Smooth-Start

During startup, the laser does not emit light, and the

APC loop is not closed. The smooth-start circuit pulls

the MD pin to approximately 2.5V during the POR delay

and while TX_DISABLE is high. This causes the power-

control amplifier to shut off the bias transistor. When

POR delay is over and TX_DISABLE is low, the MD pin

is no laser power and thus no monitor diode current.

The output voltage of the power-control amplifier then

begins to increase. A capacitor attached to COMP

increase in bias current (Figure 11). Maxim recom-

Modulation Circuitry

The modulation circuitry consists of an input buffer, a

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

5). The modulator drives up to 30mA of modulation cur-

rent into a 25

Ω load.

Many of the modulator performance specifications

depend on total modulator current. To ensure good driver

performance, the voltage at either OUT+ or OUT- must

The amplitude of the modulation current is set with resis-

tors at the MODSET and temperature coefficient (TC)

temperature-stable portion of the modulation current,

increasing portion of the modulation current. Figure 6

shows modulation current as a function of temperature

(the modulation temperature coefficient is 4000ppm/°C).

Intermediate temperature coefficient values of the mod-

ulation current can be obtained as described in the

selection table.

Safety Circuitry

The safety circuitry contains a disable input, a fault

latch, and fault detectors (Figure 7). This circuitry moni-

tors the operation of the laser driver and forces a shut-

down if a single-point fault is detected. A single-point

3.0V to 5.5V, 2.5Gbps VCSEL

and Laser Driver

8

_______________________________________________________________________________________

PIN

FAULT CONDITION

MON2

BIAS

TC, MODSET

Table 1. Typical Fault Conditions

MAX3996

100

Ω

IN+

IN-

INPUT BUFFER

50

Ω

50

Ω

OUT+

OUT-

1.2V REFERENCE

0ppm/

°C

MODSET

FAULT

200mV

MODSET

1.2V REFERENCE

4000ppm/

°C

TC FAULT

200mV

TC

Σ

CURRENT AMPLIFIER

96X

ENABLE

MODULATION

CURRENT

GENERATOR

CURRENT

SWITCH

Figure 5. Modulation Circuitry

0.6

0.8

0.7

1.0

0.9

1.2

1.1

1.3

020 30 40

10

50 60 70 80 90 100 110

JUNCTION TEMPERATURE (

°C)

TEMPCO = 4000ppm/

°C

TEMPCO = 50ppm/

°C

Figure 6. Modulation Current vs. Temperature for Maximum

and Minimum Temperature Coefficient