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LMR51420: Maximum Current Rating of Inductor for 5V output application

Kartavya Agarwal
Prodigy 90 points
Part Number: LMR51420

Tool/software:

Hi,

I am using the LMR51420YDDCR as a buck converter. Here are my system specs:

Vin Range - 23 - 25 V

Vout = 5 V

Max Iout = 1.5 A

I am trying to determine the optimal current rating of the inductor needed. The online design tool suggests a current rating of 7 A, which I think is very large and is resulting in a significantly large component.

What I am trying to figure out is the minimum current rating required to guarantee that I'm within the components safe operating region and ensuring output voltage regulation for the entire operating range of the IC.

Based on the above specs and Figure 8-5 of the datasheet (page 13), it seems that the IC will always operate at 1.1 MHz. If so, according to Equation 10 (on Page 18), I estimate the maximum ripple current to be 0.8 A. At the maximum load of 2A, this would yield a max current of less than 3 A.

Moreover, the critical current at which the converter goes into DCM is 0.4 A based on the above for a buck converter.

As a result, can I select an inductor with saturation and RMS current rating of anything greater than 4 A (accounting for safety margin from the peak CCM current expected)?

  • 0
    TI__Guru 73585 points

    Hello

    The highest peak current the inductor will see is the high side current limit value; about 4A or so in your case.

    An inductor with a saturation current of about 4A will be safe.

    In general, we prefer that the inductor is rated for the peak current limit of the regulator.  This will ensure that the inductor does not saturate during a current limit event.  It is best to select an inductor with a "soft" saturation characteristic.  With this type of characteristic, the inductance will not fall appreciable when the current rating is reached.  This allows the customer to select an inductor with a current rating somewhat smaller than the peak current limit of the regulator.  Inductors with a "hard" saturation characteristic should be avoided, as their inductance falls very rapidly as the rated current is approached.  In this case, the inductor current will rise so quickly that the current limit protection of the regulator may not be able to respond fast enough to protect the application.  For soft saturating inductors, the current rating can be closer to your peak inductor current in the application.

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 90 points

    Hi,

    Thank you very much for the clarification.

    I found this inductor on Digikey: www.digikey.com/.../12141053

    Would this be ok? Also, I want to ensure that I'm not compromising any aspect of the switching regulator's performance by choosing this inductor. Please feel free to suggest any considerations I might have missed.

  • +1 in reply to Kartavya Agarwal
    TI__Guru 73585 points

    Hello

    That is a good choice.

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 90 points

    Hi Frank,

    Thank you very much for your feedback.

    Final question, should I anticipate any challenges with this inductor and switching regulator at light loads. My minimum output current with this switching regulator would be around 0.5 A?

  • 0 in reply to Kartavya Agarwal
    TI__Guru 73585 points

    Hello

    You should have not trouble at light loads.

    Your efficiency at light loads will depend on the device option you select as shown in the efficiency curves.

    Thanks