WEBENCH® Tools/LM5088: IBB supply

Hi Zam:

I opened the wenbench files, it is LM25116 buck design.

can you let me know where is the difference? 

Thanks

Hi Daniel,

I think there is some issue with file attachment. I have again attached the files.

Excel file has calculations according to SNVA856A app note and with datasheets while pdf file has design according to TI WEBENCH. Both doesn't match, both recommends different values of components on same input parameters.

Regards

Zain

webench.ti.com/.../67

Attachment.xlsxWBDesign67.pdf

HI

I looked at xls,is the inductor value different confuse you?

based on,  SNVA856A L=190uH(choose 220uH) but based on datashee, it is 83uH(choose 22uH)

the difference is SNVA856A is based on buck boost topology calculating the inductor value but datasheet is based buck topology based on the excel formula of E16 & E48.

If you used as IBB, you can use SNVA856A result.

Thanks 

Hi Daniel,

Agreed! That datasheets calculation are for buck and snva856a calculations are for IBB. but also have a look at webench calculations for IBB, it recommends different values for IBB then snva856a. can you please tell us which calculations are correct, webench or snva856a?

Regards

Zain

Hi 

can you send the picture of webench schematic ? I only can see the LM25116 schematic when I open your link

Hi Daniel,

Please find the WEBENCH schematic in attachment.

Regards

Zain

WEBENCH Schematic.pdf

Hi Zain:

thank you for your sharing.

I notice there is one erro in the excel of how it calculated the ripple of inductor, for buckboost, the inductor dc current is equal Iin+Io, so the dc current should be (Io+Io*Vo/eff/Vin) which is around 4A, then you pick 30%-60% ripple factor, the inductor value should be from 50uH-100uH, I take look at wenbench result, it doesn't calculated as this way. 

You can follow the SNVA856A(pdf file) way to calculated the inductor value and LM5088 has TINA mode, you can sim the circuit in TINA to verify the design.

Thanks

Hi Daniel, 

Thanks for your time. I will correct that excel file for ripple current error. Secondly I will sim LM5088 with tina and get back to you soon.

Best Regards, 

Zain

Hi Daniel,

I have recalculated the excel file ripple current and it looks good to me. In this excel file ripple current is assumed in E14 to calculate the inductor value, then in E19 the actual ripple current is calculated according to chosen inductor value.

Can you explain me the error if I am wrong?

Regards

Zain

Hi Zain:

yes, you are right, cause the practical inductance is different from the calculated the inductance .  

Thanks

Hi Daniel,

Really appreciate your support. We were agreed on that the webench calculation have error for IBB topology and we should calc according to SNVA856A document and do simulation.

I have done simulation of LM5088 in IBB configuration. Calculations are according to SNVA856A. But regulator won't work properly. I have attached the simulation and calc data. Can you please tell me that is there any problem with simulation or there is some thing else?

Regards

Zain

IBB Calc - Copy.xlsx

SNVA856A_-22.8.TSC

3438.Working with inverting buck-boost converters.pdf

HI Zain:

I have modified you simulation files, i can do the sim right now.

you can compare with your original, I change the ground 0 to the GND of the chip

Thanks

1070.SNVA856A_-22.8.TSC