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I am designing a PCB with a TPS61090 synchronous boost converter. The datasheet recommends a ceramic 2.2uF and a tantalum polarized capacitor on the output of the chip. They discuss calculating the capacitance of the polarized capacitor and the influence of the ESR on it. But they don't actually mention the voltage rating of that capacitor, and neither do the articles I found when googling. I am planning to output 5.2V on the output (to allow a small voltage drop due to downstream components) so the capacitor should be at least that. But I also read that you need to size your capacitors to double the max voltage, because the capacitance drops with voltage. So what voltage rating would people recommend for the tantalum capacitor? Would 6.3V be enough? Tantalum caps higher than 6.3V seem to get expensive quickly.

Secondly, adafruit makes a board that uses this chip and although their schematic says it's a polarized capacitor, it looks more like a large ceramic capacitor to me. Why would you prefer a ceramic capacitor over a tantalum capacitor or vice-versa for this application?

Adafruit Powerboost 1000C Adafruit Powerboost 1000C schematic

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No. Ceramic caps other than C0G/NP0 have reduced capacitance with DC bias. Tantalums do not have DC bias but explode at the slightest overvoltage.

I don't see why you can't use an polymer electrolytic.

Tantalums have higher volumetric capacity than ceramics, but better frequency characteristics than electrolytics...but as mentioned, the are very sensitive to overvoltage. I don't think they are that popular anymore.

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For the ceramic capacitor (X7R), you need to use either a much larger value than recommended (i.e. 10uF ceramic) or a voltage rating about 4x higher than your output voltage. I doubt you'll find an NPO cap at 2.2uF that you could afford to buy; especially since you seem to be sensitive to the price of a tantalum cap.

https://www.maximintegrated.com/en/design/technical-documents/tutorials/5/5527.html

Go to 10V on the tantalum or spend lots of time analyzing your application with regards to overshoot for input voltage steps, output load steps, startup, etc. You can always buy smaller values and put them in parallel as long as you are mindful of the equivalent ESR when caps are placed in parallel (i.e. ESR drops).

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