VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
SONET/SDH 2.5Gb/s Laser Diode Driver
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 V
SS
current plus the operating I
and I
currents.
BIAS
MOD
The power of the chip is determined by the following formula:
P =(-VSS * I ) + ((V – V ) * I
) + ((V
– V ) * I
)
BIAS
D
SS
IOUT
SS
MOD
IBIAS
SS
For example with:
V
= -5.2V
= 40mA
= 20mA
= -2.0V
= -2.0V
SS
I
I
MOD
BIAS
V
V
IBIAS
IOUT
P
= 5.2 * 220mA) + ((5.2 - 2.0) * 40mA) + ((5.2-- 2.0) * 20mA)
D
PD = 1144mW + 128mW + 64mW = 1.336W
The thermal rise from junction to case is θ * P . For the metal glass package, θ = 32 °C/W. Thus the ther-
JC
D
JC
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:
V
= -5.5V
= 60mA
= 50mA
= 0V
SS
I
I
MOD
BIAS
V
V
IBIAS
IOUT
= 0V
P
P
= (5.5 * 220mA) + (5.5 * 60mA) + (5.5mA * 50mA)
D
D
=
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
© VITESSE SEMICONDUCTOR CORPORATION • 741 Calle Plano • Camarillo, CA 93012
Tel: (800) VITESSE • FAX: (805) 987-5896 • Email: prodinfo@vitesse.com
Internet: www.vitesse.com
Page 4
G52203-0, Rev 3.0
05/11/01
VITESSE [ VITESSE SEMICONDUCTOR CORPORATION ]
