Since the PDJ will change with changes in loop gain, it

is important to choose capacitor values that are as

large as is physically possible. Since each capacitor

The time constant associated with the DC blocking

capacitor on IMOD can also have an effect on PDJ. It is

important that this time constant produce minimum

droop for long consecutive bit streams.

Referring to Figure 3, the droop resulting from long time

periods without transitions can be represented by the

following equation:

APC operation assures that the discharge level for

τ is

be solved for

τ as follows:

transition, then the time constant associated with the

DC blocking capacitor needs to be longer than:

Operation without APC (open loop)

When operating without APC, be sure to configure the

MAX3667 as follows:

1) Disconnect APC from BIASSET.

2) Force a voltage of 1V to 2V at APC to prevent the

OTA from saturating.

3) Disconnect the monitor diode.

5) Pull down the COMP pin to ground through a 30k

Ω

resistor.

Remember that the bias-current output is programmed

by adjusting the BIASSET resistor when the APC loop is

disconnected.

Output Current Limits

The MAX3667 is equipped with output current limiting

is limited to approximately 140mA (see

Typical

to approximately 140mA.

limited. If either APCSET or MODSET is shorted to

ground, the MAX3667 output is turned off. Note that in

300mA. Care should be taken if the MAX3667 is being

used with a laser diode that is sensitive to this current

level.

Interface Suggestions

and Laser Compensation

Adding damping resistance in series with the laser

diode (typically 3

Ω to 5Ω) raises the load resistance,

reduces the load frequency dependence and improves

output aberrations. A series damping resistor of 4.7

Ω is

suggested for the MAX3667.

Series inductance at the cathode of the laser results in

put aberrations. Because of reduced headroom, the

output performance of the transmitted eye diagram can

be significantly impacted during 3.3V operation.

Assuming that laser package series inductance can not

be completely eliminated, a compensation network is

required. With a laser diode load of approximately 4

Ω

and 4nH, a series damping resistor of 4.7

Ω, and a cou-

pling capacitor of 0.1µF, a shunt R-C compensation

network of 22

Ω and 0.01µF is recommended (see

Typical Operating Circuit). These values may need to

be adjusted depending on the style of laser used. Note

that it is important to place the compensation network

as close to the load as possible.

[1 - 0.12]

-t

τ =

= 7.8t

ln

[100% - DROOP] = e

-t

τ

+3.3V, 622Mbps SDH/SONET Laser Driver

with Automatic Power Control

_______________________________________________________________________________________

9

DROOP

t

τ = ∞

Figure 3. Droop