MAX3740AETG [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： | MAX3740AETG |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | 3.2Gbps SFP VCSEL Driver with Diagnostic Monitors |
| 文件： | 总14页 (文件大小：286K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-3118; Rev 1; 6/04
3 .2 Gb p s S FP VCS EL Drive r w it h Dia g n o s t ic
Mo n it o rs
Ge n e ra l De s c rip t io n
Fe a t u re s
The MAX3740A 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 MAX3740A 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 com-
pensation is provided to keep the optical extinction
ratio within specifications over the operating tempera-
ture range. The MAX3740A interfaces with the Dallas
DS1858 to me e t SFF-8472 timing a nd d ia g nos tic
requirements. The MAX3740A accommodates various
VCSEL packages, including low-cost TO-46 headers.
✕ Automatic Power Control
✕ Safety Circuits Compliant with SFF and SFP
MSAs
✕ 4mm ✕ 4mm 24-Pin Thin QFN Package
Ord e rin g In fo rm a t io n
The MAX3740A 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
MAX3740AETG
-40°C to +85°C 24 Thin QFN (4mm x 4mm)
MAX3740AETG+ -40°C to +85°C 24 Thin QFN (4mm x 4mm)
+Denotes lead-free package.
The MAX3740A is available in a compact 4mm ✕ 4mm,
24-pin thin QFN package and operates over the -40°C
to +85°C temperature range.
Ap p lic a t io n s
Typ ic a l Ap p lic a t io n Circ u it
Multirate (1Gbps to 3.2Gbps) SFP/SFF Modules
Gigabit Ethernet Optical Transmitters
Fibre Channel Optical Transmitters
Infiniband Optical Transmitters
+3.3V
†
4.7kΩ
V
CC
FAULT
PWRMON
MODSET
TX_DISABLE
SQUELCH
R
MODSET
REF
MAX3740A
0.1µF
0.1µF
IN+
COMP
R
PWRSET
0.047µF
MD
IN-
BIAS
TC1
†
R
TC
L1*
0.01µF
0.01µF
TC2
OUT+
†
C
F
BIASSET
OUT-
R
BIASSET
GND
PEAKSET
BIASMON
†
R
F
50Ω
†
R
PEAKSET
R
BIASMON
†
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 .2 Gb p s S FP VCS EL Drive r w it h Dia g n o s t ic
Mo n it o rs
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (V ) ..............................................-0.5V to 6.0V
Continuous Power Dissipation (T = +85°C)
A
CC
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)
CC
Voltage at OUT+, OUT-.........................(V - 2V) to (V + 2V)
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
P-P
TX_DISABLE set low,
peaking is not used
(Note 1)
I
= 15mA
55
15
68
20
MOD
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
TX_DISABLE INPUT
Input Impedance
V
R
R
= 10kΩ to 2.97V
= 4.7kΩ to 3.63V
2.4
V
V
OH
LOAD
V
0.4
10.0
0.8
OL
LOAD
4.7
2.0
kΩ
V
Input High Voltage
Input Low Voltage
V
IH
V
V
IL
The time for I to reach I
TX_DISABLE transitions high
when
CC
CC-SHDN
Power-Down Time
50
µs
SQUELCH
Squelch Threshold
25
10
85
mV
P-P
Squelch Hysteresis
mV
P-P
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 .2 Gb p s S FP VCS EL Drive r w it h Dia g n o s t ic
Mo n it o rs
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
1mA < I
< 3mA
0.0875
0.085
-10
0.105 0.1375
BIAS
BIAS
BIASMON Gain
mA/mA
%
3mA ≤ I
≤ 15mA
0.105
0.125
+10
BIASMON Stability
(Notes 5,6)
AUTOMATIC POWER CONTROL (APC)
V
-
REF
MD Nominal Voltage
V
APC loop is closed
1
2
V
MD
0.2
1.8
0
Voltage at REF
V
REF
1.2
2.2
V
V
MD Voltage During Fault
MD Input Current
Normal operation (FAULT = low)
-2
5
0.7
20
+2
µA
µs
APC Time Constant
C
= 0.047µF (Note 6)
COMP
PWRMON Nominal Gain
LASER MODULATOR (Note 7)
V
/ (V
- V )
MD
1.85
2.15
2.45
250
V/V
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
ps
Modulation Transition Time
Deterministic Jitter
Random Jitter
t , t
R
5mA ≤ I
5mA ≤ I
≤ 15mA, 20% to 80% (Note 6)
≤ 15mA, 3.2Gbps (Notes 6, 8)
65
12
95
20
4
F
MOD
DJ
RJ
ps
P-P
MOD
(Note 6)
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
IN
V
_______________________________________________________________________________________
3
3 .2 Gb p s S FP VCS EL Drive r w it h Dia g n o s t ic
Mo n it o rs
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 > V causes a fault
Fault Threshold
V
V
0.7
-0.250
0.7
0.8
-0.2
0.8
0.9
-0.150
0.9
V
V
V
BMTH
BIASMON
BMTH
V
BIAS
V
V
referenced to V
BTH
BIAS CC
Power-Monitor Fault Threshold
V
PMTH
V
> V
causes a fault
PWRMON
PMTH
Time from rising edge of TX_DISABLE to
= I and I = I
(Note 6)
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
TX Disable Negate Time
t_
I
500
ON
BIAS
MOD
(Note 6)
Time to set V
after rising edge of TX_DISABLE (Note 6)
= low after power-on or
FAULT
Fault Reset Time
Power-On Time
t_
t_
1
60
60
200
200
ms
ms
INIT
Time after power-on to transmitter-on with
TX_DISABLE low (Note 6)
2
INIT
Time from fault occurrence to V
=
FAULT
Fault Assert Time
t_
high; C
(Note 6)
< 20pF, R = 4.7kΩ
FAULT
1.4
1
50
µs
FAULT
FAULT
Time from fault to I
= I
BIAS_OFF
and
BIAS
Fault Delay Time
t_
5
1
µs
µs
FLTDLY
I
= I
(Note 6)
MOD
MOD_OFF
Time TX_DISABLE must be held high to
reset FAULT (Note 6)
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.
CC
BIAS
Note 5: Variation of bias monitor gain for any single part over the range of V , temperature, 3mA < I
< 15mA.
CC
BIAS
Note 6: Guaranteed by design and characterization.
Note 7: Measured electrically with a 50Ω load AC-coupled to OUT+.
Note 8: Deterministic jitter is the peak-to-peak deviation from the ideal time crossings measured with a K28.5 bit pattern at 3.2Gbps
4
_______________________________________________________________________________________
3 .2 Gb p s S FP VCS EL Drive r w it h Dia g n o s t ic
Mo n it o rs
Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s
(V = +3.3V, R = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, T = +25°C, unless otherwise
CC
TC
A
noted.)
ELECTRICAL EYE WITH MAX PEAKING
MAX3740A toc03
ELECTRICAL EYE
ELECTRICAL EYE WITH PEAKING
MAX3740A toc02
MAX3740A toc01
3.2Gbps, K28.5, 10mA MODULATION,
R
= 500Ω
PEAKSET
3.2Gbps, K28.5, 10mA MODULATION,
PEAKING OFF
3.2Gbps, K28.5, 10mA MODULATION,
R
= 2.4kΩ
PEAKSET
73mV/div
73mV/div
73mV/div
50ps/div
50ps/div
50ps/div
I
vs. BIAS CURRENT
OPTICAL EYE
OPTICAL EYE
BIASMON
MAX3740A toc04
MAX3740A toc05
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
E = 8.2dB, 2.125Gbps, K28.5,
R
850nm VCSEL, WITH 2.3GHz
O-TO-E CONVERTER
E = 8.2dB, 2.5Gbps, K28.5,
R
850nm VCSEL SONET MASK
WITH +20% MARGIN
EMCORE SC-TOSA-8585-3420 VCSEL
68ps/div
EMCORE SC-TOSA-8585-3420 VCSEL
58ps/div
0
4
8
12
16
BIAS CURRENT (mA)
TRANSITION TIME
vs. MODULATION CURRENT
DETERMINISTIC JITTER
vs. MODULATION CURRENT
RANDOM JITTER
vs. MODULATION CURRENT
100
90
80
70
60
50
40
40
35
30
25
20
15
10
5
7
6
5
4
3
2
1
0
RISE
FALL
0
2
4
6
8
10
(mA
12
14
16
0
5
10
15
0
5
10
15
I
)
I
(mA
)
I
(mA )
P-P
MOD
P-P
MOD
P-P
MOD
_______________________________________________________________________________________
5
3 .2 Gb p s S FP VCS EL Drive r w it h Dia g n o s t ic
Mo n it o rs
Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )
(V = +3.3V, R = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, T = +25°C, unless otherwise
CC
TC
A
noted.)
MONITOR DIODE CURRENT
BIAS CURRENT vs. R
MODULATION CURRENT vs. R
vs. R
BIASSET
MODSET
PWRSET
10m
1m
100m
10m
1m
100m
10m
1m
MEASURED WITH A
50Ω ELECTRICAL LOAD
100µ
10µ
1µ
100µ
100
1k
10k
1k
10k
100k
100
1k
10k
R
(Ω)
R
(Ω)
R
(Ω)
PWRSET
BIASSET
MODSET
SUPPLY CURRENT vs. TEMPERATURE
INPUT RETURN LOSS
OUTPUT RETURN LOSS
80
70
60
50
40
30
20
10
0
-5
0
-2
DIFFERENTIAL
MEASUREMENT
SINGLE-ENDED
MEASUREMENT
I
= 15mA
MOD
-4
-10
-15
-20
-25
-30
-35
-40
-6
-8
-10
-12
-14
-16
-18
I
= 2mA
60
MOD
-40
-15
10
35
85
100M
1G
10G
100M
1G
10G
TEMPERATURE (°C)
FREQUENCY (Hz)
FREQUENCY (Hz)
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
R
= 100Ω
TC
REFERENCED TO +25°C
R
= 1.35kΩ
MOD
4500
3500
2500
1500
500
R
= 1kΩ
TC
R
TC
= 5kΩ
8
R
TC
= 10kΩ
7
R
= 60kΩ
TC
R
TC
= 100kΩ
6
R
= 500kΩ
TC
5
4
-500
0
10 20 30 40 50 60 70 80 90
-40
-15
10
35
60
85
100
1k
10k
(Ω)
100k
1M
TEMPERATURE (°C)
TEMPERATURE (°C)
R
TC
6
_______________________________________________________________________________________
3 .2 Gb p s S FP VCS EL Drive r w it h Dia g n o s t ic
Mo n it o rs
Typ ic a l Op e ra t in g Ch a ra c t e ris t ic s (c o n t in u e d )
(V = +3.3V, R = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, T = +25°C, unless otherwise
CC
TC
A
noted.)
TX_DISABLE NEGATE TIME
HOT PLUG WITH TX_DISABLE LOW
STARTUP WITH SLOW RAMPING SUPPLY
MAX3740A toc21
MAX3740A toc19
MAX3740A toc20
3.3V
3.3V
3.3V
V
CC
V
CC
V
CC
OV
OV
FAULT
FAULT
FAULT
LOW
HIGH
LOW
LOW
t_INIT = 62ms
LOW
TX_DISABLE
TX_DISABLE
TX_DISABLE
t_INIT = 60ms
20ms/div
t_ON = 54µs
20µs/div
LOW
LOW
LASER
OUTPUT
LASER
OUTPUT
LASER
OUTPUT
20ms/div
TRANSMITTER DISABLE
RESPONSE TO FAULT
MAX3740A toc22
MAX3740A toc23
3.3V
EXTERNALLY
FORCED
FAULT
V
CC
V
PWRMON
t_OFF = 1.86µs
t_FAULT = 245ns
FAULT
FAULT
LOW
LOW
LOW
LOW
HIGH
TX_DISABLE
TX_DISABLE
HIGH
LASER
LASER
OUTPUT
OUTPUT
1µs/div
200ns/div
FAULT RECOVERY TIME
FREQUENT ASSERTION OF TX_DISABLE
MAX3740A toc24
MAX3740A 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
LASER
OUTPUT
OUTPUT
40µs/div
200µs/div
_______________________________________________________________________________________
7
3 .2 Gb p s S FP VCS EL Drive r w it h Dia g n o s t ic
Mo n it o rs
P in De s c rip t io n
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 with ESD protection. FAULT is asserted high during a
fault condition.
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
CC
+3.3V Supply Voltage
Temperature Compensation Set Pin 1. A resistor placed between TC1 and TC2 (R ) programs the
TC
temperature coefficient of the modulation current.
TC1
Temperature Compensation Set Pin 2. A resistor placed between TC1 and TC2 (R ) programs the
TC
temperature coefficient of the modulation current.
9
TC2
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-
OUT+
Inverted Modulation-Current Output
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
PWRMON
Ground. Must be soldered to the circuit board ground for proper thermal and electrical
performance. See the Layout Considerations section.
EP
Exposed Pad
8
_______________________________________________________________________________________
3 .2 Gb p s S FP VCS EL Drive r w it h Dia g n o s t ic
Mo n it o rs
PWRMON
REF
CURRENT
AMPLIFIER
2X
1.8V
R
PWRSET
MAX3740A
ENABLE
I
BIAS
POWER-
CONTROL
AMPLIFIER
34
MD
BIAS
BIAS GENERATOR
FERRITE
BEAD
SMOOTH-
START
BIASMON
I
PD
1.6V
I
BIAS
9
(2V )
BE
1.2V
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:
De t a ile d De s c rip t io n
The MAX3740A contains a bias generator with automat-
ic power control (APC), safety circuit, and a laser mod-
ulator 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
Bia s Ge n e ra t o r
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.
S a fe t y Circ u it
The s a fe ty c irc uit c onta ins a n inp ut d is a b le
(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
V
BIAS
CC
V
= 2 ✕ I
PD
✕ R
PWRSET
PWRMON
BIASMON
PWRMON
V
> 0.8V
BIASMON
V
> 0.8V
PWRMON
_______________________________________________________________________________________
9
3 .2 Gb p s S FP VCS EL Drive r w it h Dia g n o s t ic
Mo n it o rs
Table 2. Circuit Response to Various Single-Point Faults (Closed-Loop APC Configuration)
PIN NAME
FAULT
TX_DISABLE
IN+
CIRCUIT RESPONSE TO V
SHORT
CIRCUIT RESPONSE TO GND SHORT
CC
Does not affect laser power.
Does not affect laser power.
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.
Mo d u la t io n Circ u it
The modulation circuitry consists of an input buffer, a
current mirror, and a high-speed current switch (Figure
3). The modulator drives up to 15mA of modulation into
a 50Ω VCSEL load.
De s ig n P ro c e d u re
S e le c t La s e r
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
ring ing , e d g e s p e e d , a nd volta g e s wing (s e e the
Designing the Compensation Filter Network section).
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
MODSET
TC
the Typical Operating Characteristics section.
10 ______________________________________________________________________________________
3 .2 Gb p s S FP VCS EL Drive r w it h Dia g n o s t ic
Mo n it o rs
TX_DISABLE
BIAS
V
BIAS
FAULT
FAULT
V
CC
- 0.2V
0.8V
OUTPUT
FAULT
BIASMON
HIGH-CURRENT FAULT
R
S
Q
ENABLE
R-S LATCH
PWRMON
HIGH-POWER FAULT
POR
0.8V
MAX3740A
TX_DISABLE
SAFETY CIRCUIT
Figure 2. Safety Circuit
V
CC
MAX3740A
R
OUT+
R
OUT-
INPUT BUFFER
OUT+
OUT-
CURRENT SWITCH
IN+
SIGNAL
DETECT
100Ω
PEAKING
CONTROL
IN-
PEAKSET
SQUELCH
MODULATION
CURRENT GENERATOR
R
PEAKSET
ENABLE
CURRENT AMPLIFIER 30x
TEMPERATURE
COMPENSATION
1V
200Ω
TC1
TC2
MODSET
R
MODSET
R
TC
Figure 3. Modulation Circuit
______________________________________________________________________________________ 11
3 .2 Gb p s S FP VCS EL Drive r w it h Dia g n o s t ic
Mo n it o rs
0.021mW/mA at +25°C, which reduces to 0.018mW/mA
at +85°C. The temperature coefficient is given by the
following:
P ro g ra m m in g Mo d u la t io n Cu rre n t
The modulation current output of the MAX3740A is con-
trolle d b y a re s is tor (R
) p la c e d b e twe e n
MODSET
MODSET and ground. The R
the amount of current being sourced to the VCSEL. The
modulation current is given by the following:
resistor controls
MODSET
(SE − SE
)
25
85
Laser tempco =
×1E6
SE ×(85 −25)
25
= −2380ppm/°C
R
OUT+
+ R
I
= I
× 30 ×
(
[
)
]
MOD
MODSET
R
From the Typical Operating Characteristics, the value
OUT+
LOAD
R
of R , which offsets the tempco of the laser, is 9kΩ. If
TC
1
OUT+
+ R
modulation temperature compensation is not desired,
short TC1 and TC2.
I
=
× 30 ×
MOD
200 + R
R
OUT+
MODSET
LOAD
P ro g ra m m in g t h e AP C Lo o p
Prog ra m the a ve ra g e op tic a l p owe r b y a d jus ting
. To s e le c t the re s is ta nc e , d e te rmine the
desired monitor current to be maintained over tempera-
ture and lifetime. See the Monitor Diode Current vs.
It is important to note that the modulation current being
s ourc e d b y the MAX3740A is a ffe c te d b y the loa d
impedance of the VCSEL. The Modulation Current vs.
R
PWRSET
R
graph in the Typical Operating Characteristics
MODSET
shows the current into a 50Ω electrical load.
R
graph in the Typical Operating Characteristics
PWRSET
section, and select the value of R
sponds to the required current.
that corre-
PWRSET
P ro g ra m m in g Bia s Cu rre n t
The bias current output of the MAX3740A is controlled
by a resistor (R ) placed between BIASSET and
In p u t Te rm in a t io n Re q u ire m e n t s
The MAX3740A 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 MAX3740A inputs self-bias to the
proper operating point to accommodate AC-coupling.
BIASSET
ground. In open-loop operation the R
controls
BIASSET
the bias current level of the VCSEL. In closed-loop
operation the R controls the maximum bias cur-
BIASSET
rent provided by the APC. The bias current is given by
the following:
I
= I
(
× 34
)
BIAS
BIASSET
1.2
I
=
× 34
BIAS
200 + R
BIASSET
V
CC
The Bias Current vs. R
graph is also shown in
BIASSET
the Typical Operating Characteristics.
MAX3740A
PACKAGE
1nH
16kΩ
P h o t o d io d e S e le c t io n
To ensure stable operation of the APC circuit, the time
constant of the MD node should be shorter than the
V
CC
IN+
IN-
APC time constant. (t
= 5µs if C
= 0.047µF).
APC
APC
0.5pF
50Ω
50Ω
t
5µs
APC
20
t
≤
, R
× C ≤
MD
= 250ns
MD
MD
V
CC
20
1nH
For typical I
= 400µA, R
= 500Ω, select a
PWRSET
PD
photodiode with capacitance less than 500pF.
0.5pF
24kΩ
P ro g ra m m in g Mo d u la t io n -Cu rre n t Te m p c o
Compute the required modulation tempco from the
slope efficiency of the laser at T = +25°C and at a
A
higher temperature. Then select the value of R
from
TC
the Typical Operating Characteristics. For example,
s up p os e a la s e r ha s a s lop e e ffic ie nc y (SE) of
Figure 4. Simplified Input Structure
12 ______________________________________________________________________________________
3 .2 Gb p s S FP VCS EL Drive r w it h Dia g n o s t ic
Mo n it o rs
V
CC
UNCOMPENSATED
PACKAGE
1nH
R
OUT-
R
OUT+
CORRECTLY COMPENSATED
OVERCOMPENSATED
OUT-
0.5pF
1nH
OUT+
0.5pF
TIME
MAX3740A
Figure 7. Laser Compensation
The compensation components (R and C ) are most
F
F
easily determined by experimentation. Begin with R
=
F
Figure 5. Simplified Output Structure
50Ω and C = 1pF. Increase C until the desired trans-
F
F
mitte r re s p ons e is ob ta ine d (Fig ure 7). Re fe r to
Application Note HFAN-2-0: Interfacing Maxim Laser
Drives with Laser Diodes for more information.
V
CC
MAX3740A
Ex p o s e d -P a d (EP ) P a c k a g e
FAULT
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 MAX3740A 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.
Figure 6. Fault Circuit Interface
Ap p lic a t io n s In fo rm a t io n
La s e r S a fe t y a n d IEC 8 2 5
In t e rfa c e Mo d e ls
The International Electrotechnical Commission (IEC)
determines standards for hazardous light emissions from
fiber optic transmitters. IEC 825 defines the maximum
light output for various hazard levels. The MAX3740A
provides features that facilitate compliance with IEC 825.
A common safety precaution is single-point fault toler-
ance, 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 825. The entire transmitter
circuit and component selections must be considered.
Customers must determine the level of fault tolerance
required by their applications, recognizing that Maxim
products are not designed or authorized for use as com-
ponents in systems 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.
Figures 4 and 5 show simplified input and output circuits
for the MAX3740A laser driver. Figure 6 shows the fault
circuit interface.
La yo u t Co n s id e ra t io n s
To minimize inductance, keep the connections between
the MAX3740A 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.
De s igning the Com pe ns a tion Filte r
Ne tw ork
Laser package inductance causes the laser impedance
to increase at high frequencies, leading to ringing, over-
shoot, and degradation of the laser output. A laser com-
pensation filter network can be used to reduce the laser
impedance at high frequencies, thereby reducing output
ringing and overshoot.
______________________________________________________________________________________ 13
3 .2 Gb p s S FP VCS EL Drive r w it h Dia g n o s t ic
Mo n it o rs
Fu n c t io n a l Dia g ra m
BIASMON
COMP MD
REF
PWRMON
FAULT
BIAS
BIAS
GENERATOR
WITH APC
SAFETY
CIRCUITRY
TX_DISABLE
BIASSET
ENABLE
V
CC
LASER
MODULATOR
MAX3740A
SQUELCH
IN+
OUT-
OUT+
SIGNAL
DETECT
PEAKING
CONTROL
100Ω
IN-
MODULATION CURRENT
GENERATOR
ENABLE
TC1
TC2
MODSET
PEAKSET
P in Co n fig u ra t io n
Ch ip In fo rm a t io n
TRANSISTOR COUNT: 3806
PROCESS: SiGe BIPOLAR
TOP VIEW
GND
1
2
3
4
5
6
18 BIAS
TX_DISABLE
IN+
17 BIASSET
16
V
CC
P a c k a g e In fo rm a t io n
MAX3740A
IN-
15 OUT+
14 OUT-
For the latest package outline information, go to
www.maxim-ic.com/packages.
FAULT
SQUELCH
13
GND
PART
PACKAGE TYPE
PACKAGE CODE
24 Thin QFN
(4mm x 4mm x 0.8mm)
MAX3740AETG
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 ____________________Ma x im In t e g ra t e d P ro d u c t s , 1 2 0 S a n Ga b rie l Drive , S u n n yva le , CA 9 4 0 8 6 4 0 8 -7 3 7 -7 6 0 0
© 2004 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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