How to generate clean +/-12V for op-amps from +5/3.3V?

Yes. A boost converter for +12V and an inverting switcher for -12V. Art of Electronics and switcher app notes show how. You can use PWM chips but for cost reasons I prefer doing it with 74HC14 as duty cycle controlled oscillators.

Most opamps have good psrr at low frequencies, so dc/dc converters, followed by adequate bypassing (or lc filters, in the extreme) is generally good enough. So you can usually dump the ldo's.

You can buy little SIP +5 to +-12 or +-15 converters from stock, and are often worth using if your volume isn't very high. Mouser and Digikey have lots of them.

John

1. Configure a single DCDC for + and - outputs. There are many ways to do this with the addition of only rectifiers and capacitors.
2. Operate the DCDC at a reasonably high frequency, so that filter components are small.
3. Use a low-EMI layout, and position the DCDC as far from the analog circuits as possible.
4. Use additional stages of filtering, including common mode filtering if necessary. Consider using RC filters for light loads, LC otherwise.

There is no reason why you can't end up with very clean rails, although regulation will probably not be as precise as with the additional LDO stages. Paul Mathews

Right - page 359 - uses MAX633 and MAX637. I avoid Maxim at all costs due to their inability to actual sell the parts that they list on their website. But still - the problem remains that you end up with 4 ICs, unless you skip the linear regulation which in some cases is probably OK. But even two chips - it seems like this could easily fit on one?

-Michael

There is no silver bullet. You will have to provide for a switcher with the sufficient filtering after it.

However:

1. If the supply load is only a few milliamps, you can use MAX-232 (or similar) as a power convertor.

1. Depending on your device, it could be possible to use the +/-5V rails or a single supply topology.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

As previously mentioned, I need about 50ma on each.

Unfortunately, that isn't possible in this case. I'm driving a device with +-10V. The op-amps (powered by the +-12V) are generating the

-Michael

Can you give an example of this? I am unclear in how you would use a single output DC/DC switcher to individually regulate two different outputs.

-Michael

Linear's LT1533 could be an option. Noise in the 10's of microvolt range. Not the cheapest of solutions

Martin

Yes, I usually use one 74HC14, then rent out the remaining inverters to the highest bidder ;-)

I'd give the opamps a wee bit more than +/-12V then. RR types are pricey, unless you need them for other reasons.

I think the transformer required would make it just as large if not larger than other solutions.

Gotta love those old datasheets - they don't say anything about the various transformers used (besides model numbers) and those models seem to have vanished off of the face of the earth.

-Michael

I'm using Analog OP747s. This is not a cost sensitive application, and I have alot of them around as I often use them for single supply applications.

-Michael

Yikes, you must have a nice cushy BOM budget. AFAIK those can drive to within a volt from the rails. Probably not very fast up there close to the rails but maybe it doesn't matter in your application.

I think midcom/ midicom (?) make off the shelf transformers of the correct ratios, they are quite small AFAIR.

found it

Martin

The development time for the electronics here normally costs about

-Michael

So why do you care much about the parts count? Just curious..

Best regards, Spehro Pefhany

I was just going to ask that :-)

Maybe space reasons. Anyhow, if cost doesn't matter, why not buy some tiny switcher modules in the can and whoopdidou, Michael's R&D costs would reduce down to making a model for the CAD program?

Exactly.

If you can point out some really small modules I'd be all ears - but it seems like all the ones I ever see are really tall or consume a large amount of board space.

-Michael

You can't, but Paul didn't say "regulated" outputs. He just said "+ and - outputs".

You can't regulate two independent outputs with one control loop. You can regulate one of them, or the average of the two, for instance.

Best,