R5975D datasheet(28/47 Pages) STMICROELECTRONICS

28 / 47 page

Application information

R5975D

28/47

DocID026878 Rev 1

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



Conduction losses due to the not insignificant RDS(on), which are equal to:

Equation 23

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 in the overall application. For this reason, the switching losses

related to the RDS(on) increase compared to an ideal case.



Switching losses due to turning on and off. These are derived using the following

equation:

Equation 24

where TRISE and TFALL represent the switching times of the power element that cause the

switching losses when driving an inductive load (see Figure 14). TSW is the equivalent

switching time.

Figure 14. Switching losses



Quiescent current losses.

Equation 25

where IQ is the quiescent current.

PON

RDS on



IOUT





2 D



PSW

VIN IOUT

TON TOFF



------------------------------------ FSW VIN

IOUT TSW FSW





VIN IQ



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