TPS82140: TPS82140SILT

Hi Gilad, it's not clear to me what you are showing in the oscilloscope captures. I see a high frequency noise on the yellow Ch1, but not the steady state switching ripple of the module. Can you please zoom in on the 3.3Vouput, AC-coupled with 20MHz bandwidth, with x-scale of 1us per division? Are you measuring using either the pigtal or coax approaches as mentioned in this article: https://e2e.ti.com/blogs_/b/powerhouse/archive/2016/07/27/how-you-measure-your-ripple-can-make-you-or-break-you? 

Hi.

Thanks for helping.

attached are the measurements you asked. I'm using  "pigtail" approach.

Gilad.

Hi.

Just to clarify, my main concern is from the 1.2V output. This causes my Load to under perform.

also attached are 1.2V output.

Gilad.

Hi Gilad,

Thanks for the additional information. I see your scope is set to 350MHz bandwidth. With this wide bandwidth, you will see higher frequency (HF) switching noise and not just the low frequency ripple at the module switching frequency. WEBENCH does not model the HF noise, only the low frequency ripple (what you will normally see with 20MHz oscilloscope bandwidth limit).

You can try adding small ceramic caps (nanoFarad range) to provide HF bypass to help filter out this switching noise. You can place these closer to the load, and measure the ripple at the load as well. You may find this video on this topic helpful: https://training.ti.com/power-tips-measuring-vout-ripple-dcdc-converters

Here is another video on reducing output noise that you may also find helpful: https://training.ti.com/understanding-measuring-and-reducing-output-noise-dcdc-switching-regulators

Regards,
Kris

Hi Kris.

Thank you so much for helping. Attached are two measurement scope captures.

#1:
yellow = 12V bulk cap to 3.3V DC-DC, Blue = 3.3V output cap, red = 1.2V measured on a 10nF cap near load. 350Mhz bandwidth, "pigtail" measurement style.
as you can clearly see, the HF switching noise is the main cause for our problems. we don't understand where is this noise coming from. This is my main concern. maybe
by looking at this picture you can see if the TPS82140SILT or the TPS82084SILT are working properly? maybe on of the peripheral components is causing this issues? it looks like the 3.3V DC-DC (TPS82140SILT) is causing the HF noise problems (by comparing it to the 12V source graph).

#2
1:
yellow = 12V bulk cap to 3.3V DC-DC, Blue = 3.3V output cap, red = 1.2V measured on a 10nF cap near load. 20Mhz bandwidth, "pigtail" measurement style.
the low frequency ripple looks OK on the 1.2V output (TPS82084SILT).

Please advise,

Gilad.

We are discussing offline directly.