R5973D datasheet(25/39 Pages) STMICROELECTRONICS

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R5973D

Application information

8.3

Thermal considerations

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



Conduction losses due to the not insignificant RDSON, which are equal to:

Equation 22

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 RDSON increases compared to an ideal case.



Switching losses due to turning ON and OFF. These are derived using the following

equation:

Equation 23

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 24

Where IQ is the quiescent current.

PON

RDSON IOUT





PSW

VIN IOUT

TON TOFF



------------------------------------



FSW VIN



IOUT TSW



FSW



VIN IQ



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