AN423

AN423
Maxim > Design Support > Technical Documents > Application Notes > Microprocessor Supervisor Circuits > APP 423
Keywords: LDO, low dropout regulator, power fail comparator, pfi, pfo, battery protection
APPLICATION NOTE 423
Latching Regulator Prevents Deep Discharge of
Battery
Mar 16, 1995
Abstract: In systems employing rechargeable batteries, it is important to disconnect the load before the
battery enters deep discharge, which may destroy or damage the cell. This circuit turns off the circuit
before the battery enters deep discharge and provides enough time for µC housekeeping activities.
A rechargeable battery's load should be removed at the point of complete discharge, to avoid a further
(deep) discharge that can shorten its life or destroy it altogether. Because a battery's terminal voltage
recovers when its load is removed, you can't simply disconnect the load when the terminal voltage dips
below the established threshold and then re-connect it when the voltage returns above that threshold.
Such action may produce chatter in the disconnect switch.
The voltage of a discharged cell returns almost to the level of a fully charged cell, so hysteresis can't
necessarily compensate for the recovery effect either. What's needed is a circuit that disconnects the
load from the battery and keeps them separate until an external signal (such as that from a battery
charger or pushbutton switch) indicates that the battery has been recharged or replaced.
Such a circuit can enlist the low-battery comparator in a low-dropout linear regulator (Figure 1). 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 (LBO) goes low and disconnects
both the battery and load when the output falls eight percent below its nominal value. The battery and
load then remain disconnected until commanded otherwise by S1.
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Figure 1. To protect the battery, this circuit disconnects the load before the battery enters deep
discharge. To reconnect, you must press S1.
Two factors enable the latching action in this circuit: the low-battery comparator remains active during
shutdown (most regulators deactivate this comparator during shutdown), and the circuit monitors the
regulated output voltage instead of the battery voltage (regulator voltage can't recover until the regulator
is turned back on).
The circuit also provides a active-low POWER FAIL signal (LBO, pin 1) that goes low 50ms before the
output is turned off (Figure 2). This signal can provide a controlling microprocessor time to perform
housekeeping and shutdown functions. When LBO goes low, C1 discharges through R3 until the activelow STBY input reaches its threshold (1.15V). The IC then enters its standby mode and disconnects the
battery. IC1 is a linear regulator capable of sourcing 150mA with a 350mV dropout voltage. It has a
10µA standby current and accepts input voltages to 11.5V.
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Figure 2. These waveforms illustrate timing relationships in the circuit of Figure 1.
A related idea appeared in the 3/16/95 issue of EDN.
Related Parts
MAX882
5V/3.3V or Adjustable, Low-Dropout, Low-IQ , 200mA
Linear Regulator with Standby Mode
Free Samples More Information
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