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TPS54386: inductor value selection for 2.5V and 1.8V generation from 5V

prasanna kumara G
Expert 1585 points
Part Number: TPS54386

Hello Team,

We are using TPS54386PWPR to generate 2.5V and 1.8V from 5V,

How much inductor value i must choose for proper functioning.

Please help me for the selection of inductor.

Regards,

Prasanna G

  • 0
    TI__Guru**** 191445 points
    See section 9.2.1.2.2 of the datasheet on page 32. It details the inductor selection.
  • 0 in reply to JohnTucker
    Expert 1585 points
    Hello John,

    Thanks for the support,

    For ripple current do i require to consider max current capability of the IC or the max load current ?

    Regards,
    Prasanna G
  • 0 in reply to prasanna kumara G
    TI__Guru**** 191445 points

    Generally, you design based on your actual load current.  The design example uses 2A output for each channel and 30% ripple current for a total ripple of 600 mV.  If you are designing for a current way less than the 3 A total, then the inductor value can get quite large.  TPS54386 is a non-synchronous converter.  If the inductor current falls to zero it will operate in discontinuous conduction mode (DCM).  It is perfectly ok to operate in that mode, but some like to avoid it.  In that case the criteria is the the p-p inductor ac ripple should be less than 2 times the minimum load current you expect.

  • 0 in reply to JohnTucker
    Expert 1585 points
    Dear John,

    Actually load current in our case is max 300mA, So while calculating it comes more than 20uH but we have given in design is 4.7uH, Is there any problem on functionality.

    As you told "If the inductor current falls to zero it will operate in discontinuous conduction mode (DCM)" Can please explain me elaborately.

    Thanks and regards,
    Prasanna G
  • 0 in reply to prasanna kumara G
    TI__Guru**** 191445 points
    Ripple current for your 2.5 and 1.8V rails would be 443 mA and 409 mA respectively, so the converters will enter DCM at load currents below 222 mA and 205 mA. The ripple currents are 136% and 148% of the load current rather than the recommended 30%. Theoretically you can operate under those conditions. Make sure your capacitors are rated for the ripple current and that the resulting ripple voltage is within your requirements.

    See the datasheet section 8.3.15 on page 24 for a discussion of light load (DCM) operation.
  • 0 in reply to JohnTucker
    Expert 1585 points
    Hello John,

    Thank you for your valuable information,

    1)May i know how to calculate ripple voltage in our case?

    Regards,
    Prasanna G
  • 0 in reply to prasanna kumara G
    TI__Guru**** 191445 points
    ILp-p = (Vout/Vin) * (Vin-Vout)/(Fsw*Lout)
  • 0 in reply to JohnTucker
    Expert 1585 points

    Hi john,

    Actually i need to know how to calculate VLp-p( ripple voltage ) from ILp-p .

    I have attached my schematics, can you please help me in choosing right inductor and capacitor.

    load current of 2.5V = 300mA

    load current of 1.8V = 250mA

    Regards,

    Prasanna G

  • 0 in reply to prasanna kumara G
    TI__Guru**** 191445 points
    A lot of this depends on your personal preferences and needs (which I do not know). If I design this, I would just follow the datasheet design procedure exactly. I think you can do that as easily as I can. Remember TPS54386 is internally compensated, so once you decide on the inductor, you will need to pick the output capacitor for stability.

    If you need for me to go thru the design and recalculate it, I can do that, but it will probably be next week. In the mean time, let me see if there is a design calculator available.
  • 0 in reply to JohnTucker
    Expert 1585 points
    Thank you ,

    I will seek your help ,Since this part is used is 1st time all other parts are used in my old projects.

    As of now can you tell me how to calculate VLp-p( ripple voltage ) from ILp-p.

    Regards,
    Prasanna G
  • 0 in reply to JohnTucker
    TI__Guru**** 191445 points
    I did locate a rough excel calculator. I got it from the original systems engineer. It has not been touched in 9 years and I cannot guarantee it is completely error free as it is not an officially released tool. Let me know if you would like a copy.
  • 0 in reply to JohnTucker
    Expert 1585 points
    provide me the excel sheet, i believe this is for calculating ripple voltage.

    Regards,
    Prasanna G
  • 0 in reply to prasanna kumara G
    TI__Guru**** 191445 points
    Looking at this calculator, it does not calculate the output voltage ripple. I suppose that does not surprise me. It is a little complicated. let me explain:
    The basic ripple is comprised of two main components, the ripple due to the ac current flowing thru the ESR of the output capacitor and the ripple due to the charging and recharging of the capacitance. These two components are not in phase, so they cannot be directly added together (except as a rough worst case estimate). It looks like you have only ceramic capacitors. I typically ignore the ESR effect in that case. If you really want an accurate estimate, I would probably model your output in pspice or similar. Let me know if you still want the spreadsheet. You can contact me directly at j(dash)tucker(at)ti(dot)com