and MODSET. See the

Design Procedure section for

more details on open-loop operation.

Optional Data Input Latch

To minimize input data pattern-dependent jitter, the dif-

ferential clock signal should be connected to the data

input latch, which is selected by an external LATCH

control. If LATCH is high, the input data is retimed by

the rising edge of CLK+. If LATCH is low, the input data

is directly connected to the output stage. When this

leave CLK- unconnected.

Enable Control

The MAX3867 incorporates a laser driver enable func-

tion. When ENABLE is low, both the bias and modulation

currents are off. The typical laser enable time is 250ns

and the typical disable time is 25ns.

Slow-Start

For laser safety reasons, the MAX3867 incorporates a

slow-start circuit which provides a programmable delay

time for enabling a laser diode. An external capacitor

grams the delay by the equation:

APC Failure Monitor

The MAX3867 provides an APC failure monitor

(TTL/CMOS) to indicate an APC loop tracking failure.

FAIL is set low when the APC loop can no longer adjust

the bias current to maintain the desired monitor current.

Short-Circuit Protection

The MAX3867 provides short-circuit protection for the

modulation, bias and monitor current sources. If either

BIASMAX, MODSET, or APCSET is shorted to ground,

the bias and modulation output will be turned off.

Design Procedure

When designing a laser transmitter, the optical output is

usually expressed in terms of average power and extinc-

tion ratio. Table 1 gives the relationships that are helpful

in converting between the optical average power and the

modulation current. These relationships are valid if the

average duty cycle of optical waveform is 50%

Programming the Modulation Current

in the

Typical Operating Characteristics and select the

rent at +25°C.

Programming the Bias Current

When using the MAX3867 in open-loop operation, the

select this resistor, determine the required bias current

the

Typical Operating Characteristics and select the

current at +25°C.

When using the MAX3867 in closed-loop operation, the

able to the laser diode over temperature and life. The

APC loop can subtract from this maximum value but

graph in the

Typical Operating Characteristics and

end-of-life bias current at +85°C.

Programming the APC Loop

When the MAX3867’s APC feature is used, program the

average optical power by adjusting the APCSET resistor.

To select this resistor, determine the desired monitor cur-

rent to be maintained over temperature and life. Refer to

responds to the required current.

Interfacing with the Laser Diode

To minimize optical output aberrations due to the laser

parasitic inductance, an RC shunt network is required

adjusted until the optical output waveform is optimized.

A bypass capacitor should also be placed as close to

the laser anode as possible, for the best performance.

Pattern-Dependent Jitter (PDJ)

When transmitting NRZ data with long strings of con-

secutive identical digits (CID), LF droop can occur and

contribute to pattern-dependent jitter. To minimize this

+3.3V, 2.5Gbps SDH/SONET Laser Driver

with Automatic Power Control

_______________________________________________________________________________________

9

PARAMETER

SYMBOL

RELATION

Average Power

Extinction Ratio

Optical Power High

Optical Power Low

Optical Amplitude

Pp-p

Laser Slope

Efficiency

η

Modulation Current

Table 1. Optical Power Definition