+12 -> -12 buck converter design? - Page 1
Forum section: electronics / beginners

author: nsayer
author_rank: Regular Contributor
timestamp: May 29, 2014, 11:18:37 pm
content: I've been using DC-DC converter modules on my Hydra boards, but I've decided to see if I can save a couple bucks (see what I did there?) by making my own buck converters. I was able to design one for +12->+5 @ .5A without too much difficulty, but I'm not finding a lot out there that will make -12 @ 50 mA from +12 (or +5 for that matter). The designs I have seen have easily twice as many components as the +12->+5 that I've got queued up, and they make me give second thoughts to replacing the bipolar DC-DC and just going from 2 down to 1 instead of removing them altogether.

author: mariush
author_rank: Super Contributor
timestamp: May 29, 2014, 11:28:14 pm
content: MC34063 ? (edit : just search digikey/mouser/farnell/newark for 34063, or 33063 or 3063 as in az34063, ncp3064, ncp3063 etc etc it's made by lots of companies)Freeware calculator:http://sourceforge.net/projects/mc34063uc/Online calculators (less complex) :http://www.nomad.ee/micros/mc34063a/

author: nsayer
author_rank: Regular Contributor
timestamp: May 30, 2014, 02:15:05 pm
content: Hey, thanks to you both - between the two of you, I was able to see section 5 of this document:http://www.ti.com/lit/an/slva252b/slva252b.pdf,which looks like just what the doctor ordered.

author: rob77
author_rank: Super Contributor
timestamp: May 30, 2014, 02:30:01 pm
content: couldn't agree more. the mc34063 is a versatile (buck,boost,inverting) and CHEAP. it's avilable in DIP and SMT (actually it's dirty cheap here ~25 cents for SMT and <50cent for DIP). you can build a DC converter under 2-3Euro using it.

author: nsayer
author_rank: Regular Contributor
timestamp: May 30, 2014, 05:13:53 pm
content: I've got one more question. I think I may have an answer, but want to make sure I'm right.http://www.st.com/web/en/resource/technical/document/datasheet/CD00001232.pdfFigure 17 looks like nearly *exactly* what I want.I could drive it from my +5 rail, but that rail is generated by another dc-dc converter from +12. Cutting out the middle-man would seem to be a good idea.But what are the implications of raising Vin on that sample circuit from +5 to +12 volts?Looking at the math just below, it *appears* that I would need to raise the value of L1 from 90 µH to, oh, I dunno, maybe something closer to 150 µH. Other than that, the resulting chip-apparent Vcc of 24 volts is in spec, and provided the correct voltage specs are kept for the caps... am I missing anything crucial?

author: krivx
author_rank: Frequent Contributor
timestamp: May 30, 2014, 05:26:33 pm
content: Another option if you need inversion and not a huge amount of current is a charge-pump inverter. You can avoid using inductors, you usually just need two external capacitors which are specified in the datasheet. Linear have a few nice ones, you might need to shop around if you need a very cheap solution:http://www.linear.com/product/LTC3261http://www.linear.com/product/LTC3260

author: nsayer
author_rank: Regular Contributor
timestamp: May 30, 2014, 05:34:57 pm
content: The LTC3261lookslike a great solution, but it's a dollar more costly than the DC-DC converter I'm already using.

author: krivx
author_rank: Frequent Contributor
timestamp: May 30, 2014, 06:31:56 pm
content: Quote from: nsayer on May 30, 2014, 05:34:57 pmMicrochip do some cheaper partshttp://www.digikey.com/product-detail/en/TC962COE/TC962COE-ND/418719

author: nsayer
author_rank: Regular Contributor
timestamp: May 30, 2014, 08:25:36 pm
content: Quote from: nsayer on May 30, 2014, 05:13:53 pmI watched the video and found that there were a couple of formulae missing from the ST data sheet (for individually figuring out Ton and Toff). On Semi's is much better (http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF).I've decided to make both the +5 and -12 supplies using this chip.For the +12 -> -12 @ 50mA, I got Rsc=2.4?, Ct=200pF, L=470µH and Co=68µF.For the +12 -> +5 @ 500mA, I got Rsc=0.16?, Ct=200pF, L=15µH and Co=68µF. I must make sure the diode will handle 2A peak current.(some of those part values are rounded to the most convenient nearby)Of all of the parts, it appears that the 0.16? resistor is the fiddliest one. Why that can't just be a jumper is beyond me. I mean, I found one at DigiKey, but it's an order of magnitude more expensive than the other 0805 resistors I buy.

author: mariush
author_rank: Super Contributor
timestamp: May 30, 2014, 08:52:30 pm
content: Use that calculator I linked to in a previous post.Here's some examples:Vin = 11-13v , Vout = 5v +/- 0.1v @ 500mA , Rsc = 0.27 , Lmin = 38.5 uH 1A , Ct = 330pF, diode 1n5819 , Cout = 330uF 0.081 ohm esr or better (pretty much anything low esr 330uF 10v or better should do it)Vin = 11-13v , Vout = -12v +/- 0.1v @ 200mA , Rsc = 0.27 , Lmin = 87uH 1A , Ct = 330pF diode 1n5819 , Cout = 150uF 0.11 ohm esr or better0.27 ohm is relatively standard size, if not you can get two 0.56 ohm and put them in parallel, or get 3 1ohm resistors and put them in parallel (it will be close enough).The inductor values are the minimum, use standard values... 39uH, 47uH, 68uH, 100uH ... if you get better price buying in volume and size isn't an issue, get 3 x 47uH , use two in series for the -12v to get that 87uH. Make sure the ones you get have a peak current of 1A or better and lowThe capacitance is the minimum to get a low ripple, you can use more capacitance without any problems. you don't save money going with the lowest 68uF capacitor (hence why i chose two standard values, 330uF and 150uF)

author: rob77
author_rank: Super Contributor
timestamp: May 30, 2014, 09:09:46 pm
content: that calculator is nice, i was not aware of it's existence until you pasted the imagesi'm using the mc34063 every time i need to make small power DC converter, but i was always kind off "eyeball-ing" the initial values and then adjusting them based on measurements

author: nsayer
author_rank: Regular Contributor
timestamp: May 31, 2014, 04:59:22 pm
content: In case anyone's interested, here's the final (?) outcome. The 34063 is now doing all of the DC-DC conversions necessary at a cost savings of close to $6 (in unit quantities)!

author: mariush
author_rank: Super Contributor
timestamp: May 31, 2014, 05:52:16 pm
content: I'd be interested to see a picture with just the top layer and one with just the bottom one... or a schematic.It doesn't look so well layed out to me, especially that ground trace going all around the board.. and do you have to place the ISP header so far away from the atmega and use vias to route those wires?

author: nsayer
author_rank: Regular Contributor
timestamp: May 31, 2014, 06:32:19 pm
content: Quote from: mariush on May 31, 2014, 05:52:16 pmThe schematic was attached.QuoteThere's actually a ground plane on both sides that isn't displayed. The thick traces are either the +12 or +5 rail, depending on which one.QuoteYes. There is a 2x16 LCD module that dominates the other side of the board. The connectors (including ISP) mount on the "bottom" along with the LCD, and so must be placed out of the way. That part of the design is fairly well proven. And, of course, programming happens only very, very occasionally, and in isolation with all of the rest of the functionality of the board.I'll attach a picture of the finished device. The board isn't the exact same variant, but you'll get the idea.

author: Skimask
author_rank: Super Contributor
timestamp: June 01, 2014, 05:37:37 am
content: LTM8045, my newish goto inverting/buck regulator. Piece of cake to use as long as you have a BGA adapter board

author: nsayer
author_rank: Regular Contributor
timestamp: June 01, 2014, 07:05:51 am
content: Quote from: Skimask on June 01, 2014, 05:37:37 amGoodness! That's a tremendously expensive part, isn't it?

author: nsayer
author_rank: Regular Contributor
timestamp: June 02, 2014, 12:51:29 am
content: Something just suddenly occurred to me and has me a bit concerned.The resistor between Vcc and SEN, OVR and SWE is going to take Ipk amount of current, isn't it? For the 500 mA 12->5 converter, that's an amp.Um.That's alot, isn't it?P=I^2*R... That's going to have to be a 1/3 watt resistor, no?

author: nsayer
author_rank: Regular Contributor
timestamp: June 21, 2014, 02:43:20 pm
content: The boards arrived, and it all works! The +5 supply shows around 40 mV of ripple and the -12 supply shows something like maybe 100 MICROvolts of ripple! I don't know for sure how much the 5 volt current is, but the 12 volt current for the whole board is just under 50 mA.I'm quite satisfied for my first try!