1gbps to 4.25gbps multirate vcsel driver with diagnostic … · 2004-08-12 · max3795 1gbps to...

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

MA

X3

79

5

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.

MA

X3

79

5


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

%

MA

X3

79

5


_______________________________________________________________________________________ 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.)


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

MA

X3

79

5


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.)


- 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).

MA

X3

79

5


_______________________________________________________________________________________ 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


34ps/div

4.25Gbps, K28.5, -7dBm, 850nm VCSEL, ADVANCED OPTICAL COMPONENTS

HFE4191-541


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.)

MA

X3

79

5


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

MA

X3

79

5


_______________________________________________________________________________________ 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

MA

X3

79

5


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.

MA

X3

79

5


_______________________________________________________________________________________ 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

MA

X3

79

5


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


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




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)

MA

X3

79

5


______________________________________________________________________________________ 11

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

MA

X3

79

5


12 ______________________________________________________________________________________

MAX3795

IN+

IN- 1nH

1nH

0.5pF

0.5pF 15pF

VCC

50Ω

50Ω

1kΩVCC

VCC

PACKAGE

Figure 4. Simplified Input Structure

MA

X3

79

5


______________________________________________________________________________________ 13

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.

MA

X3

79

5


14 ______________________________________________________________________________________

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

MA

X3

79

5


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

4.7kΩ†

RPWRSET

RMODSET

MAX3795

SINGLE-ENDED DRIVE DIFFERENTIAL DRIVE

1gbps to 4.25gbps multirate vcsel driver with diagnostic … · 2004-08-12 · max3795 1gbps to...

Documents