L5985 datasheet(25/36 Pages) STMICROELECTRONICS

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L5985

Application informations

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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 RDSON of the power switch; these are

equal to:

Equation 27

Where D is the duty cycle of the application and the maximum RDSON is 300mΩ.

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 28

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 29

where IQ is the quiescent current. (IQ = 20uA)

The junction temperature TJ can be calculated as:

Equation 30

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

RthJA is the equivalent thermal resistance juction 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

of heat. The RthJA measured on the application demo board described in the following

paragraph is about 60°/W.

DSON

OUT

()

⋅⋅

OUT

RISE

FALL

()

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

⋅⋅

⋅

OUT

⋅⋅

⋅

Rth

TOT

⋅

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