I have used AP6xxxx switching converters in several projects and never had any problem. They work very well. In particular, I've used the AP64501, it's rock-solid.
Yes your layout should be fine, unless we missed something.
Possibly you had damaged or non-genuine chips. I don't know. From your initial schematic, I think you had placed output capacitors after a fuse, which wasn't such a good idea as it would add some series resistance, but apart from that, I don't know. Actually given the package those converters come in, you could re-prototype just the step-down part on a piece of proto board, just to make sure.
Guess what, this was already my second attempt, but I made a third PCB in desperate before going to bed (Only the step-down components,) and it works perfectly. It has 10mv pk-pk output ripple under a 10ohm load, stable, even voltage even after drastically changing from 7v to 17v. This is with one 22uF Cap not two!
so now my question is, how on earth did I manage to burn through 3 ICs each time, despite using Low temp solder?! is it my storage method (dumping them in a PLA 3d printer container) ESD from my body (I do put the IC under my finger to double check the alignment before putting it on the board) or perhaps just bad luck?!!! the components are from LCSC.
Were they soldered fine and burned on a specific start condition? I do have to note that my pencil does make my LEDs glow as I use it to solder, but I assume it's such a low voltage that it'd not matter. I did have to rework a pin on my PCB all 3 times, including this one. The only thing I did differently was my 195c (vs 250c) temp, and discharging my body with ground before starting. I also did use another power supply with my pinecil (140w capable)
are these ICs just super heat sensitive?!
This little white LED makes me so happy...
I tried to kill it by sparking the input (suddenly connecting the alligator like I did first instead of connecting, then turning on my PSU) and a few other stuff but it didn't die.