Application cameo - 4,5 µA protection circuit lengthens Li-ion battery life

22 November 2000 Circuit & System Protection

If a Li-ion battery discharges below the recommended end-of-discharge voltage, the cycle life shortens and the battery dies prematurely.

Linear Technology describes the circuit in Figure 1 that monitors a Li-ion battery and disconnects the load to protect the battery from deep discharge when the battery voltage drops below a preset lockout voltage, while itself consuming only 4,5 µA.

For example, a lockout voltage of 3 V, set by the ratio of R1 and R2, is sensed at node A. When the battery voltage falls below the lockout voltage, node A falls below the threshold at node B, which is equal to:

V_B = 1,25 V + I • R4 = 1,37 V

where:

I = (V_T - 1,25 V)/(R3 + R4) = 800 nA,

and V_T = lockout voltage.

The output of the LT1495 will then swing high, turning off SW1 and disconnecting the load from the battery. To avoid oscillation due to the battery recovering when the load is disconnected, R5 is added to provide hysteresis around the trip point (Figure 2). With the resistor values shown, the hysteresis for the circuit is 92 mV. V_BATT must climb back above 3,092 V before the load is reconnected.

Hysteresis = V_B' • R1/R2 + V_B' - V_T where:

V_B' = (V_OMAX - I • R4) • R4/R5 + V_REF + I • R4

and:

V_OMAX = maximum output swing (high) of op-amp when V_BATT is equal to the lockout voltage.

The desired amount of hysteresis may be determined by multiplying the maximum ESR recommended for the battery by the recommended discharge current.

A lockout of too high a voltage will decrease the maximum capacity of a battery and reduce its operating range. With too low a lockout voltage, the battery is discharged too deeply, reducing its performance and life. With the 0,4% overall accuracy of the circuit shown, the lockout voltage has a narrow range of 2,988 to 3,012 V.

For further information on Linear Technology products, contact Insight, (021) 674 4103.

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