Average power is regulated using APC, which keeps

constant current from a photodiode coupled to the

laser. Peak-to-peak power is maintained by compen-

sating the modulation current for reduced slope effi-

ciency (h) of laser over time and temperature:

Modulation compensation from bias increases the mod-

ulation current by a user-selected proportion (K) need-

ed to maintain peak-to-peak laser power as bias

current increases with temperature. Refer to Maxim

Application Note HFAN-02.21 for details:

This provides a first-order approximation of the current

increase needed to maintain peak-to-peak power.

Slope efficiency decreases more rapidly as tempera-

ture increases. The MAX3646 provides additional tem-

perature compensation as temperature increases past

155Mbps to 622Mbps SFF/SFP

Laser Driver with Extinction Ratio Control

8

_______________________________________________________________________________________

MAX3646

IN+

IN-

OUT-

OUT+

INPUT BUFFER

DATA

PATH

MODSET

APCSET

APCFILT1

APCFILT2

BIAS

MD

ENABLE

x1

TX_FAULT

TX_DISABLE

SAFETY LOGIC

AND

POWER DETECTOR

BC_MON

PC_MON

SHUTDOWN

SHUTDOWN

x268

T

xK

xTC

ENABLE

x1/2

MODTCOMP

TH_TEMP

MODBCOMP

82

1

Figure 4. Functional Diagram