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Monitor circuit saves battery energy

18 Jan 2013  | Brian Huffman

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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 5µA of standby current and 30µA during low-battery indication.

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

The controller's VPP output switches to VCC 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=VCC) when VININ>SET POINT+DELTA.

When the controller reaches the lower trip point, the flip-flop latches, turning on Q1. Once latched, the VPP output drives Q2, causing the LED to flash at each sampling cycle. The circuit drives the LED with 75 mA for 80µsec every 220 msec. This operation results in an average current drain of 27µA. 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.

About the author
Brian Huffman is from Linear Technology Corp.

To download the PDF version of this article, click here.

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