Need help TLV431 Low Voltage Cutoff Circuit

Thread Starter

bigjoncoop

Joined Feb 1, 2019
129
Been looking for a low voltage cutoff circuit for lithium battery powered projects and came across this one. ( schematic below )

Couple Questions :

1. Does it appear to be Correct ?

2. Why do I need to set Hysteresis? Description said "R4 adjusts the hysteresis (difference between high and low voltage trip points" I only care about low voltage...

3. Where my load is at, is that where it gets connected? I assume so since the circuit has a resistor added for a simulated load. I replace that resistor with my load correct?

Thx

Here is Original Description of circuit and below is original circuit and Mine.

" The circuit uses a TLV431 low-power voltage-reference IC as a comparator, driving a high-side power P-MOSFET to switch the load voltage. The P-MOSFET is ON when the TLV431 Ref voltage is above 1.24V, and OFF when it is below 1.24V, as determined by the resistor divider network at that node. Pot U2 adjusts the trip point and R4 adjusts the hysteresis (difference between high and low voltage trip points. The simulation shows a lower trip point of 11.79V and and upper trip point of 13.1V for the 50% pot setting and resistor values shown.

The circuit maximum current drawn from the battery is about 1/2 mA.

The maximum load current is determined by the MOSFET used. To avoid the use of a heatsink, select a P-MOSFET with an ON resistance low enough to keep the transistor dissipation below about a watt for the maximum load current (Rds(on) ≤ 1 / Ild²).

Circuit desigh by @crutschow @ allaboutcircuits.com "

TL431 voltage cutoff.JPG
 

schmitt trigger

Joined Jul 12, 2010
320
Always leave some hysteresis, however small, in any comparator circuit, otherwise it might oscillate as it crosses the threshold point.
And yes, modifying R4’s value will change the hysteresis, with higher valued resistors decreasing the hysteresis.
 

Ian0

Joined Aug 7, 2020
491
1. IRF7309 is a dual MOSFET. Seems an odd choice when only the P-channel half is required. And just to be pedantic, the reverse diode is shown the wrong way round on your diagram.
2. The TLV431 starts to pass current at 0.6V between adj and cathode, rising linearly to 50uA at the 1.24V switching point (see figure 6 of the datasheet)
The threshold of the IRF7309 is only 1V, so with a 50k resistor gate to source, only 20uA will be required to start turning the MOSFET on, and that will happen at about 0.9V on the TLV431. If the MOSFET gets biassed partially on, it will dissipate a lot of heat and blow up.
Reduce R9 to below 20k to stop this happening.
3. This circuit might be safety-critical to stop a Lithium battery catching fire on over-discharge. Probably not the best circuit in which to use a preset. As drawn it will fail safe, and switch the battery off if the wiper lifts off the track, but don't think of putting the preset in series with R10 instead of in series with R7.
You're already using E96 resistors which must be better than 1% tolerance. Just use a fixed value resistor and ditch the preset. Your preset allows for adjustment between 11.5V and 14.5V, which is more than plenty, I bet you can't even adjust that to within 1%!
 
Last edited:

crutschow

Joined Mar 14, 2008
25,423
Yes, the P-MOSFET is backwards (notice that the substrate diode is forward biased).
If you are going to copy my circuit, please copy it exactly. ;)
 
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