Not only did my on-board boost not work (TPS61085), but the current it pulled from the supply was so weird in transient that it was the least I could do to 1) Turn off power, 2) Take that chip out. I saw straightaway some mistakes I had made in my design and approach :
1) I had left the FREQ pin floating. Wonder how you catch stuff like that without an LVS?
2) I didn't put in a jumper on the supply to the boost - I should have made it easy to cut this thing out - as it turned out I had to use the heat stream to get the chip off and there were still problems after that.
3) Shame on me - knowing about the backgate diode and having PMOS pullups, I didn't put in a jumper to cut the PMOS pullup out - now that backgate diode is clamping on of my comm pins - but, somehow, though the scope doesn't pick it up, the interface still works.
4) I should have put a jumper cutting the inverting boost off from the output of the boost. Childishly simple, but it's amazing what you can not-think of when you're under pressure. Lesson - think modular.
5) I should have put more pads on the board to let me solder some wires to drive these functions from the outside in the event they didn't work. Lesson - think modular and what-if!
6) In soldering (assembly), I should have thought modular too - start off with the boost and see if it's working.
Weird, with an input supply of 3.5, and the chip removed, and supply passing through the boost inductor (3 uH) and then going into the Schottky, the other side of the Schottky diode was 0.45V. Wha?
Also, the value for the FB node advertised on the TPS61085 datasheet is 1.24V, I was seeing a 0.6V that scaled with supply. What's going on?
Atleast I didn't get 2A of current when I turned on the supply:)
Otherwise, the interface works - the Maker has landed - in one week, a GUI running on a PC with it's own serial interface code, talking to the MCU and getting ACKs, MCU commanding the chip in test-mode. Not bad. I feels good.
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