I'm powering a project with a 2-cell 7.4V LiPo battery. What circuit do I add to make the unit stop drawing power when the voltage from the battery drops below 6.4V? The goal is to protect the LiPo battery from discharging below 3V/cell.
Answer
In all the following a TLV431 1.25V reference is specified.
This requires < 100 uA minimum regulation current compared to about 500 uA for the 2.5V TL431.
When on the TLV431 on voltage is about equal to the reference voltage - NOT 0V.
TLV431 current is a battery load even when output is off. At about 100 uA this drains about 2.5 mAh/day.
Voltage sensing divider is also a battery load. This can be small.
Hysteresis is not used in any of the following circuits - except the one copied from internet. A whiff of hysteresis could be used to stop battery on/off cycling when load is removed. Ask if unclear how to do this.
(1) Opamp based
P Channel FET high side switch.
TLV431 1.25 V reference.
R2/R3 divide Vmin to = 1.25V.
(2) N Channel FET, low side switch.
TLV431 1.25V reference.
FET Vth << Vmin
(3) P Channel FET, high side switch.
TLV431 = 1.25V reference.
FET Vth << (Vmin-1.25)V
(4) From web - similar to my N Channel low side circuit.
From This discussion page.
Here R6 adds hysteresis.
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