I am using an ADP1613 in a SEPIC configuration to convert either a 3.6V battery or a 9V power input into a 5V output. I used the ADP161x SEPIC designer to get the correct component values. A power path circuit chooses the 9V if present or the 3.6V if it isn't.
What I am seeing is that noise on the 9V input is showing spikes on the 5V output. I rigged up a relay circuit to switch the 9V input on and off continuously as a test, frequency about 14 times a second. The circuit should then switch between the 3.6V and 9V inputs automatically through the power path circuit. When the battery is low - about 2.5 to 3.0 V - there is a lot of noise on the 5V output, with spikes reaching 7 or 8 V at times, not good for sensitive ICs.
With a decent voltage on the battery, > 3.3V the spikes are not evident. If the battery is removed, the spikes are still there.
I have put filtering on the 5V near my sensitive ICs to protect them, but a better solution would be to stop the spikes in the first place.
Is this something that others have seen, and what can I do to stop the problem?