General Description
The MAX3646 is a +3.3V laser driver designed for mul-
tirate transceiver modules with data rates from
155Mbps to 622Mbps. Lasers can be DC-coupled to
the MAX3646 for reduced component count and ease
of multirate operation.
Laser extinction ratio control (ERC) combines the features
of automatic power control (APC), modulation compensa-
tion, and built-in thermal compensation. The APC loop
maintains constant average optical power. Modulation
compensation increases the modulation current in pro-
portion to the bias current. These control loops, com-
bined with thermal compensation, maintain a constant
optical extinction ratio over temperature and lifetime.
The MAX3646 accepts differential data input signals.
The wide 5mA to 60mA (up to 85mA AC-coupled) mod-
ulation current range and up to 100mA bias current
range, make the MAX3646 ideal for driving FP/DFB
lasers in fiber optic modules. External resistors set the
required laser current levels. The MAX3646 provides
transmit disable control (TX_DISABLE), single-point
fault tolerance, bias-current monitoring, and photocur-
rent monitoring. The device also offers a latched failure
output (TX_FAULT) to indicate faults, such as when the
APC loop is no longer able to maintain the average
optical power at the required level. The MAX3646 is
compliant with the SFF-8472 transmitter diagnostic and
SFP MSA timing requirements.
The MAX3646 is offered in a 4mm x 4mm, 24-pin thin
QFN package and operates over the extended -40°C to
+85°C temperature range.
Applications
Multirate OC-3 to OC-12 FEC Transceivers
125Mbps Ethernet SFP, GBIC, and 1 x 9
Transceivers
Features
♦Single +3.3V Power Supply
♦47mA Power-Supply Current
♦85mA Modulation Current
♦100mA Bias Current
♦Automatic Power Control (APC)
♦Modulation Compensation
♦On-Chip Temperature Compensation
♦Self-Biased Inputs for AC-Coupling
♦Ground-Referenced Current Monitors
♦Laser Shutdown and Alarm Outputs
♦Enable Control and Laser Safety Feature
MAX3646
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
________________________________________________________________
Maxim Integrated Products
1
Ordering Information
19-3161; Rev 2; 6/11
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
EVALUATION KIT
AVAILABLE
BIAS
APCFILT2
VCC
VCC
TX_DISABLE
MODTCOMP
TH_TEMP
OUT+
APCFILT1
SHUTDOWN
GND
BC_MON
TX_FAULT
GND
VCC
VCC
MD
MODSET
APCSET
1
2
3
4
5
6
7 8 9 10 11 12
24 23 22 21 20 19
13
14
15
16
17
18
TOP VIEW
*THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY
GROUND ON THE CIRCUIT BOARD.
IN+
IN-
PC_MON
OUT-
MODBCOMP
MAX3646
*EP
Pin Configuration
Typical Application Circuit appears at end of data sheet.
+
Denotes a lead-free/RoHS-compliant package.
*
EP = Exposed pad.
T = Tape and reel.
PART TEMP RANGE PIN-PACKAGE
MAX3646ETG -40°C to +85°C 24 Thin QFN-EP*
MAX3646ETG-T -40°C to +85°C 24 Thin QFN-EP*
MAX3646ETG+ -40°C to +85°C 24 Thin QFN-EP*
MAX3646ETG+T -40°C to +85°C 24 Thin QFN-EP*
MAX3646
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VCC = +2.97V to +3.63V, TA= -40°C to +85°C. Typical values are at VCC = +3.3V, IBIAS = 60mA, IMOD = 60mA, TA= +25°C, unless
otherwise noted.) (Notes 1, 2)
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.
Supply Voltage VCC...............................................-0.5V to +6.0V
IN+, IN-, TX_DISABLE, TX_FAULT, SHUTDOWN,
BC_MON, PC_MON, APCFILT1, APCFILT2,
MD, TH_TEMP, MODTCOMP, MODBCOMP,
MODSET, and APCSET Voltage.............-0.5V to (VCC + 0.5V)
OUT+, OUT-, BIAS Current.............................-20mA to +150mA
Continuous Power Dissipation (TA= +70°C)
24-Pin TQFN (derate 27.8mW/°C above +70°C) .......2222mW
Operating Junction Temperature Range...........-55°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Soldering Temperature (reflow)
Lead (Pb)-free packages..............................................+260°C
Packages containing lead (Pb).....................................+240°C
Storage Temperature Range .............................-55°C to +150°C
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
POWER SUPPLY
Supply Current ICC (Note 3) 47 60 mA
Power-Supply Noise Rejection PSNR f ≤ 1MHz, 100mAP-P (Note 4) 33 dB
I/O SPECIFICATIONS
Differential Input Swing VID DC-coupled, Figure 1 0.2 2.4 VP-P
Common-Mode Input VCM 1.7 VCC -
VID / 4 V
LASER BIAS
Bias-Current-Setting Range 1 100 mA
Bias Off Current TX_DISABLE = high 0.1 mA
Bias-Current Monitor Ratio IBIAS / IBC_MON 62 76 90 mA/mA
LASER MODULATION
Modulation Current-Setting
Range IMOD (Note 5) 5 85 mA
Output Edge Speed 20% to 80%
(Notes 6, 7) 5mA ≤ IMOD ≤ 85mA 100 200 ps
Output Overshoot/Undershoot (Note 7) ±6%
Random Jitter (Notes 6, 7) 1.1 2.5 psRMS
622Mbps, 5mA ≤ IMOD ≤ 85mA 24 46
Deterministic Jitter (Notes 6, 8) 155Mbps, 5mA ≤ IMOD ≤ 85mA 45 100 psP-P
5mA ≤ IMOD ≤ 10mA ±175 ±600
Modulation-Current Temperature
Stability (Note 6) 10mA < IMOD ≤ 85mA ±125 ±480 ppm/°C
5mA ≤ IMOD ≤ 10mA ±20
Modulation-Current-Setting Error 15Ω load,
TA = +25°C10mA < IMOD ≤ 85mA ±15 %
Modulation Off Current TX_DISABLE = high 0.1 mA
MAX3646
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
_______________________________________________________________________________________ 3
Note 1: AC characterization is performed using the circuit in Figure 2 using a PRBS 223 - 1 or equivalent pattern.
Note 2: Specifications at -40°C are guaranteed by design and characterization.
Note 3: Excluding IBIAS and IMOD. Input data is AC-coupled. TX_FAULT open, SHUTDOWN open.
Note 4: Power-supply noise rejection (PSNR) = 20log10(Vnoise (on VCC) / ΔVOUT). VOUT is the voltage across the 15Ωload when IN+
is high.
Note 5: The minimum required voltage at the OUT+ and OUT- pins is +0.75V.
Note 6: Guaranteed by design and characterization.
Note 7: Tested with 00001111 pattern at 622Mbps.
Note 8: DJ includes pulse-width distortion (PWD).
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
AUTOMATIC POWER AND EXTINCTION RATIO CONTROLS
Monitor-Diode Input Current
Range IMD Average current into the MD pin 18 1500 µA
MD Pin Voltage 1.4 V
MD Current Monitor Ratio IMD / IPC_MON 0.85 0.93 1.15 mA/mA
APC Loop Time Constant CAPC_FILT = 0.01µF, ΔIMD / ΔIBIAS = 1/70 3.3 µs
APC Setting Stability (Note 6) ±100 ±480 ppm/°C
APC Setting Accuracy TA = +25°C±15 %
IMOD Compensation-Setting
Range by Bias K K = ΔIMOD / ΔIBIAS 0 1.5 mA/mA
IMOD Compensation-Setting
Range by Temperature TC TC = ΔIMOD / ΔT (Note 6) 0 1.0 mA/°C
Threshold-Setting Range for
Temperature Compensation TTH (Note 6) +10 +60 °C
LASER SAFETY AND CONTROL
Bias and Modulation Turn-Off
Delay
CAPC_FILT = 0.01µF, ΔIMD / ΔIBIAS = 1/80
(Note 6) 5µs
Bias and Modulation Turn-On
Delay
CAPC_FILT = 0.01µF, ΔIMD / ΔIBIAS = 1/80
(Note 6) 600 µs
Threshold Voltage at Monitor Pins VREF Figure 5 1.14 1.3 1.39 V
INTERFACE SIGNALS
TX_DISABLE Input High VHI 2.0 V
TX_DISABLE Input Low VLO RPULL = 45kΩ (typical) 0.8 V
VHI = VCC 15
TX_DISABLE Input Current VLO = GND -70 -140 µA
TX_FAULT Output Low Sinking 1mA, open collector 0.4 V
Shutdown Output High Sourcing 100µA VCC - 0.4 V
Shutdown Output Low Sinking 100µA 0.4 V
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +2.97V to +3.63V, TA= -40°C to +85°C. Typical values are at VCC = +3.3V, IBIAS = 60mA, IMOD = 60mA, TA= +25°C, unless
otherwise noted.) (Notes 1, 2)
MAX3646
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
4 _______________________________________________________________________________________
Typical Operating Characteristics
(VCC = +3.3V, CAPC = 0.01µF, IBIAS = 20mA, IMOD = 30mA, TA= +25°C, unless otherwise noted.)
OPTICAL EYE DIAGRAM
(622.08Mbps, 27 - 1 PRBS, 467MHz FILTER)
MAX3646 toc01
270ps/div
1310nm FP LASER
re = 8.2dB
OPTICAL EYE DIAGRAM
(155Mbps, 27 - 1 PRBS, 117MHz FILTER)
MAX3646 toc02
1ns/div
1310nm FP LASER
re = 8.2dB
ELECTRICAL EYE DIAGRAM
(IMOD = 30mA, 622.08MHz, 27 - 1 PRBS)
MAX3646 toc03
320ps/div
75mV/div
2pF BETWEEN OUT+
AND OUT-
SUPPLY CURRENT (ICC) vs. TEMPERATURE
(EXCLUDES BIAS AND MODULATION CURRENTS)
MAX3646 toc04
TEMPERATURE (°C)
SUPPLY CURRENT (mA)
80706050403020100-10-20-30
35
40
45
50
55
60
30
-40 90
3.63V
2.97V
3.3V
BIAS-CURRENT MONITOR RATIO
vs. TEMPERATURE
MAX3646 toc05
TEMPERATURE (°C)
IBIAS/IBC_MON (mA/mA)
807050 60-10 0 10 20 30 40-30 -20
72
74
76
78
80
82
84
86
88
90
70
-40 90
PHOTOCURRENT MONITOR RATIO
vs. TEMPERATURE
MAX3646 toc06
TEMPERATURE (°C)
IMD/IPC_MON (mA/mA)
807050 60-100 10203040-30 -20
0.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
0.80
-40 90
MODULATION CURRENT vs. RMODSET
MAX3646 toc07
RMODSET (kΩ)
IMOD (mA)
10
10
20
30
40
50
60
70
80
90
0
1 100
PHOTODIODE CURRENT vs. RAPCSET
MAX3646 toc08
RAPCSET (kΩ)
IMD (mA)
101
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0
0.1 100
DETERMINISTIC JITTER vs.
MODULATION CURRENT
MAX3646 toc09
IMOD (mA)
DJ (psP-P)
807050 6020 30 4010
30
40
50
60
70
80
90
100
20
090
155mbps
MAX3646
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
_______________________________________________________________________________________
5
Typical Operating Characteristics (continued)
(VCC = +3.3V, CAPC = 0.01µF, IBIAS = 20mA, IMOD = 30mA, TA= +25°C, unless otherwise noted.)
RANDOM JITTER vs.
MODULATION CURRENT
MAX3646 toc10
IMOD (mA)
RJ (psRMS)
807050 6020 30 4010
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0
090
COMPENSATION (K) vs. RMODBCOMP
MAX3646 toc11
RMODBCOMP (kΩ)
K (mA/mA)
1010.10.01
0.1
1
10
0.01
0.001 100
TEMPERATURE COMPENSATION vs.
RTH_TEMP (RMODTCOMP = 500Ω)
MAX3646 toc12
TEMPERATURE (°C)
IMOD (mA)
80706050403020100
40
50
60
70
80
90
100
30
-10 90
RTH_TEMP = 12kΩ
RTH_TEMP = 7kΩ
RTH_TEMP = 4kΩ
RTH_TEMP = 2kΩ
HOT PLUG WITH TX_DISABLE LOW
MAX3646 toc14
20ms/div
VCC
FAULT
TX_DISABLE
LASER
OUTPUT
0V
3.3V
t_init = 59.6ms
LOW
LOW
TRANSMITTER ENABLE
MAX3646 toc15
10μs/div
VCC
FAULT
TX_DISABLE
LASER
OUTPUT
3.3V
t_on = 23.8μs
LOW
HIGH
LOW
TRANSMITTER DISABLE
MAX37646 toc16
20ns/div
VCC
FAULT
TX_DISABLE
LASER
OUTPUT
3.3V
91.2ns
LOW
HIGH
LOW
RESPONSE TO FAULT
MAX3646 toc17
400ns/div
VPC_MON
FAULT
TX_DISABLE
LASER
OUTPUT
t_fault = 160ns
EXTERNALLY
FORCED FAULT
FAULT RECOVERY TIME
MAX3646 toc18
40ms/div
VPC_MON
FAULT
TX_DISABLE
LASER
OUTPUT
t_init = 58ms
EXTERNALLY
FORCED FAULT
LOW
LOWLOW
HIGH
HIGH
MAX3646
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
6 _______________________________________________________________________________________
PIN NAME FUNCTION
1 MODTCOMP
Modulation-Current Compensation from Temperature. A resistor at this pin sets the temperature
coefficient of the modulation current when above the threshold temperature. Leave open for zero
temperature compensation.
2, 5, 14, 17 VCC +3.3V Supply Voltage
3 IN+ Noninverted Data Input
4 IN- Inverted Data Input
6 TX_DISABLE
Transmitter Disable, TTL. Laser output is disabled when TX_DISABLE is asserted high or left
unconnected. The laser output is enabled when this pin is asserted low.
7 PC_MON
Photodiode-Current Monitor Output. Current out of this pin develops a ground-referenced voltage
across an external resistor that is proportional to the monitor diode current.
8 BC_MON
Bias-Current Monitor Output. Current out of this pin develops a ground-referenced voltage across
an external resistor that is proportional to the bias current.
9 SHUTDOWN
Shutdown Driver Output. Voltage output to control an external transistor for optional shutdown
circuitry.
10, 12 GND Ground
11 TX_FAULT Open-Collector Transmit Fault Indicator (Table 1)
13 BIAS Laser Bias-Current Output
15 OUT- Inverted Modulation-Current Output. IMOD flows into this pin when input data is low.
16 OUT+ Noninverted Modulation-Current Output. IMOD flows into this pin when input data is high.
18 MD
Monitor Photodiode Input. Connect this pin to the anode of a monitor photodiode. A capacitor to
ground is required to filter the high-speed AC monitor photocurrent.
19 APCFILT1
Connect a capacitor (CAPC) between pin 19 (APCFILT1) and pin 20 (APCFILT2) to set the dominant
pole of the APC feedback loop.
20 APCFILT2 (See Pin 19)
21 APCSET
A resistor connected from this pin to ground sets the desired average optical power. The total
capacitive load at the APCSET pin should be no more than 10pF. Minimize metal resistance for
ground connections.
22 MODSET
A resistor connected from this pin to ground sets the desired constant portion of the modulation
current. The total capacitive load at the MODSET pin should be no more than 10pF. Minimize
metal resistance for ground connections.
23 MODBCOMP
Modulation-Current Compensation from Bias. Couples the bias current to the modulation current.
Mirrors IBIAS through an external resistor. Leave open for zero-coupling.
24 TH_TEMP
Threshold for Temperature Compensation. A resistor at this pin programs the temperature above
which compensation is added to the modulation current.
— EP
Exposed Pad. Solder the exposed pad to the circuit board ground for specified thermal and
electrical performance.
Pin Description
Detailed Description
The MAX3646 laser driver consists of three main parts:
a high-speed modulation driver, biasing block with
ERC, and safety circuitry. The circuit design is opti-
mized for high-speed, low-voltage (+3.3V) operation
(Figure 4).
High-Speed Modulation Driver
The output stage is composed of a high-speed differ-
ential pair and a programmable modulation current
source. The MAX3646 is optimized for driving a 15Ω
load. The minimum instantaneous voltage required at
OUT- is 0.7V for modulation currents up to 60mA and
0.75V for currents from 60mA to 85mA. Operation
above 60mA can be accomplished by AC-coupling or
with sufficient voltage at the laser to meet the driver
output voltage requirement.
To interface with the laser diode, a damping resistor
(RD) is required. The combined resistance damping
resistor and the equivalent series resistance (ESR) of
the laser diode should equal 15Ω. To further damp
aberrations caused by laser diode parasitic induc-
tance, an RC shunt network may be necessary. Refer to
Application Note 274:
HFAN-02.0: Interfacing Maxim
Laser Drivers with Laser Diodes
for more information.
Any capacitive load at the cathode of a laser diode
degrades optical output performance. Because the
BIAS output is directly connected to the laser cathode,
minimize the parasitic capacitance associated with the
pin by using an inductor to isolate the BIAS pin para-
sitics form the laser cathode.
Extinction Ratio Control
The extinction ratio (re) is the laser on-state power
divided by the off-state power. Extinction ratio remains
constant if peak-to-peak and average power are held
constant:
re= (2PAVG + PP-P) / (2PAVG - PP-P)
MAX3646
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
_______________________________________________________________________________________ 7
VIN+
VIN-
200mV (min)
2400mV (max)
100mV (min)
1200mV (max)
IMOD
VOLTAGE
CURRENT
TIME
SINGLE ENDED
DIFFERENTIAL
(VIN+) - (VIN-)
IOUT+
Figure 1. Required Input Signal and Output Polarity
MAX3646
21Ω
21Ω
50Ω
OUT+
OUT-
OSCILLOSCOPE
50Ω
1μF
BIAS-T
BIAS-T
1μF
Figure 2. Test Circuit for Characterization
L1
1μH
C1
0.1μF
C3
0.1μF
C2
10μF
VOLTAGE
SUPPLY
SOURCE
NOISE OPTIONAL
OPTIONAL
FILTER DEFINED BY SFP MSA
HOST BOARD MODULE
TO LASER
DRIVER VCC
Figure 3. Supply Filter
MAX3646
Average power is regulated using APC, which keeps
constant current from a photodiode coupled to the
laser. Peak-to-peak power is maintained by compen-
sating the modulation current for reduced slope effi-
ciency (η) of laser over time and temperature:
PAVG = IMD/ρMON
PP-P = ηx IMOD
Modulation compensation from bias increases the mod-
ulation current by a user-selected proportion (K) needed
to maintain peak-to-peak laser power as bias current
increases with temperature. Refer to Application Note
1119:
HFAN-02.2.1: Maintaining the Extinction Ratio of
Optical Transmitters Using K-Factor Control
for details:
K = ΔIMOD / ΔIBIAS
This provides a first-order approximation of the current
increase needed to maintain peak-to-peak power.
Slope efficiency decreases more rapidly as tempera-
ture increases. The MAX3646 provides additional tem-
perature compensation as temperature increases past
a user-defined threshold (TTH).
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
8 _______________________________________________________________________________________
MAX3646
IN+
IN-
OUT-
OUT+
INPUT BUFFER
DATA
PATH
IMOD
VBG
RMODSET
MODSET
VBG RAPCSET
APCSET
IAPCSET
CAPC
APCFILT1 APCFILT2
IBIAS
BIAS
MD
VCC
IBIAS
ENABLE
x1
TX_FAULT
TX_DISABLE
SAFETY LOGIC
AND
POWER DETECTOR
RBC_MON
BC_MON
VCC
RPC_MON
PC_MON
RD
VCC
IMD
CMD
SHUTDOWN
SHUTDOWN
x268
RPULL = 45kΩ
T
xKxTC
T > TTH
IBIAS
IMOD
ENABLE
x1/2
MODTCOMP
RMODTCOMP
TH_TEMP
RTH_TEMP
MODBCOMP
RMODBCOMP
IBIAS
82
IMD
1
Figure 4. Functional Diagram
MAX3646
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
_______________________________________________________________________________________ 9
1If any of the I/O pins are shorted to GND or VCC (single-point failure; see Table 2), and the bias current or the photocurrent
exceeds the programmed threshold.
2 End-of-life (EOL) condition of the laser diode. The bias current and/or the photocurrent exceed the programmed threshold.
3 Laser cathode is grounded and photocurrent exceeds the programming threshold.
4No feedback for the APC loop (broken interconnection, defective monitor photodiode), and the bias current exceeds the
programmed threshold.
Table 1. Typical Fault Conditions
RBC_MON
BC_MON
VCC
VCC
RPC_MON
PC_MON
COMP
VREF
VREF
TTL
OPEN COLLECTOR
CMOS
SHUTDOWN
TX_FAULT
R
S
Q
RS
LATCH
COUNTER
60ms DELAY
POR AND COUNTER
60ms DELAY
100ns DELAY IBIAS
ENABLE
IMOD
ENABLE
VCC
TX_DISABLE
COMP
EXCESSIVE
APC CURRENT
SETPOINT
EXCESSIVE
MOD CURRENT
SETPOINT
IMD
1
IBIAS
82
Figure 5. Simplified Safety Circuit
MAX3646
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
10 ______________________________________________________________________________________
PIN CIRCUIT RESPONSE TO OVERVOLTATGE OR
SHORT TO VCC
CIRCUIT RESPONSE TO UNDERVOLTAGE OR
SHORT TO GROUND
TX_FAULT Does not affect laser power. Does not affect laser power.
TX_DISABLE Modulation and bias currents are disabled. Normal condition for circuit operation.
IN+
The optical average power increases and a fault occurs
if VPC_MON exceeds the threshold. The APC loop
responds by decreasing the bias current.
The optical average power decreases and the APC loop
responds by increasing the bias current. A fault state
occurs if VBC_MON exceeds the threshold voltage.
IN-
The optical average power decreases and the APC loop
responds by increasing the bias current. A fault state
occurs if VBC_MON exceeds the threshold voltage.
The optical average power increases and a fault occurs
if VPC_MON exceeds the threshold. The APC loop
responds by decreasing the bias current.
MD This disables bias current. A fault state occurs. The APC circuit responds by increasing the bias current
until a fault is detected, then a fault* state occurs.
SHUTDOWN Does not affect laser power. If the shutdown circuitry is
used, the laser current is disabled. Does not affect laser power.
BIAS In this condition, the laser forward voltage is 0V and no
light is emitted.
Fault state* occurs. If the shutdown circuitry is used, the
laser current is disabled.
OUT+ The APC circuit responds by increasing the bias current
until a fault is detected, then a fault state* occurs.
Fault state* occurs. If the shutdown circuitry is used, the
laser current is disabled.
OUT- Does not affect laser power. Does not affect laser power.
PC_MON Fault state* occurs. Does not affect laser power.
BC_MON Fault state* occurs. Does not affect laser power.
APCFILT1 IBIAS increases until VBC_MON exceeds the threshold
voltage.
IBIAS increases until VBC_MON exceeds the threshold
voltage.
APCFILT2 IBIAS increases until VBC_MON exceeds the threshold
voltage.
IBIAS increases until VBC_MON exceeds the threshold
voltage.
MODSET Does not affect laser power. Fault state* occurs.
APCSET Does not affect laser power. Fault state* occurs.
Table 2. Circuit Responses to Various Single-Point Faults
*
A fault state asserts the TX_FAULT pin, disables the modulation and bias currents, and asserts the SHUTDOWN pin.
Safety Circuitry
The safety circuitry contains a disable input
(TX_DISABLE), a latched fault output (TX_FAULT), and
fault detectors (Figure 5). This circuitry monitors the
operation of the laser driver and forces a shutdown if a
fault is detected (Table 1). The TX_FAULT pin should
be pulled high with a 4.7kΩto 10kΩresistor to VCC as
required by the SFP MSA. A single-point fault can be a
short to VCC or GND. See Table 2 to view the circuit
response to various single-point failure. The transmit
fault condition is latched until reset by a toggle or
TX_DISABLE or VCC. The laser driver offers redundant
laser diode shutdown through the optional shutdown
circuitry as shown in the
Typical Application Circuit
.
This shutdown transistor prevents a single-point fault at
the laser from creating an unsafe condition.
Safety Circuitry Current Monitors
The MAX3646 features monitors (BC_MON, PC_MON)
for bias current (IBIAS) and photocurrent (IMD). The
monitors are realized by mirroring a fraction of the cur-
rents and developing voltages across external resistors
connected to ground. Voltages greater than VREF at
PC_MON or BC_MON result in a fault state. For exam-
ple, connecting a 100Ωresistor to ground at each mon-
itor output gives the following relationships:
VBC_MON = (IBIAS / 82) x 100Ω
VPC_MON = IMD x 100Ω
External sense resistors can be used for high-accuracy
measurement of bias and photodiode currents. On-chip
isolation resistors are included to reduce the number of
components needed to implement this function.
Design Procedure
When designing a laser transmitter, the optical output is
usually expressed in terms of average power and
extinction ratio. Table 3 shows relationships that are
helpful in converting between the optical average
power and the modulation current. These relationships
are valid if the mark density and duty cycle of the opti-
cal waveform are 50%.
For a desired laser average optical power (PAVG) and
optical extinction ratio (re), the required bias and modu-
lation currents can be calculated using the equations in
Table 3. Proper setting of these currents requires
knowledge of the laser to monitor transfer (ρMON) and
slope efficiency (η).
Programming the Monitor-Diode
Current Set Point
The MAX3646 operates in APC mode at all times. The
bias current is automatically set so average laser power
is determined by the APCSET resistor:
PAVG = IMD / ρMON
The APCSET pin controls the set point for the monitor
diode current. An internal current regulator establishes
the APCSET current in the same manner as the
MODSET pin. See the IMD vs. RAPCSET graph in the
Typical Operating Characteristics
and select the value
of RAPCSET that corresponds to the required current at
+25°C:
IMD = 1/2 x VREF / RACPSET
The laser driver automatically adjusts the bias to main-
tain the constant average power. For DC-coupled laser
diodes:
IAVG = IBIAS + IMOD / 2
Programming the Modulation
Current with Compensation
Determine the modulation current form the laser slope
efficiency:
IMOD = 2 x PAVG / ηx (re- 1)/(re+ + 1)
The modulation current of the MAX3646 consists of a
static modulation current (IMODS), a current proportion-
al to IBIAS, and a current proportional to temperature.
The portion of IMOD set by MODSET is established by
an internal current regulator, which maintains the refer-
ence voltage of VREF across the external programming
resistor. See the IMOD vs. RMODSET graph in the
Typical Operating Characteristics
and select the value
MAX3646
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
______________________________________________________________________________________ 11
PARAMETER SYMBOL RELATION
Average power PAVG PAVG = (P0 + P1) / 2
Extinction ratio rere = P1 / P0
Optical power of a one P1P1 = 2PAVG x re / (re + 1)
Optical power of a zero P0P0 = 2PAVG / (re + 1)
Optical amplitude PP-P PP-P = P1 - P0
Laser slope efficiency ηη = PP-P / IMOD
Modulation current IMOD IMOD = PP-P / η
Threshold current ITH P0 at I ≥ ITH
Bias current
(AC-coupled) IBIAS IBIAS ≥ ITH + IMOD / 2
Laser to monitor
transfer ρMON IMD / PAVG
Table 3. Optical Power Relations
Note: Assuming a 50% average input duty cycle and mark
density.
MAX3646
of RMODSET that corresponds to the required current
at +25°C:
IMOD = IMODS + K x IBIAS + IMODT
IMODS = 268 x VREF / RMODSET
IMODT = TC x (T - TTH) | T > TTH
IMODT = 0 | T < TTH
An external resistor at the MODBCOMP pin sets current
proportional to IBIAS. Open circuiting the MODBCOMP
pin can turn off the interaction between IBIAS and IMOD:
K = 1700 / (1000 + RMODBCOMP) +10%
If IMOD must be increased from IMOD1 to IMOD2 to
maintain the extinction ratio at elevated temperatures,
the required compensation factor is:
K = (IMOD2 - IMOD1) / (IBIAS2 - IBIAS1)
A threshold for additional temperature compensation
can be set with a programming resistor at the
TH_TEMP pin:
TTH = -70°C + 1.45MΩ/ (9.2kΩ+ RTH_TEMP)°C +10%
The temperature coefficient of thermal compensation
above TTH is set by RMODTCOMP. Leaving the
MODTCOMP pin open disables additional thermal
compensation:
TC = 1 / (0.5 + RMODTCOMP(kΩ)) mA/°C +10%
Current Compliance (IMOD
≤
60mA),
DC-Coupled
The minimum voltage at the OUT+ and OUT- pins is
0.7V.
For:
VDIODE = Diode bias point voltage (1.2V typ)
RL= Diode bias point resistance (5Ωtyp)
RD= Series matching resistor (20Ωtyp)
For compliance:
VOUT+ = VCC - VDIODE - IMOD x (RD+ RL) -
IBIAS x RL≥0.7V
Current Compliance (IMOD > 60mA),
AC-Coupled
For applications requiring modulation current greater
than 60mA, headroom is insufficient from proper opera-
tion of the laser driver if the laser is DC-coupled. To
avoid this problem, the MAX3646’s modulation output
can be AC-coupled to the cathode of a laser diode. An
external pullup inductor is necessary to DC-bias the
modulation output at VCC. Such a configuration isolates
laser forward voltage from the output circuitry and allows
the output at OUT+ to swing above and below the sup-
ply voltage (VCC). When AC-coupled, the MAX3646
modulation current can be programmed up to 85mA.
Refer to Application Note 274:
HFAN-02.0: Interfacing
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
12 ______________________________________________________________________________________
VCC
VCC
VCC
0.11pF
0.7nH
IN+
0.11pF
0.7nH
IN-
PACKAGE
5kΩ
5kΩ
24kΩ
16kΩ
MAX3646
Figure 6. Simplified Input Structure
MAX3646
PACKAGE
0.7nH OUT-
0.7nH OUT+
0.11pF
0.11pF
VCC
Figure 7. Simplified Output Structure
Maxim Laser Drivers with Laser Diodes
for more informa-
tion on AC-coupling laser drivers to laser diodes.
For compliance:
VOUT+ = VCC - IMOD / 2 x (RD+ RL) ≥0.75V
Determine CAPC
The APC loop filter capacitor (CAPC) must be selected
to balance the requirements for fast turn-on and mini-
mal interaction with low frequencies in the data pattern.
The low-frequency cutoff is:
CAPC(µF) ≅68 / (f3dB(kHz) x (ηx ρMON)1.1
High-frequency noise can be filtered with an additional
cap, CMD, from the MD pin to ground:
CMD ≅CAPC / 4
The MAX3646 is designed so turn-on time is faster than
1ms for most laser gain values (ηx ρMON). Choosing a
smaller value of CAPC reduces turn-on time. Careful
balance between turn-on time and low-frequency cutoff
may be needed at low data rates for some values of
laser gain.
Interface Models
Figures 6 and 7 show simplified input and output cir-
cuits for the MAX3646 laser driver. If dice are used,
replace package parasitic elements with bondwire par-
asitic elements.
Layout Considerations
To minimize loss and crosstalk, keep the connections
between the MAX3646 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 data inputs, as well as
the module output.
Laser Safety and IEC 825
Using the MAX3646 laser driver alone does not ensure
that a transmitter design is IEC 825 compliant. The
entire transmitter circuit and component selections must
be considered. Each customer must determine the level
of fault tolerance required by their application, recogniz-
ing 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 sup-
port 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.
Exposed-Pad (EP) Package
The exposed pad on the 24-pin QFN provides a very low
thermal resistance path for heat removal from the IC. The
pad is also electrical ground on the MAX3646 and should
be soldered to the circuit board ground for proper ther-
mal and electrical performance. Refer to Application Note
862:
HFAN-08.1: Thermal Consideration of QFN and
Other Exposed-Paddle Packages
at www.maxim-ic.com
for additional information.
Chip Information
PROCESSS: SiGe/BIPOLAR
MAX3646
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
______________________________________________________________________________________ 13
MAX3646
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
14 ______________________________________________________________________________________
MAX3646
IN+
IN-
REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE.
VCC
SHUTDOWN
+3.3V
OPTIONAL SHUTDOWN
CIRCUITRY
+3.3V
15Ω
10Ω
OUT-
OUT+
BIAS
MD
BC_MON
APCFILT1
APCFILT2
GND
APCSET
MODSET
TX_DISABLE
TX_FAULT
+3.3V
CDR
CAPC
RPC_MON
CMD
0.01μF
FERRITE BEAD
PC_MON
RBC_MON
RMODSET
RAPCSET
MODBCOMP
MODTCOMP
TH_TEMP
RTH_TEMP
RMODTCOMP
RMODBCOMP
0.1μF
0.1μF
Typical Application Circuit
Package Information
For the latest package outline information and land patterns (footprints), go to www.maxim-ic.com/packages. Note that a “+”, “#”, or
“-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing per-
tains to the package regardless of RoHS status.
PACKAGE TYPE PACKAGE CODE OUTLINE NO. LAND PATTERN NO.
24 TQFN-EP T2444-3 21-0139 90-0021
MAX3646
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________
15
© 2011 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
Revision History
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
0 1/04 Initial release —
1 7/04 Added the lead(Pb)-free package option to the Ordering Information 1
2 6/11
Updated Ordering Information; changed Absolute Maximum Ratings to reflect lead
and soldering specs; changed continuous power dissipation specs; updated bias-
current monitor ratio in Electrical Characteristics table; changed the APCSET and
MODSET function description in Pin Description table; replaced Figure 2
1, 2, 6, 7