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19-1249; Rev 0b; 10/97
622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown
________________General Description
The MAX3766 is a complete, easy-to-program laser driver for fiber optic LAN transmitters, optimized for operation at 622Mbps. It includes a laser modulator, automatic power control (APC), and a failure indicator with latched shutdown. Laser modulation current can be programmed up to 60mA at 622Mbps. A programmable modulation temperature coefficient can be used to keep the transmitted extinction ratio nearly constant over a wide temperature range. The modulator operates at data rates up to 1.25Gbps at reduced modulation current. APC circuitry uses feedback from the laser's monitor photodiode to adjust the laser bias current, producing constant output power regardless of laser temperature or age. The MAX3766 supports laser bias currents up to 80mA. The MAX3766 provides extensive laser safety features, including a failure indicator with latched shutdown and a smooth start-up bias generator. These features help ensure that the transmitter output does not reach hazardous levels. The MAX3766 is available in a compact 20-pin QSOP and dice.
KIT ATION EVALU BLE AVAILA
____________________________Features
o 60mA Modulation Current o 80mA Bias Current o 200ps Edge Speed o Modulation-Current Temperature Compensation o Automatic Power Control o Laser-Fail Indicator with Latched Shutdown o Smooth Laser Start-Up
MAX3766
Ordering Information
PART MAX3766EEP MAX3766E/D TEMP. RANGE -40C to +85C -40C to +85C PIN-PACKAGE 20 QSOP Dice*
*Dice are designed to operate over this range, but are tested and guaranteed at TA = +25C only. Contact factory for availability.
________________________Applications
622Mbps ATM Transmitters 1.25Gbps Fiber Optic LAN Transmitters 1.25Gbps Ethernet Transmitters
TOP VIEW
BIASMAX 1 TC 2 REF2 3 20 REF1 19 POWERSET 18 MD 17 GNDOUT
Pin Configuration
Typical Application Circuits appear at end of data sheet.
MOD 4 GND 5 IN- 6 IN+ 7 GND 8 VCC 9 ENABLE 10
MAX3766
16 BIAS 15 OUT+ 14 OUT13 VCCOUT 12 FAIL 11 SAFETY
QSOP
________________________________________________________________ Maxim Integrated Products
1
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown MAX3766
ABSOLUTE MAXIMUM RATINGS
Supply Voltage, VCC, VCCOUT .................................-0.5V to 7.0V Voltage at IN+, IN-, ENABLE, SAFETY, FAIL ...........................................-0.5V to (VCC + 0.5V) Voltage at MOD, BIASMAX, POWERSET, TC ..........-0.5V to 4.0V Current out of REF1, REF2 .................................-0.1mA to 10mA Current into OUT+, OUT- ....................................-5mA to 100mA Current into BIAS.................................................-5mA to 130mA Current into MD .......................................................-5mA to 5mA Current into FAIL ...................................................-5mA to 30mA Current into SAFETY..............................................-5mA to 10mA Continuous Power Dissipation (TA = +85C) QSOP (derate 9.1mW/C above +85C) .......................590mW Operating Junction Temperature Range ...........-40C to +150C Processing Temperature (dice) .......................................+400C Storage Temperature Range .............................-55C to +150C Lead Temperature (soldering, 10sec) .............................+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.
ELECTRICAL CHARACTERISTICS
PARAMETER RECOMMENDED OPERATING CONDITIONS Supply Voltage, VCC Ambient Operating Temperature Differential Input Signal Amplitude Input Common-Mode Voltage Enable Input High Enable Input Low Voltage at OUT+, OUTVoltage at BIAS FAIL Load Data Rate Data Duty Cycle Laser to PIN Coupling DC PARAMETERS Supply Current FAIL Output High FAIL Output Low Bias-Current Range Bias Current when Driver is Disabled or Shut Down Modulation-Current Programmable Range Modulation Current Modulation Current when Driver is Disabled or Shut Down Minimum Modulation-Current Temperature Compensation Maximum Modulation-Current Temperature Compensation RTC = 0 RTC = open -50 5600 Input data high (Note 3) Input data low (Note 2) 2 1 ICC (Note 2) R F AIL = 5.1k IMD > 15A, R F AIL = 5.1k (Note 3) 0.5 0.1 21 4.3 0.33 0.44 80 10 60 200 10 25 32 mA V V mA A mA A A ppm/C ppm/C While using APC Not tested All DC testing uses 5.1k load VCC - 2.5 VCC - 2.5 2.7 to 20 DC to 1.25 50 0.001 to 0.1 (Note 1) VIN+ - VIN-, common-mode input = VCC - 1.3V, Figure 1 Referenced to VCC 4.5 -40 500 -1.4 2.0 0.8 5.0 25 1000 -1.3 5.5 85 1800 -1.19 V C mV V V V V V k Gbps % mA/mA CONDITIONS MIN TYP MAX UNITS
2
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown
ELECTRICAL CHARACTERISTICS (continued)
PARAMETER Monitor-Diode Current Programmable Range Monitor-Diode Bias Voltage Upper MD Voltage for Failure Lower MD Voltage for Failure Range of MD for No Failure REF1 Voltage for Failure REF1 Reference Voltage REF2 Reference Voltage AC PARAMETERS (Notes 4, 5, and 6) IMOD = 60mA Output Edge Speed (20% to 80%) Output Aberrations Pulse-Width Distortion Random Jitter IMOD = 30mA IMOD = 10mA IMOD = 30mA IMOD = 60mA IMOD = 30mA IMOD = 10mA RMS, TA = +25C, VCC = +5V, IMOD = 30mA 210 160 125 10 5 20 80 2 80 80 120 3 ps ps 400 300 250 % ps VCC - VMD Referenced to VCC Referenced to VCC Width of operating window, centered at nominal VMD Referenced to nominal VREF1 TA = +25C, VREF1 TA = +25C, VREF2 0.5 2.8 2.1 3.1 2.4 3.4 2.7 300 CONDITIONS MIN 15 1.5 -1.2 -2.8 2.1 TYP MAX 2000 2.3 UNITS A V V V mV V V V
MAX3766
3
Note 1: Dice are tested at room temperature only (TA = +25C). Note 2: VCC = +5.5V, RBIASMAX = 887, RMOD = 887, RPOWERSET = 287, RTC = 0, VBIAS = VOUT+ = VOUT- = 3.0V. Supply current excludes IBIAS, IOUT+, IOUT-, and IFAIL. Note 3: Total output current must be reduced at high temperatures with packaged product to maintain maximum junction temperature of Tj = +150C. See the Design Procedure section. Note 4: All AC parameters are measured with a 25 load. IMOD is the AC current amplitude at either OUT pin. The AC voltage at OUT is greater than VCC - 2.5V. Note 5: Pulse-width distortion is measured at the 50% crossing point. Data input is a 155MHz square wave, with tR 300ps. Note 6: AC specifications are guaranteed by design and characterization.
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown MAX3766
Typical Operating Characteristics
(Typical Operating Characteristics are measured on the MAX3766 evaluation kit, VCC = +5.0V, TA = +25C, unless otherwise noted.)
SUPPLY CURRENT vs. TEMPERATURE (EXCLUDES OUTPUT CURRENTS AND IFAIL)
MAX3766-01
MODULATION CURRENT TEMPCO vs. RTC
MAX3766-02
DIE MODULATION CURRENT vs. RMOD (TJ = +25C)
LASER MODULATION CURRENT (mAp-p) TEMPCO = 3000ppm/C TEMPCO = 0ppm/C
MAX3766-03
40
6000
100
SUPPLY CURRENT (mA)
30 TEMPCO (ppm/C) 4000
TEMPCO = 5600ppm/C 10
20
2000
10 0 0 -40 -20 0 20 40 60 80 100 1 10 100 1k RTC () 10k 100k 1M AMBIENT TEMPERATURE (C)
1 1 10 100 1k 10k 100k RMOD ()
MODULATION CURRENT vs. RMOD (20 QSOP, TA = +25C)
LASER MODULATION CURRENT (mAp-p) TEMPCO = 0ppm/C
MAX3766-04
MODULATION CURRENT vs. TEMPERATURE
RTC = 100 MODULATION CURRENT (mAp-p) 60 RTC = 330 50 RTC = 1k RTC = 3.3k 40 RTC = 10k RTC = 100k
MAX3766-05
MODULATION EDGE SPEED AND PWD vs. AMPLITUDE
MAX3766-06
100
70
250
20% TO 80% EDGE SPEED (ps)
200
TA = +85C TA = +25C
TEMPCO = 3000ppm/C 10 TEMPCO = 5600ppm/C
150
100 TA = -40C 50 PULSE-WIDTH DISTORTION (ps) 0 10 20 30 40 50 60
30
1 1 10 100 1k 10k 100k RMOD ()
20 -40 -20 0 20 40 60 80 100 AMBIENT TEMPERATURE (C)
0 MODULATION CURRENT (mAp-p)
EYE DIAGRAM (622Mbps, 1300nm LASER, 470MHz FILTER)
MAX3766-07
EYE DIAGRAM (1.244Gbps, 25 LOAD, IMOD = 60mA)
2 - 1 PRBS
31
MAX3766-08
MONITOR CURRENT vs. RPOWERSET
MAX3766-09
10
2 - 1 PRBS
31
MD CURRENT (mA)
1
125W/div
60mA/div
0.1
0.01 161ps/div 81ps/div 0.1 1 10 RPOWERSET (k) 100 1000
4
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown
Typical Operating Characteristics (continued)
(Typical Operating Characteristics are measured on the MAX3766 evaluation kit, VCC = +5.0V, TA = +25C, unless otherwise noted.)
MAX3766
BIAS CURRENT vs. RBIASMAX (NO APC, OPEN-LOOP CONFIGURATION)
MAX3766-10
MONITOR CURRENT vs. TEMPERATURE
MAX3766-11
DATA-DEPENDENT JITTER vs. TEMPERATURE (CMD = 0.1F)
240 220 DDJ (ps) 200 180 160 140 2 PRBS PATTERN 72 CONSECUTIVE ZEROS
13
MAX3766-12
100
10
260
1 IBIAS (mA) IMD (mA)
NOMINAL = 1mA
223- 1 PRBS PATTERN
10
0.1
NOMINAL = 200A NOMINAL = 20A
0.01 NOMINAL = 2A 1 1 10 100 1k 10k 100k RBIASMAX () 0.001 -40 -20 0 20 40 60 80 100 AMBIENT TEMPERATURE (C)
120 100 -40 -20 0 20 40 60 80 100 AMBIENT TEMPERATURE (C)
UNSUCCESSFUL STARTUP
VENABLE VMD FAIL
MAX3766-13
SUCCESSFUL STARTUP
MAX3766-14
SMOOTH STARTUP
MAX3766-15
VENABLE VMD FAIL
OPTICAL OUTPUT WITH DATA ON 100W/ div
VSAFETY ( = 30s)
VSAFETY ( = 1500s)
OPTICAL OUTPUT WITH DATA OFF
DATA OUT (AC COUPLED)
DATA OUT (AC COUPLED) 50s/div 5s/div
10s/div
ABRUPT SHUTDOWN
MAX3766-16
REFERENCE VOLTAGE vs. TEMPERATURE
MAX3766-17
3.40 3.20 3.00 VOLTAGE (V) 2.80 2.60 2.40 2.20 2.00 VREF2 RTC = OPEN
VREF1
OPTICAL OUTPUT WITH DATA ON 100W/ div
OPTICAL OUTPUT WITH DATA OFF
5s/div
-45
-25
-5
20
40
60
80
100
AMBIENT TEMPERATURE (C)
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown MAX3766
Pin Description
PIN 1 NAME BIASMAX FUNCTION The current into BIASMAX sets the maximum laser bias current. Connecting BIASMAX directly to REF1 allows the largest possible bias current. The resistance (RTC) between TC and REF1 programs the temperature coefficient of REF2. Connecting TC directly to REF1 produces the minimum tempco. Leaving TC unconnected produces the maximum tempco. REF2 is the reference voltage used to program the modulation current. The tempco of REF2 is programmed by RTC. The current into MOD programs the laser modulation current. Connect MOD to REF2 with a resistor or potentiometer. Ground. All grounds must be connected. Inverting Data Input Noninverting Data Input Positive Supply Voltage. All VCC pins must be connected. ENABLE is a TTL-compatible input. When low or open, this pin disables the output modulation and bias current. A capacitor to ground at SAFETY determines the turn-on delay for the safety circuits. If SAFETY is grounded or TTL low, internal safety shutdown features are disabled. A TTL high at SAFETY enables the internal safety shutdown features. The FAIL output asserts low if the voltage at MD is above or below nominal. FAIL also asserts if REF1 is inadvertently tied to the positive supply. FAIL has TTL-compatible output voltage levels. Supply Voltage for the Output Current Drivers Inverting Modulation-Current Output Noninverting Modulation-Current Output Connection for the DC Laser Bias Current Ground for the Output Current Drivers Input for the laser monitor photodiode current. The current into POWERSET programs the average optical output power when automatic power control is used. REF1 is a voltage reference used to program laser bias current and average power.
2
TC
3 4 5, 8 6 7 9 10
REF2 MOD GND ININ+ VCC ENABLE
11
SAFETY
12 13 14 15 16 17 18 19 20
FAIL VCCOUT OUTOUT+ BIAS GNDOUT MD POWERSET REF1
6
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown
PECL Input Buffer
VOLTS VIN+ VINVIN+ VINSINGLE-ENDED INPUT 500mV MIN 1800mV MAX DIFFERENTIAL INPUT 250mV MIN 900mV MAX
MAX3766
The differential PECL input signals are connected to the high-speed PECL input buffer at IN+ and IN-. The input impedance at IN+ and IN- is greater than 100k, and the input bias current is less than 10A. The MAX3766's data inputs are not self-biasing. The common-mode input should be set by the external PECL termination circuitry. To obtain good AC performance, inputs should always be greater than 2.2V and less than VCC.
RESULTING SIGNAL VIN+ - VIN500mV MIN 1800mV MAX
Laser Modulation-Current Driver
The laser modulation-current driver consists of a current mirror and an emitter coupled pair. The mirror has a gain of +30mA/mA. Modulation-current amplitude is programmed with external resistor RMOD connected from REF2 to MOD. RMOD can be estimated as follows: 1.55V 30 RMOD =
TIME
IOUT+ IMOD
IMOD
( ) - 520
Figure 1. Required Input Signal and Output Polarity
_______________Detailed Description
Figure 2 is a functional block diagram of the MAX3766 laser driver. The major functional blocks are the reference generator, PECL input buffer, laser-bias circuit, modulation-current driver, automatic power control (APC), failure detection, and safety circuit.
Reference Generator
The MAX3766 provides adjustments for maximum laser-bias current, laser modulation current, and average laser power. To program these adjustments, simply use the currents obtained by inserting a resistor in series with integrated voltage references REF1 and REF2. The temperature coefficient (tempco) of REF1 compensates for the tempco of the bias, modulation, and APC current mirrors. Therefore, a programming current derived from REF1 is constant with temperature. REF2 provides a positive tempco, which can be applied to the modulation current. A positive modulation-current tempco will compensate for the thermal characteristics of typical laser diodes. The modulationcurrent tempco is programmed by an external resistor (RTC), which is connected from REF1 to TC. RTC and an internal 2k resistor form a weighted sum of the temperature-compensated reference (REF1) and the temperature-increasing reference, which is buffered and output at REF2. REF1 and REF2 are stable with no bypass capacitance. Bypass filtering REF1 or REF2 is not required.
with RTC = 0. The MAX3766 AC output drives up to 60mA of laser current. Pulse-width distortion and overshoot are lowest between 30mA and 60mA. However, output edge speed increases at lower currents. When the output current is between 2mA and 60mA, the edge speed is suitable for communications up to 622Mbps. Edge speeds below 30mA are suitable for communications up to 1.25Gbps (see Typical Operating Characteristics). The modulation-current tempco can be programmed with an external resistor R TC , as described in the Reference Generator section. An internal 520 resistor is included to limit the maximum modulation current if MOD is connected directly to REF2. If the MAX3766 is shut down or disabled, the modulation programming current is shunted to ground. Any remaining modulation current is switched to OUT-. For optimum performance, the voltage at OUT+ and OUT- must always exceed VCC - 2.5V.
Laser Bias Circuit
The laser bias circuit is a current mirror with a gain of +40mA/mA. Redundant controls disable the bias current during a shutdown or disable event: the programming current is switched off, and any remaining bias output current is switched away from the laser. Ensure that the voltage at BIAS always remains above VCC - 2.5V. If the bias circuit is not used, connect BIAS to VCC.
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown MAX3766
RPOWERSET 5.1k VCC FAIL MOD REF2 TC REF1 BANDGAP 2k V REFERENCE GENERATOR V IBIASMAX TEMP TEMP POWERSET MIRROR 1X MD VCC BIASMAX POWERSET CMD RMOD RTC RBIASMAX
VCC
VCC
520
520
300
MONITOR PHOTODIODE IAPC MONITORDIODE AMPLIFIER
SAFETY ENABLE SAFETY START-UP CIRCUIT SHDN FAILURE FAILURE DETECTION
APC
VCC - 2V AUTOMATIC POWER CONTROL BIAS LASER
MAX3766
ILBP
VCC
SHDN IN+
BIAS MIRROR 40X LASER BIAS CIRCUIT
INPECL INPUT BUFFER MODULATION MIRROR 30X MODULATIONCURRENT DRIVER OUT+ OUTVCC
GNDOUT
VCCOUT
Figure 2. Functional Diagram
The available laser bias current is programmed by connecting external resistor R BIASMAX from REF1 to BIASMAX. The BIASMAX programming current is adjusted by the APC circuit and amplified by the laser bias circuit. An internal 520 resistor between BIASMAX and the mirror input at internal node APC limits the maximum laser bias current when BIASMAX is connected directly to REF1. BIASMAX can be directly connected to REF1 in space-constrained designs, causing the maximum
8
programming current (about 2.5mA) to flow into BIASMAX. Selecting a BIASMAX resistor saves power and limits the transmitter's maximum light output. RBIASMAX can be estimated as follows: 1.55V 40 RBIASMAX =
()
I BIASMAX
- 520
This equation applies to maximum bias currents above 10mA.
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown
Automatic Power Control
Transmitters employing a laser with monitor photodiode can use the APC circuit to maintain constant power, regardless of laser threshold changes due to temperature and aging. The APC circuit consists of the POWERSET current mirror and the monitor diode amplifier. The POWERSET current mirror provides an accurate method of programming the back facet monitor photodiode current, which is assumed to be proportional to laser output power. An external resistor from REF1 to POWERSET programs the current in the unity-gain current mirror. RPOWERSET can be estimated as follows: RPOWERSET = 1.55V I MOD - 300 the reference voltage would rise, the current at POWERSET would increase, and the APC loop would attempt to add laser current beyond the intended value. Either failure condition causes the FAIL output to assert TTL low. The FAIL output buffer is an open-collector output and is designed to operate with a 5.1k external pull-up resistor.
MAX3766
Safety/Start-Up Circuit
The safety circuit includes the digital logic needed to provide a latched internal shutdown signal (SHDN) for disabling the laser if a failure condition exists. The MAX3766 produces less than 20A of total laser current when disabled by safety features or by the ENABLE input. Figure 4 is a simplified schematic of the safety circuit. If ENABLE is low or open, the laser bias and modulation outputs are disabled by SHDN, regardless of the state of the safety logic. The TTL-compatible ENABLE input is internally pulled low with a 100k resistor. There are two useful safety configurations: failure indication and latched shutdown.
The monitor-diode amplifier senses the current from the monitor photodiode at MD, provides gain, and adjusts the laser bias programming current (ILBP). The monitordiode amplifier forces the monitor-diode current to equal the current programmed at POWERSET. The monitor-diode amplifier can reduce the laser bias programming current, but cannot increase it. Therefore, the APC circuit can adjust laser bias current between 0 and the setting determined by RBIASMAX. When the APC feedback loop is closed, the voltage at MD is approximately 2V below VCC. If the loop cannot close due to excess or insufficient photocurrent, a failure is detected by the failure-detection circuit. Internal circuitry prevents the voltage at MD from dropping below VCC - 3.2V. The stability and time constant of the APC feedback loop is determined by an external compensation capacitor (CMD) of at least 0.1F. Connect the compensation capacitor from V CC to MD, as shown in Typical Application Circuits, to ensure a smooth startup at power-on or transmitter enable. If a monitor diode is not available, the APC feature can be disabled by connecting RPOWERSET to GND and leaving MD unconnected.
Failure-Indication Configuration Select the failure-indication configuration by connecting SAFETY to ground. In this configuration, a failure condition is reported at FAIL, but does not cause a latched shutdown. This configuration requires no additional circuitry for start-up.
VMD VCC 2V 200mV 200mV FAILURE (INTERNAL)
Failure Detection
Figure 3 shows a simplified schematic of the failuredetection circuit. The failure-detection circuit senses two conditions. First, if the APC control loop cannot control the monitor current due to laser undercurrent, overcurrent, or a fault condition, a window comparator detects that V MD is above or below V CC - 2V and asserts the failure signal. Second, if REF1 is shorted to the positive supply (or any another voltage above the normal operating level), a comparator detects this condition and asserts the failure signal. If left undetected,
0.5V VREF1 VBANDGAP
Figure 3. Failure-Detection Circuit (Simplified)
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown MAX3766
OPEN COLLECTOR FAIL SIMPLIFIED OPEN-COLLECTOR OUTPUT CIRCUIT
FAILURE (INTERNAL) VCC 200k SAFETY
S
RESET DOMINATE RS FLIP-FLOP Q
OUT
R CSAFETY OPEN COLLECTOR SHDN (INTERNAL) IN
ENABLE 100k
Figure 4. Simplified Safety Circuit Schematic
Latched Shutdown Configuration This configuration is shown in the Typical Application Circuits (configured for best performance), and can be selected by connecting a capacitor (C SAFETY) to ground at SAFETY. In this configuration, the transmitter is shut down when a failure is detected. It can be restarted only by a power-on cycle or a toggle of the ENABLE input.
During start-up, FAIL is asserted until laser power reaches the programmed level. The safety circuit must be disabled at power-on or at transmitter enable, providing enough time for the APC circuit to reach the programmed laser power level. In space-constrained designs, CSAFETY can be selected to provide a shutdown delay. When power is initially applied, or when the ENABLE signal is toggled from a logic 0 to a logic 1, the voltage at SAFETY is low, and rises with a time constant set by CSAFETY and an internal 200k pull-up resistor. The SAFETY signal is inverted and resets the input of a reset-dominant RS flip-flop. The internal signal FAILURE from the failure-detection circuit is connected to the set input of the flip-flop. After SAFETY has gone high (allowing time for the APC feedback loop to settle) and if internal signal FAILURE is low, the flip-flop output is low, and the bias and modulation outputs are allowed to remain on. Refer to Figure 5 for a timing diagram of start-up in the latched shutdown configuration.
VCC
VCC ON (OR ENABLE SWITCHED TO ON STATE)
tON LASER BIAS AND MODULATION CURRENT
OUTPUT CURRENTS ENABLED AFTER A FIXED DELAY
FAIL OUTPUT
tAPC
FAIL DEASSERTS WHEN THE APC LOOP SETTLES
SAFETY
tSAFETY
SAFETY FEATURES START CHECKING THE FAILURE SIGNAL AFTER A TIME SET BY A CAPACITOR ON THE SAFETY INPUT. AFTER THIS TIME, THE LASER DRIVER IS DISABLED IF A FAILURE OCCURS.
Figure 5. Start-Up Sequence Timing
The duration of tSAFETY must be about 10 times tAPC for a successful start-up. After start-up, the transmitter operates normally until a failure is detected, causing the output currents to be shut down. The laser-current outputs remain off until the failure condition is eliminated and the ENABLE input is toggled, or until the power is cycled. A potential problem with this transmitterenable method is that a slow-rising power supply may not enable the transmitter.
10
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown
VCC OR ENABLE
__________________Design Procedure
Select Laser
VCC ENABLE
MAX3766
MAX809MEUR-T
RESET CSAFETY
MAX3766*
SAFETY
Select a communications-grade laser with a rise time of 0.5ns or better for 622Mbps applications. The voltage swing at the OUT+ pin affects the output waveform, and is largely determined by the laser resistance, inductance, and modulation current. To obtain the MAX3766's AC specifications, the output voltage at OUT+ must remain above VCC - 2.5V at all times. An approximation for the minimum voltage at OUT+ is given by the following equation (Table 1): VOUT(MIN) = VCC(MIN) - VLASER - IMOD L RL + RD + t r Select a laser that meets the output voltage criteria. A high-efficiency laser requires low modulation current and generates low voltage swing at OUT+. Laser package inductance can be reduced by trimming leads. Typical package leads have inductance of 25nH per inch (1nH/mm). A compensation filter network can also be used to reduce ringing, edge speed, and voltage swing.
*IN LATCHED SHUTDOWN CONFIGURATION
(
)
Figure 6. Reset Pulse Generator
If PC board space is not a constraint, Maxim recommends enabling the transmitter with a reset-pulse generator, such as the MAX809, which generates a reset signal after VCC reaches 4.5V (Figure 6). This method ensures that the transmitter starts correctly, even if the supply ramps very slowly.
Table 1. Output Voltage Approximation
VARIABLE VOUT(MIN) VCC(MIN) VLASER RL IOUT RD L tr DESCRIPTION Approximation for the lowest voltage at the OUT+ pin Minimum power supply Laser forward voltage at operating power Laser dynamic resistance Laser modulation current Any damping resistance or line termination in series with the laser (but not in series with BIAS) Total series inductance of laser, laser package, and board traces to the MAX3766 20% to 80% rise time of the laser modulation current, filtered by a compensation network TYPICAL VALUE 2.2V 4.5V 1.3V 3 30mA 10 6nH 300ps (20% to 80%)
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11
622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown MAX3766
Set Modulation-Current Tempco
Compute the required modulation tempco from the slope efficiency of the laser at TA = +25C and at a hot temperature. Then select the value of RTC from the Typical Operating Characteristics. For example, suppose a laser has a slope efficiency (SE) of 0.021mW/mA at +25C, which reduces to 0.018mW/mA at +85C. The temperature coefficient is given by the following: Laser tempco = - SE25 SE25 * 85 - 25 = -2380ppm / C Select RBIASMAX to provide sufficient current for a hot laser at its end of life. For example, if the expected laser threshold at +85C and end of life is 40mA, then from the Typical Operating Characteristics, RBIASMAX should be 1k or less. If APC is not used, the laser bias current is programmed by R BIASMAX . Select R BIASMAX from the Typical Operating Characteristics.
(SE85
(
)
Set APC Time Constant
Capacitor CMD determines the APC time constant, and must be large enough not to cause data-dependent jitter. For 622Mbps SONET/ATM applications, Maxim recommends selecting CMD 0.1F.
)
* 106
From the Typical Operating Characteristics, the value for RTC, which offsets the tempco of the laser, is 3k. If modulation temperature compensation is not desired, connect TC directly to REF1.
Select CSAFETY When using the latched shutdown configuration, determine the minimum value of CSAFETY from the Typical Operating Characteristics. Calculate CSAFETY as follows:
CSAFETY =
CMD 20k * IMD
Set Modulation Current
The modulation-current amplitude can be programmed with a fixed resistor or adjusted with a potentiometer. A small internal resistance is provided to prevent damage if the potentiometer is adjusted to the end of its range. The value of RMOD can be selected from the Typical Operating Characteristics. Example: A transmitter requires average power of -8dBm (160W), with an extinction ratio of 15. The optical signal output is 280W (see Optical Power Relations). If the slope efficiency is 0.021mW/mA at +25C, then the required modulation current is 0.280mW / 0.021mW/mA = 13.3mA. From the Typical Operating Characteristics, the value of RMOD is selected to be 3k.
For example: If CMD is 0.1F and typical monitor current (IMD) is 100A, then the value of CSAFETY should be 50nF or larger. This ensures that tSAFETY is at least 10 times the tAPC.
Design Bias Filter
To reduce data-dependent jitter, add a filter at BIAS (see Typical Operating Circuit). Maxim recommends a 1H inductor or ferrite bead with a self-resonance frequency of 200MHz or more.
Design Laser-Compensation Filter Network
Laser package lead inductance causes the laser impedance to increase at high frequencies, which leads to ringing, overshoot, and degradation of the output eye. A laser-compensation filter network can be used to reduce the output load seen by the MAX3766 at high frequencies, thereby reducing output ringing and overshoot. The compensation components (RCOMP and CCOMP) are most easily determined by experimentation. Begin with a no-compensation network, and observe the ring frequency (fn) of the laser and laser driver (Figure 7). Begin with RCOMP = 25 and CCOMP = 1/(2fn RCOMP). Increase C COMP until the desired transmitter eye is obtained.
Set Average Laser Power and Maximum Bias Current
When APC is used, the average power control is programmed by RPOWERSET, which is typically a potentiometer. The value of RPOWERSET can be estimated from the Typical Operating Characteristics. Example: Suppose a transmitter's output power will be adjusted to -8dBm (160W) average power during manufacturing. The coupling efficiency from laser to monitor photodiode varies from 0.4A/W to 0.8A/W for the selected laser, causing monitor current to vary between 64A and 128A. From the Typical Operating Characteristics, R POWERSET should be adjustable between 12k and 24k.
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown MAX3766
UNCOMPENSATED OPTICAL POWER P1
CORRECTLY COMPENSATED PAVE OVERCOMPENSATED
P0 500ps/div TIME
Figure 7. Example Laser Compensation
Figure 8. Optical Power Relations
Calculate Power Consumption
The MAX3766's junction temperature must be kept below +150C at all times. Calculate total power dissipated on the MAX3766 by laser power as follows: Power = VCC (ICC + IBIAS + IMOD) - (IMOD / 2 + IBIAS) VLASER. where IBIAS is the maximum bias current allowed by RBIASMAX, IMOD is the AC modulation current, VLASER is the typical laser forward voltage. Junction temperature = power (Watts) * 110 (C/W).
Table 2. Optical Power Definitions
PARAMETER Average Power Extinction Ratio Optical Power of a "1" Optical Power of a "0" Signal Amplitude SYMBOL PAVE re P1 P0 RELATION
PAVE = (P0 + P1) / 2 re = P1 / P0 P1 = 2PAVE re re + 1
__________Applications Information
Optical Power Relations
Many MAX3766 specifications relate to output current amplitude. When working with fiber optic transmitters, the output is normally expressed in terms of average optical power and extinction ratio (Figure 8). Table 2 lists relations that are helpful in converting optical power to output signal amplitude when designing with the MAX3766. The relations are true if the average duty cycle of the input data is 50%.
P0 = 2PAVE / re + 1
(
)
re - 1 re + 1
PINPUT
PINPUT = P1 - P0 = 2PAVE
Input Terminations
The MAX3766's data inputs must be biased externally. Refer to Figure 9 for common input terminations.
Laser Safety and IEC 825
The International Electrotechnical Commission (IEC) determines standards for hazardous light emissions from fiber optic transmitters. Specification IEC 825 defines the maximum light output for various hazard levels. The MAX3766 provides features that aid compliance with IEC 825.
A common safety requirement is single-point fault tolerance, whereby one unplanned short, open, or resistive connection does not cause excess light output. When the MAX3766 is used in the latched shutdown configuration, as shown in Typical Application Circuits, the circuit responds as shown in Table 3. Using the MAX3766 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, 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.
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown MAX3766
Table 3. MAX3766 Fault Response in Latched Shutdown Configuration
PIN BIASMAX REF1 CIRCUIT RESPONSE TO OVERVOLTAGE OR SHORT TO VCC Depending upon the setting of RPOWERSET, there is either no effect, or a latched shutdown. High voltage on REF1 causes a failure and latched shutdown. Modulation current is increased. Either the APC circuit will reduce power levels, or an overcurrent will be detected at MD, causing a failure signal and latched shutdown. Normal condition for circuit operation. Forces output to either constant 1 or 0. APC maintains the power level at the programmed level. Normal condition for circuit operation. No effect on circuit. CIRCUIT RESPONSE TO UNDERVOLTAGE OR SHORT TO GROUND Bias current reduction causes a low laser output, resulting in a latched shutdown. Modulation and bias currents are reduced or off; no hazard exists.
REF2, TC, MOD
Modulation current is reduced; no hazard exists.
ENABLE IN+, INSAFETY FAIL
Modulation and bias currents are shut down. Forces output to either constant 1 or 0. APC maintains the power level at the programmed level. Safety shutdown features are disabled, but a hazard is not created. No effect on circuit. High laser output asserts FAIL. A complete short will destroy the laser, eliminating the hazard. A resistive short may cause a hazard. External circuitry combined with the FAIL signal may be used to protect against a resistive short (Figure 10). Forces output to be logic 1. APC maintains the power level at the programmed level. Voltage decrease at MD causes a failure and output current shutdown. Laser output decreases.
OUT+, BIAS
Voltage increase at these pins will turn off the laser.
OUTMD POWERSET
Normal condition for circuit operation. Voltage increase at MD causes a failure and output current shutdown. Laser output increases, but is limited by the setting of RBIASMAX.
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown MAX3766
VCC PECL OUTPUT 50 VCC - 2V VCC
MAX3766
IN 5.1k
VCC 68
VCC LASER OUT+ 100k FAIL
RF OR NONPECL OUTPUT 180 VCC R2 2.87k
MAX3766
IN FAIL
5.1k
RESET
MAX3766
TTL OR CMOS OUTPUT
R1 10k
MAX3766
IN
Figure 10. External Laser Shutdown Circuit
R3 11.8k
2) Low-frequency oscillation on the bias-current output. Ensure CMD 0.1F. 3) Modulation driver is not needed. If only the bias-current driver and safety circuits are needed, connect IN+ to VCC, and leave IN- unconnected. Connect OUT+ and OUT- to the supply. Leave MOD, TC, and REF2 unconnected. 4) APC is not needed. If only the high-speed modulation driver is used, connect BIAS to VCC, and leave POWERSET, MD, FAIL, and BIASMAX unconnected. Connect SAFETY to ground. 5) Laser edge switching speed is low. Refer to the Design Bias Filter section. It may be necessary to select LBIAS with a higher self-resonating frequency.
SINGLE-ENDED TERMINATION IS SHOWN. THE OTHER INPUT SHOULD BE TERMINATED SIMILARLY, OR CONNECTED TO VCC - 1.3V.
Figure 9. Input Terminations
Layout Considerations
The MAX3766 is a high-frequency product. The performance of the circuit is largely dependent upon the layout of the circuit board. Use a multilayer circuit board with a dedicated ground plane. Use short laser package leads placed close to OUT+ and OUT- to keep output inductance low. Power supplies should be capacitively bypassed to the ground plane with surface-mount capacitors placed near the power-supply pins.
Wire Bonding Die
The MAX3766 uses bondpads with gold metalization. Make connections to the die with gold wire only, using ball bonding techniques. Wedge bonding is not recommended. Pad size is 4 mils (0.1mm) square. Die thickness is typically 15 mils (0.38mm).
Solutions to Common Problems
1) Laser output is ringing and contains overshoot. This is often caused by inductive laser packaging. Try reducing the lead length of the laser pins. Modify the compensation network to reduce the driver's output edge speed (see Design Procedure). This problem can also occur if the voltage at OUT+, OUT-, or BIAS is below VCC - 2.5V. Test this by increasing the supply voltage, or reducing the modulation current.
Interface Models
Figure 11 shows typical models for the inputs and outputs of the MAX3766, including package parasitics. If dice are used, replace the package parasitic elements with bondwire parasitic elements.
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown MAX3766
INPUT OUTPUT BIAS
OUT+ PACKAGE 1.5nH VIN+ 0.2pF 0.4pF VCC VCC 1.5nH VIN0.2pF 0.4pF Q3 VCC - 2.5 250 Q2 VCC 1pF VCC 250 Q1 VCC 0.2pF 1.5nH 1.5nH
OUTPACKAGE 0.2pF
PACKAGE 0.2pF 1.5nH
VCC 1pF
2pF
VCC - 2.5 Q4 VCC - 2.5
Q5
IBIAS IMOD
Q1, Q2 INPUT BIAS CURRENT 1A Q1, Q2 INPUT RESISTANCE 1M
Q3, Q4 OUTPUT RESISTANCE 100k
Q5 OUTPUT RESISTANCE 100k
Figure 11. Interface Models
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown
Typical Application Circuits
FAILURE-INDICATION CONFIGURATION
0.01F VCC CMD VCC LASER
MAX3766
VCCOUT VCC IN+ IN-
ENABLE OUT-
MD OUT+
BIAS VCC
BIASMAX RPOWERSET POWERSET REF1 TC REF2
MAX3766
FAIL SAFETY MOD GNDOUT GND
RFAIL 5.1k
RMOD CONFIGURED FOR MINIMUM COMPONENT COUNT. VCC LASER
LATCHED SHUTDOWN CONFIGURATION
0.01F
CMD RD ROUTMD OUT+ BIAS VCC RCOMP CCOMP LBIAS
VCC
VCCOUT VCC ENABLE OUTIN+ INRBIASMAX BIASMAX RPOWERSET POWERSET REF1 RTC TC REF2 MOD
MAX3766
FAIL SAFETY GNDOUT GND
RFAIL 5.1k
CSAFETY (OPTIONAL--SEE TEXT)
RMOD CONFIGURED FOR BEST PERFORMANCE. RTC SETS THE TEMPERATURE COEFFICIENT OF THE MODULATION CURRENT.
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown MAX3766
___________________Chip Topography
IN+ INGND MOD
GND REF2 TC VCC BIASMAX ENABLE REF1 SAFETY FAIL VCCOUT POWERSET MD 0.056" (1.422mm)
OUT- OUT+
BIAS GNDOUT
0.045" (1.143mm)
TRANSISTOR COUNT: 725 SUBSTRATE CONNECTED TO GND AND GNDOUT.
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown
________________________________________________________Package Information
QSOP.EPS
MAX3766
______________________________________________________________________________________
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622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown MAX3766
NOTES
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.
20 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 1997 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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