Analysis of Monitoring Circuits for Battery Powered Systems

Many battery -powered systems require a visual indicator to show when the battery needs to be replaced. LEDs are commonly used for this purpose, but they consume at least 10mA of current. This considerable current can accelerate the discharge of the battery and shorten its useful life. Figure 1 uses a sampled data technique to reduce the average power consumption of the monitoring circuit. The circuit has a standby current of 5μA and consumes 30μA when indicating low voltage.

During a sampling cycle, the LTC1041 set range controller powers up its two internal comparators; samples the VIN, SET POINT, and DELTA inputs; stores the comparison results in an output latch; and then powers down. This process takes about 80μs. The external RC network formed by R1 and C1 determines the sampling rate.

Figure 1: Rather than continuously draining battery power, this monitoring circuit samples its input to achieve 5μA standby power consumption and 30μA low-voltage indication power consumption.

The controller’s VPP output switches to VCC during the controller’s effective 80μs on-time and switches to high impedance during the off-time. A fast settling reference sets the trip point. R2 must be small enough to provide the minimum current required for the LT1009. R3, R4, and R5 divide the battery voltage and feed it into the input of a comparator. The resistors provide a lower trip point of 5.5V and an upper trip point of 5.95V. The internal comparator has a low current bias point, allowing the use of large value resistors for the divider. R5 sets the comparator’s hysteresis. The comparator drives an internal RS flip-flop that resets (ON/OFF=ground) when VINSETPOINT+DELTA.

When the controller reaches the lower trigger, the trigger latches, turning Q1 on. Once latched, the VPP output drives Q2, causing the LED to flash on each sampling period. The circuit drives the LED for 80μs every 220ms at 75mA. This operation results in an average current consumption of 27μA. The LED may flash once during power-up because the latch voltage cannot be determined. Bypass capacitor C2 ensures low source impedance under transient loads.