Any good resources for DC power supply design?

Hi - I end up making a lot of simple power supplies for my PCBs. Most everything has been DC to DC - lots of linear regs, a couple dc-dc step downs, even one step up. But I normally just choose a part and go by what its datasheet says. Are there any good resources for what sorts of considerations should go into choosing parts to give a clean, stable, accurate power supply for a PCB? I'm talking about low voltage, low current stuff. Normally I have a somewhat noisy 12V signal coming into a PCB and I'll need 3.3V or 5V at a couple hundred ma from it - so supplies for that sort of situation are what I'm most interested in.

Any books/websites/app notes/etc.?

Thanks!

Have a look at the HP Power Supply Cookbook:

Dave :)

If you are really after a "nice, clean and stable" supply it's hard to beat some of the "low noise" linear regs available. It's common to use say a normal DC-DC converter to get your base efficiency, and follow that with a nice low noise linear reg for powering sensitive stuff like high end ADC's. The linear reg need only drop a volt or two, and current requirements for parts that require low noise supplies are usually small, so the linear reg doesn't affect your overall efficiency much.

Dave :)

All the usual suspects have good app notes.

Linear tech TI Analog Devices Maxim (but see the old thread about availability) National Semi On Semi

and more, of course.

Each manufacturer naturally targets their own devices, but as a good general view, Linear Tech does the best (imo, ymmv) app notes on such things.

Cheers

PeteS

Hi Dave - so for example on a current project I have a 12V power source. It's a li-poly battery - so it fluctuates around that a bit. It has a widely varying load on it, so it will not be particuarly clean. I need to use this to power a 16b ADC board that has many sensitive analog sensors on it. The board needs a 5V supply. It shouldn't draw more than 200ma peak. I think 50-100ma will be the normal load. I also need to maintain as much efficiency as possible - I'd really like to see 90% or better. I'd like noise on this supply to be in the single digit micro volt range, ideally. (being that it's powering a 16b ADC)

So are you saying that the ideal solution would be to use a DC/DC converter to bring the voltage down to, say, 5.5V, and then use a linear regulator to bring that down to 5V? Can DC/DC converters just not be accurate enough?

Thanks,

-Mike

Yes. It might be overkill, or it might not, depending on your application. Lots of system factors are involved, but it can't hurt to start with a nice low noise regulated supply. I have done this with high end 24bit sigma-delta converters using a low noise low dropout reg like the Micrel 5255:

In one case, I used a high efficiency custom designed 70-90VDC to 5V DC-DC converter, and then the extra low-noise regs from the 5V rail for the 24bit converters which didn't comsume much power on their own. The 5V powered some higher power digitial stuff where the noise was not critical. I still got >90% system power efficiency.

As a rule you would not power a 16bit ADC directly from a regular DC- DC supply, you are begging for trouble.

They can be, but other system factors come into play. For instance, PCB layout can be critical. Get this wrong and it doesn't matter how low-noise your power supply is. You can blow it all with one bad ground track placement.

Dave :)

alas poor femptoFarad, it buggered him, Horatio

Cheers Terry

I'm currently looking at some integrated DC-DC stepdowns for both final 5V load and also as pre-regs to some linears.

RECOM (in deutchland) make some TO-220-pin-compatible switchers. I'm hoping to find other sources for these, as I also have a couple of 12V->5V linear regs in need of thermal relief.

Hi Mike, check this link

Srinivas EMITT Solutions

I always seem to be doing the same thing, my solution of choice now is to use integral swith type lm2594, and lowest esr caps you can and get the layout extremly tight, a second LC filter next to this helps a lot too and another LC next to the adc or as already suggested use low drop/low noise linear regs, but be aware linear regs arnt that good at rejecting switching frequency noise.

For higher power/efficiency a synchronous rectifier type is good such as dual mosfet driver type tps40054.

I always seem to need far more different voltages than I would like.

Colin =^.^=