3.0V to 5.5V, 1.25Gbps/2.5Gbps

LAN Laser Drivers

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13

with photodiode. The POL and POL inputs determine

the laser pinning (common cathode, common anode)

and affect the smooth-start circuits (Table 4).

Common Cathode with Photodiode

(Optical Feedback)

In the common cathode with photodiode configuration,

a servo control loop is formed by external PNP Q1, the

control amplifier (Figure 9). The voltage at MD is stabi-

noise operation and to reject power-supply noise. The

time constant governs how quickly the laser bias current

reacts to a change in the average total laser current

to obtain a loop time constant in excess of 1µs, provid-

might be necessary to ensure the APC loop’s stability

when low bias currents are desired.

250mV at maximum bias current.

The discrete components used with the common cath-

ode with photodiode configuration are:

POL

DESCRIPTION

MAX3286/MAX3296

VCC

Common cathode with

photodiode

DEVICE

Table 4. POL Pin Setup for Each Laser Configuration Type

POL

GND

LASER PINNING

MAX3287/MAX3297

—

—

MAX3286/MAX3296

VCC

Common cathode without

photodiode

MAX3288/MAX3298

—

GND

—

MAX3286/MAX3296

GND

Common anode with

photodiode

MAX3289/MAX3299

—

VCC

—

MAX3286/MAX3296

VCC

VCC

Not allowed; fault occurs

—

MAX3286/MAX3296

GND

GND

Not allowed; fault occurs

—

1000

1

1

100

1000

10

10

3500ppm

3000ppm

2000ppm

2500ppm

1000ppm

1500ppm

500ppm

5mA

10mA

15mA

20mA

25mA

30mA

0

10

5

20

15

35

30

25

40

04

26

8

10

12

14

10Ω

LOAD

25Ω

LOAD

50Ω

LOAD