ST1S14 datasheet(26/46 Pages) STMICROELECTRONICS

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Application information

ST1S14

26/46

DocID17977 Rev 2

7.3

Thermal considerations

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 24

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 conduction losses

related to the RDSON increase compared to an ideal case.

•

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

equation:

Equation 25

where TRISE and TFALL represent the switching times of the power element that cause the

switching losses when driving an inductive load (see Figure 15). TSW is the equivalent

switching time.

Figure 15. Switching losses

•

Quiescent current losses.

Equation 26

DSON

OUT

()

⋅

OUT

RISE

FALL

()

-----------------------------------------

OUT

SW_EQ

⋅⋅

⋅

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