MAX3869 Просмотр технического описания (PDF) - Maxim Integrated

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MAX3869

+3.3V, 2.5Gbps SDH/SONET Laser Driver with Current Monitors and APC

Maxim Integrated 

+3.3V, 2.5Gbps SDH/SONET Laser Driver

with Current Monitors and APC

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 MAX3869 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.

Current Monitors

The MAX3869 features bias- and modulation-current

monitor outputs. The BIASMON output sinks a current

equal to 1/37 of the laser bias current (IBIAS / 37). The

MODMON output sinks a current equal to 1/29 of the

laser modulation current (IMOD / 29). BIASMON and

MODMON should be connected through a pull-up resis-

tor to VCC. Choose a pull-up resistor value that ensures a

voltage at BIASMON greater than VCC - 1.6V and a volt-

age at MODMON greater than VCC - 1.0V.

Slow-Start

For laser safety reasons, the MAX3869 incorporates a

slow-start circuit that provides a delay of 250ns for

enabling a laser diode.

APC Failure Monitor

The MAX3869 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 MAX3869 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

mark density and duty cycle of the optical waveform are

50%.

Programming the Modulation Current

For a given laser power PAVG, slope efficiency (η), and

extinction ration (re), the modulation current can be cal-

culated using Table 1. See 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 MAX3869 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. See 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 MAX3869 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. See 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 MAX3869’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. See 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 Laser Diodes

To minimize optical output aberrations caused by signal

reflections at the electrical interface to the laser diode, a

series damping resistor (RD) is required (Figure 4).

Additionally, the MAX3869 outputs are optimized for a

25Ω load. Therefore, the series combination of RD and

RL (where RL represents the laser-diode resistance)

Table 1. Optical Power Definition

PARAMETER SYMBOL

RELATION

Average Power

PAVG PAVG = (P0 + P1) / 2

Extinction Ratio

re

re = P1 / P0

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

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

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

Laser Slope

Efficiency

η

η = Pp-p / IMOD

Modulation Current IMOD IMOD = Pp-p / η

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