VITESSE

SEMICONDUCTOR CORPORATION

Preliminary Data Sheet

VSC7940

SONET/SDH 3.125Gb/s

Laser Diode Driver with Automatic Power Control

Page 12

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

Applications Information

The following is a typical design example for the VSC7940 assuming 5V operation with APC.

Select a Laser

The Table 8 provides specifications for a typical communication-grade laser capable of operating at

2.5Gb/s.

Table 6: Typical Laser Characteristics

Select Resistor for APCSET

Select Resistor for MODSET

To ensure some minimum extinction ratio over temperature and lifetime, assume an optimal extinction ratio

of 20 (13dB) at 25

°C. The modulation current may be calculated from the following equation:

should be 8.5k

Ω.

Select Resistor for BIASMAX

The maximum threshold current at +85°C and end of life must be determined. A graph of a typical laser’s

approximated by:

should be 5k

Ω.

Symbol

Parameter

Value

Units

λ

Wavelength

1310

nm

Average Optical Output Power

6

mW

Threshold Current

6

mA

ρ

MON

Laser to Monitor Transfer

0.04

mA/mW

η

Laser Slope Efficiency

0.4

mW/mA

Operating Temperature Range

-40 to +85

°C