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Topic: Voltage Reg Caps (Read 530 times) previous topic - next topic

baum

What are good values for a 5V voltage regulator? I am using the 5V to power a micro, so it needs to be fairly accurate. Right now I am using .1uf for both Caps, but I have seen many people use caps in the 10u or 100u range.

Right now I am using ceramic, and therefore saving space on my PCB, but electrolytic caps would be better if I need to use 10uf or greater.


Thanks!
baum

MarkT

The correct answer is read the datasheet for the device.  Note that low-drop-out regulators can be fussy about capacitors.  Normally 1uF on the input and 10uF on the output is fine for low current.  Increase the output decoupling for higher currents.  Ceramics are not required here - they belong right next to the _logic_ chips, not the regulator.
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RuggedCircuits

The "good" values depend more on what's before and after the regulator and doesn't so much affect accuracy. If what's coming in to the regulator is far away from the source, or the source has lots of ripple, a "bigger" capacitor at the input (10uF or so) can help with line transient response, i.e., keeping the output steady even if the input "bounces".

Conversely, putting a bigger capacitor on the output helps with load transient response, i.e., keeping the output steady even if the load (the thing that the regulator powers) instantly kicks up its current demands (fast switching load transient).

Basically, if your circuit works with 0.1uF caps on both input and output there's really no need to change it IMHO. Many regulators work fine with no caps at all (but check your regulator's datasheet....some have very specific requirements).

I generally start with 0.1uF on the input and output then add if necessary. Ceramic is a good choice.

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baum

I'll try it on a bboard and see...

MarkT

Just be careful with ceramics and low-dropout regulators - some oscillate if the decoupling has too low (or too high) an ESR - you might not notice the oscillation without a scope and it can cause various odd errors.  Basically check the datasheet if its a modern low-dropout device, if its a good old 7805 clone then no problem.
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baum

http://www.sparkfun.com/datasheets/Components/LM7805.pdf


Page 7 says 1/3 uF (.33) on input .1u on out. I'm using that one.

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