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WEBENCH® Tools/LM3478: Li-Po battery to 40V, 100mA

Andrej Pajnic25
Prodigy 60 points
Part Number: LM3478
Other Parts Discussed in Thread: LM3481, LM5022, TPS55340

Tool/software: WEBENCH® Design Tools

Hello,

working on project, where I need 40V for pulsed device (pulse is 60ns, 40V, peak 20A). Till now I was using AMS AS1340 boost device and switched to TI's LM3478 - need slightly more power.

Here I have big problem with inrush current - when LM3478 is switched ON, battery goes to "safe mode". Design is done in accordance to Workbench:

Is there any workaround to overcome this "first current" problem? I was looking to switch to LM3481 but Workbench says, it will not work...

Thank you for support and best regards,

Andrej

  • 0
    TI__Mastermind 21720 points

    Hi Andrej,

    In a boost converter the initial rush current is inherent due to the output capacitor being below the input voltage. If you are trying to pulse the the output voltage it might be better to used a MOSFET H bridge that is supplied by a stable 40V rail. This should allow for the 60ns pulse you require.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 60 points

    Hi Garett,

    there is no problem with "pulsed" use - each pulsed output is having own loESR capacitor. So capacitor in boost DC/DC is only storage.

    But problem is not on pulsed side. If I start boost DC/DC only, so without any part that is using this 40V, situation is the same. I am attaching my design and hope, that there will be some idea that can help. :-)

    Thank you and best regards,

    Andrej

  • 0 in reply to Andrej Pajnic25
    TI__Mastermind 21720 points

    Hi Andrej,

    Thanks for posting the schematic. This helps to clear up the application.

    When you mention the "first current" problem is it when the battery is first connected or when the UVLO signal is above the standby threshold and switching starts?

    If it when the battery  is first connected then this is called inrush current and is related to the output capacitor. This article talks about where inrush comes from and some solutions to combat it.

    If it is the when the device starts switching it is related to the soft-start time. This can be a challenge with the LM3481 but is possible with some external components. If the SS functionality is needed the LM5022 might be another option.

    Please let me know if you have any questions.

    -Garrett

  • 0 in reply to Garrett Roecker
    Prodigy 60 points
    Hi Garret,

    problem is with soft start - when boost device is enabled and starts switching.
    LM3481 is out of calculation since it have 85% DC and LM5022 is having Vin=6V when I use Li-Po battery. :-)

    Anyway, I will try with TPS55340 and go for 38V. Hope it will work better since there is controlled SS.

    Thank you for all support,
    Andrej