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LM2675 WEBENCH results?

martin b
Intellectual 560 points
Other Parts Discussed in Thread: LM2675

Hi.

I have just finished simulating the LM2675 in WEBENCH,

VinMin = 25.0V

VinMax = 40.0V

Vout = 18.0V

Iout = 0.8A

According to the BOM Cin and Cout are ceramic caps with ESR in the mOhms, and as I intend to use the LM2675 in a split power design similar design as outlined in (Application Note 1118.pdf), can I swap out the electrolytics for smaller ceramic caps, or isn't it that simple?

I understand these Simple Switcher series regs need some ESR for stability, but why is WEBENCH advising on ceramics?

Is it better to stick with electrolytic capacitors?

1538.WebenchReportsServlet.pdf

Martin.

  • 0
    TI__Genius 15810 points
    Hello Martin,
    Webench does a sweep on the ESR requirement of the output capacitors to check for stability. It starts with a low value assuming ceramic caps because the ceramic caps are generally more desired because of space constraints. If the design is stable with ceramic caps then Webench will usually promote those.

    For your design, what I see is that the design will regulate the negative voltage and keep the positive voltage a few milivolts below. The positive is not tightly regulated. This design is essentially a inverting buckboost if viewed from a control loop standpoint. It might help to have electrolytic caps like those shown in the application note. Webench shows low value ceramics because it is designing for a buck design.

    Hope this answers your question.
    Regards,
    Akshay