Maxim > Design Support > Technical Documents > Application Notes > Amplifier and Comparator Circuits > APP 1872
Maxim > Design Support > Technical Documents > Application Notes > Automotive > APP 1872
Maxim > Design Support > Technical Documents > Application Notes > Battery Management > APP 1872
Keywords: comparator, low battery, LED, low cost, low power consumption, comparators
APPLICATION NOTE 1872
Low-Battery Indicator Is Low Cost, Highly Efficient,
and Environmentally Friendly
Feb 03, 2003
Abstract: This application note presents a circuit that uses a low-power CMOS comparator to provide an
LED visual indication of a low-battery condition. This LBO (low-battery-indicator) is achieved by pulsing
the LED at a low frequency and low duty cycle. The circuit accomplishes this without draining excessive
battery current that can lead to permanent battery damage and, ultimately, hazardous waste disposal.
This circuit also helps conserve battery current in the off cycle by placing the comparator in shutdown.
Equations and circuit analysis are included for determining duty cycle and comparator trip points.
The usual method for implementing the low-battery warning featured in most battery-operated equipment
is to illuminate an LED. By drawing current, however, the LED exacerbates the low-battery condition.
You can greatly reduce the LED's power consumption by operating it at a low frequency and a low duty
cycle.
An existing low-battery output (LBO) like that found on DC-DC converters offers a convenient way to
light the LED (Figure 1). The MAX9030 is a small, inexpensive comparator with shutdown capability in a
6-pin SC70 package. The device remains in shutdown while the battery voltage is at normal operating
levels; it asserts LBO when battery voltage falls below a preset threshold.
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