MAX3996 Ver la hoja de datos (PDF) - Maxim Integrated

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Fabricante

MAX3996

3.0V to 5.5V, 2.5Gbps VCSEL and Laser Driver

Maxim Integrated 

3.0V to 5.5V, 2.5Gbps VCSEL

and Laser Driver

Table 1. Typical Fault Conditions

PIN

MON2

BIAS

TC, MODSET

FAULT CONDITION

VMON2 > 400mV

VBIAS < 400mV

VMODSET or VTC < 200mV

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 released and pulled to GND by RSET because there

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

(CCOMP) slows the slew rate and allows a controlled

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

mends CCOMP = 0.1µF.

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 25Ω 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

not be less than VCC - 1V.

The amplitude of the modulation current is set with resis-

tors at the MODSET and temperature coefficient (TC)

pins. The resistor at MODSET (RMOD) programs the

temperature-stable portion of the modulation current,

and the resistor at TC (RTC) programs the temperature-

increasing portion of the modulation current. Figure 6

shows modulation current as a function of temperature

for two extremes: RTC is open (the modulation current

has zero temperature coefficient), and RMOD is open

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

Intermediate temperature coefficient values of the mod-

ulation current can be obtained as described in the

Design Procedure section. Table 2 is the RTC and RMOD

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

fault can be a short to VCC or GND, or between any two

VCC

MAX3996

50Ω

50Ω

OUT+

IN+

INPUT BUFFER

CURRENT

SWITCH

OUT-

100Ω

IN-

ENABLE

Σ

CURRENT AMPLIFIER

96X

MODULATION

CURRENT

GENERATOR

1.2V REFERENCE

4000ppm/°C

1.2V REFERENCE

0ppm/°C

200mV

TC

RTC

TC FAULT

200mV

MODSET

RMOD

MODSET

FAULT

Figure 5. Modulation Circuitry

1.3

RTC ≥ 1.9kΩ

1.2

RMOD = OPEN

TEMPCO = 4000ppm/°C

1.1

1.0

0.9

RTC = OPEN

TEMPCO = 50ppm/°C

0.8

0.7

0.6

0 10 20 30 40 50 60 70 80 90 100 110

JUNCTION TEMPERATURE (°C)

Figure 6. Modulation Current vs. Temperature for Maximum

and Minimum Temperature Coefficient

8 _______________________________________________________________________________________

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