Thermal information

STA540SA

16/30

Doc ID 16066 Rev 2

7

Thermal information

In order to avoid the thermal protection intervention that is placed at Tj =150 °C (thermal

muting) or Tj=160 °C (thermal shutdown), it is important to design the heatsink Rth (°C/W)

value correctly.

The parameters that influence the design are:

Maximum dissipated power for the device (Pdmax)

Maximum thermal resistance junction to case (Rth_j-case)

Maximum ambient temperature Tamb_max

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

negligible in this case.

Example 1: 4-channel single-ended amplifier

VCC =14.4 V, RL = 4 Ω x 4 channels, Rth_j-case = 2.5 °C/W, Tamb_max = 50 °C, Pout = 4 x 7 W

The required thermal resistance for the heatsink is

Example 2: 2-channel single-ended plus 1-channel (BTL) amplifier

VCC = 14.4 V, RL = 2 x 2 Ω (SE) + 1 x 4 Ω (BTL), Pout = 2 x 12 W + 1 x 26 W

The required thermal resistance for the heatsink is

Design notes on examples 1 and 2

The values found give a heatsink that is designed to sustain the maximum dissipated power.

But as explained in the applications note (AN1965) the heatsink can be smaller when a

realistic application is considered where a musical program is used.

When the average listening power concept is considered, the dissipated power is about 40%

less than the Pdmax. Therefore, in examples 1 and 2, the resulting average dissipated power

is reduced as follows:

–

Example 1: 10.5 W - 40% = 6.3 W giving Rth_c-amb = 13.4 °C/W

–

Example 2: 21 W - 40% = 12.6 W giving Rth_c-amb = 5.4 °C/W

Figure 27 below shows the power derating curve for the device.

P

dmax

NChannel

V

CC

2

2

Π

2

R

L

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

4 2.62

10.5W

=

⋅

=

⋅

=

"" R

th_c-amb

150

T

amb_max

–

P

dmax

--------------------------------------- R

th_j-case

150

50

–

10.5

---------------------- 2.5

7

°C/W

=

–

=

–

=

P

dmax

2

V

CC

2

2

Π

2

R

L

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

2V

CC

2

Π

2

R

L

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

2 5.25

10.5

21W

=

+

⋅

=

+

⋅

=

"" R

th_c-amb

150

T

amb_max

–

P

dmax

--------------------------------------- R

th_j-case

150

50

–

21

---------------------- 2.5

2.2

°C/W

=

–

=

–

=

Obsolete

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- Obsolete

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