It's known that RS-232 port has -12V (approx.) on DTR and RTS pins relatively to the port GND.
It's known that the supply of these pins enough for few tens mA (30-50mA in different sources). However, it was enough to power computer mouse. Total: 2 lines * 12V * 50mA = 1200mW (optimistic).
I use MAX3232 level converter to connect RS232 port to MCU's UART. It works well. However, I must supply +3.3V to the MAX3232. MAX3232 uses about 30-50 mA on 3.3V (165mW pessimistic), so RS232 port should have enough power for the level converter.
The problem is: GND is the same net for RS232 port and for MAX3232 power. That means to draw the power from the port I must convert -12V into +3.3V.
- I know how to convert Positive to Negative power using Cuk converter.
- I know that I could use negative line LDO to reduce -12V to -3.3V.
I am looking for a method or component which allows me to convert Negative -12V to Positive +3.3V. Tens milliamps is enough. The switching regulator is preferred over LDO due to high efficiency of power conversion: on RS232 port we don't have too much spares.
To rephrase (and maybe simplify) the question: how to separate/isolate ground plane/net of inverting step down converter?
Any idea, please?
There are many questions about inverting voltage for measurement/sensing. This question is not about measurement; it's about power.