" Why .1uF ceramic capacitor on VCC pin of microcontroller "

Discussion in 'The Projects Forum' started by mishra87, Jul 12, 2016.

  1. mishra87

    Thread Starter Member

    Jan 17, 2016
    170
    2
    I have basic question.

    Why can only .1uF ceremic capacitor we put on the VCC pin of microcontroller of any other IC chip.

    Why is the value only .1uF, why its not more or less than .1uF.
    What is the basic fundamentals behind this.
    Why can we not use other dielectric make like electrolytic etc...!!!
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    906
    It is for decoupling. Digital transitions are usually quite rapid and the supply rail is not able to respond timely. Thus, they cause a quick dip, which can affect other parts of the circuit. A fast acting capacitor (usually ceramic) filters that dip. Some applications may need a 0.1 uF and 1 uF in parallel. There are many discussions of the matter. Here are a few:
    http://www.murata.com/~/media/webre...atalog/products/emc/emifil/c39e.ashx?la=en-us
    http://www.ti.com/lit/an/sloa069/sloa069.pdf
    http://ultracad.com/mentor/esr and bypass caps.pdf
    Cypress pdf uploaded.

    John
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    You're a designer and you can do anything you want. Supply bypassing is not a simple thing because there are multiple objectives. The executive summary is that you want to:
    1. Supply immediate current demands to a chip whose outputs are switching.
    2. Short high frequency AC noise to AC ground.
    #1 is satisfied by having a low impedance path to a bulk capacitance on the order of tens of μF.
    #2 is satisfied by the parallel combination of several lower values like .1 μF or even .01 μF

    The values used are somewhat arbitrary, but the decade values may be more plentiful and cheaper that other values. You do not all three capacitors on each chip, but if you can stand the cost put the smallest value on each chip, a value ten time bigger on every 5 chips, and a big one on every 25 chips, or one per board. Some is better than none, and more is better than some.

    It may be instructive to compute the reactance of several decade capacitors at 1, 10, 100, 1000 MHz and compare those numbers. Smaller capacitors make better AC shorts.
     
  4. Kjeldgaard

    Member

    Apr 7, 2016
    73
    17
    A year ago there was an article series here on the site, starting at: Clean Power for Every IC, Part 1

    The articles begin by explaining current draw from integrated circuits, continues to explain the parasitic inductors in capacitors and the parasitic capacitors in inductors etc.
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
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