MAX3766EEP+ ,622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety ShutdownELECTRICAL CHARACTERISTICSPARAMETER CONDITIONS MIN TYP MAX UNITSRECOMMENDED OPERATING CONDITIONSSup ..
MAX3766EEP+ ,622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety ShutdownApplications622Mbps ATM Transmitters1.25Gbps Fiber Optic LAN TransmittersPin Configuration1.25Gbps ..
MAX3768CUB ,+3.0 V to +5.5 V, 1.25 Gbp/2.5Gbp limiting amplifierApplications†MAX3265EUE -40°C to +85°C 16 TSSOP-EP†Gigabit Ethernet Optical ReceiversMAX3268CUB 0°C ..
MAX3768CUB+T ,+3.0V to +5.5V, 1.25Gbps/2.5Gbps Limiting Amplifiersfeatures include RMS power detectors withprogrammable loss-of-signal (LOS) indication, an ♦ Program ..
MAX3770CEE ,2.125Gbps/1.063Gbps / 3.3V Fibre Channel RepeatersFeaturesThe MAX3770 is a 2.125Gbps Fibre Channel repeater Meet Fibre Channel Jitter Tolerance Requ ..
MAX3772CEE ,Dual-Rate Fibre Channel RepeatersMAX3772–MAX377519-2192; Rev 0; 10/01Dual-Rate Fibre Channel Repeaters
MAX734ESA+ ,12V, 120mA Flash Memory Programming Supply
MAX734ESA+T ,12V, 120mA Flash Memory Programming Supply
MAX735 ,-5V, Inverting, PWM DC-DC ConverterFeatures
. Converts +2.7V to +9V In
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MAX7359BETG+ ,2-Wire Interfaced Low-EMI Key Switch Controller/GPOApplicationsMAX7359EWA+ -40°C to +85°C 25 WLPCell Phones+Denotes a lead(Pb)-free/RoHS-compliant pac ..
MAX7359ETG+ ,2-Wire Interfaced Low-EMI Key Switch Controller/GPOfeatures autosleep and autowake to fur- ♦ FIFO Queues Up to 16 Debounced Key Eventsther minimize th ..
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MAX3766EEP+
622Mbps LAN/WAN Laser Driver with Automatic Power Control and Safety Shutdown
________________General DescriptionThe 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 tem-
perature coefficient can be used to keep the transmit-
ted 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 haz-
ardous levels. The MAX3766 is available in a compact
20-pin QSOP and dice.
________________________Applications622Mbps ATM Transmitters
1.25Gbps Fiber Optic LAN Transmitters
1.25Gbps Ethernet Transmitters
____________________________Features60mA Modulation Current80mA Bias Current200ps Edge SpeedModulation-Current Temperature CompensationAutomatic Power ControlLaser-Fail Indicator with Latched ShutdownSmooth Laser Start-Up
MAX3766
622Mbps LAN/WAN Laser Driver with
Automatic Power Control and Safety Shutdown19-1249; Rev 1; 11/04
EVALUATION KIT
AVAILABLE
Ordering Information*Dice are designed to operate over this range, but are tested and
guaranteed at TA= +25°C only. Contact factory for availability.
+Denotes lead-free package.
Typical Application Circuits appear at end of data sheet.
Pin ConfigurationREF1
POWERSET
GNDOUTMOD
REF2
BIASMAX
TOP VIEW
BIAS
OUT+
OUT-
VCCOUTGND
IN+
IN-
GND
FAIL
SAFETYENABLE
VCC
MAX3766
QSOP
PARTTEMP RANGEPIN-PACKAGEMAX3766EEP-40°C to +85°C20 QSOP
MAX3766EEP+-40°C to +85°C20 QSOP
MAX3766E/D-40°C to +85°CDice*
MAX3766
622Mbps LAN/WAN Laser Driver with
Automatic Power Control and Safety Shutdown
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICSStresses 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.
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= +85°C)
QSOP (derate 9.1mW/°C above +85°C).......................590mW
Operating Junction Temperature Range...........-40°C to +150°C
Processing Temperature (dice).......................................+400°C
Storage Temperature Range.............................-55°C to +150°C
Lead Temperature (soldering, 10sec).............................+300°C
CONDITIONSUNITSMINTYPMAXPARAMETER4.55.05.5Supply Voltage, VCC
(Note 1)°C-402585Ambient Operating Temperature
Referenced to VCCV-1.4-1.3-1.19Input Common-Mode Voltage0.8Enable Input Low
VIN+- VIN-, common-mode input =
VCC- 1.3V, Figure 12.0Enable Input High50010001800Differential Input Signal AmplitudeVCC- 2.5Voltage at BIAS
While using APC%50Data Duty Cycle
GbpsDC to 1.25Data RateVCC- 2.5Voltage at OUT+, OUT-
Not testedmA/mA0.001 to 0.1Laser to PIN Coupling
RFAIL= 5.1kΩV4.3FAILOutput High
(Note 3)mA0.580Bias-Current Range
ICC(Note 2)
IMD> 15µA, RFAIL= 5.1kΩV0.330.44FAILOutput Low212532Supply Current0.110Bias Current when Driver is
Disabled or Shut Down
Input data low (Note 2)µA1200Modulation Current
Input data high (Note 3)mA260Modulation-Current Programmable
Range
RTC= 0Ωppm/°C-50Minimum Modulation-Current
Temperature Compensation10Modulation Current when Driver is
Disabled or Shut Down
All DC testing uses 5.1kΩloadkΩ2.7 to 20FAILLoad
RTC= openppm/°C5600Maximum Modulation-Current
Temperature Compensation
RECOMMENDED OPERATING CONDITIONS
DC PARAMETERS
MAX3766
622Mbps LAN/WAN Laser Driver with
Automatic Power Control and Safety Shutdown
ELECTRICAL CHARACTERISTICS (continued)
Note 1:Dice are tested at room temperature only (TA= +25°C).
Note 2:VCC= +5.5V, RBIASMAX= 887Ω, RMOD= 887Ω, RPOWERSET= 287Ω, RTC= 0Ω, 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 = +150°C. See the Design Proceduresection.
Note 4:All AC parameters are measured with a 25Ωload. IMODis 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.
VCC - VMDV1.52.12.3Monitor-Diode Bias Voltage
Referenced to VCCV-2.8Lower MD Voltage for Failure
Referenced to VCCV-1.2Upper MD Voltage for Failure
Referenced to nominal VREF1V0.5REF1 Voltage for Failure= +25°C, VREF2V2.12.42.7REF2 Reference Voltage
Width of operating window, centered at
nominal VMD= +25°C, VREF1V
CONDITIONS2.83.13.4REF1 Reference Voltage300Range of MD for No Failure
IMOD= 60mA210400
Output Edge Speed (20% to 80%)
IMOD= 10mA
IMOD= 30mA160300
IMOD= 30mA%10Output Aberrations152000Monitor-Diode Current
Programmable Range
UNITSMINTYPMAXPARAMETERIMOD= 60mA580
IMOD= 30mA2080
IMOD= 10mA120
Pulse-Width Distortion
RMS, TA= +25°C, VCC= +5V, IMOD= 30mAps23Random Jitter
AC PARAMETERS (Notes 4, 5, and 6)
MAX3766
622Mbps LAN/WAN Laser Driver with
Automatic Power Control and Safety Shutdown
Typical Operating Characteristics(Typical Operating Characteristics are measured on the MAX3766 evaluation kit, VCC= +5.0V, TA = +25°C, unless otherwise noted.)
SUPPLY CURRENT vs. TEMPERATURE
(EXCLUDES OUTPUT CURRENTS AND IFAIL)
MAX3766-01
AMBIENT TEMPERATURE (°C)
SUPPLY CURRENT (mA)
11k1010010k100k1M
MODULATION CURRENT TEMPCO
vs. RTCMAX3766-02
RTC (Ω)
TEMPCO (ppm/°C)
11k1010010k100k
DIE MODULATION CURRENT vs. RMOD
(TJ = +25°C)MAX3766-03
RMOD (Ω)
LASER MODULATION CURRENT (mAp-p)
TEMPCO = 0ppm/°C
TEMPCO = 5600ppm/°C
TEMPCO = 3000ppm/°C
11k1010010k100k
MODULATION CURRENT vs. RMOD
(20 QSOP, TA = +25°C)MAX3766-04
RMOD (Ω)
LASER MODULATION CURRENT (mAp-p)
TEMPCO = 0ppm/°C
TEMPCO = 3000ppm/°C
TEMPCO = 5600ppm/°C
125μW/div
161ps/div
EYE DIAGRAM
(622Mbps, 1300nm LASER, 470MHz FILTER)MAX3766-07231- 1 PRBS
MODULATION CURRENT
vs. TEMPERATURE
MAX3766-05
AMBIENT TEMPERATURE (°C)
MODULATION CURRENT (mAp-p)
RTC = 100Ω
RTC = 330Ω
RTC = 1kΩ
RTC = 3.3kΩ
RTC = 10kΩ
RTC = 100kΩ
MODULATION EDGE SPEED AND
PWD vs. AMPLITUDE
MAX3766-06
MODULATION CURRENT (mAp-p)
20% TO 80% EDGE SPEED (ps)
TA = +85°C
TA = +25°C
TA = -40°C
PULSE-WIDTH
DISTORTION (ps)
60mA/div
81ps/div
EYE DIAGRAM
(1.244Gbps, 25Ω LOAD, IMOD = 60mA)MAX3766-0831- 1 PRBS
MONITOR CURRENT vs. RPOWERSET
MAX3766-09
RPOWERSET (kΩ)
MD CURRENT (mA)0.1
MAX3766
622Mbps LAN/WAN Laser Driver with
Automatic Power Control and Safety Shutdown11k1010010k100k
BIAS CURRENT vs. RBIASMAX
(NO APC, OPEN-LOOP CONFIGURATION)MAX3766-10
RBIASMAX (Ω)
BIAS
(mA)
MONITOR CURRENT vs. TEMPERATURE
MAX3766-11
AMBIENT TEMPERATURE (°C)
IMD
(mA)0.1NOMINAL = 1mA
NOMINAL = 200μA
NOMINAL = 20μA
NOMINAL = 2μA
DATA-DEPENDENT JITTER
vs. TEMPERATURE (CMD = 0.1μF)
MAX3766-12
AMBIENT TEMPERATURE (°C)
DDJ (ps)
-40-2002040608010013 PRBS PATTERN
72 CONSECUTIVE ZEROS23- 1 PRBS PATTERN
10μs/div
UNSUCCESSFUL STARTUPMAX3766-13
VENABLE
VMD
VSAFETY (τ = 30μs)
FAIL
DATA OUT (AC COUPLED)
5μs/div
100μW/
div
ABRUPT SHUTDOWNMAX3766-16
OPTICAL OUTPUT WITH DATA ON
OPTICAL OUTPUT WITH DATA OFF
50μs/div
SUCCESSFUL STARTUPMAX3766-14VENABLE
VMD
FAIL
VSAFETY (τ = 1500μs)
DATA OUT (AC COUPLED)
5μs/div
100μW/
div
SMOOTH STARTUPMAX3766-15
OPTICAL OUTPUT WITH DATA ON
OPTICAL OUTPUT WITH DATA OFF
REFERENCE VOLTAGE
vs. TEMPERATURE
MAX3766-17
VOLTAGE (V)
RTC = OPEN
VREF1
VREF2
Typical Operating Characteristics (continued)(Typical Operating Characteristics are measured on the MAX3766 evaluation kit, VCC= +5.0V, TA = +25°C, unless otherwise noted.)
MAX3766
622Mbps LAN/WAN Laser Driver with
Automatic Power Control and Safety Shutdown
Pin DescriptionENABLE is a TTL-compatible input. When low or open, this pin disables the output modulation and bias
current.ENABLE10
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.
SAFETY11
The FAILoutput asserts low if the voltage at MD is above or below nominal. FAILalso asserts if REF1 is
inadvertently tied to the positive supply. FAILhas TTL-compatible output voltage levels.FAIL12
Supply Voltage for the Output Current DriversVCCOUT13
Inverting Modulation-Current OutputOUT-14
Ground. All grounds must be connected.GND5, 8
Inverting Data InputIN-6
Noninverting Data InputIN+7
Positive Supply Voltage. All VCCpins must be connected.VCC9
The current into MOD programs the laser modulation current. Connect MOD to REF2 with a resistor or
potentiometer.MOD4
REF2 is the reference voltage used to program the modulation current. The tempco of REF2 is pro-
grammed by RTC.REF23
PINThe 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.2
The current into BIASMAX sets the maximum laser bias current. Connecting BIASMAX directly to REF1
allows the largest possible bias current.BIASMAX1
FUNCTIONNAMEGround for the Output Current DriversGNDOUT17
Input for the laser monitor photodiode current. MD18
The current into POWERSET programs the average optical output power when automatic power
control is used.POWERSET19
REF1 is a voltage reference used to program laser bias current and average power.REF120
Noninverting Modulation-Current OutputOUT+15
Connection for the DC Laser Bias CurrentBIAS16
MAX3766
622Mbps LAN/WAN Laser Driver with
Automatic Power Control and Safety Shutdown
_______________Detailed Description Figure 2 is a functional block diagram of the MAX3766
laser driver. The major functional blocks are the refer-
ence generator, PECL input buffer, laser-bias circuit,
modulation-current driver, automatic power control
(APC), failure detection, and safety circuit.
Reference GeneratorThe MAX3766 provides adjustments for maximum
laser-bias current, laser modulation current, and aver-
age 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 tempera-
ture. REF2 provides a positive tempco, which can be
applied to the modulation current. A positive modula-
tion-current tempco will compensate for the thermal
characteristics of typical laser diodes. The modulation-
current tempco is programmed by an external resistor
(RTC), which is connected from REF1 to TC. RTCand
an internal 2kΩresistor 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.
PECL Input BufferThe 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 100kΩ, and
the input bias current is less than 10µA. The
MAX3766’s data inputs are not self-biasing. The com-
mon-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.
Laser Modulation-Current DriverThe laser modulation-current driver consists of a cur-
rent mirror and an emitter coupled pair. The mirror has
a gain of +30mA/mA. Modulation-current amplitude is
programmed with external resistor RMODconnected
from REF2 to MOD. RMODcan be estimated as follows:
with RTC= 0Ω.
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 RTC, as described in the
Reference Generatorsection. An internal 520Ωresistor
is included to limit the maximum modulation current if
MOD is connected directly to REF2.
If the MAX3766 is shut down or disabled, the modula-
tion 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 CircuitThe 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.
R =
1.55V30MOD
MOD()− 520Ω
VIN+
VOLTS
250mV MIN
900mV MAX
500mV MIN
1800mV MAX
500mV MIN
1800mV MAX
IMOD
IOUT+
TIME
VIN-
VIN-
VIN+ - VIN-
VIN+
RESULTING SIGNAL
SINGLE-ENDED INPUT
DIFFERENTIAL INPUT
Figure 1. Required Input Signal and Output Polarity
MAX3766The available laser bias current is programmed by
connecting external resistor RBIASMAXfrom REF1 to
BIASMAX. The BIASMAX programming current is
adjusted by the APC circuit and amplified by the laser
bias circuit.
An internal 520Ωresistor 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
programming current (about 2.5mA) to flow into
BIASMAX. Selecting a BIASMAX resistor saves power
and limits the transmitter’s maximum light output.
RBIASMAXcan be estimated as follows:
This equation applies to maximum bias currents above
R =
1.55V40 520BIASMAX
BIASMAX()−Ω
622Mbps LAN/WAN Laser Driver with
Automatic Power Control and Safety ShutdownSAFETY
START-UP
CIRCUIT
FAILURE
DETECTION
REFERENCE
GENERATOR
TEMP
BANDGAP
POWERSET
MIRROR
AUTOMATIC
POWER CONTROL
VCC - 2V
MONITOR-
DIODE
AMPLIFIER
LASER BIAS CIRCUIT
BIAS
MIRROR
40X
APC
ILBP
IBIASMAX
IAPC
TEMP
MODULATION
MIRROR
30X
PECL
INPUT
BUFFER
ENABLE
SHDN
SAFETY
VCC
VCC
FAILMODREF2TCREF1BIASMAX
CMD
VCC
VCC
VCC
MONITOR
PHOTODIODE
LASER
BIAS
OUT+
OUT-
POWERSET
RPOWERSET
RBIASMAXRTCRMOD5.1k
520Ω520Ω300Ω
FAILURE
SHDN
GNDOUTVCCOUT
VCC
IN+
IN-
MODULATION-
CURRENT
DRIVER
MAX3766
Figure 2. Functional Diagram
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 photo-
diode 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 cur-
rent mirror. RPOWERSETcan be estimated as follows:
The monitor-diode amplifier senses the current from the
monitor photodiode at MD, provides gain, and adjusts
the laser bias programming current (ILBP). The monitor-
diode amplifier forces the monitor-diode current to
equal the current programmed at POWERSET. The
monitor-diode amplifier can reduce the laser bias pro-
gramming 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 fail-
ure 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 0.1µF. Connect the com-
pensation capacitor from VCCto MD, as shown in
Typical Application Circuits, to ensure a smooth start-
up at power-on or transmitter enable.
If a monitor diode is not available, the APC feature can
be disabled by connecting RPOWERSETto GND and
leaving MD unconnected.
Failure DetectionFigure 3 shows a simplified schematic of the failure-
detection 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 VMDis above or below VCC- 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 con-
dition and asserts the failure signal. If left undetected,
the reference voltage would rise, the current at POWER-
SET would increase, and the APC loop would attempt to
add laser current beyond the intended value.
Either failure condition causes the FAILoutput to assert
TTL low. The FAILoutput buffer is an open-collector
output and is designed to operate with a 5.1kΩexternal
pull-up resistor.
Safety/Start-Up CircuitThe 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 20µA of total laser cur-
rent 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 modula-
tion outputs are disabled by SHDN, regardless of the
state of the safety logic. The TTL-compatible ENABLE
input is internally pulled low with a 100kΩresistor.
There are two useful safety configurations: failure
indication and latched shutdown.
Failure-Indication ConfigurationSelect the failure-indication configuration by connecting
SAFETY to ground. In this configuration, a failure condi-
tion is reported at FAIL, but does not cause a latched
shutdown. This configuration requires no additional cir-
cuitry for start-up.
R = 1.55V POWERSET
MOD−300Ω
MAX3766
622Mbps LAN/WAN Laser Driver with
Automatic Power Control and Safety ShutdownVMD
FAILURE
(INTERNAL)
200mV
200mV2V
VCC
0.5V
VREF1
VBANDGAP
Figure 3. Failure-Detection Circuit (Simplified)
MAX3766
Latched Shutdown ConfigurationThis configuration is shown in the Typical Application
Circuits(configured for best performance), and can
be selected by connecting a capacitor (CSAFETY) 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, FAILis asserted until laser power reach-
es the programmed level. The safety circuit must be dis-
abled 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, CSAFETYcan be select-
ed 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 CSAFETYand an inter-
nal 200kΩpull-up resistor. The SAFETY signal is invert-
ed 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 feed-
back 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.
The duration of tSAFETY must be about 10 times tAPCfor
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 eliminat-
ed and the ENABLE input is toggled, or until the power
is cycled. A potential problem with this transmitter-
enable method is that a slow-rising power supply may
not enable the transmitter.
622Mbps LAN/WAN Laser Driver with
Automatic Power Control and Safety ShutdownRESET DOMINATE
RS FLIP-FLOP
OPEN
COLLECTORFAIL
ENABLE
CSAFETY
SAFETY
OPEN
COLLECTOR
100k
200k
VCC
FAILURE
(INTERNAL)
SHDN
(INTERNAL)
SIMPLIFIED
OPEN-COLLECTOR
OUTPUT CIRCUITOUT
Figure 4. Simplified Safety Circuit Schematic
VCCVCC ON (OR ENABLE SWITCHED TO ON STATE)
LASER BIAS AND
MODULATION CURRENT
tON
tAPC
tSAFETY
OUTPUT CURRENTS ENABLED AFTER A FIXED DELAY
FAIL DEASSERTS WHEN THE APC LOOP SETTLES
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.
FAIL
OUTPUT
SAFETY
Figure 5. Start-Up Sequence Timing
