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
**broken link removed**
-