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
MOD
and I
BIAS
currents.
The power of the chip is determined by the following formula:
P
D
=(-VSS * 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
P
D
= 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
θ
JC
* P
D
. For the metal glass package,
θ
JC
= 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:
V
SS
I
MOD
I
BIAS
V
IBIAS
V
IOUT
P
D
P
D
=
=
=
=
=
-5.5V
60mA
50mA
0V
0V
= (5.5 * 220mA) + (5.5 * 60mA) + (5.5mA * 50mA)
=
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
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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
VITESSE [ VITESSE SEMICONDUCTOR CORPORATION ]
