The JEDEC standard now 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 strictly speaking thermal resistances, but rather offer practical and relative means of estimating junction temperatures. These psi metrics are determined to be significantly independent of the copper-spreading area. The key thermal metrics (Ψ_JT and Ψ_JB) are used in accordance with Equation 14.

Equation 14.

where:

• P_D is the power dissipated as explained in Equation 11
• T_T is the temperature at the center-top of the device package, and
• T_B is the PCB surface temperature measured 1 mm from the device package and centered on the package edge