Current-level monitor with hierarchical precision

Abstract

Current-level monitoring circuitry incorporating a full-time coarse monitor and a part-time fine monitor and capable of generating control signals when the current-level being monitored reaches certain predetermined thresholds. In its preferred embodiment the invention is incorporated into battery-protection circuitry, guarding against both excess charging currents and excess discharging currents. A key concept of the invention is a hierarchical monitoring system incorporating a full-time coarse monitor that activates the fine monitor only when the battery current level enters a certain range and then deactivates it once the level falls out of that range again. Should the current level continue to rise up to the threshold of unsafe battery current, the fine monitor will disconnect the battery. In the preferred embodiment of the invention, the fine monitor operates by comparing, with a predetermined reference voltage, the voltage drop across a fine sensing resistor through which battery current is directed. The fine sensing resistor may be a precision resistor or it may be the same resistance used by the coarse monitor. The reference voltage is established in a way that permits a differing disconnect threshold to be set depending on the battery current polarity and in such a way that it changes with temperature so as to largely offset the temperature dependence of the sensing resistors. The present invention utilizes a single fine comparator for monitoring both battery-charge and battery-discharge, thereby providing high accuracy without the need for an additional fine comparator with its high space and power demands.