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LM62440-Q1: Inductor selection

Shinji Tsuji
Genius 12708 points
Part Number: LM62440-Q1

Hi team,

There is a minimum inductor requirement equation below in LM62440-Q1 datasheet to avoid sub-harmonic oscillation. 

There is also maximum inductance requirement based on minimum ripple required (K>10%) for current mode operation. 

My application is Automotive and VIN varies from 4.5V to 16V for Vout=3.3V at 2.1MHz Iout=3.7A max. 

In case of VIN=4.5V which is highest duty ratio, minimum inductance is calculated to be 0.91uH. 

I would like to select from 0.68uH/1uH/2.2uH, but neither of inductance meet both minimum and maximum required value. 

In this case, should I relax one of the requirement above? 

If so, could you advise how to select the best inductance?

regards,

  • 0
    TI__Genius 10240 points

    Hi,

    You can use webench to get a proper inductance value.  Thanks.

    Aurora

  • 0 in reply to Aurora Tie
    TI__Genius 12708 points

    Hi Aurora-san,

    Could you please advise if there is a guide line to select the inductor  value if you cannot find the value which

    meet both minimum ripple current (for proper current mode operation) and maximum ripple current (to avoid sub harmonic operation)?

    regards,

  • 0 in reply to Shinji Tsuji
    TI__Genius 10240 points

    Hi,

    Firstly, the value of inductor cannot be lower than this result. 

    Then you need to calculate the value of the inductor using this formula.

    Finally choose the largest value. For your application, when K=0.25, L is 1.35uH. So you can choose 1.2uH. This result is also the value recommended by webench. K is generally controlled at 0.2~0.4

    Aurora

  • 0 in reply to Aurora Tie
    TI__Genius 12708 points

    Hi Aurora-san,

    I calculated above 2 equation considering operating range and tolerance of the components, then I cannot find the inductor value which

    meat both of the equation.

    In that case, which parameter can I compromise?

    For example, minimum Lo=0.91uH from my calculation.

    If I select 1uH, then minimum K = 8.28%, and minimum L=0.8uH.. 

    If I select 2.2uH, then minimum K=3.76%, and minimum L=1.76uH.

    Should I select 1uH or 2.2uH? 

    regards,

  • 0 in reply to Shinji Tsuji
    TI__Genius 10240 points

    Hi，

    K is the value you give, usually 0.2~0.4. For the second formula, at the maximum input voltage,  you can get a the inductance. If this sense value is less than the value calculated by the first formula, then it is taken according to the first formula. If it is greater than the value of the first formula, then use this value.

    1uH and 2.2uH are both acceptable, and their K is 0.15 and 0.33 respectively.

    Thanks.

  • 0 in reply to Aurora Tie
    TI__Genius 12708 points

    Hi Aurora-san,

    From my worst case condition, 

    1uH : K=min 8.28%, tolerance of inductance is -20%, so worst case inductance is 0.8uH. 

    2.2uH : K=min 3.76%, inductance value is 1.76uH

    VINmin=4.5V, Vout=3.3V, fsw=2.2MHz, Ioutmax=3.658A, L=2.2uH

    In this condition, K=~5%

    regards,

  • 0 in reply to Shinji Tsuji
    TI__Genius 12708 points

    Do you mean the maximum inductor value should only be calculated at VIN max condition?

    Since my input is car battery, it varies from 4.5V to 16V. 

    If I only care about Max Vin=16V, I will take 2.2uH.

    regards,

  • 0 in reply to Shinji Tsuji
    TI__Genius 10240 points

    Hi，

    I understand your confusion. 

    Under the minimum Vin, you need to use the max inductance to check K.

    For example, if you choose L=1uH, tolerance of inductance is ±20%, so K=0.094 @ L=1.2uH, K=0.11@ L=1uH, K=0.14@L=0.8uH. You need to ensure that the K corresponding to the minimum input voltage is not less than 0.1. 

    So for your application, I think L=1uH will be better. Thanks.

    Aurora

  • 0 in reply to Aurora Tie
    TI__Genius 12708 points

    Hi Aurora-san,

    If I select 1uH, then it violates the equation for subharmonic operation at Low Vin condition.

    Should I priorities K>0.1 than keeping L>05*Vout/Fsw? 

    I am afraid if subharmonic oscillation occur at Low Vin condition. 

    regards,

  • 0 in reply to Shinji Tsuji
    TI__Genius 10240 points

    Hi,

    L>05*Vout/Fsw=0.5*3.3/2100000=0.79uH. 1uH will not violate the equation. This equation has nothing to do with Vin.

    Aurora