VSC7924KFL 查看數據表（PDF） - Vitesse Semiconductor

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VSC7924KFL

SONET/SDH 2.5Gb/s Laser Diode Driver

Vitesse Semiconductor 

VITESSE

SEMICONDUCTOR CORPORATION

SONET/SDH 2.5Gb/s Laser Diode Driver

Preliminary Data Sheet

VSC7924

Calculation of the Maximum Case Temperature

The VSC7924 is designed to operate with a maximum junction temperature of 125°C. The rise from the

case to junction is determined by the power dissipation of the device. The power dissipation is determined by

the VSS current plus the operating IMOD and IBIAS currents.

The power of the chip is determined by the following formula:

PD = (-VSS * ISS) + ((VIOUT – VSS) * IMOD) + ((VIBIAS – VSS) * IBIAS)

For example with:

VSS

IMOD

IBIAS

VIBIAS

VIOUT

= -5.2V

= 40mA

= 20mA

= -2.0V

= -2.0V

PD = (-5.2 * 220mA) + ((5.2 - 2.0) * 40mA) + ((5.2-- 2.0) * 20mA)

PD = 1144mW + 128mW + 64mW = 1.336W

The thermal rise from junction to case is θJC * PD. For the ceramic package, θJC = 25°C/W. Thus the ther-

mal rise is:

25°C/W * 1.336W = 33.4°C

The maximum case temperature is:

125°C – 33.4°C = 91.6°C

The absolute maximum power dissipation of the device is at:

VSS

= -5.5V

IMOD = 60mA

IBIAS = 50mA

VIBIAS = 0V

VIOUT = 0V

PD = (5.5 * 220mA) + (5.5 * 60mA) + (5.5mA * 50mA) PD = 1.815W

This will net a maximum junction to case thermal rise of: 1.815W * 25°C/W = 45.4°C

This situation will allow maximum case temperature of: 125°C – 45.4°C = 79.6°C

Page 4

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

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

Internet: www.vitesse.com

G52156-0, Rev 3.0

05/01/01

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