# Split rail supply needed from single 12v supply

#### johneaston

##### New Member
Hi
I am working on a circuit to be fitted into my car which needs a split rail 5v+ 0v 5v- , i need to get this from the cars single 12v supply. I am not drawing lots of power just enough for an Exor ic and 3 maxim video multiplexers. Any information or ideas would be appreciated.
John

#### eng1

##### New Member
You can use a linear regulator for the positive voltage, an LM78L05 for example. Then a charge pump like the ICL7660 can invert the regulated voltage. The negative output would be non-regulated, though.
You can also take a look at Maxim's converters.

#### ThermalRunaway

##### New Member
eng1 said:
You can use a linear regulator for the positive voltage, an LM78L05 for example. Then a charge pump like the ICL7660 can invert the regulated voltage. The negative output would be non-regulated, though.
You can also take a look at Maxim's converters.
He could use a 7905 for the negative supply rail. Depending on the output current requirements, I would suggest using lower current versions such as the 78L05 which is good for 100mA.

Brian

#### eng1

##### New Member
ThermalRunaway said:
He could use a 7905 for the negative supply rail. Depending on the output current requirements, I would suggest using lower current versions such as the 78L05 which is good for 100mA.

Brian
I agree with you about the 78L05 and I suggested it for the positive rail in my previous post.
As far as the negative rail is concerned, a 79L05 would require at least 2 V between its output and its input and the maximum input voltage of a charge pump IC is usually 10 V or less, so he can't convert the voltage from the battery.

Maxim has very-low-dropout regulators for negative voltages, but they might be quite expensive.

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#### ThermalRunaway

##### New Member
eng1 said:
I agree with you about the 78L05 and I suggested it for the positive rail in my previous post.
As far as the negative rail is concerned, a 79L05 would require at least 2 V between its output and its input and the maximum input voltage of a charge pump IC is usually 10 V or less, so he can't convert the voltage from the battery.

Maxim has very-low-dropout regulators for negative voltages, but they might be quite expensive.
Oh yes, so you did. Sorry about that, I read "LM7805" which I'm sure you're aware is the 1A version. Still, maybe it was a good idea to point this out regardless so that he's aware there are different versions of the device.

Brian

#### jbeng

##### Member
Another solution would be to get a ready-made dc-dc converter module. You could buy them new (expensive) or maybe find them at a surplus place for a more reasonable price. Here's one at Marlin P. Jones & Associates, a place I've bought from for years. Only thing is, this one converts +5v to +/-5v. I do like the price, though.
Jeff

#### justDIY

##### Active Member
Microchip has a handy charge pump voltage doubling inverter, which while not directly regulated, the datasheet has lots of information for handling ripple and transients (basically garbage in = garbage out, clean power in = clean power out). TC682

I use them for generating -10v bias voltage for cold-weather LCD displays.

here's a shorthand version of Microchip's verbose URL for the TC682 product page:
http://tinyurl.com/2mp7w4

How much current do your video thingys need? If its just microamperes, the 682 is a good solution. If it's more than a few milliamperes, Jeff's suggestion of a SMPS is the way to go.

#### Russlk

##### New Member
If you dont need chassis ground for your circuit to work, you can regulate to +10 volts and +5 volts then use the +5 as common and have relative +-5V from there.

#### kchriste

##### New Member
Forum Supporter
You can build this:

And follow it with a 79L05 to get -5v regulated. Good for under 100ma.

##### Banned
Russlk's suggestion seems easiest. The OP did state a large amount of current wouldn't be needed, and virtual grounds are a piece of cake with low current requirements.

#### johneaston

##### New Member
Many thanks for all the replies so far. I feel that perhaps i had better elaborate on what i am trying to do.
I am building a video switching circuit to be used in a car so the supply available is the 12v car supply. From the car supply i need to run a couple of IC's directly and 3 maxim Max453 video multiplexers, these are the devices that need the split supply -5v,0v,+5v, each IC draws approx 660mW so total of 1.8W @5v . Unfortunately i am not able to isolate the ground points so i cannot simply use a LM78L05 and LM79L05 combination so i am still trying to find a suitable solution. Still hoping for more suggestions from what appears to be a very helpful forum.
Regards
John

#### gootee

##### New Member
johneaston said:
Many thanks for all the replies so far. I feel that perhaps i had better elaborate on what i am trying to do.
I am building a video switching circuit to be used in a car so the supply available is the 12v car supply. From the car supply i need to run a couple of IC's directly and 3 maxim Max453 video multiplexers, these are the devices that need the split supply -5v,0v,+5v, each IC draws approx 660mW so total of 1.8W @5v . Unfortunately i am not able to isolate the ground points so i cannot simply use a LM78L05 and LM79L05 combination so i am still trying to find a suitable solution. Still hoping for more suggestions from what appears to be a very helpful forum.
Regards
John
John,

I'm not entirely sure what you mean by not being able to isolate the ground points, unless it means that the -5v needs to be relative to the car's ground/0v level.

If that's the case, it might be easiest to simply use a second battery, with its plus terminal connected to the car's ground, with a regulator to get your -5V, if it's not a 5V battery.

Otherwise, a Cuk-topology inverting switch-mode power supply should work well, and would be relatively easy and cheap to contruct. Check out something like the LT1931A. You'd only need to add a couple of 1A-capable 10uH inductors (small toroids, preferably), a 1A schottky diode, a couple of electrolytic capacitors (22uF and 4.7uF), two resistors, and two smaller capacitors, although you might also want to try to add a linear post-regulator and or an LC filter on the output, depending on your needs. If you needed more power, you could also look at the LT1377, LT1372, and LT1534.

If you download LTSpice (aka "SwitcherCad"), from http://www.linear.com, run it, and select File --> Switch Selector Guide, it will give you several circuit designs, for what you want to do, and simulate them for you. [Although I've heard rumors that the Switch Selector Guide part might be a separate download, now.]

You could also go to national.com and use their Webench app's "power" option, and probably get it designed automatically, on line, but using a national IC.

However, if you CAN use a virtual ground, then something similar to the "rail-splitter" scheme presented in the LM675 datasheet, at national.com, can work well, even for higher current and power requirements, if a capable-enough amplifier is used. I have a production test-instrument design that uses the LM1875, similarly.

Good luck.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

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#### kchriste

##### New Member
Forum Supporter
Unfortunately i am not able to isolate the ground points so i cannot simply use a LM78L05 and LM79L05 combination so i am still trying to find a suitable solution.
You don't need to isolate the grounds. That's the whole point of the 555 charge pump circuit; to provide a negative supply with the same ground as the positive supply. I've drawn in the regulators for you:

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#### gootee

##### New Member
What was the minimum current requirement? 360 mA per rail?

By going to national.com and selecting Webench-->Power, then entering an input voltage range and an output voltage and max current, you can get a list of recommended regulator ICs for your application, with basic specs and links to their product pages.

Each product page has a datasheet link, and maybe some application note links, etc.

Better yet, many product pages have buttons you can click on to order free samples of the ICs, and very often a button to order an "evaluation board" PCB, to build your power supply on, which typically costs $25 to$30 for one board.

Anyway, I went and looked at some of them:

The LM2611 datasheet, http://www.national.com/ds.cgi/LM/LM2611.pdf , has a 12V to -5V converter schematic (see Fig 12, and Fig 11), that's good for up to 375 mA. It's a 1.4MHz Cuk-topology converter, which apparently has only 1 mV of output ripple, and should be very cheap to build, since its high frequency means that the inductors will be quite small. The LM2611 product page is at http://www.national.com/pf//LM/LM2611.html . An evaluation board and free samples of the IC are available.

This national.com appnote, http://www.national.com/an/AN/AN-1157.pdf , has a schematic for a 12V to -5V converter that's good for up to 1.5 Amps load current, using an LM2673-5 IC (Product page is at: http://www.national.com/pf//LM/LM2673.html ). It's in Figure 6 of AN-1157 (although note that the caption under Fig 6 is incorrect). It only needs one inductor, 33 uH rated for up to 3 Amps, which should be very cheap to buy. An evaluation board and free samples of the IC are available.

I'd use some small toroids, for the inductors. Mouser.com and Digikey.com both have lots of them.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

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