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. Section 5.5 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 the following equations. 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. 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.

where:

• P_D is the dissipated power
• T_T is the temperature at the center-top of the device package

where:

• T_B is the PCB surface temperature measured 1 mm from the device package and centered on the package edge

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