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MAX3867

+3.3V, 2.5Gbps SDH/SONET Laser Driver with Automatic Power Control

Maxim Integrated 

+3.3V, 2.5Gbps SDH/SONET Laser Driver

with Automatic Power Control

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

latch function is not used, connect CLK+ to VCC and

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

(CSLWSTRT) connected from this pad to ground pro-

grams the delay by the equation:

tENABLE ≅ 100kΩ · (CSLWSTRT + 2.5pF)

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

For a given laser power PAVE, slope efficiency η, and

extinction ration re, the modulation current can be calcu-

lated by Table 1. Refer to the IMOD vs. RMODSET graph

in the Typical Operating Characteristics and select the

value of RMODSET that corresponds to the required cur-

rent at +25°C.

Programming the Bias Current

When using the MAX3867 in open-loop operation, the

bias current is determined by the RBIASMAX resistor. To

select this resistor, determine the required bias current

at +25°C. Refer to the IBIASMAX vs. RBIASMAX graph in

the Typical Operating Characteristics and select the

value of RBIASMAX that corresponds to the required

current at +25°C.

When using the MAX3867 in closed-loop operation, the

RBIASMAX resistor sets the maximum bias current avail-

able to the laser diode over temperature and life. The

APC loop can subtract from this maximum value but

cannot add to it. Refer to the IBIASMAX vs. RBIASMAX

graph in the Typical Operating Characteristics and

select the value of RBIASMAX that corresponds to the

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

the IMD vs. RAPCSET graph in the Typical Operating

Characteristics and select the value of RAPCSET that cor-

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

(Figure 4). If RL represents the laser diode resistance,

the recommended total resistance for RD + RL is 25Ω.

Starting values for coaxial lasers are RF = 75Ω and

CF = 3.3pF. RF and CF should be experimentally

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

Table 1. Optical Power Definition

PARAMETER SYMBOL

RELATION

Average Power

PAVE PAVE = (P0 + P1) / 2

Extinction Ratio

re

re = P1 / P0

Optical Power High P1 P1 = 2PAVE · re / (re + 1)

Optical Power Low P0 P0 = 2PAVE / (re + 1)

Optical Amplitude Pp-p Pp-p = 2PAVE (re - 1) / (re + 1)

Laser Slope

Efficiency

η

η = Pp-p / IMOD

Modulation Current IMOD IMOD = Pp-p /η

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