Hey there,
I'm working on a buck-boost design utilizing the LM34936 with the following parameters:
Vin: 6-26v
Vout: 12v
Iout: 5A
It's a pretty standard setup, pretty much all of my component values came from Webench. Here's a picture of the schematic:
The performance isn't perfect, but it's nothing that can't be solved with some component tuning - not a big deal there. However, above around 22.9v on the input, the output becomes unstable at low load (below ~200-300mA).
Here's a scope trace of the output at Vin=22.8v and Iout=~10mA. There seems to be a lot of output voltage ripple, but 200mV p-p is okay for my applications:
However, when Vin is increased to 23v or so (and Iout=10mA), the output enters what looks like an unstable mode, where the converter drastically overshoots the 12v vout target, then disables output until the output capacitor voltage decays (due to load and leakage current), at which point the converter overshoots the output voltage again. The power good LED momentarily blinks whenever the regulator turns on and boosts the output voltage back up. I understand that some converters will pulse skip at light loads, but this behavior is raising the average output voltage from the nominal 12v or so to around 13.05v, which is unacceptable for this application. When I re-apply the output load, typical operation resumes.
Here's a scope trace of the output in this unstable mode - notice the large timescale.
I've tried different feedback compensation networks and slope compensation networks, to no avail. Increasing the output capacitance simply reduces the frequency of the overshoot-decay cycle.
Any help diagnosing or fixing this problem would be incredibly incredibly appreciated - I'd be happy to run some further tests or upload traces of different nodes or upload the PCB layout or anything like that, if that would be helpful.
