Reference voltage switching circuit with hysteresis - Page 1
Forum section: electronics / projects

author: Phelix
author_rank: Newbie
timestamp: February 21, 2025, 10:32:49 am
content: Hello all,I have a product operating by battery where I need the power source to be shut off if the battery is below a certain level of voltage. (3V for exemple)For cost ans space available reasons I would like to manage this in a simple way.Knowing the trig value of the EN pin of the voltage regulator I'm using is not controlled I made the following schematic (see attached). Where the zener is determing the voltage to trig around.I would like to intagrate a small hysteresis. But I'm not sure the best way to make it...Let me know your thoughts.Best regards,

author: moffy
author_rank: Super Contributor
timestamp: February 21, 2025, 11:12:14 am
content: If running off a battery, then low power drain is a major consideration, something like the ATL431:https://www.ti.com/product/ATL431?keyMatch=tl431&tisearch=universal_search&usecase=partmatchesonly needs 20uA and there are plenty of voltage monitoring chips that use very low current which would be less power hungry than a zener.

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 21, 2025, 12:53:47 pm
content: It would be difficult to add any significant hysteresis to that circuit, as D14 and the BE junction of Q11 form a very low impedance path when conducting. Also that path means that one of those two components will likely die when your input voltage goes over the limit by more than a volt. Should at least add a base resistor to Q11. Also agree with moffy's comment on quiescent power drain.As for suggesting alternate circuits, there's a range of options depending on how willing you are to use specialized chips:1. Use a single open-collector comparator (like LM311), a voltage reference (zener, TL431, etc), and some resistors.2. Use an open-collector with a built-in reference (like LT6703HV).3. Use a voltage supervisor chip (which is effectively the same as above).4. Use a linear regulator whose enable pin has a precise threshold and adjustable hysteresis built-in.What's the maximum Vin your circuit needs to withstand, and what threshold do you want?

author: Phelix
author_rank: Newbie
timestamp: February 21, 2025, 01:16:05 pm
content: Thank you Moffy and mtwieg fopr you considerations.I updated the circuit accdording your recemmandation using the shunt voltage reference. I calculated to have it around 3V voltage trig.See attached.I also add the R35 to bring hysteresis effect on the resistor bridge... The value is not yet calculated.Is this correct with your initial view?Thanks,

author: HVManiacc
author_rank: Newbie
timestamp: February 21, 2025, 01:19:39 pm
content: Hello phelixIf this can be done as simply as possible, use two TL431s in a slightly modified voltage window detector circuit configuration.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: February 21, 2025, 08:28:58 pm
content: Quote from: mtwieg on February 21, 2025, 12:53:47 pmHow about a MAX809T, or maybe MAX809S?The MAX809 (and its many clones) is a dedicated-purpose voltage supervisor IC that will keep its output low whenever the VCC voltage is below the threshold (3.08V for the 'T', 2.94V for 'S' - a range of thresholds are available). It has some hysteresis in the form of tolerating voltage dips below the threshold for a certain period of time relative to the amount of excursion below the threshold. For instance, 100 mV below will be allowed for up to approx. 50 us; 200 mV for perhaps 25 us - see the graph curve in the datasheet. Because its primary purpose is as a reset supervisor for MCUs, etc. it also features a delay of a couple hundred milliseconds before asserting the output after the voltage rises above the threshold, but I suspect that won't be a problem for OP's application. Current consumption is quite low too - a few uA - so should be fine for battery power.

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 21, 2025, 08:57:12 pm
content: Quote from: Phelix on February 21, 2025, 01:16:05 pmYes that should work well (assuming resistor values are selected properly). However I still strongly recommend adding a resistor in series with the base of Q11 to limit current into the anode of U4.Quote from: HwAoRrDk on February 21, 2025, 08:28:58 pmYep that's one example of what I was referring to. Though without knowing the max Vin range and the desired threshold, it's impossible to recommend a specific part. Most supervisor chips, including that one, are limited to 5-6V max.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: February 21, 2025, 09:30:30 pm
content: Quote from: mtwieg on February 21, 2025, 08:57:12 pmGiven the stated tentative 3V threshold and use of an LDO regulator, I was assuming the OP is using a lithium battery with a working voltage range of at most perhaps 4.3V. I'd be surprised if OP had a higher-voltage battery like, say, 12V; an LDO wouldn't be good in terms of efficiency in that scenario.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: February 21, 2025, 09:59:08 pm
content: Quote from: Phelix on February 21, 2025, 10:32:49 amDid you check how much more is a regulator with a precision Enable pin? Saves a lot of parts !

author: PCB.Wiz
author_rank: Super Contributor
timestamp: February 22, 2025, 01:28:01 am
content: Quote from: Phelix on February 21, 2025, 01:16:05 pmThe circuit still has significant battery drain.If the battery voltage is withing range of a XC6219, it is probably within range of a Voltage supervisor/ reset generator mentioned above.Those Voltage supervisors can come in 10uA to sub 1uA and in 3%, 2%, 1% regions.Some have millisecond enable delays and some are a few microseconds.

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 24, 2025, 01:24:50 pm
content: Quote from: HwAoRrDk on February 21, 2025, 09:30:30 pmYou're probably correct. In fact if they're using the XC6219 then that proves it, since its max Vin is just 6V.

author: David Hess
author_rank: Super Contributor
timestamp: February 24, 2025, 02:53:08 pm
content: You need a different circuit because as the input voltage rises, the current is unlimited through the zener diode and PNP base-emitter junction. Another problem is that the zener diode is unlikely to be happy with only about 100 microamps.Hysteresis could be added by adding a low value resistance in series with the emitter, and then feedback from the output of the regulator to the emitter, but for the reason above, I would use a different circuit.If you do not want to use a comparator, then I suggest a bipolar transistor differential pair in place of your single transistor.

author: Peabody
author_rank: Super Contributor
timestamp: February 24, 2025, 03:30:29 pm
content: If it's a lithium cell, what about a protection chip. Everybody is used to the DW01 with dual external mosfets, but they now make these with the mosfets in the chip. Typically they shut off power at 2.5V, and re-enable at 3.V. Of course they have other protection shutdowns that you would need to be ok with. Look at the FM5056 and the XB7608A. They switch the low side to the battery, and run on a couple microamps. Or just use a protected battery.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: February 24, 2025, 09:47:30 pm
content: Quote from: David Hess on February 24, 2025, 02:53:08 pmThere are Zeners from multiple sources these days, spec'd for 50uA - eg BZX38450 etc even too 2% spec.These are 'normal' zeners, but the tested operating point is changed.Low voltage zeners are naturally soft, and have poor tempcos, which means they needexactly 50uAto give the rated voltage.If the OP can match the voltage, a simple voltage monitor/reset IC will be lower power.