MAX3668 查看數據表（PDF） - Maxim Integrated

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MAX3668

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

Maxim Integrated 

+3.3V, 622Mbps SDH/SONET

Laser Driver with Automatic Power Control

Automatic Power Control

To maintain constant average optical power, the

MAX3668 incorporates an APC loop to compensate for

the changes in laser threshold current over temperature

and lifetime. A back-facet photodiode mounted in the

laser package is used to convert the optical power into a

photocurrent. The APC loop adjusts the laser bias cur-

rent so the monitor current is matched to a reference cur-

rent set by RAPCSET. The time constant of the APC loop

is determined by an external capacitor (CAPC). To elimi-

nate the pattern-dependent jitter associated with the

APC loop-time constant and to guarantee loop stability,

the recommended value for CAPC is 0.1µF.

When the APC loop is functioning, the maximum allowable

bias current is set by an external resistor, RBIASMAX. An

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

no longer be adjusted to achieve the desired average

optical power.

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 MAX3668 is fully operational without

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

external resistors connected from ground to BIASMAX

and MODSET. Connect a 100kΩ resistor from APCSET

to ground and leave MD open for open-loop operation.

Enable Control

The MAX3668 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.

APC Failure Monitor

The MAX3668 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. This output

is internally pulled up to VCC through a 6kΩ resistor.

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

Laser Slope Efficiency

Modulation Current

Pp-p

η

IMOD

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

η = Pp-p / IMOD

IMOD = Pp-p / η

Short-Circuit Protection

The MAX3668 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 outputs 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 the optical waveform is 50%.

Programming the Modulation Current

In addition to being a function of RMODSET, the modula-

tion current delivered to the laser (IMODL) also depends

on the values of the series damping resistor (RD), the

shunt compensation resistance (RFILT), and the laser

diode’s resistance (see Typical Operating Circuit).

The modulation current (assuming CFILT<<CD) into the

laser diode can be represented by the following:

( ) 

IMODL

=

IMOD



 20Ω

+

20Ω





RD + r LASER 

Assuming RD = 5Ω and rLASER = 5Ω, this equation is

simplified to:

IMODL = IMOD(0.67)

For RD = 5.0Ω and a laser resistance of approximately

5Ω, refer to the Modulation Current vs. Modulation Set

Resistor 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

When using the MAX3668 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 Bias Current vs. Maximum Bias

Set Resistor graph in the Typical Operating Character-

istics and select the value of RBIASMAX that corre-

sponds to the required current at +25°C.

When using the MAX3668 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 Bias Current vs. Maximum

Bias Set Resistor graph in the Typical Operating

Characteristics and select the value of RBIASMAX that

corresponds to the end-of-life bias current at +85°C.

_______________________________________________________________________________________ 7

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