3.0V to 5.5V, 1.25Gbps/2.5Gbps

LAN Laser Drivers

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9

SMOOTH-

START

IN+

IN-

TC

MODSET

MON

SHDNDRV

REF

POR

PORDLY

POL

POL

FAULT

FAULT

FLTDLY

EN

EN

1.97V

LV

1.53V

+1.7V

50Ω

50Ω

CONTROLLED

REFERENCE

GENERATOR

LASER

MODULATOR

MODULATION CURRENT

GENERATOR

1.7V

REF

BIASDRV

MD

OUT-

OUT+

POR CIRCUIT

SAFETY

CIRCUITRY

INPUT BUFFER

MAX3286

MAX3296

BIAS GENERATOR

Figure 2b. Laser Driver Functional Diagram

LASER

MODULATOR

POR CIRCUIT

SAFETY

BIAS GENERATOR

MODSET

TC

BIASDRV

MD

SHDNDRV

FAULT

FAULT

POR

LV

PORDLY

EN

EN

FLTDLY

POL

POL

MON

MD

OUT-

OUT+

REF

IN+

IN-

Figure 2a. Simplified Laser Driver Functional Diagram

the anode or cathode of a monitor photodiode or to a

resistor-divider, depending on the specific application

circuit. Three application circuits are supported:

common-cathode laser with photodiode, common-

cathode laser without photodiode, and common-anode

laser with photodiode (as shown in the Design Procedure

section). The POL and POL inputs determine the laser

pinning (common cathode, common anode) (Table 4).

The smooth-start circuitry prevents current spikes to the

laser during power-up or enable; this ensures compliance

with safety requirements and extends the life of the laser.

The power-control amplifier drives an external transistor

to control the laser bias current. In a fault condition, the

power-control amplifier’s output is disabled (high