3.2Gbps SFP VCSEL Driver with Diagnostic

Monitors

10

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PIN NAME

CIRCUIT RESPONSE TO GND SHORT

FAULT

Does not affect laser power.

Does not affect laser power.

TX_DISABLE

Modulation and bias current are disabled.

Normal condition for circuit operation.

IN+

Does not affect laser power.

Does not affect laser power.

IN-

Does not affect laser power.

Does not affect laser power.

SQUELCH

Does not affect laser power.

Does not affect laser power.

TC1

Does not affect laser power.

Does not affect laser power.

TC2

The laser modulation is increased, but average power

is not affected.

Modulation current is disabled.

MODSET

Modulation current is disabled.

The laser modulation is increased, but average power

is not affected.

PEAKSET

Does not affect laser power.

Does not affect laser power.

OUT+

Modulation current is disabled.

Modulation current is disabled.

OUT-

Does not affect laser power.

Does not affect laser power.

BIASSET

Laser bias is disabled.

Fault state* occurs.

BIAS

Fault state* occurs. Note that VCSEL emissions may

continue; care must be taken to prevent this condition.

Disables VCSEL.

BIASMON

Fault state* occurs.

Does not affect laser power.

COMP

The bias current is reduced, and the average power of

the laser output is reduced.

the bias monitor fault threshold is exceeded, a fault is

signaled.

MD

the bias-monitor fault threshold is exceeded, a fault is

signaled.

The bias current is reduced, and the average power of

the laser output is reduced.

REF

the bias-monitor fault threshold is exceeded, a fault is

signaled.

The bias current is reduced, and the average power of

the laser output is reduced.

PWRMON

Fault state* occurs.

Does not affect laser power.

Table 2. Circuit Response to Various Single-Point Faults (Closed-Loop APC Configuration)

*

A fault state asserts the FAULT pin, disables the modulator output, and disables the bias output.

Modulation Circuit

The modulation circuitry consists of an input buffer, a

current mirror, and a high-speed current switch (Figure

3). The modulator drives up to 15mA of modulation into

a 50

Ω VCSEL load.

The amplitude of the modulation current is set with

resistors at MODSET and temperature coefficient (TC1,

grams the temperature-stable portion of the modulation

programs the temperature coefficient of the modulation

the

Typical Operating Characteristics section.

Design Procedure

Select Laser

Select a communications-grade laser with a rise time of

260ps or better for 1.25Gbps, or 130ps or better for

2.5Gbps applications. Use a high-efficiency laser that

requires low modulation current and generates a low-

voltage swing. Trim the leads to reduce laser package

inductance. The typical package leads have induc-

tance of 25nH per inch (1nH/mm). This inductance

causes a large voltage swing across the laser. A com-

pensation filter network can also be used to reduce

ringing, edge speed, and voltage swing (see the

Designing the Compensation Filter Network section).