AN1872 Datasheet, PDF (1/2 Pages) Maxim Integrated Products – Low-Battery Indicator Has Low Cost and High Efficiency

AMPLIFIER AND COMPARATOR CIRCUITS
Application Note 1872: Feb 03, 2003
Low-Battery Indicator Has Low Cost and High Efficiency
The circuit described uses a low-power CMOS comparator to provide an LED visual
indication of a low-battery condition, without excessive battery current drain. This is
achieved by pulsing the LED at a low frequency and low duty cycle and conserving
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). IC1 is a small, inexpensive comparator with shutdown
capability in a 6-pin SC70 package. It remains in shutdown while the battery voltage is at
normal operating levels, but asserts LBO when battery voltage falls below a preset threshold.
Figure 1. Operating this low-battery-warning LED at low frequency and low duty cycle
saves power and extends battery life.
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