L7981 datasheet(28/44 Pages) STMICROELECTRONICS

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Application informations

L7981

28/44

Doc ID 15182 Rev 3

6.5

Thermal considerations

The thermal design is important to prevent the thermal shutdown of device if junction

temperature goes above 150°C. The three different sources of losses within the device are:

conduction losses due to the not negligible RDSon of the power switch; these are

equal to:

Equation 29

Where D is the duty cycle of the application and the maximum RDSon over temperature is

220 m

Ω. Note that the duty cycle is theoretically given by the ratio between V

OUT and VIN,

but actually it is quite higher to compensate the losses of the regulator. So the conduction

losses increases compared with the ideal case.

switching losses due to power MOSFET turn ON and OFF; these can be

calculated as:

Equation 30

Where TRISE and TFALL are the overlap times of the voltage across the power switch (VDS)

and the current flowing into it during turn ON and turn OFF phases, as shown in Figure 16.

TSW is the equivalent switching time. For this device the typical value for the equivalent

switching time is 30 ns.

Quiescent current losses, calculated as:

Equation 31

where IQ is the quiescent current (IQ=2.4 mA).

The junction temperature TJ can be calculated as:

Equation 32

Where TA is the ambient temperature and PTOT is the sum of the power losses just seen.

RthJA is the equivalent thermal resistance junction to ambient of the device; it can be

calculated as the parallel of many paths of heat conduction from the junction to the ambient.

For this device the path through the exposed pad is the one conducting the largest amount

DSON

OUT

()

⋅⋅

OUT

RISE

FALL

()

------------------------------------------- Fsw

⋅⋅

⋅

OUT

⋅⋅

⋅

Rth

TOT

⋅

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