Supply capacitances

Hi all,

I'm working on a small project in which I will use two separate PCBs each having an atmega328. Total power consumption will not peak over 150mA. PCB1 will get power from a distant (10m or so) switched power supply (I'm thinking along the lines of 5V). On that PCB I will put a MCP1702 3.3V because some components need 3.3V and thus I would like to supply everything with 3.3V. PCB2 will then also be supplied from this 3.3V. PCB1 will not have a RFM12B and probably nothing else. PCB2 will have a bunch of LEDs, buttons and a DB18S20.

Looking around and reading through the MCP1702 datasheet and looking at several arduino schematics I figured I should use:

  • 0.1u ceramic on the unregulated side
  • 10u electrolyt with 0.1u ceramic in parallel on the regulated side

My questions:

  • Am I correct in my assumptions
  • Why the 0.1u on the regulated side
  • Most important: does PCB2 need decoupling

Thanks in advance, Jef

You get better regulation out of 10uf + 0.1uf than just a 10uf electrolytic, because the electrolytic has higher inductance and esr than the ceramic does.

You always need decoupling. There’s a reason the decoupling caps go right next to the power pins of the part they’re decoupling the supply for.

You should have a cap on the 5v supply on PCB1 to ensure that the power supply is stable enough, considering the long wires and potential for rapidly shifting load from PCB1. I always do this, whether I think I need to or not.

Capacitors on regulators are there for two reasons.

a/ Recommended types and values from the datasheet stop the regulator from self-oscillating.

b/ Added capacitance can improve load regulation when the current varies a lot, like LEDs blinking or motors starting.