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ADP238x as inverting buck-boost - output capacitor

Question asked by EVC on Jul 6, 2017
Latest reply on Jul 23, 2018 by LucaV

I am using the spreadsheet (ADP 238x Inverting Buck Boost Designer) to calculate the components.

 

Specs: input 5-7V, output -6V/1A, ripple 0.5%. One thing is bothering me though: the recommended output capacitor is electrolytic. I am trying to avoid using eletrolytics in this kind of circuit. With the parameters bellow:

 

It returns a Nichicon RR71C680MDN1 (68µF, 16V, 7mΩ) for Cout1. If I click on Cout1 link and pick a lower ESR capacitor, say 2xMurata's GRM21BC81A106KE18 (10µF, 10V, 4mΩ), it adds Cout2, Panasonic EEEFP1C470AP (47µF, 16V, 260mΩ).

 

What is the reason for this? Why can't I just pick two ceramic capacitors of say 33uF instead? I thought at first it had to do with stability requiring a minimum ESR but the two 10uF capacitors in parallel have 2mOhms, much lower than the 7mOhms of the original capacitor.

 

Thanks in advance for your support.

 

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