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LM76003: Inductor and capacitor selection

Shuhei Goto
Intellectual 2100 points
Part Number: LM76003

Hi team,

Customer has questions regarding to selection inductor & capacitor.

(The following references the equations in the data sheet.)

  1. Since the output is variable, there is no L that always satisfies equation (18).
    There are higher or lower VIN-Vout. Which should they choose the L for?
  2. What is the best inductor rating?
    Is it OK if it is greater than 6.8A (high-side current limit)
  3. For equation (23), is Δ Vout Voutmax-Voutmin?
    What are the typical values to set?

Best regards,

Goto

  • 0
    TI__Genius 10330 points

    Hi Goto,

    I think you can use webench to get a proper value. You can find it in our website. There are also some recommended values ​​​​in our datasheet to choose from. For your questions, yes, ΔVout=Voutmax-Voutmin. This needs to be selected by the customer based on the output voltage and application requirements. Typically 1% of the output voltage. Thanks.

    Aurora

  • 0 in reply to Aurora Tie
    TI__Intellectual 2100 points

    Hi Aurora,

    Thanks for your kind support.

    In this case, Customer wants to change Vout. Should we use a value of Voutmax or Voutmin in WEBENCH to estimate the cap? 

    Also, let me ask additional question.

    Is it possible to adjust the ripple with an external cap? 

    Best regards,

    Goto

  • 0 in reply to Shuhei Goto
    TI__Intellectual 2100 points
    Shuhei Goto said:
    In this case, Customer wants to change Vout. Should we use a value of Voutmax or Voutmin in WEBENCH to estimate the cap?

    Not cap, L

  • 0 in reply to Shuhei Goto
    TI__Genius 10330 points

    Hi,

    For L, it need to meet the formula below. So it will get the max value when Vout= 0.5Vin, D=0.5.

    For Cout, if D is larger, D' will smaller. Cout will be smaller. 

    So you need to consider all your working conditions comprehensively. And considering the piezoelectric effect of ceramic capacitors, we generally recommend that the capacitance value be larger. Thanks.

    Aurora

  • 0 in reply to Aurora Tie
    TI__Intellectual 2100 points

    Hi Aurora,

    Thanks for your kind support.

    I'm sorry for the confusion. Let me write specific.

    • Vin, 34V
    • fsw, 0.5 Mhz
    • Iout, 3A

    In case of Vout, 11V: 4.6<= L<= 9.1

    In case of Vout, 31V: 12.4<= L<=24.8  

    In this case (variable Vout)
    What value should I choose for L?

    Also, What is the best inductor maximum rating?
    Is it OK if it is greater than 6.8A (high-side current limit)?

    Best regards,

    Goto

  • 0 in reply to Shuhei Goto
    TI__Genius 10330 points

    Hi Goto,

    Did you write it wrong? I did the math, 

    In case of Vout, 31V: 4.6<= L<= 9.1

    In case of Vout, 11V: 12.4<= L<=24.8 

    Anyway, for inductors, the following formula needs to be satisfied, where K should not be less than 0.1 in any case. So when Vo=31V, the maximum of L is 18.2uH, So you can let 12.4<= L<=18.2uH. Maybe you can choose 15uH. The peak current should not exceed high-side current limit. Thanks.

    Aurora