VSC7938 Ver la hoja de datos (PDF) - Vitesse Semiconductor

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componentes Descripción

Fabricante

VSC7938

SONET/SDH 3.125Gb/s Laser Diode Driver with Automatic Power Control

Vitesse Semiconductor 

VITESSE

SEMICONDUCTOR CORPORATION

Preliminary Data Sheet

VSC7938

SONET/SDH 3.125Gb/s

Laser Diode Driver with Automatic Power Control

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

Symbol

λ

PAVE

Ith

ρMON

η

TC

Parameter

Wavelength

Average Optical Output Power

Threshold Current

Laser to Monitor Transfer

Laser Slope Efficiency

Operating Temperature Range

Value

1310

6

6

0.04

0.4

-40 to +85

Units

nm

mW

mA

mA/mW

mW/mA

°C

Select Resistor for APCSET

The monitor diode current is estimated by IMD = PAVE * ρMON = 6mW * 0.04mA/mW = 0.24mA. The IMD

vs. RAPCSET in Typical Operating Characteristics shows the resistor at APCSET should be 5kΩ.

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:

IMOD = Pp-p / η= 2 * PAVE * (re-1) / (re+1) / η = 2 * 6mA * (20-1) / (20 + 1) / 0.4 = 27.1mA

The graph of IMODSET vs. RMODSET in Typical Operating Characteristics shows the resistor for MODSET

should be 8.5kΩ.

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

Ith versus TC reveals a maximum threshold current of 30mA at 85°C. Therefore, the maximum bias can be

approximated by:

IBIASMAX = ITH-MAX + IMOD / 2 = 30mA + 27.1mA / 2 = 43.6mA

The graph of IBIASMAX vs. RBIASMAX in Typical Operating Characteristics shows the resistor for BIASMAX

should be 5kΩ.

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.

G52349-0, Rev 3.0

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

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