MAX3646 [MAXIM]
155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control; 155Mbps至622Mbps的SFF / SFP激光驱动器，带有消光比控制
| 型号： | MAX3646 |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | 155Mbps to 622Mbps SFF/SFP Laser Driver with Extinction Ratio Control |
| 文件： | 总14页 (文件大小：831K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-3161; Rev 1; 7/04
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
General Description
Features
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.
♦ Single +3.3V Power Supply
♦ 47mA Power-Supply Current
♦ 85mA Modulation Current
♦ 100mA Bias Current
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.
♦ 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
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.
♦ Enable Control and Laser Safety Feature
Ordering Information
TEMP
RANGE
PIN-
PACKAGE
PKG
CODE
PART
MAX3646ETG
-40°C to +85°C 24 Thin QFN T2444-1
MAX3646ETG+ -40°C to +85°C 24 Thin QFN T2444-1
+Denotes lead-free package.
Pin Configuration
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.
TOP VIEW
24 23 22 21 20 19
Applications
Multirate OC-3 to OC-12 FEC Transceivers
1
2
3
4
5
6
MODTCOMP
18
17
MD
V
V
CC
CC
125Mbps Ethernet SFP, GBIC, and 1 x 9
Transceivers
IN+
IN-
16 OUT+
MAX3646
OUT-
15
14
13
V
CC
V
CC
BIAS
TX_DISABLE
7
8
9
10 11 12
THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY
GROUND ON THE CIRCUIT BOARD.
Typical Application Circuit appears at end of data sheet.
________________________________________________________________ Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
ABSOLUTE MAXIMUM RATINGS
Supply Voltage V ...............................................-0.5V to +6.0V
OUT+, OUT-, BIAS Current.............................-20mA to +150mA
CC
IN+, IN-, TX_DISABLE, TX_FAULT, SHUTDOWN,
BC_MON, PC_MON, APCFILT1, APCFILT2,
MD, TH_TEMP, MODTCOMP, MODBCOMP,
Continuous Power Dissipation (T = +85°C)
A
24-Pin QFN (derate 20.8mW/°C above +85°C) .........1805mW
Operating Junction Temperature Range...........-55°C to +150°C
Storage Temperature Range.............................-55°C to +150°C
MODSET, and APCSET Voltage.............-0.5V to (V
+ 0.5V)
CC
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
(V
= +2.97V to +3.63V, T = -40°C to +85°C. Typical values are at V
= +3.3V, I
= 60mA, I = 60mA, T = +25°C, unless
MOD A
CC
A
CC
BIAS
otherwise noted.) (Notes 1, 2)
PARAMETER
POWER SUPPLY
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Supply Current
I
(Note 3)
f ≤ 1MHz, 100mA
47
33
60
mA
dB
CC
Power-Supply Noise Rejection
I/O SPECIFICATIONS
Differential Input Swing
PSNR
(Note 4)
P-P
V
DC-coupled, Figure 1
0.2
1.7
2.4
V
ID
P-P
V
V
V
-
CC
Common-Mode Input
V
CM
/ 4
ID
LASER BIAS
Bias-Current-Setting Range
Bias Off Current
1
100
0.1
95
mA
mA
TX_DISABLE = high
Bias-Current Monitor Ratio
LASER MODULATION
I
/ I
68
79
mA/mA
BIAS BC_MON
Modulation Current-Setting
Range
I
(Note 5)
5
85
mA
MOD
20% to 80%
(Notes 6, 7)
Output Edge Speed
5mA ≤ I
≤ 85mA
100
200
ps
%
MOD
Output Overshoot/Undershoot
Random Jitter
(Note 7) (with 2pF between OUT+ and OUT-)
(Notes 6, 7)
±6
1.1
24
2.5
46
ps
RMS
622Mbps, 5mA ≤ I
≤ 85mA
≤ 85mA
MOD
MOD
Deterministic Jitter (Notes 6, 8)
ps
P-P
155Mbps, 5mA ≤ I
45
100
600
480
±20
±15
0.1
5mA ≤ I
≤ 10mA
MOD
175
125
Modulation-Current Temperature
Stability
(Note 6)
ppm/°C
10mA < I
≤ 85mA
MOD
5mA ≤ I
≤ 10mA
MOD
15Ω load,
Modulation-Current-Setting Error
Modulation Off Current
%
T
= +25°C
A
10mA < I
≤ 85mA
MOD
TX_DISABLE = high
mA
AUTOMATIC POWER AND EXTINCTION RATIO CONTROLS
Monitor-Diode Input Current
Range
I
Average current into the MD pin
18
1500
µA
MD
MD Pin Voltage
MD Current Monitor Ratio
1.4
V
I
/ I
0.85
0.93
1.15
mA/mA
MD PC_MON
2
_______________________________________________________________________________________
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
ELECTRICAL CHARACTERISTICS (continued)
(V
= +2.97V to +3.63V, T = -40°C to +85°C. Typical values are at V
= +3.3V, I
= 60mA, I = 60mA, T = +25°C, unless
MOD A
CC
A
CC
BIAS
otherwise noted.) (Notes 1, 2)
PARAMETER
APC Loop Time Constant
APC Setting Stability
SYMBOL
CONDITIONS
MIN
TYP
3.3
MAX
UNITS
µs
C
= 0.01µF, ∆I
/ ∆I
= 1/70
APC_FILT
MD
BIAS
(Note 6)
= +25°C
±100
±480
±15
ppm/°C
%
APC Setting Accuracy
T
A
I
Compensation-Setting
MOD
K
K = ∆I
/ ∆I
0
0
1.5
1.0
mA/mA
mA/°C
°C
MOD
BIAS
Range by Bias
I
Compensation-Setting
MOD
TC
TC = ∆I
/ ∆T (Note 6)
MOD
Range by Temperature
Threshold-Setting Range for
Temperature Compensation
T
(Note 6)
+10
+60
TH
LASER SAFETY AND CONTROL
Bias and Modulation Turn-Off
Delay
C
= 0.01µF, ∆I
= 0.01µF, ∆I
/ ∆I
/ ∆I
= 1/80
= 1/80
APC_FILT
MD
MD
BIAS
BIAS
5
µs
(Note 6)
Bias and Modulation Turn-On
Delay
C
APC_FILT
(Note 6)
600
µs
V
Threshold Voltage at Monitor Pins
INTERFACE SIGNALS
V
Figure 5
1.14
2.0
1.3
-70
1.39
REF
TX_DISABLE Input High
TX_DISABLE Input Low
V
V
V
HI
V
R
= 45kΩ (typical)
0.8
15
LO
PULL
V
V
= V
HI
CC
TX_DISABLE Input Current
µA
= GND
-140
0.4
LO
TX_FAULT Output Low
Shutdown Output High
Shutdown Output Low
Sinking 1mA, open collector
Sourcing 100µA
V
V
V
V
- 0.4
CC
Sinking 100µA
0.4
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 I
and I
. Input data is AC-coupled. TX_FAULT open, SHUTDOWN open.
BIAS
MOD
Note 4: Power-supply noise rejection (PSNR) = 20log (V
) / ∆V
). V
is the voltage across the 15Ω load when IN+
OUT
10 noise (on VCC
OUT
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).
_______________________________________________________________________________________
3
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
Typical Operating Characteristics
(V
= +3.3V, C
= 0.01µF, I
= 20mA, I
= 30mA, T = +25°C, unless otherwise noted.)
CC
APC
BIAS
MOD
A
OPTICAL EYE DIAGRAM
ELECTRICAL EYE DIAGRAM
OPTICAL EYE DIAGRAM
(622.08Mbps, 27 - 1 PRBS, 467MHz FILTER)
(155Mbps, 27 - 1 PRBS, 117MHz FILTER)
(I
= 30mA, 622.08MHz, 27 - 1 PRBS)
MOD
MAX3646 toc01
MAX3646 toc03
MAX3646 toc02
1310nm FP LASER
2pF BETWEEN OUT+
AND OUT-
1310nm FP LASER
r = 8.2dB
e
r = 8.2dB
e
75mV/div
270ps/div
320ps/div
1ns/div
BIAS-CURRENT MONITOR RATIO
vs. TEMPERATURE
PHOTOCURRENT MONITOR RATIO
vs. TEMPERATURE
SUPPLY CURRENT (I ) vs. TEMPERATURE
(EXCLUDES BIAS AND MODULATION CURRENTS)
CC
90
88
86
84
82
80
78
76
74
72
70
1.20
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
60
55
3.63V
50
45
2.97V
3.3V
40
35
30
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
DETERMINISTIC JITTER vs.
MODULATION CURRENT
MODULATION CURRENT vs. R
PHOTODIODE CURRENT vs. R
APCSET
MODSET
90
80
70
60
50
40
30
20
10
0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
100
90
80
70
60
50
40
30
20
155mbps
1
10
100
0.1
1
10
100
0
10 20 30 40 50 60 70 80 90
(mA)
R
(kΩ)
R
(kΩ)
APCSET
I
MODSET
MOD
4
_______________________________________________________________________________________
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
Typical Operating Characteristics (continued)
(V
= +3.3V, C
= 0.01µF, I
= 20mA, I
= 30mA, T = +25°C, unless otherwise noted.)
CC
APC
BIAS
MOD
A
TEMPERATURE COMPENSATION vs.
(R = 500Ω)
RANDOM JITTER vs.
MODULATION CURRENT
COMPENSATION (K) vs. R
MODBCOMP
R
TH_TEMP MODTCOMP
10
1
100
90
80
70
60
50
40
30
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
R
= 12kΩ
TH_TEMP
R
= 7kΩ
TH_TEMP
R
= 4kΩ
TH_TEMP
R
= 2kΩ
0.1
0.01
TH_TEMP
0.001
0.01
0.1
1
10
100
-10
0
10 20 30 40 50 60 70 80 90
0
10 20 30 40 50 60 70 80 90
(mA)
R
(kΩ)
TEMPERATURE (°C)
I
MODBCOMP
MOD
TEMPERATURE COMPENSATION vs.
HOT PLUG WITH TX_DISABLE LOW
TRANSMITTER ENABLE
R
(R
= 10kΩ)
TH_TEMP MODTCOMP
MAX3646 toc14
MAX3646 toc15
44
42
40
38
36
34
32
30
V
3.3V
CC
3.3V
R
= 12kΩ
TH_TEMP
V
CC
LOW
t_init = 59.6ms
FAULT
0V
R
= 7kΩ
TH_TEMP
HIGH
LOW
R
= 4kΩ
= 2kΩ
TH_TEMP
FAULT
TX_DISABLE
t_on = 23.8µs
R
TH_TEMP
TX_DISABLE
LOW
LOW
LASER
OUTPUT
LASER
OUTPUT
20ms/div
10µs/div
-10
0
10 20 30 40 50 60 70 80 90 100
TEMPERATURE (°C)
RESPONSE TO FAULT
TRANSMITTER DISABLE
FAULT RECOVERY TIME
MAX3646 toc17
MAX37646 toc16
MAX3646 toc18
V
PC_MON
FAULT
V
V
CC
PC_MON
EXTERNALLY
FORCED FAULT
EXTERNALLY
FORCED FAULT
3.3V
LOW
FAULT
t_fault = 160ns
FAULT
HIGH
HIGH
t_init = 58ms
HIGH
LOW
91.2ns
TX_DISABLE
LOW
TX_DISABLE
TX_DISABLE
LASER
OUTPUT
LASER
OUTPUT
LOW
LOW
LASER
OUTPUT
400ns/div
20ns/div
40ms/div
_______________________________________________________________________________________
5
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
Pin Description
PIN
NAME
FUNCTION
Modulation-Current Compensation from Temperature. A resistor at this pin sets the temperature
1
MODTCOMP coefficient of the modulation current when above the threshold temperature. Leave open for zero
temperature compensation.
2, 5, 14,
17
V
+3.3V Supply Voltage
CC
3
4
IN+
IN-
Noninverted Data Input
Inverted Data Input
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.
6
7
8
9
TX_DISABLE
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.
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.
BC_MON
Shutdown Driver Output. Voltage output to control an external transistor for optional shutdown
circuitry.
SHUTDOWN
10, 12
11
GND
TX_FAULT
BIAS
Ground
Open-Collector Transmit Fault Indicator (Table 1)
Laser Bias-Current Output
13
15
OUT-
Inverted Modulation-Current Output. I
MOD
Noninverted Modulation-Current Output. I
flows into this pin when input data is low.
16
OUT+
flows into this pin when input data is high.
MOD
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.
18
19
MD
Connect a capacitor (C
) between pin 19 (APCFILT1) and pin 20 (APCFILT2) to set the dominant
APC
APCFILT1
pole of the APC feedback loop.
20
21
APCFILT2
APCSET
(See Pin 19)
A resistor connected from this pin to ground sets the desired average optical power.
A resistor connected from this pin to ground sets the desired constant portion of the modulation
current.
22
23
24
EP
MODSET
MODBCOMP
TH_TEMP
Modulation-Current Compensation from Bias. Couples the bias current to the modulation current.
Mirrors I
BIAS
through an external resistor. Leave open for zero-coupling.
Threshold for Temperature Compensation. A resistor at this pin programs the temperature above
which compensation is added to the modulation current.
Ground. Solder the exposed pad to the circuit board ground for specified thermal and electrical
performance.
Exposed Pad
6
_______________________________________________________________________________________
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
VOLTAGE
V
V
CC
CC
SINGLE ENDED
V
IN+
100mV (min)
V
1200mV (max)
IN-
30Ω
30Ω
Z = 30Ω
OUT-
0
DIFFERENTIAL
(V ) - (V
)
200mV (min)
IN+
IN-
30Ω
0.5pF
2400mV (max)
MAX3646
OSCILLOSCOPE
I
OUT+
CURRENT
Z = 30Ω
0
Z = 50Ω
0
OUT+
I
75Ω
50Ω
OUT+
I
MOD
TIME
Figure 1. Required Input Signal and Output Polarity
Figure 2. Test Circuit for Characterization
HOST BOARD
MODULE
FILTER DEFINED BY SFP MSA
TO LASER
DRIVER V
L1
1µH
SOURCE
NOISE
CC
OPTIONAL
OPTIONAL
VOLTAGE
SUPPLY
C1
0.1µF
C2
10µF
C3
0.1µF
Figure 3. Supply Filter
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
Maxim Application Note HFAN 0.0: Interface Maxim’s
Laser Driver to Laser Diode for more information.
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).
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.
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.
Extinction Ratio Control
The extinction ratio (r ) is the laser on-state power
e
divided by the off-state power. Extinction ratio remains
constant if peak-to-peak and average power are held
constant:
r = (2P
e
+ P ) / (2P
- P
)
P-P
AVG
P-P
AVG
To interface with the laser diode, a damping resistor
(R ) is required. The combined resistance damping
D
resistor and the equivalent series resistance (ESR) of
_______________________________________________________________________________________
7
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
V
CC
SHUTDOWN
MAX3646
INPUT BUFFER
IN+
DATA
PATH
OUT-
OUT+
R
D
IN-
I
MOD
ENABLE
I
MOD
SHUTDOWN
TX_FAULT
SAFETY LOGIC
AND
POWER DETECTOR
I
BIAS
BIAS
ENABLE
V
CC
I
BIAS
TX_DISABLE
R
= 45kΩ
PULL
V
R
I
CC
APCSET
MD
1
V
APCSET
MD
BG
I
BIAS
PC_MON
x1/2
R
PC_MON
xTC
x268
xK
I
APCSET
I
BIAS
82
T > T
I
MD
TH
BC_MON
C
MD
R
BC_MON
T
x1
V
BG
TH_TEMP
MODTCOMP
MODSET MODBCOMP
APCFILT1
APCFILT2
R
R
R
R
MODBCOMP
TH_TEMP
MODTCOMP
MODSET
C
APC
Figure 4. Functional Diagram
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 (h) of laser over time and temperature:
K = ∆I
/ ∆I
MOD BIAS
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
P
AVG
= I /ρ
MD MON
a user-defined threshold (T ).
TH
P
P-P
= η x I
MOD
Modulation compensation from bias increases the mod-
ulation current by a user-selected proportion (K) need-
ed to maintain peak-to-peak laser power as bias
current increases with temperature. Refer to Maxim
Application Note HFAN-02.21 for details:
8
_______________________________________________________________________________________
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
V
CC
POR AND COUNTER
60ms DELAY
I
MOD
TX_DISABLE
ENABLE
COUNTER
60ms DELAY
I
BIAS
100ns DELAY
ENABLE
V
CC
I
MD
1
V
V
REF
REF
PC_MON
Q
R
S
COMP
COMP
V
R
R
CC
PC_MON
RS
LATCH
I
BIAS
82
BC_MON
SHUTDOWN
TX_FAULT
BC_MON
CMOS
EXCESSIVE
APC CURRENT
SETPOINT
EXCESSIVE
MOD CURRENT
SETPOINT
TTL
OPEN COLLECTOR
Figure 5. Simplified Safety Circuit
Table 1. Typical Fault Conditions
If any of the I/O pins are shorted to GND or V
exceeds the programmed threshold.
(single-point failure; see Table 2), and the bias current or the photocurrent
CC
1
2
3
End-of-life (EOL) condition of the laser diode. The bias current and/or the photocurrent exceed the programmed threshold.
Laser cathode is grounded and photocurrent exceeds the programming threshold.
No feedback for the APC loop (broken interconnection, defective monitor photodiode), and the bias current exceeds the
programmed threshold.
4
_______________________________________________________________________________________
9
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
Table 2. Circuit Responses to Various Single-Point Faults
CIRCUIT RESPONSE TO OVERVOLTATGE OR
SHORT TO V
CIRCUIT RESPONSE TO UNDERVOLTAGE OR
SHORT TO GROUND
PIN
CC
TX_FAULT
Does not affect laser power.
Does not affect laser power.
TX_DISABLE Modulation and bias currents are disabled.
The optical average power increases and a fault occurs
Normal condition for circuit operation.
The optical average power decreases and the APC loop
responds by increasing the bias current. A fault state
IN+
if V
exceeds the threshold. The APC loop
PC_MON
responds by decreasing the bias current.
The optical average power decreases and the APC loop The optical average power increases and a fault occurs
responds by increasing the bias current. A fault state if V exceeds the threshold. The APC loop
occurs if V exceeds the threshold voltage.
occurs if V
exceeds the threshold voltage.
BC_MON
IN-
PC_MON
responds by decreasing the bias current.
BC_MON
The APC circuit responds by increasing the bias current
until a fault is detected, then a fault* state occurs.
MD
SHUTDOWN
BIAS
This disables bias current. A fault state occurs.
Does not affect laser power. If the shutdown circuitry is
used, the laser current is disabled.
Does not affect laser power.
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.
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+
OUT-
Does not affect laser power.
Fault state* occurs.
Does not affect laser power.
Does not affect laser power.
Does not affect laser power.
PC_MON
BC_MON
Fault state* occurs.
I
increases until V
exceeds the threshold
exceeds the threshold
I
increases until V
exceeds the threshold
exceeds the threshold
BIAS
BC_MON
BC_MON
BIAS
BC_MON
APCFILT1
APCFILT2
voltage.
voltage.
I increases until V
BIAS
I
increases until V
BIAS
BC_MON
voltage.
voltage.
MODSET
APCSET
Does not affect laser power.
Does not affect laser power.
Fault state* occurs.
Fault state* occurs.
*A fault state asserts the TX_FAULT pin, disables the modulation and bias currents, and asserts the SHUTDOWN pin.
10 ______________________________________________________________________________________
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
isolation resistors are included to reduce the number of
Table 3. Optical Power Relations
components needed to implement this function.
PARAMETER
Average power
SYMBOL
RELATION
= (P + P ) / 2
Design Procedure
P
P
AVG
AVG
0
1
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%.
Extinction ratio
r
r = P / P
e 1 0
e
Optical power of a one
Optical power of a zero
Optical amplitude
P
P
P = 2P x r / (r + 1)
AVG e e
1
0
1
P = 2P
/ (r + 1)
e
0
AVG
P
P
= P - P
P-P 1 0
P-P
Laser slope efficiency
Modulation current
Threshold current
η
η = P
/ I
P-P MOD
I
I
= P
/ η
P-P
MOD
MOD
For a desired laser average optical power (P
) and
AVG
I
P at I ≥ I
0 TH
TH
optical extinction ratio (r ), the required bias and modu-
e
Bias current
(AC-coupled)
lation currents can be calculated using the equations in
Table 3. Proper setting of these currents requires
I
I
≥ I + I
/ 2
BIAS
BIAS
TH
MOD
knowledge of the laser to monitor transfer (ρ
slope efficiency (η).
) and
MON
Laser to monitor
transfer
ρ
I
/ P
MD AVG
MON
Note: Assuming a 50% average input duty cycle and mark
density.
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:
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
P
AVG
= I
/ ρ
MD MON
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
be pulled high with a 4.7kΩ to 10kΩ resistor to V
as
CC
MODSET pin. See the I
vs. R
graph in the
APCSET
MD
required by the SFP MSA. A single-point fault can be a
short to V or GND. See Table 2 to view the circuit
Typical Operating Characteristics and select the value
of R that corresponds to the required current at
CC
APCSET
+25°C:
response to various single-point failure. The transmit
fault condition is latched until reset by a toggle or
TX_DISABLE or V . The laser driver offers redundant
CC
I
= 1/2 x V
/ R
MD
REF ACPSET
laser diode shutdown through the optional shutdown
circuitry as shown in the Typical Operation Circuit. This
shutdown transistor prevents a single-point fault at the
laser from creating an unsafe condition.
The laser driver automatically adjusts the bias to main-
tain the constant average power. For DC-coupled laser
diodes:
I
= I
+ I
/ 2
AVG
BIAS
MOD
Safety Circuitry Current Monitors
Programming the Modulation
Current with Compensation
The MAX3646 features monitors (BC_MON, PC_MON)
for bias current (I
) and photocurrent (I ). The
MD
BIAS
Determine the modulation current form the laser slope
efficiency:
monitors are realized by mirroring a fraction of the cur-
rents and developing voltages across external resistors
connected to ground. Voltages greater than V
at
I
= 2 x P / η x (r - 1)/(r + 1)
AVG e e+
REF
MOD
PC_MON or BC_MON result in a fault state. For exam-
ple, connecting a 100Ω resistor to ground at each mon-
itor output gives the following relationships:
The modulation current of the MAX3646 consists of a
static modulation current (I ), a current proportion-
MODS
al to I
, and a current proportional to temperature.
BIAS
V
= (I
/ 82) x 100Ω
The portion of I
set by MODSET is established by
BC_MON
BIAS
MOD
an internal current regulator, which maintains the refer-
ence voltage of V across the external programming
V
= I
x 100Ω
PC_MON
MD
REF
External sense resistors can be used for high-accuracy
measurement of bias and photodiode currents. On-chip
resistor. See the I
vs. R
graph in the
MODSET
MOD
Typical Operating Characteristics and select the value
______________________________________________________________________________________ 11
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
of R
that corresponds to the required current
Current Compliance (I
≤ 60mA),
MODSET
at +25°C:
MOD
DC-Coupled
The minimum voltage at the OUT+ and OUT- pins is
0.7V.
I
= I
+ K x I
+ I
MOD
MODS
BIAS MODT
I
= 268 x V
/ R
MODS
REF
MODSET
| T > T
| T < T
For:
I
I
= TC x (T - T )
TH
MODT
MODT
TH
TH
V
= Diode bias point voltage (1.2V typ)
DIODE
= 0
R = Diode bias point resistance (5Ω typ)
L
An external resistor at the MODBCOMP pin sets current
proportional to I
pin can turn off the interaction between I
R = Series matching resistor (20Ω typ)
D
. Open circuiting the MODBCOMP
BIAS
and I
:
MOD
For compliance:
BIAS
K = 1700 / (1000 + R
MODBCOMP
) +10%
to I
V
OUT+
= V
- V
- I
x (R + R ) -
CC
DIODE MOD
D
L
I
x R ≥ 0.7V
BIAS
L
If I
must be increased from I
to
MOD2
MOD
MOD1
maintain the extinction ratio at elevated temperatures,
the required compensation factor is:
Current Compliance (I
> 60mA),
MOD
AC-Coupled
K = (I
- I
) / (I
- I
)
MOD2 MOD1
BIAS2 BIAS1
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
A threshold for additional temperature compensation
can be set with a programming resistor at the
TH_TEMP pin:
T
TH
= -70°C + 1.45MΩ / (9.2kΩ + R )°C +10%
TH_TEMP
The temperature coefficient of thermal compensation
above T is set by R . Leaving the
modulation output at V . Such a configuration isolates
CC
TH
MODTCOMP
laser forward voltage from the output circuitry and allows
the output at OUT+ to swing above and below the sup-
MODTCOMP pin open disables additional thermal
compensation:
ply voltage (V ). When AC-coupled, the MAX3646
CC
modulation current can be programmed up to 85mA.
TC = 1 / (0.5 + R
(kΩ)) mA/°C +10%
MODTCOMP
Refer to Maxim Application Note HFAN 02.0: Interfacing
V
CC
V
CC
MAX3646
PACKAGE
0.7nH
16kΩ
PACKAGE
0.7nH
V
CC
OUT-
IN+
0.11pF
0.11pF
5kΩ
5kΩ
0.7nH
OUT+
0.11pF
V
CC
0.7nH
IN-
0.11pF
MAX3646
24kΩ
Figure 6. Simplified Input Structure
Figure 7. Simplified Output Structure
12 ______________________________________________________________________________________
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
Maxim’s Laser Drivers to Laser Diodes for more informa-
balance between turn-on time and low-frequency cutoff
may be needed at low data rates for some values of
laser gain.
tion on AC-coupling laser drivers to laser diodes.
For compliance:
V
= V
- I
/ 2 x (R + R ) ≥ 0.75V
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.
OUT+
CC MOD D L
Determine C
APC
The APC loop filter capacitor (C
) must be selected
APC
to balance the requirements for fast turn-on and mini-
mal interaction with low frequencies in the data pattern.
The low-frequency cutoff is:
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.
1.1
ρ
C
(µF) ≅ 68 / (f
(kHz) x (η x
3dB
)
MON
APC
High-frequency noise can be filtered with an additional
cap, C , from the MD pin to ground:
MD
C
MD
≅ C
/ 4
APC
The MAX3646 is designed so turn-on time is faster than
1ms for most laser gain values (η x ρ
smaller value of C
). Choosing a
MON
reduces turn-on time. Careful
APC
Typical Application Circuit
+3.3V
OPTIONAL SHUTDOWN
CIRCUITRY
+3.3V
0.1µF
0.01µF
+3.3V
IN+
15Ω
CDR
OUT-
0.1µF
10Ω
IN-
OUT+
R
R
MODBCOMP
MAX3646
MODBCOMP
MODTCOMP
TH_TEMP
BIAS
MD
FERRITE BEAD
MODTCOMP
R
TH_TEMP
C
MD
C
APC
REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE.
______________________________________________________________________________________ 13
155Mbps to 622Mbps SFF/SFP
Laser Driver with Extinction Ratio Control
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information,
go to www.maxim-ic.com/packages.)
PACKAGE OUTLINE
12, 16, 20, 24L THIN QFN, 4x4x0.8mm
1
C
21-0139
2
PACKAGE OUTLINE
12, 16, 20, 24L THIN QFN, 4x4x0.8mm
2
C
21-0139
2
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2004 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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