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)

So far:

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!

  • 1
    \$\begingroup\$ sounds kindof like a MAX3232 without the negative side, on the output \$\endgroup\$
    – Pete W
    Feb 17 '21 at 2:02
  • \$\begingroup\$ @PeteW Pretty much. It's frustrating that so many of these RS232 transceivers contain charge pumps that'll do 3.3V to >12V but the pumps aren't available as a separate package. \$\endgroup\$
    – Whiterat
    Feb 17 '21 at 13:40
  • \$\begingroup\$ No reason you can't use them for just the charge pump... You might even be able to get a 232 transceiver to do most of the work. Check the V_I_L and V_I_H -- some of them have input transition at a volt or so, and may well be able to receice your 0 to 3v3 levels. Then all you need to do is stop the Tx going below 0, which sounds a lot easier than doing all the level shifting from scratch. // the V+ output is often more like 10V btw, but does that really matter? \$\endgroup\$
    – Pete W
    Feb 17 '21 at 13:51
  • 1
    \$\begingroup\$ I did wonder about abusing that given it does have a charge pump AND does the inversion... but yeah also saw that most are 10V typical output which unfortunately is a no go. The other end that I'm connecting to has a voltage divider before an npn so need >=12v to get through. \$\endgroup\$
    – Whiterat
    Feb 17 '21 at 14:04

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