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I want to build a simple voltage inverter, just using two diodes and two caps to get a source of negative voltage.

enter image description here

The problem is, that my micro is running at 3.3V and I need at least -6V and about 10-15mA. I have a +8V rail availible in my circuit, so I need a way to use the 3.3V to switch the +8V. I tried simulating this in LTspice: enter image description here But it did not work very well, and it has the problem that when the mosfet is open, it is just wasting power on R41. How to do this correctly? I think I need more than one mosfet/bjt...

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  • \$\begingroup\$ Nice diagrams how accurate does the -6V have to be? \$\endgroup\$
    – RoyC
    Dec 27, 2016 at 0:42
  • \$\begingroup\$ Not very much, it can have quite a ripple, even dip down to -5V. \$\endgroup\$
    – Dan
    Dec 27, 2016 at 0:59
  • \$\begingroup\$ Btw, just wondering, should I just use a 555 timer powered directly from the 8V? \$\endgroup\$
    – Dan
    Dec 27, 2016 at 1:01
  • \$\begingroup\$ Consider replacing the 100R (R41) with an inductor. You may need a flyback diode across it if you draw no current from -Vout. \$\endgroup\$
    – glen_geek
    Dec 27, 2016 at 1:02
  • \$\begingroup\$ Erhmm... Why are you trying to build one when you don't understand how they work or have adequate specs? -1V drop / 15mA implies switches must be < 60 Ohms so any CMOS inverter will work. BAT54 Vf=0.8 @ 200mApk. \$\endgroup\$ Dec 27, 2016 at 1:27

2 Answers 2

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You only have 3 active switches ( 1 FET 2 diodes and a 100R resistor ) instead of 4.

enter image description here This is the ideal circuit. and ESR of switch and caps is key to your requirement with Load regulation error being 1V/@15mA = 67 Ohm The ESR of 3.3V 74ALSxxx drivers are 22~33 Ohm for example.

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I like the idea of building it by yourself, but it's a generally wrong thing to do, unless you are a student. In any commercial development it would be cheaper, more reliable, more universal to use a charge pump IC. It would do same work, but generally better than you can. The number of components will be lower, you will not have to deal with EMI stuff (or at least it will be much simpler than if you layout a charge pump of your own). The assembly will cost less. I think you get the point...

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