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19-2247; Rev 1; 4/02
2.5Gbps Laser Driver with Automatic Modulation Control
General Description
The MAX3865 is designed for direct modulation of laser diodes at data rates up to 2.5Gbps. It incorporates two feedback loops, the automatic power-control (APC) loop and the automatic modulation-control (AMC) loop, to maintain constant average optical output and extinction ratio over temperature and laser lifetime. External resistors or current output DACs may set the laser output levels. The driver can deliver up to100mA of laser bias current and up to 60mA laser modulation current with a typical (20% to 80%) edge speed of 84ps. The MAX3865 accepts differential clock and data input signals with on-chip 50 termination resistors. The inputs can be configured for CML or other high-speed logic. An input data-retiming latch can be enabled to reject input pattern-dependent jitter when a clock signal is available. The MAX3865 provides laser bias current and modulation current monitors, as well as a failure detector, to indicate the laser operating status. These features are all implemented on an 81mil 103mil die; the MAX3865 is also available as a 32-pin QFN package. o Single +3.3V or +5V Power Supply o 68mA Supply Current o Up to 2.5Gbps (NRZ) Operation o Feedback Control for Constant Average Power o Feedback Control for Constant Extinction Ratio o Programmable Bias Current Up to 100mA o Programmable Modulation Current Up to 60mA o 84ps Rise/Fall Time o Selectable Data Retiming Latch o Bias and Modulation Current Monitors o Failure Detector o ESD Protection
Features
MAX3865
Ordering Information
PART MAX3865EGJ MAX3865E/D TEMP. RANGE -40C to +85C -40C to +85C PIN-PACKAGE 32 QFN Dice*
Applications
SONET/SDH Transmission Systems Add/Drop Multiplexers Digital Cross-Connects Section Regenerators 2.5Gbps Optical Transmitters
*Dice are designed to operate from -40C to +85C , but are tested and guaranteed at TA = +25C only. Contact factory for availability. Pin Configuration appears at end of data sheet.
Typical Applications Circuit
+3.3V +3.3V 200 200 LP EN0 EN1 BIASMON MODMON RTEN FAIL VCC MODN LP 0.056F 20 15 MODQ 20 0.056F LED 20 LASER +3.3V
DATADATA+
50 50 +3.3V 50 50
DATADATA+ VDR VCR CLK+ CLKGND
MAX3892
2.5Gbps SERIALIZER CLK+ CLK-
MAX3865
BIAS BIAS_X MD MD_X
MODMAX
BIASMAX
AMCSET
APCSET
REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE
RMODMAX
RBIASMAX
RAPCSET
RAMCSET
Covered by U.S. Patent numbers 5,883,910, 5,850,409, and other patent pending.
________________________________________________________________ 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.
2.5Gbps Laser Driver with Automatic Modulation Control MAX3865
ABSOLUTE MAXIMUM RATINGS
Voltage at Any Pin...............................................................+7.0V Supply Voltage (VCC) ............................................-0.5V to +7.0V Voltage at VCR, VDR, DATA+, DATA-, CLOCK+, and CLOCK- Pins ..................-0.5V to (VCC + 0.5V) Voltage at DATA+ and DATA- Pins ..................................(VDR - 1.2V) to (VDR + 1.2V) Voltage at CLK+ and CLK- Pins ......(VCR - 1.2V) to (VCR + 1.2V) Voltage at MODQ and MODN Pins ................0V to (VCC + 1.5V) Voltage at Any Other Pins (RTEN, EN0, EN1, FAIL, MODMAX, BIASMAX, AMCSET, APCSET, MD_X, BIAS, BIAS_X, BIASMON, MODMON) ............-0.5V to (VCC + 0.5V) Current into BIAS Pin ......................................-20mA to +150mA Current into MODQ and MODN Pins ..............-20mA to +100mA Current into MD Pin...........................................-10mA to +10mA Operating Junction Temperature .....................-55C to +150C Storage Temperature Range .............................-55C to +150C Continuous Power Dissipation (TA = +85C) 32-Pin QFN (derate 21.2mW/C above +85C) ................1.3W Lead Temperature (soldering, 10s) .................................+300C Processing Temperature (die) .........................................+400C
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.
ELECTRICAL CHARACTERISTICS
(VCC = +3.14V to +3.6V or +4.5V to +5.5V, TA = -40C to +85C. Typical values are at VCC = +3.3V, IBIAS = 50mA, IMOD = 30mA, TA = +25C, unless otherwise noted.) (Notes 1, 2, 3)
PARAMETER Power-Supply Current Differential Input Voltage Instantaneous Input Voltage Single-Ended Input Resistance Input Return Loss, for Data+, Data-, Clock+, and ClockBias-Current Setting Range Bias Off Current Bias-Current Setting Accuracy Compliance Voltage for BIAS and BIAS_X IBIAS to IBIASMON Ratio Modulation-Current Setting Range Modulation Off Current Modulation-Current Setting Accuracy Compliance Voltage for MODQ and MODN IMOD to IMODMON Ratio Compliance Voltage for BIASMON and MODMON Voltage at MD Pin VMD (Note 5) 1.8 1.0 IMOD EN0, EN1 = low AMC off IMOD = 60mA IMOD = 5mA +3.14V VCC +3.6V +4.5V VCC +5.5V 1.8 1.8 32 VCC + 0.4 0.25 VCC + 1.2 5.5 5 EN0, EN1 = low APC off (Note 5) IBIAS = 100mA IBIAS = 1mA 1 48 60 0.1 15 0.1 VCC + 0.4 RLIN SYMBOL ICC VID CONDITIONS VCC = +3.14V to +3.6V (Note 4) VCC = +4.5V to +5.5V, typical current at VCC = +5.0V (Note 4) Data and clock inputs (Figure 2) Data and clock inputs (Figure 2) (Note 5) Input to VDR, VCR f 2.7GHz 2.7GHz < f < 4GHz 1 0.2 1.3 40 50 20 17 100 0.1 15 MIN TYP 68 69 MAX 85 90 1.6 VCC + 0.4V 60 mA Vp-p V dB mA mA % mA V
mA/mA
UNITS
mA mA % mA V
mA/mA
(Note 5)
V V
2
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2.5Gbps Laser Driver with Automatic Modulation Control
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +3.14V to +3.6V or +4.5V to +5.5V, TA = -40C to +85C. Typical values are at VCC = +3.3V, IBIAS = 50mA, IMOD = 30mA, TA = +25C, unless otherwise noted.) (Notes 1, 2, 3)
PARAMETER Bias-Setting Accuracy at MD Pin Modulation-Setting Accuracy at MD Pin EN0, EN1, and RTEN Input High EN0, EN1, and RTEN Input Low FAIL Output High FAIL Output Low FAIL Current Setup/Hold Time Output Edge Speed Output Overshoot Enable/Startup Maximum CID Deterministic Jitter Random Jitter AMC Pilot Tone Frequency fAMC tSU, tHD t R , tF Source 50A Sink 100A Low state, VOL forced to VCC (Figure 2) (Note 5) Load = 20, 20% to 80% (Notes 5, 7) (Notes 5, 7) APC and AMC off (Notes 2, 5) (Notes 2, 5) (Notes 5, 7) 80 22 1.6 1 50 100 84 9 150 130 2.4 0.4 5.0 SYMBOL IMD = 1mA IMD = 36A (Note 6) IMD = 1mA IMD = 36A 2.0 0.8 CONDITIONS MIN TYP 15 10 15 10 MAX UNITS % A % A V V V V mA ps ps % ns bit psp-p psRMS MHz
MAX3865
Note 1: Note 2: Note 3: Note 4: Note 5: Note 6: Note 7:
AC characterization performed using the circuit in Figure 1. Measured using a 2.5Gbps 213 - 1 PRBS with 80 0's and 80 1's input data pattern. Specifications at -40C are guaranteed by design and characterization. VCC current excludes the current into MODQ, MODN, BIAS, BIAS_X, MODMON, and BIASMON pins. Guaranteed by design and characterization. Measured with low-frequency data. Instantaneous current into MD pin range is 36A to 1000A. Measured using a 2.5Gbps repeating 0000 0000 1111 1111 pattern.
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3
2.5Gbps Laser Driver with Automatic Modulation Control MAX3865
VCC VCC 33 0.1F MODN DATA+ PATTERN GENERATOR CLK+50 VCC 50 DATA+ VDR VCR CLK+ MODQ 33 0.1F 50
RTEN DATA50 DATA-
VCC
OSCILLOSCOPE
MAX3865
BIAS 50 VCC
CLK-
50
CLK-
BIAS_X VCC
GND
EN1
EN0
Figure 1. Test Circuit
VCC + 0.4V CLK+ 0.1V-0.8V CLK1.3V
tSU DATA+
tHD
VCC + 0.4V
0.1V-0.8V DATA1.3V
(DATA+) - (DATA-)
VID = 0.2Vp-p-1.6Vp-p
IMODQ
5mA-60mA
Figure 2. Required Input Signal, Setup/Hold-Time Definition, and Output Polarity 4 _______________________________________________________________________________________
2.5Gbps Laser Driver with Automatic Modulation Control
Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
OPTICAL EYE DIAGRAM (EXTINCTION RATIO = 8.25dB, 213-1 PRBS AT 2.5Gbps, 1.87GHz FILTER)
MAX3865 toc01
MAX3865
ELECTRICAL EYE DIAGRAM (IMODQ = 60mA)
MAX3865 toc02
ELECTRICAL EYE DIAGRAM (IMODQ = 30mA)
MAX3865 toc03
PATTERN 213 - 1 PRBS DATA RATE = 2.5Gbps
PATTERN 213 - 1 PRBS DATA RATE = 2.5Gbps
58ps/div
58ps/div
58ps/div
SUPPLY CURRENT (ICC) vs. TEMPERATURE (EXCLUDES BIAS AND MODULATION CURRENTS)
MAX3865 toc04
DETERMINISTIC JITTER vs. TEMPERATURE (IMODQ = 60mA)
MAX3865 toc05
TYPICAL DISTRIBUTION OF RISE TIME (WORST-CASE CONDITIONS)
ELECTRICAL MEASUREMENT IMODQ = 60mA VCC = +3.14V TA = +85C
MAX 3865 toc06
80 75 SUPPLY CURRENT (mA) 70 65 60 55 50 -40 -15 10 35 60 VCC = +5.0V VCC = +3.3V
100 90 DETERMINISTIC JITTER (ps) 80 70 60 50 40 30 20 10 0
30 25 PERCENT OF UNITS (%) 20 15 10 5 0
85
-40
-20
0
20
40
60
80
100 102 104 106 108 110 112 114 116 118 120 RISE TIME (ps)
TEMPERATURE (C)
TEMPERATURE (C)
TYPICAL DISTRIBUTION OF FALL TIME (WORST-CASE CONDITIONS)
MAX 3865 toc07
DIFFERENTIAL |S11| vs. FREQUENCY
-17 -19 -21 |S11| (dB) -23 -25 -27 -29 -31 -33
MAX3865 toc08
30 25 PERCENT OF UNITS (%) 20 15 10 5 0 100 102 104 106 108 110 112 114 116 118 120 FALL TIME (ps) ELECTRICAL MEASUREMENT IMODQ = 60mA VCC = +3.14V TA = +85C
-15
-35 0 500 1000 1500 2000 2500 3000 3500 4000 FREQUENCY (MHz)
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2.5Gbps Laser Driver with Automatic Modulation Control MAX3865
Pin Description
PIN 1, 8, 19, 22, 28 2 3 4 5 6 7 9, 16, 23, 24, 25 10 11 12 13 14 15 17 18 20 21 26 27 29 30 31 NAME VCC DATADATA+ VDR VCR CLK+ CLKGND RTEN EN0 EN1 FAIL BIASMON MODMON BIAS_X BIAS MODN MODQ MD MD_X AMCSET APCSET MODMAX Positive Supply Voltage Complementary Data Input, with On-Chip Termination Data Input, with On-Chip Termination Termination Reference Voltage for Data Inputs Termination Reference Voltage for Clock Inputs Clock Input for Data Retiming, with On-Chip Termination Complementary Clock Input for Data Retiming, with On-Chip Termination No Internal Connection. Tie to ground. Data Retiming Enable Input, TTL Compatible, Active-High Operating Mode Input, TTL Compatible Operating Mode Input, TTL Compatible Fault Warning, TTL Compatible. Low for fault condition. Bias-Current Monitor. Open-collector type, tie to VCC if not used. Modulation-Current Monitor. Open-collector type, tie to VCC if not used. Bias Shunt. Always tie to the BIAS pin. Laser Bias-Current Output. Connect to the laser via an inductor. Modulation-Current Output to Dummy Load Modulation-Current Output to Laser Feedback Input from Monitor Diode Monitor Diode Shunt. Connect to GND when laser diode to monitor current gain 0.005. Connect to the MD pin for gain 0.02. For 0.005 < gain < 0.02 connect to either GND or the MD pin. Monitor Diode Modulation-Current (Peak-to-Peak) Set Point Monitor Diode Bias-Current (Average) Set Point Connect an external resistor to ground to program IMOD in the MANUAL and APC modes. The resistor sets the maximum IMOD in AMC mode. The AMC loop may reduce IMOD from its maximum but cannot add to it. Connect an external resistor to ground to program IBIAS in the MANUAL mode. The resistor sets the maximum IBIAS in the APC and AMC modes. The APC loop may reduce IBIAS from its maximum but cannot add to it. The exposed paddle and corner pins must be soldered to ground. FUNCTION
32
BIASMAX Exposed Paddle
EP
6
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2.5Gbps Laser Driver with Automatic Modulation Control MAX3865
Table 1. Mode Selection
EN0 0 0 1 1 EN1 0 1 0 1 OPERATING MODE Shutdown Manual APC AMC DESCRIPTION Bias and modulation currents off BIASMAX programs laser bias, MODMAX programs modulation APCSET programs laser bias, MODMAX programs modulation AMCSET programs modulation current and APCSET programs bias
Detailed Description
The MAX3865 laser driver consists of two main parts: a high-speed modulation driver and biasing block as shown in Figure 4. Outputs to the laser diode are a switched modulation current and a steady bias current. Two servo loops may be enabled to control bias and modulation currents for constant optical power and extinction ratio. The MAX3865 requires a laser with a built-in monitor diode to provide feedback about the optical output. The average laser power, as sensed by the monitor diode, is controlled by the APC servo loop. Peak-topeak modulation current is controlled by the AMC servo loop. The modulation output stage uses a programmable current source with a maximum current of 60mA. A high-speed differential pair switches this source to the laser diode. The clock and data inputs to the modulation driver may use CML, PECL, and other logic levels. The optional clock signal can be used to synchronize data transitions for minimum pattern-dependent jitter.
Operating Mode
The MAX3865 can be set in four operating modes, depending on applications requirements. Mode selection is by two TTL-compatible inputs (see Table 1).
APC Loop
In APC mode, a servo loop maintains the average current from the monitor diode at a level set by the APCSET input. Laser bias current is varied in this mode to maintain the monitor diode current. The BIASMAX input must be set to a value larger than the maximum expected bias current. In this mode, BIASMAX limits the maximum bias current to the laser if the control loop fails. The FAIL pin will go low if average IMD IAPCSET.
Mark-Density Compensation
Average power control assumes 50% mark density for times greater than about 100ns. For long patterns or situations where 50% mark density does not apply, the MAX3865 provides mark-density compensation. The APCSET reference is increased by an amount proportional to the mark density multiplied by the modulation amplitude. The AMCSET input is used as an estimate of the peak-to-peak modulation current when the mark density is not 50%. Mark-density compensation is active in both APC and AMC control modes.
Clock/Data Input Logic Levels
The MAX3865 is directly compatible with VCC-referenced CML. Other logic interfaces are possible. For VCC-referenced CML or AC-coupled logic, tie VDR and VCR to VCC. For other DC-coupled differential signals, float VDR and VCR (Figure 5). To prevent excess power dissipation in the input matching resistors, keep the instantaneous input voltage within 1.2V of VDR or VCR as specified in the electrical characteristics.
AMC Loop
In AMC mode, a servo loop maintains the peak-to-peak current from the monitor diode at a level set by the AMCSET input. Laser modulation current is varied in this mode to maintain the monitor diode current. The MODMAX input must be set to a value larger than the maximum expected modulation current. In this mode, MODMAX limits the maximum modulation current to the laser if the control loop fails. The FAIL pin will go low if peak-to-peak IMD IAMCSET. The APC loop is active when in the AMC mode. In AMC mode, mark-density compensation is automatic.
Optional Input Data Retiming
To eliminate pattern-dependent jitter in the input data, a synchronous differential clock signal should be connected to the CLK+ and CLK- inputs, and the RTEN control input should be tied high. Input data retiming occurs on the rising edge of CLK+. If RTEN is tied low, the retiming function is disabled and the input data is directly connected to the output stage. When no clock is available, tie CLK+ to VCC, ground CLK- through a 1.5k resistor, and leave VCR open.
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7
2.5Gbps Laser Driver with Automatic Modulation Control MAX3865
Warning Outputs
A TTL-compatible, active-low warning flag, FAIL, is set when: * One or more of the programmable currents is set at greater than 150% of the rated maximum for the chip. A shorted programming resistor would cause this warning. In this case, the bias and modulation outputs are shut down to protect the laser. * Average IMD IAPCSET in the APC or AMC mode. This could be caused by too low a setting for maximum IBIAS or by a laser that has exceeded its useful life. * Peak-to-peak IMD IAMCSET in the AMC mode. This could be caused by too low a setting for IMODMAX or by a laser which has exceeded its useful life. The FAIL flag also is set for a few microseconds following power-up, until the servo loops settle. The BIASMON and MODMON pins can be used to monitor the laser current and predict the end of the useful laser life before a failure occurs.
Table 2. Optical Power Relations
PARAMETER Average Power Extinction Ratio Optical Power of a "1" Optical Power of a "0" Optical Amplitude Laser Slope Efficiency Laser to Monitor Diode Transfer SYMBOL PAVG re P1 P0 Pp-p MON RELATION PAVG = (P0 + P1)/2 r e = P1 / P 0 P1 = 2PAVG re/( re + 1) P0 = 2PAVG/( re + 1) Pp-p = P1 - P0 = Pp-p/IMOD MON = IMD / PAVG
Note: Assuming a 50% average input duty cycle and mark density.
Design Procedure
When designing a laser transmitter, the optical output is usually expressed in terms of average power and extinction ratio. Table 2 gives relationships that are helpful in converting between the optical power and the
+5V
modulation current. These relationships are valid if the mark density and duty cycle of the optical waveform are 50%. For a desired laser average optical power, PAVG, and optical extinction ratio, re, the required modulation current can be calculated based on the laser slope efficiency, , using the equations in Table 2.
+5V
RTEN
EN1
EN0
VCC
20 MODN IMODQ MODQ 20 IBIAS 20 15
DATA-
DATA+
MAX3865
CLK+
BIAS BIAS_X IMD MD
CLKMODMAX BIASMAX AMCSET RAMCSET APCSET RAPCSET
RMODMAX
RBIASMAX
Figure 3. DC-Coupled Laser Circuit 8 _______________________________________________________________________________________
2.5Gbps Laser Driver with Automatic Modulation Control
Laser Current Requirements
Bias and modulation current requirements can be determined from the laser threshold current and slope efficiency. The modulation and bias currents under a single operating condition are: P r -1 IMOD = 2 x AVG x e re + 1 * For DC-coupled laser diodes: IBIAS > ITH where ITH is the laser threshold current. * For AC-coupled laser diodes: I IBIAS > ITH + MOD 2 Given the desired parameters for operation of the laser diode, the programming of the MAX3865 is explained in the following text. formed by the LC circuit must be low enough to limit the droop. Droop = Number _ CID Data _ Rate x LP x C
MAX3865
If droop = 6.7%, number_CID = 100 and data_rate = 2.5Gbps, then possible values for LP and C may be LP = 6H and C = 0.056F. Both L and C must be increased in value to reduce droop without ringing.
Programming the Maximum Bias Current
In AMC (or APC) mode, the bias current needs a limit if the loop becomes open. RBIASMAX sets the maximum allowed bias current. The bias current is proportional to the current through RBIASMAX. An internal current regulator maintains the band-gap voltage of 1.2V across the programming resistors. Select the maximum IBIAS programming resistor as follows: IBIASMAX = 480 x 1.2V RBIASMAX + 2k
Current Limits
To keep the modulation current in compliance with the programmed value, the following constraint on the total modulation current must be made: DC-Coupled Laser Diodes: VCC - VDIODE - IMOD (RD + RL) - IBIAS RL 1.8V * For VDIODE--Laser diode bias point voltage (1.2V typ) RL--Laser diode bias-point resistance (5 typ) RD--Series matching resistor (15 typ) AC-Coupled Laser Diodes: To allow larger modulation current, the laser can be AC-coupled to the MAX3865 as shown in the Typical Application Circuit. In this configuration, a constant current is supplied from the inductor LP. The requirement for compliance in the AC-coupled circuit is as follows: I VCC - MOD x (RD + RL ) 1.8V 2 The AC-coupling capacitor and bias inductor form a second-order high-pass circuit. Pattern-dependent jitter results from the low-frequency cutoff of this high-pass circuit. To prevent ringing:
Alternatively, a current DAC forcing I DAC from the BIASMAX pin may set the current maximum: IBIASMAX = 480 IDAC When the AMC or APC loop is enabled, the actual bias current is reduced below the maximum value to maintain a constant average current from the monitor diode. With closed-loop control, the bias current will be determined by the transfer function of the monitor diode to laser-diode current. For example, if the transfer function to the monitor diode is 10.0A/mA, then setting IMD for 500A will result in IBIAS equal to 50mA. In manual mode, the bias current IBIAS is IBIASMAX as set by RBIASMAX.
Programming the Average Monitor Diode-Current Set Point
The APCSET pin controls the set point for the average monitor diode current when in AMC or APC mode. The APCSET current is externally established in the same manner as the BIASMAX pin. The average monitor diode current IMD can be programmed with a resistor as follows: average _ IMD = 5 x 1.2V RAPCSET + 2k
(RD + RL ) 2 x
LP C
For deviation from 50% duty cycle or for runs of consecutive identical digits (CID), the low-frequency corner
Alternatively, a current DAC at the APCSET pin can set the monitor diode current by: average IMD = 5 IDAC
9
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2.5Gbps Laser Driver with Automatic Modulation Control MAX3865
Mark-Density Compensation in APC Mode
When mark density is expected to deviate from 50% for periods exceeding 5% of the APC time constant, the AMCSET pin should be programmed to compensate the APC set point. The time constant is determined by the laser to monitor diode gain. APC = 1.5ns GMD
Table 3. Connection of the MD_X Pin
LASER-TO-MONITOR DIODE-CURRENT GAIN <0.005 0.005 to 0.02 >0.02 MD_X SHUNT CONNECTION GND or Open (Open or GND) or MD MD
Programming the Peak-to-Peak Monitor Diode-Current Set Point
The AMCSET pin controls the set point for the peak-topeak monitor diode current in AMC mode. The peak-topeak value of the monitor diode current can be programmed with a resistor as follows: IMD(p-p) = 5 x 1.2V RAMCSET + 2k
GMD =
IMONITOR ILASER
(For example, APC = 150ns for GMD = 0.01mA/mA.) Set the estimated peak-to-peak monitor diode current by the following equation: Estimated IMD(p-p) = 5 x 1.2V RAMCSET + 2k
Alternatively a current DAC at the AMCSET pin can set the monitor diode current by: IMD(p-p) = 5 IDAC
Alternatively, a current DAC at the AMCSET pin can set the monitor diode current by: Estimated IMD(p-p) = 5 IDAC
Laser Gain Compensation
The MAX3865 may be used in closed-loop operation with a wide variety of laser-to-monitor diode gains. Table 3 shows the connection of the MD_X pin for different current-gain ranges.
Programming the Maximum Modulation Current
In AMC mode, the modulation current needs a limit if the loop becomes open. RMODMAX sets the maximum allowed modulation current. The modulation current is proportional to the current through RMODMAX. Select the maximum IMOD programming resistor as follows: IMODMAX = 320 x 1.2V RMODMAX + 2k
Current Monitor Outputs
The MAX3865 provides bias and modulation current monitors. The BIASMON output sinks a current proportional to the bias current: I IBIASMON = BIAS 48 The MODMON pin sinks a current proportional to the laser modulation current: I IMODMON = MOD 32 The BIASMON and MODMON pins should not be allowed to drop below 1.8V. They should be tied to VCC when not in use.
Alternatively, a current DAC forcing I DAC from the MODMAX pin may set the current maximum IMODMAX = 320 IDAC When the AMC loop is enabled, the actual modulation current is reduced from the maximum value to maintain constant peak-to-peak current from the monitor diode. With closed-loop control, the modulation current will be determined by the transfer function of the monitor diode to laser diode current. For example, if the transfer function to the monitor diode is 10.0A/mA, then setting IMD for 500A will result in IMOD equal to 50mA. In manual mode, the modulation current IMOD is set by RMODMAX.
10
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2.5Gbps Laser Driver with Automatic Modulation Control MAX3865
VCC
RTEN o DATA D MUX Q 1 MODN MODQ IMODO
20
CD CLK BIAS MONITOR DIODE FEEDBACK MODULATION CONTROL MD IBIAS IMD
RD
CONTROL LOGIC
-
SHUTDOWN EN0 EN1 FAIL LOOP MONITOR OVERCURRENT x5 x5
+ -
BIAS CONTROL x320
+
x480
VBG
VBG
MAX3865
VBG
VBG
RAMCSET
RAPCSET
RMODMAX
RBIASMAX
Figure 4. Functional Diagram
VCC
VCC MODQ MODN GND
VDR
50 DATA+
50
DATA-
IMOD
DATA AND CLOCK INPUT CIRCUITS ARE EQUIVALENT
GND
GND
Figure 5. Equivalent Input Circuit
Figure 6. Equivalent Modulation Output Circuit 11
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2.5Gbps Laser Driver with Automatic Modulation Control MAX3865
Applications Information
Layout Considerations
To minimize loss and crosstalk, keep the connections between the MAX3865 output and the laser diode as short as possible. Use good high-frequency layout techniques and multilayer boards with uninterrupted ground plane to minimize EMI and crosstalk. Circuit boards should be made using low-loss dielectrics. Use controlled-impedance lines for the clock and data inputs as well as the modulation output. 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.
Chip Information
TRANSISTOR COUNT: 1690 Substrate Connected To GND PROCESS: Bipolar DIE SIZE: 81mil 103mil
References
For further information, refer to the application notes for fiber optic circuits, HFAN-02, on the Maxim web page. Laser Safety and IEC 825 Using the MAX3865 laser driver alone does not ensure that a transmitter design is compliant with IEC 825. The entire transmitter circuit and component selections must be considered. Each customer must determine the level of fault tolerance required by their application,
Pin Configuration
MODMAX BIASMAX
MD_X
VCC
MD
32
31
30
29
28
27
26
25 24 23 22 21
GND
TOP VIEW
AMCSET
APCSET
VCC DATADATA+ VDR VCR CLK+ CLKVCC
1 2 3 4 5 6 7 8 10 11 12 13 14 15 16
GND GND VCC MODQ MODN VCC BIAS BIAS_X
MAX3865
20 19 18 17
9
EN0
EN1
FAIL
BIASMON
MODMON
GND
THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY GROUND ON THE CIRCUIT BOARD
12
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RTEN
GND
2.5Gbps Laser Driver with Automatic Modulation Control
Chip Topography
MODMAX BIASMAX AMCSET APCSET MD_X
MAX3865
GND
GND
N.C.
N.C.
BP10
BP11
BP12
BP13
BP14
BP15
BP16
BP17
BP18
BP19
BP20
BP9
N.C.
VCC
MD
VCC
BP8
BP21
N.C.
DATA-
BP7
BP22
N.C.
DATA+
BP6
BP23
VCC
VDR
BP5
BP24
MODQ 81mil 2.06mm
VCR
BP4
BP25
MODN
CLK+
BP3
BP26
VCC
CLK-
BP2
BP27
BIAS
VCC
BP1
BP28
BIAS_X
BP40
BP39
BP38
BP37
BP36
BP35
BP34
BP33
BP32
BP31
BP30
RTEN
FAIL
EN0
EN1
BIASMON
GND
GND
GND
GND
MODMON
GND
103mil 2.62mm
Note: N.C. means no external connection permitted. Leave these pads unconnected.
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GND
BP29
13
2.5Gbps Laser Driver with Automatic Modulation Control MAX3865
Pad Coordinates
NAME VCC CLKCLK+ VCR VDR DATA+ DATAVCC N.C. GND BIASMAX MODMAX APCSET AMCSET VCC MD_X MD GND N.C. N.C. PAD BP1 BP2 BP3 BP4 BP5 BP6 BP7 BP8 BP9 BP10 BP11 BP12 BP13 BP14 BP15 BP16 BP17 BP18 BP19 BP20 COORDINATES (m) 46, 46 46, 241 46, 435 46, 629 46, 824 46, 1018 46, 1213 46, 1407 151, 1607 346, 1607 540, 1607 735, 1607 929, 1607 1123, 1607 1318, 1609 1512, 1609 1707, 1607 1901, 1607 2095, 1607 2290, 1607 NAME N.C. N.C. VCC MODQ MODN VCC BIAS BIAS_X GND GND MODMON BIASMON FAIL GND GND EN1 EN0 RTEN GND GND PAD BP21 BP22 BP23 BP24 BP25 BP26 BP27 BP28 BP29 BP30 BP31 BP32 BP33 BP34 BP35 BP36 BP37 BP38 BP39 BP40 COORDINATES (m) 2382, 1423 2382, 1229 2382, 1034 2382, 840 2382, 646 2382, 451 2382, 257 2382, 62 2287, -153 2093, -153 1898, -153 1704, -153 1510, -153 1315, -153 1121, -153 926, -153 732, -153 538, -153 343, -153 149, -153
Coordinates are for the center of the pad. Coordinate 0, 0 is the lower left corner of the passivation opening for pad 1.
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2.5Gbps Laser Driver with Automatic Modulation Control
Package Information
MAX3865
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2.5Gbps Laser Driver with Automatic Modulation Control MAX3865
Package Information (continued)
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.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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