BCM4354 Data Sheet
Junction Temperature Estimation and PSIJT Versus ThetaJC
Junction Temperature Estimation and PSIJT Versus ThetaJC
The package thermal characterization parameter PSIJT (JT) yields a better estimation of actual junction
temperature (TJ) than using the junction-to-case thermal resistance parameter ThetaJC (θJC). The reason for
this is that θJC is based on the assumption that all the power is dissipated through the top surface of the package
case. In actual applications, however, some of the power is dissipated through the bottom and sides of the
package. JT takes into account the power dissipated through the top, bottom, and sides of the package. The
equation for calculating the device junction temperature is:
TJ = TT + P x JT
Where:
•
•
•
•
TJ = Junction temperature at steady-state condition (°C)
TT = Package case top center temperature at steady-state condition (°C)
P = Device power dissipation (Watts)
JT = Package thermal characteristics; no airflow (°C/W)
Environmental Characteristics
For environmental characteristics data, see Table 30: “Environmental Ratings,” on page 126.
Broadcom®
October 15, 2014 • 4354-DS109-R
BROADCOM CONFIDENTIAL
Page 185
CYPRESS [ CYPRESS ]
