How to calculate values for decoupling capacitor

Thread Starter

selva97

Joined Nov 30, 2020
21
Hi,
For decoupling we just use capacitor like 1uF Bulk and 0.1uF ceramic caps at each power pin, But how these values are calculated?? How many caps we need use in parallel? Can anyone explain it with example of any datasheet or any?
 

Marc Sugrue

Joined Jan 19, 2018
218
Hi,
For decoupling we just use capacitor like 1uF Bulk and 0.1uF ceramic caps at each power pin, But how these values are calculated?? How many caps we need use in parallel? Can anyone explain it with example of any datasheet or any?
Normally for digital devices the datasheet will advise values but if you wanted to calculate your own you could measure the changes in current, time periods of any pulses, and select a delta V (dV) to give you a value of C.

C = (i*dt)/dv

You can also select the capacitor based on the chips EMI signature. MLCC Capacitors typcially have a frequency response as shown here https://en.wikipedia.org/wiki/Ceramic_capacitor#/media/File:MLCC-Imp-versus-Freqenz.engl.png which means above the SRF the devices inducance will have more and more of an effect making its capacitive reactance increase. You can select devices such that the SRF co-insides with particular areas of interest or areas you want to minimise emissions. If you have multilple points of interest then possibly parallel different values of capacitor to take advantage of the changes in resonant frequency to eliminate emissions of particular frequencies.

Your example of a 1uF with 0.1u is exactly this approach. The 1uF will have a low SRF but will deal with the larger low frequency currents. The 0.1uF will have a much higher SRF meaning it will deal with more of the high frequency emissions, in parallel they work together to widen the bandwidth.
 
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