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WEBENCH® Tools/LM2596: LM2596 input capacitor WBENCH vs datasheet

Magnus Pihl67
Intellectual 260 points
Part Number: LM2596
Other Parts Discussed in Thread: LMR33630

Tool/software: WEBENCH® Design Tools

According to the datasheet, the input capacitor is 680uF (front sheet), or 470uF (other pages). 

WBENCH recommends 10uF as input capacitor (Vin=12V, Iout=3A, Vout=5V). 

Why is this?

(Also output capacitor is a huge difference between them)

  • 0
    TI__Mastermind 23845 points

    Hi,

    The value of 10uF seems too low for this operating frequency, you might see a higher ripple at the input if you choose 10uF ceramic. 

    Upon checking with the evaluation module, you can use 100uF or 2x47uF as your input capacitor. Just make sure the electrolytic capacitor can handle 1.5A rms current total if you choose 100uF aluminum electrolytic. 

    http://www.ti.com/lit/ug/snvu536/snvu536.pdf

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Intellectual 260 points

    Thanks, but what do you mean with "seems to low"? I was expecting I could trust the results of Webench. Not have to multiply with factor 10.

    Please explain.

  • 0 in reply to Magnus Pihl67
    TI__Mastermind 23845 points

    Hi 

    What i meant is that the value is relatively low compared to what is mounted on the EVM (Evaluation Module)

    The input capacitor selection depends on how much input ripple required as well as the switching frequency of operation. 

    There is one article that might help to understand the requirements 

    http://www.ti.com/lit/an/slyt670/slyt670.pdf

    Webench normally is good as a guideline, but i normally also check evaluation module as a 2nd source of check

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Intellectual 260 points

    Hi.

    If so, why is as much as 680uF used in the LM2596 datasheet (first page schematic)?

    The snvu536.pdf document have a formula for calculating this capacitor.

    Cin = D*(1-D)*Io / (dVin_pp*fsw)
    Cin = 0.46*(1-0.46)*3 / (0.24*150k) =
    Cin = 6.9uF

    (D is 5/(12*0.9 = 0.46)

    This is far, far away from both 100uF and 680uF.

    The only reason I can see for such a large capacitance, is to get a large package size, as this is needed to high a current rating on electrolytic caps. However, a ceramic cap does not have this current rating vs. size, so a 6.9uF ceramic cap, might be just as good, keeping a lower size and price?

    BR,
    Magnus

    PS/ snvu536.pdf uses 0,24V for ΔVIN_PP, but does not state where it comes from /DS

  • 0 in reply to Magnus Pihl67
    TI__Mastermind 23845 points

    Hi Magnus,

    The calculation seems reasonable, but i dont think we tested before with the 6.9uF input capacitor before.

    It seems small since this is an older generation of part, i would be cautious with the approach. 

    If you like a smaller solution size, i can recommend use a newer generation part such LMR33630 which can works with ceramic input and output capacitor and very simple to use (compensation is all internal).

    http://www.ti.com/product/LMR33630

    Thanks

    -Arief