VITESSE

SEMICONDUCTOR CORPORATION

Preliminary Data Sheet

VSC7940

SONET/SDH 3.125Gb/s

Laser Diode Driver with Automatic Power Control

Page 8

G52357-0, Rev 3.2

05/11/01

© VITESSE SEMICONDUCTOR CORPORATION • 741 Calle Plano • Camarillo, CA 93012

Tel: (800) VITESSE • FAX: (805) 987-5896 • Email: prodinfo@vitesse.com

Internet: www.vitesse.com

Detailed Description

The VSC7940 is a high-speed laser driver with Automatic Power Control. The device is designed to operate

up to 3.125Gb/s with a 5V supply. The data and clock inputs support PECL inputs as well as other inputs that

meet the common-mode voltage and differential voltage swing specifications. The differential pair output stage

is capable of sinking up to 100mA into the laser with typical rise and fall times of 60ps. The VSC7940 is

designed to be DC-coupled. The key features of the VSC7940 are its Automatic Power Control, low power sup-

ply current, and fast rise and fall times. The VSC7938 and VSC7939 are similar Vitesse laser drivers designed

for 60mA maximum output modulation currents. These laser drivers may be powered from a 3.3V or 5V supply

and may be AC- or DC-coupled to the laser diode. The VSC7939 is available in the same 32-pin TQFP package

as the VSC7940. The VSC7938 is available in a 48-pin TQFP package.

Automatic Power Control

To ensure constant average optical power, the VSC7940 utilizes an Automatic Power Control (APC) loop.

A photodiode mounted in the laser package provides optical feedback to compensate for changes in average

laser output power due to changes that affect laser performance such as temperature and laser lifetime. The laser

bias current is adjusted by the APC loop according to the reference current set at APCSET by an external resis-

tor. An external capacitor at CAPC controls the time constant for the APC feedback loop. The recommended

value for CAPC is 0.1

µF. This value reduces pattern-dependent jitter associated with the APC feedback loop

and guarantees stability. Because the APC loop noise is internally filtered, APCFILT is not internally connected

and does not need to be connected to any external components. The device’s performance will not be affected if

a capacitor is connected to APCFILT. If the APC loop cannot adjust the bias current to track the desired monitor

current, FAIL is set low.

The device may be operated with or without APC. To utilize APC, a capacitor must be connected at CAPC

(0.1

µF) and a resistor must be connected at APCSET to set the average optical power. For open-loop operation

(no APC), a 100k

Ω resistor should be connected between APCSET and GND. CAPC has no effect on open-

loop operation. In both modes of operation, resistors to ground should be placed at BIASMAX and MODSET to

set the bias and modulation currents.

Data Retiming

The VSC7940 provides inputs for differential PECL clock signals for data retiming to minimize jitter at

GND.

Short-Circuit Protection

If BIASMAX or MODSET are shorted to ground, the output modulation and bias currents will be turned

off.

Modulation and Bias Current Monitors

The VSC7940 provides monitoring of the modulation and bias currents vias BIASMON and MODMON.

These pins sink a current proportional to the actual modulation and bias currents. MODMON sinks approxi-

mately 1/29th of the amount of modulation current and BIASMON sink approximately 1/37th of the amount of

1.6V.