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Design Ideas |
Modern low-dropout (LDO) voltage regulators for battery-powered
or other micropower applications offer a variety of monitoring and diagnostic
functions, such as low-battery and out-of-regulation signals. Dedicated LED
indicators for these diagnostic functions, plus a typical power-on indicator,
consume relatively large amounts of power. This design integrates several
power-monitoring/diagnostic functions into one bicolor LED, thus conserving
power and extending battery life. The smart power monitor in
Figure 1 exploits the visual attributes,
including continuous glow, blinking, and color variance, of the bicolor LED to
display the normal operating condition and the low-battery and out-of-regulation
conditions.
The circuit uses an LP2953 LDO regulator from National Semiconductor in a typical suggested battery-power application. The regulator provides a 5V regulated output and low-battery and out-of-regulation signals. The power-monitor section in the dashed-line box comprises a standard, inexpensive quad NOR IC and a bicolor LED from Siemens. In a normal condition, both the low-battery and out-of-regulation outputs are high, causing the green LED to glow continuously. (The outputs of IC2D and IC2C are low.)
When the battery voltage sinks below the predetermined threshold (approximately 5.8V for the R1/R2 ratio shown), the low-battery output switches low, starting oscillation in the free-running multivibrator comprising IC2A and IC2B. The oscillation frequency depends on the R6C3 time constant; for the values shown, it's approximately 2 to 4 Hz. As the output of IC2D stays low and the output of IC2C periodically changes its state, the green LED blinks, indicating a low-battery condition.
If the battery voltage continues to drop, the LDO will go out of linear-regulation mode. In this case, the out-of-regulation signal switches low, causing the output of IC2D to go high and the output of IC2C to go low. The red LED then glows continuously, indicating the out-of-regulation condition. The 5V output from the LDO powers the NOR IC. Other LDOs, such as the LP2960 or LP2956, are also suitable for this application. (DI #1940)
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