question about decoupling capacitor

Question

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?

koudelad · Accepted Answer

Hello,

have a look at this Decoupling Tutorial by Analog Devices. Although there are CMOS circuits that can work with 12 V, logic circuits are rarely powered with this voltage. The most common are 5 V, 3,3 V, 2,5 V and 1,8 V. As you mentioned, 3,3 V in your case.

The LCD module you mention definitely has some capacitors, they need to be as close to the power pins as possible, so they can't rely on the designer of a custom board.

David

question about decoupling capacitor

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