multi-voltage dc power supply /  LT3479 - Page 1
Forum section: electronics / projects

author: grips03
author_rank: Contributor
timestamp: February 24, 2014, 09:07:24 pm
content: I'm looking for a power supply with 5vdc or 12vdc input and outputs 5vdc @1a, 12vdc @ 1a, 10vdc @ 1a and -5vdc @ 1a. Any recommendations?If no such thing exists or something close, I know finding 10vdc and -5vdc might be a pain. Then I would likely build using LT3479. So the second question is there a similar part to the LT3479 that you would recommend over this part?thanks

author: mariush
author_rank: Super Contributor
timestamp: February 24, 2014, 09:57:39 pm
content: You could start with a simple 12v 4-7a power supply - plenty of these for laptops and monitors.The 10v 1A you can get with a 7810 or a LM317:http://www.digikey.com/product-detail/en/UA7810CKCS/296-21621-5-ND/1494014http://www.digikey.com/product-detail/en/LM7810ACT/LM7810ACT-ND/1954729http://www.digikey.com/product-detail/en/LM7810CT/LM7810CT-ND/1954730The regulator has a voltage drop of 2v at 1A, which works out just fine for 12v input. It will only waste 2 watts as heat, which means a simple heatsink will be fine. Switchers almost don't make sense, you wouldn't get much higher efficiency.For the 5v, a simple lm2576 will do the job.For the -5v, at 1A it's a bit tough. A plain MC34063 will do about 500mA from 12v input...not aware of other ICs that could do 1A off the top of my head.

author: grips03
author_rank: Contributor
timestamp: February 25, 2014, 03:44:00 am
content: thanks for the quick reply. 12v vdc adapter input seems like the way to go for sure.for 12v, just take from the adapterfor 10v, I like your idea of 7810 and the price is good too7805 or lm2576? Is the lm2576 better? Seems to have higher current rating which is good as I think the product it will power most uses +5vdc.MC34063 - seems to be very flexible for sure, boost, buck, inverting etc.Since device exists that it will power I'll measure the current this weekend and perhaps MC34063 will work.thanks

author: don.r
author_rank: Frequent Contributor
timestamp: February 25, 2014, 04:07:46 am
content: The 7805 will see a 5v drop off the 10v line or a 7v drop off the 12v line. So you are either looking at 2 extra watts for the 7810 and 3 watts for the 7805 or 7 watts for the 7805. The LM2576 will be more efficient (probably about 80% so 2 watts) but will add some 52Khz noise and require a few extra parts.

author: mariush
author_rank: Super Contributor
timestamp: February 25, 2014, 07:29:17 am
content: 7810 can only output 1A-1.5A, depending on who you go with. LM317 may do 2A.So if you go with 7810, you can't use its output as an input for 7805 because if you pull 1A on 5v and 1A on 10v, your 7810 needs to produce 2A of power (and it can't).The LM317 may be able to but it's still not a good idea.A 7805 will drop the voltage difference as heat, so if you give it 10v and output 5v at 1A, you'll have 5w wasted as heat, and 5w is harder to spread and keep the IC within reasonable temperatures.Your 7805 will have basically 50% efficiency - ouput 5v @ 1A and consume 1A from 10v .. wasting 5w for 5w produced.A lm2576 or lm2596 or a similar switching regulator will have about 70-80% efficiency, so it will consume about 8 watts to produce 5w (5v @ 1A). So from the start, you're only doing 3w of heat at max 1A output, which is more easily managed. If powered from 12v, you'll basically use around 12v @ 0.7A to produce 5v@1a. The efficiency is also better if you power it from higher voltage, so no need to worry about the 7810 having to do its 10v @ 1a job AND feed the 5v regulator.Those 3watts will also be spread on the switching chip, the schottky diode and the inductor so it should be just as easily to cool as the 7810 regulator. You can get ready made boards with LM2576 or LM2596, just search eBay for these chip names, they're cheap and even if the chips themselves are chinese clones, even the cheapest clones can do 1A of current with no problems.If you want smaller size,LM2679runs at higher frequency (500kHz) so it needs smaller inductors and capacitors but it's a bit more expensive and you only gain about 5-8% efficiency (85-88% efficiency overall)Yes, MC34063 is quite versatile but it doesn't have as many protections as a proper switching regulator and it's also kind of limited due the maximum current limit of the internal transistor, which is 1.5A. So yeah, the chip may say 1.5A max but that's not under any condition, you have to use the formulas and determine if that peak current is reached. For 12v, -5v out @ 500mA, that peak current is almost reached ... 1491 mAIt also can't do 5v@1A from 10v or 12v, it can probably do 5v@700 mA from 12v with about 65-70% efficiency. LM25x6 chips are better, they can do up to 3A or so.You have a "calculator" of sorts here :http://www.nomad.ee/micros/mc34063a/and there's also a calculator on sourceforge for this chip if you want to play around with values :http://sourceforge.net/projects/mc34063uc/If you want to try other chips, a possible suggestion would be downloading LTSpice from linear technology and then try simulating some of their inverting switching regulators , the models should be included in the LTSpice package. LT chips are a bit expensive though (but very good), usually 4-10$ in one piece quantity.For example see LT1070, LT1170, LT1270, LT1071, LT1076, LT1172 etc etc

author: grips03
author_rank: Contributor
timestamp: February 25, 2014, 04:58:10 pm
content: very helpful stuff. Thank you