Design Ideas: July 6, 1995

The circuit in Fig 1 protects a battery-operated system in two ways: Q₁ prevents damage from the flow of reverse current that could occur if you install the battery backward, and Q₃ prevents the excessive current flow that could occur with a sudden load increase or a short circuit. A properly installed battery fully enhances Q₁ by pulling its gate voltage more than 5V below the source voltage. If you install the battery backward, Q₁ stays off because the gate voltage is positive with respect to the source voltage. Regardless of battery polarity, the orientation of the body diodes of Q₁ and Q₃ ensures that no current can flow when either device is off.

IC₂ is a current-sensing amplifier that senses the load current flowing between its RS+ and RS terminals. IC₂'s output is a proportional but smaller (1.5 mA max) current at the OUT pin, which develops a voltage across R₈ proportional to the load current. During normal operation, both comparator outputs are high, and Q₃ remains on.

When the load current exceeds a limit set by R₈ (I_LIMIT= 2000V_TH/R₈), where 2000 is the sense amplifier's gain, and V_TH is the comparator's input threshold of 1.182V ñ2%, the B-comparator output switches low and turns off Q₄, which turns off Q₃ and disconnects the battery from its load. At the same time, Q₆ provides positive feedback by pulling the comparator input to the level of the collapsing supply rail, latching Q₃ off as the supply voltage drops.

An output short circuit turns off IC₂ by removing the voltage at pins 6 and 7 (3V is the minimum for proper operation). Control via the B comparator disappears because the R₈ voltage goes to zero for this short-circuit condition, but comparator A then shuts off Q₃ by turning off Q₅. Q₂ speeds the Q₃ turn-off time to about 10 µsec. When Q₃ is off, the circuit draws about 2 µA. During normal operation, the battery current varies with its terminal voltage: 200 µA at 5V, 230 µA at 6V, 300 µA at 8V, and 310 µA at 10V. To restore power, press S₁. (DI ##1730)