19-3848; Rev 0; 11/05 Burst-Mode Laser Driver Features The MAX3643 burst-mode laser driver provides bias and modulation current drive for PON burst-mode ONT applications. It is specifically designed for use with a low-cost external controller for the APC (and if desired, AMC) loop. A high-speed differential burst-enable input enables the driver to switch the laser from a dark (output off) condition to full on-condition in less than 2ns. When BEN is inactive, typical modulation and bias currents are 5µA each. Laser modulation current can be set from 10mA to 85mA and bias current can be set from 1mA to 70mA using the MODSET and BIASSET inputs. A sample-and-hold circuit is provided to capture the monitor diode output during short PON bursts, if needed, and the BEN high-speed signal is mirrored on an LVCMOS output to be used by the controller operating the APC/AMC loop. The MAX3643 burst-mode laser driver is packaged in a 4mm x 4mm, 24-pin thin QFN package. It operates from -40°C to +85°C. ♦ 10mA to 85mA Modulation Current ♦ 1mA to 70mA Bias Current ♦ Monitor Diode Sample and Hold ♦ 45ps Output Transition Time ♦ 2ns Turn-On/-Off Time ♦ Reference Voltage Generator ♦ LVPECL High-Speed Inputs (Data, Burst Enable) Ordering Information TEMP RANGE PINPACKAGE MAX3643ETG -40°C to +85°C 24 TQFN-EP* T2444-3 MAX3643ETG+ -40°C to +85°C 24 TQFN-EP* T2444-3 PART PKG CODE *EP = Exposed paddle. +Denotes lead-free package. Applications Pin Configuration appears at end of data sheet. 155/622/1244Mbps A/B/GPON ONT Modules 1.25Gbps IEEE EPON ONT Modules Typical Application Circuit +3.3V 0.1µF 24Ω 27nH 15Ω +3.3V FILTER VCC OUT10Ω IN+ OUT+ IN- BIAS+ 10Ω MAX3643 15Ω BEN+ BIAS- BEN- 3kΩ VCC CCOMP MDIN EN IMAX RCOMP MDOUT BCMON 1kΩ VREF VMSET MODSET 1kΩ VBSET 1kΩ BIASSET BENOUT GND 1kΩ ANALOG OUT START CONVERT ANALOG OUT ANALOG IN ANALOG IN ANALOG IN CONTROL CONTROLLER CONTINUOUS OPERATION ________________________________________________________________ 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 MAX3643 General Description MAX3643 Burst-Mode Laser Driver ABSOLUTE MAXIMUM RATINGS Supply Voltage VCC ..............................................-0.5V to +4.0V Current into BIAS-, BIAS+, OUT-, OUT+ .......-20mA to +150mA Voltage at VMSET, VBSET, IN+, IN-, BEN+, BEN-, EN, MDIN, MDOUT, BENOUT, BIASMON ................-0.5V to (VCC + 0.5V) Voltage at MODSET, BIASSET, VREF, IMAX ........-0.5V to +3.0V Voltage at OUT-, OUT+, BIAS-, BIAS+ .....+0.3V to (VCC + 0.5V) Continuous Power Dissipation (TA = +85°C) 24-Pin TQFN, Multilayer Board (derate 27.8mW/°C above +85°C) ............................1807mW Operating Temperature Range ...........................-40°C to +85°C Storage Temperature Range ............................-55°C to +150°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. OPERATING CONDITIONS PARAMETER SYMBOL Supply Turn-On Time CONDITIONS 10% to 90% Ambient Temperature MIN TYP MAX UNITS 0.001 1000 ms -40 +85 °C 1250 Mbps Data Rate Voltage at VMSET, VBSET 0 1.4 V Voltage at BIASMON 0 1.4 V Voltage at MDIN 0 2.56 V ELECTRICAL CHARACTERISTICS (VCC = +3.0V to +3.6V, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, TA = +25°C, IBIAS = 20mA, IMOD = 30mA, unless otherwise noted.) (Note 1) PARAMETER SYMBOL CONDITIONS ICC Excluding laser bias and mode currents, max at IMOD = 85mA, IBIAS = 70mA LVPECL Differential Input Voltage VIN VIN = (VIN+) - (VIN-) LVPECL Common-Mode Input Voltage VCM Supply Current MIN TYP MAX UNITS 32 51 mA 1600 mVP-P VCC VIN / 4 V I/O SPECIFICATIONS LVCMOS Output High Voltage VCC 1.49 IOH = -100µA LVCMOS Output Low Voltage IOL = 100µA BENOUT Propagation Delay CL = 20pF, from BEN zero crossing to 67% CMOS level Td 200 LVCMOS Input Pullup Resistance VCC 1.32 VCC 0.2 V 0.2 30 ns 75 LVCMOS Input Current V kΩ VIN = 0V or VIN = VCC 50 µA LVCMOS Input High Voltage 2.0 VCC V LVCMOS Input Low Voltage 0.2 0.8 V 1 70 mA 50 µA mA/mA BIAS GENERATOR SPECIFICATIONS Bias Current Range Bias Current, Burst Off BIASSET Current Gain 2 IBIAS IBIAS, OFF GBIAS VBIAS+, VBIAS- ≥ 0.6V BEN = low or EN = high 5 1mA ≤ IBIAS < 2mA, VBSET = VREF 88 2mA ≤ IBIAS < 10mA, VBSET = VREF 70 88 110 10mA ≤ IBIAS < 70mA, VBSET = VREF 82.5 88 94.5 _______________________________________________________________________________________ Burst-Mode Laser Driver (VCC = +3.0V to +3.6V, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, TA = +25°C, IBIAS = 20mA, IMOD = 30mA, unless otherwise noted.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS BIASSET Current Gain Stability 5mA ≤ IBIAS ≤ 70mA (Note 13) -4.4 +4.4 % BIASSET Current Gain Linearity 5mA ≤ IBIAS ≤ 70mA (Note 14) -3.75 +3.75 % Bias Current Overshoot VCC turn-on/-off < 1s 10 % 17 mA/A Bias Current Monitor Gain GBSM 11 1mA ≤ IBIAS < 5mA Bias Current Monitor Gain Stability BIASSET Resistor 2mA ≤ IBIAS ≤ 70mA, VBSET = VREF 5mA ≤ IBIAS ≤ 70mA 14 ±4 -5 RBIAS 40 IMOD 10 % +5 50 60 Ω 85 mA 120 µA MODULATOR SPECIFICATIONS Modulation Current Range Modulation Current Off IMOD, OFF Instantaneous Voltage at OUT+ MODSET Current Gain GMOD BEN = low or EN = high or VIN = low 5 10mA ≤ IMOD < 60mA 0.6 60mA ≤ IMOD ≤ 85mA 0.75 V 10mA < IMOD < 85mA, VMSET = VREF 82.5 MODSET Current Gain Stability (Note 13) -4.4 +4.4 % MODSET Current Gain Linearity (Note 14) -2.2 +2.2 % IBIASSET = 0.15mA, IMODSET = 0.7mA MODSET, BIASSET Gain Matching (Note 15) 88 94.5 mA/mA 0.5 IMODSET = IBIASSET = 0.15mA 1.7 IMODSET = IBIASSET = 0.4mA 1 IMODSET = IBIASSET = 0.55mA % 1 Modulation Current Rise Time tR 20% to 80% 45 85 ps Modulation Current Fall Time tF 20% to 80% 45 85 ps Deterministic Jitter (Note 3) 17 45 psP-P Random Jitter (Note 4) 0.8 1.4 psRMS 50 60 Ω 1.4 V MODSET Resistor RMOD 40 MODSET, BIASSET OPERATIONAL AMPLIFIER SPECIFICATIONS MODSET, BIASSET Voltage Range 0.005 Voltage Error (Note 5) Input Leakage VMSET and VBSET pins 0.1 ±5 mV 1.5 µA TURN-OFF/-ON SPECIFICATIONS Burst-Enable Time (Notes 2, 6, 7) 2.3 ns Burst-Disable Time (Notes 2, 6, 8) 2.0 ns _______________________________________________________________________________________ 3 MAX3643 ELECTRICAL CHARACTERISTICS (continued) MAX3643 Burst-Mode Laser Driver ELECTRICAL CHARACTERISTICS (continued) (VCC = +3.0V to +3.6V, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, TA = +25°C, IBIAS = 20mA, IMOD = 30mA, unless otherwise noted.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS 2.56 V SAMPLE/HOLD SPECIFICATIONS MDIN Voltage Range 0.05 MDOUT Settling Relative to final value at 3µs, CL < 20pF Sample/Hold Droop After 100µs (Note 9) Sampling Error Final value measured after 10µs (MDOUT - MDIN), burst width > 576ns ±1 mV ±2.56 mV 3 ±14 mV 1.235 1.295 V µs BANDGAP VOLTAGE REFERENCE SPECIFICATIONS VREF Output RL > 10kΩ, CL < 50pF 1.175 MODULATION/BIAS CURRENT DISABLE Enable Time 5mA < IBIAS, 10mA < IMOD (Note 10) 5.5 Disable Time (Notes 2, 11) 375 ns 15 kΩ mA RIMAX Range Current Limit (Tested with IBIAS = IMOD) 3 IBIAS+IMOD RIMAX = 3kΩ 155 RIMAX = 5kΩ 100 150 RIMAX = 10kΩ 50 75 OPTICAL EVALUATION Eye Margin 1.25Gbps (Note 12) 33 % Note 1: DC parameters are production tested at TA = +25°C, guaranteed by design and characterization at TA = -40°C. AC parameters are guaranteed by design and characterization. Note 2: For 10mA ≤ IMOD ≤ 85mA and 4mA ≤ IBIAS ≤ 70mA. Note 3: Deterministic jitter measured with a continuous pattern of 27-1 PRBS, 80 ones, 27-1 PRBS, 80 zeros at 1.25Gbps, and both LVPECL inputs terminated by the network shown in Figure 3. Note 4: Random jitter, rise time, fall time measured with 0000011111 pattern at 1.25Gbps. Note 5: Voltage difference between VMSET and MODSET or VBSET and BIASSET excluding IR drops. The maximum operating voltage at VMSET or VBSET must be less than 1.4V for proper operation. Note 6: Turn-on/-off time is when the BEN+/BEN- LVPECL inputs are used to control modulation and bias currents. Note 7: Burst-enable delay is measured between the time at which the rising edge of the differential burst-enable input reaches the midpoint, and the time at which the combined output currents (bias plus modulation) reach 90% of their final level. Note 8: Burst-disable delay is measured between the time at which the falling edge of the differential burst-enable input reaches the midpoint, and the time at which the combined output currents (bias plus modulation) fall below 10% of the bias-on current. Note 9: Droop measured with sample/hold output load of 10MΩ. Note 10: Enable delay is measured between the time at which the falling edge of the EN input reaches ≤ 0.8V, and the time at which the combined output currents (bias plus modulation) reach 90% of their final level. Note 11: Disable delay is measured between the time at which the rising edge of the EN input reaches ≥ 2V, and the time at which the combined output currents (bias plus modulation) fall below 10% of the bias-on current. Note 12: Excelight SXT5731-Q laser. Note 13: Current gain stability = [(Gain – nominal Gain) / nominal Gain], nominal Gain at VCC = 3.3V, TA = +25°C. Note 14: Gain linearity = (Gainmax – Gainmin), Gainavg = Gainmax - Gainmin Gainavg/2 2 Note 15: Gain matching = Gain mod / Gainbias − Gain mod nom / Gainbiasnom nominal at VCC = 3.3V, TA = +25°C. Gain mod nom / Gainbiasnom 4 _______________________________________________________________________________________ Burst-Mode Laser Driver OPTICAL EYE DIAGRAM (1.25Gbps, 933MHz FILTER, PATTERN 213-1 + 80 CID) SUPPLY CURRENT vs. TEMPERATURE (IBIAS = 20mA, IMOD = 30mA) 27 24 45 21 40 18 DJ (psP-P) ICC (mA) 50 35 30 25 6 15 3 0 -40 -20 0 20 40 60 10 80 20 30 TEMPERATURE (°C) AVERAGE OPTICAL POWER = 3dBm EXTINCTION RATIO = 15dB GbE MASK MARGIN = 33% RANDOM JITTER vs. MODULATION AMPLITUDE 50 60 40 IMOD (mA) 70 80 IMOD vs. IMODSET 1.1 1.0 MAX3643 toc05 90 MAX3643 toc04 1.2 80 70 0.9 60 0.8 IMOD (mA) 0.7 0.6 50 40 30 0.5 0.4 20 0.3 10 0 0.2 10 20 30 40 50 60 70 0 80 200 400 600 IMOD (mA) IMODSET (µA) IBIAS vs. IBIASSET IMOD vs. VMSET (RMODSET = 1kΩ) 90 MAX3643 toc06 90 80 70 80 70 IMOD (mAP-P) 60 50 40 800 1000 MAX3643 toc07 RANDOM JITTER (psRMS) 12 9 10 IBIAS (mA) 15 20 EXCELIGHT SXT5731-Q LASER MAX3643 toc03 EXCLUDES BIAS AND MODULATION CURRENT 55 30 MAX3643 toc02 MAX3643 toc01 60 DETERMINISTIC JITTER vs. MODULATION AMPLITUDE 60 50 40 30 30 20 20 10 10 0 0 0 200 400 600 IBIASSET (µA) 800 1000 0 0.2 0.4 0.6 0.8 1.0 1.2 VMSET (V) _______________________________________________________________________________________ 5 MAX3643 Typical Operating Characteristics (Typical values are at VCC = +3.3V, TA = +25°C, data pattern = 27-1 PRBS + 80 ones + 27-1 PRBS + 80 zeros, unless otherwise noted.) Typical Operating Characteristics (continued) (Typical values are at VCC = +3.3V, TA = +25°C, data pattern = 27-1 PRBS + 80 ones + 27-1 PRBS + 80 zeros, unless otherwise noted.) BIAS CURRENT MONITOR GAIN (IBCMON/IBIAS) vs. TEMPERATURE IBIAS vs. VBSET (RBIASSET = 1kΩ) 70 MAX3643 toc09 80 TIMING DIAGRAM, BURST-ON (933MHz FILTER ON OUT+) MAX3643 toc10 17 MAX3643 toc08 90 16 VOUT+ GAIN (mA/A) 60 IBIAS (mA) 50 40 15 VBIAS+ 14 30 VBEN+ 20 13 10 0 12 0.2 0.4 0.6 0.8 1.0 -40 -20 0 20 40 60 80 VBSET (V) TEMPERATURE (°C) TIMING DIAGRAM, BURST-OFF (933MHz FILTER ON OUT+) SAMPLE/HOLD (10MΩ LOAD, 2.8µs SAMPLE AND 100µs HOLD, MDIN = 1.56V) MAX3643 toc11 500ps/div IBIAS + IMOD CURRENT THRESHOLD vs. RIMAX MAX3643 toc12 160 VOUT+ MAX3643 toc13 0 140 1.56V MDOUT 1.22V VBIAS+ BEN+ VBEN+ CURRENT THRESHOLD (mA) MAX3643 Burst-Mode Laser Driver 120 100 80 60 40 20 0 500ps/div 20µs/div 0 5 10 15 RIMAX (kΩ) 6 _______________________________________________________________________________________ 20 25 30 Burst-Mode Laser Driver PIN NAME FUNCTION 1 VCCA 2 IN+ Noninverted Data Input, LVPECL Compatible 3 IN- Inverted Data Input, LVPECL Compatible 4 VCCS Signal Supply Voltage 5 BEN+ Noninverted Burst-Enable Input, LVPECL Compatible 6 BEN- Inverted Burst-Enable Input, LVPECL Compatible 7 BENOUT 8 EN 9 BCMON 10 IMAX 11 MDOUT 12 MDIN Monitor Diode In. Analog Input for sample/hold. 13 BIAS- Connect BIAS- to VCC Through a 15Ω Resistor (or 5Ω Resistor and Switching Diode) Analog Supply Voltage Burst-Enable Output, LVCMOS. Signal replicates BEN input. Enable Input LVCMOS. Active low enables BIAS± and MOD± outputs. Bias Current Monitor. Current out of this pin develops a ground-referenced voltage across an external resistor proportional to the bias current. Current-Limit Reference. Connect a resistor from IMAX to GND to set maximum IBIAS plus IMOD. Monitor Diode Out. Analog Output for sample/hold. 14 BIAS+ Laser Bias Current Output. Modulation current flows into this pin when BEN input is high. 15, 18 VCCO Output Supply Voltage 16 OUT+ Laser Modulation Current Output. Modulation current flows into this pin when both BEN and IN inputs are high. 17 OUT- Connect OUT- to VCC Through a 15Ω Resistor (or 5Ω Resistor and Switching Diode) 19 GND Supply Ground. This pin must be connected to ground. 20 MODSET 21 VMSET 22 VREF 23 VBSET 24 BIASSET EP EP Modulation Current Set. Current from this pin to ground sets the laser modulation current. MODSET Reference. A ground-referenced voltage at this point establishes the MODSET reference. Reference Voltage Output. May be used for VMSET, VBSET. BIASSET Reference. A ground-referenced voltage at this point establishes the BIASSET reference. Bias Current Set. Current from this pin to ground sets the laser bias current. Exposed Paddle (Ground). The exposed pad must be soldered to the circuit board ground for proper thermal and electrical operation. _______________________________________________________________________________________ 7 MAX3643 Pin Description MAX3643 Burst-Mode Laser Driver VCC 0.1µF 27nH 24Ω 15Ω OUT+ MAX3643 OUT- IN+ 10Ω RCOMP 10Ω BIAS+ VCC LVPECL BIAS- IN- 15Ω BEN+ LVPECL BENCMOS BENOUT IMAX LIMIT LIMIT VCC VCC MDIN CMOS EN I1 IM = 88 x I1 IB = 88 x I2 I2 MDOUT VCC IBCMON = IB x 0.014 1.2V RMOD VREF VMSET MODSET VBSET RBIAS BIASSET BCMON Figure 1. Functional Diagram 8 _______________________________________________________________________________________ CCOMP Burst-Mode Laser Driver The MAX3643 laser driver includes a bias current generator, bias current monitor, modulation current generator, laser drive outputs, and monitor diode sample and hold. LVPECL-compatible inputs are provided for both high-speed data and burst enable. The high-speed burst-enable input signal is replicated on an LVCMOS output for use by the controller. Laser Diode Modulation and Bias Current Generators Laser diode modulation current amplitude is controlled by the current out of the MODSET pin, and bias current by the current out of the BIASSET pin, according to: IMOD = IMODSET x 88 IBIAS = IBIASSET x 88 A voltage source and two op amps are provided to enable IMODSET and IBIASSET to be set using either a resistor to ground or a current digital-to-analog converter (DAC). The high-impedance op amp reference input can be externally controlled, so that the modulation and bias currents can also be set using voltage DACs. Laser Diode Modulation and Bias Current Limiter Typical laser diodes have an absolute maximum rating of 150mA. To reduce the possibility of laser damage, the modulation current and bias current are shut off if the sum IMOD + IBIAS attempts to exceed the limit set by RIMAX; see the Typical Operating Characteristics. tion current output OUT+ is optimized to drive a 15Ω load, and must be DC-coupled. A damping resistor, RD, provides impedance matching to the laser diode. The combined value of the series damping resistor and the laser diode equivalent series resistance should be close to 15Ω. An RC shunt network, RCOMP/CCOMP, should also be provided to reduce optical output aberrations and duty-cycle distortion. The values of RCOMP and CCOMP can be adjusted to match the laser and PC board layout characteristics for optimal optical eye performance. The OUT- pin is normally connected through a 15Ω resistor to VCC or through a switching diode and series resistor to VCC. With some laser diodes, the use of a switching diode at OUT can improve the optical output eye by better matching the laser characteristics. The bias current ranges from 1mA to 70mA, as set by the current through BIASSET. Current in the BIAS output also switches at high speed when bursting; therefore, the BIAS+ pin should be connected directly through a resistor, equal to RD as determined above, to the laser cathode. The BIAS- pin must also be connected through a 15Ω resistor or through a switching diode and series resistor to VCC. When the BEN input is high, the laser driver sinks bias and modulation current according to the settings at MODSET and BIASSET. When the BEN input is low, the BIAS+ and OUT+ currents both shut off within 2ns. Note that when BEN is low, the bias current is shunted through the BIAS- output and the modulation current through the OUT- output. Bias Current Monitor Monitor Diode Sample and Hold The laser diode bias current can be monitored by measuring the voltage across an external load resistor connected from BCMON to ground. For example, a 1kΩ resistor from BCMON to ground gives the following relationship: Laser monitor diode current is only generated when there is an optical output (BEN is active). When BEN is inactive, the monitor current is zero, reflecting the fact that the laser is off. A sample-and-hold circuit, triggered by the state of the BEN input, is provided in the MAX3643. During the burst-enable active period, the voltage present at MDIN is stored on an internal sample-and-hold capacitor; and during the burst-enable inactive period, that voltage is output on MDOUT; see the timing diagram in Figure 2. While the internal sample-and-hold is sampling (BEN active), MDOUT voltage takes a 1.2V reference level. VBCMON = IBIAS × GBSM × 1kΩ The voltage at BCMON must be below 1.4V for proper operation. Output Drivers The modulation current ranges from 10mA to 85mA, as set by the current through MODSET. The laser modula- _______________________________________________________________________________________ 9 MAX3643 Detailed Description MAX3643 Burst-Mode Laser Driver BURST-ENABLE (BEN±) IN ≤ 125µs FOR BPON/GPON INTERNAL MDIN VOLTAGE SAMPLER BENOUT OUTPUT 3µs MDOUT < 2.56mV 1.2V 100µs Figure 2. Sample-and-Hold Timing Diagram Enable Input An LVCMOS input, EN, is provided to disable both bias and modulation currents under external control. The maximum time to disable laser current with the EN control is 375ns. IMOD = IMODSET x 88 To program the laser modulation current using a resistor or digital potentiometer, connect VMSET to VREF, attach a resistor from the MODSET pin to ground, and set the current according to: Setting the Current Limit A current limiter is provided to protect the laser diode by shutting down both bias and modulation currents when total current exceeds a value set by the resistor connected from IMAX to ground. Do not use less than 3kΩ IMAX. See the IBIAS + IMOD Current Threshold vs. RIMAX graph in the Typical Operating Characteristics. Programming the MODSET and BIASSET Inputs To program the laser modulation current using a current DAC, connect VMSET to VREF, attach the DAC to the MODSET pin and set the current according to: 10 IMOD = 1.2V × 88 RMODSET + RMOD To program the laser modulation current using a PWM voltage DAC (requiring a high-impedance load), attach a DAC output to the VMSET pin, connect a resistor from the MODSET pin to ground as shown in the Typical Applications Circuit, and set the current according to: IMOD = VDAC × 88 RMODSET + RMOD ______________________________________________________________________________________ Burst-Mode Laser Driver VCC 130Ω Laser diode bias current is set in the same manner as modulation current. LVPECL Data/Burst-Enable Inputs 130Ω MAX3643 ZO = 50Ω IN+/BEN+ ZO = 50Ω IN-/BEN- LVPECL The MAX3643 data and BEN inputs are biased with an on-chip, high-impedance network. When DC-coupled, the MAX3643 operates properly with signals that meet the EC table input-swing and common-mode requirements, including LVPECL and most CML. See Figure 3 for a termination network that can be used to connect the data and BEN inputs to LVPECL data outputs. Other termination networks may also be used, as long as both the input swing and common limits are met. MAX3643 This approach can also be used for a conventional voltage DAC output, if desired. In all cases, the voltage at MODSET must be kept ≤ 1.4V, which limits the range of acceptable values for RMODSET depending on the maximum modulation current. 82Ω 82Ω Figure 3. LVPECL High-Speed Inputs Sample-and-Hold Operation When the MAX3643 internal sample-and-hold is not required, the MDIN pin should be connected to ground and the MDOUT pin unconnected. If the internal sampleand-hold is required, then it is necessary to ensure that the time constant resulting from the monitor diode load resistance and the total load capacitance is compatible with the desired minimum burst interval. It is also necessary to make certain that the load at MDOUT does not exceed the capability of the MDOUT pin. Because the voltage at MDIN is not reflected to MDOUT until after the end of the laser burst, systems using the internal sample-and-hold alone cannot support continuous mode operation, often a required feature for module calibration. In this case, the voltage at MDIN can also be connected directly to a mux input as shown in the Typical Applications Circuit. As long as the total capacitance (including monitor diode intrinsic capacitance, MDIN capacitance, mux off-capacitance, and wiring parasitics) is less than 50pF, and the monitor diode load resistor is less than 2kΩ, then the sample-and-hold captures a 576ns minimum burst. The MAX3643 typical MDIN capacitance is 5pF, typical monitor diode maximum capacitance is 25pF, and the typical capacitance of a mux input in the off-state is 3pF to 5pF. When the IN+ INVCMBEN + (100mV to 800mV) VCMBEN BEN+ VCMBEN - (100mV to 800mV) 3.3V BENR1 = 1.65kΩ MAX3643 VCMBEN = +2.0V R2 = 2.54kΩ Figure 4. Single-Ended Biasing for Burst Enable mux is in the on-state, the capacitance at the input is typically 10pF to 20pF. If the minimum burst duration is longer than 576ns, it may be useful to connect an external capacitor in parallel with the monitor diode load to limit the effects of the data pattern on the monitor diode output. ______________________________________________________________________________________ 11 MAX3643 Burst-Mode Laser Driver Applications Information Running Burst-Enable Single-Ended Layout Considerations To minimize inductance, keep the connections between the MAX3643 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. Take extra care to minimize stray parasitic capacitance on the BIAS and MD pins. Use good high-frequency layout techniques and multilayer boards with uninterrupted ground planes to minimize EMI and crosstalk. IN+ VCC See Figure 5 for setting up the single-ended LVTTL or LVCMOS biasing for burst enable. IN- R3 = 5kΩ LVTTL OR LVCMOS HIGH BEN+ R4 = 3kΩ LVTTL OR LVCMOS LOW VCC BENR5 = 5kΩ MAX3643 R6 = 9kΩ Laser Safety and IEC 825 Using the MAX3643 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 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 can occur. Figure 5. Single-Ended LVCMOS or LVTTL Biasing for Burst Enable Interface Model VCC VCC VCC 16kΩ 5kΩ Exposed-Paddle Package The exposed paddle on the 24-pin TQFN provides a very low thermal resistance path for heat removal from the IC. The pad is also electrical ground on the MAX3643 and must be soldered to the circuit board ground for proper thermal and electrical performance. Refer to Maxim Application Note HFAN-08.1: Thermal Considerations for QFN and Other Exposed-Paddle Packages for additional information. IN+/BEN+ VCC 5kΩ IN-/BEN- 24kΩ MAX3643 Figure 6. Simplified Input Circuit Schematic 12 ______________________________________________________________________________________ Burst-Mode Laser Driver Chip Information TRANSISTOR COUNT: 2771 PROCESS: SiGe BiPOLAR 16 BIAS- 17 VCCO OUT+ 18 BIAS+ VCCO OUT- TOP VIEW 15 14 13 12 MDIN GND 19 11 MDOUT MODSET 20 10 IMAX VMSET 21 MAX3643 VREF 22 VBSET 23 *EP IN- 4 5 6 BEN- 3 VCCS 2 BEN+ 1 IN+ + VCCA BIASSET 24 9 BCMON 8 EN 7 BENOUT THIN QFN (4mm x 4mm) *THE EXPOSED PAD MUST BE CONNECTED TO GROUND FOR PROPER THERMAL AND ELECTRICAL PERFORMANCE. ______________________________________________________________________________________ 13 MAX3643 Pin Configuration Package Information (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.) 24L QFN THIN.EPS MAX3643 Burst-Mode Laser Driver PACKAGE OUTLINE, 12, 16, 20, 24, 28L THIN QFN, 4x4x0.8mm 21-0139 14 ______________________________________________________________________________________ E 1 2 Burst-Mode Laser Driver PACKAGE OUTLINE, 12, 16, 20, 24, 28L THIN QFN, 4x4x0.8mm 21-0139 E 2 2 Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 15 © 2005 Maxim Integrated Products M. QUIJANO Printed USA is a registered trademark of Maxim Integrated Products, Inc. MAX3643 Package Information (continued) (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)