AN1330 datasheet(20/31 Pages) STMICROELECTRONICS

20 / 31 page

Application information

AN1330

20/31

5.3

Thermal considerations

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

●

switching losses due to the not negligible RDSON. These are equal to:

Equation 21

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 of the overall application. For this reason, the switching 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 22

where TON and TOFF are the overlap times of the voltage across the power switch and the

current flowing into it during the turn ON and turn OFF phases. TSW is the equivalent

switching time.

●

Quiescent current losses.

Equation 23

Where IQ is the quiescent current.

●

Example:

–VIN = 5 V

–VOUT = 3.3 V

–IOUT = 1 A

RDSON has a typical value of 0.25 Ω @ 25 °C and increases up to a maximum value of 0.5 Ω

@ 150 °C. We can consider a value of 0.4

Ω.

TSW is approximately 120 ns. IQ has a typical value of 2.5 mA @ VIN = 12 V. The overall

losses are:

Equation 24

The junction temperature of the device will be:

Equation 25

where TA is the ambient temperature and RthJ-A is the thermal resistance junction-to-

ambient.

DS on

()

OUT

()

•

OUT

•

OFF

()

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

•

OUT

•

TOT

DSON

OUT

()

OUT

⋅

⋅⋅

⋅

⋅⋅

0.4 1

0.7

5 1 120 10

–

250 10

52.5 10

–

0.44W

≅

⋅⋅

⋅

⋅⋅

Rth

–

•

TOT

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