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UVLO circuit design for NiMH battery system

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Hello EEVbloggers,

I wanted to design an under voltage lockout circuit for some project. The system is powered using a 3 NiMH cells connected in series. The circuit needs to cut off the power when the voltage of cells drop to about 2.8V~3V and connect it back only if the voltage is roughly > 3.6V. The NiMH cells always bounce back to 3.4-3.5V when they are fully discharged (at least in my case). therefore, i want to mitigate that effect.
So my idea was to go with an hystersis based circuit (see the circuit in the attached picture below). The idea is technically to use a rail-to-rail low voltage op amp (LMV358 my case) to do the trick since i would need the output to drive some TTL stage ahead to disconnect the battery. The simulations were fine. However, when i tried to implement it using a regular LM358, the hysterisis was a bit off as the cut off occured at about 3.2V not 2.8V~3V as wished and i do not currently have a low voltage rail to rail Op amp.

So my questions are:
- Does the rebouce effect of the cells mess with the thresholds of the hystersis loop ?
- Are there any alternative solutions (using the minimum of components) to implement this ?

Thank you in advance,


--- Quote from: _Vendetta_ on January 25, 2022, 09:15:42 am ---- Are there any alternative solutions (using the minimum of components) to implement this ?

--- End quote ---

Depends of what you are planning to do with the battery. For example, if your system requires a stable power rail many DC-DC converters already contain UVLO circuitry. The most you'd have to do in this case to use it is to calculate a voltage divider (2 extra components) and some of them don't even require any external parts (0 extra components).

A few remarks.

Perhaps some Li-ion protection IC could work?

TL431 is a big power hog and it seems you will keep it powered during shutdown to wait until the battery is recharged. Research low power substitutes, I know they exist but can't give concrete part numbers.

TL431 power consumption decreases when its reference input is less than 2.5V. So I recommend connecting it as below, to further reduce power consumption in shutdown. The low power variants may or may not have this property too, so RTFM ;)

With some trickery, it may be possible to do away with the opamp completely. See below. This requires a logic level MOSFET. TL431 can only pull its cathode to ~2V, which doesn't really leave much Vgs when supply is down to 3V. Look for 431 alternatives with 1.25V rather than 2.5V reference voltage - I think these could work (and yes, they exist).

In the example:
R1/R2 is sized to turn-on the xx431 chip at 3.6V input.
R4 reduces the threshold to keep it going down to 3V input.
R3 limits current drawn by the 431. Current needs to be at least the minimum spec in the datasheet (1mA for original TL431).

Isn't the problem that the simulations use a rail-to-rail opamp, but the LM358 used in the physical circuit isn't rail-to-rail?  With positive feedback, the ouput of the opamp in simulation will be either at ground or at the battery voltage.  But in practice the LM358 outputs don't extend to that range, particularly on the high side.  So it seems that would affect the hysteresis points.  It seems your circuit should work with either type of opamp, but not with the same resistor values.

The other thing that might affect the switching points is input offset voltage.  You can simulate that by adding a voltage source of +/- a few mV in series with the non-inverting input, and see if that changes anything.

Also, you might look at the MCP6041 or MCP6141 opamps, or the equivalent MCP6541 comparator with push-pull output and some internal hysteresis.  These are all very low power parts - 1uA quiescent current, and Digikey actually has some in stock.

I wonder if something like this actually requires the precision of the TL431.  Perhaps a zener, or even an LED, would work well enough.

The original circuit may work wrong with LM358 because of input common mode range too.

Something like LM385-1.2 is infinitely better than any LED for power consumption.
The input needs not be rail to rail if the reference is lower (like 1.25V).
And an opamp isn't needed, just a comparator.

LM385-ADJ looks like it could replace TL431 in my circuit above, if they still make it.


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