+3.3V, 622Mbps SDH/SONET

Laser Driver with Current Monitors and APC

8

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Automatic Power Control

To maintain constant average optical power, the

MAX3669 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-

nate the pattern-dependent jitter associated with the

APC loop-time constant and to guarantee loop stability,

When the APC loop is functioning, the maximum allowable

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.

Bias and Modulation Monitors

The MAX3669 includes pins to monitor the output levels

of bias and modulation current. BIASMON and MOD-

MON sink current proportional to laser bias current and

modulation current, respectively. By monitoring the cur-

monitor the levels of bias and modulation current in the

laser (Figure 3).

Open-Loop Operation

If necessary, the MAX3669 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 100k

Ω resistor from APCSET

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

Enable Control

The MAX3669 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 MAX3669 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

Ω resistor.

Short-Circuit Protection

The MAX3669 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

diode’s resistance (see Typical Operating Circuit).

laser diode can be represented by the following:

simplified to:

Ω and a laser resistance of approximately

5

Ω, see the Modulation Current vs. Modulation Set

Resistor graph in the Typical Operating Characteristics

the required current at +25°C.

Programming the Bias Current

When using the MAX3669 in open-loop operation, the

select this resistor, determine the required bias current

at +25°C. See the Bias Current vs. Maximum Bias Set

I

= I

2

2

+ R + r

MODL

MOD

D

LASER

0

0

Ω

Ω

()

⎡

⎣

⎢

⎢

⎤

⎦

⎥

⎥

Table 1. Optical Power Definition

η

Laser Modulation

Current

η

Laser Slope

Efficiency

Optical Amplitude

Optical Power Low

Optical Power High

Extinction Ratio

Average Power

RELATION

SYMBOL

PARAMETER