VITESSE
SEMICONDUCTOR CORPORATION
Data Sheet
2.488GHz SONET/SDH
Clock Generator
VSC8121
Thermal Considerations
This package has been enhanced with a copper heat slug to provide a low thermal resistance path from the
die to the exposed surface of the heat spreader. The thermal resistance is shown in the Table 7.
Table 7: Thermal Resistance
Symbol
Description
Thermal resistance from junction-to-case.
oC/W
θJC
2.0
Thermal resistance from case-to-ambient with no airflow, including conduction through
the leads.
θCA
35.0
Thermal Resistance With Airflow
Shown in Table 8 is the thermal resistance with airflow. This thermal resistance value reflects all the thermal
paths including through the leads in an environment where the leads are exposed. The temperature difference
between the ambient airflow temperature and the case temperature should be the worst-case power of the device
multiplied by the thermal resistance.
Table 8: Thermal Resistance With Airflow
Airflow
θ
CA (oC/W)
100 lfpm
200 lfpm
400 lfpm
600 lfpm
28
25
21
18
Maximum Ambient Temperature Without Heatsink
The worst case ambient temperature without use of a heatsink is given by the equation:
T
= T
- P θ
(MAX) CA
A(MAX)
C(MAX)
where:
θ
T
T
P
= Theta case to ambient at appropriate airflow
= Ambient Air temperature
= Case temperature (85oC for VSC8121)
= Power (0.7W for VSC8121)
CA
A(MAX)
C(MAX)
(MAX)
Table 9: Maximum Ambient Air Temperature Without Heatsink
Airflow
TA(MAX) oC
None
60
65
68
70
72
100 lfpm
200 lfpm
400 lfpm
600 lfpm
Note that ambient air temperature varies throughout the system based on the positioning and magnitude of
heat sources and the direction of air flow.
© 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 12
G52163-0, Rev 4.2
04/16/01
VITESSE [ VITESSE SEMICONDUCTOR CORPORATION ]
