A5974AD datasheet(29/49 Pages) STMICROELECTRONICS

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A5974AD

Application information

Doc ID 018762 Rev 1

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Equation 28

8.3.2

Thermal impedance ZTHJ-A(t)

The thermal impedance of the system, considered as the device in the HSO8 package

soldered on the application board, takes on an important rule when the maximum output

power is limited by the static thermal performance and not by the electrical performance of

the device. Therefore, the embedded power elements could manage a higher current but the

system is already taking away the maximum power generated by the internal losses.

In case the output power increases, the thermal shutdown is triggered because the junction

temperature triggers the designed thermal shutdown threshold.

The RTH is a static parameter of the package; it sets the maximum power loss which can be

generated from the system given the operation conditions.

If we suppose, as an example, TA = 80 °C, 140 °C is the maximum operating temperature

before triggering the thermal shutdown and RTH = 40 °C/W, therefore, the maximum power

loss achievable with the thermal performance of the system is:

Figure 15. represents the estimation of power losses for different output voltages at VIN=5 V

and TAMB=80 °C. The calculations are performed considering the RDS(on) of the power

element equal to 0.4 A

Figure 15.

Power losses estimation (VIN = 5 V, fSW = 500 kHz)

The red trace represents the maximum power which can be taken away, as calculated

above, whilst the other traces are the total internal losses for different output voltage.

The embedded conduction losses are proportional to the duty cycle required for the

conversion. Assuming the input voltage constant, the switching losses are proportional to

the output current while the quiescent losses can be considered as constant.

As a consequence, in Figure 15, the maximum power loss is for VOUT=3.3 V, where the

system can manage a continuous output current up to 1.8 A. The device could deliver a

1.38 42

128

°C

≅

⋅

MAX DC

ΔT

-----------

J MAX

AMB

–

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

------

1.5W

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