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LMR16006: LMR16006 output inductor value calculation by using formula from LMR16006 datasheet

Mladen Bruck
Prodigy 70 points
Part Number: LMR16006

Hello,

I having trouble getting same value for output inductor value by using formula from  LMR16006 datasheet. Please refer to 9.2.1 Custom Design procedure.

Parameters:

Input Voltage: 16V

Output Voltage: 5.0V

Output Current 0.6 A

Switching Frequency: 0.7 MHz

On page 12 (9.2.2.2) for calculating output inductor selection, below formula is used to calculate:

 

 Explanation for calculation Kind is confusing "... 30%-40% of the DC output current."

For example I chose 33% (with this percentage  I got matching value)

  1. Should deduct Iout by 33% then Kind= 0.402 A and Lo= 0.00002036H

or

  1. Should get 33% of Iout then Kind= 0.198 A and Lo= 0.00004134

 For me case 1 resulting  value explained in datasheet, but does not follow common understanding of phrase "30%-40% of the DC output current."

What was value for Kind  used in using formula 1 datasheet from page 12, where value of 20.4 µH is get as result?

Thanks,

Mladen

  • 0
    TI__Mastermind 27130 points

    Hi 

    Both value 20uH or 40uH works fine for LMR16006. it is more flexible when you use peak current mode control like LMR16006.  you can extend K range to 0.25-0.6. but too small inductor will make the current ripple large and core loss will be significant increase. and Vin change will also make the Kind change, that's why we recommend 0.3-0.4 but not force user to choose inductor value in this range.

    and you can also use webench to simulate the circuit with different inductor value in different input voltage before you making your board.

    Thanks

  • 0 in reply to Daniel Li14
    Prodigy 70 points

    Hi Danilel,

    Thank you for support.

    But that was not answer I expected sorry.  My confusion is how particular inductor value of 22uH is calculated in that explanation, since  Kind value is not shown :(. What was the K factor in 9.2.2.2 used to get Lo=20.4 uH as result ?

    I am the user :).

    My input voltage is defined from 8V min  to 16V max, and  I already used webench to generate schematic and components values. Value for inductor is 22uH and both Cin and Cout are 10uH.

    I am not very experienced working with TI voltage convertors, so I am not familiar with defining right K factors, more relying to datasheet calculations.  Where can I find more info about what is the K factor and how to properly chose to particular application?

    Thank,

    Mladen

  • +1 in reply to Mladen Bruck
    TI__Mastermind 27130 points

    Hi

    first, choose K=0.3 as initial value , using Vinmax=16V, Vo=5V, Io=0.6A and fsw=700Khz to calculated the inductor value which is 27.8uH. 

    second, choosing the proper inductor, you will see typical value for inductors are 22uH, 33uH, sometime you can find 27uH. Here, we choosing 22uH. 

    third, go back check full input range inductor ripple factor K. if you choose 22uH, Vin=12V, K=0.31, Vin=16V, K=0.37 which is in the recommend range.

    Thanks

  • 0 in reply to Daniel Li14
    Prodigy 70 points

    Hi Daniel,

    Thank you for support Slight smile