I'm in a bit of a quandary as to if I've approached something the right way or if there's an obvious solution I'm missing.
(Sorry for lack of schematic, still WIP).
Self-powered isolated USB > UART adapter with RX @ 3.3V and inverted TX signal @ 12V. (Weird, I know)
Starting off with an FT232RL @ 5V.
CBUS#4 (SLEEP) on the FT232RL going through a NOT gate (SN74LVC1G04) - more on this later.
ADuM5401 isolator with integrated DC-DC doing 5V > 3.3V.
5V, TXD, RXD and the now inverted SLEEP piped through the ADuM.
(So far so good)
For the 12V feed, I thought an inductor-less charge pump like LTC1263 would be a good choice. However this requires 5V and on the isolated side I now only have 3.3V (the level RX is working at)
So I then figured adding an LTC1751-5V would get the 3.3V > 5V > 12V - I don't need much current so efficiency isn't critical.
Then tying in the isolated SLEEP signal to the shutdown (SHDN) pins on the LTC1263 and LTC1751.
And ultimately shifting the 3.3V TX signal upto 12V with something like a MMBT3904 which also achieves the inversion. (Welcome to suggestions on alternatives!)
Alternative thought was to drop the LTC1751-5V, throw 5V through the ADuM and then use an LDO to get a 3.3V source on the isolated side and chuck a BSS138 (or similar) to handle the 3.3V > 5V pre ADuM for the RX.
Any thoughts much appreciated!