Unregulated dc-dc isolated converters

[Asim]

Hello everyone,

I am making a project that needs +5v,+12v,-12v supplies. I need it to be portable and small so I had the idea of using a power bank that will give me the 5v. I will also use a DC-DC isolated converter that will take the 5v and give an output of +15&-15 isolated with 1 watts for each rail.  These high rails will only power a dual op amp.
My question is:
Can i get away without regulating the rails?
Expected current is less than 40 mA for the high rails

[Asim]

The op amp will be the front end of a simple function generator

[kizzap]

It depends on the op-amp. Also it depends on the noise that the DC-DC converter puts out.

[Seekonk]

It is likely that it can, but do you REALLY need two rails?. 

[T3sl4co1l]

Does it have to be isolated?  Can you not stack 3V above and below the 12V rails?

Also, why does it require 12V rails if you obviously require 15V rails?

Tim

[macboy]

Does it really need to be isolated, that is, floating with respect to the +5V supply? Do they really not share a common ground?

Old thin net (10Base-2) ethernet cards often had an isolated 5 V to 9 V (or 12) converter. This converter powered the 10Base-2 Phy, which could not be ground referenced to the computer, otherwise there would be a huge ground loop created by the ethernet cable. With 10Base-T and other twisted pair ethernet, the isolation uses a transformer on the ethernet signal instead. If you can find one of these old ethenet cards, you could salvage the converter. You can of course buy these too.

After you have one 9 V supply, getting the other is easy. There are common small ICs to provide a negative rail from a positive one, like MAX1044, TC0144, LT1054, etc.

If you don't need an isolated supply, and if you can operate from +/-5V, then just use the converter IC above to create the -5 V rail. Using XR-2206? Try it this way.

[Asim]

Actually it is DDS, the 5 v for the microcontroller. The other rails for the op amp so i can have a signal that i can vary its amplitude and bias

[Jeroen3]

You might get away with an inverting supply.
+12V input, LDO to +5V, and than you invert some to -12V.

An isolated supply (such as recom RB-0505S) makes it flexible, since they don't care if they are boosting or inverting, or just isolating.
Those isolated recom modules aren't regulated very strictly, but you can get very efficiënt LDO supply IC's for a little bit of power.