Monitor circuit conserves battery energy

Brian Huffman, Linear Technology Corp, Milpitas, CA - April 19, 2012

Originally published in the August 20, 1990, issue of EDN

Many battery-powered systems require a visual indication when the battery needs replacement. LEDs often serve as the indicator, but they can draw as much as 10 mA of current. This excessive current drain unduly accelerates the battery’s discharging and curtails the battery’s useful life. Figure 1 uses a sampled-data technique to lower the monitor circuit’s average power consumption. The circuit draws 5 μA of standby current and 30 μA during low-battery indication.

Monitor circuit conserves battery energy figure 1

During a sampling cycle, the LTC1041 bang-bang controller applies power to both of its internal comparators; samples the V_IN, SET POINT, and DELTA inputs; stores the results of the comparisons in an output latch; and turns off power. This process takes approximately 80 μsec. An external RC network consisting of R₁ and C₁ sets the sampling rate.

The controller’s V_PP output switches to V_CC during the controller’s active 80-μsec on time and switches to a high impedance during off time. A fast-settling reference sets the trip points. R₂ must be small enough to supply the LT1009’s minimum required current. R₃, R₄, and R₅ divide the battery voltage and feed it into a comparator input. The resistors provide a lower trip point of 5.5V and an upper trip point of 5.95V. The internal comparators’ low-current bias point permits using high-valued resistors for the divider. R₅ sets the comparator’s hysteresis. The comparators drive an internal RS flip-flop; the flip-flop is set (ON/OFF=V_CC) when V_IN<SET POINT−DELTA. The flip-flop is reset (ON/OFF=ground) when V_IN>SET POINT+DELTA.

When the controller reaches the lower trip point, the flip-flop latches, turning on Q₁. Once latched, the V_PP output drives Q₂, causing the LED to flash at each sampling cycle. The circuit drives the LED with 75 mA for 80 μsec every 220 msec. This operation results in an average current drain of 27 μA. The LED may flash once during power up because the latch output is temporarily indeterminate. A bypass capacitor, C₂, ensures low-supply impedance under transient loads.