A5974D datasheet(28/46 Pages) STMICROELECTRONICS

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Application information

A5974D

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DocID018774 Rev 2

Figure 16 plots the power losses for VIN = 12 V and main output rails.

Figure 16. Power losses estimation (VIN = 12V, fSW = 250 kHz)

At VIN = 12 V and VOUT = 5 V can deliver 2.5 A continuously (see Figure 17) because the

total power loss is now lower than 2 W [(

switching loss + quiescent loss) < conduction

loss].

As a consequence, the calculation of the internal power losses must be done for each

specific operating condition given by the final application.

In applications where the current to the output is pulsed, the thermal impedance should be

considered instead of the thermal resistance.

The thermal impedance of the system could be much lower than the thermal resistance,

which is a static parameter. As a consequence the maximum power losses can be higher

than 2 W if a pulsed output power is requested from the load:

Equation 30

So, depending on the pulse duration and its frequency, the maximum output current can be

delivered to the load.

The characterization of the thermal impedance is strictly dependent on the layout of the

board. In Figure 17 the measurement of the thermal impedance of the evaluation board of

the A5974D is provided.

PMAX t

T

ZTH t

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

TJ MAX TAMB

–

ZTH t

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

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