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I have an MCU on a board that is turned ON/OFF using a P channel MOSFET switch. All it does is enable power to the MCU or disconnects it.

Basically this:

schematic

simulate this circuit – Schematic created using CircuitLab

The 5V is the 'global supply' which is fed from another PCB. It has several decoupling caps as shown. Should I add additional caps near the MCU or would these caps do? The physical distance between the current caps and MCU is about 10mm to 15mm.

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No good. The MCU needs its own 10~100nF caps.

C1 and C2 are also pointless. They don't do anything useful because the PMOS doesn't need decoupling capacitors and the PCB itself doesn't benefit from such tiny decoupling for an offboard supply. If what you are trying to do is decouple the offboard supply then you need a large bulk capacitance (47-100uF or more).

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  • \$\begingroup\$ Thanks! This sounds about right. If the external power supply comes from a board that is very close (connected via headers), I assume a large cap. won't be required for this board as the other board has caps. on the power rails. \$\endgroup\$
    – user733606
    Feb 13, 2022 at 5:38

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