Current-level monitor with hierarchical precision
First Claim
1. A circuit for monitoring current level magnitudes, polarity, or both associated with the charging and discharging of a battery having a positive terminal and a negative terminal, wherein the circuit acts as a switch for interrupting battery current when the battery current exceeds preset levels, the circuit comprising:
- a. a first power transistor having a first turn-on resistance, wherein said first power resistor is coupled to the positive terminal of the battery;
b. a second power transistor having a second turn-on resistance, wherein said second power transistor is coupled between said first power transistor and the negative terminal of the battery;
c. a first coarse comparator having a first input coupled between said first power transistor and the positive terminal and a second input coupled between said second power transistor and the negative terminal;
d. a second coarse comparator having a first input coupled between said second power transistor and the negative terminal and a second input coupled between said first power transistor and the positive terminal; and
e. a fine comparator activated by an output of either of said first coarse comparator and said second coarse comparator, said fine comparator having a first input coupled to a first resistance terminal of a resistive element and a second input coupled to a second resistance terminal of said resistive element, wherein an output of said fine comparator controls operation of said first power transistor and said second power transistor.
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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.
44 Citations
20 Claims
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1. A circuit for monitoring current level magnitudes, polarity, or both associated with the charging and discharging of a battery having a positive terminal and a negative terminal, wherein the circuit acts as a switch for interrupting battery current when the battery current exceeds preset levels, the circuit comprising:
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a. a first power transistor having a first turn-on resistance, wherein said first power resistor is coupled to the positive terminal of the battery; b. a second power transistor having a second turn-on resistance, wherein said second power transistor is coupled between said first power transistor and the negative terminal of the battery; c. a first coarse comparator having a first input coupled between said first power transistor and the positive terminal and a second input coupled between said second power transistor and the negative terminal; d. a second coarse comparator having a first input coupled between said second power transistor and the negative terminal and a second input coupled between said first power transistor and the positive terminal; and e. a fine comparator activated by an output of either of said first coarse comparator and said second coarse comparator, said fine comparator having a first input coupled to a first resistance terminal of a resistive element and a second input coupled to a second resistance terminal of said resistive element, wherein an output of said fine comparator controls operation of said first power transistor and said second power transistor. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)
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15. A method of protecting a rechargeable battery from overcharging or overdischarging, the battery including a positive terminal and a negative terminal, the method comprising the steps of:
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a. directing current associated with the battery through a resistive switch coupled between the positive terminal and the negative terminal of the battery; b. comparing a voltage across said resistive switch with a pair of coarse comparators; c. controlling activation of a fine comparator with an output of said coarse comparators as a function of said voltage across said switch; and d. controlling operation of said resistive switch with an output of said fine comparator. - View Dependent Claims (16, 17, 18, 19, 20)
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Specification