Keep the ripple to about 2% of voltage.
The ripple can be calculated by V = Q/C, where q is the charge that gets pumped into the cap every switching cycle.
Remember to account for the esr related ripple too, if that is significant.
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woops , i didnt realise Klaus had written that before i wrote mine.
Also, ensure the caps are'nt over-ripple-currented.
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If you do an v=ir calculation for the esr related ripple , then the overall voltage ripple is SQRT(v1^2 + v2^2)
Where v1 = peak current into cap * esr
V2 = Q/C as above