MAX3667(1998) View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX3667
(Rev.:1998)

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

Maxim Integrated 

+3.3V, 622Mbps SDH/SONET Laser Driver

with Automatic Power Control

_______________Detailed Description

Low-voltage operation of laser diodes and optical

transmitters produces stringent headroom conditions

for laser drivers. Fast changes in modulation current

produce large inductive voltage spikes, creating device

saturation problems. Therefore, for +3.3V operation, the

MAX3667’s modulation current should be AC coupled

to the cathode of a laser diode. The recommended DC

blocking capacitor value is 1µF. A simplified block dia-

gram of the modulation driver is shown in Figure 1.

The IMOD pin is internally biased through a 31Ω pull-up

resistor. This design decouples the headroom associat-

ed with the modulation driver from the forward voltage

drop of the laser diode, allowing the circuit to tolerate

greater di/dt voltage transients. The design of the

MAX3667 assumes a maximum DC forward-voltage

drop of 1.6V across the laser diode. Bias current is DC

coupled to the laser diode separately at the IBIAS out-

put. In most applications, some small amount of resis-

tance should be added in series with the DC blocking

capacitor to help damp out the aberrations created by

parasitic elements.

Automatic Power Control

The automatic power control (APC) feature allows an

optical transmitter to maintain constant power, despite

changes in laser efficiency due to temperature and

aging. The APC loop requires the use of a PIN monitor

photodiode, which generates a current proportional to

the laser diode output power. A scaled version of the

current flowing into the MD pin is compared to a scaled

version of the current flowing out of the APCSET pin.

When these currents are of equal value, the inputs of

the operational transconductance amplifier (OTA) are

balanced, and COMP is forced to approximately 1V.

When the average value of the monitor diode current

exceeds the value established by the APCSET current,

the COMP voltage is forced lower. If the average value

of the monitor diode current is less than the value

established by the APCSET current, the COMP node

voltage is forced higher. The output of the OTA (the

APC pin), when connected directly to BIASSET (closed-

loop condition), is used as an error signal to adjust the

bias current flowing into BIASSET. The maximum OTA

output current is approximately ±250µA.

INPUT

(MODULATION)

VCC

IMOD

RFILT 1µF RDAMP

22Ω

4.7Ω

0.01µF

VCC

LASER

DIODE

IBIAS

31Ω

IQMOD

QMOD

MAX3667

VCC

INPUT

(BIAS)

MODMON

Figure 1. Simplified Modulation Driver Block Diagram

BIASMON

6 _______________________________________________________________________________________

Share Link: