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LM51581: Webench vs Design calculation sheet = confused me

Martin Likavec
Prodigy 90 points
Part Number: LM51581

Tool/software:

Hello,

I want to design small boost converter from powered from USB 5V to 48Vout, Iout = 15mA using the LM51581.

I used Calculation design tool (attached my edited excel sheet) - https://www.ti.com/tool/download/SNVR511

Copy of LM5157-58_Excel_Quickstart_Calculator_for_Boost_Converter_Design_V1_0_0.xlsx

Also, I used webench for circuit design. Picture below:

I am confused why webench recommended me so different inductor value vs calculation sheet? I tried my general boost calculator and calculated inductor value was close to your excel sheet.

My application is for audio. I will be happy if you can advise me on any improvements to design best possible source with minimal noise. I have also Pi filter on the input to minimalize noise from general USB wall adapters.

I want to run LM51581 in CCM mode. Is CCM better for audio? Could you tell me if the converter will run in CCM according to my excel sheet values? Is 47uH for CCM?

I am not sure according to info in datasheet so I want to confirm.. Is the set switching frequency constant?

Thank you
Best regards,
Martin

  • +1
    TI__Genius 11925 points

    Hi Martin,

    Thanks for reaching out.

    The difference between webench and excel comes from the fact, that webench also considers DCM operation. In the excel sheet you entered that you want to have a CCM operation with 60%ripple max. Thats how the sheet calculated the inductor value. I recommend to try out the TI powerstage designer: https://www.ti.com/tool/POWERSTAGE-DESIGNER  Here you can easylie plot the curves of a design and see exactly when the change between CCm dand DCM happens.

    I would, regarding noise, also recommend to go for CCM. For your 47uH inductor, the change to DCM will happen at a load of around 5mA. Above this load you will have CCM.

    This converter has fixed frequency. However, in very low load, it will skip pulses.

    Best regards

    Moritz