A5974AD datasheet(30/49 Pages) STMICROELECTRONICS

30 / 49 page

Application information

A5974AD

30/49

Doc ID 018762 Rev 1

continuous output current up to 2 A to the load, however, the maximum power loss of 1.5 W

is reached with an output current of 1.8 A, so the maximum output power is derated if

TAMB=80 °C.

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

Figure 16.

Power losses estimation (VIN = 12 V, fSW = 500 kHz)

At TAMB=80°C and VIN=12 V, the device is thermally limited only when VOUT=5 V (see

Figure 16).

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

Therefore, 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 demonstration board

of the A5974AD is provided.

MAX t

()

ΔT

TH t

()

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

J MAX

AMB

–

TH t

()

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

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