STPD01 datasheet(34/43 Pages) STMICROELECTRONICS

34 / 43 page

PLS=RDSon IOUT21−D

(7)

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 whole 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:

PSW=VINIOUTTrise+Tfall

2 FSW

(8)

where Trise and Tfall 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.

•

Quiescent current losses:

PQ=VIN×IIN

(9)

where IN is the supply current.

PTOT=PQ+PSW+PHS+PLS

(10)

The junction temperature of the device is:

TJ=TA+RtℎJ−A⋅PTOT

(11)

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

Considering that the device, mounted on the board with a good ground plane, has a thermal resistance junction to

ambient (RthJ-A) of about 31 °C/W.

It is also possible to estimate the junction temperature directly from the efficiency measures acquired on a

stationary application condition.

Considering that the power losses are given by:

PLOSS=PIN−POUT

(12)

Neglecting the AC losses of the selected inductor, the power losses are given by:

PLOSS=VIN⋅IIN−VOUT⋅IOUT−DCR1⋅IOUT

(13)

Therefore, the junction temperature TJ can be calculated as:

TJ=TA+RtℎJA⋅PLOSS

(14)

STPD01

Selecting components for the application

DS13473 - Rev 1

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