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LMR43620-Q1: Inductor selection

Youhei Miyaoka
Expert 3865 points
Part Number: LMR43620-Q1

Tool/software:

Hi team,

My customer want to check their inductance selection. Here is their calculation.

And they want to use inductor which have 20% variation. So the worst case is;

2.75uH > 3.3uHx0.8 = 2.64uH

2.75uH < 4.7uHx0.8 = 3.76uH.

So, they will choose 4.7uH for inductor. OTOH, our recommendation is 2.2uH. 4.7uH is still our recommendation?

Regards,

Youhei MIYAOKA

  • +1
    TI__Expert 3400 points

    Hello,

    A 4.7uH inductor selection seems to violate the minimum ripple current allowed by this device, especially considering the inductance tolerance could add +20% (to 5.64uH).

    3.3uH should satisfy your listed specifications while maintaining 15% ripple at the lowest. Ripple must not fall below 10% of the max current of the device if you are using the 2-amp variant. Reducing your typical switching frequency to 2.1MHz using a very low tolerance resistor is also a often recommended for such a concern.

    Best,

    Ryan

  • 0 in reply to Ryan
    TI__Expert 3865 points

    Hi Ryan,

    Thank you. Could you explain your last sentence in detail please? Sorry, I'm not sure what you are saying. Is that option possible to fixed frequency version?

    Ryan said:
    Reducing your typical switching frequency to 2.1MHz using a very low tolerance resistor is also a often recommended for such a concern.

    Regards,

    Youhei

  • +1 in reply to Youhei Miyaoka
    TI__Expert 3400 points

    Hello,

    My apologies, I assumed you were using an adjustable frequency variant. What I mean to say is that using an adjustable frequency resistor, RT in the data sheet for this device, allows you more flexibility to ensure the switching frequency does not exceed maximum tolerances for your chosen inductor. 

    This will not be relevant to your use case, as you have a fixed frequency variant, which should fall under the tolerances you listed above. The option to change the switching frequency is not available in a fixed frequency version.

    Best,

    Ryan

  • 0 in reply to Ryan
    TI__Expert 3865 points

    Hi Ryan,

    Np. OK, I'll recommend customer to use 3.3uH inductor instead of 4.7uH.

    Regards,

    Youhei

  • 0 in reply to Youhei Miyaoka
    TI__Expert 3400 points

    Great! I'll close this thread for now. Please feel free to open again to continue support.

  • 0 in reply to Ryan
    TI__Expert 3865 points

    Yes, Thank you.