VITESSE

SEMICONDUCTOR CORPORATION

Preliminary Data Sheet

VSC7938

SONET/SDH 3.125Gb/s

Laser Diode Driver with Automatic Power Control

G52349-0, Rev 3.0

Page 13

01/20/01

© 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 VSC7938 assuming 3.3V operation with APC.

Select a Laser

The Table 5 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.

Wire Bonding

For best performance, gold ball-bonding techniques are recommended. Wedge bonding is not recom-

mended. For best performance and to minimize inductance keep wire bond lengths short.

PCB Layout Guidelines

Use high frequency PCB layout techniques with solid ground planes to minimize crosstalk and EMI. Keep

high speed traces as short as possible for signal integrity. The output traces to the laser diode must be short to

minimize inductance. Short output traces will provide best performance.

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