LM4950TS power dissipation for an 8ohm / 3Wrms maximum

Hi Ti friends and other designers,

I'm using LM4950 for a audio power design and I confront an issue about the power dissipation.

In Mono BTL mode, the power dissipation can be calculated as follow formula, but I'd say it’s only suitable for calculating the maximum value:

Directly capture the power dissipation for 1W and 3W from the diagram, 1W load dissipates 3W power from the supply while 3W load dissipates 3.7W power:

As my design would be put into a sealed box, It would be very hot if i just want a 1W output to the speaker, the efficiency would be so low, 33%.

Is this the real thing that would happen?

Hi Minggang:

               How about adding some holes on your box? In real application, the box usually will have some Ventilation hole.

               And if the temperature is too high. The chip will thermal shut down. 

Hi Derek,

Thanks for your advice, because of the limitation of the application, the box has to be sealed without any holes.

And I still want to make sure if the dissipation is so high, 3W power dissipation even i only need 1W output on load?

Thanks,

Minggang

Hi Minggang:

            Yes. You are right. But it's normal in Class-AB amplifier. The effeciency is usually below 50%. If you want to fnd a high efficiency power amplifier. You can choose Class-D. This is why the class- D is so popular.