Keeping battery-powered systems alert

Low battery detection is important inbattery-powered systems to alert the user to replace the battery.In some cases, a secondary level of detection is necessary to alertthe system to a critical low battery level. A nanopower voltagedetector device can be implemented that combines a low-powerreference and a comparator to build a simple low battery detectioncircuit.

Measurement set-up

As shown in Figure 1, a low-battery detectionapplication circuit can be designed using two low-power voltagedetectors, such as the Touchstone TS12001. The detectors arepowered by a single 1.5V lithium AA battery whereas the other partsof the circuit are powered from 3V supplied by a separate boostconverter. A threshold voltage of 1.356V and 1.288V is set for thetop voltage detector IC and the bottom voltage detector IC,respectively. When the battery drops below a programmed thresholdvoltage, the detectors’ open drain outputs switch from aLOW state to a HIGH state. Detector output voltages are thenapplied to NAND gates that generate a LOW state to forward bias theLEDs. The NAND gate outputs are also connected to GPIO inputs of amicrocontroller. When the yellow LED detector is triggered, a lowbattery level (1.356V) is indicated. When the red LED detector istriggered, a critical low battery level (1.288V) is indicated.

Figure 1. TS12001 Low Battery DetectionCircuit

For testing purposes, the 1.5V lithium AAbattery was replaced with a power supply. Figure 2 shows thecomplete circuit where two demo boards are used.

Results

When the power supply voltage applied to the SETpin reached approximately 1.356V, the yellow LED turned on and thered LED remained off, representing a low battery level condition.However, when the power supply voltage was reduced further andreached approximately 1.288V, the red LED turned on indicating acritical low battery level. At this point, both LEDs were on asexpected.

Figure 2. TS12001 Low Battery DetectionCircuit Picture

Other considerations

Some voltage detectors, like the TS12001, makeavailable a comparator latch function. In this IC, the(“LHDET” ) ̅ pin is useful forcapturing one-time events. While this function was not explicitlyused in this application, the (“LHDET” )̅ pin was connected to VIN thereby disablingone-time-latch operation. For detailed explanation of thisfunction, please refer to the video referenced below.The open drainoutput is useful for voltage level translation. In this circuit, itwas used to translate a 1.5V supply to the 3V supply environmentrequired by the downstream microcontroller.

In summary, the application circuit describedherein achieves a number of goals: a) user-programmable thresholdvoltages; b) low-supply-voltage operation from 0.65V to 2V; c)low-monitor-mode supply current of 2µA; and d) smallactive pcb area less than 5mm2, including external components ifthe discrete NAND gates are already part of a system gatearray.

For additional information, please follow thelinks to the corresponding product pages:

• TS12001 Voltage Detectorhttp://touchstonesemi.com/products/ts12001

• TS12001 Application Videohttp://touchstonesemi.com/videos/ts12001-application-video