AN2603 datasheet(12/23 Pages) STMICROELECTRONICS

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Thermal considerations

AN2603

12/23

Doc ID 13778 Rev 3

Thermal considerations

The dissipated power of the device is determined by three separate factors:

●

Switching losses due to the RDS(ON). These are equal to:

Equation 10

and

Equation 11

where D is the duty cycle of the application.

Note:

the duty cycle is theoretically given by the ratio between VOUT and VIN, but in practice it is

significantly higher than this value to compensate for the losses of the overall application.

For this reason, the switching losses related to the RDS(ON) increase compared to an ideal

case.

●

On and OFF switching losses. These are given by the following relation:

Equation 12

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 13

where IQ is the quiescent current.

Example: VIN = 5 V, VOUT = 3.3 V, IOUT = 1.5 A

The RDS(ON) has a typical value of 0.12 Ω @ 25 °C and increases up to a maximum value of

0.16

Ω @ 150 °C. Considering a value of 0.15 Ω, T

SW is approximately 20 ns and IQ has a

typical value of 1.5 mA @ VIN = 5 V. The overall losses are:

Equation 14

The junction temperature of the device will be:

Equation 15

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

resistance.

ON P

DSON P

OUT

⋅⋅

ON N

DSON N

OUT

–

()

⋅⋅

OUT

OFF

()

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

OUT

⋅⋅

⋅

⋅⋅

⋅

TOT

DSON P I

OUT DRDSON N I

OUT

–

() V

OUT

⋅

⋅⋅

⋅

⋅⋅

0.15 1.5

0.73

0.12 1.5

10.73

–

() 51.5 20 10

–

1.5 10

51.5 10

–

0.552W

≅

⋅⋅

⋅

⋅⋅

Rth

–

TOT

⋅

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