MC34063 dc-dc inverting - capacitor question
You can use smaller capacitors, depending on your current draw and ripple requirements.

The capacitor you're probably talking about is \$C_O\$, the output smoothing capacitor. You ought to look into ripple smoothing capacitors to see why frequency has entered your 'DC power' situation.



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The output capacitor can be calculated with the equation from the datasheet - 

$$ C_O = 9*\frac{I_{out}\;t_{on}}{V_{ripple(pp)}}$$

Here's what that means. You know \$I_{out}\$, it's 20mA. You should probably know \$t_{on}\$ as well, it's the frequency part you mentioned.

The engineering enters the playing field when you need to pick a value with \$V_{ripple(pp)}\$ that fits your application. There is going to be some ripple on the DC voltage, how much is ok? Enter that value in the equation.

That equation will give you the capacitor value you can have within the manufacturer's design constraints. If you perform the calculation and come up with too big of a value, then you need to weigh that against the ripple value that you chose.