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decoupling capacitor voltage rating

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sapphire_2010

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Hi,
I am using a voltage regulator to convert DC 5V to 3.3V & want to use a decoupling capacitor 0.1uF/1uF at INPUT & OUTPUT of regulator. what should be the recommended voltage rating for capacitor for input & output? Any formula to calculate? I heard caps voltage should be 2X or 3X of the voltage at that point. Is that right?
any help is appreciated.
SA
 

Edit: Looks like I'm talking rubbish again...

I do not believe that using a capacitor rated very much above the operating voltage is in any way beneficial. For electrolytic capacitors it could even be detrimental as the rated voltage is gradually reduced to the operating voltage as the dielectric layer is not being maintained at the rated voltage.

So, simply choose whatever voltage rating is next up (in the make of capacitor you choose) from the operating voltage, and remember to include possible voltage peaks from ripple etc. in the operating voltage. Providing the rated voltage is not exceeded, you are good to go.

Also ensure not to exceed other rated parameters, such as maximum ripple current, temperature, etc.
 
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There is probably another reason for picking a voltage rating well above the actual voltage but lifetime is one. Capacitors run close to their rated voltage don't last as long as ones that are a lot further from the rated voltage. Whether this is really a practical issue for general decoupling, I am not sure, but it certainly is for SMPS capacitors for example.

Keith.
 

It is based on (very old) MIL REL ratings. I don't know if they are accurate and relevant now.

Keith.
 

Ceramic caps will have less effective capacitance when operated close to their rated voltage. How much less depends on the dielectric. For instance, see figure 4 on page 13 of the attachment. The Y5V cap is down about -80% at its rated voltage (16 V), the X5R is down about -40%. If you're using ceramic caps, I think 2-3x is a reasonable rule of thumb depending on the dielectric.
 

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  • Linear LT1962.pdf
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I seem to remember reading documents recommending 10V decouplers for 3V3 chips somewhere, rather than 4V or 6V3 ones.

Keith
 
For any reasonable MTBF figures, use 2X voltage.
Any ceramic below X7 series are pretty rubbish and should not be used in my opinion (X5S etc) they are absolutly terrible. If you have to use X5 series then I would try for 3X the supply voltage.
 
thanks for your comments:
I will conclude 2-3X is suitable to select caps voltage rating. The X5R and X7R dielectrics result in more stable characteristics and are more suitable.
 

I usually use a x2 rated capacitor but there two exceptions. When I use a cap for an AC mains input voltage (whatever it is 220/230 or 400/440) I try to use something over x2 due to spikes or high voltage noise at the input. The other exception is when I am looking for a very low ESR capacitor (like the ones you would use after an SMPS). While working with electrolytics capacitors, the higher the voltage rating, the lower the ESR it will have.
 

For 0.1 or 0.01 ceramic cap, you can choose from 10V onto about 50V. Don't use high voltage cap because the size become larger and lead inductance and internal dimension cause more parasitic inductance that work against you in high frequency range. I use ceramic cap with 0.1 spacing for through hole. For SMD, get 0603 or whatever size you are using.

For larger polar cap, anything 10V to 25V is plenty. This govern by the size and the price.

I never seen a ceramic bypass cap burn yet. Polarized cap do burn not just by over voltage, they do die regardless of how much over rating you put into. If you use 50% over the operating voltage, it should not be an issue.
 

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