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LM51772: Inductor de-rating setting

Masashi Mineda
Expert 1325 points
Part Number: LM51772


Tool/software:

Dear Sirs and Madams,

I would like to know about the inductor derating settings (INC_INDUCT_DE-RATE).

Table 7-6 of the data sheet provides inductor derating settings.

What is the difference in effect between "Disable" and "Enable", and between "30%" and "40%"?

Regards,

Masashi

  • 0
    TI__Mastermind 31880 points

    Hi Masashi,

    Thanks for using the e2e forum.
    Our expert for the LM51772 device is out of office today.
    We will into your question and get back to you within 1-2 days.

    Thank you very much for your patience.
    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    TI__Guru 61011 points

    Hi Masashi,

    inductor de-rate could be set via I2C on 4 different levels.

    With the R2D interface only two of the available options could be selected:

    30% and 40% (depending if the setting of the INC_IND_DE-RATE in the R2D)

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Expert 1325 points

    Hello Stefan,

    I don't understand the relationship between the slope compensation equation (29) and INC_INDUC_DERATE in Table 7-6.

    Referencing the example in the datasheet.

    By back-calculating from equation (46), we can see that fsw is 600kHz.

    We also understand that Leff = 2.5uH is the inductance taking saturation into account.

    I don't really understand the relationship between the calculated coefficient of 1.04 and the high coefficient of 1.5 stated in the text.

    Furthermore, from the 24% inductor derating mentioned in the last sentence, we can understand that this is when a 3.3 uH inductor drops to 2.5 uH due to a large current.

    From "settting for 30% derating could be used", I learned that it is good to set a derating of 30% for 24%, but I don't really understand how it relates to the coefficient calculated by MSC.

    Regards,
    Masashi

  • +1 in reply to Masashi Mineda
    TI__Guru 61011 points

    Hi Masashi,

    please have a look at the Table: ADDR/Slope Pin (R2D-CH1) Configuration Overview"

    You will see that the next higher value from 1.05 is 1.5 - as stated in the description from the calculated value you should select the next higher possible value which can be selected -> 1.5

    Due to the inductor de-rate (inductance get less) the inductor ripple current will get higher.
    MSC defines the slope compensation, so a value which will be added to the measured inductor peak current to avoid sub harmonic oscillations.
    Due to the inductor de-rate you get a higher inductor peak current and therefore do not need to add so much slope compensation -> MSC need be selected lower if not using the inductor de-rate setting.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Expert 1325 points

    Hello Stefan,

    I was able to understand it thanks to your explanation.

    Regards,

    Masashi