MAX3286CSLASHD datasheet(10/28 Pages) MAXIM

10 / 28 page

3.0V to 5.5V, 1.25Gbps/2.5Gbps

LAN Laser Drivers

______________________________________________________________________________________

impedance). This ensures that the PNP or NPN transistor

is turned off, removing the laser-bias current. (See the

Applications Information section.)

The REF pin provides a controlled reference voltage

dependent upon the voltage at MON. The voltage at REF

is VREF = 2.65 - 2.25(VCC - VMON). A resistor connected

at REF determines the laser power when APC is used

with common-cathode lasers. See the Design Procedure

section for information about setting the laser power.

Modulation Circuitry

The modulator circuitry consists of an input buffer, current

generator, and high-speed current switch (Figure 4). The

modulator drives up to 30mA of modulation current into

a 25Ω load.

Many of the modulator performance specifications

depend on the total modulator current (IOUT) (Figure 1a).

To ensure good driver performance, the voltage at

OUT+ and OUT- must not be less than VCC - 1V.

The amplitude of the modulation current is set with

resistors at the MODSET and temperature coefficient (TC)

pins. The resistor at MODSET (RMOD) programs the

temperature-stable portion of modulation current, while

the resistor at TC (RTC) programs the temperature-

increasing portion of the modulation current. Figure 5

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).

Intermediate tempco values of modulation current can

be obtained as described in the Design Procedure sec-

tion. Table 3 is the RTC and RMOD selection table.

Safety Circuitry

The laser driver can be used with two popular safety

systems. APC maintains laser safety using local feed-

back. Safety features monitor laser driver operation and

SMOOTH-

START

POLARITY_FAULT

ENABLE

WINDOW

COMPARATOR

FAULT

REF_FAULT

MONITOR_FAULT

VCC - 540mV

BIASDRV

MON

POL

REF

POWER-

CONTROL

AMPLIFIER

+1.7V

CONTROLLED REFERENCE VOLTAGE

VREF = 2.65 - 2.25 (VCC - VMON)

+1.97V

2.95V

+1.53V

GLITCH

REJECT

ENABLE

Figure 3. Bias Generator Circuitry

VCC

CURRENT

SWITCH

INPUT

BUFFER

OUT+

OUT-

IN+

IN-

50Ω

400Ω

VCC - 0.3V

CURRENT AMPLIFIER

4000ppm/°C

REFERENCE

ENABLE

0.8V

MODSET

RTC

RMOD

0.8V

TC_FAULT

MOD_FAULT

MODULATION CURRENT

GENERATOR

1.2V

REFERENCE

MAX3286

MAX3296

Figure 4. Laser Modulator Circuitry

53.6

1.69

RMOD

(kΩ)

RTC

(kΩ)

IMOD = 15mA

11.5

26.7

3500

162

RMOD

(kΩ)

RTC

(kΩ)

IMOD = 30mA

RMOD

(kΩ)

IMOD = 5mA

3.65

RTC

(kΩ)

TEMPCO

(ppm/°C)

8.06

3.16

11.3

2.49

18.7

2.0

20.0

4.12

2000

24.9

5.76

2500

16.2

9.53

3000

34.8

13.3

57.6

6.49

5.23

4.32

4.22

13.3

5.11

6.49

6.19

4.32

80.6

2.26

500

13.3

2.67

1000

40.2

3.24

1500

15.8

26.7

19.1

26.7

13.3

8.87

Table 3. RTC and RMOD Selection Table

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