Many rechargeable battery types can be damaged by deep discharge. This circuit performs under-voltage protection (UVP) to prevent this, as well as acting as a load switch. With little or no modification, it can work for almost any battery type, with voltage from 4.5V - 19V. Standby current is under 1µA.
High-side N-channel MOSFET Q2 reduces cost compared to a P-channel part. Turn-on and -off are inherently soft, so switching spikes are avoided.
Figure 1 The circuit is set up for a 12V lead accumulator with gelled electrolyte at 20°C. Another type of accumulator may require component value changes.
The first time the battery/accumulator is connected to +Acc, the circuit is off. C3 is discharged, so the programmable reference TL431A (D1) will be off, with leakage under 1µA. Thus, all other elements of the circuit are disabled, and Q2 is off because its gate is discharged through R5.
In this state the circuit is waiting for an enabling positive pulse at the On input – via a button or other controller. During this pulse, the TL431A turns on, providing power to the TLC555, configured as an oscillator. Through C2 & D3, the oscillator produces a bootstrap voltage on the gate capacitance of Q2, turning it on.
After the On signal is removed, the circuit remains in the active state because the voltage from divider R1-R2 charges C3, maintaining the loop from D1 to Q2 back to D1.
The circuit disconnects the load and turns itself off if the /Off input goes low or the under-voltage trip point is reached.
The trip point expression is:
Vt = (1 + R1/R2) Vref
(Vref is the TL431 2.5V reference voltage)
So R1/R2 = Vt / Vref - 1