Multiple power supplies from 12V dc source

Your plan to use switchers for the 5 and 3.3 Volt supplies is good, then Select LDO regulators for the 2.5, 1.8, and 1.2 Volt outputs.

I did this for a board recently but the currents were much worse, with the 1.2 V requiring 900 mA. So I had a lot of power lost, but the scheme is good.

I'd look at making this a parasite supply on the 3.3V regulator producing about 6V and then a LDO making it into 5V

(12-5)*0.3 = 2.1W so a linear would need a heat sink. More electronics is often cheaper than adding a heat sink.

This really needs to be a switcher.

I'd look for a multiple LDO.

Yes that was the scheme I was thinking off. Just that it's a lot of components.

I also need some sort of supply/brown out protection to reset the CPU. The

I've tried this on an earlier design which worked apart from introducing a lot of noise on the +3.3V rail. I assume this was because current was no longer nominally constant, as in a straighforward step down voltage converter, where chunks of current were now taken out by another winding.

I am not very keen on this level of heat dissipation!

I haven't seen any. Where can I get one? Do they really work out cheaper than separates?

Many thanks.

It is best to make the parasite winding the lower powered one and to put a significant amount of bypass on it. There is a funny non-linear step responce issue in supplies like this. Bepending on the sizes of the filter caps, The 3.3V supply can't swing up as fast as it swings down or down as fast as up. Generally the servo loop of the regulator has to be designed so that it can have its gain cross over anywhere in a widish span of frequencies.

Intentionally making the coupling to the parasite winding a bit sloppy can help loop stablity.

The variations in current on the 5V supply will effect the 3.3V supply if you are not careful. Since the 5V supplies regulation doesn't need to be so tight, you may want to slip a resistor somewhere in that part of the design to force any current spikes to be taken from the capacitors and not from the windings.

I think Linear has one. They cost more but the stuffing cost could pay it back.