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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.