Have bypass cap recommendations already taken derating into consideration?

Question

Datasheets often specify specific values for bypass caps. My assumption is that these values reflect the actual value (after derating), however, I often see on evaluation boards the same nominal value components being used. X5R caps can lose up to 80% of their nominal value under normal dc biases. How are recommended values in datasheets supposed to be interpreted? I can upload specific examples if necessary.

Answer 1

I would say as a general rule that data sheet examples and application note circuits are there to show you how to make something "work" however more advanced options such as making it work reliably under all conditions is left to you the designer.

The level of quality if app notes etc varies by vendor but you should keep in mind that the person who provided that schematic to you may not be as experienced an engineer as you, and may only have ever made circuits for eval boards and not actual production. Or my personal favorite marketing may have had a say in scaling down the component values or quantity to make the solution look cheaper compared to a competitor...

In other words treat the datasheet and app notes as a guide and use your skill, research and hard work to make a quality product ;)

Edit--- I'd add to more directly answer your question that I'd bet most bypass cap recommendations don't even think about aging or derating and further probably are based on some outdated rule of thumb (again vendor dependent...). I'd recommend looking at what they suggest and then doing your own analysis.

Answer 2

EDIT:

I had written before that I remember a section about this topic in a Microchip datasheet, but didn't have a way to search for it.

Now, I have, an here it is.

Section 2.4.1 of Microchip's PIC18(L)F2X/45K50 datasheet says:

So, if the internals of your MCU require 220nF (as in this case for V_USB3V3), the simplest way is to just connect a 16V 220nF cap. This is by far simpler than studying datasheets to find out what 6.3V cap has about 220nF at 3.3V (here, you would use a 470nF cap, but other caps may behave different...)