Design Ideas: March 16, 1995
To avoid a deep discharge that can shorten or destroy a rechargeable battery's life, you need to remove the load at the point of complete discharge. Because a battery's terminal voltage recovers after load removal, you can't simply disconnect the load when the voltage dips below a threshold and reconnect it when the voltage returns above that threshold. This would cause the disconnect switch to chatter. The voltage of a discharged cell recovers almost to the level of a fully charged cell, so simple hysteresis can't compensate for the recovery effect.
What you need is a circuit that disconnects the load from the battery and keeps them separate until an external signal, such as from a battery charger or pushbutton switch, indicates that the battery has been recharged or replaced.
Fig 1a shows such a circuit that enlists the low-battery comparator in IC1, a low-dropout linear regulator. In this circuit, the low-battery comparator and error amplifier share the internal reference and the external resistor divider. With the resistor values shown, the low-battery output goes low and disconnects the battery from the load when the output falls 8% below its nominal value. The load and battery then remain disconnected until S1 commands them otherwise.
Two features enable the latching action in this circuit. The low-battery comparator remains active during shutdown, whereas most regulators deactivate this comparator during shutdown. The circuit monitors the regulated output's voltage instead of the battery's voltage. The regulator's voltage can't recover until the regulator turns back on.
The circuit also provides a POWER FAIL signal, IC1's LBO output, that goes low 50 msec before the output turns off (Fig 1b). This signal can give a controlling µP time to perform housekeeping and shutdown functions. When LBO goes low, C1 discharges through R1 until the STBY input reaches its threshold of 1.15V. The IC then enters its standby mode and disconnects the battery. (DI #1675)