I am designing a board to be used outdoors and the enclosure can get quite warm in the sun. Therefore I want to avoid using electrolytic caps to ensure long life of the board. The electrolytics were to be used for decoupling and be around 47uF.
From research on the web, it would seem that I can use a few ceramics in series to get the same capacitance (while lowering the ESR too) and get the robustness/reliability that I am after.
Are there any drawbacks to using several ceramics rather than one big cap? (other than space).
I think you are overstating the problem. How long is long life? Will five years do you powered up 24/7?
And remember a capacitor generally does not fail due to the wear out effect but loose it's capacitance. The life time is defined as loosing 20%.
DVDdoug:
It would take 470 0.1uF ceramics to get 47uF!!! If you can find a 1uF ceramic, it would take 47 of them.
And if you could get 10uF ceramic capacitor, as you can, it would only take five of them.
Mind you when you get into such a large ceramic capacitor the capacitance is very dependent on the voltage across it and the temprature variations are huge.
Are we talking decoupling logic chips or analog circuitry? Ceramic are much better
for logic than electrolytics I think. Larger capacitance values are needed when the loads are
higher current or sensitive analog signals such as audio.
Mike is correct. You should use ceramics to decouple the device AT THE CHIP and 1uF ceramics are fairly available, although 0.1 ceramics are fine as well. Coming onto the board at the connector should be your 10uF.
Remember, it's much wiser to distribute that 47uF as several smaller caps. Just good layout practice.
