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TPS54620 using Webench

I am using Webench to setup 54620 to convert 12V to 1.0V @ 3A. For whatever reasons, the tool keep limiting the choices of inductor with IDC rated at minimum of 9.857A, which made no sense since my load is only at 3A.

I like to use the Coilcraft XAL40xx small form factor power inductor, are they in the Webench tool? Thanks.

9 Replies

  • I have not really had a chance to check out webench.  Switcherpro will give you better control over your inductor choices.  If you cannot find the part in the library, you canalways simulate it in the "what if" section.  You do need to balance saturation current with your expected fault conditions.  The TPS54620 will current limit at 11 A typical.

  • In reply to JohnTucker:

    Switcherpro doesn't seem to let me increase the switch frequency, thus can lead to the use of a smaller inductor.

  • In reply to Albert Ng:

    While I do not know webench too well, I can help you with switcherpro.  The adjustable switching frequency is accessible using the "advanced inputs" option when you start your design.  Once you set it, you wil have to start over to change it though.

  • In reply to JohnTucker:

    Ok, thanks. Actually 54620 is not supported in Switchpro, but I can use 54320 with it instead.

  • In reply to Albert Ng:

    TPS54620 is in swtcherpro...I just did a design using it today.  Let me know if you need further assistance

  • Hello Albert,

    The TPS54620 has an internal FET which is rated for 6A. Webench currently rates the inductor for the device's max load capacity. This is currently done so that the inductor will not saturate in case of a short circuit event. We intend to install a user preferred option for the inductor current rating, but that is currently in the works. I hope this information helps. The Webench database does include the Coilcraft series of inductors you mentioned. 

    Regards,
    Akshay 

  • In reply to Akshay Mehta:

    f your intent is to size the inductor to prevent saturation in a short circuit, then you should probably be looking at current limit rather than the nominal load current rating.  Typically, the TPS54620 can deliver 11 A into a short circuit.  I usually pick a saturation current somewhat less than current limit but ensure to use an inductor with a soft saturation characteristic so that the inductance does not go to zero under short circuit conditions.  One other benefit of a large saturation current rating is that typically it wil have lower series resistance as well, so efficiency is improved.

  • In reply to JohnTucker:

    John,

    I agree. The options would be to rate for device current limit or for the design load current. The software would have picked the max load current of the device because that was entered as a max current.

    We usually provide an additional margin on the saturation current which will ensure an inductor with higher current rating. The 11A current limit value is nearly double the max load capability of the part. This definitely warrants the user preferred option of inductor current rating. We will be working on that.

    Regards,
    Akshay 

  • In reply to Akshay Mehta:

    John and Albert,

    In my earlier post I forgot to mention that you can control the total footprint of the design using the Optimization knob. Turning the knob to position 1 will optimize the design for a smaller footprint and the resulting efficiency could be lower. The position 5 would put more emphasis on higher efficiency which might result in larger components and thus increasing the footprint. 

    The DCR and current limit might not be directly proportional and would have a dependency on the size. On Albert's design, if I turn the knob to '1', I get an inductor (Wurth 744310095) rated for 11A which has a footprint of 80mm2 and DCR of 6.5mohms. The resulting power dissipation is about 70mW. When I turn the knob all the way to '5', I get an inductor (Coilcraft SERxx series) rated for 12A, which has a footprint of 241mm2 and DCR of 1.3mohms. The resulting power dissipation is about 10mW as seen from the Op Vals section. 

    Regards,
    Akshay