VSC7923KF datasheet(4/14 Pages) VITESSE

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VITESSE

SEMICONDUCTOR CORPORATION

Preliminary Data Sheet

VSC7923

SONET/SDH 2.5Gb/s Laser Diode Driver

Page 4

G52203-0, Rev 3.0

05/11/01

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

Internet: www.vitesse.com

Calculation of the Maximum Case Temperature

The VSC7923 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

= -5.2V

IMOD

= 40mA

IBIAS

= 20mA

VIBIAS

= -2.0V

VIOUT

= -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

JC * PD. For the metal glass package, θJC = 32 °C/W. Thus the ther-

mal rise is:

32°C/W * 1.336W = 42.7°C

The maximum case temperature is:

125°C – 42.7°C = 82.3°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 * 32°C/W = 58°C

This situation will allow maximum case temperature of: 125°C – 58°C = 67°C

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