Advice sought re. voltage regulators for FPGA/CPLD and othe LV digital circuits. - Page 1
Forum section: electronics / general

author: elcomtel
author_rank: Regular Contributor
timestamp: September 16, 2013, 03:51:46 am
content: I am after your opinions and advice.I've noticed some concerns expressed on electronics discussion forums about voltage regulators used on various FPGA development boards especially about 'cheap' development boards coming out of China using SOT-223 footprint devices such as AMS1117-1.2V, AMS1117-1.5V, AMS1117-1.8V, AMS1117-2.5V, AMS1117-3.3V, AMS1117-5.0V, fake LT1117 and LM1117,etc.I often buy a lot of electronic components from China (If element14, Jaycar, Altronics can source from China then why shouldn't I.) and I was planing to buy a quantity of (SOT-223 footprint) AMS1117-1.2V, AMS1117-1.5V,AMS1117-1.8V, AMS1117-2.5V, AMS1117-3.3V, AMS1117-5.0V (100 of each) for my in-house design and development activities.My instincts tell me that those cheap SOT-223 AMS1117 voltage regulators should be ok to use, but in light of some of the concerns (opinions on various electronic discussion forums) that have been expressed I thought I would address this question to the EEVblog audience.Your experiences and opinions about the use of 'cheap' voltage regulators sold on eBay for use with Xilinx and Altera FPGA/CPLD families would be highly regarded. Many thanks in advance.

author: marshallh
author_rank: Unknown
timestamp: September 16, 2013, 05:38:53 am
content: Buy Diodes Inc(Zetex) or STmicro branded parts from a legit source. 50 cents in singles.Unless you're shipping a huge volume why bother.If your power rail is screwed then you will run into problems down the line debugging wasting time. Would you put $30 tires on your brand new Ferrari?These regs (usually 800mA or 1A) will work to power small FPGAs but for anything bigger than pea-sized you will want a proper push/pull buck converter or in extreme cases a multiphase buck converter.Also I had a laugh at the marquee. I think it's been years since I've seen one. It is pretty tacky and unnecessary IMO.

author: free_electron
author_rank: Super Contributor
timestamp: September 16, 2013, 06:57:32 am
content: What fpga's are we talking avout ? A sot223 cant dissipate much .any fpga design with a bit of size is going to send those simple regs in thermal shutdown. Look for dpak regulators , for larger fpgas use switchers

author: elcomtel
author_rank: Regular Contributor
timestamp: September 16, 2013, 10:25:17 am
content: Quote from: marshallh on September 16, 2013, 05:38:53 amI have to agree with you and admit that I posted the heading with the marquee because I am a tacky sort of a guy. On the other hand an avatar of a cigar smoking cat is perfectly good taste. Have you ever considered the cruelty and suffering that poor animal suffered in order to create that avatar?Since I agree with you about the degree of my tackiness of my heading, I decided in the interest of maintaining harmony on this forum to change it to something more acceptable.

author: elcomtel
author_rank: Regular Contributor
timestamp: September 16, 2013, 11:19:24 am
content: Quote from: marshallh on September 16, 2013, 05:38:53 amQuote from: free_electron on September 16, 2013, 06:57:32 amThis is good...you guys have got me thinking. I was essentially copying the voltage regulation that exists on some development boards that I ordered. One is based on a Xilinx XC2C64A CPLD and the other is based on an Altera EPM570. These CPLD are in the featherweight division. I also wanted to design some very rudimentary 'proof of concept' FPGA development boards with the aim of providing alternative FPGA loading mechanisms (slave serial from a SPI EEPROM/FLASH or a uProcessor). The FPGA will be some kind of mid-range Spartan 3 or Spartan 6.The subject of voltage regulation for FPGA is more complex than I thought. Using switchers and buck converters circuits is therefore inevitable.Given I am a relative newbie (an 'old fart' who knows most of the 74 series off the top of his head, but has not dared to venture into anything beyond 5V logic levels) to designing FPGA products, I have some difficulties getting off the ground.Therefore, is there a painless way of incorporating my needs for 1.2V, 1.5V, 1.8V, 2.5V, 3.3V supply voltages for Xilinx Spartan 3 & 6 and possibly Virtex. I simply want an easy switcher or buck circuit design environment (EDA software or an on-line resource) that is not going to cause me hassles.I also need reliable access to the parts (ICs, inductors, etc). Something that is as common as the ubiquitous LM555 or LM741. No 'flash in the pan' technology please.I just need something that makes my life easy.

author: mrflibble
author_rank: Super Contributor
timestamp: September 16, 2013, 12:11:31 pm
content: Take a look at for example the LT3501. Spartan-6 board I have here in front of me (Atlys from digilent) use that one.Seethe schematicsfor some inspiration.

author: Hardcorefs
author_rank: Regular Contributor
timestamp: September 16, 2013, 01:48:46 pm
content: If you are designing An FPGA board from scratch. (I really do not recommend it)1.First thing is to decide on the FPGA.2. Read the STARTUP requirements for the device, many of these chips will NOT startup correctly if you are unable to supply the initial current, in many cases it can be 4-6 Amps (the newer xilinx devices require > 20Amps!!!!) this then drops off to significantly lower levels.3. The running Current depends on the HDL design, gate utilization frequency and MORE importantly logic state changes.Many of these cheap Chinese boards don't function reliably ,because they do not take into account 1,2,3Unless you are implementing really slow or small logic devices, simple regulation will not generally work, Buck convertors are the way to go.And don't just copy the development board setups........MANY of these designs are totally unsuitable/under-rated for the FPGA, if you really hammer the logic.I have a number of logic designs that have <5 I/O pins switching <10mA total, but I can totally destroy the Buck convertors even on some of the biggest Xilinx development boards.worse still.. many of the cheap buck convertors have a positive temperature coefficient.... as you push them they get hotter and drift upwards, which in-turn takes the FPGA out of spec.A drift of .01-.1 of a volt is significant (when the core voltage is speced down at 0.95 of a volt)For example a design loaded into various development boards, can max out at >200Mhz on a professional board, but only meet 20-40Mhz on a cheap one, same code, same FPGA.. Different power circuit design.Ultimately it depends WHAT you are doing inside the FPGA... and what your overall requirements are.

author: free_electron
author_rank: Super Contributor
timestamp: September 16, 2013, 02:32:07 pm
content: Altera has a current calculator spreadsheet. You pick the fpga, feed the clock frequency, how many flipflops are attached to it and the io utilisation (you get that report after synthesis) the spreadsheet then tells you how much current the core and io banks will needA mid sized fpga will draw a few ampere when it is pretty full and running at a few hundred MHz...I have used lp3904 regulators (national semi) this chip has two switchers and two ldo on board and they are programmable.If you want to make your life easy take a look at the simpleswitcher family.Go to ti.com and select their development tool webench.Fill in your i put voltage range, output voltages and current and the thing does all the work for you.It will give you a list of chips. Pick one and it will do all calculations , give you a schematic, bom and you can even save the schematic in altium or orcad or some other formats.You can pick alternate parts from within the toolAs for 'no flash in the pan'. You can't drive fpga's with vacuum tubes.You are dealing with cutting edge technology. Deal with it.Now, since you are apperently new to this fpga world: have you though abot other 'flash in the pan' stuff you will need ?A medium size fpga will require a 4 or 6 layer board ...It may be a ball grid array .. Who's going to solder it ? The same goes of it is a 240 pin tqfp monster with a copper slug on its back for heat stpreader.What are you going to hook up to it ? If you will attach some fast ram you better perform length equalisation on the traces and pick the right io banks.Can you give us an idea of which fpga you have in mind so we can form a better idea of what you are trying to do and point you at the problems you will run i to and how to solve em.Ive done many fpga designs some got away with a double sided board, some with a 4 layer, some needed 12 layers... (1451 pin xilinx... There were 3 on the board...)

author: elcomtel
author_rank: Regular Contributor
timestamp: September 17, 2013, 01:43:35 am
content: Quote from: free_electron on September 16, 2013, 02:32:07 pmWow !!! That was excellent advice. I did exactly as you suggested and I hit the jackpot.This is where I ended up and is exactly what I was after. Thank you indeed !!!http://www.ti.com/lsds/ti/analog/webench/fpga-power-architect.pageQuote from: free_electron on September 16, 2013, 02:32:07 pmThat's a bit rough !!!In my original request I was suggesting that I wanted to stay 'generic' and to do as most of the industry does, thereby guaranteeing that parts would be available in the future. 'Flash in the pan' meant that I wanted to avoid using technology developed that 'seemed like a good idea at the time' but the industry took a different direction.Quote from: free_electron on September 16, 2013, 02:32:07 pmWhile I did admit to being a bit of a 'FPGA newbie' the truth is that ten years ago I worked on a development team as a PCB designer where I often designed telecommunications PCB around a PowerQUICC MPC860 (BGA) and a Spartan II (TQFP-208) on multilayer PCBs. Unfortunately I didn't do any VHDL/Verilog or embedded software development. My FPGA PCB design skills (disigning for BGA, TQFP and all other SMD footprints) are quite good. So you shouldn't be too surprised about my original posting considering I am an old fart with top gun 74XX TTL 5V logic knowledge. I'm simply trying to build on things I already know.Quote from: free_electron on September 16, 2013, 02:32:07 pmI haven't decided exactly what FPGA I want to use. My first design will be a FPGA module for a FPGA development motherboard, so this can be any FPGA. The idea is to do something similar to an Altium Nanoboard (Series I and II) where you have plug in FPGA modules. I might buy an existing 'el-cheapo' Chinese FPGA development board that has a plug-in FPGA module and use the footprint of the Chinese plugin FPGA module as the layout for my 'simple FPGA design'. This means that my FPGA design would only have a FPGA, a slave serial SPI memory device, one or two JTAG ports and 2-3 connectors to connect my FPGA module to the development motherboard.

author: dannyf
author_rank: Super Contributor
timestamp: September 17, 2013, 11:02:32 am
content: You have a few questions to evaluate:1) will a brand-name genuine 1117 do the job?2) will an off-brand genuine 1117 (multi-sourced) do the job?3) will a fake 1117 do the job?Each has its own answers.

author: elcomtel
author_rank: Regular Contributor
timestamp: September 17, 2013, 11:29:36 am
content: Quote from: mrflibble on September 16, 2013, 12:11:31 pmThank you for the link.Without any further analysis (looking up datasheets) the zener diode D4 looks like it's connected to the wrong supply rail.

author: mrflibble
author_rank: Super Contributor
timestamp: September 17, 2013, 12:45:52 pm
content: Well, those schottky diodes are connected as per the "Typical Application" schematic in the datasheet. So with a bit of luck that's the correct way to connect them.

author: dannyf
author_rank: Super Contributor
timestamp: September 17, 2013, 12:48:00 pm
content: QuoteIt is a high-speed diode meaning to charge up the bootstrap capacitor so the upper half of the bridge can swing above the positive rail.

author: JoeyP
author_rank: Frequent Contributor
timestamp: September 17, 2013, 08:12:00 pm
content: Article just published on this subject:http://www.eejournal.com/archives/articles/20130917-powerpuzzle/

author: free_electron
author_rank: Super Contributor
timestamp: September 17, 2013, 09:22:54 pm
content: Quote from: elcomtel on September 17, 2013, 01:43:35 amApparently i misunderstood 'flash in the pan' ... i thought you meant 'no tssop or fancy things'. Too many times on forums like these we find questions of people that want an fpga board but it has to be dip and thru-hole parts. Hence my 'strong' reaction.Quotemake sure you use GOOD connectors for the interconnect otherwise the module will be useless ! and with good connectors i mean good in terms of signal integrity. 100 mil pitch dual row pinheaders just don't cut it these days. Look at samtec QMS QFS series. perfect for mezzanine boards. if you get the versions with -PCx extention you can feed all the power you want thru the connector. these connectors are designed for thi skind of stuff. i've been using em for at least 7 years. not a single problem.they have versions specifiaclly made to carry even differential signalling like LVD (pin order is GND DPx DMx GND ...) and they have a solid ground blade inbetween the columns .the power pins can handle serious currents.

author: elcomtel
author_rank: Regular Contributor
timestamp: September 18, 2013, 04:20:39 am
content: Quote from: free_electron on September 17, 2013, 09:22:54 pmStop making me feel like an old man.Here I was day dreaming in my "old fart" ways. I even bought a large stash on eBay of various 0.1" connectors to pursue this FPGA development dream of mine. Give this old fart a fair go. I cut my teeth on PDP-11/LSI-11 systems (that tells you old I am), none of this new stuff. The last time I saw 0.1" connectors was on a asynchronous VME bus.Seriously I do understand where I would want to use an old school 0.1" connector and where I would want to use a 'fancy pants' high density connector.Quote from: free_electron on September 17, 2013, 09:22:54 pmNow that's a 'flash in the pan' suggestion if I ever heard one.That Samtec QMS series connector is one seriously good connector:http://www.samtec.com/technical-specifications/default.aspx?seriesMaster=QMSOperation up to 9.5 GHz / 19 Gbps is very impressive.From an economic and practical perspective I am inclined to use VHDCI connectors as used on SCSI-3 68pin disk drive connectors. You can always find cheap SCSI-3 cables and 68p-to-50p SCSI cable adapters if you need to work with 'valve era' 0.1" (100mil) connectors. Besides, think of the practicality of being able to use a standard SCSI cable as a FPGA system I/O cable. Altium uses VHDCI connectors to connect their FPGA module to their Nanoboard product.So I will take your advice, but I'll go with the VHDCI connectors.

author: free_electron
author_rank: Super Contributor
timestamp: September 18, 2013, 01:18:27 pm
content: Vhdci is perfectly fine. They just are a bit bulky to my liking and lack dedicated power rails.That is the beauty of the samtec connectors . You have 4 or 8 dedicated pins , per corner for power rails.So if you use a hi end fpga you can send those differential pairs over without problems. Sata, pci express , usb 3.0 is all no problem for the samtec connector. Try feeding that through the vhdci and ot will jump out of its holes, scamper across the board to a far corner, curl up it is little pins and croak..Anyway. Up to you.And no, those connectors are bot flash in the pan. They have been around for a very long time. They were designed for mezzanine board. They are frequently found in hi end routers to plug in the media adapters.

author: jahonen
author_rank: Super Contributor
timestamp: September 18, 2013, 02:29:47 pm
content: I'd like to add that while those Samtec connectors are very nice for signal integrity, they are pain to solder manually. Even more so for the center ground tab, as it is completely inaccessible below connector plastic. Using hot air will likely melt the connector body.But of course, if you use reflow techniques such as IR or vapour phase, then it is a completely different story.Regards,Janne