ONET4201LDRGER_技术文档

ONET4201LD

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SLLS677–NOVEMBER 2005

155-Mbps to 4.25-Gbps LASER DRIVER

FEATURES

APPLICATIONS

•

SONET/SDH Transmission Systems

•

Multirate Operation From 155 Mbps up to 4.25

Gbps

Fibre Channel Optical Modules

Fiber Optic Data Links

Digital Cross-Connects

Optical Transmitters

Bias Current Programmable From 1 mA

to 100 mA

Modulation Current Programmable From 5 mA

to 85 mA

DESCRIPTION

•

APC and Fault Detection

Fault Mode Selection

The ONET4201LD is a laser driver for multiple fiber

optic applications up to 4.25 Gbps. The device

accepts CML input data and provides bias and

modulation currents for driving a laser diode. Also

provided are automatic power control (APC),

temperature compensation of modulation current,

fault detection, and current monitor features.

Bias and Photodiode Current Monitors

CML Data Inputs

Temperature Compensation of Modulation

Current

•

Single 3.3-V Supply

The device is available in a small-footprint, 4-mm ×

4-mm, 24-pin, QFN package. The circuit requires a

single 3.3-V supply.

Active Back-Termination at the Output

Surface-Mount, Small-Footprint, 4-mm ×

4-mm, 24-Lead QFN Package

This power-efficient laser driver is characterized for

operation from –40°C to 85°C.

Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas

Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.

PRODUCTION DATA information is current as of publication date.

Products conform to specifications per the terms of the Texas

Instruments standard warranty. Production processing does not

necessarily include testing of all parameters.

ONET4201LD

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SLLS677–NOVEMBER 2005

These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam

during storage or handling to prevent electrostatic damage to the MOS gates.

DETAILED DESCRIPTION

BLOCK DIAGRAM

A simplified block diagram of the ONET4201LD is shown in Figure 1.

This compact, low-power, 4.25-Gbps laser driver circuit consists of a high-speed data path and a bias and control

block.

The function of the data path is to buffer the input data and then modulate the laser diode current according to

the input data stream.

The bias and control block generates the laser diode bias current, contains automatic power control (APC) to

maintain constant optical output power, generates a modulation current that can be temperature compensated

and controls power-on during start-up and shutdown after failure detection. The circuit design is optimized for

high-speed and low-voltage operation (3.3 V).

The main circuit blocks are described in detail below.

OUTPOL

Current Modulator

MOD+

Input Buffer Stage

DIN+

DIN–

MOD–

Modulation Current Generator

MODSET

MODCTRL

OUT+ OUT–

Active Termination

MODTC

IMODEN IMODMON

Bias Current Generator

IBMAX

BIAS

MONB

IBEN IBMON

IBSET

4

VCC

Reference

Voltage

and Bias

3

Generation

Automatic Power Control

(APC)

GND

IBSET

CAPC

MONP

PD

CAPC

MONP

PD

IMODEN IMODMON IBEN IBMON

APCCTRL

Control

DISABLE

APCMON

APCSET

SDOWN

FLTMODE

SDOWN

B0092-02

Figure 1. Simplified Block Diagram of the ONET4201LD

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DETAILED DESCRIPTION (continued)

HIGH-SPEED DATA PATH

The high-speed data path consists of an input buffer stage and a current modulator.

The input buffer stage takes CML compatible differential signals. It provides on-chip 50-Ω termination to VCC.

AC-coupling may be used at the DIN+ and DIN– inputs.

The laser diode current modulator mainly consists of two common-emitter output transistors and the required

driver circuitry. Depending on the input data stream, the modulation current is sunk at the MOD+ or the MOD–

pin.

Modulation current setting is performed by means of the modulation current generator block, which is supervised

by the control circuit block.

The laser diode can be either ac- or dc-coupled. In both cases, the maximum modulation current is 85 mA. The

modulation output is optimized for driving a 20-Ω load.

For optimum performance when driving a laser diode over a 20-Ω transmission line, the ONET4201LD provides

active 20-Ω back-termination, which minimizes jitter caused by reflections.

BIAS AND CONTROL

The bias and control circuitry consists of the bandgap voltage and bias generation block, the bias current

generator, the automatic power control block and the supervising control circuitry.

BANDGAP VOLTAGE AND BIAS GENERATION

The bandgap voltage reference provides process and temperature-independent reference voltages needed to set

bias current, modulation current, and photodiode reference current. Additionally, this block provides the biasing

for all internal circuits.

AUTOMATIC POWER CONTROL

The ONET4201LD laser driver incorporates an APC loop to compensate for the changes in laser threshold

current over temperature and lifetime. The internal APC is enabled when resistors are connected to the IBMAX

and APCSET pins. A back-facet photodiode mounted in the laser package is used to detect the average laser

output power. The photodiode current I_PD, which is proportional to the average laser power, can be calculated by

using the laser-to-monitor transfer ratio, ρ_MON, and the average power, P_AVG

:

I

[A] + P [W] ò [AńW]

PD

AVG

MON

(1)

In closed-loop operation, the APC modifies the laser diode bias current by comparing I_PDwith a reference current

I_APCSETand generates a bias compensation current. I_PDcan be programmed by selecting the external resistor

R_APCSETaccording to:

4.69 V

[W] ò

R

[W] +

+

APCSET

I

[A]

P

[AńW]

MON

PD

AVG

(2)

The bias compensation current subtracts from the maximum bias current to maintain the monitor photodiode

current. The maximum bias current is programmed by the resistor connected to IBMAX:

343 V

I

[A] +

BIASMAX

R

[W]

BIASMAX

(3)

This current limit establishes the maximum bias current available in closed loop mode, as well as in transient

fault conditions such as shorts at the PD pin to ground or delayed laser power up.

An external pin MONB is provided as a bias current monitor output. A fraction of the bias current (1/68) is

mirrored and develops a voltage drop across an external resistor to ground, R_MONB. The voltage at MONB is

given as:

R

[W]

I

[A]

MONB

BIAS

V

[V] +

MONB

68

(4)

3

If the voltage at MONB is greater than the programmed threshold, a fault mode occurs.

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DETAILED DESCRIPTION (continued)

The MONP is also provided as a photocurrent monitor output. The photodiode current, I_PD, is mirrored and

develops a voltage across an external resistor to ground, R_MONP. The voltage at MONP is given as:

V

[V]

R

[W]

I

[A]

MONP

PD

(5)

If the voltage at MONP is greater than the programmed threshold, a fault mode occurs.

As with any negative-feedback system design, care must be taken to assure stability of the loop. The loop

bandwidth must not be too high in order to minimize pattern-dependent jitter. The dominant pole is determined by

the capacitor C_APC. The recommended value for C_APCis 200 nF. The capacitance of the monitor photodiode C_PD

adds another pole to the system, and thus it must be small enough to maintain stability. The recommended value

for this capacitance is C_PD≤ 50 pF.

The internal APC loop can be disabled by connecting a 100-kΩ resistor from APCSET to VCC and leaving PD

open. In open-loop operation, the laser diode current is set by I_BIASMAXand I_MODSET

.

MODULATION-CURRENT GENERATOR

The modulation-current generator defines the tail current of the modulator, which is sunk from either MOD+ or

MOD–, depending on the data pattern. The modulation current consists of a modulation current I_MOD0at a

reference temperature T₀= 60°C (set by the resistor R_MODSET) and a temperature-dependent modulation current

defined by the resistor R_MODTC. The modulation current can be estimated as follows:

265 V

24 W

o

ǒ_{T[ C] * T [ C]}

Ǔ

I

[A] +

MOD

1 )

) 630 ppm

ǒ

Ǔ

ǒ

Ǔ

0

R

[W]

R

[W]

MODSET

MODTC

(6)

Note that the reference temperature, T₀, and the temperature compensation set by R_MODTCvary from part to part.

To reduce the variation, I_MODcan be calibrated over temperature and set with a microcontroller DAC or digital

potentiometer.

CONTROL

The functions of this block are to control the start-up sequence, detect faults, detect tracking failure of the APC

loop, and provide disable control. The laser driver has a controlled start-up sequence, which helps prevent

transient glitches from being applied to the laser during power on. At start-up, the laser diode is off, SDOWN is

low, and the APC loop is open. Once V_CCreaches ~2.8 V, the laser diode bias generator and modulation current

generator circuitry are activated (if DISABLE is low). The slow-start circuitry gradually brings up the current

delivered to the laser diode. From the time that V_CCreaches ~2.8 V until the modulation current and bias current

reach 95% of their steady state value, is considered the initialization time. If DISABLE is asserted during power

on, the slow-start circuitry does not activate until DISABLE is negated.

FAULT DETECTION

The fault-detection circuitry monitors the operation of the ONET4201LD. If FLTMODE is set to a low level,

(hard-fault mode) this circuitry disables the bias and modulation circuits and latches the SDOWN output on

detection of a fault. The fault mode is reset by toggling DISABLE (for a minimum time of T_RES) or by toggling V_CC

.

Once DISABLE is toggled, SDOWN is set low and the circuit is re-initialized.

If FLTMODE is set to a high level (soft-fault mode), a fault is indicated at the SDOWN output; however, the bias

and modulation circuits are not disabled. The SDOWN output is reset once the fault causing condition

disappears. Toggling DISABLE or VCC is not required.

A functional representation of the fault detection circuitry is shown in Figure 2.

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DETAILED DESCRIPTION (continued)

T

Counter

RES

IMODEN

DISABLE

RES

START

IBEN

Inverter

Comparator

Inverter

Flipflop

VCC

+

Q

-

CMOS

Buffer

R

Q

MUX

I0

S

Q

+

2.8 V

SDOWN

-

I1

MUX

I1

I

PD

I

BIAS/68

Q

Comparator

I0

MONB

+

Q

-

Comparator

MONP

+

Q

-

+

1.25 V

-

MODTC

MODSET

APCSET

IBMAX

MODTC

MODSET

APCSET

IBMAX

SHORT

Short Circuit

to VCC or

GND Detect

FLTMODE

B0093-01

Figure 2. Functional Representation of the Fault Detection Circuitry

A fault mode is produced if the laser cathode is grounded and the photocurrent causes MONP to exceed its

programmed threshold. Another fault mode can be produced if the laser diode end-of-life condition causes

excessive bias current and photodiode current that results in monitor voltages (MONP, MONB) being greater

than their programmed threshold. Other fault modes can occur if there are any I/O pin single-point failures (short

to V_CCor GND) and the monitor voltages exceed their programmed threshold (see Table 1).

5

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DETAILED DESCRIPTION (continued)

Table 1. Response to I/O-Pin Shorts to VCC or GND

FLTMODE = LOW

FLTMODE = HIGH

Response to Short to GND Response to Short to V_CCResponse to Short to GND

Response to Short to V_CC

APCSET

BIAS

SDOWN latched high, I_BIASand No fault, I_MODunaffected

I_MODdisabled

SDOWN high, I_BIASand I_MODNo fault

unaffected

SDOWN latched high, I_MOD

disabled

No fault, I_BIASgoes to zero

SDOWN high, I_MOD

unaffected

No fault, I_MODunaffected

CAPC

DIN+

DIN–

No fault

No fault, I_BIASgoes to zero

No fault

No fault, I_MODunaffected

No fault, I_MODdisabled

Normal circuit operation

No fault, I_BIASgoes to zero

No fault

No fault, I_MODdisabled

No fault

DISABLE Normal circuit operation

Normal circuit operation

SDOWN high, I_MODunaffected

IBMAX

MOD+

MOD–

SDOWN latched high, I_BIASand SDOWN latched high, I_BIAS

I_MODdisabled and I_MODdisabled

SDOWN high, I_MOD

unaffected

SDOWN latched high, I_BIASand No fault

I_MODdisabled

SDOWN high, I_BIAS

unaffected

No fault

SDOWN latched high, I_BIASand No fault

I_MODdisabled

SDOWN high, I_BIAS

unaffected

No fault

MODSET SDOWN latched high, I_BIASand No fault, disables I_MOD

I_MODdisabled

SDOWN high, I_BIAS

unaffected

No fault, disables I_MOD

MODTC

MONB

MONP

SDOWN latched high, I_BIASand No fault

I_MODdisabled

SDOWN high, I_BIASand I_MODNo fault

unaffected

No fault

SDOWN latched high, I_BIAS

No fault

SDOWN high, I_BIASand I_MOD

and I_MODdisabled

unaffected

No fault

SDOWN latched high, I_BIAS

and I_MODdisabled

No fault

SDOWN high, I_BIASand I_MOD

unaffected

OUTPOL

PD

No fault, polarity reverses

No fault, I_MODunaffected

No fault

No fault, polarity reverses

No fault, I_MODunaffected

No fault

No fault, I_BIASgoes to zero

No fault

No fault, I_BIASgoes to zero

No fault

SDOWN

PACKAGE

For the ONET4201LD, a small-footprint, 4-mm × 4-mm, 24-lead QFN package is used, with a lead pitch of 0,5

mm. The pinout is shown in Figure 3.

In order to achieve the required low thermal resistance of about 38 K/W, which keeps the maximum junction

temperature below 115°C, a good thermal connection of the exposed die pad is mandatory.

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RGE PACKAGE

(TOP VIEW)

24 23 22 21 20 19

GND

VCC

DIN+

DIN–

VCC

GND

VCC

1

18

2

17

16

15

14

13

3

4

5

6

MOD–

MOD+

VCC

BIAS

7

8

9

10 11 12

P0024-03

Figure 3. Pinout of the ONET4201LD in a 4-mm × 4-mm, 24-Lead QFN Package (Top View)

TERMINAL FUNCTIONS

TERMINAL

I/O

DESCRIPTION

NAME

APCSET

BIAS

NO.

23

13

20

3

Analog-in

Set photodiode reference current with resistor to GND.

Analog-out Laser diode bias current sink. Connect to laser cathode.

CAPC

Analog

CML-in

APC loop capacitor

DIN+

Noninverted data input. On-chip, 50-Ω terminated to VCC.

Inverted data input. On-chip, 50-Ω terminated to VCC.

Disable modulation and bias current outputs.

DIN–

4

CML-in

DISABLE

FLTMODE

24

10

LVTTL-in

CMOS-in

Fault mode selection input. If a low level is applied to this pin, any fault event is latched and the

bias and modulation currents are disabled in a fault condition. Toggling of DISABLE or VCC

resets the fault condition. If pin is set to a high level, fault events are flagged at the SDOWN

output but not latched. The bias and modulation currents are not disabled. SDOWN is reset

once the fault condition disappears.

GND

1, 6, 18, EP

Supply

Circuit ground. The exposed die pad (EP) must be grounded.

Set maximum laser diode current with resistor to GND.

IBMAX

MOD+

MOD–

21

15

16

Analog-in

Analog-out Laser modulation current output. Connect to laser cathode. Avoid usage of vias on board.

Analog-out Complementary laser modulation current output. Connect to VCC adjacent to anode of laser

diode. Avoid usage of vias on board.

MODSET

MODTC

MONB

11

12

8

Analog-in

Set temperature-independent modulation current with resistor to GND.

Set modulation-current temperature compensation with resistor to GND.

Analog-out Bias current monitor sources 1/68 of the bias current

MONP

7

Analog-out Photodiode current monitor sources a current identical to the photodiode current

OUTPOL

22

LVTTL-in

Alters modulation current output polarity. Open or high: normal polarity, low: inverted polarity.

OUTPOL is pulled up internally. Normal polarity: when DIN+ is high, current is sunk into MOD+.

PD

19

Analog-in

Monitor photodiode input. Connect to photodiode anode for APC. Sinks the photodiode current

to GND.

SDOWN

VCC

9

LVTTL-out Fault detection flag

Supply 3.3 V ±10% supply voltage

2, 5, 14, 17

7

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ABSOLUTE MAXIMUM RATINGS

over operating free-air temperature range (unless otherwise noted)⁽¹⁾

VALUE

–0.3 to 4

–20 to 120

–5 to 5

UNIT

V

V_CC

Supply voltage⁽²⁾

I_IBIAS

Current into BIAS

mA

V

I_IMOD+, I_IMOD–

I_PD

Current into MOD+, MOD–

Current into PD

V_DIN+, V_DIN–,V_DISABLE

,

Voltage at DIN+, DIN–, DISABLE, MONB, MONP, FLTMODE, SDOWN⁽²⁾

–0.3 to 4

V_MONB, V_MONP, V_FLTMODE

V_SDOWN

,

V_CAPC, V_IBMAX, V_MODSET

V_APCSET, V_MODTC

,

Voltage at CAPC, IBMAX, MODSET, APCSET, MODTC⁽²⁾

–0.3 to 3

V

V_MOD+, V_MOD-

V_BIAS

Voltage at MOD+, MOD–⁽²⁾

Voltage at BIAS⁽²⁾

0.6 to VCC+1.5

V

1 to 3.5

ESD rating at all pins except MOD+, MOD–

ESD rating at MOD+, MOD-

2

1

ESD

kV (HBM)

T_J,max

T_stg

Maximum junction temperature

150

°C

Storage temperature range

–65 to 150

–40 to 85

260

T_A

Characterized free-air operating temperature range

Lead temperature 1,6 mm (1/16 inch) from case for 10 seconds

T_LEAD

(1) 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 under recommended operating

conditions is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.

(2) All voltage values are with respect to network ground terminal.

RECOMMENDED OPERATING CONDITIONS

over operating free-air temperature range (unless otherwise noted)

MIN

3

NOM

MAX

3.6

UNIT

V

V_CC

T_A

Supply voltage

3.3

Operating free-air temperature

–40

85

°C

8

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DC ELECTRICAL CHARACTERISTICS

over recommended operating conditions (unless otherwise noted)

PARAMETER

TEST CONDITIONS

MIN

TYP

3.3

32

MAX

UNIT

V

V_CC

I_VCC

Supply voltage

3

3.6

I_MOD= 30 mA, I_BIAS= 20 mA (excluding I_MOD, I_BIAS

)

mA

µA

Supply current

I_MOD= 60 mA, I_BIAS= 100 mA (excluding I_MOD, I_BIAS

)

55

100

25

I_BIAS

Bias current range

Bias off-current

Bias overshoot

I_BIAS-OFF

DISABLE = high or hard-fault mode; V_BIAS≤ 3.5 V

During module hot plugging. V_CCturn on time must be

10%

≤ 0.8 s

Bias current temperature

stability

APC open loop

–480

480

ppm/°C

Bias current absolute

accuracy⁽¹⁾

I_BIAS≥ 1 mA

–15%

15%

I_BIAS= 1 mA, T_A= 25°C

I_BIAS/I_MONB

±15%

68

Bias current monitor gain

mA/ mA

V

MONB and MONP threshold

range

A fault is never detected for V_MONB/P≤ 1 V and a fault

always occurs for V_MONB/P≥ 1.35 V

1

1.25

1.35

PD current monitor gain

I_PD/I_MONP

1

mA/mA

V_ID

Differential input signal

200

2.4

1600

mVp-p

SDOWN output high voltage

SDOWN output low voltage

DISABLE input impedance

DISABLE input high voltage

DISABLE input low voltage

Monitor diode voltage

I_OH= 100 µA sourcing

V

I_OL= 1 mA sinking

0.4

10

4.7

2

7.4

kΩ

V

0.8

1.6

V

V_PD

V

Monitor diode dc current range

18

1500

µA

(1) Absolute accuracy refers to part-to-part variation.

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AC ELECTRICAL CHARACTERISTICS

Typical operating condition is at V_CC= 3.3 V, I_MOD= 30 mA, I_BIAS= 20 mA and T_A= 25°C,

over recommended operating conditions (unless otherwise noted)

PARAMETER

Data Rate

Modulation current range

TEST CONDITIONS

MIN

4.25

5

TYP MAX

UNIT

Gbps

mA

I_MOD

Current into MOD+/MOD– pin;

V_MOD+, V_MOD–≥ 0.6 V

85

25

I_MOD-OFFModulation off-current

Modulation current stability

DISABLE = high or hard-fault occurred

µA

–600

600 ppm/°C

I_MOD= 10 mA

±40%

±25%

±20%

8300

630

Modulation current absolute

accuracy⁽¹⁾

I_MOD= 50 mA

I_MOD= 80 mA

R_MODTC= 3.125 kΩ

Modulation current

ppm/°C

temperature compensation⁽²⁾

R_MODTC= Open

t_r

Output rise time (20% to 80%)

Output fall time (20% to 80%)

Disable assert time (see Figure 4)

V_MOD+≥ 1 V, V_MOD–≥ 1 V, I_MOD= 30 mA

55

75

5

ps

µs

t_f

55

t_OFF

Time from rising edge of DISABLE to when output

currents fall below the maximum limits of I_MOD-OFF

and I_BIAS-OFF

0.06

t_ON

Disable negate time (see Figure 5)

Time to initialize

Time from falling edge of DISABLE to when output is

90% of nominal

200

3.3

µs

t_INIT

From power on or negation of SDOWN using

DISABLE

t_FAULT

Fault assert time

Time from fault to SDOWN rising edge

50

10

µs

Maximum spike pulse length at DISABLE being

ignored

t_RESET

DISABLE reset (see Figure 6)

Time DISABLE must be high to reset SDOWN

20

µs

Output overshoot/undershoot

Random jitter

–13.5%

13.5

%

I_MOD= 60 mA

0.6

15

0.9 ps_RMS

10 mA ≤ I_MOD≤ 60 mA, with K28.5 pattern

30

ps_p-p

at 4.25 Gbps

10 mA ≤ I_MOD≤ 60 mA, with 2²³– 1 PRBS or

equivalent pattern at 2.67 Gbps

13

32

ps_p-p

DJ

Deterministic jitter⁽³⁾

K28.5 pattern at 1.06 Gbps

2²³– 1 PRBS or equivalent pattern at 155 Mbps

5

ps_p-p

10

(1) Absolute accuracy refers to part-to-part variation.

(2) For a given external resistor connected to the MODTC pin, the modulation current temperature compensation will vary due to

part-to-part variations.

(3) Jitter measured at positive edge and negative edge crossing of eye diagram.

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V

HIGH

SDOWN

V

LOW

t

V

HIGH

DISABLE

V

LOW

I

MOD

I

MOD

I

MOD-OFF

I

BIAS

I

BIAS

I

t

BIAS-OFF

t

OFF

T0102-01

Figure 4. DISABLE Assert Time t_OFF

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V

HIGH

SDOWN

V

LOW

t

V

HIGH

DISABLE

V

LOW

I

MOD

I

MOD

I

MOD-OFF

I

BIAS

I

BIAS

I

t

BIAS-OFF

t

ON

T0103-01

Figure 5. DISABLE Negate Time t_ON

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V

HIGH

SDOWN

V

LOW

t

V

HIGH

DISABLE

V

LOW

I

MOD

I

MOD

I

MOD-OFF

I

BIAS

I

BIAS

I

t

BIAS-OFF

t

RESET

ON

T0104-01

Figure 6. SDOWN Reset Time t_RESET

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TYPICAL CHARACTERISTICS

Typical operating condition is at V_CC= 3.3 V, I_MOD= 30 mA, I_BIAS= 20 mA and T_A= 25°C (unless otherwise noted)

ELECTRICAL EYE-DIAGRAM AT 4.25 Gbps

WITH K28.5 PATTERN, I_MOD= 30 mA

ELECTRICAL EYE-DIAGRAM AT 2.125 Gbps

WITH K28.5 PATTERN, I_MOD= 30 mA

Time [50ps/Div]

Time [100ps/Div]

G001

G002

Figure 7.

Figure 8.

ELECTRICAL EYE-DIAGRAM AT 1.0625 Gbps

WITH K28.5 PATTERN, I_MOD= 30 mA

DETERMINISTIC JITTER

vs

MODULATION CURRENT

60

50

40

30

20

10

0

10

15

20

25

30

35

40

45

50

55

60

Time [200ps/Div]

Modulation Current − mA

G003

G004

Figure 9.

Figure 10.

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TYPICAL CHARACTERISTICS (continued)

Typical operating condition is at V_CC= 3.3 V, I_MOD= 30 mA, I_BIAS= 20 mA and T_A= 25°C (unless otherwise noted)

RANDOM JITTER

vs

MODULATION CURRENT

RANDOM JITTER

vs

TEMPERATURE

3.0

2.5

2.0

1.5

1.0

0.5

0.0

3.0

2.5

2.0

1.5

1.0

0.5

0.0

10

15

20

25

30

35

40

45

50

55

60

−40 −30 −20 −10

0

10 20 30 40 50 60 70 80 90

Modulation Current − mA

T

A

− Free-Air Temperature − °C

G005

G006

Figure 11.

Figure 12.

RISE TIME AND FALL TIME

vs

MODULATION CURRENT

BIAS-MONITOR CURRENT GAIN I_MONB/I_BIAS

vs

BIAS CURRENT I_BIAS

80

70

60

50

40

30

20

19

18

17

16

15

14

13

12

11

10

Rise Time

Fall Time

10

15

20

25

30

35

40

45

50

55

60

10

15

20

25

30

35

40

45

50

55

60

Modulation Current − mA

Bias Current − mA

G007

G008

Figure 13.

Figure 14.

15

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TYPICAL CHARACTERISTICS (continued)

Typical operating condition is at V_CC= 3.3 V, I_MOD= 30 mA, I_BIAS= 20 mA and T_A= 25°C (unless otherwise noted)

BIAS CURRENT I_BIASIN OPEN LOOP MODE

MODULATION CURRENT I_MOD

vs

EXTERNAL RESISTOR R_MODSET

vs

EXTERNAL RESISTOR R_BIASMAX

120

100

80

60

40

20

0

100

90

80

70

60

50

40

30

20

10

0

10

20

30

40

50

60

70

80

90 100

0

10

20

30

40

50

60

70

80

90 100

R

− External Resistor − kΩ

R

− External Resistor − kΩ

BIASMAX

MODSET

G009

G010

Figure 15.

Figure 16.

MONITOR DIODE CURRENT I_PD

vs

EXTERNAL RESISTOR R_APCSET

PHOTODIODE MONITOR GAIN I_MONP/I_PD

vs

TEMPERATURE

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

3.0

2.5

2.0

1.5

1.0

0.5

0.0

0

10

20

30

40

50

60

70

80

90 100

−40 −30 −20 −10

0

10 20 30 40 50 60 70 80 90

R

− External Resistor − kΩ

T

A

− Free-Air Temperature − °C

APCSET

G011

G012

Figure 17.

Figure 18.

16

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TYPICAL CHARACTERISTICS (continued)

Typical operating condition is at V_CC= 3.3 V, I_MOD= 30 mA, I_BIAS= 20 mA and T_A= 25°C (unless otherwise noted)

BIAS CURRENT MONITOR GAIN I_MONB/I_BIAS

SUPPLY CURRENT (excl. I_MODand I_BIAS)

vs

TEMPERATURE

20

18

16

14

12

10

80

70

60

50

40

30

20

10

−40 −30 −20 −10

0

10 20 30 40 50 60 70 80 90

−40 −30 −20 −10

0

10 20 30 40 50 60 70 80 90

T

A

− Free-Air Temperature − °C

T

A

− Free-Air Temperature − °C

G013

G014

Figure 19.

Figure 20.

DISABLE ASSERT TIME t_OFF

DISABLE NEGATE TIME t_ON

∆t = 2.21 µs

∆t = 240 µs

V

SDOWN

V

DISABLE

I

MOD+

I

BIAS

Time [100 µs/Div]

Time [500 ns/Div]

G016

G017

Figure 21.

Figure 22.

SHUTDOWN RESET TIME t_RESET

∆t = 12.8 µs

V

SDOWN

V

DISABLE

I

MOD+

I

BIAS

Time [5 µs/Div]

G018

Figure 23.

17

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APPLICATION INFORMATION

Figure 24 shows the ONET4201LD connected with a dc-coupled interface to the laser diode, alternatively the

ONET4201LD laser driver can be ac-coupled.

OUTPOL

DISABLE

VCC

GND

VCC

DIN+

DIN–

VCC

GND

Monitor

Photodiode

VCC

20 W

MOD–

MOD+

VCC

DIN+

ONET4201LD

24-Lead QFN

R_D

DIN–

Laser-

Diode

BIAS

MONP

FLTMODE

MONB

SDOWN

S0154-02

Figure 24. Basic Application Circuit With DC-Coupled Interface Between

the ONET4201LD and the Laser Diode

APC loop instability may occur with large inductive loading on the BIAS pin. To ensure loop stability in this case,

it is recommended to connect a 1-nF capacitor to ground at the BIAS pin.

18

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APPLICATION INFORMATION (continued)

SELECT A LASER

In the design example according to Figure 24, the ONET4201LD is dc coupled to a typical communication-grade

laser diode capable of operating at 4.25 Gbps with the specifications shown in Table 2.

Table 2. Laser Diode Specifications

PARAMETER

VALUE

1310

5

UNITS

nm

λ

Wavelength

P_AVGAverage optical output power

I_THThreshold current

ρ_MONLaser-to-monitor transfer

Laser slope efficiency

mW

10

mA

0.05

0.2

mA/mW

mW/mA

η

SELECT APCSET RESISTOR

When the APC loop is activated, the desired average optical output power P_AVGis defined by characteristics of

the monitor diode and by the APCSET resistor R_APCSET. The relation between the monitor photodiode current IPD

and the average optical output power P_AVGis given by Equation 7:

I

[A] + P

[W] ò

[AńW]

PD

AVG

MON

(7)

The R_APCSETresistor is calculated by Equation 8:

4.69 V 4.69 V

[W] ò

R

[W] +

+

APCSET

I

[A]

P

[AńW]

MON

PD

AVG

(8)

For the laser diode specified in Table 2 and the desired average optical output power of 5 mW, R_APCSETis

calculated as seen in Equation 9:

4.69 V

[W] ò

4.69 V

5 mW 0.05 mAńmW

R

[W] +

+

+ 18.75 kW

APCSET

P

[AńW]

MON

AVG

(9)

Note that the monitor photodiode current I_PDmust not exceed 1.5 mA corresponding to a minimum APCSET

resistor R_APCSET,MIN= 3.1 kΩ.

SELECT MODSET RESISTOR

Modulation current I_MODis dependent on the required optical output peak-to-peak power P_p-por the average

optical power P_AVG. I_MODcan be calculated using the laser slope efficiency η and the desired extinction ratio r_e:

r

e^*1

e⁾¹

2 P

[W]

P

[W]

p*p

r

AVG

h[WńA]

I

[A] +

MOD

+

h[WńA]

(10)

Using the laser diode parameters from Table 2 and assuming an extinction ratio r_e= 8 dB (≈6.3) for an average

optical power P_AVG= 5 mW the required modulation current results as:

6.3*1

2 5 mW

6.3)1

I

+

+ 36.3 mA

MOD

0.2 mWńmA

(11)

(12)

The modulation current is adjustable with a selectable temperature coefficient TC according to the relation:

o

_[A]ǒ_{1 ) TC}ǒ_{T[ C] * T [ C]}ǓǓ

0

I

[A] + I

MOD

MOD0

where T is the ambient temperature in °C and T₀is the reference temperature (T₀= 60°C).

The temperature coefficient of the modulation current TC is typically adjustable between 630 ppm/°C and 8300

ppm/°C.

For calculation of the required external resistor R_MODSETfor a given modulation current and a given temperature,

the formula can be modified as follows:

19

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SLLS677–NOVEMBER 2005

265 V

o

ǒ_{1 ) TC}ǒ_{T[ C] * T [ C]}ǓǓ

0

R

[W] +

MODSET

I

[A]

MOD

(13)

If 4000 ppm/°C is the desired temperature coefficient and the modulation current from the example above,

36.3 mA, is required at a temperature of 25°C, the MODSET resistor R_MODSETis given by Equation 14.

4000 ppm

^oC

265 V

36.3 mA

o

ǒ_{1 )}

_{25 C *}_{60 C)}Ǔ_{+ 6.3 kW}

(

R

[W] +

MODSET

(14)

Note that the modulation current I_MODmust not exceed 85 mA over the complete temperature range,

corresponding to a minimum MODSET resistor R_MODSET,MIN= 3.1 kΩ.

SELECT MODTC RESISTOR

The R_MODTCresistor is used to program a modulation temperature coefficient that can be used to compensate for

the decreased slope efficiency of the laser at a higher temperature. The temperature coefficient TC_LDof the laser

can be calculated using the slope efficiency η₁at temperature T₁and η₂at temperature T₂as shown in

Equation 15:

h [WńA] * h [WńA]

1

2

1

10⁶

_LDƪ ƫ₊

TC

^oC

o

_h1[WńA]ǒ_{T [ C] * T [ C]}Ǔ

2 1

(15)

As an example, for the laser in Table 2, the slope efficiency at temperature T₁= 25°C is η₁= 0.2 mW/mA. At

temperature T₂= 85°C the slope efficiency is η₂= 0.15 mW/mA. The corresponding temperature coefficient TC_LD

laser can be calculated:

0.15 mWńmA * 0.2 mWńmA

0.2 mWńmA (85^oC * 25^oC)

1

TC

+

10⁶+ * 4167

^oC

LD

(16)

The MODTC resistor R_MODTCcan be used to compensate the laser temperature coefficient TC_LDin order to

maintain the same optical output swing within a range of 630 ppm up to 8300 ppm. For this, R_MODTCmay be

programmed as follows:

24 W

R

+

MODTC

1

^oC

_{(TC * 630 ppm)}ƪ ƫ

o

C

(17)

To compensate for the decreased slope efficiency of the laser in Table 2, T_Cmust be 4167 ppm/°C.

This leads to the following MODTC resistor R_MODTC

:

24 W

R

+

+ 6.8 kW

MODTC

4167 ppm * 630 ppm

^oC

(18)

SELECT BIASMAX RESISTOR

The BIASMAX resistor R_BIASMAXis used to limit the bias current applied to the laser diode.

To calculate R_BIASMAX, the maximum threshold current at 85°C and end of life must be determined. The

maximum bias current for the dc-coupled interface can be approximated by Equation 19.

I

[A]

I

[A]

BIASMAX

THMAX

(19)

R_BIASMAXcan be set by Equation 20.

343 V

R

[W] +

+

BIASMAX

I

[A]

I

[A]

BIASMAX

THMAX

(20)

For the example laser diode, the maximum threshold current is 40 mA at 85°C. Therefore, R_BIASMAXcan be

approximated by Equation 21.

20

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343 V

40 mA

R

+

+ 8.6 kW

BIASMAX

(21)

SELECT V_MONBAND V_MONPRANGE

Monitoring the bias current is achieved by taking the fractional (1/68) bias current and developing a voltage

across an external resistor to ground. Equation 22 provides the value for V_MONBfor a resistor value equal to

768 Ω.

R

[W]

I

[A]

768 W

I

[A]

MONB

BIAS

68

V

[V] +

+

+ 11.29 W I

[A]

MONB

BIAS

68

(22)

Monitoring of the photodiode current is achieved by taking a mirror of I_PDand developing a voltage across an

external resistor to ground. Equation 23 provides the value for V_MONPfor a resistor equal to 200 Ω.

V

[V]

R

[W]

I

[A]

200 W

I

[A]

MONP

PD

(23)

LASER DIODE INTERFACE

The output stage of the ONET4201LD is optimized for driving a 20-Ω load. The combination of a damping

resistor, R_D, along with the resistance of the laser diode must be 20 Ω for impedance matching. The suggested

typical value for R_Dis 6 Ω to 15 Ω. A bypass capacitor of 10 nF placed close to the laser anode also helps to

optimize performance.

21

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PACKAGE OPTION ADDENDUM

www.ti.com

16-Dec-2005

PACKAGING INFORMATION

Orderable Device

ONET4201LDRGER

ONET4201LDRGERG4

ONET4201LDRGET

ONET4201LDRGETG4

Status ⁽¹⁾

ACTIVE

Package Package

Pins Package Eco Plan ⁽²⁾Lead/Ball Finish MSL Peak Temp ⁽³⁾

Qty

Type

Drawing

QFN

RGE

24

3000 Green (RoHS & CU NIPDAU Level-2-260C-1 YEAR

no Sb/Br)

QFN

RGE

3000 Green (RoHS & CU NIPDAU Level-2-260C-1 YEAR

no Sb/Br)

250 Green (RoHS & CU NIPDAU Level-2-260C-1 YEAR

no Sb/Br)

250 Green (RoHS & CU NIPDAU Level-2-260C-1 YEAR

no Sb/Br)

⁽¹⁾The marketing status values are defined as follows:

ACTIVE: Product device recommended for new designs.

LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.

NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in

a new design.

PREVIEW: Device has been announced but is not in production. Samples may or may not be available.

OBSOLETE: TI has discontinued the production of the device.

(2)

Eco Plan

-

The planned eco-friendly classification: Pb-Free (RoHS) or Green (RoHS

&

no Sb/Br)

-

please check

http://www.ti.com/productcontent for the latest availability information and additional product content details.

TBD: The Pb-Free/Green conversion plan has not been defined.

Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements

for all 6 substances, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered

at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.

Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame

retardants (Br or Sb do not exceed 0.1% by weight in homogeneous material)

(3)

MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder

temperature.

Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is

provided. TI bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the

accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and continues to take

reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on

incoming materials and chemicals. TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited

information may not be available for release.

In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI

to Customer on an annual basis.

Addendum-Page 1

IMPORTANT NOTICE

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型号：	ONET4201LDRGER
厂家：	TEXAS INSTRUMENTS
描述：	155-Mbps to 4.25-Gbps LASER DRIVER 155 - Mbps到4.25 Gbps的激光驱动器驱动器
文件：	总26页 (文件大小：1191K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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