Inverting negative-to-positive step-down switching converter

Question

• 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.

Answer 1

Negative input power can be converted to positive output with following ICs: This schematics was found on TI forum (notice the source link on the scheme).

Following method uses less components and found in the datasheet of each chip accordingly.

LT1070, LT1071, LT1170, LT1171, LT1172:

And minimal possible components with small coil and handy package S8E (For 3.3V output set R4 approx. 32k):