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LM3429 the minimum switching frequency

Tomoaki Yoshida
Expert 2200 points
Other Parts Discussed in Thread: LM3429

Hi,

 

I have a question about minimum switching frequency of LM3429.

I can find the minimum switching frequency on web page.

http://www.ti.com/product/LM3429

 

However I can't find it in datasheet.

I know that maximum switching frequency is limited by minimum on time.

I think that the minimum switching frequency is not limited.

 

Why is the minimum switching frequency limited to 100kHz?

I would like to know the reason.

 

Best regards,

Tomoaki Yoshida

  • 0
    TI__Mastermind 49960 points

    HI Yoshida-san,

    The LM3439 is a minimum off time boost regulator. The datasheet is a bit confusing on this as is states:

    But the key point is the minimum Off time would be 35ns, for lower switching frequencies your simply can keep increase the off time. Apparently, there is no limit. For Boost application, the following equation applies:

    So we should be able to switch at very low frequencies if desired. Please note: this increases solution size by inductor and capacitance required to maintain a stable output.

     


    I hope this helps.

  • 0 in reply to JCHK
    Intellectual 590 points
    Hi John,

    I'm also interested in this setting problem.
    It looks like LM3429 is not the Fixed frequency, it depends on the RT and CT to desire the toff.

    As the BKBST EVM, the RC and CT connect to Vsw.
    Would the frequency drift if we have different Vin ?

    I've tried to test with this case, and I found the frequency could no be fixed. It will be difficult to solve the EMI issue in different frequency.
    Especially for automotive application.

    Is this correct, or do you have any suggestion ?

    thanks
    Mac.
  • 0 in reply to Mac Chen
    TI__Mastermind 49960 points

    Hi Mac,

    For Boost and Buck-boost configurations, the frequency should be independent of VIN. How much frequency variation are you seeing on the BCKBST EVM?

    Thanks,

  • 0 in reply to JCHK
    Intellectual 590 points

    Hi John,

    When the Vin=12V, the Feq. is ~620KHz.
    and if Vin=24V, the Freq. will be ~680KHz.

    It shifted 60KHz, but it might be worse if Vin up to 36 or more.

    Thanks!

    Mac.