Rick wrote above:
"I think the best approach here will be to rig up linear 3.3/5V supplies "
Hello Rick and group,
I have a similar tought on another thread in this group. Check for the message:
There already is a 3,3V linear regulator in the rig. One should only disable the "handover" from the linear regulator to the buck converter. I seriously intend to do so, at least until I understand what's up with all this happening that makes me feel rather unsure when handling the rig in so many experimental ways.
The rig is small and frail; my hands are big and clumsy... ;-)
For a 5V rail there is no such linear regulator in the rig, so one might add it somewhere where there is enough groundplane on the PCB to solder it to. I would refrain from using one of those scarry xx1117 5V LDO's needlessly having the output electrode on PIN 2 (middle pin) and the cooling tab. In case of a thermic failure, the input gets toasted directly to the output. Whoever designed this chip... (censored)
More civil linear regulators have the cooling tab connected to the ground electrode and if "melt-down" happens, the input gets connected to the ground and there might be hope that a fuse somewhere gets blown before the trace on the PCB burns out. But in most cases, the output stays safely isolated from the ugly happening.
And do not forget to connect the 3,3V regulator to the 5V rail, to avoid unnecessary loss. By doing so, there might be problems with the wake-up timing of the whole combo. As designed, the 3,3V rail feeding the main professor is alive and kicking long before the 5V rail is up. Reversing the order of wake-ups, some latch-up might occur that was not foreseen in the initial sequence. A big electrolitic cap here and there or even some sort of wise watch-dog reset-policy might be needed if wake-up gets bitchy.?
Let's try and please keep us informed of your progress. The more I think, the better I find the idea of replacing the buck converters with linear regulators, if not for good, at least for the testing period, until things settle down. But the more I think, the more I believe that, if I would succeed in replacing the buck converters with LDOs... I would just leave it stay put this way.?
It is also very refreshing to know that the buck converter boards can be tested without endangering the main professor, the one and only, the unreplaceable.
Yours friendly, Razvan dl2arl
DL2ARL