Precision Voltage Source from LI-Batteries - Page 1
Forum section: electronics / general

author: Noobster
author_rank: Contributor
timestamp: October 24, 2022, 06:37:21 pm
content: Hi,my problem might be too specific but i did not find an adequate solution in my search. I am looking for the following:-Stable voltage source from rechargeable batteries (got those) for measuring applications- fixed voltage in the common Range 2.5-5V would be ok, adjustable better-output 50-200mA-short term (<2 hours) low drift (temperature range should be +-2°C during that time)-budget < 5 DollarSadly i can´t give an answer on what precision i want to achieve, it is more like this design challenge has come up a few times and i would like to have a go to solution that is not too complicated. The Idea is as precise as possible so i have one thing less to worry about.

author: DavidKo
author_rank: Frequent Contributor
timestamp: October 25, 2022, 04:06:43 am
content: What means stable? First what came into my mind was TL431 (or equivalent). See datasheet for recommended connections.

author: opa627bm
author_rank: Regular Contributor
timestamp: October 25, 2022, 08:49:48 am
content: Use Ref5050 will be a good starting point.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: October 25, 2022, 08:58:23 pm
content: How about an AD584 module from eBay?Or if you can afford 10x times better goodness at 10x price then ADR1399H-EBZ.

author: MasterT
author_rank: Frequent Contributor
timestamp: October 25, 2022, 09:25:08 pm
content: 1. High current OPA :TCA0372 or LA65002. LDO:AMS1117, LD1117

author: Noobster
author_rank: Contributor
timestamp: October 26, 2022, 10:36:12 am
content: Hi,thanks for all the suggestions. What would be the best way to buffer the voltage reference (opa as voltage follower ?). Does it (for thermal and load behavior) make more sense to use voltage reference plus follower or a linear regulator ? Also i am concerned with the voltage drifting as the battery voltage decreases.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 26, 2022, 04:44:33 pm
content: Cheapest thing I can think of would be some voltage reference that hits whatever sweet spot between performance and price that you like. Follow that with a voltage divider made from inexpensive 1% metal film resistors. Switches are expensive, so use a header with jumpers to set the voltage. Follow that with an opamp that, again, hits whatever sweet spot you like, as a buffer. Put a simple NPN transistor at the output, in the feedback loop, so you get enough current and not heat up the upstream circuits.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 26, 2022, 05:16:16 pm
content: For <$5 budget you can try a LM317 with a resistive voltage divider, something like this:https://www.st.com/resource/en/datasheet/lm317.pdf

author: magic
author_rank: Super Contributor
timestamp: October 26, 2022, 05:33:06 pm
content: LM317 could work as a power buffer, but I'm not sure if it will perform well as a reference combined with a buffer.Staying in the realm of ultra low cost solutions, it's very easy to put LM317 in the feedback loop of TL431 so that the latter acts as the reference. You pretty much need just the two ICs and one resistor (from LM317 output to adjust, to supply TL431 bias, 1kΩ will do). Minimum output voltage is 3.75V.There are some reference ICs with output force and sense pins, perhaps these could drive an external buffer to eliminate a separate opamp from the BOM.Important question: what supply voltage is available?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 26, 2022, 05:54:42 pm
content: 5$ is a very tight budget and would not really allow for much precision.A voltage regulator would be cheap, but when loaded the regulator heats up and this hampers accuracy. So one should have a separate reference / regulator and power transistor of some kind.A cheap version may be with a TL431 and power transistor.

author: Noobster
author_rank: Contributor
timestamp: October 28, 2022, 04:27:05 pm
content: Thanks again for the suggestions.@magic: I have multiple packs with 3 cells so 11-12.6V also typical 18v batteries from power tools.@Conrad Hoffman: that sounds very goodHere is also where my rudimentary knowledge stops, what to look for in the single components: Are all OPAs precise buffers? etc. Concrete component suggestions would also be helpful (i doubt i would find the best fit).Generally it probably makes sense to spacially (thermally) divide the "power stage" from the reference (and the buffer?).

author: magic
author_rank: Super Contributor
timestamp: October 28, 2022, 05:34:36 pm
content: This is what I suggested. I have never characterized this circuit for accuracy because I have only used a variant of it to build a CC/CV float battery charger (LM317 was doing CC part, not included here), but it should be much better than LM317 alone.With such high input voltage LM317 works fine.TL431 could be replaced with something better. It's certainly possible to have separate reference and opamp chips, but perhaps some integrated reference with force/sense capability could suffice. Alternatively, a 5V ref and opamp at unity gain, then no particularly accurate resistors are needed. I wonder if those AD584 from eBay that somebody mentioned above would work, as this covers everything - reference, opamp, feedback divider.

author: iMo
author_rank: Super Contributor
timestamp: October 28, 2022, 05:42:20 pm
content: An uA723 oldie again? With a PNP power transistor for those 200mA?Not sure it fits <$5 today..The TC will be rather low, perhaps comparable with 317 or 431..

author: magic
author_rank: Super Contributor
timestamp: October 28, 2022, 05:50:09 pm
content: Well, of course.Not sure how you would use a PNP, but NPN emitter follower (or LM317 - yes, I know I'm boring) is an obvious option. Current limiting is possible using the 723's facilities.I recall a whole thread about getting the most out of that chip.Also, National's version was the winner for long term stability.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 28, 2022, 06:01:57 pm
content: IIRC 431 is better in thermal stability, and 723 was tricky for lower voltages.TI has many SMD voltage references, some might be slightly better than TL431 and still cheap, e.g. REF35 $1 in kpcshttps://www.ti.com/power-management/voltage-reference/overview.htmlthought that's a little more than $3/1pcs at mouser.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 28, 2022, 08:19:31 pm
content: Not sure how good the performance might be, but you could make a discrete bandgap reference with a couple transistors glued together to keep the temperature the same. Cost would be pennies. Google bandgap references.

author: iMo
author_rank: Super Contributor
timestamp: October 28, 2022, 09:21:31 pm
content: Quote from: magic on October 28, 2022, 05:50:09 pmYep, there is NPN or PNP possible with 723.. I've been using in my floating +/-15V source the PNPs there, as well as I added the foldback current limiting which works nice too with 723..

author: magic
author_rank: Super Contributor
timestamp: October 29, 2022, 07:59:25 am
content: Quote from: Conrad Hoffman on October 28, 2022, 08:19:31 pmUnlikely to be competitive with IC references, because for any 1°C of ΔT you get 2mV of ΔVbe and this is amplified over 10x by typical bandgap circuits by the time you reach 1.2V, and OP wanted 2.5~5V.However, you can select two TL431 such that the sum of their drift is close to zero at the temperature of interest, glue them together, and build my circuit using the second TL431 in place of R2.

author: iMo
author_rank: Super Contributor
timestamp: October 29, 2022, 08:16:51 am
content: I think the OP should have refined his/her requirements a bit, as we may suggest almost anything here in this section

author: RoGeorge
author_rank: Super Contributor
timestamp: October 29, 2022, 09:08:24 am
content: For discrete components, maybe a low noise, LED based reference then?https://www.eevblog.com/forum/chat/leds-as-a-low-noise-shunt-regulator/https://duckduckgo.com/?q=walt+jung+led+regulatorNot mine, saved from diyaudio forum IIRC:

author: Noobster
author_rank: Contributor
timestamp: October 29, 2022, 09:11:25 pm
content: "I think the OP should have refined his/her requirements a bit, as we may suggest almost anything here in this section". Yes, and i love the suggestionsLet me try to narrow it down, i think most important is-temperature drift-drift from input voltage decreasinghas anyone tries simple ways of temperature "control"? like a ptc resistor and make one expoxy block out of the circuit, put in styrofoam ?

author: iMo
author_rank: Super Contributor
timestamp: October 29, 2022, 10:33:34 pm
content: Quote from: Noobster on October 29, 2022, 09:11:25 pmYou have to make the specification like this, for example:0. input voltage 8V to 15V1. output voltage 5V2. output current max 200mA3. output voltage temp drift <20ppm/C4. long term drift <100ppm/1kHr5. line regulation <40ppm/V6. load regulation (10mA to 100mA) <40ppm7. output noise (1Hz to 1kHz) <10uV rmsetc.

author: Noobster
author_rank: Contributor
timestamp: October 30, 2022, 11:40:49 am
content: 0-2 should be answered. For the rest: 4 doesn´t matter, the rest as low as possible for the budget with special emphasis on thermal drift and line regulation. One example i use this on is distance measurement with hall effect sensors where i measure <0.5mV (this sucks with just a cheapo linear regulator). I am a mechanical engineer so i don´t enjoy building a new circuit everytime to fit my needs exactly, therefore i want something that works everytime. For example for mosfets i buy just one type that is overkill for most of my applications but i have them here and don´t need to order new ones/check datasheets each time. I know this seems strange, sadly for me electronics is something i have to deal with but don´t enjoy. Thanks for all the suggestions, to me the solution voltage reference->opa +transistor seems the most versatile. What do i look for in opa and transistor selection?

author: lyxmoo
author_rank: Contributor
timestamp: October 30, 2022, 11:57:54 am
content: LT1372 typical application Li-Ion Cell to 5V SEPIC Converter.take a look.

author: Noobster
author_rank: Contributor
timestamp: October 31, 2022, 08:34:52 am
content: I did some more searching and stumbled across the LT6658 which would fit the budget. What do you guys think of it ? I doubt it is easy to achieve a better result with a diy circuit (at least for me)?@lyxmoo looks good for high loads but seems noisy to me (again maybe im wrong)

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 31, 2022, 09:42:28 pm
content: Good specs and looks like it's under $5!

author: lyxmoo
author_rank: Contributor
timestamp: November 01, 2022, 05:41:32 am
content: measuring circuit mainly depends on Higher PSRR measurement compents, power source itself is secondary factors.

author: magic
author_rank: Super Contributor
timestamp: November 01, 2022, 06:27:19 am
content: The specs are bad because load regulation doesn't include effects of die heating by the output bufferThis is going to be considerable - 200mA with 10V input and 5V output is 1W. If you apply the load, wait for things to settle, then start your measurement and keep the load constant, things may work out. But if loading (edit: or input voltage for that matter) will be changing during measurement, the output voltage will wander around like a drunkard.Also, the chip is hard to solder - lots of tiny pins and a thermal pad underneath which must be soldered down to remove heat from those internal buffers.Maybe it would be possible to put LM317 (or some NPN) in the feedback loop of that thingBut no 100% guarantee that it will work and still a PITA to solder.

author: Noobster
author_rank: Contributor
timestamp: November 01, 2022, 11:52:45 am
content: @magic: I think i am missing something here, they are giving the output voltage/temperature graph on the first page of the datasheet for 150mA load (looks like 0.2mV from 25 to 50°C). Wouldn´t any kind of external buffer also suffer from a temperature coefficient ? The TL431 you mentioned seems to have at least as bad thermal performance, it wouldn´t heat up as much ofc. Worst case i mount the chip directly to a block of aluminium or a metal housing. Small soldering i am used to.

author: iMo
author_rank: Super Contributor
timestamp: November 01, 2022, 01:30:14 pm
content: There is the "load regulation" like output voltage changes XX ppm/mA (mA of output current), and there is the TC in YY ppm/degC output voltage change.Both apply. The LG and the TC you'll get from the datasheet.For the change of the chip temperature you may, for example, calculate it easily likedT = (Vin - Vout)*Iout*Rth, where Rth is the thermal resistance of the chip against pcb (or the cooler) for a specific package (the datasheet value), like for example 80degC/Watt.The absolute temperature of the chip will beTchip=dT + TambientWith 1Watt heat loss and say Rth=80C/W the dT change will be 80C, with TC=10ppm/C the voltage change with dT=80C will be 800ppm (approximately as the TC with Temp is usually highly nonlinear for large dT, as usually shown in the datasheet).

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 01, 2022, 02:58:37 pm
content: So buffer the thing to keep the heat out. You can't expect metrology lab grade results for $5 without some additional engineering. Or even with it. Lots of hard to solder leads is the way of the world now, so best to get a microscope and learn to deal with it.

author: Noobster
author_rank: Contributor
timestamp: November 01, 2022, 06:24:25 pm
content: I do get that part, what i don´t quite know/understand is: There is no "perfect" buffer (if i understand correctly) so if you thermally distance the reference and the buffer (as i already suggested btw.) the reference stays almost perfect but the buffer (will also heat up from 200mA) ads thermal/load coefficient ? So is that error much lower and can be ignored ? My first instinct would be that the guys from Analog Devices know a lot more about the topic than me and slapping a big heatsink on it does not add to cost (few cents in aluminium maybe). Like i said soldering small pins doesn´t bother me, i have done that (stereo microscope).I quess what i am trying to say: how big is the error in the buffer compared to the reference?

author: Noobster
author_rank: Contributor
timestamp: November 01, 2022, 06:26:44 pm
content: trying to learn here, pretty sure the precision we are talking about is starting to exceed my needs

author: magic
author_rank: Super Contributor
timestamp: November 01, 2022, 07:18:28 pm
content: The effective technique is to include the buffer in the feedback loop of an opamp, which compares the buffer's output with the correct reference voltage and drives the buffer's input so as to get the reference voltage on the output.Opamps have plenty of open loop gain - less than 1µV change in the difference between output voltage and reference voltage (the final error) suffices to move the opamp's output many mV, enough to compensate for the drift of any reasonable buffer.Ideally, both the reference and the opamp would be somewhat isolated from the worst heat. If you are lucky, you may get both components on one chip - LT6658 actually is an example of that. LTC6655 is another, and this thread reminded me that there is an MCP1501 somewhere in my parts box which looks very similar too (better specs than TL431 for $1), I will have to find it. And there are a few voltage references which aren't strictly opamps but still behave quite like opamps, such as TL431 or AD584.Some/many/all of those chips may be usable in such manner, without external opamp chips and the extra cost of such, but it's an experimental affair because this kind of use is usually not documented or characterized by the manufacturer. Most of the time, they just tell you that the output of the chip should be connected directly to the feedback input of the chip, so that the internal opamp produces all the load current by itself.A potential problem is oscillation, particularly if capacitors are present on the output. Having a scope at hand would be a good idea. This is also a concern when using separate reference and opamp plus a buffer. Only with fully integrated chips do you get some assurance that it will be stable as long as specified conditions regarding output capacitance are met.

author: iMo
author_rank: Super Contributor
timestamp: November 01, 2022, 07:59:45 pm
content: Quote from: Noobster on November 01, 2022, 06:24:25 pmWhen the reference source is thermally at distance from the "hot" buffer the influence of the buffer is small - as the buffer is usually wired in a control loop of an opamp. When the buffer heats up by say 50 degrees and drifts "internally" it has a little impact on the output voltage as the "buffer" is within a control loop.. See an example below - the reference is far away from the power transistor (the buffer), the opamp creates the control loop, where the hot buffer darlington transistor is inside the control loop, the opamp keeps the output voltage at the reference level constant, such the buffer's drift has almost none influence..

author: Noobster
author_rank: Contributor
timestamp: November 01, 2022, 09:12:35 pm
content: Quote.Thank you that cleared it up perfectly!QuoteQuoteI am prette sure that sums up the solutionThank you very much. Concrete chip suggestions are still welcome (although there are a lot here already).

author: magic
author_rank: Super Contributor
timestamp: November 02, 2022, 09:14:51 am
content: Quote from: magic on November 01, 2022, 07:18:28 pmI played with this chip on a breadboard. Specifically, the 3.3V version.Without feedback connection, the output goes to the top rail.With feedback pulled up to Vdd with 1kΩ, the output goes down to ground and some tens of µA go into the feedback pin.With feedback shorted to the output and Vdd open circuit (only 100nF on it) it works in shunt mode, from ~2.7V supply delivered by internal protection diodes.I take the above as confirmation that the chip consists of a ~1.2V bandgap reference, a rail to rail opamp and a fairly high resistance internal divider in the feedback path of the opamp. No interactions (like clamping diodes) are apparent between the output and feedback pins.So I tried BC547 emitter follower and LM317 in the feedback loop, both appear to work.Output voltages were fairly similar, maybe 1mV difference.Variations of similar magnitude were caused by touching things on the breadboardHaven't checked for oscillation yet...