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
SS
current plus the operating I
MOD
and I
BIAS
currents.
The power of the chip is determined by the following formula:
P
D
=
(-V
SS
*
I
SS
) + ((V
IOUT
– V
SS
) *
I
MOD
) + ((V
IBIAS
– V
SS
) *
I
BIAS
)
For example with:
V
SS
I
MOD
I
BIAS
V
IBIAS
V
IOUT
=
=
=
=
=
-5.2V
40mA
20mA
-2.0V
-2.0V
(-5.2 *
220mA)
+ ((5.2
- 2.0)
*
40mA)
+ ((5.2
-
- 2.0)
*
20mA)
1144mW + 128mW + 64mW = 1.336W
P
D
=
P
D
=
The thermal rise from junction to case is
θ
JC
*
P
D
. For the ceramic package,
θ
JC
= 25°C/W. Thus the ther-
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
SS
I
MOD
I
BIAS
V
IBIAS
V
IOUT
=
=
=
=
=
-5.5V
60mA
50mA
0V
0V
(5.5 *
220mA)
+ (5.5 *
60mA)
+ (5.5mA *
50mA)
P
D
=
1.815W
P
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
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
G52156-0, Rev 3.0
05/01/01
VITESSE [ VITESSE SEMICONDUCTOR CORPORATION ]
