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LMR36510: About inductor selection

Ryoma Ohashi
Expert 3730 points
Part Number: LMR36510

Hi team,

My customer has several questions about the inductor selection. Could you kindly answer to these?

9.2.1.2.3 Inductor Selection,

  • Is I_ OUTmax the maximum current of the circuit? Or is it the maximum current of the inductor rating?
  • Is the K=0.4 determined by the range of 20% to 40%. How did this number selected?

  • Is K*I OUTmax in below the same with the ΔI_L in a general DC-DC?
  • When input voltage: 27V~48 V, output voltage: 24 V, output current: 0.05A, K = 0.4. With the above formula, it calculates the ideal value for the inductor is L=1500uH. But when we use the WEBENCH® power designer, the inductor used in the recommended circuit is 68uH~100uH. Why does the L value differs from the calculation?

Regards,

Ohashi

  • +1
    TI__Mastermind 18275 points

    Hello Ohashi,

    The constant K represents the percent current ripple for the design. Values of K from 0.2 to 0.4 have been established as an industry standard.  It has been found that values in this range give the best compromise between inductor size and inductor ripple current. 

    IOUTmax is used in the datasheet as the maximum current of the device. The paragraph above equation 4 recommends using the maximum device current to design for light loads. Since your proposed load is 0.05A, the device will operate in PFM mode for increased efficiency.

    A value of 75uH is reasonable for your application based on K=0.4 and the full load current of the device; 1A 

    K*IOUTmax would be your current ripple: ΔI_L.

    Best regards,

    Ridge