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HIGH-SPEED DATA PATH

BIAS AND CONTROL

BANDGAP VOLTAGE AND BIAS GENERATION

AUTOMATIC POWER CONTROL

I

(1)

R

APCSET

[W]

+

4.69 V

I

PD

[A]

+

4.69 V

P

AVG

[W]

ò

MON

[A W]

(2)

I

BIASMAX

[A] +

343 V

R

BIASMAX

[W]

(3)

V

MONB

[V] +

R

MONB

[W]

I

BIAS

[A]

68

(4)

ONET4211LD

SLLS688 – NOVEMBER 2005

DETAILED DESCRIPTION (continued)

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.

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.

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.

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

using the laser-to-monitor transfer ratio,

ρ

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

given as:

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

3