AM8068

Motor Driver ICs

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Practically, most of the device heat goes into the PCB, there is a very low heat flow through top of the

package, So the temperature difference between Tj and Tt shall be small, that is any error caused by PCB

variation is small.

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This constant represents that

Ψjt is completely PCB independent and could be used to predict the Tj in the

environment of the actual PCB if Tt is measured properly.

Step 1 : Used the simulated

Ψjt value listed above.

Step 2 : Measure Tt value by using

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Thermocouple Method

We recommend use of a small ~40 gauge(3.15mil diameter) thermocouple. The bead and

thermocouples wires should touch the top of the package and be covered with a minimal amount of

thermally conductive epoxy. The wires should be heat-insulated to prevent cooling of the bead due to

heat loss into wires. This is important towards preventing “too cool” Tt measurements, which would

lead to the calculated Tj also being too cool.

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IR Spot Method

An IR Spot method should be utilized only when using a tool with a small enough spot area to acquire

the true top center “hot spot”.

Many so-called “small spot size” tools still have a measurement area of 0~100+mils at “zero” distance

of the tool from the surface. This spot area is too big for many smaller packages and likely would

result in cooler readings than the small

thermocouple method. Consequently, to match between

spot area and package surface size is important while measuring Tt with IR sport method.

Step 3 : calculating power dissipation by

P

Step 4 : Estimate Tj value by

Tj=

Ψjt x P+Tt

Step 5: Calculated

Θja value of actual PCB by the known Tj

Θja(actual) = (Tj-Ta)/P

AMtek semiconductors

May. 2018 V1.1

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