8.1.7 Estimating Junction Temperature

The JEDEC standard now recommends the use of psi (Ψ) thermal metrics to estimate the junction temperatures of the linear regulator when in-circuit on a typical PCB board application. These metrics are not thermal resistance parameters and instead offer a practical and relative way to estimate junction temperature. These psi metrics are determined to be significantly independent of the copper area available for heat-spreading. The Thermal Information table lists the primary thermal metrics, which are the junction-to-top characterization parameter (ψ_JT) and junction-to-board characterization parameter (ψ_JB). These parameters provide two methods for calculating the junction temperature (T_J). As described in Equation 8, use the junction-to-top characterization parameter (ψ_JT) with the temperature at the center-top of device package (T_T) to calculate the junction temperature. As described in Equation 9, use the junction-to-board characterization parameter (ψ_JB) with the PCB surface temperature 1 mm from the device package (T_B) to calculate the junction temperature.

For detailed information on the thermal metrics and how to use them, see the Semiconductor and IC Package Thermal Metrics application report.