laser package is used to convert the optical power into

a photocurrent. The APC loop adjusts the laser bias

current so that the monitor current is matched to a ref-

the APC loop is determined by an external capacitor

ciated with the APC loop-time constant, and to guaran-

0.1µF.

When the APC loop is functioning, the maximum allow-

An APC failure flag (FAIL) is set low when the bias current

can no longer be adjusted to achieve the desired aver-

age optical power. To filter out the APC loop noise, use

an external capacitor at APCFILT with a recommended

value of 0.1µF.

APC closed-loop operation requires the user to set three

currents with external resistors connected between

ground and BIASMAX, MODSET, and APCSET. Detailed

guidelines for these resistor settings are described in

the

Design Procedure section.

Open-Loop Operation

If necessary, the MAX3867 is fully operational without

APC. In this case, the laser current is directly set by two

external resistors connected from ground to BIASMAX

+3.3V, 2.5Gbps SDH/SONET Laser Driver

with Automatic Power Control

8

_______________________________________________________________________________________

LATCH

1000pF

APCSET

CAPC

BIASMAX

MD

MODSET

FAIL

BIAS

25

Ω

MUX

DATA

CLK

0

1

ENABLE

OUT+

OUT-

DQ

172X

FAILURE

DETECTOR

40X

5X

MAX3867

Figure 4. Functional Diagram