MAX3740ETG [MAXIM]
3.2Gbps SFP VCSEL Driver with Diagnostic Monitors; 的3.2Gbps SFP VCSEL驱动器，带有诊断监视器
| 型号： | MAX3740ETG |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors |
| 文件： | 总14页 (文件大小：817K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-2679; Rev 2; 7/03
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
General Description
Features
The MAX3740 is a high-speed VCSEL driver for small-
form-factor (SFF) and small-form-factor pluggable (SFP)
fiber optic LAN transmitters. It contains a bias genera-
tor, a laser modulator, and comprehensive safety fea-
tures. The automatic power control (APC) adjusts the
laser bias current to maintain average optical power
over changes in temperature and laser properties. The
driver accommodates common cathode and differential
configurations.
ꢀ Supports all SFF-8472 Digital Diagnostics
ꢀ 2mA to 15mA Modulation Current
ꢀ 1mA to 15mA Bias Current
ꢀ Optional Peaking Current to Improve VCSEL Edge
Speed
ꢀ Supports Common Cathode and Differential
Configuration
The MAX3740 operates up to 3.2Gbps. It can switch up
to 15mA of laser modulation current and source up to
15mA of bias current. Adjustable temperature compen-
sation is provided to keep the optical extinction ratio
within specifications over the operating temperature
range. The MAX3740 interfaces with the Dallas DS1858
to meet SFF-8472 timing and diagnostic requirements.
The MAX3740 accommodates various VCSEL pack-
ages, including low-cost TO-46 headers.
ꢀ Automatic Power Control
ꢀ Safety Circuits Compliant with SFF and SFP
MSAs
ꢀ 4mm ✕ 4mm 24-Pin Thin QFN Package
Ordering Information
The MAX3740 safety circuit detects faults that could
cause hazardous light levels and disables the VCSEL
output. The safety circuits are compliant with SFF and
SFP multisource agreements (MSA).
PART
TEMP RANGE
PIN-PACKAGE
MAX3740ETG -40°C to +85°C 24 Thin QFN (4mm ✕ 4mm)
The MAX3740 is available in a compact 4mm ✕ 4mm,
24-pin thin QFN package and operates over the -40°C
to +85°C temperature range.
Applications
Typical Application Circuit
Multirate (1Gbps to 3.2Gbps) SFP/SFF Modules
Gigabit Ethernet Optical Transmitters
Fibre Channel Optical Transmitters
Infiniband Optical Transmitters
+3.3V
FAULT
V
4.7kΩ
CC
OUT1
IN1
FAULT
PWRMON
MODSET
REF
TX_DISABLE
SQUELCH
MON2
H0
H1
DS1858
L0 L1
MON1
MAX3740
BIASMON
COMP
0.1µF
R
IN+
IN-
BIASMON
0.047µF
MD
0.1µF
BIAS
TC1
L1*
†
R
TC
0.01µF
OUT+
OUT-
TC2
†
C
F
BIASSET
GND
PEAKSET
R
0.01µF
50Ω
R
BIASSET
†
†
R
F
PEAKSET
†
OPTIONAL COMPONENT
*FERRITE BEAD
________________________________________________________________ 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.
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (V ) ..............................................-0.5V to 6.0V
Continuous Power Dissipation (T = +85°C)
CC
A
Voltage at TX_DISABLE, IN+, IN-, FAULT,
SQUELCH TC1, TC2, MODSET, PEAKSET, BIASSET,
BIAS, BIASMON, COMP, MD, REF,
24-Lead Thin QFN
(derate 20.8mW/°C above +85°C).................................1354mW
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range.............................-55°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
PWRMON ...............................................-0.5V to (V
+ 0.5V)
+ 2V)
CC
Voltage at OUT+, OUT-.........................(V
- 2V) to (V
CC
CC
Current into FAULT ............................................ -1mA to +25mA
Current into OUT+, OUT-....................................................60mA
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, TC1 and TC2 are shorted, PEAKSET open, T =
CC A
CC
A
+25°C, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
SQUELCH set low,
I
= 2mA
32
MOD
MOD
P-P
TX_DISABLE set low,
peaking is not used
(Note 1)
I
= 15mA
55
15
67
20
P-P
I
CC
Supply Current
mA
Additional current when peaking is used
(Note 2)
Additional current when SQUELCH is high
Total current when TX_DISABLE is high
5
10
5
I
3.9
CC-SHDN
FAULT OUTPUT
Output High Voltage
Output Low Voltage
V
R
R
= 10kΩ to 2.97V
= 4.7kΩ to 3.63V
2.4
V
V
OH
LOAD
V
0.4
40
OL
LOAD
Current into FAULT pin with V = 0V and
CC
Output Leakage
0.5
µA
V
= 3.3V
FAULT
TX_DISABLE INPUT
Input Impedance
Input High Voltage
Input Low Voltage
4.7
2.0
10.0
0.8
kΩ
V
V
IH
V
V
IL
The time for I
TX_DISABLE transitions high
to reach I
when
CC
CC-SHDN
Power-Down Time
50
µs
SQUELCH
Squelch Threshold
25
10
85
mV
mV
P-P
P-P
Squelch Hysteresis
Time to Squelch Data
Time to Resume from Squelch
BIAS GENERATOR (Note 4)
(Note 3)
(Note 3)
0.02
0.02
5.00
5.00
µs
µs
Minimum
Maximum
1
Bias Current
I
mA
%
BIAS
15
-8
5mA ≤ I
1mA ≤ I
≤ 15mA
≤ 5mA
+8
BIAS
BIAS
Accuracy of Programmed Bias
Current
∆BIAS
-12
+12
2
_______________________________________________________________________________________
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
ELECTRICAL CHARACTERISTICS (continued)
(V
= +2.97V to +3.63V, T = -40°C to +85°C. Typical values are at V
= +3.3V, TC1 and TC2 are shorted, PEAKSET open, T =
CC A
CC
A
+25°C, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Bias Current During Fault
I
Current out of the BIAS pin
1.5
10
µA
BIAS_OFF
I
< 3mA
0.0925 0.105 0.1375
BIAS
BIASMON Nominal Gain
mA/mA
3mA ≤ I
≤ 15mA
0.085
0.105
0.125
BIAS
AUTOMATIC POWER CONTROL (APC)
V
-
REF
0.2
MD Nominal Voltage
V
APC loop is closed
1
2
V
MD
Voltage at REF
V
1.2
1.8
0
2.2
V
V
REF
MD Voltage During Fault
MD Input Current
Normal operation (FAULT = low)
-2
5
0.7
10
+2
2.4
250
µA
µs
V/V
APC Time Constant
C
= 0.047µF (Note 5)
COMP
PWRMON Nominal Gain
LASER MODULATOR (Note 6)
V
/ (V
- V )
MD
1.9
2.15
PWRMON
REF
Minimum
Maximum
Data Input Voltage Swing
Output Resistance
V
mV -
P P
ID
2200
15
Single-ended resistance at OUT+
Single-ended resistance at OUT-
Minimum
80
72
105
100
2
Ω
Modulation Current
I
mA -
P P
MOD
Maximum
Minimum Peaking Current Range
Maximum Peaking Current Range
Peaking Current Duration
0.2
2
mA
mA
ps
80
Tolerance of Programmed
Modulation Current
TC1 is shorted to TC2
-10
+10
%
Minimum Programmable
Temperature Coefficient
0
ppm/°C
Maximum Programmable
Temperature Coefficient
Temperature range 0°C to +70°C
+5000
ppm/°C
Modulation Transition Time
Deterministic Jitter
Random Jitter
t , t
R
5mA ≤ I
5mA ≤ I
≤ 15mA, 20% to 80% (Note 5)
≤ 15mA, 3.2Gbps (Notes 5, 7)
65
12
95
20
4
ps
F
MOD
MOD
DJ
RJ
ps
P-P
(Note 5)
1.3
ps
RMS
Laser Modulation During Fault or
while Squelch is Active
I
15
50
µA
P-P
MOD_OFF
Input Resistance
Differential resistance
85
100
115
Ω
V
0.3
-
CC
Input Bias Voltage
V
V
IN
_______________________________________________________________________________________
3
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
ELECTRICAL CHARACTERISTICS (continued)
(V
= +2.97V to +3.63V, T = -40°C to +85°C. Typical values are at V
= +3.3V, TC1 and TC2 are shorted, PEAKSET open, T =
CC A
CC
A
+25°C, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
SAFETY FEATURES (see the Typical Operating Characteristics section)
High-Current Fault Threshold
Fault Threshold
V
V
V
V
> V causes a fault
BMTH
0.7
-0.250
0.7
0.8
-0.2
0.8
0.9
-0.150
0.9
V
V
V
BMTH
BIASMON
V
V
referenced to V
BIAS CC
BIAS
BTH
Power-Monitor Fault Threshold
V
> V
causes a fault
PMTH
PMTH
PWRMON
Time from rising edge of TX_DISABLE to
= I and I = I
(Note 5)
TX Disable Time
t_
I
1.8
55
5
µs
µs
OFF
BIAS
BIAS_OFF
MOD
MOD_OFF
Time from rising edge of TX_DISABLE to
and I at 99% of steady state
I
TX Disable Negate Time
t_
500
BIAS
MOD
ON
(Note 5)
Time to set V
after rising edge of TX_DISABLE (Note 5)
= low after power-on or
FAULT
Fault Reset Time
Power-On Time
t_
t_
1
60
60
200
200
ms
ms
INIT
INIT
Time after power-on to transmitter-on with
TX_DISABLE low (Note 5)
2
Time from fault occurrence to V
=
FAULT
Fault Assert Time
t_
high; C
(Note 5)
< 20pF, R = 4.7kΩ
1.4
1
50
µs
FAULT
FAULT
FAULT
Time from fault to I
= I
BIAS_OFF
and
BIAS
Fault Delay Time
t_
5
1
µs
µs
FLTDLY
I
= I
(Note 5)
MOD
MOD_OFF
Time TX_DISABLE must be held high to
reset FAULT (Note 5)
TX_DISABLE Reset
t_
RESET
Note 1: Supply current measurements exclude I
from the total current.
BIAS
Note 2: Tested with R
= 1.18kΩ.
PEAK
Note 3: Measured by applying a pattern that contains 20µs of K28.5, followed by 5µs of zeros, then 20µs of K28.5, followed by 5µs
of ones. Data rate is equal to 2.5Gbps, with inputs filtered using 1.8GHz Bessel filters.
Note 4: V
< V
- 0.7V.
BIAS
CC
Note 5: Guaranteed by design and characterization.
Note 6: Measured electrically with a 50Ω load AC-coupled to OUT+.
Note 7: Deterministic jitter is the peak-to-peak deviation from the ideal time crossings measured with a K28.5 bit pattern at 3.2Gbps
(00111110101100000101).
4
_______________________________________________________________________________________
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
Typical Operating Characteristics
(V = +3.3V, R = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, T = +25°C, unless otherwise
CC
noted.)
TC
A
ELECTRICAL EYE
ELECTRICAL EYE WITH PEAKING
ELECTRICAL EYE WITH MAX PEAKING
MAX3740 toc01
MAX3740 toc02
MAX3740 toc03
3.2Gbps, K28.5, 10mA MODULATION,
PEAKING OFF
3.2Gbps, K28.5, 10mA MODULATION,
3.2Gbps, K28.5, 10mA MODULATION,
R
= 2.4kΩ
R
= 500Ω
PEAKSET
PEAKSET
73mV/div
73mV/div
73mV/div
50ps/div
50ps/div
50ps/div
OPTICAL EYE
I
vs. BIAS CURRENT
OPTICAL EYE
BIASMON
MAX3740 toc05
MAX3740 toc04
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
E
= 8.2dB, 2.5Gbps, K28.5,
E
= 8.2dB, 2.125Gbps, K28.5,
850nm VCSEL, WITH 2.3GHz
O-TO-E CONVERTER
R
R
850nm VCSEL SONET MASK
WITH +20% MARGIN
EMCORE SC-TOSA-8585-3420 VCSEL
58ps/div
EMCORE SC-TOSA-8585-3420 VCSEL
68ps/div
0
4
8
12
16
BIAS CURRENT (mA)
DETERMINISTIC JITTER
vs. MODULATION CURRENT
TRANSITION TIME
vs. MODULATION CURRENT
RANDOM JITTER
vs. MODULATION CURRENT
40
35
30
25
20
15
10
5
7
6
5
4
3
2
1
0
100
90
80
70
60
50
40
RISE
FALL
0
0
5
10
15
0
5
10
15
2
4
6
8
10
(mA
12
14
16
I
(mA
P-P
)
I
(mA
P-P
)
I
)
MOD
MOD
MOD
P-P
_______________________________________________________________________________________
5
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
Typical Operating Characteristics (continued)
(V = +3.3V, R = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, T = +25°C, unless otherwise
CC
noted.)
TC
A
MONITOR DIODE CURRENT
BIAS CURRENT vs. R
MODULATION CURRENT vs. R
MODSET
vs. R
BIASSET
PWRSET
16
14
12
10
8
18
16
14
12
10
8
2000
1800
1600
1400
1200
1000
800
6
6
600
4
4
400
2
2
200
0
0
0
0
10
20
30
40
0
2
4
6
8
10
0
2
4
6
8
10
R
(kΩ)
R
(kΩ)
R
(kΩ)
BIASSET
MODSET
PWRSET
SUPPLY CURRENT vs. TEMPERATURE
INPUT RETURN LOSS
OUTPUT RETURN LOSS
0
-5
0
80
70
60
50
40
30
20
10
DIFFERENTIAL
MEASUREMENT
SINGLE-ENDED
MEASUREMENT
-2
-4
I
= 15mA
MOD
-10
-15
-20
-25
-30
-35
-40
-6
-8
-10
-12
-14
-16
-18
I
= 2mA
60
MOD
100M
1G
10G
100M
1G
FREQUENCY (Hz)
10G
-40
-15
10
35
85
FREQUENCY (Hz)
TEMPERATURE (°C)
MODULATION CURRENT
vs. TEMPERATURE
MONITOR DIODE CURRENT
vs. TEMPERATURE
MODULATION CURRENT TEMPCO
vs. R
TC
11
10
9
300
275
250
225
200
175
150
125
100
5500
4500
3500
2500
1500
500
R
TC
= 100Ω
REFERENCED TO +25°C
R
MOD
= 1.35kΩ
R
TC
= 1kΩ
R
TC
= 5kΩ
8
R
TC
= 10kΩ
7
R
TC
= 60kΩ
R
TC
= 100kΩ
6
R
TC
= 500kΩ
5
4
-500
0
10 20 30 40 50 60 70 80 90
-40
-15
10
35
60
85
100
1k
10k
100k
1M
TEMPERATURE (°C)
TEMPERATURE (°C)
R
TC
(Ω)
6
_______________________________________________________________________________________
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
Typical Operating Characteristics (continued)
(V = +3.3V, R = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, T = +25°C, unless otherwise
CC
noted.)
TC
A
HOT PLUG WITH TX_DISABLE LOW
TX_DISABLE NEGATE TIME
MAX3740 toc21
STARTUP WITH SLOW RAMPING SUPPLY
MAX3740 toc19
MAX3740 toc20
3.3V
3.3V
3.3V
V
CC
V
CC
V
CC
OV
OV
FAULT
FAULT
FAULT
LOW
LOW
HIGH
LOW
t_INIT = 62ms
TX_DISABLE
TX_DISABLE
TX_DISABLE
t_INIT = 60ms
t_ON = 54µs
LOW
LOW
LOW
LASER
OUTPUT
LASER
OUTPUT
LASER
OUTPUT
20ms/div
20µs/div
20ms/div
TRANSMITTER DISABLE
RESPONSE TO FAULT
MAX3740 toc22
MAX3740 toc23
3.3V
EXTERNALLY
FORCED
FAULT
V
V
CC
PWRMON
t_OFF = 1.86µs
t_FAULT = 245ns
FAULT
FAULT
LOW
LOW
LOW
LOW
HIGH
TX_DISABLE
TX_DISABLE
HIGH
LASER
OUTPUT
LASER
OUTPUT
1µs/div
200ns/div
FAULT RECOVERY TIME
FREQUENT ASSERTION OF TX_DISABLE
MAX3740 toc24
MAX3740 toc25
EXTERNAL
FAULT
REMOVED
EXTERNALLY
FORCED FAULT
V
V
PWRMON
PWRMON
FAULT
FAULT
HIGH
LOW
LOW
HIGH
TX_DISABLE
TX_DISABLE
LOW
t_INIT = 54µs
LASER
OUTPUT
LASER
OUTPUT
40µs/div
200µs/div
_______________________________________________________________________________________
7
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
Pin Description
PIN
NAME
FUNCTION
1, 10, 13
GND
Ground
Transmit Disable. Driver output is disabled when TX_DISABLE is high or left unconnected. The
driver output is enabled when the pin is asserted low.
2
TX_DISABLE
3
4
IN+
IN-
Noninverted Data Input
Inverted Data Input
Fault Indicator. Open-drain output. FAULT is asserted high during a fault condition. Note: This pin
does not have ESD protection.
5
FAULT
Squelch Enable. Squelch is enabled when the pin is set high. Squelch is disabled when the pin is
set low or left open.
6
7, 16, 20
8
SQUELCH
V
+3.3V Supply Voltage
CC
Temperature Compensation Set Pin 1. A resistor placed between TC1 and TC2 (R ) programs the
TC
temperature coefficient of the modulation current.
TC1
TC2
Temperature Compensation Set Pin 2. A resistor placed between TC1 and TC2 (R ) programs the
TC
temperature coefficient of the modulation current.
9
Modulation Set. A resistor connected from MODSET to ground (R
modulation current amplitude.
) sets the desired
MODSET
11
12
MODSET
PEAKSET
Peaking Current Set. A resistor connected between PEAKSET and ground (R
peaking current amplitude. To disable peaking, leave PEAKSET open.
) programs the
PEAKSET
14
15
OUT-
Inverted Modulation-Current Output
OUT+
Noninverted Modulation-Current Output
Bias Current Set. When a closed-loop configuration is used, connect a 1.7kΩ resistor between
ground and BIASSET to set the maximum bias current. When an open configuration is used,
17
18
19
BIASSET
BIAS
connect a resistor between BIASSET and ground (R
) to program the VCSEL bias current.
BIASSET
Bias Current Output
Bias Current Monitor. The output of BIASMON is a sourced current proportional to the bias current.
A resistor connected between BIASMON and ground (R ) can be used to form a ground-
BIASMON
referenced bias monitor.
BIASMON
Compensation Pin. A capacitor between COMP and MD compensates the APC. A typical value of
0.047µF is recommended. For open-loop configuration, short the COMP pin to GND to deactivate
the APC.
21
COMP
22
23
MD
Monitor Diode Connection
Reference Pin. Reference monitor used for APC. A resistor between REF and MD (R
photo monitor current when the APC loop is closed.
) sets the
PWRSET
REF
Average Power Monitor. The pin is used to monitor the transmit optical power. For open-loop
configuration, connect PWRMON to GND.
24
EP
PWRMON
Ground. Must be soldered to the circuit board ground for proper thermal and electrical
performance. See the Layout Considerations section.
Exposed Pad
8
_______________________________________________________________________________________
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
PWRMON
REF
CURRENT
AMPLIFIER
2X
1.8V
R
PWRSET
MAX3740
ENABLE
I
BIAS
40
POWER-
CONTROL
AMPLIFIER
MD
BIAS
BIAS GENERATOR
FERRITE
BEAD
SMOOTH-
START
BIASMON
I
PD
1.6V
BE
I
BIAS
9
(2V
)
0.8V
R
BIASMON
200Ω
COMP
BIASSET
R
BIASSET
C
COMP
Figure 1. Bias Generator
The BIASMON output provides a current proportional to
the laser bias current given by:
Detailed Description
The MAX3740 contains a bias generator with automatic
power control (APC), safety circuit, and a laser modula-
tor with optional peaking compensation.
I
= I
/ 9
BIASMON
BIAS
When APC is not used (no monitor diode, open-loop
configuration) connect the COMP and PWRMON pins
to GND. In this mode, the bias current is set by the
Bias Generator
Figure 1 shows the bias generator circuitry that contains
a power-control amplifier and smooth-start circuitry. An
internal PNP transistor provides DC laser current to bias
the laser in a light-emitting state. The APC circuitry
adjusts the laser-bias current to maintain average power
over temperature and changing laser properties. The
smooth-start circuitry prevents current spikes to the laser
during power-up or enable, ensuring compliance with
safety requirements and extending the life of the laser.
resistor R
. When a closed-loop configuration is
BIASSET
used, connect a 1.7kΩ resistor between ground and
BIASSET to set the maximum bias current.
Safety Circuit
The safety circuit contains an input disable
(TX_DISABLE), a latched fault output (FAULT), and fault
detectors (Figure 2). This circuit monitors the operation
of the laser driver and forces a shutdown (disables
laser) if a fault is detected (Table 1). Table 2 contains
the circuit’s response to various single-point failures.
The transmit fault condition is latched until reset by a
The MD input is connected to the cathode of a monitor
diode, which is used to sense laser power. The BIAS
output is connected to the anode of the laser through an
inductor or ferrite bead. The power-control amplifier dri-
ves a current amplifier to control the laser’s bias current.
During a fault condition, the bias current is disabled.
toggle of TX_DISABLE or V . The FAULT pin should
CC
be pulled high with a 4.7kΩ to 10kΩ resistor.
Table 1. Fault Conditions
The PWRMON output provides a voltage proportional to
average laser power given by:
PIN
FAULT CONDITION
> V - 0.2V
BIAS CC
BIAS
V
V
V
V
= 2 ✕ I
PD
✕ R
PWRSET
PWRMON
BIASMON
PWRMON
> 0.8V
BIASMON
PWRMON
> 0.8V
_______________________________________________________________________________________
9
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
Table 2. Circuit Response to Various Single-Point Faults (Closed-Loop APC Configuration)
PIN NAME
FAULT
CIRCUIT RESPONSE TO V
SHORT
CIRCUIT RESPONSE TO GND SHORT
Does not affect laser power.
CC
Does not affect laser power.
TX_DISABLE
IN+
Modulation and bias current are disabled.
Does not affect laser power.
Does not affect laser power.
Does not affect laser power.
Does not affect laser power.
Normal condition for circuit operation.
Does not affect laser power.
Does not affect laser power.
Does not affect laser power.
Does not affect laser power.
IN-
SQUELCH
TC1
The laser modulation is increased, but average power
is not affected.
TC2
Modulation current is disabled.
The laser modulation is increased, but average power
is not affected.
MODSET
Modulation current is disabled.
PEAKSET
OUT+
Does not affect laser power.
Modulation current is disabled.
Does not affect laser power.
Laser bias is disabled.
Does not affect laser power.
Modulation current is disabled.
Does not affect laser power.
Fault state* occurs.
OUT-
BIASSET
Fault state* occurs. Note that VCSEL emissions may
continue; care must be taken to prevent this condition.
BIAS
Disables VCSEL.
BIASMON
Fault state* occurs.
Does not affect laser power.
I
increases to the value determined by R
; if
BIAS
BIASSET
The bias current is reduced, and the average power of
the laser output is reduced.
COMP
MD
the bias monitor fault threshold is exceeded, a fault is
signaled.
I
increases to the value determined by R
; if
BIAS
BIASSET
The bias current is reduced, and the average power of
the laser output is reduced.
the bias-monitor fault threshold is exceeded, a fault is
signaled.
I
increases to the value determined by R
; if
BIAS
BIASSET
The bias current is reduced, and the average power of
the laser output is reduced.
the bias-monitor fault threshold is exceeded, a fault is
signaled.
REF
PWRMON
Fault state* occurs.
Does not affect laser power.
*A fault state asserts the FAULT pin, disables the modulator output, and disables the bias output.
Modulation Circuit
The modulation circuitry consists of an input buffer, a
current mirror, and a high-speed current switch (Figure
Design Procedure
Select Laser
Select a communications-grade laser with a rise time of
260ps or better for 1.25Gbps, or 130ps or better for
2.5Gbps applications. Use a high-efficiency laser that
requires low modulation current and generates a low-
voltage swing. Trim the leads to reduce laser package
inductance. The typical package leads have induc-
tance of 25nH per inch (1nH/mm). This inductance
causes a large voltage swing across the laser. A com-
pensation filter network can also be used to reduce
ringing, edge speed, and voltage swing (see the
Designing the Compensation Filter Network section).
3). The modulator drives up to 15mA of modulation into
a 50Ω VCSEL load.
The amplitude of the modulation current is set with
resistors at MODSET and temperature coefficient (TC1,
TC2) pins. The resistor at MODSET (R
) pro-
MODSET
grams the temperature-stable portion of the modulation
current, and the resistor between TC1 and TC2 (R
)
TC
programs the temperature coefficient of the modulation
current. For appropriate R and R values, see
TC
MODSET
the Typical Operating Characteristics section.
10 ______________________________________________________________________________________
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
FAULT
TX_DISABLE
BIAS
VBIAS FAULT
VCC - 0.2V
OPEN-
DRAIN
NMOS
BIASMON
HIGH-CURRENT FAULT
R
S
Q
ENABLE
0.8V
0.8V
PWRMON
HIGH-POWER FAULT
POR
R-S LATCH
MAX3740
TX_DISABLE
SAFETY CIRCUIT
Figure 2. Safety Circuit
V
CC
MAX3740
R
OUT-
R
OUT+
OUT+
OUT-
CURRENT
SWITCH
INPUT BUFFER
IN+
SIGNAL
DETECT
PEAKING
CONTROL
100Ω
IN-
PEAKSET
SQUELCH
MODULATION
CURRENT
GENERATION
CURRENT AMPLIFIER
30x
R
PEAKSET
ENABLE
TEMPERATURE
COMPENSATION
1V
TC1
TC2
MODSET
R
TC
R
MODSET
Figure 3. Modulation Circuit
______________________________________________________________________________________ 11
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
From the Typical Operating Characteristics, the value
Programming Modulation Current
See the Modulation Current vs. R graph in the
of R , which offsets the tempco of the laser, is 9kΩ. If
TC
MODSET
modulation temperature compensation is not desired,
short TC1 and TC2.
Typical Operating Characteristics, and select the value
of R that corresponds to the required current at
MODSET
+25°C.
Programming the APC Loop
Program the average optical power by adjusting
Programming Modulation-Current Tempco
R
. To select the resistance, determine the
Compute the required modulation tempco from the
PWRSET
desired monitor current to be maintained over tempera-
ture and lifetime. See the Monitor Diode Current vs.
slope efficiency of the laser at T = +25°C and at a
A
higher temperature. Then select the value of R
from
TC
R
graph in the Typical Operating Characteristics
the Typical Operating Characteristics. For example,
suppose a laser has a slope efficiency (SE) of
0.021mW/mA at +25°C, which reduces to 0.018mW/mA
at +85°C. The temperature coefficient is given by the
following:
PWRSET
section, and select the value of R
sponds to the required current.
that corre-
PWRSET
Input Termination Requirements
The MAX3740 data inputs are SFP MSA compatible. On-
chip 100Ω differential input impedance is provided for
optimal termination (Figure 4). Because of the on-chip
biasing network, the MAX3740 inputs self-bias to the
proper operating point to accommodate AC-coupling.
(SE − SE
)
25
85
Laser tempco =
×1E6
SE ×(85−25)
25
= −2380ppm/°C
V
CC
V
CC
MAX3740
PACKAGE
1nH
0.5pF
R
OUT-
R
OUT+
PACKAGE
1nH
0.5pF
1nH
16kΩ
V
V
CC
OUT-
OUT+
IN+
IN-
50Ω
50Ω
0.5pF
CC
1nH
0.5pF
MAX3740
24kΩ
Figure 4. Simplified Input Structure
Figure 5. Simplified Output Structure
12 ______________________________________________________________________________________
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
Applications Information
Interface Models
Figures 4 and 5 show simplified input and output cir-
cuits for the MAX3740 laser driver.
UNCOMPENSATED
CORRECTLY COMPENSATED
Layout Considerations
To minimize inductance, keep the connections between
the MAX3740 output pins and laser diode as short as
possible. Use good high-frequency layout techniques
and multilayer boards with uninterrupted ground planes
to minimize EMI and crosstalk.
OVERCOMPENSATED
Designing the Compensation Filter
Network
TIME
Laser package inductance causes the laser imped-
ance to increase at high frequencies, leading to ring-
ing, overshoot, and degradation of the laser output. A
laser compensation filter network can be used to
reduce the laser impedance at high frequencies, there-
by reducing output ringing and overshoot.
Figure 6. Laser Compensation
825. The entire transmitter circuit and component
selections must be considered. Customers must deter-
mine the level of fault tolerance required by their appli-
cations, recognizing that Maxim products are not
designed or authorized for use as components in sys-
tems intended for surgical implant into the body, for
applications intended to support or sustain life, or for
any other application where the failure of a Maxim
product could create a situation where personal injury
or death may occur.
The compensation components (R and C ) are most
F
F
easily determined by experimentation. Begin with R =
F
50Ω and C = 1pF. Increase C until the desired trans-
F
F
mitter response is obtained (Figure 6). Refer to
Application Note HFAN-2-0: Interfacing Maxim Laser
Drives with Laser Diodes for more information.
Exposed-Pad (EP) Package
The exposed pad on the 24-pin thin QFN provides a
very low thermal resistance path for heat removal from
the IC. The pad is also electrical ground on the
MAX3740 and must be soldered to the circuit board
ground for proper thermal and electrical performance.
Refer to Maxim Application Note HFAN-08.1: Thermal
Considerations for QFN and Other Exposed-Pad
Packages for additional information.
ESD Protection
The FAULT pin of the MAX3740 does not include ESD
protection. If this pin is connected to the DS1858, pro-
tection is not needed. Protection can be provided with
external diodes as shown in Figure 7.
V
CC
Laser Safety and IEC 825
The International Electrotechnical Commission (IEC)
determines standards for hazardous light emissions
from fiber optic transmitters. IEC 825 defines the maxi-
mum light output for various hazard levels. The
MAX3740 provides features that facilitate compliance
with IEC 825. A common safety precaution is single-
point fault tolerance, whereby one unplanned short,
open, or resistive connection does not cause excess
light output. Using this laser driver alone does not
ensure that a transmitter design is compliant with IEC
MAX3740
FAULT
PHILLIPS
BAV99
Figure 7. External Diode Protection
______________________________________________________________________________________ 13
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
Functional Diagram
BIASMON
COMP MD
REF
PWRMON
FAULT
BIAS
BIAS
GENERATOR
WITH APC
SAFETY
CIRCUITRY
TX_DISABLE
BIASSET
ENABLE
V
CC
LASER
MODULATOR
MAX3740
SQUELCH
IN+
OUT-
OUT+
SIGNAL
DETECT
PEAKING
CONTROL
100Ω
IN-
MODULATION CURRENT
GENERATOR
ENABLE
TC1
TC2
MODSET
PEAKSET
Pin Configuration
Chip Information
TRANSISTOR COUNT: 3806
TOP VIEW
PROCESS: SiGe BIPOLAR
GND
TX_DISABLE
IN+
1
2
3
4
5
6
18 BIAS
17 BIASSET
16
V
CC
Package Information
MAX3740
IN-
15 OUT+
14 OUT-
For the latest package outline information, go to
www.maxim-ic.com/packages.
FAULT
13
SQUELCH
GND
PART
PACKAGE TYPE
PACKAGE CODE
24 Thin QFN
(4mm ✕ 4mm ✕ 0.8mm)
MAX3740ETG
T2444-1
24 THIN QFN (4mm x 4mm)
*EXPOSED PAD IS CONNECTED TO GND
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
© 2003 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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