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LM3478: Webench doesn't seem to want to generate LM3478 design when output power > 32 watts?

Joseph Chiu
Prodigy 130 points
Part Number: LM3478

I have successful past designs with the LM3478 and would like to stay with the part.  I recently have an application where it would be useful to convert about 45  watts of power (12Vin nominal to 24Vout nominal) and Webench does not offer any design around the LM3478.  If I play around with Vout and Iout values, it looks like LM3478 designs are offered up until the output power hits 32 watts.

Is there any inherent reason why the LM3478 might not be suitable for higher wattage output?  Or is the limitation simply the limits of the modeling available in Webench?  As a controller without having an onboard switch (FET), I would think it wouldn't have an inherent limit?

  • 0
    TI__Intellectual 2880 points

    Hi Joseph,

    Thanks for reaching out.
    Which topology are thinking of using in your application using the LM3478? 

    Thank you and best regards,
    Bryan

  • 0 in reply to Bryan Radix
    Prodigy 130 points

    I am currently using a SEPIC buck-boost configuration.

    https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=9E3849861D524EA7

    Typical operating condition is 12vin, but with a desired range of nominal 12Vin to 24Vin (margined to 10V-27V) and output of about 22V-24V out with ~42W delivered at the output during continuous peak demand.

  • 0 in reply to Joseph Chiu
    Prodigy 130 points

    Just following up on this -- it looks like Webench's initial offerings of designs might not include LM3478-based designs when the conditions exceed 32W, but if I am under 32W, select a LM3478-based design for customization and then change the operating conditions, it will update the design and component choices for the new condition.

    So I suppose it's "solved" in terms of my immediate needs.  I am still a bit curious as to how Webench decides to cut off the LM3478 design though?  Thank you.

  • +1 in reply to Joseph Chiu
    TI__Mastermind 22480 points

    Hi Joseph,

    I agree that is confusing that the Webench tool sets a hidden limit of 32W output applications with SEPIC.
    The LM3478 is indeed capable of higher output power, especially as the switching FET is external.

    It might be related to the point, that the tool recommends synchronous devices for high power applications to reduces diode losses and improve overall efficiency.
    However, I cannot say this for sure just yet, as the tool does not give any explanation why no design can be generated, and even more why it does work when the settings are adjusted later on.

    Best regards,
    Niklas