MAX3766EEP View Datasheet(PDF) - Maxim Integrated

Part Name

Description

MFG CO.

MAX3766EEP

622Mbps LAN/WAN laser driver with automatic power control and safety shutdown.

Maxim Integrated 

622Mbps LAN/WAN Laser Driver with

Automatic Power Control and Safety Shutdown

Set Modulation-Current Tempco

Compute the required modulation tempco from the

slope efficiency of the laser at TA = +25°C and at a hot

temperature. Then select the value of RTC from the

Typical Operating Characteristics.

For example, suppose a laser has a slope efficiency

(SE) of 0.021mW/mA at +25°C, which reduces to

0.018mW/mA at +85°C. The temperature coefficient is

given by the following:

( ) Laser tempco = SE85 − SE25 • 106

( ) SE25 • 85 − 25

= −2380ppm / °C

From the Typical Operating Characteristics, the value

for RTC, which offsets the tempco of the laser, is 3kΩ. If

modulation temperature compensation is not desired,

connect TC directly to REF1.

Set Modulation Current

The modulation-current amplitude can be programmed

with a fixed resistor or adjusted with a potentiometer. A

small internal resistance is provided to prevent damage

if the potentiometer is adjusted to the end of its range.

The value of RMOD can be selected from the Typical

Operating Characteristics.

Example: A transmitter requires average power of

-8dBm (160µW), with an extinction ratio of 15. The opti-

cal signal output is 280µW (see Optical Power

Relations). If the slope efficiency is 0.021mW/mA at

+25°C, then the required modulation current is

0.280mW / 0.021mW/mA = 13.3mA. From the Typical

Operating Characteristics, the value of RMOD is select-

ed to be 3kΩ.

Set Average Laser Power

and Maximum Bias Current

When APC is used, the average power control is pro-

grammed by RPOWERSET, which is typically a poten-

tiometer. The value of RPOWERSET can be estimated

from the Typical Operating Characteristics.

Example: Suppose a transmitter’s output power will be

adjusted to -8dBm (160µW) average power during

manufacturing. The coupling efficiency from laser to

monitor photodiode varies from 0.4A/W to 0.8A/W for

the selected laser, causing monitor current to vary

between 64µA and 128µA. From the Typical Operating

Characteristics, RPOWERSET should be adjustable

between 12kΩ and 24kΩ.

Select RBIASMAX to provide sufficient current for a hot

laser at its end of life. For example, if the expected

laser threshold at +85°C and end of life is 40mA, then

from the Typical Operating Characteristics, RBIASMAX

should be 1kΩ or less.

If APC is not used, the laser bias current is pro-

grammed by RBIASMAX. Select RBIASMAX from the

Typical Operating Characteristics.

Set APC Time Constant

Capacitor CMD determines the APC time constant, and

must be large enough not to cause data-dependent jit-

ter. For 622Mbps SONET/ATM applications, Maxim rec-

ommends selecting CMD ≥ 0.1µF.

Select CSAFETY

When using the latched shutdown configuration, deter-

mine the minimum value of CSAFETY from the Typical

Operating Characteristics. Calculate CSAFETY as follows:

CSAFETY =

CMD

20kΩ • IMD

For example: If CMD is 0.1µF and typical monitor cur-

rent (IMD) is 100µA, then the value of CSAFETY should

be 50nF or larger. This ensures that tSAFETY is at least

10 times the tAPC.

Design Bias Filter

To reduce data-dependent jitter, add a filter at BIAS

(see Typical Operating Circuit). Maxim recommends a

1µH inductor or ferrite bead with a self-resonance fre-

quency of 200MHz or more.

Design Laser-Compensation

Filter Network

Laser package lead inductance causes the laser

impedance to increase at high frequencies, which

leads to ringing, overshoot, and degradation of the out-

put eye. A laser-compensation filter network can be

used to reduce the output load seen by the MAX3766

at high frequencies, thereby reducing output ringing

and overshoot.

The compensation components (RCOMP and CCOMP)

are most easily determined by experimentation. Begin

with a no-compensation network, and observe the ring

frequency (fn) of the laser and laser driver (Figure 7).

Begin with RCOMP = 25Ω and CCOMP = 1/(2πfn RCOMP).

Increase CCOMP until the desired transmitter eye is

obtained.

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