TPS546D24A: Noise from TPS546 crossing to other power domains

Tom Martin 

Thank you for sharing all of the details about your experiments and testing.

The first thing I would ask you to try would be to disconnect all of the ground probes except 1, and see if that lowers the noise level you are observing on that 1 rail.  Repeating that for all 4 rails that you are measuring.

Why?

The MSO64B, like almost all oscilloscopes, uses a common busbar for the ground shields of it's BNC inputs, which is also connected to the Earth ground terminal of the oscilloscopes power plug.  This creates a very low impedance connection between the BCN input shields (ground) of two channels, such that most of the resistance between the barrels of two single ended passive probes like the TPP1000 is actually in the ground shields of their wires  As a result, if the external circuits they are connected to are forced apart by a high current flowing through low impedance, most of that resulting voltage difference will appear across the probe ground wire.  With the probes operating in 10:1 input attenuation mode, the oscilloscope will gain that voltage drop up by a factor of 10 and a small ground variation can become a large signal variation.

This is almost certainly why we see VDD rise and PVIN fall during the SW on-time when there is current being drawn from the input.

By only having 1 oscilloscope ground connected at a time, we can typically remove that influence on the oscilloscope's displayed results.

The noise spikes on the waveforms appear to be switch transition noise, which is typically between 50 and 200 MHz and caused by the resonance of the PVIN and SW nodes as the Power MOSFETs turn on and off in 2-3 nano-seconds.  That noise is expected as the power MOSFETs are rapidly switching current into and out of the ground, but it can be mitigated with component selection and layout.

Bypassing PVIN to PGND with an 0402 capacitor whose self-resonant frequency matches the resonant frequency of the SW node for example helps reduce the ringing, as does adding a resistor in series with the BOOT pin to slow-down the turn-on of the MOSFET, or an R-C snubber from SW to PGND to couple any resonant noise back to the exposed pad.

Finally, good layout technique can minimize cross-inductor capacitance that can conduct high-frequency noise across the inductor, or reduce series inductance on high-frequency output capacitors, minimizing their ability to contain that noise close to the IC.

For the schematic / layout review, you can click on my profile name and send me a message with your e-mail address so we can assist you with a review.  Have you used the Schematic and Layout review checklist already provided from the TPS546D24A's product folder?

https://www.ti.com/product/TPS546D24A#design-tools-simulation - 

SLURB01 — TPS546D24A TPS546B24A TPS546A24A Excel Calculator Tool and Checklist 

Hi Peter,

Thank you for your thorough response.

RE: the scope GND issue, we will try this experiment next time we get in our lab, connecting a single ground from the barrel-tip of the signal of interest, while leaving our other scope channels "un-referenced" without a GND signal sent to the scope.

Yes, you are right that of course VR switching noise is "expected", however, my opinion is that this should be already mitigated with our Cout capacitor selection. We should review the design and I can add more/different Cout in the next rev as needed.

I will review the schematic and layout checklists provided by TI. We are in the Rev B design phase now so this is when we want to catch any issues from Rev A and ensure they are not repeated! I have two improvements so far--(1) we had thermal reliefs in Rev A, e.g. on our output inductor, so I am removing those, and although the layout appears generally good in Rev A, I am more closely following the EVM design and datasheet recommendations.

I'll reach out separately for additional review. Please note we have many TI parts in our otherwise small design as you will see, so I'd appreciate a general review of all TI parts! I hope we can pull in the relevant engineers as needed.

Thanks,

Tom