TEXAS INSTRUMENTS ONET4211LD Technical data

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ONET4211LD

SLLS688 – NOVEMBER 2005

155 Mbps to 4.25 Gbps LASER DRIVER

FEATURES APPLICATIONS

• Multirate Operation From 155 Mbps up to

4.25 Gbps

• Bias Current Programmable From 1 mA

to 100 mA

• Modulation Current Programmable From 5 mA

to 85 mA

• APC and Fault Detection

• Fault Mode Selection The ONET4211LD is a laser driver for multiple fiber

• Bias and Photodiode Current Monitors

• CML Data Inputs

• Temperature Compensation of Modulation

Current

• Single 3.3-V Supply

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

24-Lead QFN Package

• SONET/SDH Transmission Systems

• Fibre Channel Optical Modules

• Fiber Optic Data Links

• Digital Cross-Connects

• Optical Transmitters

DESCRIPTION

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.

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.

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.

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B0092-01

VCC

GND

DIN+

DIN-

MOD+

MOD-

MODSET

Current Modulator

OUTPOL

MODTC

BIAS

MONB

IBMAX

MONP

APCSET

SDOWN

DISABLE

CAPC

FLTMODE

Input Buffer Stage

Modulation Current Generator

MODCTRLMODSET

IMODEN

MODTC

IMODMON

Reference

Voltage

and Bias

Generation

VCC

GND

Bias Current Generator

BIASIBMAX

IBEN IBMON IBSET

MONB

Control

IMODEN

IMODMON

IBEN

IBMON

DISABLE

SDOWN

APCCTRL

APCMON

FLTMODE

Automatic Power Control

(APC)

IBSET

APCMON

APCCTRL

MONP

APCSET

CAPC

ONET4211LD

SLLS688 – 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 ONET4211LD 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 in the following paragraphs.

Figure 1. Simplified Block Diagram of the ONET4211LD

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IPD[A]  P

AVG

[W]  

MON

[AW]

APCSET

[] 

4.69 V

IPD[A]



4.69 V

AVG

[W]  

MON

[AW]

BIASMAX

[A] 

343 V

BIASMAX

[]

MONB

[V] 

MONB

[]  I

BIAS

[A]

ONET4211LD

SLLS688 – NOVEMBER 2005

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 consists mainly of two common-emitter output transistors and the required

driver circuitry. According to 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 either case, the maximum modulation current is 85 mA. The

modulation output is optimized for driving a 20- Ω load. For optimum power efficiency, the laser driver does not provide any on-chip back-termination.

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 the 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 ONET4211LD 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 IPDthat is proportional to the average laser power can be calculated by using the laser-to-monitor transfer ratio, ρ

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

APCSET

and generates a bias compensation current. IPDcan be programmed by selecting the external resistor

APCSET

according to:

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:

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 given as:

If the voltage at MONB is greater than the programmed threshold, a fault mode occurs. MONP is also provided as a photocurrent monitor output. The photodiode current, IPD, is mirrored and develops a

voltage across an external resistor to ground, R

and the average power, P

MON

. The voltage at MONP is given as:

MONP

AVG

. The voltage at MONB is

MONB

(1)

(2)

(3)

(4)

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MONP

[V]  R

MONP

[]  IPD[A]

MOD

[A] 

265 V

MODSET

[]





1 



24 

MODTC

[]

 630 ppm







T[oC]  T0[oC]





ONET4211LD

SLLS688 – NOVEMBER 2005

DETAILED DESCRIPTION (continued)

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 ensure 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 C

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

value for this capacitance is C The internal APC loop can be disabled by connecting a 100-k Ω resistor from APCSET to VCC and leaving PD

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

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 current I temperature T0= 60 ° C (set by the resistor R the resistor R

Note that the reference temperature, T0, and the temperature compensation set by R To reduce the variation, I potentiometer.

MODTC

. The recommended value for C

APC

≤ 50 pF.

. The modulation current can be estimated as follows:

can be calibrated over temperature and set with a microcontroller DAC or digital

MOD

MODSET

APC

) and a temperature-dependent modulation current defined by

is 200 nF. The capacitance of the monitor photodiode

BIASMAX

and I

MODSET

MODTC

at a reference

MOD0

vary from part to part.

(5)

(6)

CONTROL

The function of this block is 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 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 when V 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.

reaches ~2.8 V, the laser diode bias generator and modulation current

reaches ~2.8 V until the modulation current and bias

FAULT DETECTION

The fault detection circuitry monitors the operation of the ONET4211LD. 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 VCC.

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 .

) or by toggling

RES

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B0093-01

DISABLE

SDOWN

Inverter

MUX

IBEN

IMODEN

MONP

MONB

VCC

BIAS/68

1.25 V

2.8 V

Comparator

Flipflop

CMOS

Buffer

Comparator

APCSET

MODSET

MODTC

IBMAX

APCSET

MODSET

MODTC

RES

T Counter

RES

SHORT

START

IBMAX

FLTMODE

Short Circuit

to VCC or

GND Detect

DETAILED DESCRIPTION (continued)

ONET4211LD

SLLS688 – NOVEMBER 2005

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 photocurrent 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 VCC or

Figure 2. Functional Representation of the Fault Detection Circuitry

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

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ONET4211LD

SLLS688 – NOVEMBER 2005

DETAILED DESCRIPTION (continued)

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

Response to Short to GND Response to Short to V

APCSET SDOWN latched high, I

disabled unaffected

MOD

BIAS SDOWN latched high, I

disabled unaffected CAPC No fault No fault, I DIN+ No fault, I DIN– No fault, I

MOD MOD

DISABLE Normal circuit operation Normal circuit operation Normal circuit operation Normal circuit operation IBMAX SDOWN latched high, I

disabled and I

MOD

MOD+ SDOWN latched high, I

disabled unaffected

MOD

MOD– SDOWN latched high, I

disabled unaffected

MOD

MODSET SDOWN latched high, I

disabled unaffected

MOD

MODTC SDOWN latched high, I

disabled unaffected

MOD

MONB No fault SDOWN latched high, I

MONP No fault SDOWN latched high, I

OUTPOL No fault, polarity reverses No fault No fault, polarity reverses No fault PD No fault, I

MOD

SDOWN No fault No fault No fault No fault

FLTMODE = LOW FLTMODE = HIGH

Response to Short to GND Response to Short to V

and No fault, I

BIAS

MOD

No fault, I

disabled No fault No fault, I disabled No fault No fault, I

and SDOWN latched high, I

BIAS

and No fault SDOWN high, I

BIAS

and No fault SDOWN high, I

BIAS

and No fault, I

BIAS

and No fault SDOWN high, I

BIAS

and I

MOD

unaffected No fault, I

unaffected SDOWN high, I

MOD

goes to zero SDOWN high, I

BIAS

goes to zero No fault, I

BIAS

SDOWN high, I

disabled unaffected

disabled SDOWN high, I

MOD

disabled unaffected

goes to zero No fault, I

BIAS

No fault SDOWN high, I

BIAS

No fault SDOWN high, I

BIAS

MOD MOD MOD

MOD

and I

BIAS

MOD

No fault

No fault, I

unaffected No fault, I disabled No fault disabled No fault

MOD

SDOWN high, I

No fault

No fault, I

No fault

MOD

BIAS

and I

BIAS

unaffected No fault, I

unaffected

MOD

goes to zero

BIAS

unaffected

MOD

disabled

MOD

and I

BIAS

goes to zero

BIAS

and I

MOD

PACKAGE

For the ONET4211LD, 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 .

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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P0024-03

GND

DISABLE

VCC

APCSET

MOD-

OUTPOL

MOD+

IBMAX

VCC

CAPC

BIAS

GND

MONP

VCC

MONB

DIN+

SDOWN

DIN-

FLTMODE

VCC

MODSET

GND

MODTC

24 23 22 21 20 19

7 8 9 10 11 12

RGE PACKAGE

(TOP VIEW)

ONET4211LD

SLLS688 – NOVEMBER 2005

Figure 3. Pinout of the ONET4211LD in a 4-mm × 4-mm, 24-Lead QFN Package (Top View)

TERMINAL FUNCTIONS

TERMINAL

NAME NO.

APCSET 23 Analog-in Set photodiode reference current with resistor to GND. BIAS 13 Analog-out Laser-diode bias-current sink. Connect to laser cathode. CAPC 20 Analog APC loop capacitor DIN+ 3 CML-in Non-inverted data input. On-chip, 50- Ω terminated to VCC. DIN– 4 CML-in Inverted data input. On-chip, 50- Ω terminated to VCC. DISABLE 24 LVTTL-in Disable modulation and bias-current outputs. FLTMODE 10 CMOS-in Fault mode selection input. If a low level is applied to this pin, any fault event is latched and the

GND 1, 6, 18, EP Supply Circuit ground. The exposed die pad (EP) must be grounded. IBMAX 21 Analog-in Set maximum laser diode current with resistor to GND. MOD+ 15 Analog-out Laser modulation current output. Connect to laser cathode. Avoid usage of vias on board. MOD– 16 Analog-out Complementary laser modulation current output. Connect to VCC adjacent to anode of laser

MODSET 11 Analog-in Set temperature-independent modulation current with resistor to GND. MODTC 12 Analog-in Set modulation-current temperature compensation with resistor to GND. MONB 8 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.

PD 19 Analog-in Monitor photodiode input. Connect to photodiode anode for APC. Sinks the photodiode current

SDOWN 9 LVTTL-out Fault detection flag VCC 2, 5, 14, 17 Supply 3.3-V, ± 10% supply voltage

TYPE DESCRIPTION

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.

diode. Avoid usage of vias on board.

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

to GND.

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ONET4211LD

SLLS688 – NOVEMBER 2005

ABSOLUTE MAXIMUM RATINGS

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

IBIAS

, I

IMOD+

IMOD–

, V

DIN+

DIN–,VDISABLE

, V

MONP

FLTMODE

, V

CAPC

V V V

IBMAX

, V

APCSET MOD+ BIAS

MODTC

, V

MOD–

, V

MONB

, V

SDOWN

, V

, Voltage at CAPC, IBMAX, MODSET, APCSET, MODTC

MODSET

ESD

J,max

STG

LEAD

Supply voltage Current into BIAS –20 mA to 120 mA Current into MOD+, MOD– – 20 mA to 120 mA Current into PD –5 mA to 5 mA

, Voltage at DIN+, DIN–, DISABLE, MONB, MONP, FLTMODE, SDOWN

Voltage at MOD+, MOD– Voltage at BIAS ESD rating at all pins except MOD+, MOD– 2 kV (HBM) ESD rating at MOD+, MOD- 1 kV (HBM) Maximum junction temperature 150 ° C Storage temperature range –65 ° C to 150 ° C Characterized free-air operating temperature range –40 ° C to 85 ° C Lead temperature 1,6 mm (1/16 inch) from case for 10 seconds 260 ° C

(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.

(2)

(1)

(2)

0.6 V to VCC + 1.5 V

–0.3 V to 4 V

–0.3 V to 4V

–0.3 V to 3 V

1 V to 3.5 V

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