June 5, 1997

Battery-protection circuit allows surges

Ron Lenk, Temic/Siliconix, Santa Clara, CA

The circuit in Figure 1 monitors two lithium-ion batteries and protects both of them against overcharging, overdischarging, and undervoltage. Undervoltage is a condition in which a load draws the cells down so far that it's dangerous to draw any current at all from them. In normal-discharge operation, the circuit draws only 30 µA, and, in undervoltage conditions, the circuit draws only 1 µA. IC₁ is an eight-pin device that senses the direction of current flow and adjusts the drive to the two MOSFETs, Q₁ and Q₂, to ensure that they turn off in any of the fault conditions. The common-source connection of the two MOSFETs ensures that when IC₁'s DCO pin is low, current can flow in neither direction, with each MOSFET blocking the other's body diode.

By monitoring the voltage drop in R₁, IC₁ also accurately senses the magnitude of the current that the load draws from the battery. IC₁ turns off the MOSFETs within 25 µsec from the time the voltage across R₁ reaches 28 mV. You can adjust the overcurrent limit by selecting R₁. In some applications, it may be desirable to allow a short peak of power without tripping the overcurrent protection. The filter comprising R₂ and C₁ allows short power peaks. It takes time to charge C₁ through R₂; thus, a short pulse of high current does not turn off IC₁. In Figure 1, the time extension is approximately 47 kilohms×470 nF=25 msec. The upper limit is a function of the leakage current into IC₁'s I_S pin. R₂ should not exceed 100 kilohms, but you can increase C₁. (DI #2041)

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