TDA8954

[johnlevreault]

I don't quite know where to post this, but I'm looking for some advice on the TDA8954. I have a circuit board with 2 devices, both of which are running in full-bridge mode. They are "cut and paste" designs: same exact schematic, same exact layout. One channel works and the other oscillates! I suspect there's something funky in the thermal design, but it may be exacerbated by the layout. Is there a reference design for this product that would show me recommended techniques?

Thanks for any help. If this isn't the right place to post this, please let me know that, too!

[johnlevreault]

Hi, Jose. Thank you for your response. The application note was very interesting and helpful.

The problem seems to be related to the heatsink. We are using the 8954J (through-hole) version with an Aavid 530101B00150G heatsink. We were originally using some thermal grease between the part and the heatsink, but the thickness was too thick and very variable. We are now using some Bergquist Sil-Pad, so the problem is very similar across different boards.We are also seeing shutdown with low supply voltages, like +/-32V, so I am very suspicious about the heatsink.

My production engineer has discovered that he can make the problem go away but raising the heatsink up off the board. This suggests to me that there is some coupling between the circuit board and the heatsink. The data sheet for the 8954 says that the heat spreader is at VSS and to use a 0.1uf cap to either ground or VSS, so I would think that this "decouples" the heatsink from any noise emanating from the board. The output traces run directly underneath the heatsink, but moving them to the underside of the board does not help.

The cap at the PROT pin seems very sensitive. If I increase it about 10pf, say to 100pf, the problem becomes very bad.

I am very confused, so any thoughts you might have would be very helpful.

[reyes]

Hi,

Please make sure that the external heatsink is properly calculated according to the requirements mentioned in section 14.5 of the TDA8954 datasheet: http://www.nxp.com/documents/data_sheet/TDA8954.pdf

As extra recommendations:

Use a HF decoupling capacitor of about 100nF .. 220nF nearby the de between each Vp and power ground pin.

When for the HF decoupling capacitors SMD components are used, be aware differences in behavior according to the capacitor material. Good results are found with NPO capacitors which have a low ESR, next are X7R capacitors and last are Y5V capacitors which have a considerable ESR.

Have a great day,
Jose

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[johnlevreault]

Hi, Jose. Thanks for your response. We have solved the problem. It turns out that the bypass cap for VDDA was returned to the system ground plane instead of directly to SGND. (There is a separate trace from SGND to the ground plane.) Thank you for your patience and advice. As usual, it's something stupid on my part, but there were some mysteries in the chip that I did not understand that were leading me astray.

Best regards,

John

[reyes]

Hi,

I also believe that the problems could be related by a problem during the manufacturing process, probably by the temperature when soldering the devices or something similar.

There is an application note that may help, it is not specifically designed for the TDA8954, but for other Class-D audio amplifiers from the NXP portfolio, however, this document mention different recommendation and techniques for the layout design and thermal considerations that can also be used for the TDA8954.

You can find this application note in the following link: www.nxp.com/documents/application_note/AN10436.pdf

Have a great day,
Jose

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