Estimating TJ: Basic Calculation Equations

Up till now, to facilitate understanding of thermal resistance data, we have explained how θ_JA and Ψ_JT are used, or can be used, in calculations to estimate T_J, and have discussed the characteristics of Ψ_JT and the usefulness of θ_JA and Ψ_JT in estimating T_J. From here, we will present examples of calculations to estimate T_J using thermal resistance data.

Basic Calculation Equations Used in Estimation of T_J

We begin with the basic calculation equations used in estimation of T_J. T_J can be estimated using either of the following two equations.

① Using the ambient temperature T_A

② Using the temperature T_T at the center of the top surface of the IC package in the state of actual use

Whichever equation is used, the power consumption P is essentially the product of the power supply voltage and the power supply current for the IC, but the IC functions (whether it is a power supply IC, an op-amp, etc.), the output load current, and other factors must be considered. In all cases, however, the power consumption is determined from the current and voltage consumed by the IC. Shown below is an example of a linear regulator.

This is the simplest example; the power consumption of the IC itself is V_IN×I_CC, the power consumption of the load is the input/output difference (V_IN-V_OUT) ×I_OUT, and the sum of the two is the power consumption for the IC as a whole.

From the next article, examples are used in actual determination of T_J.