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I have a +3.3V supply and ground through usb on a board. I want to get a -3.3v supply from it for giving reference to an adc. Do you know how to do it?

Rick_2047
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3 Answers3

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LMC7660 is a standard part for this. If you don't require any more than a few mA, this circuit should work fine.

enter image description here

It can operate from +1.5V to +10V and provide an output of -1.5V to -10V.

stevenvh
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Thomas O
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As others have said, a charge pump is the simplest way to get a negative voltage if you don't need much power. Since you say this is for "reference", you apparently only need a little power but also some accuracy. In that case you probably want to use a charge pump followed by a negative linear regulator.

For completeness, I want to show the switching topology to use when higher power is needed:

When the switching element SW1 is closed, inductor L1 builds up current thru it. When the switch is opened, that current can only come thru D1 which charges up C1 and makes a more negative voltage on -V. Of course in a real case SW1 would generally be a transistor and there would be some feedback controlling the switch to regulate -V to the appropriate level. I am just trying to show the basic switching power supply topology to make a negative power rail from a positive one.

Olin Lathrop
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The device you need is called a Charge Pump Inverter

There are many different ones around. For example, Maxim has a wide selection:

http://para.maxim-ic.com/en/search.mvp?fam=chargepumps&828=Inverter&tree=powersupplies

And all the other major IC manufacturers do their own equivalents.

Majenko
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  • Any way to make this charge pump inverter using discrete parts? – Rick_2047 Oct 04 '11 at 12:20
  • I tried making one once - not very good I'm afraid (I stick with ICs now), but there are some great suggestions on improving my design: http://electronics.stackexchange.com/questions/15010/can-you-offer-suggestions-for-this-voltage-interverting-circuit – Majenko Oct 04 '11 at 13:41