I need to build a power supply for my colecovision, it requires 12v, 5v, and -5v. I plan on powering it off of a 12v powerbrick and use a step down converter to get 5v. My question is how do i get a -5v?

 

You need an isolated dc to dc converter.

converter

 

Right, think of it like adding a 5V battery. Ground of the first circuit is connected to +5 coming from the new 5V source. The opposite pole is now -5V relative to ground of the first circuit. The key word is isolated. Neither pole of the new source is tied to the first circuit's power supply.

 

Hello,

How much current is needed for the -5 Volts.
If it is a few mAmps an ICL7660 might be enough.

It might also be possible to convert a second 5 Volts down converter to -5 Volts, depending on the feedback construction.
If it has a transformer it might be possible.

Bertus

 

Use an ICL7660 converter IC. Converts +5v to -5V.

 

I just need .1A for the -5v line.

 

I just need .1A for the -5v line.

Your going to need a transformer with more than one secondary, or a CT secondary for that much current, or another brick converter to provide the -5V.

 

Many DC/DC buck converters can be run in an inverting boost configuration. +12V in, -5V out is a net voltage differential of 17 V, and most of these parts are rated for at least 24 V. National Semiconductor Simple Switcher datasheets have the app circuit. You have given zero information about what you are using to go from +12 V to +5 V, but the odds are pretty good that another piece can go from +12 V to - 5 V.

Or, for only 100 mA, use a bipolar 555 square wave oscillator running on the +12V, and couple the output through an electrolytic capacitor to a half-wave rectifier. Output will be around -9V, easy to regulate down to -5 V.

ak

 

The switched capacitor voltage inverters (ICL7660, LTC1144, 555) can not provide the current you are looking for. AK make a good suggestion about the simple switchers. A simple and cheap way to generate a negative voltage.

 

I just need .1A for the -5v line.

That's probably pushing it for a 7660, you could build a charge-pump inverter on the output of a bipolar 555 - it can source/sink 200mA (I think the CMOS type can only do half that). Because of losses you won't get the full -5V from 5V - I'd go for a burst mode controller running off the 12V supply. You just need a simple voltage sensor to pull down the 555 reset pin at the required voltage level.

 

That's probably pushing it for a 7660...

The ICL7660A has a max current output of only 100μA.

 

The LT1054 will give 0.1A out.