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LM22679 - Vin 36V, Vout 3.3V to 30V@5A max

Pallav Aggarwal
Prodigy 20 points
Other Parts Discussed in Thread: LM22679

Hello Support,

I am planning to use LM22679 for an application where Vin is 36VDC and Vout is variable from 3.3V to 30V@5A max.

how should i select the input output capacitor and inductor. I tried many option in webench but all results in different values obvioulsy, so should i take the highest?

Could you please guide.

Regards/Pallav

  • 0
    TI__Genius 15810 points
    You'll need to calculate the worst case ripple ratio for both ends of the output voltage and select that inductor that gives you a compromise for the extreme ends. You should aim for a range of 10 to 50% ripple ratio.

    For the output capacitance, you'd have to choose enough so that the lowest VOUT is stable. That said, the LM22679 is a voltage mode device and compensating it is going to be a bit tricky. You might have to find a balance for COUT also. By that I mean, you may have to reduce COUT enough to have stability but resulting in a tad higher voltage ripple.
  • 0 in reply to Akshay Mehta
    Prodigy 20 points

    Dear Akshay,

    Thank you for the details.

    I have used webench and found various Inductor and Cout values based on Vout and IMax out:

    Vin (V) Cin Vout (V) Inductor Cout
    36 4.7u/1u 3 10uH/1.5mR 100uF
    36 4.7u/1u 5 15uH/1.5mR 180uF
    36 4.7u/1u 10 22uH/1.9mR 82uF
    36 4.7u/1u 15 22uH/1.9mR 56uF
    36 4.7u/1u 20 22uH/1.9mR 39uF
    36 4.7u/1u 25 22uH/1.9mR 27uF
    36 4.7u/1u 30 15uH/1.5mR 39uF
    36 4.7u/1u 32 15uH/1.5mR 39uF

    I have few questions,

    - Is there an option in webench or somewhere i can put my Inductor /Cout values and see how DC-dc will perform? say i select 22uH/1.0mR & 82uF for entire range now i need to check if it will work properly and will be stable in any current output (few milliamps upto 5A)

    When i checked the values of Inductor and cout at lower current say 0.5 or 1A, values were very different. Should i stick to Imax for my calculations?

    Sorry for asking basic questions but i am new this.


    Thanks for your time and support.

    Regards/Pallav

  • 0 in reply to Pallav Aggarwal
    TI__Genius 15810 points

    For question 1, you will have to ask the webench folks by posting that question in their forum.

    For question 2, the value of the inductor will be higher at lighter loads because the software is trying to keep the design running in CCM. You do not need to force it to run in CCM and a smaller inductor value corresponding to a heavier load should be fine. This will operate your design in DCM when the load is very light.