General Description
The MAX3738 is a +3.3V laser driver designed for multirate transceiver modules with data rates from 1Gbps
to 2.7Gbps. Lasers can be DC-coupled to the
MAX3738 for reduced component count and ease of
multirate operation.
Laser extinction ratio control (ERC) combines the features
of automatic power control (APC), modulation compensation, and built-in thermal compensation. The APC loop
maintains constant average optical power. Modulation
compensation increases the modulation current in proportion to the bias current. These control loops, combined with thermal compensation, maintain a constant
optical extinction ratio over temperature and lifetime.
The MAX3738 accepts differential data input signals.
The wide 5mA to 60mA (up to 85mA AC-coupled) modulation current range and up to 100mA bias current
range, make the MAX3738 ideal for driving FP/DFB
lasers in fiber optic modules. External resistors set the
required laser current levels. The MAX3738 provides
transmit disable control (TX_DISABLE), single-point
fault tolerance, bias-current monitoring, and photocurrent 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 MAX3738 is
compliant with the SFF-8472 transmitter diagnostic and
SFP MSA timing requirements.
The MAX3738 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-24 to OC-48 FEC Transceivers
Gigabit Ethernet SFF/SFP and GBIC
Transceivers
1Gbps/2Gbps Fibre Channel SFF/SFP and GBIC
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
MAX3738
1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
________________________________________________________________ Maxim Integrated Products 1
Ordering Information
19-3162; Rev 0; 1/04
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.
PART
TEMP
RANGE
PINPACKAGE
PKG
CODE
MAX3738ETG
T2444-1
BIAS
APCFILT2
V
CC
V
CC
TX_DISABLE
MODTCOMP
TH_TEMP
OUT+
APCFILT1
SHUTDOWN
GND
BC_MON
TX_FAULT
GND
V
CC
V
CC
MD
MODSET
APCSET
1
2
3
4
5
6
7891011 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
MAX3738
Pin Configuration
Typical Application Circuit appears at end of data sheet.
-40°C to 85°C 24 Thin QFN
MAX3738
1Gbps to 2.7Gbps 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, I
BIAS
= 60mA, I
MOD
= 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 (V
CC
+ 0.5V)
OUT+, OUT-, BIAS Current.............................-20mA to +150mA
Continuous Power Dissipation (T
A
= +85°C)
24-Pin TQFN (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
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
POWER SUPPLY
Supply Current I
CC
(Note 3) 47 60 mA
Power-Supply Noise Rejection PSNR f ≤ 1MHz, 100mA
P-P
(Note 4) 33 dB
I/O SPECIFICATIONS
Differential Input Swing V
ID
DC-coupled, Figure 1 0.2 2.4
V
P-P
Common-Mode Input V
CM
1.7
V
CC
V
LASER BIAS
Bias-Current-Setting Range 1
mA
Bias Off Current TX_DISABLE = high 0.1 mA
Bias-Current Monitor Ratio I
BIAS
/ I
BC_MON
68 79 95
mA/mA
LASER MODULATION
Modulation Current-Setting
Range
I
MOD
(Note 5) 5 85 mA
Output Edge Speed
20% to 80%
(Notes 6, 7)
5mA ≤ I
MOD
≤ 85mA 65 80 ps
Output Overshoot/Undershoot With 1pF between OUT+ and OUT- ±6%
Random Jitter (Notes 6, 7) 0.62 1.3
ps
RMS
2.7Gbps, 5mA ≤ I
MOD
≤ 85mA 18 40
Deterministic Jitter (Notes 6, 8)
1.25Gbps, 5mA ≤ I
MOD
≤ 85mA 20 41
ps
P-P
5mA ≤ I
MOD
≤ 10mA
Modulation-Current Temperature
Stability
(Note 6)
ppm/°C
5mA ≤ I
MOD
≤ 10mA
Modulation-Current-Setting Error
15Ω load,
T
A
= +25°C
%
Modulation Off Current TX_DISABLE = high 0.1 mA
AUTOMATIC POWER AND EXTINCTION RATIO CONTROLS
Monitor-Diode Input Current
Range
I
MD
Average current into the MD pin 18
µA
MD Pin Voltage 1.4 V
MD Current Monitor Ratio IMD / I
PC_MON
0.85 0.93
mA/mA
10mA ≤ I
10mA < I
≤ 85mA ±125 ±480
MOD
≤ 85mA ±15
MOD
±175 ±600
VID / 4
100
±20
1500
1.15
MAX3738
1Gbps to 2.7Gbps 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 I
BIAS
and I
MOD
. Input data is AC-coupled. TX_FAULT open, SHUTDOWN open.
Note 4: Power-supply noise rejection (PSNR) = 20log
10(Vnoise (on VCC
) / ∆V
OUT
). V
OUT
is the voltage across the 15Ω load 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 2.7Gbps.
Note 8: DJ includes pulse-width distortion (PWD).
PARAMETER
SYMBOL
CONDITIONS MIN TYP
MAX
UNITS
APC Loop Time Constant
3.3 µs
APC Setting Stability (Note 6)
ppm/°C
APC Setting Accuracy TA = +25°C
%
I
MOD
Compensation-Setting
Range by Bias
KK = ∆I
MOD
/ ∆I
BIAS
0 1.5
mA/mA
I
MOD
Compensation-Setting
Range by Temperature
TC TC = ∆I
MOD
/ ∆T (Note 6) 0 1.0
mA/°C
Threshold-Setting Range for
Temperature Compensation
T
TH
(Note 6) +10
°C
LASER SAFETY AND CONTROL
Bias and Modulation Turn-Off
Delay
C
APC_FILT
= 0.01µF, ∆IMD / ∆I
BIAS
= 1/80
(Note 6)
5µs
Bias and Modulation Turn-On
Delay
C
APC_FILT
= 0.01µF, ∆IMD / ∆I
BIAS
= 1/80
(Note 6)
µs
V
REF
Figure 5 1.14 1.3
V
INTERFACE SIGNALS
TX_DISABLE Input High V
HI
2.0 V
TX_DISABLE Input Low V
LO
R
PULL
= 45kΩ (typ) 0.8 V
VHI = V
CC
15
TX_DISABLE Input Current
V
LO
= GND -70
µA
TX_FAULT Output Low Sinking 1mA, open collector 0.4 V
Shutdown Output High Sourcing 100µA
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, I
BIAS
= 60mA, I
MOD
= 60mA, TA= +25°C, unless
otherwise noted.) (Notes 1, 2)
Threshold Voltage at Monitor Pins
C
APC_FILT
= 0.01µF, ∆IMD / ∆I
BIAS
= 1/70
±100 ±480
±15
+60
600
1.39
VCC - 0.4
-140
MAX3738
1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
4 _______________________________________________________________________________________
Typical Operating Characteristics
(VCC= +3.3V, C
APC
= 0.01µF, I
BIAS
= 20mA, I
MOD
= 30mA, TA= +25°C, unless otherwise noted.)
OPTICAL EYE DIAGRAM
(2.7Gbps, 2
7
- 1 PRBS, 2.3GHz FILTER)
MAX3738 toc01
54ps/div
1310nm FP LASER
r
e
= 8.2dB
OPTICAL EYE DIAGRAM
(1.25Gbps, 2
7
- 1 PRBS, 940MHz FILTER)
MAX3738 toc02
116ps/div
1310nm FP LASER
r
e
= 8.2dB
ELECTRICAL EYE DIAGRAM
(I
MOD
= 30mA, 2.7Gbps, 27 - 1 PRBS)
MAX3738 toc03
52ps/div
75mV/div
1pF BETWEEN OUT+
AND OUT-
SUPPLY CURRENT (ICC) vs. TEMPERATURE
(EXCLUDES BIAS AND MODULATION CURRENTS)
MAX3738 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
MAX3738 toc05
TEMPERATURE (°C)
I
BIAS
/I
BC_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
MAX3738 toc06
TEMPERATURE (°C)
I
MD
/I
PC_MON
(mA/mA)
807050 60-10 0 10 20 30 40-30 -20
0.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
0.80
-40 90
MODULATION CURRENT vs. R
MODSET
MAX3738 toc07
R
MODSET
(kΩ)
I
MOD
(mA)
10
10
20
30
40
50
60
70
80
90
0
1100
PHOTODIODE CURRENT vs. R
APCSET
MAX3738 toc08
R
APCSET
(kΩ)
I
MD
(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
MAX3738 toc09
I
MOD
(mA)
DJ (ps
P-P
)
807050 6020 30 4010
5
10
15
20
25
30
35
40
45
50
0
090
2.7Gbps
MAX3738
1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
_______________________________________________________________________________________ 5
RANDOM JITTER
vs. MODULATION CURRENT
MAX3738 toc10
I
MOD
(mA)
RJ (ps
RMS
)
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. R
MODBCOMP
MAX3738 toc11
R
MODBCOMP
(kΩ)
K (mA/mA)
1010.10.01
0.1
1
10
0.01
0.001 100
TEMPERATURE COMPENSATION vs.
R
TH_TEMP
(R
MODTCOMP
= 500Ω)
MAX3738 toc12
TEMPERATURE (°C)
I
MOD
(mA)
80706050403020100
40
50
60
70
80
90
100
30
-10 90
R
TH_TEMP
= 12k
Ω
R
TH_TEMP
= 7k
Ω
R
TH_TEMP
= 4k
Ω
R
TH_TEMP
= 2k
Ω
TEMPERATURE COMPENSATION vs.
R
TH_TEMP
(R
MODTCOMP
= 10kΩ)
MAX3738 toc13
TEMPERATURE (°C)
I
MOD
(mA)
80706050403020100
32
34
36
38
40
42
44
30
-10 10090
R
TH_TEMP
= 12k
Ω
R
TH_TEMP
= 7k
Ω
R
TH_TEMP
= 4k
Ω
R
TH_TEMP
= 2k
Ω
HOT PLUG WITH TX_DISABLE LOW
MAX3738 toc14
20ms/div
V
CC
FAULT
TX_DISABLE
LASER
OUTPUT
0V
3.3V
t_init = 59.6ms
LOW
LOW
TRANSMITTER ENABLE
MAX3738 toc15
10µs/div
V
CC
FAULT
TX_DISABLE
LASER
OUTPUT
3.3V
t_on = 23.8µs
LOW
HIGH
LOW
TRANSMITTER DISABLE
MAX3738 toc16
20ns/div
V
CC
FAULT
TX_DISABLE
LASER
OUTPUT
3.3V
91.2ns
LOW
HIGH
LOW
RESPONSE TO FAULT
MAX3738 toc17
400ns/div
V
PC_MON
FAULT
TX_DISABLE
LASER
OUTPUT
t_fault = 160ns
EXTERNALLY
FORCED FAULT
FAULT RECOVERY TIME
MAX3738 toc18
40ms/div
V
PC_MON
FAULT
TX_DISABLE
LASER
OUTPUT
t_init = 58ms
EXTERNALLY
FORCED FAULT
LOW
LOWLOW
HIGH
HIGH
Typical Operating Characteristics (continued)
(VCC= +3.3V, C
APC
= 0.01µF, I
BIAS
= 20mA, I
MOD
= 30mA, TA= +25°C, unless otherwise noted.)
MAX3738
1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
6 _______________________________________________________________________________________
PIN
FUNCTION
1
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
V
CC
+3.3V Supply Voltage
3 IN+ Noninverted Data Input
4 IN- Inverted Data Input
6
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 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. I
MOD
flows into this pin when input data is low.
16 OUT+ Noninverted Modulation-Current Output. I
MOD
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 (C
APC
) 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.
22 MODSET
A resistor connected from this pin to ground sets the desired constant portion of the
modulation current.
23
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.
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
Ground. Solder the exposed pad to the circuit board ground for specified thermal and electrical
performance.
Pin Description
NAME
MODTCOMP
TX_DISABLE
SHUTDOWN
MODBCOMP
Exposed Pad
Detailed Description
The MAX3738 laser driver consists of three main parts:
a high-speed modulation driver, biasing block with
ERC, and safety circuitry. The circuit design is optimized for high-speed, low-voltage (+3.3V) operation
(Figure 4).
High-Speed Modulation Driver
The output stage is composed of a high-speed differential pair and a programmable modulation current
source. The MAX3738 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
(R
D
) 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 inductance, 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.
At data rates of 2.7Gbps, 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 parasitics 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 = (2P
AVG
+ P
P-P
) / (2P
AVG
- P
P-P
)
MAX3738
1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
_______________________________________________________________________________________ 7
V
IN+
V
IN-
200mV (min)
2400mV (max)
100mV (min)
1200mV (max)
I
MOD
VOLTAGE
CURRENT
TIME
SINGLE ENDED
DIFFERENTIAL
(V
IN+
) - (V
IN-
)
I
OUT+
Figure 1. Required Input Signal and Output Polarity
MAX3738
30Ω 30Ω
30Ω
75Ω 50Ω
OUT+
OUT-
V
CCVCC
OSCILLOSCOPE
Z
0
= 30Ω
Z
0
= 30Ω Z0 = 50Ω
0.5pF
I
OUT+
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 V
CC
Figure 3. Supply Filter
MAX3738
Average power is regulated using APC, which keeps
constant current from a photodiode coupled to the
laser. Peak-to-peak power is maintained by compensating the modulation current for reduced slope efficiency (h) of laser over time and temperature:
P
AVG
= I
MD
/
ρ
MON
P
P-P
= η x I
MOD
Modulation compensation from bias increases the modulation current by a user-selected proportion (K) needed to maintain peak-to-peak laser power as bias
current increases with temperature. Refer to Maxim
Application Note HFAN-02.21 for details:
K = ∆I
MOD
/ ∆I
BIAS
This provides a first-order approximation of the current
increase needed to maintain peak-to-peak power.
Slope efficiency decreases more rapidly as temperature increases. The MAX3738 provides additional temperature compensation as temperature increases past
a user-defined threshold (TTH).
1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
8 _______________________________________________________________________________________
MAX3738
IN+
IN-
OUT-
OUT+
INPUT BUFFER
DATA
PATH
I
MOD
V
BG
R
MODSET
MODSET
V
BG
R
APCSET
APCSET
I
APCSET
C
APC
APCFILT1 APCFILT2
I
BIAS
BIAS
MD
V
CC
I
BIAS
ENABLE
x1
TX_FAULT
TX_DISABLE
SAFETY LOGIC
AND
POWER DETECTOR
R
BC_MON
BC_MON
V
CC
R
PC_MON
PC_MON
R
D
V
CC
I
MD
C
MD
SHUTDOWN
SHUTDOWN
x268
R
PULL
= 45k
Ω
T
xKxTC
T > T
TH
I
BIAS
I
MOD
ENABLE
x1/2
MODTCOMP
R
MODTCOMP
TH_TEMP
R
TH_TEMP
MODBCOMP
R
MODBCOMP
I
BIAS
82
I
MD
1
Figure 4. Functional Diagram
MAX3738
1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
_______________________________________________________________________________________ 9
1
If 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.
4
No feedback for the APC loop (broken interconnection, defective monitor photodiode), and the bias current exceeds the
programmed threshold.
Table 1. Typical Fault Conditions
R
BC_MON
BC_MON
V
CC
V
CC
R
PC_MON
PC_MON
COMP
V
REF
V
REF
TTL
OPEN COLLECTOR
CMOS
SHUTDOWN
TX_FAULT
R
S
Q
RS
LATCH
COUNTER
60ms DELAY
POR AND COUNTER
60ms DELAY
100ns DELAY
I
BIAS
ENABLE
I
MOD
ENABLE
V
CC
TX_DISABLE
COMP
EXCESSIVE
APC CURRENT
SETPOINT
EXCESSIVE
MOD CURRENT
SETPOINT
I
MD
1
I
BIAS
82
Figure 5. Simplified Safety Circuit
MAX3738
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 4.7kΩ to 10kΩ resistor to VCCas
required by the SFP MSA. A single-point fault can be a
short to VCCor 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 Applications Circuit.
This shutdown transistor prevents a single-point fault at
the laser from creating an unsafe condition.
Safety Circuitry Current Monitors
The MAX3738 features monitors (BC_MON, PC_MON)
for bias current (I
BIAS
) and photocurrent (IMD). The
monitors are realized by mirroring a fraction of the currents and developing voltages across external resistors
connected to ground. Voltages greater than V
REF
at
PC_MON or BC_MON result in a fault state. For example, connecting a 100Ω resistor to ground at each monitor output gives the following relationships:
V
BC_MON
= (I
BIAS
/ 82) x 100Ω
V
PC_MON
= IMDx 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.
1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
10 ______________________________________________________________________________________
PIN
CIRCUIT RESPONSE TO OVERVOLTATGE OR
SHORT TO V
CC
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 V
PC_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 V
BC_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 V
BC_MON
exceeds the threshold voltage.
The optical average power increases and a fault occurs
if V
PC_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
I
BIAS
increases until V
BC_MON
exceeds the threshold
voltage.
I
BIAS
increases until V
BC_MON
exceeds the threshold
voltage.
APCFILT2
I
BIAS
increases until V
BC_MON
exceeds the threshold
voltage.
I
BIAS
increases until V
BC_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.
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 optical waveform are 50%.
For a desired laser average optical power (P
AVG
) and
optical extinction ratio (re), the required bias and modulation currents can be calculated using the equations in
Table 3. Proper setting of these currents requires
knowledge of the laser to monitor transfer (ρ
MON
) and
slope efficiency (η).
Programming the Monitor-Diode Current
Set Point
The MAX3738 operates in APC mode at all times. The
bias current is automatically set so average laser power
is determined by the APCSET resistor:
P
AVG
= I
MD
/ ρ
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
MODSET pin. See the I
MD
vs. R
APCSET
graph in the
Typical Operating Characteristics and select the value
of R
APCSET
that corresponds to the required current
at +25°C.
IMD= 1/2 x V
REF
/ R
ACPSET
The laser driver automatically adjusts the bias to maintain the constant average power. For DC-coupled
laser diodes:
I
AVG
= I
BIAS
+ I
MOD
/ 2
Programming the Modulation Current with
Compensation
Determine the modulation current from the laser slope
efficiency:
I
MOD
= 2 x P
AVG
/ η x (re- 1) / (r
e+
+ 1)
The modulation current of the MAX3738 consists of a
static modulation current (I
MODS
), a current proportion-
al to I
BIAS
, and a current proportional to temperature.
The portion of I
MOD
set by MODSET is established by
an internal current regulator, which maintains the reference voltage of V
REF
across the external programming
resistor. See the I
MOD
vs. R
MODSET
graph in the
Typical Operating Characteristics and select the value
of R
MODSET
that corresponds to the required current
at +25°C:
I
MOD
= I
MODS
+ K x I
BIAS
+ I
MODT
I
MODS
= 268 x V
REF
/ R
MODSET
I
MODT
= TC x (T - TTH) | T > T
TH
I
MODT
= 0 | T < T
TH
An external resistor at the MODBCOMP pin sets current
proportional to I
BIAS
. Open circuiting the MODBCOMP
pin can turn off the interaction between I
BIAS
and I
MOD
:
K = 1700 / (1000 + R
MODBCOMP
) ±10%
If I
MOD
must be increased from I
MOD1
to I
MOD2
to
maintain the extinction ratio at elevated temperatures,
the required compensation factor is:
K = (I
MOD2
- I
MOD1
) / (I
BIAS2
- I
BIAS1
)
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
TH_TEMP
)°C ±10%
The temperature coefficient of thermal compensation
above T
TH
is set by R
MODTCOMP
. Leaving the
MODTCOMP pin open disables additional thermal
compensation:
TC = 1 / (0.5 + R
MODTCOMP
(kΩ)) mA/°C ±10%
MAX3738
1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
______________________________________________________________________________________ 11
PARAMETER
RELATION
Average Power P
AVG
P
AVG
= (P0 + P1) / 2
Extinction Ratio r
e
re = P1 / P
0
Optical Power of a One
P
1
P1 = 2P
AVG
x re / (re + 1)
Optical Power of a Zero
P
0
P0 = 2P
AVG
/ (re + 1)
Optical Amplitude P
P-P
P
P-P
= P1 - P
0
Laser Slope Efficiency
ηη = P
P-P
/ I
MOD
Modulation Current I
MOD
I
MOD
= P
P-P
/ η
Threshold Current I
TH
P0 at I ≥ I
TH
Bias Current
(AC-Coupled)
I
BIAS
I
BIAS
≥ ITH + I
MOD
/ 2
Laser to Monitor
Transfer
ρ
MON
IMD / P
AVG
Table 3. Optical Power Relations
Note: Assuming a 50% average input duty cycle and mark
density.
SYMBOL
MAX3738
Current Compliance (I
MOD
≤ 60mA),
DC-Coupled
The minimum voltage at the OUT+ and OUT- pins
is 0.7V.
For:
V
DIODE
= Diode bias point voltage (1.2V typ)
RL= Diode bias point resistance (5Ω typ)
RD= Series matching resistor (20Ω typ)
For compliance:
V
OUT+
= VCC- V
DIODE
- I
MOD
x (RD+ RL) -
I
BIAS
x RL≥ 0.7V
Current Compliance (I
MOD
> 60mA),
AC-Coupled
For applications requiring modulation current greater
than 60mA, headroom is insufficient from proper operation of the laser driver if the laser is DC-coupled. To
avoid this problem, the MAX3738’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 supply voltage (VCC). When AC-coupled, the MAX3738
modulation current can be programmed up to 85mA.
Refer to Maxim Application Note HFAN 02.0: Interfacing
Maxim’s Laser Drivers to Laser Diodes for more information on AC-coupling laser drivers to laser diodes.
For compliance:
V
OUT+
= VCC- I
MOD
/ 2 x (RD+ RL) ≥ 0.75V
Determine C
APC
The APC loop filter capacitor (C
APC
) must be selected
to balance the requirements for fast turn-on and minimal interaction with low frequencies in the data pattern.
The low-frequency cutoff is:
C
APC
(µF) ≅ 68 / (f
3dB
(kHz) x (η x
ρ
MON
)
1.1
)
High-frequency noise can be filtered with an additional
cap, CMD, from the MD pin to ground.
CMD≅ C
APC
/ 4
The MAX3738 is designed so turn-on time is faster than
1ms for most laser gain values (η x ρ
MON
). Choosing a
smaller value of C
APC
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 circuits for the MAX3738 laser driver. If dice are used,
replace package parasitic elements with bondwire parasitic elements.
1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
12 ______________________________________________________________________________________
V
CC
V
CC
V
CC
0.11pF
0.7nH
IN+
0.11pF
0.7nH
IN-
PACKAGE
5kΩ
5kΩ
24kΩ
16kΩ
MAX3738
Figure 6. Simplified Input Structure
MAX3738
PACKAGE
0.7nH OUT-
0.7nH OUT+
0.11pF
0.11pF
V
CC
Figure 7. Simplified Output Structure
Layout Considerations
To minimize loss and crosstalk, keep the connections
between the MAX3738 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 MAX3738 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, 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 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 TQFN provides a very
low thermal resistance path for heat removal from the IC.
The pad is also electrical ground on the MAX3738 and
should be soldered to the circuit board ground for proper
thermal and electrical performance. Refer to Maxim
Application Note HFAN-08.1: Thermal Consideration
for QFN and Other Exposed-Pad Packages at
www.maxim-ic.com for additional information.
Chip Information
TRANSISTOR COUNT: 1884
PROCESSS: SiGe/Bipolar
MAX3738
1Gbps to 2.7Gbps SFF/SFP Laser Driver with
Extinction Ratio Control
______________________________________________________________________________________ 13
MAX3738
IN+
IN-
REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE.
V
CC
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
C
APC
R
PC_MON
C
MD
0.01µF
FERRITE BEAD
PC_MON
R
BC_MON
R
MODSET
R
APCSET
MODBCOMP
MODTCOMP
TH_TEMP
R
TH_TEMP
R
MODTCOMP
R
MODBCOMP
0.1µF
0.1µF
Typical Application Circuit
MAX3738
1Gbps to 2.7Gbps 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.
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.
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 QFN THIN, 4x4x0.8 mm
21-0139
1
B
2
24L QFN THIN.EPS
PACKAGE OUTLINE
12,16,20,24L QFN THIN, 4x4x0.8 mm
21-0139
2
B
2
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