I/P and output capacitor for Linear Regulator

Discussion in 'The Projects Forum' started by rakeshm55, Feb 14, 2011.

  1. rakeshm55

    Thread Starter Member

    Oct 19, 2010
    68
    0
    Hi,
    I designed a 3.3V Linear regulator using On chip's 5203A-3.
    How to fix the values of i/p and o/p capacitors??
    I just followed the recommendations in Data sheet and applied 22uF (High ESR-electrolytic) capacitor at the output and a 100nF ceramic capacitor at the input.
    What is method used to calculate these values ??
    How to arrive at the apt values of capacitors needed from the load requirements??
    What is the role played by this high value o/p capacitor?? its ESR is large and they dont supress ripple or not a good vicinity source , then y use them....
    please help
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Here's a link to ONsemi's datasheet for the CS5203A series of regulators (obsolete):
    http://www.onsemi.com/pub_link/Collateral/CS5203A-D.PDF
    Look on page 5 where it talks about stability issues; low-ESR output caps can cause stability problems.

    You might look at Linear Technology's notes for the LT1085, which is similar to the 5203A series.
     
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  3. rakeshm55

    Thread Starter Member

    Oct 19, 2010
    68
    0
    hi,
    Actually i wanted to know the technique to decide on the values of input and output capacitance based on the load requirements.... especially linear regulators....
    how do we arrive at capacitance values??

    The second part of the question was pertaining to "Tank Capacitors" what for they are used in PCB's?? as a good energy source ?? To reduce the parasitic inductance value on the board?? But dont the decoupling capacitor do the job....What is the need of huge tantalum capacitors in there??... how to arrive at their values
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Have a read through some PCB design tutorials:
    http://www.smps.us/pcb-design.html

    Deciding on how many caps and how much capacitance to use can get pretty involved. Basically, just about any linear or digital IC that I can think of requires a minimum of an 0.1uF cap across its' supply pin(s); two if dual-rail. Many require more, such as 555 timers, opamps, etc. If your power rail(s) are long, thin and meandering, you'll have a lot of parasitic inductance, so you'll need a cap every so often.

    You'll have to review the datasheets for the IC's you're using, and analyze your circuit paths on the board to determine what you'll need.
     
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