AN834 A L K A L I N E B ATTERY L OW -VOLTAGE I NDICATOR 1. Overview* In many battery-powered systems, a user would like to know when it is time to replace the batteries before they are completely discharged, causing the device being powered to fail completely. Alkaline batteries have an open cell voltage of about 1.5 V. As they are discharged, the voltage slowly drops as shown in Figure 1. When the cell voltage reaches about 1.25 V, they have delivered about 90% of their stored energy. Constant Load Starting Drain Temperature 8.25 k 181.8 µA 20 °C 1.6 1.5 Volume [V] 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0 200 400 Time [h] 600 800 Figure 1. Alkaline Battery Voltage Discharge Characteristic The circuit shown in Figure 2 monitors the voltage of two alkaline cells in series and begins flashing an efficient red LED when the total battery voltage drops below 2.5 V, alerting the user that the batteries need to be replaced. *Note: This application note has previously been published on the Discover Circuits web site and is republished here with the permission of the author, David Johnson of Discover Circuits. Rev. 1.0 1/15 Copyright © 2015 by Silicon Laboratories AN834 AN834 "" & ! ' $ !"&# "' "! "! $ 6,/,&21/$%6 $"" %$" $ ' (!" HVLJQHGE\'DYH-RKQVRQ !$ #"! OO LJKWV HVHUYHG Figure 2. Voltage Monitoring Circuit This circuit takes advantage of some new voltage comparators from Silicon Labs. The TSM9118 has an internal 1.25 V reference and an open drain output while drawing an astoundingly low 0.7 µA. That circuit is configured as a classic voltage monitoring circuit, which changes state at a 2.5 V battery voltage. A second TSM9119 part is wired as a 0.5 Hz oscillator which is configured to produce short 20 ms output pulses every 2 seconds. The TSM9119 is a conventional comparator with a push-pull output, which also draws a very low current. Using the two low power parts, the total standby current of the circuit is kept at a low 3 µA. When the LED starts flashing, the average current will increase to about 50 µA. This is still low enough that the circuit should continue flashing for some time, giving the user time to notice the warning and replace the batteries. For additional information, contact Silicon Labs. 2 Rev. 1.0 Smart. Connected. Energy-Friendly Products Quality Support and Community www.silabs.com/products www.silabs.com/quality community.silabs.com Disclaimer Silicon Laboratories intends to provide customers with the latest, accurate, and in-depth documentation of all peripherals and modules available for system and software implementers using or intending to use the Silicon Laboratories products. Characterization data, available modules and peripherals, memory sizes and memory addresses refer to each specific device, and "Typical" parameters provided can and do vary in different applications. Application examples described herein are for illustrative purposes only. Silicon Laboratories reserves the right to make changes without further notice and limitation to product information, specifications, and descriptions herein, and does not give warranties as to the accuracy or completeness of the included information. Silicon Laboratories shall have no liability for the consequences of use of the information supplied herein. This document does not imply or express copyright licenses granted hereunder to design or fabricate any integrated circuits. The products must not be used within any Life Support System without the specific written consent of Silicon Laboratories. A "Life Support System" is any product or system intended to support or sustain life and/or health, which, if it fails, can be reasonably expected to result in significant personal injury or death. Silicon Laboratories products are generally not intended for military applications. Silicon Laboratories products shall under no circumstances be used in weapons of mass destruction including (but not limited to) nuclear, biological or chemical weapons, or missiles capable of delivering such weapons. Trademark Information Silicon Laboratories Inc., Silicon Laboratories, Silicon Labs, SiLabs and the Silicon Labs logo, CMEMS®, EFM, EFM32, EFR, Energy Micro, Energy Micro logo and combinations thereof, "the world’s most energy friendly microcontrollers", Ember®, EZLink®, EZMac®, EZRadio®, EZRadioPRO®, DSPLL®, ISOmodem ®, Precision32®, ProSLIC®, SiPHY®, USBXpress® and others are trademarks or registered trademarks of Silicon Laboratories Inc. ARM, CORTEX, Cortex-M3 and THUMB are trademarks or registered trademarks of ARM Holdings. Keil is a registered trademark of ARM Limited. All other products or brand names mentioned herein are trademarks of their respective holders. Silicon Laboratories Inc. 400 West Cesar Chavez Austin, TX 78701 USA http://www.silabs.com