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Need for parallel capacitors across mains(VCC) of a CHIP

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mura_krish

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Hi.,

I Need to know, what is the reason for connecting a huge some of parallel capacitors with different ranges across an VCC Pin of an IC.

And also how many capacitors i want to connect is this is depends on any calculation or depends on the number of Outputs?:idea::?:
 

raka200

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Each time an output of your chip toggle, there is a inrush current.
Your power supply hasn't a zero ohm source resistor (at least L-R of the copper trace, at more, the bandwith of the control loop of your power supply...), so your chip will see a decrease at is power supply pin. This decrease will change the decision level of your chip.... and so on...
You will need a capacitor to act as a energy storage next to your component, and cancel these effects.
We use several type of capacitor because the capacitor are not perfect.
For example, COG capacitor have a good frequency response (I mean, it act like a capacitor at high frequency...) but you couldn't have the necessary capacitor value.
with chemical (tantalum, or aluminuim) capacitors, you have the sufficient storage, but the frequency response is quite bad (act like a inductor at 100MHz, GHz).
Luckily, most of the time, for high frequency, you will need low energy storage, and for low frequency, you will need high energy storage, so we use the 2 capacitors in parrallel.
How to calculate the corret capacitor value ???? It depends on the working frequency of your chip, the power source, the PCB, etc.... Quite difficult to evaluate.
A thumb rule is to use 1nF/COG, and 100nF/X7R on each power supply of your chip. If it works a low frequency (<10MHz) 100nF/X7R is sufficient...

Hope it help you...
 

max0412

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It's also a good idea to scale package sizes for bypassing.read this:

Code:
http://www.intersil.com/data/an/an1325.pdf
How many you use depend on the freq and magnitude of the signal you are trying to bypass,and what is acceptable.
 

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