A5974AD datasheet(27/49 Pages) STMICROELECTRONICS

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A5974AD

Application information

Doc ID 018762 Rev 1

27/49

Figure 13.

Layout example

8.3

Thermal considerations

8.3.1

Thermal resistance RTHJ-A

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 RTHJ-A measured on the application is about 40 °C/W.

The junction temperature of device is:

Equation 22

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

●

Conduction losses due to the not insignificant RDS(on), 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 RDS(on) increases compared to an ideal case.

●

Switching losses due to turning on and off. These are derived using the following

equation:

Equation 24

thJA

TOT

⋅

DS on

()

OUT

()

⋅

OUT

OFF

()

------------------------------------ F

OUT

⋅⋅

⋅

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