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

•

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

PSW=VINIOUTTrise+Tfall

2 FSW

(8)

during the turn-on and turn-off phases.

•

Quiescent current losses:

PQ=VIN×IIN

(9)

PTOT=PQ+PSW+PHS+PLS

(10)

The junction temperature of the device is:

TJ=TA+RtℎJ−A⋅PTOT

(11)

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

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)

TJ=TA+RtℎJA⋅PLOSS

(14)

STPD01

Selecting components for the application

DS13473 - Rev 1

page 34/43