Analysis of CR2477 battery-powered system monitoring circuit

Many CR2477 battery-powered systems require a visual indicator to indicate when the batteries need to be replaced. LEDs are generally used for such indicators, but they consume at least 10mA of current. This large current will accelerate the discharge of the CR2477 battery and shorten the useful life of the CR2477 battery. 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 low voltage is indicated.

During a sampling period, the LTC1041 sets the range controller to power up its two internal comparators; samples the VIN, SETpOINT, 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.

This monitoring circuit does not continuously consume CR2477 battery power, but achieves 5μA standby power consumption and 30μA low-voltage indication power consumption by sampling the input.

The controller's Vpp output switches to VCC during the controller's active 80μs on-time and to high impedance during the off-time. A quick settling baseline sets the trigger point. R2 must be small enough to provide the minimum required current to the LT1009. R3, R4 and R5 divide the CR2477 battery voltage and feed it to 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 voltage divider. R5 sets the comparator hysteresis. The comparator drives an internal RS flip-flop. When VINSETpOINT+DELTA, the flip-flop is reset (ON/OFF=ground).

When the controller reaches the lower flip-flop, the flip-flop latches, causing Q1 to conduct. Once latched, the Vpp output drives Q2, causing the LED to blink on each sample cycle. The circuit drives the LED for 80μs every 220ms with 75mA current. This operation results in an average power consumption of 27μA. The LED may flash once during power-up because the latch voltage cannot be determined. Bypass capacitor C2 ensures low supply impedance under transient loads.

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