L5983 datasheet(26/37 Pages) STMICROELECTRONICS

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Application information

L5983

26/37

5.5

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 RDS(on) of the power switch; these are

equal to:

Equation 26

Where D is the duty cycle of the application and the maximum RDS(on) is 220 mΩ. Note that

the duty cycle is theoretically given by the ratio between VOUT an 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 27

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 50ns.

Quiescent current losses, calculated as:

Equation 28

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

The junction temperature TJ can be calculated as:

Equation 29

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.

DS on

()

OUT

()

⋅⋅

OUT

RISE

FALL

()

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

⋅⋅

⋅

OUT

⋅⋅

⋅

Rth

TOT

⋅

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