Maxim MAX3740AETG+ 3.2gbps sfp vcsel driver with diagnostic monitor Datasheet

19-3118; Rev 3; 1/10
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
The MAX3740A is a high-speed VCSEL driver for smallform-factor (SFF) and small-form-factor pluggable (SFP)
fiber optic LAN transmitters. It contains a bias generator, a laser modulator, and comprehensive safety features. 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.
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 compensation is provided to keep the optical extinction
ratio within specifications over the operating temperature range. The MAX3740A interfaces with the Dallas
DS1858 to meet SFF-8472 timing and diagnostic
requirements. The MAX3740A accommodates various
VCSEL packages, including low-cost TO-46 headers.
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).
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.
Features
♦ 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
♦ Automatic Power Control
♦ Safety Circuits Compliant with SFF and SFP
MSAs
♦ 4mm ✕ 4mm, 24-Pin Thin QFN Package
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
MAX3740AETG
-40°C to +85°C
24 Thin QFN-EP*
MAX3740AETG+
-40°C to +85°C
24 Thin QFN-EP*
+Denotes a lead(Pb)-free/RoHS-compliant package.
*EP = Exposed pad.
Applications
Typical Application Circuit
Multirate (1Gbps to 3.2Gbps) SFP/SFF Modules
+3.3V
Gigabit Ethernet Optical Transmitters
4.7kن
VCC
Fibre Channel Optical Transmitters
FAULT
PWRMON
MODSET
Infiniband Optical Transmitters
TX_DISABLE
RMODSET
SQUELCH
MAX3740A
0.1μF
REF
IN+
RPWRSET
COMP
0.047μF
IN-
MD
TC1
BIAS
0.1μF
RTC†
L1*
0.01μF
TC2
OUT+
CF†
BIASSET
RBIASSET
OUTGND PEAKSET
BIASMON
RPEAKSET†
0.01μF
50Ω
RF†
RBIASMON
†OPTIONAL COMPONENT
*FERRITE BEAD
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
1
MAX3740A
General Description
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (VCC) ..............................................-0.5V to 6.0V
Voltage at TX_DISABLE, IN+, IN-, FAULT,
SQUELCH, TC1, TC2, MODSET, PEAKSET, BIASSET,
BIAS, BIASMON, COMP, MD, REF,
PWRMON ...............................................-0.5V to (VCC + 0.5V)
Voltage at OUT+, OUT- .........................(VCC - 2V) to (VCC + 2V)
Current into FAULT ............................................ -1mA to +25mA
Current into OUT+, OUT- ....................................................60mA
Continuous Power Dissipation (TA = +85°C)
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
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
(VCC = +2.97V to +3.63V, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA =
+25°C, unless otherwise noted.)
PARAMETER
Supply Current
SYMBOL
ICC
ICC-SHDN
CONDITIONS
MIN
SQUELCH set low,
IMOD = 2mAP-P
TX_DISABLE set low,
peaking is not used
IMOD = 15mAP-P
(Note 1)
TYP
MAX
UNITS
32
55
68
Additional current when peaking is used
(Note 2)
15
20
Additional current when SQUELCH is high
5
10
3.9
5
mA
Total current when TX_DISABLE is high
FAULT OUTPUT
Output High Voltage
VOH
RLOAD = 10kΩ to 2.97V
Output Low Voltage
VOL
RLOAD = 4.7kΩ to 3.63V
2.4
V
0.4
V
10.0
kΩ
TX_DISABLE INPUT
Input Impedance
4.7
Input High Voltage
VIH
Input Low Voltage
VIL
2.0
The time for ICC to reach ICC-SHDN when
TX_DISABLE transitions high
Power-Down Time
V
0.8
50
V
µs
SQUELCH
Squelch Threshold
25
Squelch Hysteresis
10
85
mVP-P
mVP-P
Time to Squelch Data
(Note 3)
0.02
5.00
µs
Time to Resume from Squelch
(Note 3)
0.02
5.00
µs
BIAS GENERATOR (Note 4)
Bias Current
Accuracy of Programmed Bias
Current
2
IBIAS
ΔBIAS
Minimum
Maximum
1
15
5mA ≤ IBIAS ≤ 15mA
-8
+8
1mA ≤ IBIAS ≤ 5mA
-12
+12
_______________________________________________________________________________________
mA
%
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
(VCC = +2.97V to +3.63V, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA =
+25°C, unless otherwise noted.)
PARAMETER
Bias Current During Fault
SYMBOL
IBIAS_OFF
BIASMON Gain
BIASMON Stability
CONDITIONS
MIN
Current out of the BIAS pin
TYP
MAX
UNITS
1.5
10
µA
1mA < IBIAS < 3mA
0.0875
0.105
0.1375
3mA ≤ IBIAS ≤ 15mA
0.085
0.105
0.125
(Notes 5,6)
-10
mA/mA
+10
%
AUTOMATIC POWER CONTROL (APC)
MD Nominal Voltage
VMD
Voltage at REF
VREF
APC loop is closed
1
VREF 0.16
2
V
1.2
1.8
2.2
V
+2
µA
MD Voltage During Fault
0
MD Input Current
Normal operation (FAULT = low)
-2
0.7
APC Time Constant
CCOMP = 0.047µF (Note 6)
5
20
PWRMON Nominal Gain
VPWRMON / (VREF - VMD)
1.85
2.15
V
µs
2.45
V/V
LASER MODULATOR (Note 7)
Data Input Voltage Swing
VID
Output Resistance
Modulation Current
IMOD
Minimum
Maximum
250
2200
Single-ended resistance at OUT+
80
105
Single-ended resistance at OUT-
72
100
Minimum
Maximum
2
15
mVP-P
Ω
mAP-P
Minimum Peaking Current Range
0.2
mA
Maximum Peaking Current Range
2
mA
Peaking Current Duration
80
ps
Tolerance of Programmed
Modulation Current
TC1 is shorted to TC2
-10
Minimum Programmable
Temperature Coefficient
Maximum Programmable
Temperature Coefficient
Temperature range 0°C to +70°C
+10
%
0
ppm/°C
+5000
ppm/°C
t R , tF
5mA ≤ IMOD ≤ 15mA, 20% to 80% (Note 6)
65
95
ps
Deterministic Jitter
DJ
5mA ≤ IMOD ≤ 15mA, 3.2Gbps (Notes 6, 8)
12
20
psP-P
Random Jitter
RJ
(Note 6)
1.3
4
psRMS
15
50
µAP-P
100
115
Ω
Modulation Transition Time
Laser Modulation During Fault or
while Squelch is Active
IMOD_OFF
Input Resistance
Input Bias Voltage
Differential resistance
VIN
85
VCC 0.3
V
_______________________________________________________________________________________
3
MAX3740A
ELECTRICAL CHARACTERISTICS (continued)
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +2.97V to +3.63V, TA = -40°C to +85°C. Typical values are at VCC = +3.3V, TC1 and TC2 are shorted, PEAKSET open, TA =
+25°C, unless otherwise noted.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
SAFETY FEATURES (see the Typical Operating Characteristics section)
High-Current Fault Threshold
VBIAS Fault Threshold
VBMTH
VBTH
VBIASMON > VBMTH causes a fault
VBIAS referenced to VCC
0.7
0.8
0.9
V
-0.250
-0.2
-0.150
V
0.7
0.8
0.9
V
Power-Monitor Fault Threshold
VPMTH
VPWRMON > VPMTH causes a fault
TX Disable Time
t_OFF
Time from rising edge of TX_DISABLE to
IBIAS = IBIAS_OFF and IMOD = IMOD_OFF
(Note 6)
1.8
5
µs
TX Disable Negate Time
t_ON
Time from rising edge of TX_DISABLE to
IBIAS and IMOD at 99% of steady state
(Note 6)
55
500
µs
Fault Reset Time
t_INIT1
Time to set VFAULT = low after power-on or
after rising edge of TX_DISABLE (Note 6)
60
200
ms
Power-On Time
t_INIT2
Time after power-on to transmitter-on with
TX_DISABLE low (Note 6)
60
200
ms
Fault Assert Time
t_FAULT
Time from fault occurrence to VFAULT =
high; CFAULT < 20pF, RFAULT = 4.7kΩ
(Note 6)
1.4
50
µs
Fault Delay Time
t_FLTDLY
Time from fault to IBIAS = IBIAS_OFF and
IMOD = IMOD_OFF (Note 6)
1
5
µs
TX_DISABLE Reset
t_RESET
Time TX_DISABLE must be held high to
reset FAULT (Note 6)
1
µs
Note 1: Supply current measurements exclude IBIAS from the total current.
Note 2: Tested with RPEAK = 1.18kΩ.
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: VBIAS < VCC - 0.7V.
Note 5: Variation of bias monitor gain for any single part over the range of VCC, temperature, 3mA < IBIAS < 15mA.
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
(00111110101100000101).
4
_______________________________________________________________________________________
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
ELECTRICAL EYE
ELECTRICAL EYE WITH MAX PEAKING
ELECTRICAL EYE WITH PEAKING
MAX3740A toc01
MAX3740A toc03
MAX3740A toc02
3.2Gbps, K28.5, 10mA MODULATION,
PEAKING OFF
3.2Gbps, K28.5, 10mA MODULATION,
RPEAKSET = 500Ω
3.2Gbps, K28.5, 10mA MODULATION,
RPEAKSET = 2.4kΩ
73mV/div
73mV/div
73mV/div
50ps/div
50ps/div
50ps/div
OPTICAL EYE
MAX3740A toc04
IBIASMON vs. BIAS CURRENT
MAX3740A toc05
ER = 8.2dB, 2.5Gbps, K28.5,
850nm VCSEL SONET MASK
WITH +20% MARGIN
1.6
1.4
IBIASMON (mA)
ER = 8.2dB, 2.125Gbps, K28.5,
850nm VCSEL, WITH 2.3GHz
O-TO-E CONVERTER
1.8
MAX3740A toc06
OPTICAL EYE
1.2
1.0
0.8
0.6
0.4
EMCORE SC-TOSA-8585-3420 VCSEL
0.2
EMCORE SC-TOSA-8585-3420 VCSEL
0
68ps/div
0
58ps/div
4
8
16
12
BIAS CURRENT (mA)
DETERMINISTIC JITTER
vs. MODULATION CURRENT
25
20
15
10
5
4
3
2
0
0
0
5
10
IMOD (mAP-P)
15
MAX3740A toc09
80
RISE
70
FALL
60
50
1
5
90
TRANSITION TIME (ps)
6
RANDOM JITTER (psRMS)
30
100
MAX3740A toc08
35
DETERMINISTIC JITTER (psP-P)
7
MAX3740A toc07
40
TRANSITION TIME
vs. MODULATION CURRENT
RANDOM JITTER
vs. MODULATION CURRENT
40
0
5
10
IMOD (mAP-P)
15
2
4
6
8
10
12
14
16
IMOD (mAP-P)
_______________________________________________________________________________________
5
MAX3740A
Typical Operating Characteristics
(VCC = +3.3V, RTC = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, TA = +25°C, unless otherwise
noted.)
Typical Operating Characteristics (continued)
(VCC = +3.3V, RTC = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, TA = +25°C, unless otherwise
noted.)
100μ
10m
1m
100μ
1m
10k
100k
100
1k
RBIASSET (Ω)
SUPPLY CURRENT vs. TEMPERATURE
OUTPUT RETURN LOSS
INPUT RETURN LOSS
DIFFERENTIAL
MEASUREMENT
-5
0
-4
40
-6
-15
S22 (dB)
S11 (dB)
50
-20
-25
30
IMOD = 2mA
10
-15
10
35
85
60
-16
-40
100M
-18
100M
10G
1G
MODULATION CURRENT
vs. TEMPERATURE
MODULATION CURRENT TEMPCO
vs. RTC
MONITOR DIODE CURRENT
vs. TEMPERATURE
9
RTC = 5kΩ
RTC = 10kΩ
RTC = 60kΩ
RTC = 100kΩ
6
RTC = 500kΩ
5
4500
3500
2500
1500
500
10
20
30
40
50
60
TEMPERATURE (°C)
70
80
90
275
250
225
200
175
150
125
100
-500
4
300
MAX3740A toc18
RTC = 1kΩ
REFERENCED TO +25°C
MONITOR DIODE CURRENT (μA)
10
7
5500
TEMPCO (ppm/°C)
RMOD = 1.35kΩ
MAX3740A toc17
FREQUENCY (Hz)
RTC = 100Ω
0
10G
1G
FREQUENCY (Hz)
11
8
-14
-35
TEMPERATURE (°C)
MAX3740A toc16
-40
-8
-10
-12
-30
20
SINGLE-ENDED
MEASUREMENT
-2
-10
60
10k
RPWRSET (Ω)
0
MAX3740A toc13
IMOD = 15mA
1k
100
RMODSET (Ω)
80
70
10μ
1μ
10k
MAX3740A toc14
1k
SUPPLY CURRENT (mA)
10m
MAX3740A toc12
MEASURED WITH A
50Ω ELECTRICAL LOAD
MAX3740A toc15
1m
MAX3740A toc11
MAX3740A toc10
10m
100m
MODULATION CURRENT (AP-P)
BIAS CURRENT (A)
100m
6
MONITOR DIODE CURRENT
vs. RPWRSET
MODULATION CURRENT vs. RMODSET
MONITOR DIODE CURRENT (A)
BIAS CURRENT vs. RBIASSET
MODULATION CURRENT (mAP-P)
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
100
1k
10k
RTC (Ω)
100k
1M
-40
-15
10
35
TEMPERATURE (°C)
_______________________________________________________________________________________
60
85
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
TX_DISABLE NEGATE TIME
STARTUP WITH SLOW RAMPING SUPPLY
HOT PLUG WITH TX_DISABLE LOW
MAX3740A toc21
MAX3740A toc20
MAX3740A toc19
3.3V
3.3V
VCC
FAULT
3.3V
VCC
OV
VCC
OV
FAULT
LOW
TX_DISABLE
LOW
t_INIT = 62ms
TX_DISABLE
t_INIT = 60ms
FAULT
LOW
TX_DISABLE
LOW
HIGH
t_ON = 54μs
20μs/div
20ms/div
20ms/div
LOW
LASER
OUTPUT
LASER
OUTPUT
LASER
OUTPUT
LOW
RESPONSE TO FAULT
TRANSMITTER DISABLE
MAX3740A toc22
3.3V
MAX3740A toc23
EXTERNALLY
FORCED
VPWRMON
FAULT
VCC
t_OFF = 1.86μs
FAULT
FAULT
HIGH
LOW
TX_DISABLE
TX_DISABLE
t_FAULT = 245ns
LOW
LOW
LOW
HIGH
LASER
OUTPUT
LASER
OUTPUT
200ns/div
1μs/div
FREQUENT ASSERTION OF TX_DISABLE
FAULT RECOVERY TIME
MAX3740A toc25
MAX3740A toc24
EXTERNAL
FAULT
REMOVED
VPWRMON
FAULT
VPWRMON
EXTERNALLY
FORCED FAULT
FAULT
HIGH
LOW
HIGH
TX_DISABLE
LOW
LOW
t_INIT = 54μs
LASER
OUTPUT
40μs/div
TX_DISABLE
LASER
OUTPUT
200μs/div
_______________________________________________________________________________________
7
MAX3740A
Typical Operating Characteristics (continued)
(VCC = +3.3V, RTC = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, TA = +25°C, unless otherwise
noted.)
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
MAX3740A
Pin Description
8
PIN
NAME
FUNCTION
1, 10, 13
GND
2
TX_DISABLE
3
IN+
Noninverted Data Input
4
IN-
Inverted Data Input
5
FAULT
6
SQUELCH
7, 16, 20
VCC
+3.3V Supply Voltage
8
TC1
Temperature Compensation Set Pin 1. A resistor placed between TC1 and TC2 (RTC) programs the
temperature coefficient of the modulation current.
9
TC2
Temperature Compensation Set Pin 2. A resistor placed between TC1 and TC2 (RTC) programs the
temperature coefficient of the modulation current.
11
MODSET
Modulation Set. A resistor connected from MODSET to ground (RMODSET) sets the desired
modulation current amplitude.
12
PEAKSET
Peaking Current Set. A resistor connected between PEAKSET and ground (RPEAKSET) programs
the peaking current amplitude. To disable peaking, leave PEAKSET open.
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.
Fault Indicator. Open-drain output with ESD protection. FAULT is asserted high during a
fault condition.
Squelch Enable. Squelch is enabled when the pin is set high. Squelch is disabled when the pin is
set low or left open.
14
OUT-
Inverted Modulation-Current Output
15
OUT+
Noninverted Modulation-Current Output
17
BIASSET
18
BIAS
Bias Current Set. When a closed-loop configuration is used, connect a 1.7k resistor between
ground and BIASSET to set the maximum bias current. When an open configuration is used,
connect a resistor between BIASSET and ground (RBIASSET) to program the VCSEL bias current.
Bias Current Output
19
BIASMON
Bias Current Monitor. The output of BIASMON is a sourced current proportional to the bias current.
A resistor connected between BIASMON and ground (RBIASMON) can be used to form a groundreferenced bias monitor.
21
COMP
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.
22
MD
Monitor Diode Connection
23
REF
Reference Pin. Reference monitor used for APC. A resistor between REF and MD (RPWRSET) sets the
photo monitor current when the APC loop is closed.
24
PWRMON
—
EP
Average Power Monitor. The pin is used to monitor the transmit optical power. For open-loop
configuration, connect PWRMON to GND.
Exposed Pad. Ground. Must be soldered to the circuit board ground for proper thermal and
electrical performance. See the Layout Considerations section.
_______________________________________________________________________________________
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
MAX3740A
PWRMON
REF
1.8V
RPWRSET
CURRENT
AMPLIFIER
2X
MAX3740A
POWERCONTROL
AMPLIFIER
MD
ENABLE
IBIAS
34
BIAS
BIAS GENERATOR
FERRITE
BEAD
SMOOTHSTART
IPD
BIASMON
1.6V
(2VBE)
IBIAS
9
1.2V
RBIASMON
200Ω
COMP
BIASSET
RBIASSET
CCOMP
Figure 1. Bias Generator
Detailed Description
The MAX3740A contains a bias generator with automatic power control (APC), safety circuit, and a laser modulator with optional peaking compensation.
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.
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 drives a current amplifier to control the laser’s bias current.
During a fault condition, the bias current is disabled.
The PWRMON output provides a voltage proportional to
average laser power given by:
VPWRMON = 2 ✕ IPD ✕ RPWRSET
The BIASMON output provides a current proportional to
the laser bias current given by:
IBIASMON = IBIAS / 9
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
resistor RBIASSET. When a closed-loop configuration is
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
toggle of TX_DISABLE or VCC. The FAULT pin should
be pulled high with a 4.7kΩ to 10kΩ resistor.
Table 1. Fault Conditions
PIN
FAULT CONDITION
BIAS
VBIAS > VCC - 0.2V
BIASMON
VBIASMON > 0.8V
PWRMON
VPWRMON > 0.8V
_______________________________________________________________________________________
9
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
Table 2. Circuit Response to Various Single-Point Faults (Closed-Loop APC Configuration)
PIN NAME
FAULT
TX_DISABLE
CIRCUIT RESPONSE TO VCC SHORT
CIRCUIT RESPONSE TO GND SHORT
Does not affect laser power.
Does not affect laser power.
Modulation and bias current are disabled.
Normal condition for circuit operation.
IN+
Does not affect laser power.
Does not affect laser power.
Does not affect laser power.
IN-
Does not affect laser power.
SQUELCH
Does not affect laser power.
Does not affect laser power.
TC1
Does not affect laser power.
Does not affect laser power.
TC2
The laser modulation is increased, but average power
is not affected.
Modulation current is disabled.
Modulation current is disabled.
The laser modulation is increased, but average power
is not affected.
MODSET
PEAKSET
Does not affect laser power.
Does not affect laser power.
OUT+
Modulation current is disabled.
Modulation current is disabled.
OUT-
Does not affect laser power.
Does not affect laser power.
Laser bias is disabled.
Fault state* occurs.
Fault state* occurs. Note that VCSEL emissions may
continue; care must be taken to prevent this condition.
Disables VCSEL.
Fault state* occurs.
Does not affect laser power.
COMP
The bias current is reduced, and the average power of
the laser output is reduced.
IBIAS increases to the value determined by RBIASSET; if
the bias monitor fault threshold is exceeded, a fault is
signaled.
MD
IBIAS increases to the value determined by RBIASSET; if
the bias-monitor fault threshold is exceeded, a fault is
signaled.
The bias current is reduced, and the average power of
the laser output is reduced.
REF
IBIAS increases to the value determined by RBIASSET; if
the bias-monitor fault threshold is exceeded, a fault is
signaled.
The bias current is reduced, and the average power of
the laser output is reduced.
Fault state* occurs.
Does not affect laser power.
BIASSET
BIAS
BIASMON
PWRMON
*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
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 (RMODSET) programs the temperature-stable portion of the modulation
current, and the resistor between TC1 and TC2 (RTC)
programs the temperature coefficient of the modulation
current. For appropriate RTC and RMODSET values, see
the Typical Operating Characteristics section.
10
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 lowvoltage swing. Trim the leads to reduce laser package
inductance. The typical package leads have inductance of 25nH per inch (1nH/mm). This inductance
causes a large voltage swing across the laser. A compensation filter network can also be used to reduce
ringing, edge speed, and voltage swing (see the
Designing the Compensation Filter Network section).
______________________________________________________________________________________
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
MAX3740A
TX_DISABLE
BIAS
VBIAS FAULT
FAULT
OUTPUT
VCC - 0.2V
FAULT
BIASMON
HIGH-CURRENT FAULT
R
0.8V
Q
ENABLE
S
R-S LATCH
PWRMON
HIGH-POWER FAULT
0.8V
POR
TX_DISABLE
SAFETY CIRCUIT
MAX3740A
Figure 2. Safety Circuit
VCC
MAX3740A
ROUT+
ROUT-
INPUT BUFFER
OUT+
CURRENT SWITCH
IN+
OUTSIGNAL
DETECT
100Ω
IN-
PEAKING
CONTROL
PEAKSET
SQUELCH
ENABLE
CURRENT AMPLIFIER 30x
MODULATION
CURRENT GENERATOR
RPEAKSET
TEMPERATURE
COMPENSATION
1V
200Ω
TC2
TC1
MODSET
RMODSET
RTC
Figure 3. Modulation Circuit
______________________________________________________________________________________
11
MAX3740A
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
Programming Modulation Current
The modulation current output of the MAX3740A is controlled by a resistor (R MODSET ) placed between
MODSET and ground. The RMODSET resistor controls
the amount of current being sourced to the VCSEL. The
modulation current is given by the following:
⎡
⎤
ROUT+
IMOD = (IMODSET ) × 30 × ⎢
⎥
R
+
R
LOAD ⎦
⎣ OUT+
⎡⎛
⎤ ⎡
⎞
⎤
ROUT+
1
IMOD = ⎢⎜
× 30⎥ × ⎢
⎥
⎟
⎢⎣⎝ 200 + RMODSET ⎠
⎥⎦ ⎣ ROUT+ + RLOAD ⎦
[
]
0.021mW/mA at +25°C, which reduces to 0.018mW/mA
at +85°C. The temperature coefficient is given by the
following:
(SE85 − SE25 )
× 1E6
SE25 × (85 − 25)
= −2380ppm / °C
Laser tempco =
From the Typical Operating Characteristics, the value
of RTC, which offsets the tempco of the laser, is 9kΩ. If
modulation temperature compensation is not desired,
short TC1 and TC2.
Programming the APC Loop
It is important to note that the modulation current being
sourced by the MAX3740A is affected by the load
impedance of the VCSEL. The Modulation Current vs.
RMODSET graph in the Typical Operating Characteristics
shows the current into a 50Ω electrical load.
Programming Bias Current
The bias current output of the MAX3740A is controlled
by a resistor (RBIASSET) placed between BIASSET and
ground. In open-loop operation the RBIASSET controls
the bias current level of the VCSEL. In closed-loop
operation the RBIASSET controls the maximum bias current provided by the APC. The bias current is given by
the following:
Program the average optical power by adjusting
R PWRSET . To select the resistance, determine the
desired monitor current to be maintained over temperature and lifetime. See the Monitor Diode Current vs.
RPWRSET graph in the Typical Operating Characteristics
section, and select the value of RPWRSET that corresponds to the required current.
Input Termination Requirements
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.
IBIAS = (IBIASSET ) × 34
⎛
⎞
1.2
IBIAS = ⎜
⎟ × 34
⎝ 200 + RBIASSET ⎠
VCC
The Bias Current vs. RBIASSET graph is also shown in
the Typical Operating Characteristics.
MAX3740A
Photodiode Selection
To ensure stable operation of the APC circuit, the time
constant of the MD node should be shorter than the
APC time constant. (tAPC = 5µs if CAPC = 0.047µF).
PACKAGE
IN+
16kΩ
VCC
1nH
0.5pF
50Ω
5μs
t MD ≤
, RMD × CMD ≤
= 250ns
20
20
t APC
VCC
50Ω
For typical IPD = 400µA, RPWRSET = 500Ω, select a
photodiode with capacitance less than 500pF.
Programming Modulation-Current Tempco
Compute the required modulation tempco from the
slope efficiency of the laser at TA = +25°C and at a
higher temperature. Then select the value of RTC from
the Typical Operating Characteristics. For example,
suppose a laser has a slope efficiency (SE) of
12
___________________________________________________
IN-
1nH
0.5pF
24kΩ
Figure 4. Simplified Input Structure
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
UNCOMPENSATED
PACKAGE
ROUT+
1nH
OUT-
0.5pF
1nH
OUT+
CORRECTLY COMPENSATED
POWER
ROUT-
MAX3740A
VCC
OVERCOMPENSATED
0.5pF
TIME
MAX3740A
Figure 7. Laser Compensation
The compensation components (RF and CF) are most
easily determined by experimentation. Begin with RF =
50Ω and CF = 1pF. Increase CF until the desired transmitter response is obtained (Figure 7). Refer to
Application Note HFAN-2-0: Interfacing Maxim Laser
Drives with Laser Diodes for more information.
Figure 5. Simplified Output Structure
VCC
MAX3740A
Exposed-Pad (EP) Package
FAULT
Figure 6. Fault Circuit Interface
Applications Information
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.
Interface Models
Laser Safety and IEC 825
Figures 4 and 5 show simplified input and output circuits
for the MAX3740A laser driver. Figure 6 shows the fault
circuit interface.
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 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 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 components 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.
Layout Considerations
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.
Designing the Compensation Filter
Network
Laser package inductance causes the laser impedance
to increase at high frequencies, leading to ringing, overshoot, and degradation of the laser output. A laser compensation filter network can be used to reduce the laser
impedance at high frequencies, thereby reducing output
ringing and overshoot.
______________________________________________________________________________________
13
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
MAX3740A
Functional Diagram
COMP
FAULT
MD
REF
BIAS
BIAS
GENERATOR
WITH APC
SAFETY
CIRCUITRY
TX_DISABLE
BIASMON
PWRMON
BIASSET
ENABLE
VCC
LASER
MODULATOR
MAX3740A
SQUELCH
OUTOUT+
IN+
SIGNAL
DETECT
100Ω
PEAKING
CONTROL
INMODULATION CURRENT
GENERATOR
TC2
PEAKSET
MODSET
Pin Configuration
REF
MD
COMP
VCC
BIASMON
22
21
20
19
BIAS
17
BIASSET
IN+
3
16
VCC
15
OUT+
14
OUT-
13
GND
*EP
12
6
PEAKSET
SQUELCH
11
5
10
4
9
INFAULT
MAX3740A
TC2
21-0139
18
2
8
T2444-4
DOCUMENT NO.
1
7
24 TQFN-EP
(4mm x 4mm x
0.75mm)
PACKAGE CODE
GND
TX_DISABLE
VCC
PACKAGE TYPE
PWRMON
Package Information
For the latest package outline information and land patterns,
go to www.maxim-ic.com/packages. Note that a “+”, “#”, or
“-” in the package code indicates RoHS status only. Package
drawings may show a different suffix character, but the drawing
pertains to the package regardless of RoHS status.
23
TOP VIEW
TC1
TRANSISTOR COUNT: 3806
PROCESS: SiGe BIPOLAR
24
Chip Information
GND
TC1
MODSET
ENABLE
THIN QFN (4mm x 4mm)
*EXPOSED PAD IS CONNECTED TO GND
14
______________________________________________________________________________________
3.2Gbps SFP VCSEL Driver with Diagnostic
Monitors
REVISION
NUMBER
REVISION
DATE
0
12/03
1
2
3
DESCRIPTION
PAGES
CHANGED
Initial release.
—
6/04
Added a lead-free package to the Ordering Information table.
1
In the Electrical Characteristics table, modified the MD Nominal Voltage parameter
of VREF - 0.2V (typ) to VREF - 0.16V (typ).
3
5/06
Modified Figure 1 to clarify the meaning of the arrow labeled IPD.
9
Updated the Package Information section to correct the package code.
14
1/10
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 15
© 2010 Maxim Integrated Products
Maxim is a registered trademark of Maxim Integrated Products, Inc.
MAX3740A
Revision History
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