Using a LDO to go from 13v DC to 3.3v DC, bad idea? - Page 1
Forum section: electronics / beginners

author: ZeroAviation
author_rank: Contributor
timestamp: January 26, 2017, 05:45:22 pm
content: All,I am building a little project, it will be used in the automotive environment so I know there will be a lot of AC noise from the alternator.The project will consist of several LED's, STM32F4 MCU, Can Transceiver, and FT232... Should be about 800-900ma of current draw.My question is, would it be ok to use a LDO such as MIC39100-3.3WS with a wide input voltage range to step down from 13-14vDC to 3.3V for VCC with plenty of filter caps? I'm thinking i will have about 9w of dissipation, which I think is probably too much to dissipate with the pcb.I am tight on space, is there a better switching solution with a small inductor?Thanks

author: Dave_PT
author_rank: Frequent Contributor
timestamp: January 26, 2017, 05:53:55 pm
content: 9W is a lot!This regulator will not take it!Use a step-down circuit. There are a lots to suit all tastes and prices.Look for in this link the circuit that best suits what you want to do.http://www.digikey.pt/products/en/integrated-circuits-ics/pmic-voltage-regulators-dc-dc-switching-regulators/739?FV=ffe002e3&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=0&pbfree=0&rohs=0&quantity=0&ptm=0&fid=0&pageSize=25

author: retrolefty
author_rank: Super Contributor
timestamp: January 26, 2017, 05:55:58 pm
content: Quote from: ZeroAviation on January 26, 2017, 05:45:22 pmAt that current requirement only a SM DC/DC regulator makes sense. Size of the inductor is dictatedby the current requirement, not negotiable. So it will either fit or have to go off board.

author: Dave_PT
author_rank: Frequent Contributor
timestamp: January 26, 2017, 05:58:32 pm
content: For example the MC34063 is very cheap and simple to use.I use it frequently...

author: Dave_PT
author_rank: Frequent Contributor
timestamp: January 26, 2017, 06:01:03 pm
content: Quote from: ZeroAviation on January 26, 2017, 05:45:22 pmIf you find a DC/DC with a high working frequency (1 MHz for example), the inductors are smaller ...

author: Dave_PT
author_rank: Frequent Contributor
timestamp: January 26, 2017, 06:08:48 pm
content: Have you looked at this regulator?AP3211KTR-G1http://www.diodes.com/_files/datasheets/AP3211.pdfDo the math and see if it suits your project.

author: JPortici
author_rank: Super Contributor
timestamp: January 26, 2017, 06:12:03 pm
content: one i'm going to play with is AP3211 (and the companion app note for design)SOT23-61.5A peak1 MHz frequencythen as already said inductor size is also a of acceptable ripple current/voltageyou may also do 3.6 - 4V then LDO

author: mariush
author_rank: Super Contributor
timestamp: January 26, 2017, 07:00:48 pm
content: Use a switching regulator at a high frequency, which allows you to use small inductors and ceramic capacitors or polymer capacitors. If you go with ceramic capacitors, make sure you keep in mind the big fluctuations in temperature that may happen inside the box that will be in your car... the capacitance of ceramic capacitors could vary a lot with temperature.Don't use cheap mc34063 and variations, it's ancient chip running at low frequency and with barely any protections.Get something that can handle voltages higher than 13-15v, because you may have higher voltages at some moments due to car's alternator and other things.Have a look at something likeMIC4680 from Micrel / MicrochiporCS5173 from ON Semi(which is pin-to-pin compatible with LT1372/1373 (that are great but more expensive)

author: ZeroAviation
author_rank: Contributor
timestamp: January 26, 2017, 08:26:06 pm
content: Thank you all for your input!I will go with the MIC4680, I have limited knowledge of switching regulators, but this one seems to be the best option! I will probably use a UP2B-680-R inductor, the size isn't terrible for my PCB footprintCheers!

author: suicidaleggroll
author_rank: Super Contributor
timestamp: January 26, 2017, 08:41:28 pm
content: Make sure you put in PLENTY of protection on that input rail. Automotive electrical systems are horrible environments for electronics. The circuit should be able to handle up to 60V, down to ~6V, reverse voltage, and should be able to withstand brief pulses of +/- a couple hundred volts. See here for more recommendations, especially regarding fusing, ceramics on the input rail, and large package parts (eg: 1206).https://www.eevblog.com/forum/projects/design-of-automotive-electronics/

author: langwadt
author_rank: Super Contributor
timestamp: January 26, 2017, 10:09:43 pm
content: Quote from: ZeroAviation on January 26, 2017, 05:45:22 pmfor a one-off a module like thishttp://www.ebay.com/itm/321829513465is cheaper than the components,just make a footprint for it and solder on board

author: JPortici
author_rank: Super Contributor
timestamp: January 26, 2017, 11:10:02 pm
content: Ah! i missed the automotive part.where will the board be located? all those LEDs and current make me think that it will be inside the body, not under the hood.if this was the case the environment is much more forgiving900 mA seems a lot even if you have a screen