I am using a buck converter to drop from 5V to 1.8V to be able to drive both of these voltage levels in a single design. The buck converter works correctly to generate a clean 1.8V supply, but the input side of the buck converter, the 5V side, is being contaminated with switching noise spikes of around 1.5 volts as a result of the buck converter being in the circuit. I have used the FT440A and AP3429 chips.
I have tried playing with the capacitance on the 5V input to the buck converter as well as adding diodes and inductors, but so far I have had no real effect on the spike amplitude. Is there a set of standard approaches to removing this sort of input noise that someone could highlight?
The default circuit is all that I am using with the exception of trying numerous modifications on the Vin side to get rid of the noise being seen:
Here are the input spikes that I see:
As far as layout is concerned, C1 is as close to Vin as possible. I have tried from 4.7 to 22uF capacitors. I also tried adding 3.3uH and 4.7uH inductors between before Vin.