WEBENCH® Tools/TPS92691: WEBENCH® Tools/TPS92691

vinod sharma

Intellectual 450 points

Part Number: TPS92691

Tool/software: WEBENCH® Design Tools

Hi,

I am not able to match webench design result with datasheet calculation.

If Inductor p-p current is calculated from datasheet section 8.1.2, it is much lesser than webench results.

Input & output parameter are given in attached pdf.6114.WebenchReportsServlet6V-36V.pdf

over 7 years ago

0 Atul_Kulkarni over 7 years ago

Genius 4910 points

Hi Vinod,

We are looking into your query. Will get back to you on this soon.

Thanks and Regards,
Atul

0 Atul_Kulkarni over 7 years ago in reply to Atul_Kulkarni

Genius 4910 points

Hi Vinod,

Please find the below equations used for calculating Inductor p-p current.

IL_pp = (VinMax) * DMin* tsw/ L.

Substituting VinMax = 36V, Dmin=0.80987 ,tsw = 1/fsw = 1/416834 and L = 47u in above equation, we get

where DMin = (Vout) / (Vout+VinMin*Estimated_Efficiency)

IL_pp = 1.488A

Thanks and Regards,

Atul

0 vinod sharma over 7 years ago in reply to Atul_Kulkarni

Intellectual 450 points

Hi Atul,

As per datasheet equation 8, use VIN(MIN) for this calculation. But in your calculation it is VinMax.
Please see below link also for Boost converter calculation, equation 2 on page 2.

www.ti.com/.../slva372c.pdf

regards
Vinod Sharma

0 Atul_Kulkarni over 7 years ago in reply to vinod sharma

Genius 4910 points

Hi Vinod,

Yes the datasheet uses Vinmin and Dmax for calculating Ilpp, where as the webench uses Vinmax and Dmin for calculating Ilpp.

Thanks and Regards,

Atul

0 vinod sharma over 7 years ago in reply to Atul_Kulkarni

Intellectual 450 points

Hi Atul,

Why it is so? Which method is correct.
If we follow webbench then unnecessary we have to use high current rating part. This will increase cost & space of our board.

regards
Vinod Sharma

0 Atul_Kulkarni over 7 years ago in reply to vinod sharma

Genius 4910 points

Hi Vinod,

For the input conditions 6,36,12.8,0.18 the highest inductor ripple occurs at VinMax which is the buck condition.
Hence Vinmax is being used in calculations.

D_buck = 0. 355 and Vin Max = 36 => D*Vin = 12.8
D_boost = 0.532 and Vin Min = 6 => D*Vin = 3.192

With fixed fsw and L values you can observe the ripple is maximum at VinMax and Dmin. Hence the worst case ripple is shown.

However you can also change the optimizer to position 2 for low BOM cost.

Thanks and Regards,
Atul

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