The JEDEC standard recommends the use of psi (Ψ) thermal metrics to estimate the junction temperatures of the LDO when in-circuit on a typical PCB board application. These metrics are not thermal resistances, but offer practical and relative means of estimating junction temperatures. These psi metrics are independent of the copper-spreading area. The key thermal metrics (Ψ_JT and Ψ_JB) are used in accordance with Equation 4 and are described in the Thermal Information table.

Equation 4.

where:

• P_D is the power dissipated as shown in Equation 2
• T_T is the temperature at the center-top of the device package
• T_B is the PCB surface temperature measured 1mm from the device package and centered on the package edge