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**Estimating T**_{J}:

Calculation Example Using θ_{JA}

2022.08.24 Thermal Design

Calculation Example Using θ

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}.

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.

・Input power supply voltage: up to 42V |
HTSOP-J8 package: |

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.

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}.

・To estimate T_{J} using the thermal resistance θ_{JA}, the values of the power consumption P and of T_{A} are necessary.

・T_{J} is found from calculation equations, and is confirmed to be within T_{JMAX}.