VITESSE
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
SONET/SDH 2.5Gb/s Laser Diode Driver
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 V current plus the operating I
and I
currents.
BIAS
SS
MOD
The power of the chip is determined by the following formula:
= (-V * I ) + ((V – V ) * I ) + ((V
P
– V ) * I
)
BIAS
D
SS
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
-
P
1144mW + 128mW + 64mW = 1.336W
D
The thermal rise from junction to case is θ * P . For the ceramic package, θ = 25°C/W. Thus the ther-
JC
D
JC
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:
V
=
=
=
=
=
-5.5V
60mA
50mA
0V
SS
I
I
MOD
BIAS
V
V
IBIAS
IOUT
0V
P
=
(5.5 * 220mA) + (5.5 * 60mA) + (5.5mA * 50mA)
P
= 1.815W
D
D
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
© 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
G52156-0, Rev 3.0
05/01/01
VITESSE [ VITESSE SEMICONDUCTOR CORPORATION ]
