WEBENCH® Tools/TPS54340B-Q1: Op-Val & Bode plot cross freq. not matching for TPS54340B-Q1 based design.

Hi David,

Please note that I have shared the design with above said email ID with details written to it.

Regards,
jagdish

Hi Jagdish,

Can you please share you design with "Share with Public" option? If you can share the design, we can check if you are running into any of the 3 issues mentioned below.

Some reasons why the opvals and bode plot sim might not match:

1. If Cout has been changed, is it a custom cap? If so, the “Derated Cap”, “Total Derated Cap” and “Cap” all need to match up and set the number of parallel caps to 1. This is an issue with custom cap form today. 
2. Bode plot simulation runs at VinNom = (VinMin+VinMax)*0.5. so need to make sure to recalculate the “Opvals” at VinNom to compare with bode plot sim values.
3. Also, when using “Recomp”, make sure the power stage components has the correct derated value of Cout that is expected.

Regards,
Amod 

Hi Amod,

Please my response as below,

=> Design is shared with link details as below,
webench.ti.com/.../SDP.cgi

=> Yes Cout is changed to ceramic, these are 2 in parallel, I checked total derated cap matches to value in Re-comp tool. However, parallel caps aren't set to 1. I will check this.

=> As of now I am not using Re-comp tool.

Thanks,
jagdish

Hi Amod,

Tried using single capacitor of 94uF instead of 2X47uF but results are same. Cross over in bode plot is 16K while in Op-Val is 28K.

Awaiting your further response.

Thanks,
jagdish

Thanks for the additional info Jagdish. I am taking a look at your design.

Regards,
Amod

Hi Jagdish,

It seems the issue is arising due to discrepancy in the sim and opval model parameters. Someone from our team is looking into it and will get back to you with further updates. Apologize for the inconvenience caused and appreciate your patience on this matter.

For now, I would recommend using the opvals and recomp values to compensate and confirm the results and stability of your design.

Regards,
Amod

Hi Amod,

Thanks for your response. OK, I will await the results from your team.

However, using recomp components as per datasheet equations results in stable design as per opval, so I will proceed ahead with this as of now. I believe you are talking same " recomp values to compensate and confirm the results and stability of your design".

Do I understand correct, please advice.

Regards,
jagdish

Hi Jagdish,

Seems like the difference between Simulation Bode plot & Opval Bode Plot was due to edited Rload (20ohm).

Could you please let us know if you had manually changed Rload to 20ohm ?

Note: Any changes made to Input source & Rload in the schematic will not affect Opvals. Hence we might be observing discrepancy.

I made following changes in the schematic (Below is the shared design FYR) :

https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=D9FCFEA65079651C

1. Cout : Made Cap, Total Derated cap and Derated Cap as 52uF with Quantity = 1

2. Rload: Changed to 1.6ohm [Iout = 2.5A]

Thanks & Regards,

Harish

Hi Harish,

Even without changing Rload in bode plot fco was 16K (in simulated bode plot) rather than 28K. After changing Rload to 20E it gave fco near 5K.

BTW, I can't open the link, nor I can see shared design in my list. Can you please look into this.

Also while checking in opval, I was just changing values in top tab of op-val (VIN & IOUT) & not checking there after doing any schematic modification. This is FYI.

Regards,
jagdish