Decoupling capacitor

Discussion in 'General Electronics Chat' started by matty204359, Dec 13, 2013.

  1. matty204359

    Thread Starter Member

    Apr 6, 2011
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    suppose I have a rather large circuit with many elements that are switching, and maybe the input supply is a bit noisy. however I want to run a MCU(Microchip PIC) that will ultimately be the one controlling the switching. the basic question is can I just slap a really big tank capacitor across the voltage rail near the IC to avoid any random noise from effecting its input voltage? Is there a limit of an acceptable size? another concern was ESR of the capacitor limiting the DC current.

    since the MCU will be switching at a rather high frequency some thing near 80 MHz CPU/DSP oscillator. and even higher yet the PWM peripheral modules at 120 MHz.

    for example:

    xc=\frac{1}{(2)(\pi)(f)(C)}

    something massive like a 1 farad capacitor at a frequency of 100 MHz gives you (1/some huge number) meaning you have extremely low capacitive reactance. which would lead me to believe bigger is better! I'm still concerned with the DC ESR.

    Extra info:
    My goal is to implement a DC-DC buck converter, using a single chip. the single MCU/DSP, would be responsible for everything. except the power mosfets and their respective gate drivers.

    Most of my concern is in the on chip ADC. noise in the input will make the reading garbage. I want to limit the buck converter to as few IC's as possible. Initially I was gonna use a precision 4.096 volt ADC reference for an external ADC. the idea of reduced complexity that a single chip controller could provide grew on me. Obviously with the DSP functionally I can do some digital filtering.
     
    Last edited: Dec 13, 2013
  2. eeabe

    Member

    Nov 30, 2013
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    9
    It's pretty common practice to use a variety of capacitor sizes for decoupling. Generally, capacitors of different sizes are best at decoupling different frequencies because ESR is not really constant across frequencies.

    One way to do it is to put a large capacitor anywhere in the circuit, and then put small ones near the points that have high frequency current requirement, such as at the Vcc pins of an MCU, or near other IC's or switching devices.
     
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  3. BobTPH

    Active Member

    Jun 5, 2013
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    The datasheet should tell you what bypassing capacitors you need. Typically, PICs call for a 100nF at each Vss/Vdd pair.

    Bob
     
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  4. ian field

    Distinguished Member

    Oct 27, 2012
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    ESR can be an issue with large electrolytics, with aluminium electrolytics; the 2 plates are usually rolled up strips of foil, so there's usually significant parasitic inductance as well.

    Its common practice to have some bulk decoupling where power enters the board, and also at the opposite corner or at all 4 corners.

    Usually; you should give every single chip on the board its own local decoupling capacitor, 10 - 100nF is typical, disk ceramics are good enough, but multilayer ceramic chip capacitors have the lowest ESR - some manufacturers offer resin-dipped leaded parts if you're not doing SMD layout.

    I've seen brochures offering ceramic chip as high as 180uF!
     
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  5. #12

    Expert

    Nov 30, 2010
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    At the top of the "chat" page is a permanent post about this.
    http://forum.allaboutcircuits.com/showthread.php?t=45583

    Switching circuits always have fast transients, aluminum capacitors aren't fast enough, and circuit board traces always have some inductance. Even a great design will kind of look like a capacitor forest if you do it right.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    matty204359 and #12 like this.
  7. matty204359

    Thread Starter Member

    Apr 6, 2011
    105
    3
    I did read that section of the data sheet

    "On boards with power traces running longer than six
    inches in length, it is suggested to use a tank capacitor
    for integrated circuits including DSCs to supply a local
    power source. The value of the tank capacitor should
    be determined based on the trace resistance that connects
    the power supply source to the device, and the
    maximum current drawn by the device in the application.
    In other words, select the tank capacitor so that it
    meets the acceptable voltage sag at the device. Typical
    values range from 4.7 μF to 47 μF."

    I'm not really worried about the voltage sag on the actual cpu of the chip, it can handle a lot of ripple, I'm worried about the ADC voltage ripple. stable ADC voltage reference is what I'm looking for.

    [​IMG]
     
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
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    the normal practice is to place a .1 or so cap across the power rail at each IC. One large cap where the power comes in is usually sufficient, 100 mf or so. for that high frequency, a smaller cap shoud be used in paralell with the .1 mfd, unless the internal inductance of thde .1 is real low. a problem with caps is that they also have an inductive effect, and can be resonant at unwanted frequencies too.
    cliff
     
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  9. BobTPH

    Active Member

    Jun 5, 2013
    789
    114
    You might try using Vcap, the output of the internal 2.5V regulator, as the ADC reference.

    Bob
     
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  10. matty204359

    Thread Starter Member

    Apr 6, 2011
    105
    3
    thanks guys I think I got it now.


    I'm gonna just put the decoupling caps on the IC pins as per Microchip's instruction and your recommendations. If that doesn't do it I might employ some sort of decade capacitor bank, ie. 0.01μ , 0.1μ, 1.0μ, 10.0μ in parallel and see how that performs.


    Microchips pinout
    [​IMG]


    Microchips LC filter?
    [​IMG]
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    In some demanding applications you might see about three capacitors in parallel immediately local to a supply rail pin.

    You might see a 1n & 100n disc ceramic and a 10u tant, generally at least one bulk smoothing where power enters the board; 100 - 470u is often seen.

    If you're working with ADCs - watch out where you ground decoupling caps - there's often both digital and analogue grounds.

    Generally speaking; the decoupling capacitor traces should take the shortest possible route from the Vcc to the GND pin.
     
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  12. matty204359

    Thread Starter Member

    Apr 6, 2011
    105
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    I was thinking of making it a two sided PCB and basically flooding the bottom layer with a ground plane. common ground on both analogue and digital, maybe separate them and have a star ground(single point) connection. so basically have a Dground and an Aground, but connected at a single point to keep them common.
     
  13. matty204359

    Thread Starter Member

    Apr 6, 2011
    105
    3
    good idea. after thumbing through a few datashees/ reference manuals

    [​IMG]

    Any ideas what will happen with their external LC filter since the pins are connected internally anyway?
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    If you use a substantial groundplane that probably isn't necessary - just take care not to ground digital supply decoupling capacitors closer to the A GND pin than the D GND pin.
     
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  15. matty204359

    Thread Starter Member

    Apr 6, 2011
    105
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    wow thats a hell of a high capacitance for a ceramic, must be low voltage and high cost!
     
  16. Veracohr

    Well-Known Member

    Jan 3, 2011
    552
    76
    The DAC datasheet I'm using for my current project specifically says to use a 10μF tantalum and a 0.1μF ceramic as close as possible to the supply pin. It also goes into some rather confusing stuff about ground planes and "fencing", but you might try similar values.
     
  17. matty204359

    Thread Starter Member

    Apr 6, 2011
    105
    3
    I'd leave fencing to the professionals unless your experiencing a lot of cross coupling, between your signal lines. digital signals like to put out a lot of emi since the signal wire is basically a inductor and dc signals are basically giving rapid rise and fall times in it.
     
  18. ian field

    Distinguished Member

    Oct 27, 2012
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    More or less - the bigger the capacitor; the more parasitic degrading qualities, if you're really worried about logic-hash on the supply rails you can go to the extreme of also adding capacitors in parallel of tens or hundreds of pF - but its rarely necessary to go that far.

    A 100uF on its own would have too much ESR and parasitic inductance to do any good and a 100pF on its own would be too small to do any good.

    Its all about paralleling various types and sizes of capacitors to get the best of both (or more) worlds.
     
  19. matty204359

    Thread Starter Member

    Apr 6, 2011
    105
    3
    Its still in the prototype stage, I'm going to work my way up and see what works, the board right now is just open loop controlled by a frigging 555 timer just to make sure my n channel fet drivers are working properly.(as far as I can tell they are) its a long way from a finished project, I still need to wait for my microcontroller and oscilloscope to be delivered.

    I've been learning a lot about design, its not as easy. work arounds and failures happen more often than I'd like. i'm hoping its just something that comes with experience.
     
  20. ian field

    Distinguished Member

    Oct 27, 2012
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    If you're using a bipolar 555 on the same board as DAC/ADC circuitry - you may have to go to "extremes" on its supply pins. The bipolar totem-pole output tends to crowbar Vcc at the crossover point!
     
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