A5974D datasheet(24/46 Pages) STMICROELECTRONICS

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Application information

A5974D

24/46

DocID018774 Rev 2

Figure 13. Layout example

8.3

Thermal considerations

8.3.1

Thermal resistance RthJA

RthJ-A is the equivalent static thermal resistance junction to ambient of the device; it can be

calculated as the parallel of many paths of heat conduction from the junction to the ambient.

For this device the path through the exposed pad is the one conducting the largest amount

of heat. The static RthJA measured on the application is about 40 °C/W.

The junction temperature of the device will be:

Equation 22

The dissipated power of the device is tied to three different sources:



Conduction losses due to the not insignificant RDSON, which are equal to:

Equation 23

Where D is the duty cycle of the application. Note that the duty cycle is theoretically given by

the ratio between VOUT and VIN, but in practice it is substantially higher than this value to

compensate for the losses in the overall application. For this reason, the switching losses

related to the RDSON increase compared to an ideal case.

TA RthJA

–

PTOT



PON

RDSON IOUT





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