AN1518 datasheet(19/30 Pages) STMICROELECTRONICS

19 / 30 page

AN1518

Application information

19/30

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 24

Where IQ is the quiescent current.

Example:

–VIN = 5 V

–VOUT = 3.3 V

–IOUT = 2 A

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

150 °C. We can consider a value of 0.4

Ω.

TSW is approximately 70 ns.

IQ has a typical value of 2.5 mA @ VIN = 12 V.

The overall losses are:

Equation 25

The junction temperature of device will be:

Equation 26

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

ambient. Considering that the device is mounted on board with a good groundplane, that it

has a thermal resistance junction-to-ambient (RthJ-A) of about 42 °C/W, and an ambient

temperature of about 70 °C:

Equation 27

•

TOT

DSON

OUT

()

•

OUT

•

0.4

•

0.7

•

–

•

250

•

–

52.5

•

–

•

13W

≅

Rth

–

TOT

•

1.3

125

° C

≅

•

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