The waveforms with the li-ion pack are interesting. The input is stable, but output has occasional oscillations that do look like control loop instability. Have you considered doing a loop magnitude/phase measurement to get the system phase/gain margin? I think it's a linear control loop so it should be valid. It's internally compensated though, so not sure what you can do to fix it if that's the issue.
I've done some more testing powering the board with two three-cell Li-ion battery packs in parallel to reduce the input impedance as much as possible, including adding a 2200uF aluminum polymer cap to the input of the buck converter. The converter definitely still oscillates under these conditions. I think this rules out the input supply as the cause of the oscillation. Perhaps this regulator is just not designed to be stable with this little output capacitance? Or maybe it needs a higher inductor ripple current?
Unfortunately I don't have an injection transformer to attempt to measure the system gain and phase margin.
I have looked at the datasheet and don't understand the way you have connected Vsen+ and Vsen-. That doesn't look correct to me.
There are few different things going on with Vsen+ and Vsen-. The datasheet shows them directly connected to the output, however they have an appnote (
https://www.vishay.com/docs/66843/anpowersupply.pdf) which suggests adding an RC filter to improve transient response and jitter when using long lead for the remote voltage sense, which applies to me. This is where the 1R + 2.2uF components come from. My calculation of the values could be suspect though. The board also needs to be able to operate with the remote sense wires disconnected, which is what the 220R resistors allow. If the remote sense wires are disconnected, the feedback will come through those resistors, however if the remote sense wires are connected, that signal will dominate. Vishay told me that the internal impedance between the Vsen+ and - terminals was 33k +/- 30%, indicating the 220R resistors should induce minimal error.
To rule out these parts as potential causes of the oscilation, I removed the 2.2uF caps very early on. Then I just now removed the 220R resistors and shorted on their terminals, with no change. The remote sense wires have always been left disconnected.
I've attached a schematic of the current circuit configuration for clarity.
Dual li-ion battery pack powering board at roughly 12.3V. 1A to 12A load step at 2.5A/us. Some oscillation of output.
Yellow = Vout, Green = Vin, Blue = inductor current
Exact same configuration, nothing changed, sometimes it will be stable after a load step like the last image, sometimes it does this
Same configuration but with 2200uF aluminum-polymer cap on input. Vin doesn't undershoot as much, but oscillation can still occur.
Zoomed in on start of oscillation same li-ion battery + 2200uF input configuration. I think you can almost see how the converter control is unstable, where it skips a pulse, then undershoots, then overcompensates, then skips pulses for longer so it undershoots further, etc.
Now stable with the 2200uF cap is moved to the output. I cannot get this to oscillate. Pictured is a 1A to 25A load step at 2.5A/us. Those small wiggles during the recovery from the undershoot still concern me greatly. Does this mean it is still borderline unstable?