19-4799; Rev 4; 2/05 +3.3V, 622Mbps SDH/SONET Laser Driver with Automatic Power Control The MAX3668 is a complete, +3.3V laser driver with automatic power control (APC) circuitry for SDH/SONET applications up to 622Mbps. It accepts differential PECL inputs, provides bias and modulation currents, and operates over a temperature range of -40°C to +85°C. An APC feedback loop is incorporated to maintain a constant average optical power over temperature and lifetime. The wide modulation current range of 5mA to 75mA and bias current of 1mA to 80mA are easy to program, making this product ideal for use in various SDH/SONET applications. The MAX3668 also provides enable control and a failuremonitor output to indicate when the APC loop is unable to maintain the average optical power. The MAX3668 is available in a 5mm 32-pin TQFP package as well as in dice. Features ♦ Single +3.3V or +5.0V Operation ♦ 38mA Supply Current at +3.3V ♦ Programmable Modulation Current from 5mA to 75mA ♦ Programmable Bias Current from 1mA to 80mA ♦ Rise/Fall Time <200ps ♦ Automatic Average Power Control with Failure Monitor ♦ Complies with ANSI, ITU, and Bellcore SONET/SDH Specifications ♦ Enable Control Ordering Information PART Applications 622Mbps SDH/SONET Access Nodes Laser Driver Transmitters TEMP RANGE PIN-PACKAGE MAX3668EHJ -40°C to +85°C 32 TQFP (5mm x 5mm) MAX3668EHJ+ MAX3668E/D -40°C to +85°C Note 32 TQFP (5mm x 5mm) Dice* Note: Dice are designed to operate over a -40°C to +140°C junction temperature (Tj) range, but are tested and guaranteed at TA = +25°C. *Contact factory for availability. +Denotes lead-free package. Pin Configuration appears at end of data sheet. Section Regenerators Typical Operating Circuit +3.3V +3.3V 130Ω 130Ω FAIL ENABLE LASER VCC R6.3Ω DATA+ MAX3693 RD 5Ω OUT- PECL OUT+ DATA- CD 1.0µF MAX3668 BIAS GND CAPC APCSET 82Ω MODSET 82Ω BIASMAX 4:1 SERIALIZER WITH CLOCK GEN R+ 20Ω RFILT 20Ω CFILT 5pF FERRITE BEAD MD CMD 1000pF 0.1µF †C0 ________________________________________________________________ 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 MAX3668 † General Description MAX3668 +3.3V, 622Mbps SDH/SONET Laser Driver with Automatic Power Control ABSOLUTE MAXIMUM RATINGS Supply Voltage, VCC .............................................-0.5V to +7.0V Current into BIAS ............................................-20mA to +150mA Current into OUT+, OUT- ............................... -20mA to +100mA Current into MD....................................................-5mA to +5mA Voltage at DATA+, DATA-, ENABLE, FAIL .......................................................-0.5V to (VCC + 0.5V) Voltage at OUT+, OUT- .............................+1.5V to (VCC + 1.5V) Voltage at MODSET, APCSET, BIASMAX, CAPC............................................................... -0.5V to +3.0V Voltage at BIAS .........................................+1.0V to (VCC + 0.5V) Continuous Power Dissipation (TA = +85°C) 32-Pin TQFP (derate 14.3mW/°C above +85°C)........ 929mW Operating Junction Temperature Range ...........-55°C to +150°C Processing Temperature (Die).........................................+400°C Storage Temperature Range ........................... -65°C to +165°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 = +3.14V to +5.5V, TA = -40°C to +85°C, unless otherwise noted. Typical values are at VCC = +3.3V, TA = +25°C.) (Note 1) PARAMETER SYMBOL Supply Current Bias Current Range MIN (Note 2) IBIAS Bias Off Current 38 VBIAS = VCC - 1.6V APC open loop Bias Current Absolute Accuracy VID Common-Mode Input Voltage VICM DATA+, DATA- Input Current IIN Monitor Diode Current Stability 1 IBIAS = 80mA 390 IBIAS = 1mA 920 MAX UNITS 60 mA 80 mA 100 µA ppm/°C APC open loop -15 15 % Figure 1 200 1600 mVp-p VCC 1.32 VCC VID/4 V 10 µA 10 480 VCC 1.49 PECL compatible -1 (Note 4) Monitor Diode Current Absolute Accuracy IMD = 1mA -480 IMD = 18µA (Note 5) 70 ppm/°C -15 15 % 18 1000 µA DC Monitor Diode Current IMD Monitor Diode Input Voltage (MD Pin) VMD TTL Input High Voltage VIH TTL Input Low Voltage VIL TTL Output High Voltage (FAIL) VOH Sourcing 50µA 2.4 TTL Output Low Voltage (FAIL) VOL Sinking 100µA 0.1 2 TYP ENABLE = low (Note 3) Bias Current Stability Differential Input Voltage CONDITIONS 0.8 V 2.0 V VCC - 0.3 _______________________________________________________________________________________ 0.8 V VCC V 0.44 V +3.3V, 622Mbps SDH/SONET Laser Driver with Automatic Power Control (VCC = +3.14V to +5.5V, load as shown in Figure 2, TA = -40°C to +85°C, unless otherwise noted. Typical values are at VCC = +3.3V, TA = +25°C.) (Note 6) PARAMETER Modulation Current Range SYMBOL IMOD Modulation Off Current CONDITIONS MIN (Note 7) ENABLE = low (Note 3) IMOD = 75mA Modulation Current Stability -620 tR/tF 20% to 80%, RL = 10Ω | | 20Ω load (Note 8) Pulse-Width Distortion (peak-to-peak) (Notes 9, 10) Enable/Start-Up Delay Open loop MAX UNITS 75 mA 200 µA 620 300 -15 Jitter Generation (peak-to-peak) Maximum Consecutive Identical Digits at 622Mbps 175 IMOD = 5mA (Note 5) Modulation Current Absolute Accuracy Output Rise/Fall Time TYP 5 15 IMOD = 5mA 100 200 IMOD = 75mA 230 375 100 IMOD = 5mA 70 155 IMOD = 75mA 10 135 250 CID ppm/°C % ps ps ps ns 80 Bits Note 1: Characteristics at -40°C are guaranteed by design and characterization. Dice are tested at TA = +25°C only. Note 2: Tested with RMODSET = 5.11kΩ (IMOD ≈ 38mA), RBIASMAX = 4.56kΩ (IBIAS ≈ 52mA), excluding IBIAS and IMOD. Note 3: Both the bias and modulation currents will be disabled if any of the current set pins are shorted to ground. Note 4: This assumes that the laser to monitor diode transfer function does not change with temperature. Note 5: See Typical Operating Characteristics for worst-case distributions. Note 6: AC characteristics are guaranteed by design and characterization. Note 7: Total IMOD out of OUT+. Refer to the Design Procedure for information regarding current delivered to the laser. Note 8: Input signal is a 622Mbps, 213- 1 PRBS with 80 inserted zeros. Note 9: Input signal is a 622Mbps, 11110000 pattern. Note 10:PWD = (wider pulse – narrower pulse) / 2. VCC DATA+ 100mV MIN DATA- 800mV MAX (DATA+) (DATA-) 200mVp-p MIN 20Ω 1.0µF OUT10Ω 1600mVp-p MAX 20Ω MAX3668 1.0µF OSCILLOSCOPE OUT+ IOUT+ IMOD BIAS 12.4Ω 50Ω 15Ω VCC Figure 1. Required Input Signal and Output Polarity Figure 2. Output Termination for Characterization _______________________________________________________________________________________ 3 MAX3668 AC ELECTRICAL CHARACTERISTICS Typical Operating Characteristics (VCC = +3.3V, TA = +25°C, unless otherwise noted.) EYE DIAGRAM (622Mbps, 1300nm LASER WITH 467MHz FILTER) 622Mbps 622Mbps PATTERN = 213 - 1 + 80 CID IMOD = 75mA PATTERN = 213 - 1 + 80 CID IMOD = 35mA 193ps/div 200ps/div MONITOR DIODE CURRENT vs. APC SET RESISTOR BIAS CURRENT vs. MAXIMUM BIAS SET RESISTOR 200ps/div MODULATION CURRENT vs. MODULATION SET RESISTOR IMOD (mA) MAX3688 TOC06 100 IMD (mA) IBIAS (mA) 1 100 MAX3668 TOC05 1000 MAX3668 TOC04 10 0.1 10 10 0.01 1 1 1 10 100 0.1 1 10 0.1 100 100 10 RBIASMAX (kΩ) RMODSET (kΩ) RANDOM JITTER vs. MODULATON CURRENT PULSE-WIDTH DISTORTION vs. MODULATION CURRENT SUPPLY CURRENT vs. TEMPERATURE 35 PWD (ps) 19 18 30 25 20 15 VCC = +5.0V 45 VCC = +3.3V 40 35 10 17 50 SUPPLY CURRENT (mA) 40 MAX3668 TOC09 45 IBIAS = 48mA IMOD = 33mA 5 30 0 16 20 40 IMOD (mA) 60 80 1000 55 MAX3668 TOC08 20 50 MAX3668 TOC07 INCLUDES RANDOM JITTER DUE TO MEASUREMENT EQUIPMENT 0 1 RAPCSET (kΩ) 21 4 MAX3668 TOC03 EYE DIAGRAM (IMOD = 75mA) MAX3668 TOC02 MAX3668 TOC01 EYE DIAGRAM (IMOD = 35mA) 223 - 1 PRBS RANDOM JITTER (ps p-p) MAX3668 +3.3V, 622Mbps SDH/SONET Laser Driver with Automatic Power Control 0 20 40 IMOD (mA) 60 80 -40 -15 10 35 TEMPERATURE (°C) _______________________________________________________________________________________ 60 85 +3.3V, 622Mbps SDH/SONET Laser Driver with Automatic Power Control DISTRIBUTION OF MODULATION CURRENT STABILITY (WORST CASE) VCC = +5V 20 20 15 10 5 VCC = +3.3V 1.4 1.6 1.8 2.0 2.2 2.4 2.6 10 0 -25 65 155 245 335 425 515 MODULATION DIODE CURRENT STABILITY (ppm/°C) RBIAS (kΩ) 15 5 0 0.1 TA = -40°C TO +85°C IMD = 18µA PERCENT OF UNITS (%) CLOSED-LOOP OPERATION 1 TA = -40°C TO +85°C IMOD = 5mA PERCENT OF UNITS (%) BIAS CURRENT (mA) 100 25 MAX3668-11 VCC = +3.3V, + 5V 10 25 MAX3668 TOC10 1000 DISTRIBUTION OF MONITOR DIODE CURRENT STABILITY (WORST CASE) MAX3668-12 AVAILABLE BIAS CURRENT vs. MAXIMUM BIAS SET RESISTOR -88 -52 20 92 160 MONITOR DIODE CURRENT STABILITY (ppm/°C) Pin Description PIN NAME 1, 2, 6, 15, 17, 20, 24 FUNCTION VCC 3 DATA+ Positive PECL Data Input 4 DATA- Negative PECL Data Input 5, 7, 8, 10, 14, 21, 22, 30 GND 9 ENABLE 11 FAIL TTL Output. Indicates APC failure when low. Internally pulled high through a 6kΩ resistor. 12, 13, 26, 27, 28 N.C. No Connection. Leave unconnected. 16 BIAS Laser Bias Current Output. Isolate from laser with a ferrite bead. 18 OUT+ Positive Modulation Current Output. IMOD flows into this pad when the input signal is high. Connect this pad to AC coupling network. 19 OUT- Negative Modulation Current Output. IMOD flows into this pad when the input signal is low. Connect this pad to VCC through a 6.3Ω resistor. 23 MD Monitor Photodiode Connection. Connect this pad to the monitor photodiode anode. A capacitor to ground is required to filter high-speed AC monitor photocurrent. 25 CAPC APC Compensation Capacitor. A 0.1µF capacitor connected from this pad to ground controls the dominant pole of the automatic power control (APC) feedback loop. 29 APCSET 31 MODSET 32 BIASMAX Positive Supply Voltage Ground TTL/CMOS Enable Input. High for normal operation, low to disable laser bias and modulation currents. Internally pulled high. APC Set Resistor. A resistor connected from this pad to ground sets the desired average optical power. The resulting current is equal to the desired DC monitor diode current. Connect a 100kΩ resistor from this pad to ground if APC is not used. Modulation Set Resistor. A resistor from this pad to ground sets the laser modulation current. Maximum Bias Set Resistor. A resistor from this pad to ground sets the maximum laser bias current. The APC function can subtract from this maximum value but cannot add to it. This resistor controls the bias-current level when the APC loop is not used. _______________________________________________________________________________________ 5 MAX3668 Typical Operating Characteristics (continued) (VCC = +3.3V, TA = +25°C, unless otherwise noted.) MAX3668 +3.3V, 622Mbps SDH/SONET Laser Driver with Automatic Power Control DATA+ OUT+ OUT- DATAVCC 100k ENABLE IBIAS 165X MAX3668 BIAS 40X 5X MD IMD FAILURE DETECTOR MODSET CAPC BIASMAX APCSET FAIL RMODSET RBIASMAX CAPC RAPCSET Figure 3. Functional Diagram Detailed Description The MAX3668 laser driver consists of two main parts: a high-speed modulation driver and a laser-biasing block with Automatic Power Control (APC). The circuit is optimized for low-voltage (+3.3V) operation. The output stage is composed of a high-speed differential pair and a programmable modulation current source. Since the modulation output drives a maximum current of 75mA into the laser with a 230ps edge speed, large transient voltage spikes can be generated due to the parasitic inductance. These transients and the laser forward voltage leave insufficient headroom for the proper operation of the laser driver if the modulation output is DC-coupled to the laser diode. To solve this problem, the MAX3668’s modulation output is designed to be 6 AC-coupled to the cathode of a laser diode. A simplified functional diagram is shown in Figure 3. The MAX3668 modulation output is optimized for driving a 20Ω 10Ω load; the minimum required voltage at OUT+ is 2.0V. Modulation current swings of 75mA are possible. To interface with the laser diode, a damping resistor (RD) is required for impedance matching. An RC shunt network may be used to compensate for the laser-diode parasitic inductance, thereby improving the optical output aberrations and duty-cycle distortion. At a 622Mbps data rate, any capacitive load at the cathode of a laser diode degrades the optical output performance. Since the BIAS output is directly connected to the laser cathode, minimize the parasitic capacitance associated with this pin by using an inductor to isolate the BIAS pin from the laser cathode. _______________________________________________________________________________________ +3.3V, 622Mbps SDH/SONET Laser Driver with Automatic Power Control Short-Circuit Protection The MAX3668 provides short-circuit protection for the modulation, bias, and monitor current sources. If either BIASMAX, MODSET, or APCSET is shorted to ground, the bias and modulation outputs will be turned off. APC closed-loop operation requires the user to 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. Open-Loop Operation If necessary, the MAX3668 is fully operational without APC. In this case, the laser current is directly set by two external resistors connected from ground to BIASMAX and MODSET. Connect a 100kΩ resistor from APCSET to ground and leave MD open for open-loop operation. Enable Control The MAX3668 incorporates a laser driver enable function. When ENABLE is low, both the bias and modulation currents are off. The typical laser enable time is 250ns. Design Procedure When designing a laser transmitter, the optical output is usually expressed in terms of average power and extinction ratio. Table 1 gives the relationships that are helpful in converting between the optical average power and the modulation current. These relationships are valid if the average duty cycle of the optical waveform is 50%. Programming the Modulation Current In addition to being a function of RMODSET, the modulation current delivered to the laser (IMODL) also depends on the values of the series damping resistor (RD), the shunt compensation resistance (RFILT), and the laser diode’s resistance (see Typical Operating Circuit). The modulation current (assuming CFILT<<CD) into the laser diode can be represented by the following: 20Ω IMODL = IMOD 20Ω + (RD + r LASER ) Assuming RD = 5Ω and rLASER = 5Ω, this equation is simplified to: IMODL = IMOD(0.67) For RD = 5.0Ω and a laser resistance of approximately 5Ω, refer to the Modulation Current vs. Modulation Set Resistor graph in the Typical Operating Characteristics and select the value of RMODSET that corresponds to the required current at +25°C. APC Failure Monitor Programming the Bias Current The MAX3668 provides an APC failure monitor (TTL/CMOS) to indicate an APC loop tracking failure. FAIL is set low when the APC loop can no longer adjust the bias current to maintain the desired monitor current. This output is internally pulled up to VCC through a 6kΩ resistor. When using the MAX3668 in open-loop operation, the bias current is determined by the RBIASMAX resistor. To select this resistor, determine the required bias current at +25°C. Refer to the Bias Current vs. Maximum Bias Set Resistor graph in the Typical Operating Characteristics and select the value of R BIASMAX that corresponds to the required current at +25°C. When using the MAX3668 in closed-loop operation, the RBIASMAX resistor sets the maximum bias current available to the laser diode over temperature and life. The APC loop can subtract from this maximum value but cannot add to it. Refer to the Bias Current vs. Maximum Bias Set Resistor graph in the Typical Operating Characteristics and select the value of RBIASMAX that corresponds to the end-of-life bias current at +85°C. Table 1. Optical Power Definition PARAMETER SYMBOL Average Power PAVE RELATION PAVE = (P0 + P1) / 2 Extinction Ratio re re = P1 / P0 Optical Power High P1 P1 = 2PAVE · re / (re + 1) Optical Power Low P0 P0 = 2PAVE / (re + 1) Optical Amplitude Pp-p Pp-p = 2PAVE (re - 1) / (re + 1) Laser Slope Efficiency η η = Pp-p / IMOD Modulation Current IMOD IMOD = Pp-p / η _______________________________________________________________________________________ 7 MAX3668 Automatic Power Control To maintain constant average optical power, the MAX3668 incorporates an APC loop to compensate for the changes in laser threshold current over temperature and lifetime. A back-facet photodiode mounted in the laser package is used to convert the optical power into a photocurrent. The APC loop adjusts the laser bias current so 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, the maximum allowable bias current is set by an external resistor, RBIASMAX. An APC failure flag (FAIL) is set low when the bias current can no longer be adjusted to achieve the desired average optical power. MAX3668 +3.3V, 622Mbps SDH/SONET Laser Driver with Automatic Power Control Programming the APC Loop When the MAX3668’s APC feature is used, 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 life. Refer to the Monitor Diode Current vs. APC Set Resistor graph in the Typical Operating Characteristics and select the value of RAPCSET that corresponds to the required current. Interfacing with the Laser Diode To minimize optical output aberrations due to the laser parasitic inductance, an RC shunt network may be used (see Typical Operating Circuit). If RL represents the laser diode resistance, the recommended total resistance for RD + RL is 10Ω. Starting values for coaxial lasers are RFILT = 20Ω and CFILT = 5pF. RFILT and CFILT should be experimentally adjusted to optimize the output waveform. A bypass capacitor should also be placed as close to the laser anode as possible for best performance. Pattern-Dependent Jitter (PDJ) When transmitting NRZ data with long strings of consecutive identical digits (CID), LF droop can contribute to pattern-dependent jitter. To minimize this pattern-dependent jitter, two external components must be properly chosen: capacitor CAPC, which dominates the APC loop time constant; and AC-coupling capacitor CD. To filter out noise effects and guarantee loop stability, the recommended value for CAPC is 0.1µF. This results in an APC loop bandwidth of 20kHz. Consequently, the pattern-dependent jitter associated with an APC loop time constant can be ignored. The time constant associated with the DC blocking capacitor on I MOD will have an effect on PDJ. It is important that this time constant produce minimum droop for long consecutive bit streams. Referring to Figure 4, the droop resulting from long time periods without transitions can be represented by the following equation: If t1 equals 80 consecutive unit intervals without a transition, the time constant associated with the DC blocking capacitor needs to be longer than: τAC ≥ RACCD = 7.8 (80 bits) (1.6ns/bit) = 1.0µs RFILT can be ignored for CFILT<< CD, therefore the estimated value of RAC is: RAC = 20Ω (RD + rLASER) Assuming RD = 5Ω, and rLASER = 5Ω: RAC = 6.7Ω with CD = 1.0µF, τAC = 6.7µs. Input Termination Requirement The MAX3668 data inputs are PECL-compatible. However, it is not necessary to drive the MAX3668 with a standard PECL signal. As long as the specified common-mode voltage and differential voltage swings are met, the MAX3668 will operate properly. Calculate Power Consumption The total power dissipation of the MAX3668 can be estimated by the following: P = VCC × ICC + (VCC - Vf) × IBIAS + IMOD (VCC - 20Ω × IMOD / 2) where IBIAS is the maximum bias current set by RBIASMAX, IMOD is the modulation current, and Vf is the typical laser forward voltage. Applications Information The following is an example of how to set up the MAX3668. Select Laser A communication-grade laser should be selected for 622Mbps applications. Assume the laser output average power is PAVE = 0dBm, the minimum extinction τ=∞ -t (100% - DROOP) = e τ AC coupling of IMOD results in a discharge level for τ that is equal to PAVG. An overall droop of 6% relative to P p-p equates to a 12% droop relative to P AVG . To ensure a droop of less than 12% (6% relative to Pp-p), this equation can be solved for τ as follows: τ= τAC τ << τAC Pp-p PAVG -t = 7.8t ln(1 - 0.12) t1 Figure 4. Droop 8 DROOP _______________________________________________________________________________________ t +3.3V, 622Mbps SDH/SONET Laser Driver with Automatic Power Control Wavelength: Threshold Current: Threshold Temperature Coefficient: Laser to Monitor Transfer: Laser Slope Efficiency: λ = 1.3µm ΙTH = 22mA at +25°C βTH = 1.3%/°C ρMON = 0.2A/W η = 0.05mW/mA at +25°C Determine RAPCSET The desired monitor diode current is estimated by IMD = PAVE × ρMON = 200µA. The Monitor Diode Current vs. APC Set Resistor graph in the Typical Operating Characteristics shows that RAPCSET should be 6kΩ. Determine RMODSET To achieve a minimum extinction ratio (re) of 6.6 over temperature and lifetime, calculate the required extinction ratio at +25°C. Assuming re = 20, the peak-to-peak optical power Pp-p = 1.81mW according to Table 1. The required modulation current is 1.81(mW) / 0.05(mW/mA) = 36.2mA. The Modulation Current vs. Modulation Set Resistor graph in the (see Typical Operating Characteristics) shows that RMODSET should be 5kΩ. Determine RBIASMAX Wire Bonding Die For high current density and reliable operation, the MAX3668 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 4 mils (100µm) square, and die thickness is 12 mils (300µm) mils. Layout Considerations To minimize inductance, keep the connections between the MAX3668 output pins and LD 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. Laser Safety and IEC 825 Using the MAX3668 laser driver alone does not ensure that a transmitter design is compliant with IEC 825. The entire transmitter circuit and component selections must be considered. Customers must determine the level of fault tolerance required by their 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. Calculate the maximum threshold current (ITH(MAX)) at T A = +85°C and end of life. Assuming I TH(MAX) = 50mA, the maximum bias current should be: VCC IBIAS = ITH(MAX) + IMOD / 2 In this example, IBIAS = 68.1mA. The Bias Current vs. Maximum Bias Set Resistor graph in the Typical Operating Characteristics shows that RBIASMAX should be 3kΩ. Modulation Current More than 50mA To drive modulation currents greater than 50mA at 3.3V, external pull-up inductors (Figure 5) should be used to DC-bias the modulation output at VCC. Such a configuration isolates the laser forward voltage from the output circuitry and allows the output at OUT+ to swing above and below the supply voltage VCC. At +5V power supply, the headroom voltage for the MAX3668 is significantly improved. In this case, it is possible to achieve a modulation current of more than 50mA (using resistor pull-ups as shown in the Typical Operating Circuit). The MAX3668 can also be DC-coupled to a laser diode when operating at +5V supply; the voltage at OUT+ should be ≥ 2.0V for proper operation. 10Ω OUT- FERRITE BEADS LD CD RD 1.0µF 5Ω OUT+ RFILT MAX3668 BIAS CFILT FERRITE BEAD MD 1000pF Figure 5. Output Termination for Maximum Modulation Current _______________________________________________________________________________________ 9 MAX3668 ratio is re = 6.6 (8.2dB), the operating temperature is -40°C to +85°C, and the laser diode has the following characteristics: +3.3V, 622Mbps SDH/SONET Laser Driver with Automatic Power Control GND APCSET N.C. N.C. N.C. CAPC 31 30 29 28 27 26 25 VCC 1 24 VCC VCC 2 23 MD DATA+ 3 22 GND DATA- 4 GND 5 21 GND MAX3668 10 11 12 13 14 15 16 BIAS 9 VCC 17 VCC GND 8 N.C. GND N.C. 18 OUT+ FAIL 19 OUT- 7 GND 6 ENABLE VCC GND TQFP 10 20 VCC GND VCC N.C. VCC GND VCC DATADATA+ VCC GND GND ENABLE GND GND N.C. VCC FAIL GND N.C. N.C. GND VCC BIAS VCC BIASMAX MODSET GND APCSET N.C. 0.083" GND (2.10mm) N.C. GND N.C. CAPC VCC GND VCC N.C. OUT+ OUTN.C. VCC GND GND MD MODSET 32 N.C. BIASMAX TOP VIEW VCC Chip Topography GND MAX3668 Pin Configuration 0.070" (1.78mm) ______________________________________________________________________________________ +3.3V, 622Mbps SDH/SONET Laser Driver with Automatic Power Control 32L,TQFP.EPS PACKAGE OUTLINE, 32L TQFP, 5x5x1.0mm, EP OPTION 21-0079 F 1 2 (Package information continues on next page.) ______________________________________________________________________________________ 11 MAX3668 Package Information MAX3668 +3.3V, 622Mbps SDH/SONET Laser Driver with Automatic Power Control Package Information (continued) PACKAGE OUTLINE, 32L TQFP, 5x5x1.0mm, EP OPTION 21-0079 F 2 2 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. 12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2005 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products, Inc.