TPS62933: Audible Ringing

Hi Brandon

Please let me check and get back to you later.

Thanks and best regards.

Hi,

Have similar problem with my design. 

Input 14V

Output 9.5V, 2A

Frequency ~1.1MHZ.

It seems the audible noise is generated by the input capacitors.

Hi Tiaan,

Yes, i saw the same low frequency ripple on the input capacitors as well. Gui do you have any suggestions on what to look at?

Thanks,

Brandon

Hi Brandon,

I played around with the switching frequency and found the audible noise has resonant points on my PCB where it is loudest. Also audible noise also worse at higher switching frequencies.

On my design 500kHz had best result for audible noise but the ripple was not acceptable. Settled on ~775kHz switching frequency (27K resistor on RT pin). When inside the casing the audible noise was dampened enough to be acceptable.

Thanks for your input Tiaan! I can look at lowering the switching frequency to see if it goes away. I'm hoping Gui can explain why the ringing happens at certain frequencies when using this part. I've never seen the spread spectrum frequency appear on the input/output capacitors of a buck converter. I normally only see the buck converter switching frequency content.

From experience it is the high current with the piezo effect of ceramic input capacitors. Can also try to change your input capacitors to tantalum capacitors. Their ESR is worse but they do not have the same Piezo effect than ceramic capacitors. My design see high temperatures so prefer not to use Tantalum on my design.

Hi Brandon

Did you use 'Heater VADJ' to adjust the vout? Did you test waveform with the 'Heater VADJ'  applied? 

Did you try removing the C84?

Thanks and best regards.

Hi Gui,

I tried removing resistor R93 to remove the "Heater VADJ" from the design altogether. I still heard the ringing and saw the AC waveform on the input/output capacitors. Any other ideas on what to try?

Brandon 

Hi Brandon

Can you try removing the C84 ?

Thanks and best regards.

Hi Brandon

Is there any improvement after removing C84?

Thanks and best regards.

Hi Gui,

I tried removing C84 and saw no improvement on the audible ringing or 9kHz ringing. Did you have any other things to try?

Brandon

Hi Brandon

As you said that there is the same low frequency ripple on the input capacitors , it seems the ripple is from input side, Will the low frequency ripple on Cin still exists when disconnecting the TPS62933?

Or your are searching for methods to filter the low frequency ripple out? 

Thanks and best regards.

Hi Brandon

I will close this thread since no response, please let us know if there are any other questions,

Thank you.

Hi Gui,

I am looking for help with 2 items:

1) Why does the low frequency ripple exist to begin with?

2) Is there a way to filter it out or attenuate it so it isn't so audible? 

I am able to use the EN signal on the TPS62933 to disable the part. When disabled, the AC waveform goes away completely. The amplitude is also smaller when there is less load. It increases in amplitude as the load current increases.

Brandon

Hi Gui,

I have an update on this. I didn't see any noticeable improvement on the ringing when doubling the input capacitance (I also have a 47uF electrolytic capacitor in parallel with the ceramics not shown in the schematic). When I doubled the output capacitance, the AC waveform magnitude remained about the same, but the audible ringing went down. So maybe the ringing is caused by the piezoelectric effect?

Still can't explain why I am seeing the spread spectrum frequency on the input/output caps though.

Brandon

Hi Brandon

Can you please share your layout image with me?

Thanks and best regards.

Hi Gui,

See the layout below. It is a single sided layout. The voltage feedback line is on an inner layer. There is a continuous ground plane underneath the components/traces/planes that isn't shown.

Brandon