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Design Ideas |
To avoid the deep discharge that can destroy
or shorten the life of a rechargeable battery, you must disconnect its load
before the discharge is complete. This disconnect function requires a
battery-voltage monitor, which must incorporate considerable hysteresis to
prevent chatter in the disconnect switch. The chatter occurs as the battery
voltage first dips below threshold and then bounces back on load removal. The
circuit in 
Figure
1 implements the disconnect function by using a high-side,
load-disconnect transistor (the p-channel MOSFET).
The micropower MAX933 IC, containing two comparators and a voltage reference, controls Q2. R4 and R5 set a battery-voltage threshold of 5.5V for comparator A, which generates the BATT LOW output. R1 and R2 set the threshold for comparator B, which disconnects the load by turning Q2 off when the battery voltage drops to 4.8V. Comparator B also controls Q1. When Q1 is on, it adds hysteresis by connecting R3 in parallel with R2. The battery voltage must then rise to 6.6V before reconnection of the load. At that time, Q1 turns off and returns the load-disconnect threshold to 4.8V. R7 and R8 provide a few hundred millivolts of hysteresis for the BATT LOW signal; you can omit these components if hysteresis is not needed. The circuit draws 7.1 µA of battery current when the load is connected and draws 11 µA, otherwise. (DI #1926)