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Improving a TL431 Window Comparator with Hysteresis
Echo88:
I needed a cheap circuit which decides wether an input voltage is in an acceptable range and consequently switches a MOSFET to connect the input voltage to a load (Undervoltage/Overvoltage Lockout).
So i decided to adapt some TL431 in window comparator configuration for that task, based on the following circuit suggestion:
https://electronics.stackexchange.com/questions/47154/low-battery-indicator-for-2x-aa-batteries?noredirect=1&lq=1 (second circuit)
However, since the input voltage might vary a bit the circuit needs some hysteresis, which i couldnt successfully implement/the output still shows instability.
I found another TL431 comparator suggestion https://www.edn.com/shunt-regulator-monitors-battery-voltage/ with hysteresis and based on that i cobbled together another circuit, which seems to work and is stable. Might be improved in values or contain problems in real life though.
Both circuits are included in the attached .asc document and id like to know if someone can enlighten me on how to implement hysteresis on the bottom circuit, to get a stable output like the top circuit shows.
Just watch Vout/Vout2 and youll see the mentioned instability at the switching endges in the bottom circuit. The sine voltage source is used to simulate voltage variation.
Or maybe there are other suggestions for low cost/simple UV/OV-LO-circuits, without needing to use more expensive LT4356/LTC4365 for example.
Thanks. :)
magic:
Since it seems you already need two TL431, not just one...
Dunno, use a normal dual comparator with the usual hysteresis solution and open collector outputs or diodes to NOR the two outputs?
Ian.M:
A TL431 closely resembles an OPAMP with an open collector output, with its +in internally connected to a 2.5V reference and not user accessible. As the only input you have is the -in, all DC feedback is inherently negative and therefore hysteresis, which requires positive feedback is impossible without another active device acting to invert its output. If you are using it to pull down a P-MOSFET pass transistor gate, its tempting to tap additional feedback from the drain, but that can result in load dependent instability, so its much safer to use a separate small signal P-MOSFET to bypass part of the upper part of the feedback divider or switch in a high value resistor in parallel to achieve the shift in setpoint required to proviide the required hysteresis band. The EDN circuit you linked works on the same principle but increases the part count by using a PNP BJT, which needs base resistors.
The StackExchange TL431 window comparator is from a depreciated (2004) copy of the Texas Instruments TL431 datasheet [here]. The current datasheet (rev. 2018) totally omits any such circuit. I believe that would be because its fundamentally flawed - depending on tolerances there's no guarantee that the left TL431's output low level is below the reference voltage of the right TL431. To fix it you need to insert an extra resistor in the middle of the R1B, R2B divider chain with the left TL431 cathode above it and the right TL431 Ref below it.
You can apply hysteresis to the right TL431 for the undervoltage threshold easily enough, but applying hysteresis to the left one for the overvoltage threshold is much trickier.
I'm with Magic on this: If you want a window function, the TL431 is seriously sub-optimal. The cheapest option is probably a 'jellybean' dual comparator with push-pull outputs, fed from a jellybean low power regulator so the reference can be a simple divider from its supply, with its outputs NORed together with a common cathode dual diode. Three semiconductors and a sprinkling of passives.
Of course you *CAN* pay boutique prices for dual comparators with a reference in the same package but finding one with adjustable hysteresis for both thresholds isn't easy and you'll probably not be able to easily second source it.
eblc1388:
Well for discussion and comment, the following is my feeble attempt to put hysteresis to the circuit. Without them the Vout oscillate both at the low and high level limit. The output seem clean after adding the hysteresis. So it is for your reference only.
Alti:
LM339 is around $36 for 1k, LM339 from ST, LCSC
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