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General DescriptionThe MAX3795 is a high-speed VCSEL driver for small-form-factor (SFF) and small-form-factor pluggable (SFP)fiber optic transmitters. It contains a bias generator, alaser modulator, and comprehensive safety features.The automatic power control (APC) adjusts the laserbias current to maintain average optical power overchanges in temperature and laser properties. The dri-ver accommodates common-cathode and differentialconfigurations.
The MAX3795 operates up to 4.25Gbps. It can switchup to 15mA of laser modulation current and source upto 15mA of bias current. Adjustable temperature com-pensation is provided to keep the optical extinctionratio within specifications over the operating tempera-ture range. The MAX3795 interfaces with the DallasDS1856/DS1859 to meet SFF-8472 timing and diagnos-tic requirements. The MAX3795 accommodates variousVCSEL packages, including low-cost TO-46 headers.
The MAX3795 safety circuit detects faults that couldcause hazardous light levels and disables the VCSELoutput. The safety circuits are compliant with SFF andSFP multisource agreements (MSAs).
The MAX3795 is available in a compact 4mm 4mm,24-pin thin QFN package and operates over the -40°Cto +85°C temperature range. The MAX3795 is pin-for-pin compatible with the MAX3740A and is available inlead-free packages.
ApplicationsMultirate (1Gbps to 4.25Gbps) SFP/SFF Modules
Gigabit Ethernet Optical Transmitters
Fibre-Channel Optical Transmitters
Features♦ Supports All SFF-8472 Digital Diagnostics
♦ 3.3V ±10% Single Supply
♦ 2mA to 15mA Modulation Current
♦ 1mA to 15mA Bias Current
♦ 52ps Transition Time
♦ 8.4ps Deterministic Jitter
♦ Optional Peaking Current to Improve VCSEL EdgeSpeed
♦ Supports Common-Cathode and DifferentialConfiguration
♦ Safety Circuits Compliant with SFF and SFPMSAs
♦ Pin Compatible to MAX3740A
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1Gbps to 4.25Gbps Multirate VCSEL Driver with Diagnostic Monitors
________________________________________________________________ Maxim Integrated Products 1
Ordering Information
19-3387; Rev 0; 8/04
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 PIN-PACKAGE
MAX3795ETG -40°C to +85°C 24 Thi n QFN ( 4m m x 4m m )
MAX3795ETG+ -40°C to +85°C 24 Thi n QFN ( 4m m x 4m m )
24 23 22 21 20 19
PWRM
ON
REF
MD
COM
P
V CC
BIAS
MON
7 8 9 10 11 12
V CC
TC1
TC2
GND
MOD
SET
PEAK
SET
13
14
15
16
17
18
EXPOSED PAD IS CONNECTED TO GND
GND
OUT-
OUT+
VCC
BIASSET
BIAS
6
5
4
3
2
1
SQUELCH
FAULT
IN-
IN+
TX_DISABLE
GND
MAX3795
THIN QFN (4mm x 4mm)
TOP VIEW
Pin Configuration
+Denotes lead-free package.
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2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functionaloperation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure toabsolute maximum rating conditions for extended periods may affect device reliability.
Supply Voltage (VCC) ............................................-0.5V to +4.0VVoltage 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 + 1V)Current into FAULT ............................................ -1mA to +25mACurrent into OUT+, OUT- ....................................................60mA
Continuous Power Dissipation (TA = +85°C)24-Pin Thin QFN(derate 20.8mW/°C above +85°C).................................1354mW
Storage Temperature Range .............................-55°C to +150°CLead Temperature (soldering, 10s) .................................+300°C
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 SYMBOL CONDITIONS MIN TYP MAX UNITS
IMOD = 2mAP-P 35SQUELCH set low,TX_DISABLE set low,peaking is not used(Note 1) IMOD = 15mAP-P 71 81
Additional current when peaking is used,RPEAK = 1.18kΩ
15
ICC
Additional current when SQUELCH is high 5 10
Supply Current
ICC-SHDN Total current when TX_DISABLE is high 7
mA
FAULT OUTPUT
Output High Voltage VOH RLOAD = 10kΩ to 2.97V 2.4 V
Output Low Voltage VOL RLOAD = 4.7kΩ to 3.63V 0.4 V
TX_DISABLE INPUT
Input Impedance RPULL 4.7 8 10.0 kΩInput High Voltage VIH 2.0 V
Input Low Voltage VIL 0.8 V
Power-Down TimeThe time for ICC to reach ICC-SHDN whenTX_DISABLE transitions high
50 µs
SQUELCH
Squelch Threshold 25 85 mVP-P
Squelch Hysteresis 6 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
Maximum Bias Pin Voltage VBIAS-MAX Referenced to VCC -0.65 V
Minimum 1Bias Current IBIAS
Maximum 15mA
5mA ≤ IBIAS ≤ 15mA -8 +8Accuracy of Programmed BiasCurrent
∆BIAS1mA ≤ IBIAS ≤ 5mA -12 +12
%
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_______________________________________________________________________________________ 3
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
Bias Current During Fault IBIAS_OFF Current out of the BIAS pin 1.5 10 µA
1mA < IBIAS < 3mA 0.0875 0.105 0.1375BIASMON Gain GBIASMON IBIASMON / IBIAS
3mA ≤ IBIAS < 15mA 0.085 0.105 0.125mA/mA
BIASMON Stability (Notes 2, 4) -10 +10 %
AUTOMATIC POWER CONTROL (APC)
MD Nominal Voltage VMD APC loop is closed 1VREF -
0.22 V
Voltage at REF VREF 1.2 1.8 2.2 V
MD Voltage During Fault 0 V
MD Input Current Normal operation (FAULT = low) -2 0.7 +2 µA
APC Time Constant CCOMP = 0.047µF, ∆IPD / ∆ILASER = 0.02 90 µs
PWRMON Nominal Gain VPWRMON / (VREF - VMD) 1.85 2.15 2.45 V/V
LASER MODULATOR (Load is 50Ω AC-Coupled to OUT+)
Minimum 0.25Differential Input Voltage VID
Maximum 2.4VP-P
Input Common-Mode Voltage VCM 1.75 V
Differential Input Resistance RIN 85 100 115 ΩSingle-Ended Input Return Loss S11 f < 4GHz 12.7 dB
Differential Input Return Loss SDD11 f < 4GHz 11 dB
Minimum 2Modulation Current IMOD
Current into OUT+RLOAD ≤ 50Ω Maximum 15
mA
Laser Modulation During Fault orSquelch Active
IMOD_OFF DC tested 15 50 µAP-P
Tolerance of ProgrammedModulation Current
TC1 is shorted to TC2 -10 +10 %
Minimum Peaking Current RPEAKSET = 10kΩ 0.2 mA
Maximum Peaking Current RPEAKSET = 1kΩ 2 mA
Peaking Current Duration 75 ps
Output Resistance ROUT Single-ended resistance 42 50 58 Ω
Minimum ProgrammableTemperature Coefficient
0 ppm/°C
Maximum ProgrammableTemperature Coefficient
Temperature range 0°C to +70°C +5000 ppm/°C
- 40°C to + 85° C 49 72tR
50Ω load, no peaking, 5mA ≤ IMOD ≤ 15mA +100°C 58
- 40°C to + 85° C 56 79Modulation Transition Time(Note 2)
tF50Ω load, no peaking, 5mA ≤ IMOD ≤ 15mA +100°C 64
ps
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4 _______________________________________________________________________________________
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
- 40°C to + 85° C 8.4 15.6Deterministic Jitter DJ
5m A ≤ IM OD ≤ 15m A,4.25G b p s, K28.5 ( N otes 2, 5) +100°C 12.7
psP-P
APC closed loop 0.5Random Jitter RJ
APC open loop (Note 2) 0.5 0.9psRMS
SAFETY FEATURES (see the Typical Operating Characteristics)
High-Current Fault Threshold VBMTH VBIASMON > VBMTH causes a fault 0.7 0.8 0.9 V
VBIAS Fault Threshold VBTH VBIAS referenced to VCC -0.250 -0.2 -0.150 V
Power-Monitor Fault Threshold VPMTH VPWRMON > VPMTH causes a fault 0.7 0.8 0.9 V
TX Disable Time t_OFF
Time from rising edge of TX_DISABLE toIBIAS = IBIAS_OFF and IMOD = IMOD_OFF(Note 2)
1.8 5 µs
TX Disable Negate Time t_ON
Time from rising edge of TX_DISABLE toIBIAS and IMOD at 99% of steady state(Note 2)
55 500 µs
Fault Reset Time t_INIT1Time to set VFAULT = low after power-on orafter rising edge of TX_DISABLE (Note 2)
60 200 ms
Power-On Time t_INIT2Time after power-on to transmitter-on withTX_DISABLE low (Note 2)
60 200 ms
Fault Assert Time t_FAULT
Time from fault occurrence to VFAULT =high; CFAULT < 20pF, RFAULT = 4.7kΩ(Note 2)
1.4 50 µs
Fault Delay Time t_FLTDLY
Time from fault to IBIAS = IBIAS_OFF andIMOD = IMOD_OFF; measured with acontinuously occurring fault (Note 2)
1 5 µs
TX_DISABLE Reset t_RESETTime TX_DISABLE must be held high toreset FAULT (Note 2)
1 µs
Note 1: Supply current measurements exclude IBIAS from the total current.Note 2: AC characteristics guaranteed by design and characterization.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: Variation of bias monitor gain for any single part over the range of VCC, temperature, 3mA < IBIAS < 15mA.Note 5: Deterministic jitter measured at 4.25Gbps with a K28.5 pattern (00111110101100000101).
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_______________________________________________________________________________________ 5
ELECTRICAL EYE DIAGRAMMAX3795 toc01
40ps/div
75mV/div
4.25Gbps, K28.5, 10mA MODULATION, PEAKING OFF
ELECTRICAL EYE DIAGRAMMAX3795 toc02
152ps/div
75mV/div
1Gbps, K28.5, 10mA MODULATION,RPEAKSET = 1.4kΩ
OPTICAL EYE DIAGRAMMAX3795 toc03
135ps/div
1
2
3
1Gbps, K28.5, -3dBm, 850nm VCSELADVANCED OPTICAL COMPONENTS,
HFE4191-541
OPTICAL EYE DIAGRAMMAX3795 toc04
34ps/div
4.25Gbps, K28.5, -7dBm, 850nm VCSEL, ADVANCED OPTICAL COMPONENTS
HFE4191-541
OPTICAL EYE DIAGRAMMAX3795 toc05
50ps/div
3.125Gbps, K28.5, -7dBm, 850nm VCSEL, ADVANCED OPTICAL COMPONENTS
HFE4191-541
IBIASMON vs. BIAS CURRENT
MAX
3795
toc0
6
BIAS CURRENT (mA)
I BIA
SMON
(mA)
1284
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
00 16
DETERMINISTIC JITTERvs. MODULATION CURRENT
MAX
3795
toc0
7
MODULATION CURRENT (mAP-P)
DETE
RMIN
ISTI
C JIT
TER
(ps P
-P)
5
10
15
20
25
30
35
40
00 2 4 6 8 10 12 14 16 18 20
RANDOM JITTERvs. MODULATION CURRENT
MAX
3795
toc0
8
MODULATION CURRENT (mAP-P)
RAND
OM JI
TTER
(ps R
MS)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
00 2 4 6 8 10 12 14 16 18 20
IBIAS = 5mA
TRANSITION TIMEvs. MODULATION CURRENT
MAX
3795
toc0
9
MODULATION CURRENT (mA)
TRAN
SITI
ON T
IME
(ps)
10
20
30
40
50
60
70
0 2 4 6 8 10 12 14
MEASURED FROM 20%
FALL TIME
RISE TIME
Typical Operating Characteristics(VCC = +3.3V, RTC = 0Ω, PEAKSET open, measured electrically with a 50Ω load AC-coupled to OUT+, TA = +25°C, unless otherwisenoted.)
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6 _______________________________________________________________________________________
BIAS CURRENT vs. RBIASSET
MAX
3795
toc1
0
RBIASSET (Ω)
BIAS
CUR
RENT
(mA)
10
1
10
100
0.11 100
MODULATION CURRENT vs. RMODSET
MAX
3740
A to
c11
RMODSET (kΩ)
MOD
ULAT
ION
CURR
ENT
(mA P
-P)
10
10
100
110010.1
MODULATION CURRENT TEMPCOvs. RTC
MAX
3795
toc1
7
RTC (Ω)
TEM
PCO
(ppm
/°C)
100k10k1k
500
1500
2500
3500
4500
5500
-500100 1M
REFERENCED TO +25°C
MONITOR DIODE CURRENTvs. RPWRSET
MAX
3795
toc1
2
RPWRSET (Ω)
MON
ITOR
DIO
DE C
URRE
NT (A
)
1
10µ
100µ
1m
10m
1µ0 10
SUPPLY CURRENT vs. TEMPERATURE
MAX
3795
toc1
3
TEMPERATURE (°C)
SUPP
LY C
URRE
NT (m
A)
603510-1520
30
40
50
60
70
80
90
100
-40 85
IMOD = 2mA
IMOD = 15mA
INPUT RETURN LOSSM
AX37
95 to
c14
FREQUENCY (Hz)
S 11 (
dB)
1G
-30
-25
-20
-15
-10
-5
0
-35100 10G
DIFFERENTIALMEASUREMENTAT IN±
OUTPUT RETURN LOSS
MAX
3795
toc1
5
FREQUENCY (Hz)
S 22 (
dB)
1G
-40
-35
-30
-25
-20
-15
-10
-5
0
-45100 10G
SINGLE-ENDEDMEASUREMENT
MODULATION CURRENTvs. TEMPERATURE
MAX
3795
toc1
6
TEMPERATURE (°C)
MOD
ULAT
ION
CURR
ENT
(mA P
-P)
8070605040302010
5
6
7
8
9
10
11
40 90
RMODSET = 1.8kΩ
RTC = 1kΩ
RTC = 500kΩ
RTC = 100kΩ
RTC = 60kΩ
RTC = 10kΩRTC = 5kΩ
RTC = 100Ω
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 otherwisenoted.)
MONITOR DIODE CURRENTvs. TEMPERATURE
MAX
3795
toc1
8
TEMPERATURE (°C)
MON
ITOR
DIO
DE C
URRE
NT (µ
A)
6035-15 10
125
150
175
200
225
250
275
300
100-40 85
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_______________________________________________________________________________________ 7
HOT PLUG WITH TX_DISABLE LOWMAX3795 toc19
20ms/div
TX_DISABLE
VCC
LASEROUTPUT
FAULT
LOW t_INIT = 60ms
3.3V
0V
LOW
STARTUP WITH SLOW RAMPING SUPPLYMAX3795 toc20
20ms/div
TX_DISABLE
VCC
LASEROUTPUT
FAULT
LOW
t_INIT = 62ms
3.3V
LOW
0V
FAULT RECOVERY TIMEMAX3795 toc24
40µs/div
TX_DISABLE
VPWRMON
LASEROUTPUT
FAULT
LOW
t_INIT = 54µs
HIGH
EXTERNALFAULTREMOVED
LOW
LOW
HIGH
FREQUENT ASSERTION OF TX_DISABLEMAX3795 toc25
200µs/div
TX_DISABLE
VPWRMON
LASEROUTPUT
FAULT
EXTERNALLYFORCED FAULT
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 otherwisenoted.)
TX_DISABLE NEGATE TIMEMAX3795 toc21
40µs/div
TX_DISABLE
VCC
LASEROUTPUT
FAULT
t_ON = 131µs
3.3V
LOW
LOW
HIGH
TRANSMITTER DISABLEMAX3795 toc22
1µs/div
TX_DISABLE
VCC
LASEROUTPUT
FAULT
LOW
t_OFF = 2.2µs
3.3V
HIGH
LOW
RESPONSE TO FAULTMAX3795 toc23
4µs/div
TX_DISABLE
VPWRMON
LASEROUTPUT
FAULT
LOW
t_FAULT = 2.16µsHIGH
EXTERNALLYFORCEDFAULT
LOW
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8 _______________________________________________________________________________________
Pin Description
PIN NAME FUNCTION
1, 10, 13 GND Ground
2 TX_DISABLETransmit Disable. Driver output is disabled when TX_DISABLE is high or left unconnected. Thedriver output is enabled when the pin is asserted low.
3 IN+ Noninverted Data Input
4 IN- Inverted Data Input
5 FAULTFault Indicator. Open-drain output with ESD protection. FAULT is asserted high during afault condition.
6 SQUELCHSquelch Enable. Squelch is enabled when the pin is set high. Squelch is disabled when the pin isset low or left open.
7, 16, 20 VCC +3.3V Supply Voltage
8 TC1Temperature Compensation Set Pin 1. A resistor placed between TC1 and TC2 (RTC) programs thetemperature coefficient of the laser modulation current.
9 TC2Temperature Compensation Set Pin 2. A resistor placed between TC1 and TC2 (RTC) programs thetemperature coefficient of the laser modulation current.
11 MODSETModulation Set. A resistor connected from MODSET to ground (RMODSET) programs the desiredmodulation current amplitude.
12 PEAKSETPeaking Current Set. A resistor connected between PEAKSET and ground (RPEAKSET) programs thepeaking current amplitude. To disable peaking, leave PEAKSET open.
14 OUT- Inverted Modulation Current Output
15 OUT+ Noninverted Modulation Current Output
17 BIASSETBias-Current Set. When a closed-loop configuration is used, connect a 1.7kΩ resistor betweenground and BIASSET to program the maximum bias current. When an open configuration is used,connect a resistor between BIASSET and ground (RBIASSET) to program the VCSEL bias current.
18 BIAS Bias-Current Output
19 BIASMONBias-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 ground-referenced bias monitor.
21 COMPCompensation Pin. A capacitor between COMP and MD compensates the APC. A typical value of0.047µF is recommended. For open-loop configuration, short the COMP pin to GND to deactivatethe APC circuit.
22 MD Monitor Diode Connection
23 REFRefer ence P i n. Refer ence m oni tor used for AP C . A r esi stor b etw een RE F and M D ( RP WRS E T) p r og r am sthe p hotom oni tor cur r ent w hen the AP C l oop i s cl osed .
24 PWRMONAverage Power Monitor. The pin is used to monitor the transmit optical power. For open-loopconfiguration, connect PWRMON to GND.
EP Exposed PadGround. Must be soldered to the circuit board ground for proper thermal and electricalperformance. See the Layout Considerations section.
Detailed DescriptionThe MAX3795 contains a bias generator with APC,safety circuit, and a laser modulator with optional peak-ing compensation (see the Functional Diagram).
Bias GeneratorFigure 1 shows the bias-generator circuitry that containsa power-control amplifier and smooth-start circuitry. Aninternal pnp transistor provides DC laser current to biasthe laser in a light-emitting state. The APC circuitryadjusts the laser-bias current to maintain average powerover temperature and changing laser properties. Thesmooth-start circuitry prevents current spikes to the laserduring power-up or enable, ensuring compliance withsafety requirements and extending the life of the laser.
The MD input is connected to the cathode of a monitordiode, which is used to sense laser power. The BIASoutput is connected to the anode of the laser through aninductor 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.
The PWRMON output provides a voltage proportional toaverage laser power given by:
VPWRMON = 2 x IPD x RPWRSET
where VPWRMON = 0.4V (typ)
The BIASMON output provides a current proportional tothe laser bias current given by:
IBIASMON = IBIAS x GBIASMON
When APC is not used (no monitor diode), connect theCOMP and PWRMON pins to GND. In this mode, biascurrent is set by the resistor (RBIASSET) between theBIASSET pin and GND. When a closed-loop configura-tion is used, connect a 1.7kΩ resistor between groundand BIASSET to set the maximum bias current.
Safety CircuitThe safety circuit contains an input disable(TX_DISABLE), a latched fault output (FAULT), and faultdetectors (Figure 2). This circuit monitors the operationof the laser driver and forces a shutdown (disableslaser) if a fault is detected (Table 1). Table 2 containsthe circuit’s response to various single-point failures.The transmit fault condition is latched until reset by atoggle of TX_DISABLE or VCC. The FAULT pin shouldbe pulled high with a 4.7kΩ to 10kΩ resistor.
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_______________________________________________________________________________________ 9
CCOMP
COMP BIASSET
RPWRSET
MD
1.8V(2VBE + 0.2)
REFCURRENTAMPLIFIER
POWER-CONTROLAMPLIFIER
ENABLE
200Ω
RBIASSET
PWRMON
BIAS GENERATOR
SMOOTH-START
RBIASMON
BIASMON
FERRITEBEAD
BIAS
1.6V(2VBE)
2X
IBIAS34
IPD
IBIAS9
MAX3795
1V
Figure 1. Bias Generator
PIN FAULT CONDITION
BIAS VBIAS > VCC - 0.2V
BIASMON VBIASMON > 0.8V
PWRMON VPWRMON > 0.8V
Table 1. Fault Conditions
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10 ______________________________________________________________________________________
Modulation CircuitThe modulation circuitry consists of an input buffer, acurrent mirror, and a high-speed current switch (Figure3). The modulator drives up to 15mA of modulation intoa 50Ω VCSEL load.
The amplitude of the modulation current is set withresistors at MODSET and temperature coefficient (TC1,TC2) pins. The resistor at MODSET (RMODSET) pro-grams the temperature-stable portion of the modulationcurrent, and the resistor between TC1 and TC2 (RTC)programs the temperature coefficient of the modulation
PIN NAMECIRCUIT RESPONSE TO
VCC SHORTCIRCUIT RESPONSE TO
GND SHORTCIRCUIT RESPONSE TO
OPEN
FAULT Does not affect laser power. Does not affect laser power. Does not affect laser power.
TX_DISABLEModulation and bias current aredisabled.
Normal condition for circuitoperation.
Modulation and bias current aredisabled.
IN+ Does not affect laser power. Does not affect laser power. Does not affect laser power.
IN- Does not affect laser power. Does not affect laser power. Does not affect laser power.
SQUELCH Does not affect laser power. Does not affect laser power. Does not affect laser power.
TC1 Does not affect laser power. Does not affect laser power.The laser modulation is decreased,but average power is not affected.
TC2The laser modulation is increased,but average power is not affected.
Modulation current is disabled.The laser modulation is decreased,but average power is not affected.
MODSET Modulation current is disabled.The laser modulation is increased,but average power is not affected.
The laser modulation is decreased,but average power is not affected.
PEAKSET Does not affect laser power. Does not affect laser power. Does not affect laser power.
OUT+ Modulation current is disabled. Modulation current is disabled. Modulation current is disabled.
OUT- Does not affect laser power. Does not affect laser power. Does not affect laser power.
BIASSET Laser bias is disabled. Fault state* occurs. Laser bias is disabled.
BIASFault state* occurs. Note that VCSELemissions may continue. Care mustbe taken to prevent this condition.
This disables the VCSEL. This disables the VCSEL.
BIASMON Fault state* occurs. Does not affect laser power. Fault state* occurs.
COMPThe bias current is reduced, and theaverage power of the laser output isreduced.
IBIAS increases to the valuedetermined by RBIASSET. If the bias-monitor fault threshold is exceeded,a fault is signaled.
APC loop will be unstable.If the bias-monitor fault threshold isexceeded, a fault is signaled.
MD
IBIAS increases to the valuedetermined by RBIASSET. If the bias-monitor fault threshold is exceeded,a fault is signaled.
The bias current is reduced, and theaverage power of the laser output isreduced.
IBIAS increases to the valuedetermined by RBIASSET. If the biasmonitor fault threshold is exceeded,a fault is signaled.
REF
IBIAS increases to the valuedetermined by RBIASSET. If the bias-monitor fault threshold is exceeded,a fault is signaled.
The bias current is reduced, and theaverage power of the laser output isreduced.
The bias current is reduced, and theaverage power of the laser output isreduced.
PWRMON Fault state* occurs. Does not affect laser power. Does not affect laser power.
*A fault state asserts the FAULT pin, disables the modulator output, and disables the bias output.
Table 2. Circuit Response to Various Single-Point Faults (Closed-Loop APC Configuration)
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S
R Q
R-S LATCH
HIGH-POWER FAULT
HIGH-CURRENT FAULT
VBIAS FAULTBIAS
BIASMON
PWRMON
POR
TX_DISABLE
TX_DISABLE
0.8V
0.8V
VCC - 0.2V FAULT
ENABLE
SAFETY CIRCUIT
FAULTOUTPUT
MAX3795
Figure 2. Safety Circuit
VCC
ROUT
100Ω
CURRENT AMPLIFIER 40xENABLE
IN+
IN-
OUT+
OUT-
TC1 MODSET
RTC
RMODSET
MODULATIONCURRENT GENERATOR
INPUT BUFFER
CURRENT SWITCH
TEMPERATURECOMPENSATION
TC2
SIGNAL DETECT
SQUELCH
PEAKINGCONTROL
PEAKSET
200Ω
ROUT
1V
RPEAKSET
MAX3795
Figure 3. Modulation Circuit
current. For appropriate RTC and RMODSET values, seethe Typical Operating Characteristics.
Design ProcedureSelect Laser
Select a communications-grade laser with a rise time of90ps or better for 4.25Gbps applications. Use a high-efficiency laser that requires low modulation current andgenerates a low-voltage swing. Trim the leads to reducelaser package inductance. The typical package leadshave inductance of 25nH per inch (1nH/mm). Thisinductance causes a large voltage swing across thelaser. A compensation filter network can also be used toreduce ringing, edge speed, and voltage swing.
Programming Modulation CurrentA resistor (RMODSET) placed between the MODSET pinand ground controls the modulation current out of theMAX3795 to the VCSEL. The modulation current isgiven by the following:
It is important to note that the load impedance of theVCSEL affects the modulation current being sourced bythe MAX3795. The Modulation Current vs. RMODSETgraph in the Typical Operating Characteristics shows thecurrent into a 50Ω load. Capacitance at the MODSET pinshould be ≤20pF.
Programming Bias CurrentThe bias current output of the MAX3795 is controlled bya resistor (RBIASSET) placed between the BIASSET pinand ground. In open-loop operation, BIASSET controlsthe bias current level of the VCSEL. In closed-loopoperation (APC); the RBIASSET controls the maximumallowed bias current. The open-loop bias current isgiven by the following:
The Bias Current vs. RBIASSET graph in the TypicalOperating Characteristics shows the current into a 50Ωload. Capacitance at the BIASSET pin should be≤20pF.
Programming Modulation-Current TempcoCompute the required modulation tempco from theslope efficiency of the laser at TA = +25°C and at ahigher temperature. Then select the value of RTC fromthe Typical Operating Characteristics. For example,suppose a laser has a slope eff iciency (SE) of0.021mW/mA at +25°C, which reduces to 0.018mW/mAat +85°C. The temperature coefficient is given by thefollowing:
From the Typical Operating Characteristics, the valueof RTC, which offsets the tempco of the laser, is 9kΩ. Ifmodulation temperature compensation is not desired,short TC1 and TC2.
Programming the APC LoopProgram the average optical power by adjustingRPWRSET. To select the resistance, determine thedesired monitor current to be maintained over tempera-ture and lifetime. See the Monitor Diode Current vs.RPWRSET graph in the Typical Operating Characteristics,and select the value of RPWRSET that corresponds to therequired current.
Laser tempcoSE SE
SEppm C
=−
× −×
= °−
( )
( )/
85 25
25
6
85 2510
2380
IRBIAS
BIASSET=
+
×1 2200
34.
IR
RR RMOD
MODSET
OUT
OUT LOAD=
+
×
×+
1200
40
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MAX3795
IN+
IN- 1nH
1nH
0.5pF
0.5pF 15pF
VCC
50Ω
50Ω
1kΩVCC
VCC
PACKAGE
Figure 4. Simplified Input Structure
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The low frequency cutoff of a transmitter using APC isgiven by:
Input Termination Requirements The MAX3795 data inputs are SFP MSA compatible. On-chip, 100Ω differential input impedance is provided foroptimal termination (Figure 4). Because of the on-chipbiasing network, the MAX3795 inputs self-bias to theproper operating point to accommodate AC-coupling.
Applications InformationInterface Models
Figures 4 and 5 show simplified input and output circuitsfor the MAX3795 laser driver. Figure 6 shows the fault cir-cuit interface.
Layout ConsiderationsTo minimize inductance, keep the connections betweenthe MAX3795 output pins and laser diode as short aspossible. Use multilayer boards with uninterruptedground planes to minimize EMI and crosstalk.
Exposed-Pad (EP) PackageThe exposed pad on the 24-pin thin QFN provides a verylow thermal resistance path for heat removal from the IC.The pad is also electrical ground on the MAX3795 andmust be soldered to the circuit board ground for properthermal and electrical performance. Refer to MaximApplication Note HFAN-08.1: Thermal Considerations forQFN and Other Exposed-Pad Packages for additionalinformation.
Laser Safety and IEC 825The International Electrotechnical Commission (IEC)determines standards for hazardous light emissions fromfiber-optic transmitters. IEC 825 defines the maximumlight output for various hazard levels. The MAX3795 pro-vides features that facilitate compliance with IEC 825. Acommon safety precaution is single-point fault tolerance,whereby one unplanned short, open, or resistive connec-tion does not cause excess light output. Using this laserdriver alone does not ensure that a transmitter design iscompliant with IEC 825. The entire transmitter circuit andcomponent selections must be considered. Customersmust determine the level of fault tolerance required bytheir applications, recognizing that Maxim products arenot designed or authorized for use as components insystems intended for surgical implant into the body, forapplications intended to support or sustain life, or for anyother application where the failure of a Maxim productcould create a situation where personal injury or deathmay occur.
I
V VR
VRPD
REF MD
PWRSET PWRSET=
−≈ 0 2.
Figure 6. Fault Circuit Interface
MAX3795
VCC
FAULT
VCC
MAX3795
PACKAGE
1nH
1nH
0.5pF
0.5pF
OUT-
OUT+
50W 50W
Figure 5. Simplified Output Structure
f
II CdB
PD
LASER APC3
12 50
≈ ×× × ×
∆∆ π
PART PACKAGE TYPE PACKAGE CODE
MAX3795ETG24 Thin QFN
(4mm x 4mm x 0.8mm)T2444-1
MAX3795ETG+24 Thin QFN
(4mm x 4mm x 0.8mm)T2444-1
Chip InformationTRANSISTOR COUNT: 3806
PROCESS: SiGe BIPOLAR
Package InformationFor the latest package outline information, go towww.maxim-ic.com/packages.
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Functional Diagram
SAFETYCIRCUITRYTX_DISABLE
FAULT
VCC
IN+
IN-
OUT-OUT+
MODULATION CURRENTGENERATOR
BIASGENERATORWITH APC
ENABLE
ENABLE
100Ω
LASERMODULATOR
PEAKINGCONTROL
COMP MD REF PWRMON
BIAS
BIASSET
BIASMON
PEAKSETTC1 TC2 MODSET
SQUELCH
SIGNALDETECT
MAX3795
ROUTROUT
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Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses areimplied. 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
© 2004 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
Typical Application Circuit
VCC
REF
COMP
MD
BIAS
OUT+
OUT-
MODSET
BIASSET
RBIASSET
PWRMON
TC1
TC2
IN+
IN-
TX_DISABLE
FAULT
SQUELCH
GND
0.047µF
+3.3V
0.1µF
0.1µF
RTC†
†OPTIONAL COMPONENT*FERRITE BEAD
PEAKSET
RPEAKSET†
0.01µF
0.01µF50Ω
L1*
CF†
RF†
0.01µF
0.01µF56Ω
L1*
L2*
CF†
RF†
BIASMON
RBIASMON
4.7kن
RPWRSET
RMODSET
MAX3795
SINGLE-ENDED DRIVE DIFFERENTIAL DRIVE