Pre analysis of decoupling capacitor

[Bjtpower]

Hello Friends

How we can choose the decoupling capacitor before designing micro controller related circuits..??

Are there any methods which tell us to put a particular capacitor while making the PCB..??

I often understand that there should be provision for the Decoupling capacitor..?

Regards

Marx

 

[malli_1729]

Please be specific on the question. Specify which rail , for which processor/MCU/ADC/Chip. It should be appiled any of the previous combinations.

 

[KlausST]

Hi,

There are already a lot if discussions about this topic.
Just follow the links in the box "similar threads".
Or do a forum search on your own.

Klaus

 

[Bjtpower]

it is on VDD of the microcontrollers..??

Klauus:
I read the similar threds, i coudnt find my answers

 

[betwixt]

There is no exact science or formula to calculate decoupling capacitor values and in any case, unless you spend a fortune, the value tolerance of suitable capacitors is quite wide.
Think of the decoupling capacitor as a reservoir of power placed at the point where it is needed most, right at the power pins of the processor. The exact current consumption of the processor is very difficult to predict as it depends on so many factors, clock speed, instructions being executed and load on the pins for example, so the minimum capacitor value would be a moving target to find.

If you used 50 liters of water a day, you wouldn't build a 50L reservoir to hold it, you would make one much bigger to eliminate the risk of running dry. In the same way, decoupling capacitors are made big enough to cater for worst case and then a bit more. Manufacturers usually suggest minimum values and in most cases will advise you use more than one capacitor in parallel, for example 10uF electrolytic and 100nF ceramic. The 10uF is the main store of local energy but due to the construction of most electrolytic types, they are not good at releasing energy very rapidly (in the nS/uS scale) whereas ceramic types are very good in that respect. A combination of the two types is used to cover all eventualities.

Brian.