# Converting battery voltage 12 volts to -5,5 volts at about 3 mA

My team and I are trying to power a sensor. We need to get -5.5 volts and were not too familiar with making or acquiring negative voltages. So it doesn't have to be a 12 volt. Ultimately we'd like to use one battery and split out different rails from it to power two different devices.

http://www.ti.com/lit/ml/szzn001/szzn001.pdf and http://www.eetimes.com/document.asp?doc_id=1272382
seemed to be insightful. There are solutions out there to acquire negative voltages. We are looking for cheap and simple and the "Split rail" option looks like a path.

But upon looking at a few online there seems to be poor efficiency at the 3 mA load levels.

Could anyone point me in a good direction of research for a good solution to get a negative 5.5 voltage from a positive voltage at a very low amp level (3mA).

Please and thank you from Chuck a ELE student, having fun researching for senior project.

The MAX1720/MAX1721 chips are probably one of your easiest/cheapest solutions. You will have to get +5 to +5.5V from your 12 V rail using a linear or buck converter first though. This chip will simply invert the input voltage. With this single chip and a couple capacitors, you will have a negative voltage system for < \$2. No separate PWM signal needed.

It is very efficient at 3mA:

The drawback of these types of solutions is that they are not really regulated, so if you have a large dynamic current load the voltage will change a lot. For something low current like this project, this is pretty much ideal though.

One of the easiest ways to get negative voltage at low current is to simply hijack the power supply section of any MAX232-type RS-232 interface device.

These typically take the logic supply (+5V) and first double it to +10V and then invert it to produce -10V. These voltages are then used to power the RS-232 drivers in the same chip. But they also come out to their own pins, so if you're not using the drivers, you can use the current (usually several tens of milliamps) for your own purposes. Simple linear regulators can be used to reduce the ±10 V rails to whatever you actually need.

Well lets separate the need for a specific voltage from the negative voltage requirement. To make a negative voltage, you could consider the old reliable "charge pump" circuit...

You can drive the charge pump circuit from an oscillator running from your 12 volt supply. then you can run the approx. -12 volt output through a -5 volt regulator, such as a 7905, or maybe a 79L05 if you just need a few mA. And if it really needs to be -5.5, a silicon diode added between the ground terminal of the regulator and circuit ground ought to bring it down 1/2 a volt to -5.5.

There may be other nifty one chip solutions, but I'm assuming you'd like to try engineering a solution first, so this is my hat thrown in the ring. :-)