Radio MCU resets durings transmits because battery is too weak - Page 1
Forum section: electronics / projects

author: HSPalm
author_rank: Regular Contributor
timestamp: November 10, 2016, 09:22:27 am
content: Hi,I have a design with CC1310 from TI where on a CR2032 cell it will reset during transmits in cold temperatures. Also when cell voltage decreases it will reset during transmits even in warm ambient temperatures. With BR2032 cells, which I plan to use, often it never boots, just goes in to reboot cycle where it transmits on startup and resets, over and over again.I have tried to double the capacitance on the supply lines which were 22uF from the reference schematic, It hardly did anyhting better with 44uF.What can I do to store enough energy which I can deliver fast enough to my circuit? Bigger caps? Better caps? Supercaps?Attached is a trace of the VDDS supply line going below 1.8V during transmit which is minimum for my circuit. This is on a healthy cell so no reboot happened all though I can see it's close.Also attached is my circuit with power supply parts and with the caps I doubled circled.

author: HSPalm
author_rank: Regular Contributor
timestamp: November 10, 2016, 10:48:12 am
content: Some progress have been made.This is the trace with two 150uF tantal caps in parallell across the cell terminals.Why is it so that the current pull is a much milder slope now? It seems like with the caps I limit the current, but voltage drop is lower, so... Maybe it was the BR2032 characteristics that dropped the cell voltage so quick in the previous picture.

author: Howardlong
author_rank: Super Contributor
timestamp: November 10, 2016, 10:51:58 am
content: CR2032 internal resistance is in the tens of ohms. A datasheet I just looked at suggests 10 to 40 ohms.Say your total supply resistance including inductor, traces and cell etc is 50 ohms.You have a voltage drop of 1.2v according to your trace.I = V/R = 1.2/50 = 24mA.CC1310 datasheet specs Icore in Tx boost mode as 23.5mA.If you want to use a capacitor to smooth this out you need to determine the on and off times to ensure there's enough charge maintained in worst case.Back of the envelope, say you need 10ms Tx on @25mA and 10ms Tx off time (50Hz) with a ripple of 0.1v, then you can approximate using a simple rectifier smoothing calculation, C = I / (f * V) = 0.025 / ( 50 * 0.1) = 5000uF which is a BIG part compared to the rest of your circuit's components.I am not sure using a cap is necessarily the best approach anyway though, the CC1310 has an integrated DC-DC converter that I'd assume would be designed to cope with your scenario. Is there a reference design or dev kit? I see that the SmartRF06 seems to use your device and a CR2032 for example.I also strongly recommend that you test your device with a bench supply and shunt resistor, and measure the current over time by probing the voltage across the shunt.

author: Kjelt
author_rank: Super Contributor
timestamp: November 10, 2016, 11:01:16 am
content: From my own experience not all CRxxxx batteries are equally designed.I have a lot of older Somfy RF handtransmitters running on these cells and a lot of brands just do not work even brand new out of the package.Brands that I have that are working ok are Sony, GP.Brands that do not work are all B brands and Marantz, Panasonic and some others I can not recollect.

author: HSPalm
author_rank: Regular Contributor
timestamp: November 10, 2016, 12:30:52 pm
content: Quote from: Howardlong on November 10, 2016, 10:51:58 amThank you for a thorough answer. After adding the 300uF tantal I see much more stable performance, all though I know that at ~2.5V the sag will be too much for a transmit to complete.If a capacitor is the solution, which capacitor type handles below freezing temperatures better? By the way, if you think a CR2032 is "bad" look at BR2032 (which I'm using cause of temperatures). It's so "bad" that in comparison to CR series,panasonic writesthis about the CR series:QuoteThe DC-DC converter makes a 1.65V rail for the radio part when output power is up to 12dBm. Above this it's 1.8V. Maxiumum is 14dBm which I hope to use, therefore the cell voltage cannot go below 1.8V during transmit.There are several reference designs, both for theCC1310 modulethat fits the SmartRf06 (look under design files) and for thelaunchpad board(under design files). My schematic that you saw further up is a copy with only slight changes.Funny thing about the coin cell holder on the SmartRF06. TI just said on their e2e forum that they haven't tested the ref. design for CC1310 with a coin cell battery. Which I find funny for a low power device.I will try to make some current measurements.Quote from: Kjelt on November 10, 2016, 11:01:16 amThank you. Unfortunately I have to choose the BR-series from panasonic. They are the only kind that have a stable discharge curve below freezing and is rated to -40 degrees C.

author: CJay
author_rank: Super Contributor
timestamp: November 10, 2016, 01:10:45 pm
content: Does it absolutely have to be that size/form factor or can you move to a larger volume cell and/or possibly a different chemistry like LiMnO2?

author: HSPalm
author_rank: Regular Contributor
timestamp: November 10, 2016, 01:35:36 pm
content: Quote from: Howardlong on November 10, 2016, 10:51:58 amDidn't notice this. On-time is (as seen below) approximately 7ms, but off-time is 30 minutes(!).Quote from: Howardlong on November 10, 2016, 10:51:58 amHere the shunt resistor i 1 ohmQuote from: CJay on November 10, 2016, 01:10:45 pmCoin cell, yeah. It should fit inside here. I'm experimenting with BR2477, which forces me to buy the slightly taller enclosure. But it can't extend over the antenna area, which is about 1/3 of the pcb from the top.

author: Kalvin
author_rank: Super Contributor
timestamp: November 10, 2016, 02:59:53 pm
content: Have you considered using a supercapacitor in parallel with the battery. The super capacitor will hold the capacitance pretty well even at low temperatures. Here is an application note from Murata "Murata Supercapacitor Technical Note":http://www.murata.com/~/media/webrenewal/products/capacitor/edlc/techguide/electrical/c2m1cxs-053.ashx?la=enOther manufacturers have similar products and application notes.

author: HSPalm
author_rank: Regular Contributor
timestamp: November 10, 2016, 03:02:52 pm
content: I have, I actually suggested it as a possible solution in original post hereBut I have not used them before. I will certainly read from the document you provided, thank you.

author: Kalvin
author_rank: Super Contributor
timestamp: November 10, 2016, 03:19:06 pm
content: From your oscilloscope picture one could see that the transmitter will be on for about 10 ms (8ms) and the power consumption is 35 mA. You will need a capacitor of 35 millifarads if you want to achieve a maximum voltage drop of 10 mV. For a maximum allowed voltage drop of 100mV the required capacitor is 3.5 millifarads. Those numbers are worst case capacitance numbers in case of the battery cannot provide any current.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 10, 2016, 04:04:46 pm
content: The battery is clearly undersized for the application; fixing it by adding capacitance adds more weight and volume than just using a bigger battery, because the energy storage of the capacitors is several orders of magnitude worse than that of batteries. Even ultracaps are rather poor at about 0.5 Wh/kg usable when paralleled with a battery versus about 100-150 Wh/kg of a battery.Using a bigger battery instead would not only solve this particular problem better, but as a side plus, it would increase the runtime. No one likes to change batteries all the time.

author: Kalvin
author_rank: Super Contributor
timestamp: November 10, 2016, 05:05:15 pm
content: Quote from: Siwastaja on November 10, 2016, 04:04:46 pmSome button batteries just have too high internal resistance although their capacity may be suitable for the application, and thus the large, external capacitor may be needed. Using a larger battery with a smaller internal resistance may be a good solution too. The internal leakage of the battery should be take into a consideration as well.Some Li/MnO2 primary batteries are quite compact and light weight with relative small internal resistance bu they do not perform well in low temperatures ie. the peak current and capacity will be reduced in low temperatures.http://www.powerstream.com/thin-primary-lithium.htm

author: Kjelt
author_rank: Super Contributor
timestamp: November 10, 2016, 05:11:27 pm
content: Do you have room to use a double cell coin? Doubling the voltage would/could reduce the current.

author: Kalvin
author_rank: Super Contributor
timestamp: November 10, 2016, 05:21:59 pm
content: Quote from: Kjelt on November 10, 2016, 05:11:27 pmOne needs to consider this carefully as the CMOS circuits exhibit a square-voltage law in which the dynamic power consumption will increase four-fold as the supply voltage doubles:https://en.wikipedia.org/wiki/CMOS#Dynamic_dissipation

author: HSPalm
author_rank: Regular Contributor
timestamp: November 11, 2016, 09:41:05 am
content: Quote from: Kalvin on November 10, 2016, 03:19:06 pmWow, those are not small capacitors. Then I read that electrolytics can have about 1/5 of their capacitance at -40 C... Thanks for doing the calculations.Quote from: Siwastaja on November 10, 2016, 04:04:46 pmYes, it is. I am replacing the CR/BR2032 for a CR/BR2477. In the datasheet they're rated for the same continuous current but I have small hopes they will be a tad better.Quote from: Kalvin on November 10, 2016, 05:05:15 pmI posted a picture of the enclosure further up. I am taking suggestions on battery chemistries, so thank you. Another requirement is freezing temperatures, down to -40 C.Quote from: Kjelt on November 10, 2016, 05:11:27 pmI have, with a taller enclosure, but the design is running directly off of the cell so the voltage would be too high for the mcu.

author: Howardlong
author_rank: Super Contributor
timestamp: November 11, 2016, 10:53:50 am
content: It needs... a Batteriser! <joke>More seriously, the Smartrf06 reference design appears to have a separate buck/boost regulator in it, there may be good reason for that. However you don't generally want switching regulators running in a long sleep configuration.What's not clear though is that the CC1310 also states that is runs from 1.8 to 3.8v, and it includes an on-chip switching DC-DC converter and LDO, I assume to allow direct battery operation.Is your device simply triggering a brown out, and if so can you set the brown out voltage, either in a register at boot or sometimes in configuration bits/fuses?

author: mariush
author_rank: Super Contributor
timestamp: November 11, 2016, 12:21:43 pm
content: If the size of the design is flexible, couldn't you have room for a AAA battery? That would be 45mm by 10mm .. your CR2477 is already 7.7mm and with the height of the pcb and the metal bracket holding the battery you'd be at around 9mm height anyway.You could use some highly efficient switching regulator to boost the 1v ... 1.65v (depending if it's rechargeable battery or not) to at least 1.8v ... for example seeTorex XC9142(the one linked is fixed 3.3v out, from 0.9v to 2.5v input it's over 90% efficient between 1mA and 100mA and only needs a couple ceramic caps and a ceramic inductor .. the only downside is that it costs about $1, maybe about 50 cents if you buy 1000.There's alsoSemtech SC121which is also reasonably priced and just as good when it comes to extra parts required and the pcb space needed (the one linked is fixed 3.3v if feedback pin is tied to ground, or adjustable between 1.8v and 5v otherwise) or maybeams AS1329which has output adjustable between 2.5v and 5v and over 80% efficiency from 1mA and up to around 90% at 50-100mA... orTPS61071DDCRwhich is also cheap in volume...Anyway.. the above suggested chips are just from a brief search on Digikey.. there's other brands and models distributed by Farnell/Newark which could be just as good or cheaper.Sure your project would be more expensive if you have to introduce a boost regulator but then again, if you have to buy somewhat rarer batteries they'll be more expensive so in time you'd save money by buying a more common battery... and if you have to add capacitors or supercapacitor that also adds to cost and they take up pcb space ... a AAA battery will be able to give you the current you need without issues, and alkaline batteries will also last long time on low current (sleep mode)I was also thinking of suggesting1/2 AA batterieswhich can do up to 1200mAh at 3.6v (non-rechargeable) and do continuous current of 15-25mA (so some capacitor may help reduce voltage drop when transmitting but you'd probably need less capacitance) and can tolerate -60c or -55c (depends on brand)Problem is these batteries are much bigger in diameter at around 15mm but smaller in height at around 26mm but they're more expensive and not rechargeable.

author: Kalvin
author_rank: Super Contributor
timestamp: November 11, 2016, 01:19:09 pm
content: Quote from: HSPalm on November 11, 2016, 09:41:05 amAt least Murata has supercapacitors that perform very well at low temperatures down to -40 Celcius :http://www.murata.com/~/media/webrenewal/support/library/catalog/products/capacitor/edlc/o83e.ashxThe nominal capacitance will not be affected and the internal resistance (ESR) will increase only 1.5 times compared to nominal ESR at 25 Celcius (typ. ESR at 25C is 45 milliohms),If you make more searching you should be able to find suitable supercapacitors from Murata and other manufacturers to hetp the battery to provide the necessary peak current. It may be possible that you can also find suitable primary batteries that perform well at -40 Celcius.

author: CJay
author_rank: Super Contributor
timestamp: November 11, 2016, 05:56:30 pm
content: This one perhaps:http://data.energizer.com/PDFs/2l76.pdf

author: HSPalm
author_rank: Regular Contributor
timestamp: November 12, 2016, 11:49:23 am
content: Is there another way to quote multiple posts in one go without pressing quote on each post, copy the quote, press back and continue until you've combined them all by copying and pasting?Quote from: Howardlong on November 11, 2016, 10:53:50 amDo you mean the ref. schematic for the SmartRF06 board or the CC1310 module that's mounted on the SmartRF06? Yes the CC1310 incorcporates a DCDC switching converter. In deep sleep you can see it's pumped back up to ~1.7V whenever it falls under ~1.6V, tis happens every second.About the brownout detection. This is something I need to look into. Because no matter when the cell is empty or the circuit stops working, the behavior I'm seeing now is not good because it's spewing RF all over when it goes into the reboot loop.Quote from: mariush on November 11, 2016, 12:21:43 pmThe size is really not flexible. Unless a go for a re-design, of course. How do alikalines hold up in -40, though?Regarding SMPS, sure, but then again it's a redesign and a bigger circuit. It does not fit in the envelope I've been given a.t.m. but if all else fails of course something has to give. But I think I'd rather give room for a bigger/better battery than designing in a smps.Thanks for the tip on the saft cells. We've been using them before, and they're actually competitive to the 2477 size coin cells with much better cold temp. specs and current capabilities. It's just a matter of size, now.Quote from: Kalvin on November 11, 2016, 01:19:09 pmI searched on digikey for any supercapacitor with ESR lower than 1 ohm and sorted by price. The first supercap (in stock, active) with a voltage rated over 3V was 4 USD and enormous. Those you linked to are $6.5 - 10.5 and I couldn't fit them.Quote from: CJay on November 11, 2016, 05:56:30 pmThey are interesting in deed... But a tad low on capacity. Wonder if there's an easy way to pcb mount them, haven't searched for a holder for these yet.