lm 2576 or lm2678 for step down psu?

[little_carlos]

hello guys, im going to make a 1.2  to 24v 3 amp psu and I have available near my area an electronics shop that sells both ics, which one should i pick? the lm2678 its more expensive(around 10 dollars us) and the lm2576 is half the price, ive read the datasheet of the 2678 and it says it can be up to 92% efficient, the lm2576 doesnt give that info, just says "high efficient"
also, ive noticed that the lm2678 requires a few extra components and a few extra details to take care about
whats your opinion?
and please dont tell me to use other ics, since those are the best i can pick at the moment

[michaeliv]

Personally I would go for the lm2678 just to have a little extra headroom. The lm2576 is exactly 3A. So if you actually plan on using it continuously at 3A, go for the lm2678. If not, the other would do just fine.
I doubt that efficiency is any better - I can't find efficiency info for the lm2678 adjustable version. For the lm2576 there's an efficiency graph.

[little_carlos]

Personally I would go for the lm2678 just to have a little extra headroom. The lm2576 is exactly 3A. So if you actually plan on using it continuously at 3A, go for the lm2678. If not, the other would do just fine.
I doubt that efficiency is any better - I can't find efficiency info for the lm2678 adjustable version. For the lm2576 there's an efficiency graph.

thanks man, the 3A its the max value, i would push the transformer up to that for a long time, more like 2.5A continously at the max situation
i think il go for the lm2576 since its cheaper and  the efficiency is theorically  the same

[mariush]

I would go with neither, especially at ten dollars.

You can find ready made boards that do what you want, with chinese clones of LM2576 or LM2596 for 2-3$ and free shipping included, here's just one example: http://www.ebay.com/itm/DC-DC-Buck-Step-Down-Converter-Module-LM2596-Voltage-Regulator-Led-Voltmeter-/160995621268?hash=item257c163994:g:uUsAAMXQQQZRyqOI

Regarding efficiency, carefully read the datasheets for such chips. Even though you can make an "Adjustable" power supply out of these chips, in real world you're more or less supposed to know the input voltage range (for example 18-26v DC) , the output voltage (for example 12v) and approximate current output (for example 2A) your device will consume and pick parts like inductors and diodes to match these parameters.  The inductor that would get you 24v in, 12v out at 2A with let's say 90% efficiency won't be the best inductor to get 12v in , 5v out at 3A , same for the diodes... boards like the one above will have some average inductors and diodes that will get you average efficiency on the whole range of output.

Anyway, there are much better switching regulator ICs out there, if you're willing to pay $10 for a chip at the local shop you could just order one from various distributors like Digikey or Mouser or Newark / Farnell or TME.eu (both in Europe), the shipping is usually less than around 5 dollars/euro

See for example ROHM BD9781 , you have in the datasheet at page 9 and 10 some detailed description of how to choose the other parts around the chip and example circuit  : http://uk.farnell.com/rohm/bd9781hfp-tr/dc-dc-conv-buck-adj-o-p-500khz/dp/2420830 
It can still be soldered easily on a prototyping board (though since it runs at higher frequencies like 500kHz compared to LM25** 60-100kHz it should really be soldered with the inductor and diodes on a proper pcb)

Another better IC and easier to work with (TO-220 like the LM25** ICs which makes it easy to play with on prototyping board) would be MIC4576 (page 5 of datasheet shows circuit example with adjustable output) : http://uk.farnell.com/microchip/mic4576wt/dc-dc-conv-buck-200khz-to-220/dp/2510163

later edit : and as datasheet says, micrel's application note 14 explains in detail how to choose components to get the best out of the regulator chip : http://www.micrel.com/_PDF/App-Notes/an-14.pdf

You should be able to find these ICs at US distributors and in the case of MIC4576 you may even be able to request a sample from Microchip or order from Microchip's website.