Multiple buck converters from single supply - Page 1
Forum section: electronics / beginners

author: Silenus
author_rank: Contributor
timestamp: December 23, 2014, 03:04:35 am
content: Hey guys, I am trying to get both 5V and 3.3V outputs from 12V input. One of the ways I am considering is using two single fixed buck converter LM2576's however I am unsure if I have connected them in the correct way. Is thisa sound way to connect up two of these circuits with OUT1 and OUT2 being the 5V and 3.3V outputs?

author: Mad ID
author_rank: Regular Contributor
timestamp: December 23, 2014, 08:32:08 am
content: Decoupling cap is missing on U2. Also I see that you have put 200uF at U1. Since this is not a fast ceramic capacitor I recommend you put 1uF ceramic at the input of both.

author: Silenus
author_rank: Contributor
timestamp: December 23, 2014, 12:32:00 pm
content: Quote from: Mad ID on December 23, 2014, 08:32:08 amI was thinking of that earlier, and the datasheet I used didn't have a ceramic in the input decoupling so I didn't think it was necessary there. Wouldn't the decoupling cap be common to both inputs as per the circuit, and would this be okay as long as the cap is close enough to both chips? or should I have decoupling for each chip regardless?

author: rob77
author_rank: Super Contributor
timestamp: December 23, 2014, 12:42:11 pm
content: Quote from: Silenus on December 23, 2014, 12:32:00 pmno, each chip has to have it's own decoupling cap. and in general - when talking about decoupling capacitors, it's safe to assume a electrolytic + ceramic capacitor.imagine a situation when you and a friend got one bottle of water and 2 glasses - you can fill 2 glasses with water and anyone of you two can drink whenever you want.... now imagine a situation with 1 bottle and 1 glass.... you can't drink BOTH at the same timebottle = power supply, glass = decoupling cap.

author: con-f-use
author_rank: Unknown
timestamp: December 23, 2014, 02:44:31 pm
content: Or you make the glass large enough and use two big straws.

author: rob77
author_rank: Super Contributor
timestamp: December 23, 2014, 03:37:38 pm
content: Quote from: con-f-use on December 23, 2014, 02:44:31 pmnice try ! and extra points for the yellow bikinibut you would need a straw with a diameter of your mouth to achieve the same rate of flow as drinking directly

author: con-f-use
author_rank: Unknown
timestamp: December 23, 2014, 07:31:39 pm
content: Nope, time averaged flow is smaller due to a series choke.

author: Silenus
author_rank: Contributor
timestamp: December 24, 2014, 03:40:46 am
content: Quote from: rob77 on December 23, 2014, 12:42:11 pmCouldn't you just treat both chips as a single entity that has a different power requirement?

author: rob77
author_rank: Super Contributor
timestamp: December 24, 2014, 08:52:34 am
content: Quote from: Silenus on December 24, 2014, 03:40:46 amno because each chip will switch at different frequency and/or phase.btw.. the decoupling is the second cheapest part (right after resistors) of an average design, it really makes no sense to try save on this

author: Silenus
author_rank: Contributor
timestamp: December 24, 2014, 09:42:01 am
content: Quote from: rob77 on December 24, 2014, 08:52:34 amYeah makes sense, and the cost was never really an issue, I just like to understand these things better. Thanks for the help

author: dan.soethe
author_rank: Contributor
timestamp: December 24, 2014, 06:10:18 pm
content: Why can't you use 7800 series voltage regulators? They come in 5v and 3.3v versions. All you need is a big cap on the input and a small cap on the output. Nothing could be easier...

author: mariush
author_rank: Super Contributor
timestamp: December 24, 2014, 07:17:46 pm
content: The voltage difference between 12v and 5v is quite high, with a linear regulator a lot of energy would be wasted as heat with linear regulators.Ex with a 7805, to get 5v @ 1A you'd waste (12-5) x 1 = 7w as heat to produce 5w of power. That's less than 50% efficiency.A LM2576 will give you about 70-80% efficiency so it would be less heat produced, less waste. Instead of 7w wasted (for the 5v@1a example), you'll have maybe 1-2w of waste.In the case of 3.3v rail, there's two possibilities. You could go with a 12v->3.3v converter using that lym2576 but this kinda makes sense only if you need a lot of current on 3.3vFor example, if you need less than 0.5-0.75A on 3.3v, it would be much cheaper and not much less efficient to use a linear regulator to get 3.3v from the 5v produced by the first dc converter. A basic, generic, cheap 1117 linear regulator needs only 1-1.2v above output voltage to work properly so it would work fine to output 3.3v with only 5v at the input.For example, to get 3.3v @ 0.5A = 1.7w, with 12v to 3.3v dc converter you'd use about 2-2.1w to do it... with a linear regulator you'd use 5v x 0.5a = 2.5w .. so only about 0.4w difference.In addition, by producing the 3.3v from the 5v,you would basically give the 12v to 5v converter a sort of minimum load which will increase the average efficiency - you choose the components for the 12v to 5v converter thinking of the maximum output current but the efficiency will be best at a narrow range of output current. So it would good to have the output current above some minimums almost all the time.