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TAS6424R-Q1: Pi LC filter inductor value selection

wu yujiao
Prodigy 60 points
Part Number: TAS6424R-Q1

Hi TI

below schematic is from EVM user guide file, there is a Pi LC filter with 1.5uH inductor before PVDD power supply.

could you please share us how to do select this inductor?

if this inductor value is larger or smaller, where will be effected?

  • 0
    TI__Mastermind 19250 points

    Hi,

    The inductor is mainly for EMI. SRF of L1 should be as high as possible (typically >=70MHz). You can refer to below APP note:

    https://www.ti.com/lit/an/snva489c/snva489c.pdf

    The indutor is not mandatory。 If the power supply net has inductor, the PVDD inductor can be removed.

  • 0 in reply to Yanming Gong
    Prodigy 60 points

    Hi 

    could you please share the reason for this inductor need to select high SRF >=70MHz?

  • 0 in reply to wu yujiao
    TI__Mastermind 35340 points

    Hi Yujiao

      When device works under 2.1MHz, it's high order harmonic would easily reach to several dozens of MHz. Choosing a high SRF inductor here would be helpful.

  • 0 in reply to Yanming Gong
    Prodigy 60 points

    Hi

    according to this link  https://www.ti.com/lit/an/snva489c/snva489c.pdf content for L selection description,"The inductor defines the resonant frequency of the EMI filter hence its value (Lf ) is usually in the range of 1 µH to 10 µH for low and medium power applications. Choose the highest value in compliance with amperage and physical size requirements."

    Q1: shared file only provide the experience inductor value, could TI share the calculation formula for input inductor selection value?

    Q2: the lowest inductor value is 1uH, can we select lower than 1uH inductor for this input inductor?

    Q3: the file descripted "1 µH to 10 µH for low and medium power applications", now the use case in our system for high output power is  4*40W @2 ohm, efficiency is approx. 77%, calculated Iout max=14.4A. can we select 0.68uH or 0.82uH inductor? 

  • 0 in reply to wu yujiao
    TI__Mastermind 35340 points

    Hi Yujiao

      Your linked file seems prepared for consumer products, as the EMC test results in it only reach to max 30MHz. But we are talking about automotive part, the EMC will test up to several hundreds MHz, or GHz. May not supposed to use your linked file for this situation. 

      The EMC perspective can't do very precise calculation, we could share our experience using as Derek previous already mentioned. And the final value we chosen, should also based on EMC test results. Smaller than 1uH inductor also could be considered, you could prepare several value, and test it after the board is built.