MAX3667E/D 查看數據表（PDF） - Maxim Integrated

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MAX3667E/D

+3.3V, 622Mbps SDH/SONET Laser Driver with Automatic Power Control

Maxim Integrated 

+3.3V, 622Mbps SDH/SONET Laser Driver

with Automatic Power Control

bias, modulation, and monitor diode current levels.

Where necessary, the reference is adjusted by a

VBE voltage to cancel thermal errors created by the

BIASSET, MODSET, and APCSET current mirrors. This

ensures that the IBIAS and IMOD currents are nearly

constant over temperature with open-loop operation.

With the APC loop closed, this reference helps maintain

a constant average MD current (and thus a constant

average laser output power) over temperature.

Bias and Modulation Current Monitors

The BIASMON and MODMON analog output monitors

provide current levels that are directly proportional to

the IBIAS and IMOD currents levels. These currents

can be used in conjunction with other external circuitry

to supervise the performance of the laser driver system

without adding parasitics or reducing system perfor-

mance. The gains associated with these pins, relative

to IBIAS and IQMOD, are approximately 1/38 (for

BIASMON) and approximately 1/33 (for MODMON).

In addition to a scaled copy of the modulation current,

the MODMON current contains a DC offset current

used internally to keep the driver transistors functioning

at high speed, even with low modulation levels. This

current is not precisely controlled and should be

ignored when using the MODMON feature.

Design Procedure

Programming the Modulation Current

In addition to being a function of RMODSET, IMOD

is also dependent on the values of the series damp-

ing resistor (RDAMP), the shunt compensation resis-

tance (RFILT), and the resistance of the laser diode

(Figure 1).

If IQMOD represents the total current flowing into the

collector of QMOD, then the modulation current into the

laser diode can be represented by the following:



IMOD

=

IQMOD



 31Ω

( ) 31Ω RFILT





RFILT + RDAMP + rLASER 

IQMOD = (AI)(IMODSET)

AI = IMODSET to IMOD Gain

Assuming RFILT = 22Ω, RDAMP = 4.7Ω, and rLASER =

4Ω, then this equation is simplified to:

IMOD = IQMOD(0.6)

For RDAMP = 4.7Ω, RFILT = 22Ω, and a laser resistance of

approximately 4Ω, see the IQMOD Current vs. RMODSET

graph in the Typical Operating Characteristics and select

the value of RMODSET that corresponds to the required

current at +25°C.

Programming the Bias Current

(Open Loop)

When operating the MAX3667 without APC, program the

bias-current output by adjusting the BIASSET resistor.

To select this resistor, determine the desired bias cur-

rent required at +25°C. Refer to the IBIAS Current vs.

RBIASSET graph in the Typical Operating Characteristics,

and select the value of RBIASSET that corresponds to the

required current.

Programming the

Automatic Power Control (APC)

When using the MAX3667’s APC feature, program the

bias-current output by adjusting the APCSET resistor.

To select this resistor, determine the desired monitor

current to be maintained over temperature. See the MD

Current vs. RAPCSET graph in the Typical Operating

Characteristics, and select the value of RAPCSET that

corresponds to the required current.

When using the APC feature, be sure to connect the

APC pin directly to BIASSET (see the Typical Operating

Circuit). In this mode, the bias-current output level is no

longer controlled by the BIASSET resistor. The APCSET

resistor is now controlling the output bias level. Under

closed-loop conditions, RBIASSET assures that the feed-

back current range is properly centered. It is recom-

mended that RBIASSET be chosen to equal RAPCSET

during closed-loop operation.

Pattern-Dependent Jitter

To reduce pattern-dependent jitter (PDJ) effects, two

external compensation capacitors are required to

ensure that the control loop responds slowly to

changes in laser efficiency. The overall time constant of

the APC loop is set by the value of these capacitors, by

the transfer ratio between the laser diode current and

the monitor diode current, and by the MAX3667’s open-

loop gain.

CCOMP must be placed between the COMP pin and

ground; CAPC must be placed between the APC pin

and ground (see the Typical Operating Circuit).

For 622Mbps SDH/SONET applications, the recom-

mended values of CCOMP and CAPC are 1µF and 1nF,

respectively.

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