The EIA/JEDEC standard recommends the use of psi (Ψ) thermal characteristics to estimate the junction temperatures of surface-mount devices on a typical PCB board application. These characteristics are not true thermal resistance values, but rather package-specific thermal characteristics that offer practical and relative means of estimating junction temperatures. These psi metrics are determined to be significantly independent of copper-spreading area. The key thermal characteristics (Ψ_JT and Ψ_JB) are used in accordance with Equation 4 or Equation 5 and are given in the Section 6.4 table.

where:

• P_D(MAX) is explained in Equation 3
• T_TOP is the temperature measured at the center-top of the device package

where:

• P_D(MAX) is explained in the Section 8.2.2.6 section
• T_BOARD is the PCB surface temperature measured 1 mm from the device package and centered on the package edge

For more information about the thermal characteristics Ψ_JT and Ψ_JB, see the Semiconductor and IC Package Thermal Metrics application note; for more information about measuring T_TOP and T_BOARD, see the Using New Thermal Metrics application note; and for more information about the EIA/JEDEC JESD51 PCB used for validating R_θJA, see the Thermal Characteristics of Linear and Logic Packages Using JEDEC PCB Designs application note. These application notes are available at www.ti.com.