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19-2294; Rev 1; 5/03
KIT ATION EVALU ABLE AVAIL
2.7Gbps, +3.3V DC-Coupled Laser Driver
General Description 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
MAX3850
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.
Ordering Information
PART MAX3850EGJ MAX3850E/D TEMP RANGE -40C to +85C -40C to +85C PINPACKAGE 32 QFN Dice* PACKAGE CODE G3255-1 --
Applications
SDH/SONET Transmission Systems MPLS Transmitter Systems Add/Drop Multiplexers Digital Cross-Connects Section Regenerators
*Dice are designed to operate over this range, but are tested and guaranteed at TA = +25C only. Contact factory for availability. Typical Application Circuits are continued at the end of the data sheet. Pin Configuration appears at the end of the data sheet.
Typical Application Circuits
3.3V ENABLE LATCH 3.3V 0.01F LD VCC OUTDATACLOCK+ 100 CLOCKBIASMAX MODSET APCFILT APCSET BIAS MD CAPC GND BIASMON MODMON 392 392 3.3V 1000pF MAX3850 OUT+ 50 8.0pF 16
DATA+ 100 MAX3890 SERIALIZER WITH CLOCK GEN
FAIL
11
0.1F 0.1F TYPICAL APPLICATION CIRCUIT WITH DC-COUPLED INPUTS
Covered by U.S. Patent numbers 5,802,089 and 5,883,910 ________________________________________________________________ Maxim Integrated Products 1
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.
2.7Gbps, +3.3V DC-Coupled Laser Driver MAX3850
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 = +85C) 32-Pin QFN (derate 21.2mW/C above +85C) ........13.84mW Storage Temperature Range .............................-65C to +165C Operating Junction Temperature Range ...........-55C to +150C Processing Temperature (die) .........................................+400C Lead Temperature (soldering,10s) ..................................+300C
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 = -40C to +85C. Typical values are at VCC = +3.3V, IMOD = 30mA, IBIAS = 60mA, TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER Supply Voltage Supply Current Bias-Current Range Bias Off-Current Bias-Current Stability Differential Input Voltage Range Common-Mode Input Voltage TTL Input High Voltage TTL Input Low Voltage TTL Output High Voltage TTL Output Low Voltage Monitor-Diode Reverse Bias Voltage Monitor-Diode DC Current Range Monitor-Diode Set-Point Stability Monitor-Diode Bias Absolute Accuracy BIASMON to IBIAS Gain MODMON to IMOD Gain Modulation-Current Range ABIAS AMOD IMOD IMD (Note 6) (Note 5) IBIAS/IBIASMON IMOD/IMODMON VOUT+, VOUT- = 0.6V (DC-coupled) VOUT+, VOUT- = 2.0V (AC-coupled) IMD = 1mA IMD = 18A VID VICM SYMBOL VCC ICC IBIAS IBIAS-OFF (Note 2) (Note 3) ENABLE = low (Note 4) APC open loop (Figure 1) LVPECL compatible ENABLE, LATCH ENABLE, LATCH FAIL sourcing 50A Sinking 100A 2.4 0 1.5 18 -480 -1000 -15 36 25 5 5 41 30 95 295 1000 480 1000 +15 46 35 60 80 VCC 0.3 0.025 IBIAS = 100mA IBIAS = 1mA -480 -1000 200 VCC 1.49 2.0 0.8 VCC 0.4 VCC 1.32 12 456 1 CONDITIONS MIN 3.0 TYP 3.3 35 MAX 3.6 65 100 100 480 1000 1600 VCC VID/4 UNITS V mA mA A ppm/C mVP-P V V V V V V A ppm/C % A/A A/A mA
2
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2.7Gbps, +3.3V DC-Coupled Laser Driver
DC ELECTRICAL CHARACTERISTICS (continued)
(VCC = +3.0V to +3.6V, TA = -40C to +85C. Typical values are at VCC = +3.3V, IMOD = 30mA, IBIAS = 60mA, TA = +25C, unless otherwise noted.) (Note 1)
PARAMETER Modulation Off-Current Modulation-Current Stability SYMBOL IMOD = 60mA IMOD = 5mA CONDITIONS MIN -480 -1000 TYP 66 110 MAX 100 480 1000 UNITS A ppm/C
MAX3850
IMOD-OFF ENABLE = low (Note 4)
AC ELECTRICAL CHARACTERISTICS
(VCC = +3.0V to +3.6V, IMOD = 5mA to 60mA, TA = -40C to +85C. Typical values are at VCC = +3.3V, IMOD = 30mA, TA = +25C.) (Note 7)
PARAMETER Maximum Data Rate Input Latch Setup Time Input Latch Hold Time Output Rise Time Output Fall Time Output Overshoot IMOD Enable/Startup Delay IBIAS Typical Startup Delay Output Random Jitter Output Deterministic Jitter RJOUT TJOUT APC open loop, CAPC and CAPCFILT = 0 (Note 8) LATCH = high, 27- 1 PRBS with 80 inserted 0s and 80 inserted 1s tSU tH tR tF LATCH = high (Figure 3) LATCH = high (Figure 3) 20% to 80% edge speeds (Note 8) 20% to 80% edge speeds (Note 8) 30mA IMOD 60 (Note 8) IMOD = 5mA (Note 8) 90 60 66 70 2 12 270 370 0.66 8.6 1.4 50 110 100 SYMBOL CONDITIONS MIN TYP MAX 2.7 UNITS Gbps ps ps ps ps % ns ns psRMS psP-P
Note 1: Note 2: Note 3: Note 4: Note 5: Note 6:
Dice are tested at TA = +25C only. Specifications at -40C are guaranteed by design and characterization. Tested at RMODSET = 2.61k, RBIASMAX = 1.96k, 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.
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3
2.7Gbps, +3.3V DC-Coupled Laser Driver MAX3850
DATA+ DATA100mV MIN 800mV MAX
200mVP-P MIN (DATA+) - (DATA-) 1600mVP-P MAX
IOUT+ IMOD
Figure 1. Required Input Signal and Output Polarity
VCC
VCC
tCLK
CLK
MAX3850
OUT-
30
30
tSU
tH
Z0 = 30 30
1.0pF
DATA
0.5pF IOUT+ OUT+ BIAS 15 75 Z0 = 30
OSCILLOSCOPE
Figure 3. Setup/Hold Time Definition
50
VCC
Figure 2. Output Termination for Characterization
4
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2.7Gbps, +3.3V DC-Coupled Laser Driver
Typical Operating Characteristics
(DC-coupled output, TA = +25C, unless otherwise noted.)
OPTICAL EYE DIAGRAM (2.7Gbps, 130mm FP LASER 1.87Gbps FILTER, 32-QFN)
MAX3850 toc01
MAX3850
BIAS CURRENT ENABLE STARTUP DELAY vs. CAPC
MAX3850 toc02
ELECTRICAL EYE DIAGRAM (IMOD = 25mA, 213 - 1 + 80 CID, 32 QFN)
MAX3850 toc03
100
STARTUP DELAY (ms)
10
1.0
MITSUBISHI ML725C8F LASER DIODE
0.1 100p 1000p 0.01 CAPC (F) 0.1 1.0 58ps/div
ELECTRICAL EYE DIAGRAM (IMOD = 60mA, 213 - 1 + 80 CID, 32 QFN)
MAX3850 toc04
ELECTRICAL EYE DIAGRAM AC-COUPLED (IMOD = 80mA, 213 - 1 + 80 CID, 32 QFN)
MAX3850 toc05
RANDOM JITTER vs. IMOD
1.8 1.6 RANDOM JITTER (psms) 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0
MAX3850 toc06
2.0
58ps/div
58ps/div
10
20
30
40
50
60
IMOD (mA)
DETERMINISTIC JITTER vs. IMOD
MAX3850 toc07
IBIASMAX vs. RBIASMAX
150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 1k 10k RBIASMAX ()
MAX3850 toc08
IMOD vs. RMODSET
60 50 IMOD (mA) 40 30 20 10 0 100k 1k 10k RMODSET () 100k
MAX3850 toc09
30 25 TOTAL JITTER (psP-P) 20 15 10 5 0 10 20 30 40 50
70
60
IMOD (mA)
IBIASMAX (mA)
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2.7Gbps, +3.3V DC-Coupled Laser Driver MAX3850
Typical Operating Characteristics (continued)
(DC-coupled output, TA = +25C, unless otherwise noted.)
IMD vs. RAPCSET
MAX3850 toc10
SUPPLY CURRENT vs. TEMPERATURE (EXCLUDES IBIAS, IMOD, 15 LOAD)
MAX3850 toc11
BIAS-CURRENT MONITOR GAIN vs. TEMPERATURE
MAX3850 toc12
1.6 1.4 1.2 IMD (mA) 1.0 0.8 0.6 0.4 0.2 0 100 1k RAPCSET () 10k
70 60 SUPPLY CURRENT (mA) 50 40 30 VCC = 3.0V 20 10 VCC = 3.3V VCC = 3.6V
50 45 GAIN (IBIAS /IBIASMON) 40 35 30 25 20 15 IBIAS = 100mA, IMOD = 50mA IBIAS = 10mA, IMOD = 10mA
100k
-40
-15
10
35
60
85
-40
-15
10
35
60
85
TEMPERATURE (C)
TEMPERATURE (C)
MODULATION-CURRENT MONITOR GAIN vs. TEMPERATURE
MAX3850 toc13
PULSE-WIDTH DISTORTION vs. IMOD
16 14 12 PWD (ps) 10 8 6 4
MAX3850 toc14
35 30 GAIN (IMOD/IMODMON) 25 20 15 10 5 0 -40
IBIAS = 100mA, IMOD = 50mA IBIAS = 10mA, IMOD = 10mA
18
2 0 -15 10 35 60 85 10 20 30 40
VCC = 3.0V 50 60
TEMPERATURE (C)
IMOD (mA)
6
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2.7Gbps, +3.3V DC-Coupled Laser Driver
Pin Description
PIN 1, 4, 7 2 3 5 6 8 9 10 11 12 13 14 15 16, 18, 21 17 19 20 22 23 24 25 26 27 28 29 30 31 32 NAME VCC1 DATA+ DATACLK+ CLKLATCH ENABLE GND1 BIASMON MODMON FAIL APCFILT GND4 VCC4 BIAS OUT+ OUTGND4 GND3 MD VCC3 CAPC GND2 N.C. APCSET MODSET BIASMAX VCC2 Power Supply for Digital Circuits Positive Data Input Negative Data Input Positive Clock Input. Connect to VCC or leave unconnected if latch function is not used. 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 100k pullup resistor to VCC. TTL/CMOS Enable Input. High for normal operation, low to disable laser bias and modulation current. Internal 100k pullup resistor to VCC. Ground for Digital Curcuits Bias Current Monitor. Current into this pin is proportional to the laser bias current. Modulation Current Monitor. Current into this pin is proportional to the laser modulation current. TTL/CMOS Failure Output. Indicates APC failure when low. Connect a capacitor (CAPCFILT = 0.1F) from this pad to ground to filter the APC noise. Ground for Output Curcuitry Power Supply for Output Circuitry Laser Bias Current Output Positive Modulation Current Output. IMOD flows into this pad when input data is high. Negative Modulation Current Output. IMOD flows into this pad when input data is low. Ground for Modulation Current Source Ground for APC Circuitry Monitor Diode Input. Connect this pin to a monitor photodiode anode. A capacitor to ground is required to filter high-speed AC monitor photocurrent. Power Supply for APC A capacitor connected from this pad to ground controls the dominant pole for the APC feedback loop (CAPC = 0.1F). Ground for Internal Reference No Connection. Leave unconnected. A resistor connected from this pad to ground sets the desired average optical power. Connect a 100k resistor from this pad to ground if APC is not used. A resistor connected from this pad to ground sets the desired modulation current. 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. Power Supply for Internal Reference FUNCTION
MAX3850
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7
2.7Gbps, +3.3V DC-Coupled Laser Driver MAX3850
LATCH VCC
MAX3850
OUT+ 0 MUX DATA CLK D Q 1 OUTCCOMP IMOD VCC RD
RCOMP
ENABLE IBIAS IBIAS 41 BIAS
BIASMON
162X
40x 5x MD
MODMON
IMD
IMOD 30
1000pF FAILURE DETECTION
MODSET RMODSET GND4
BIASMAX RBIASMAX FAIL
CAPC
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 15 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 15. 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
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2.7Gbps, +3.3V DC-Coupled Laser Driver
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 0.1F. 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 0.1F. 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. 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.
MAX3850
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.
Open-Loop Operation
If necessary, the MAX3850 is fully operational without APC. To operate the MAX3850 open loop, connect a 100k 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.
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.
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.
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%.
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 = 0.1F, 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.
Programming the Modulation Current
For a given laser power (PAVG), slope efficiency (), 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 +25C.
Current Monitors
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
Programming the Bias Current
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 +25C. See the IBIASMAX vs. RBIASMAX graph in the Typical
9
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2.7Gbps, +3.3V DC-Coupled Laser Driver
Operating Characteristics, and select the value of RBIASMAX that corresponds to the required current at +25C. 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 +85C.
MAX3850
Table 1. Optical Power Definition
PARAMETER Average Power Extinction Ratio Optical Power High Optical Power Low Optical Amplitude Laser Slope Efficiency Modulation Current SYMBOL PAVG re P1 P0 PP-P IMOD RELATION PAVG = (P0 + P1) / 2 re = P1 / P0 P1 = 2PAVG re / (re + 1) P0 = 2PAVG / (re + 1) PP-P = P1 - P0 = PP-P / IMOD IMOD = PP-P /
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.
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.
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 15 load. Therefore, the series combination of RD and RL (where RL represents the laser-diode resistance) should equal 15. Typical values for RD are 8 to 13. For best performance, place a bypass capacitor (0.01F 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 = 50 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.)
Calculating Power Consumption
The junction temperature of the MAX3850 dice must be kept below +150C 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 -40C to +85C, and the laser diode has the following characteristics: Wavelength: = 1.3m, Threshold Current: ITH = 22mA at +25C, Threshold Temperature Coefficient: TH = 1.3%/C, Laser-toMonitor Transfer: MON = 0.2A/W, Laser Slope Efficiency: = 0.05mW/mA at +25C.
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 0.1F. 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
Determine RAPCSET The desired monitor diode current is estimated by IMD = PAVG x MON = 200A. The IMD vs. RAPCSET graph in the Typical Operating Characteristics shows RAPCSET at 6.2k.
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2.7Gbps, +3.3V DC-Coupled Laser Driver
Table 2. Bondpad Locations
PAD 1 2 3 4 5 6 7 8 9 10 11 *12 13 14 15 16 17 18 19 20 21 22 23 24 NAME GND2 GND1 VCC1 DATA+ DATAVCC1 GND1 VCC1 CLK+ CLKVCC1 GND1 LATCH ENABLE GND1 GND1 BIASMON MODMON FAIL GND4 N.C. APCFILT GND4 VCC4 COORDINATES X 46 46 46 46 46 46 46 46 46 46 46 46 205 351 484 605 727 848 970 1091 1213 1334 1456 1577 Y 1523 1334 1213 1091 970 848 727 605 484 362 241 46 46 46 46 46 46 46 46 46 46 46 46 46 PAD 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 NAME BIAS N.C. VCC4 N.C. OUT+ OUTN.C. VCC4 GND4 GND3 MD GND3 VCC3 CAPC N.C. GND3 N.C. GND3 N.C. APCSET GND2 MODSET BIASMAX VCC2 COORDINATES X 1709 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1709 1577 1456 1334 1213 1091 970 848 727 605 484 351 205 Y 46 241 373 494 616 737 859 980 1102 1223 1356 1523 1523 1523 1523 1523 1523 1523 1523 1523 1523 1523 1523 1523
MAX3850
*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 5.5k. Determine RBIASMAX Determine the maximum threshold current (ITH(MAX)) at TA = +85C 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 5k.
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.
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11
2.7Gbps, +3.3V DC-Coupled Laser Driver MAX3850
Wirebonding Die
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 (100m) square, and die thickness is 12mils (300m) square.
VCC PACKAGE
0.9nH OUT+ 0.1pF 0.9nH OUT0.1pF
Layout Considerations
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.
Laser Safety and IEC825
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.
Figure 6. Simplified Output Circuit
Pin Configuration
MODSET APCSET
TOP VIEW
VCC2
BIASMAX
GND2
CAPC 26
32
VCC PACKAGE 5k
31
30
29
28
27
VCC3 25 24 MD 23 GND3 22 GND4 21 VCC4 20 OUT19 OUT+ 18 VCC4 17 BIAS 16 VCC4
VCC1 DATA+ DATAVCC1
1 2 3 4 5 6 7 8 9 ENABLE 10 GND1 11 BIASMON 12 MODMON 13 FAIL 14 APCFILT 15 GND4
VCC 0.9nH IN+ 0.1pF 5k
MAX3850
CLK+ CLKVCC1 LATCH
VCC 5k 0.9nH IN0.1pF 24k
5mm 5mm QFN THE EXPOSED PAD MUST BE SOLDERED TO GND ON THE CIRCUIT BOARD Figure 5. Simplified Input Circuit 12 ______________________________________________________________________________________
N.C.
2.7Gbps, +3.3V DC-Coupled Laser Driver
Typical Application Circuits (continued)
3.3V 3.3V ENABLE LATCH FAIL 0.01F LD 16 DATA+ 100 MAX3890 SERIALIZER WITH CLOCK GEN. 0.1F 0.1F 0.1F 100 CLOCKBIASMAX MODSET APCFILT APCSET BIAS MD CAPC BIASMON MODMON 0.1F 392 392 3.3V 1000pF DATACLOCK+ MAX3850 OUTOUT+ 50 8.0pF 11
MAX3850
0.1F
TYPICAL APPLICATION CIRCUIT WITH AC-COUPLED INPUTS
0.1F
Chip Topography
TRANSISTOR COUNT: 1749
VCC1 GND1 CLK+ GND1 DATA- VCC1 VCC1 GND2 CLKVCC1 DATA+ GND1
Chip Information
SUBSTRATE CONNECTED TO GND DIE SIZE: 70mils 83mils DIE THICKNESS: 12mils PROCESS: SIGe Bipolar
LATCH ENABLE GND1 GND1 BIASMON MODMON FAIL GND4 N.C. APCFILT GND4 VCC4 BIAS
VCC2 BIASMAX MODSET GND2 APCSET N.C. 0.083" GND3 (2.108mm) N.C. GND3 N.C. CAPC VCC3 GND3
N.C. N.C. OUT- VCC4 GND3 VCC4 OUT+ N.C. GND4 MD 0.070" (1.778mm)
______________________________________________________________________________________
13
2.7Gbps, +3.3V DC-Coupled Laser Driver MAX3850
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.)
32L QFN.EPS
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 (c) 2003 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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