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decoupling capacitor between vcc and gnd

houandreas

hi,I'm learning from one schematic from a mezzanine card and designing a PCB for my own project. I found that on that schematic there are decoupling capacitors for power supply 3.3V and Vadj but none for the power supply 12V. On that Schematic the 3.3V is used to supply the IIC but 12V is not used. But it might be used for other high frequency IC right? So why it doesn't get a decoupling capacitor, I can't understand.

 

I'll convert the 12V to 5V to supply my 2x16 character LCD,  the connections which I saw online never contains a decoupling capacitor. Is it because the LCD already have a decoupling capacitor in itself?

 

  • 0

    If you are using a linear regulator to generate 5V from 12V, that will need some decoupling for stability - see the datasheet for the regulator for recommended minimum decoupling capacitors. If you are using step-down switched regulators, it would be necessary too (again see the datasheet).

     

    If you weren't regulating a lower voltage rail from the 12V, it would be "good practice" to include some capacitors on power lines, whether as reservoir capacitors to smooth out large switching currents, or to limit the propagation of digital/switching noise (or to provide some immunity from incoming power supply noise for sensitive circuits).

     

    Without seeing your source material it is difficult to comment whether it is not there because somebody forgot, or for another reason (such as decouplers being built into modules, or because they simply weren't using it). But if for example the 12V is merely powering a display backlight, then it will generate relatively little noise, and not be particularly sensitive to any power supply noise.

     

    So if in doubt, add some - but you can always provide the footprints for capacitors then not fit them if it becomes clear that they offer no benefit. Better than finding out afterwards that you should have. That would be my advice on what little information there is about what you are doing. Also some series inductors/ferrite beads of suitable current rating might be a good idea too, if you want to filter power supply lines properly!

     

    I hope that helps

  • 0 in reply to NigelB

    ty for your answer, I use the 12V for 2 LCDs (one is 2x16 character LCD and the other is a 320x240 display) and the DVI output chip. I looked into the datasheetof the regulator, it says it's optional to connect 2 10uF capacitor at both in/output side. To ensure it works I connected them.