microcontroller - Multiple identical parallel capacitors

Can anyone explain why some engineers use multiple parallel identical capacitors on MCU power inputs, memory power inputs... Sometimes there is up to 6, 7 or even 8! Here is one example I found for LPC4088.

This is confusing to me because I usually use one big electrolytic capacitor (to deal with low frequency ripple) and one small ceramic capacitor (to deal with high frequency noise) in parallel... Just like it is described here.

Does it have anything to do with input impedance of VDD/VDDR pins in the image? VDD is 3.3V and is supplied through a regulator LM117-3V3 like this:

I can't seem to find the info about input impedance for VDD and VDDR pins anywhere in LPC4088 datasheet.

Answer

You normally want a decoupling capacitor (usually ceramic) physically near to each power pin in order minimize the effects of parasitic inductance. This is why multiple capacitors are used.

Since the schematic isn't normally intended to reflect the physical layout, these capacitors are simply grouped together in a convenient place. Notes from the design engineer to the layout engineer (especially if they are different people) explain what is needed in terms of the physical layout. These notes can appear in the schematic itself, or in a separate design rules document.

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There are other reasons to use multiple capacitors, too. These tend to come up more with respect to the larger capacitors (e.g., electrolytic) used in power-handling circuits, such as switching power supplies.

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  Sometimes a single capacitor won't fit in the space available, while multiple smaller capacitors will.

  
  


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  Sometimes a single capacitor won't be able to handle the AC (ripple) current, while multiple smaller capacitors will.