LM3578 prelab help

Discussion in 'Homework Help' started by KCHARROIS, Jan 20, 2013.

  1. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
    1
    Hello

    So I'm trying to complete a prelab with the LM3578 switching regulator and I'm having a problem calculating C2. I'm following the example for the Buck Converter circuit and the formula for C2 is:

    C2 ≥ Vo (Vin - V0) / (8 * Fosc2 * Vin * Vripple * L1

    The given values in there example are:
    Vin = 15
    Vo = 5
    Vripple = 0.01
    L = 470u
    Fosc = 50 KHz

    C2 answer is 200uF rounded off i'm assuming

    My calculation is 5 (15 - 5) / (8 * 50KHz2 * 15 * 0.01 *470uH) = C2 35.4uF

    Where did I go wrong?

    Thanks

     
  2. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
    1
    Rethinking about my problem does mean that any value of 35.4uF or greater is an acceptable capacitor value and that they chose 220uF?
     
  3. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    It would appear that that is the case, I got the same value.

    We never want to design a circuit, particularly, a supply, to operate at the margins. It would seem that they chose this arbitrarily to give a large amount of leeway, though I can't be taken as an expert in this arena.

    For anyone wanting to verify, the formula is listed on page 16 in the datasheet.
     
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