STA541SA datasheet(16/26 Pages) STMICROELECTRONICS

16 / 26 page

Thermal information

STA541SA

16/26

Doc ID 16988 Rev 2

Thermal information

In order to avoid the premature onset of the thermal protection, see Figure 6: Electrical

characteristics on page 9, it is necessary to calculate the required thermal resistance,

Rth_HS, for the heatsink.

The parameters that influence the calculation are:

●

maximum power dissipated in the device (PdMAX)

●

maximum thermal resistance junction to case (Rth_j-case)

●

maximum ambient temperature TambMAX

There is also an additional term that depends on the quiescent current, Iq.

6.1

Heatsink specification examples

6.1.1

Rth_HS calculation

Given that VS = 17 V, RL = 2x 8 Ω , PoutMAX = 2 x 20 W

then the maximum power dissipated in the device is:

PdMAX = 2 * (2 * VS / (π

2 * R

L)) = 2 * 7.32 = 14.6 W

Using this value the required thermal resistance of the heatsink can be determined:

Rth_HS = (150 - TambMAX) / PdMAX - Rth_j-case = (150 - 70) / 14.6 - 2.5 = 3.0 °C/W

6.1.2

Calculations using music power

The thermal resistance value calculated in the above example specifies a heatsink capable

of sustaining the maximum dissipated power. Realistically, however, and as explained in the

Applications note (AN1965), the heatsink can be smaller when the application is musical

content.

When music power is considered the resulting dissipation is about 40% less than the

calculated maximum. Thus, a smaller or cheaper heatsink can be employed.

Using the values in the previous example, the maximum dissipated power reduces to:

PdMAX = 14.6 - 40% = 8.8 W

leading to a heatsink thermal resistance of Rth_HS = 6.6 °C/W.

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