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Analysis of monitoring circuit of lithium battery power supply system

Analysis of monitoring circuit of lithium battery power supply system


Many battery-powered systems require a visual indicator to indicate when the battery needs to be replaced.

Leds are commonly used for such indicators, but they consume at least 10mA of current.

This small current accelerates the discharge of the battery and shortens its useful life.

Figure 1 uses a sampling data technique to reduce the average power consumption of the monitoring circuit.

The circuit has a standby current of 5A and a power consumption of 30A when indicating low voltage.

The LTC1041 sets the range controller to power up its two internal comparators during a sampling period;

Sampling VIN, SETpOINT and DELTA inputs;

Store the comparison results in an output latch;

Then power off.

This process takes about 80 seconds.

The sampling rate is determined by the external RC network of R1 and C1.

In Figure 1, this monitoring circuit does not continuously consume battery power, but achieves 5A standby power consumption and 30A low-voltage indicating power consumption through sampling input.

When the controller was valid at the time of 80s, the Vpp output of the controller was switched to VCC, and when the controller was turned off, it was switched to high resistance.

A quick settle benchmark sets the trigger point.

R2 must be small enough to supply the minimum current required for the LT1009.

R3, R4, and R5 divide the cell voltage and feed it into the input of a comparator.

The resistor provides a lower trigger of 5.5V and a upper trigger of 5.95V.

The internal comparator has a low current bias point, enabling the use of high resistance resistors for voltage dividers.

R5 sets the hysteresis of the comparator.

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, making Q1 turn on.

Once latched, the Vpp output drives Q2, causing the LED to blink during each sampling cycle.

The circuit drives the LED80s at 75mA current every 220ms.

The result is an average power consumption of 27A.

The LED may flash once during power-on because the latch voltage cannot be determined.

The bypass capacitor C2 ensures low power supply impedance under transient loads.

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