.C. 1091 1523 18 MODMON 848 46 42 GND3 970 1523 19 FAIL 970 46 43 N.C. 848 1523 20 GND4 1091 46 44 APCSET 727 1523 21 N.C. 1213 46 45 GND2 605 1523 22 APCFILT 1334 46 46 MODSET 484 1523 23 GND4 1456 46 47 BIASMAX 351 1523 24 VCC4 1577 46 48 VCC2 205 1523 *Index pad. Orient the die with this pad in the lower-left corner. Determine RMODSET Assuming re = 20, and average power of 0dBm (1mW), then according to Table 1, the peak-to-peak optical power PP-P = 1.81mW. The required modulation current is 1.81(mW) / 0.05(mW/mA) = 36.2mA. The IMOD vs. RMODSET graph in the Typical Operating Characteristics shows RMODSET at 5.5kΩ. Determine RBIASMAX Determine the maximum threshold current (ITH(MAX)) at TA = +85°C and end of life. Assuming (ITH(MAX)) = 50mA, the maximum bias current should be: IBIASMAX = ITH(MAX) In this example, IBIASMAX = 50mA. The IBIASMAX vs. R BIASMAX graph in the Typical Operating Characteristics shows RBIASMAX at 5kΩ. Modulation Currents Exceeding 60mA For applications requiring modulation current greater than 60mA, headroom is insufficient for proper operation of the laser driver if the laser is DC-coupled. To avoid this problem, the MAX3850’s modulation output can be AC-coupled to the cathode of a laser diode. An external pullup inductor is necessary to DC-bias the modulation output at VCC. Such a configuration isolates laser forward voltage from the output circuitry and allows the output at OUT+ to swing above and below the supply voltage (VCC). Refer to Maxim Application Note HFAN 2.0 Interfacing Maxim’s Laser Drivers to Laser Diodes for more information on AC-coupling laser drivers to laser diodes. ______________________________________________________________________________________ 11 Wirebonding Die VCC For high-current density and reliable operation, the MAX3850 uses gold metalization. Make connections to the die with gold wire only, using ball-bonding techniques. Wedge bonding is not recommended. Die-pad size is 4mils (100µm) square, and die thickness is 12mils (300µm) square. PACKAGE 0.9nH OUT+ 0.1pF 0.9nH Layout Considerations OUT- To minimize inductance, keep the connections between the MAX3850 output pins and laser diode as close as possible. Optimize the laser diode performance by placing a bypass capacitor as close as possible to the laser anode. Use good high-frequency layout techniques and multilayer boards with uninterrupted ground planes to minimize EMI and crosstalk. 0.1pF Laser Safety and IEC825 VCC PACKAGE 5kΩ VCC 0.9nH IN+ 0.1pF 5kΩ Figure 6. Simplified Output Circuit BIASMAX MODSET APCSET N.C. GND2 CAPC VCC3 Pin Configuration VCC2 Using the MAX3850 laser driver alone does not ensure that a transmitter design is compliant with IEC825. The entire transmitter circuit and component selections must be considered. Each user must determine the level of fault tolerance required by the application, recognizing that Maxim products are neither designed nor authorized for use as components in systems intended for surgical implant into the body, for applications intended to support or sustain life, or for any other application in which the failure of a Maxim product could create a situation where personal injury or death may occur. 32 31 30 29 28 27 26 25 TOP VIEW VCC1 1 24 MD DATA+ 2 23 GND3 DATA- 3 22 GND4 VCC1 4 CLK+ 5 20 OUT- CLK- 6 19 OUT+ VCC1 7 18 VCC4 LATCH 8 17 BIAS 21 VCC4 MAX3850 11 12 13 14 15 16 FAIL APCFILT GND4 VCC4 24kΩ 10 MODMON 0.1pF 9 BIASMON 5kΩ 0.9nH IN- GND1 VCC ENABLE MAX3850 2.7Gbps, +3.3V DC-Coupled Laser Driver 5mm ✕ 5mm QFN THE EXPOSED PAD MUST BE SOLDERED TO GND ON THE CIRCUIT BOARD Figure 5. Simplified Input Circuit 12 ______________________________________________________________________________________ 2.7Gbps, +3.3V DC-Coupled Laser Driver 3.3V 0.01µF 16Ω 0.1µF DATA+ 100Ω OUT- 0.1µF MAX3890 11Ω OUT+ DATA- 0.1µF SERIALIZER WITH CLOCK GEN. LD FAIL LATCH ENABLE 3.3V CLOCK+ 50Ω MAX3850 8.0pF 100Ω 0.1µF BIASMON MODMON CAPC APCFILT BIAS MD APCSET TYPICAL APPLICATION CIRCUIT WITH AC-COUPLED INPUTS MODSET BIASMAX CLOCK- 0.1µF 0.1µF 392Ω 1000pF 392Ω 3.3V Chip Information Chip Topography TRANSISTOR COUNT: 1749 VCC1 GND1 CLK+ GND1 DATA- VCC1 CLK- VCC1 GND2 SUBSTRATE CONNECTED TO GND VCC1 DATA+ GND1 DIE SIZE: 70mils ✕ 83mils LATCH VCC2 ENABLE BIASMAX GND1 MODSET GND1 BIASMON GND2 APCSET MODMON N.C. DIE THICKNESS: 12mils PROCESS: SIGe Bipolar 0.083" GND3 (2.108mm) N.C. FAIL GND4 GND3 N.C. N.C. APCFILT GND4 VCC4 CAPC VCC3 BIAS GND3 N.C. N.C. OUT- VCC4 GND3 VCC4 OUT+ N.C. GND4 MD 0.070" (1.778mm) ______________________________________________________________________________________ 13 MAX3850 Typical Application Circuits (continued) 2.7Gbps, +3.3V DC-Coupled Laser Driver MAX3850 Package Information 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. 14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.