VSC7926CAL 查看數據表（PDF） - Vitesse Semiconductor

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VSC7926CAL

SDH/SONET 2.5Gbits/sec Laser Diode Driver

Vitesse Semiconductor 

VITESSE

SEMICONDUCTOR CORPORATION

SDH/SONET 2.5Gbits/sec

Laser Diode Driver

Advance Product Information

VSC7926

Calculation of the Maximum Case Temperature

The VSC7926 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 metal glass package, θJC = 32 °C/W. Thus the

thermal 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

=

IMOD

=

IBIAS

=

VIBIAS

=

VIOUT

=

-5.5V

60mA

50mA

0V

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

Page 4

© VITESSE SEMICONDUCTOR CORPORATION

741 Calle Plano, Camarillo, CA 93012 • 805/388-3700 • FAX: 805/987-5896

G52188-0, Rev 2.1

6/1/98

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