Estimating TJ: Calculation Example Using θJA

In the previous article, the basic calculation equations for estimation of T_J using θ_JA and Ψ_JT were presented. In this article, an example problem is used in calculations to estimate T_J using θ_JA.

An Example of Calculations to Estimate T_J Using θ_JA

As an example, the BD450M2EFJ-C LDO linear regulator in the BD4xxM2-C series is employed. Below are a summary of the device specifications and a block diagram.

Calculation Example

The following condition settings are used to calculate the power consumption P.

※In the case of this IC, V_CC is used for V_IN.

The values are substituted into the equation for the power consumption P.

Here T_J is calculated from the ambient temperature T_A. θ_JA is determined from the graph, assuming that this IC is mounted on the PCB indicated below.

From the graph, θ_JA is taken to be 48℃/W, a value for T_A of 85℃ is assumed, and T_J is calculated for these conditions.

As indicated in the specifications shown above, T_JMAX for this IC is 150℃, so we can judge that these usage conditions are within the allowed range.

If the estimated value were higher than T_JMAX, it would be necessary to modify the conditions. Modifications that could be made include reducing the power consumption P, lowering the ambient temperature T_A, lowering the thermal resistance θ_JA, and the like. However, electrical specifications such as the input and output voltages and the output current are required conditions, and so changing them is generally difficult. It may in some cases be possible to lower T_A by more intensive cooling or the like, but when T_A is set as an operating specification for the equipment, changes are not easy. In order to lower θ_JA, in some cases it may be possible to expand the copper foil area of the PCB. When multiple types of IC packages are available, choosing a package with a lower θ_JA is another option. In any case, modifications to the PCB layout will also be required, so it is essential that T_J be carefully estimated in the design stage.

Estimation of T_J through Actual Measurements

The above calculations are premised on the design conditions of the power supply; if the IC is already mounted on a PCB, T_J can also be estimated under close to actual conditions by measuring the power consumption P. As shown below, since I_IN is equal to I_CC+I_OUT, V_IN (V_CC) ×I_IN is the total input power to the IC, and this amount minus the output power consumption V_OUT×I_OUT is the power consumption P in the IC.

The above is an example of calculations to estimate T_J using θ_JA. The method for calculating power consumption essentially appears on the IC data sheet, which should always be consulted.

In the next article, we present an example of calculation of T_J using Ψ_JT.