VSC7923 查看數據表（PDF） - Vitesse Semiconductor
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VSC7923 Datasheet PDF : 14 Pages
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VITESSE
SEMICONDUCTOR CORPORATION
SONET/SDH 2.5Gb/s Laser Diode Driver
Preliminary Data Sheet
VSC7923
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
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 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
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
Tel: (800) VITESSE • FAX: (805) 987-5896 • Email: prodinfo@vitesse.com
Internet: www.vitesse.com
G52203-0, Rev 3.0
05/11/01
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