Abstract:
A battery protection circuit includes a moisture detection circuit, a temperature sensing circuit, and a high-temperature battery discharge circuit. The moisture detection circuit includes a pair of conductive traces closely spaced on a substrate such that a resistive path is formed between the traces when moisture forms on the substrate. The traces are connected between the positive battery terminal and a pull-down current source. When moisture forms on the substrate, pull-up current flows between the traces, and a resulting voltage change on one of the traces is detected by circuit element such as a logic inverter. The temperature sensing circuit includes a voltage reference circuit that generates a proportional-to-temperature voltage and temperature-independent voltage reference signals corresponding to various predetermined temperatures. A measuring circuit operates during a sampling interval to compare each temperature-dependent voltage to the proportional-to-temperature voltage and to store the result of each comparison until a subsequent temperature sampling interval. The measuring circuit includes multiplexing circuitry used to sequentially select each temperature-dependent voltage during a sampling interval. The high-temperature discharge circuit connects a discharge load across the battery when the voltage of the battery is above a high voltage threshold and the temperature of the battery is above the high temperature threshold. When no external charger is active, the discharge current flowing through this load works to reduce battery voltage. helping to prolong battery life.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) of provisional patent application no. 60/157,428 filed Oct. 4, 1999, entitled “Battery Protector”. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention is related to the field of battery protection circuits. 
     The need for special battery protection circuits has increased due to the widespread use of batteries such as lithium-ion batteries, which are inherently less tolerant of adverse operating conditions than are more traditional batteries such as those using nickel-based chemistries. Circuits have been used to detect whether battery voltage or current exceeds a predetermined safe level, and take certain compensatory actions upon detection of such conditions. For example, if excessively high battery current is detected, a protection circuit may turn off a protection transistor arranged in series with the battery, interrupting the flow of current. 
     For applications in which it is important to make efficient use of stored battery power, it is desirable that protection circuitry consume as little power as possible. Additionally, many battery applications are particularly cost sensitive. For such applications, it is important that the battery protection circuitry be relatively simple, compact, and easily manufactured. 
     It has been determined that the operating temperature of lithium-ion and similar batteries plays a significant role in efficient battery operation. It is desirable, for example, to charge such a battery only when the temperature is within certain ordinary limits, such as between about 0° C. and about 35° C. Also, battery life may be reduced if the battery is exposed to very high temperatures while being in a fully charged state. It is desirable to provide battery protection circuitry capable of addressing these operational concerns. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention, a battery protection circuit is disclosed that addresses several special considerations in the use of lithium-ion and similar batteries, such as temperature-dependent operation, undesirable exposure of the battery to moisture, and avoiding certain conditions that may shorten battery life. 
     The disclosed battery protection circuit includes a moisture detection circuit, a temperature sensing circuit, and a high-temperature battery discharge circuit. The moisture detection circuit includes a pair of conductive traces closely spaced on a substrate such that a resistive path is formed between the traces by moisture that forms on the substrate. The traces are connected between the positive battery terminal and a pull-down current source. When moisture forms on the substrate, a resistance is formed between the traces. The resulting pull-up current that flows between the traces causes a voltage change on one trace, which is detected by a circuit element such as a logic inverter. The output of the circuit element can be used to provide an indication to a user or to initiate other appropriate action. 
     The temperature sensing circuit includes a voltage reference circuit that generates a proportional-to-temperature voltage and temperature-independent voltage reference signals corresponding to various predetermined temperatures. A measuring circuit operates during a low-duty-cycle sampling interval to compare each temperature-dependent voltage to the proportional-to-temperature voltage and to store the result of each comparison until a subsequent temperature sampling interval. The measuring circuit includes multiplexing circuitry used to sequentially select each temperature-dependent voltage during the sampling interval, and provide the selected voltage to a comparator along with the proportional-to-temperature voltage. The output of the comparator is then selectively stored in a corresponding one of a set of storage devices such as flip/flops. The outputs of these storage devices can be used for a variety of operations that rely on information about the temperature of the battery. 
     The high-temperature discharge circuit connects a discharge load across the battery when the voltage of the battery is above a high voltage threshold and the temperature of the battery is above the high temperature threshold. When no external charger is active, the discharge current flowing through this load works to reduce battery voltage. By reducing battery voltage when the battery is exposed to high temperature, battery life can be prolonged. 
     Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The invention will be more fully understood by reference to the following Detailed Description in conjunction with the Drawing, of which: 
     FIG. 1 is block diagram of a battery protection circuit in accordance with the present invention; 
     FIG. 2 is a diagram of one side of a substrate containing the protection circuit of FIG. 1; 
     FIG. 3 is a diagram of the other side of the substrate of FIG. 2, showing a moisture transducer forming part of the battery protection circuit of FIG. 1; 
     FIG. 4 is a schematic diagram of a current bias reference circuit forming part of the protection circuit of FIG. 1; 
     FIG. 5 is a schematic diagram of a voltage reference circuit forming part of the protection circuit of FIG. 1; 
     FIG. 6 is a schematic diagram of a temperature measuring circuit forming part of the protection circuit of FIG. 1; and 
     FIG. 7 is a schematic diagram of a high-temperature discharge circuit forming part of the protection circuit of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, a number of functional circuit elements are connected to the terminals V BATT   +  and V BATT   −  of a lithium ion battery  10  by corresponding conductors. As will be appreciated from the following description, the various components are preferably located very closely to the battery  10 , and in fact in one embodiment may be located within the battery casing. In this manner, battery variables such as voltage and temperature can be more accurately detected for optimal operation of the battery protection circuitry. 
     A moisture transducer  12  has an input connected to the positive battery terminal V BATT   +  and an output labeled WET that is provided to a current bias reference circuit  14 . As described in more detail below, the resistance of the transducer  12  in the presence of moisture (referred to herein as the “wet” resistance) is substantially different from the resistance of the transducer  12  in the absence of moisture (referred to as the “dry” resistance). The current bias reference circuit  14  detects when the resistance of the transducer  12  is equal to its wet resistance, and generates a signal FAULT indicating the presence of moisture. The signal FAULT can be used by other circuitry (not shown) to alert a user that a potentially unsafe operating condition exists, enabling the user to take appropriate action. 
     In addition to its moisture-detection functionality, the current bias reference circuit  14  generates current bias reference signals PBS and NBS used by a voltage reference circuit  16  and other circuitry (not shown) that may require a precision current reference for operation. The circuitry generating the signals PBS and NBS is described with reference to FIG. 4 below. 
     The voltage reference circuit  16  of FIG. 1 generates a number of different temperature-independent reference voltages  18  and a voltage VPT whose value is proportional to the temperature of internal circuit devices. These signals are provided to a temperature detection circuit  20  as shown, and may be used by other circuitry (not shown) as well. Preferably, the voltage reference circuit  16  has sufficient thermal coupling to the battery  10  such that the temperature of its circuit devices substantially tracks the temperature of the battery  10 , in which case the voltage signal VPT accurately represents the temperature of the battery  10 . 
     The temperature detection circuit  20  periodically performs comparisons between the signal VPT and certain ones of the voltage reference signals  18 , as described in more detail below. The results of these comparisons are held in storage devices between successive sampling intervals. The outputs of the storage devices are temperature indicator signals  22  that can be used to control temperature-dependent operations. 
     In particular, one or more temperature indicator signals  22  are provided to the high temperature discharge circuit  24  used to help increase the usable life of the battery  10 . It has been found that exposing a battery to certain high-temperature and high-voltage conditions, such as described below, can significantly shorten battery life. When such conditions are detected, the high temperature discharge circuit  24  causes a small battery discharge current to flow. If there is no source of charge current, the discharge current eventually returns the battery voltage to a suitable level. 
     FIG. 2 shows a physical packaging arrangement for the battery protection circuitry of FIG.  1 . The circuits  14 ,  16 ,  20  and  24  are formed on a single integrated circuit  30 , which is mounted on a substrate  32  such as a small printed circuit board. A conductor  34  connects to the negative battery terminal V BATT   − . A bonding pad of the IC  30  is connected to a conductive circuit trace  36  which is connected to the conductor  34  by a conductor on the rear of the substrate  32 , as described below. Similarly, another bonding pad of the IC  30  is connected to a trace  38  which carries the signal WET. Additionally, one or more bonding pads of the IC  30  are connected to a conductive circuit trace  40  which is connected to the positive battery terminal V BATT   +  in a manner described below. 
     As shown in FIG. 3, the reverse side of the substrate  32  includes circuit features that implement the moisture transducer  12  of FIG.  1 . In particular, a pair of interdigitated circuit traces  42  and  44  carry the signals V BATT   +  and WET respectively. Respective edges of the traces  42  and  44  are very closely spaced apart along their entirety. In the illustrated embodiment, the trace spacing and the widths of the respective trace fingers  46  and  48  are on the order of 0.2 mm, and the total length of the mutually-facing trace edges is on the order of 30 mm. With this arrangement, when the substrate  32  comes into contact with moisture, a slightly conductive path can be created between the traces  42  and  44  when a voltage on the order of the battery voltage (3 to 4 volts) exists between the traces  42  and  44 . As described below, this change in electrical conductivity is detected by circuitry in the current bias reference circuit  14  of FIG. 1, and the presence of moisture is inferred therefrom. 
     Also shown in FIG. 3 is a conductive trace  46  used to carry the signal V BATT   −  from the conductor  34  (FIG. 2) to the trace  36  (FIG.  2 ). 
     FIG. 4 shows the current bias reference circuit  14 . Transistors P 1 , P 2 , N 1  and N 2  in combination with resistors R 1  and R 2  generate identical currents I 1 , and I 2  equal to 205 nA. Capacitors C 1  and C 2  stabilize the circuitry when transient conditions occur, such as the connecting of a charger. A third branch of the current mirror, consisting of transistor N 3 , is capable of conducting up to 205 nA to discharge the circuit node WET to substantially the voltage V BATT   −  under dry conditions. Because the node WET is a high-impedance circuit node, a capacitor C 3  is used to protect against potential glitches that may occur. The output signal FAULT is generated from the signal WET by a cascade of two inverters  50  and  52 . Under dry conditions, the signal WET is held at a low voltage, and the signal FAULT is de-asserted. 
     In the presence of moisture, the resistance of the transducer  12  (FIGS. 1 and 3) attains a value on the order of 5 Megohms or less, enabling a corresponding pull-up current to flow from the positive battery terminal V BATT   +  into the circuit  14  via the node WET. This charging current is greater than the current I 3 . The excess current flows into the capacitor C 3 , eventually charging the node WET to the positive battery terminal voltage V BATT   + . When the voltage WET becomes greater than the switching threshold at the input of the inverter  50 , the output of the inverter  50  becomes a logic low, causing the FAULT signal generated by the inverter  52  to become a logic high. As indicated above, the FAULT signal can be used by other circuitry (not shown) to signal a user or otherwise initiate appropriate responsive action. 
     While in the circuit of FIG. 4 a logic element in the form of the inverter  50  is used to discriminate between the voltages respectively indicating wet and dry conditions, in alternative embodiments it may be useful to employ a component such as a comparator having two inputs, one receiving the signal WET and the other establishing the desired threshold voltage. 
     FIG. 5 shows the voltage reference circuit  16 . In FIG. 5, several switches S 1 , S 2  and S 3  are shown being controlled by a clock signal Φ 1 , and other switches S 4  and S 5  are controlled by a clock signal Φ 2 . These clock signals are generated by clock circuitry (not shown in the Figures) preferably located on the IC  30 . The clock signals Φ 1  and Φ 2  each have a frequency on the order of 100 Hz, and they are logic complements of each other, i.e., when one is asserted, the other is deasserted. The clock signal Φ 1  preferably has a fairly low duty cycle, for example on the order of 10%. As described below, use of a low duty cycle clock minimizes the power consumption of the voltage reference circuit  16 . 
     The voltage reference circuit  16  operates when the switches S 1 , S 2  and S 3  are closed, which occurs then the clock signal Φ 1  is asserted. At these times, the main reference voltage V BG  satisfies the following relationship: 
     
       
           V   BG   =V   BEQ1   +V   T [(2 R   2   /R   1 ) ( ln A )] 
       
     
     where A is the ratio of the area of transistor Q 2  to the area of transistor Q 1 . It will be appreciated that the above equation expresses the temperature independence of the voltage V BG . The voltage V T  is directly proportional to temperature, whereas the voltage V BEQ1  is inversely proportional to temperature. Therefore, the above sum of V BEQ1  and a scaled V T  is substantially constant over temperature. 
     Because the voltage V BG  is temperature-independent, the various reference voltages VREFH, VREFM 1 , VREFM 2 , and VREFL are also temperature-independent. The resistors R 4  through R 8  are selected such that these reference voltages take on values that correspond to predetermined temperature points. VREFH corresponds to a temperature significantly higher than the normal maximum operating temperature of the battery  10 . For example, VREFH might correspond to a temperature on the order of 90° C. VREFM 1  corresponds to a substantially lower temperature, such as 45° C., which is near the highest normal battery operating temperature. VREFM 2  and VREFL correspond to high and low temperature limits, respectively, for charging the battery to avoid unduly reducing battery life. Typical values for these voltages correspond to temperatures of 35° C. and 0° C. respectively. 
     Unlike the various reference voltages VREFx, the value of the voltage VPT is proportional to temperature. This temperature dependence is due to the flow of a constant current through the resistor R 2  ADJUST, whose resistance increases with temperature. As described below, the voltage VPT is used by the temperature detection circuit  20  to determine which of the temperature ranges defined by the reference voltages VREFH, VREFM 1 , VREFM 2 , and VREFL the instantaneous temperature falls within. 
     During the non-active portion of the operating cycle, which occurs when the clock signal Φ 2  is asserted, the switches S 1 , S 2  and S 3  are open, and the switches S 4  and S 5  are closed. None of the main current-conducting components, such as the amplifier  54  and the transistors P 1 , P 2 , Q 1  and Q 2 , are conducting current. When this circuitry re-enters the normal operating state upon assertion of the clock signal Φ 1 , it can take considerable time to re-establish the circuit conditions under which the reference voltages VREFx are valid. The circuit  16  incorporates circuitry to enable a faster restart than can generally be obtained. During the non-active interval, a diode D 1  maintains a bias voltage slightly less than the voltage V BATT   +  on both inputs to the amplifier  54 . When the various devices are re-enabled upon assertion of the clock signal Φ 1 , the pre-established bias enables them to quickly reach their quiescent operating conditions, so that the reference voltages VREFx are quickly re-established. As a result, the duration of the active period can be minimized, resulting in reduced average power consumption. 
     FIG. 6 shows the temperature detection circuit  20 . During the active period established by the assertion of the clock signal Φ 1 , the temperature-indicating signal VPT is sequentially compared with the various reference signals VREFx by a comparator  60 , and the result of each comparison is stored in a corresponding flip-flop  62 ,  64 ,  66  or  68 . A control circuit  70  controls this operation using control signals MEASx. For example, the control circuit  70  may first enable a pass transistor N 1  and, after a suitable delay, clock the comparator output signal CMP into the flip-flop  62 . By this operation, the state of output signals OVH and OVH* indicate whether the temperature is greater than the temperature corresponding to the reference signal VREFH. This operation is repeated for the other three reference signals VREFM 1 , VREFM 2 , and VREFL, establishing the respective states of output signals OVM 1 , OVM 1 *, QVM 2 , OVM 2 *, UNVL and UNVL*. 
     As shown in FIG. 6, the control circuit  70  also generates a reset signal RST* that is asserted during power-up to initialize the flip-flops  62 ,  64 ,  66  and  68  to the de-asserted state. Also, an enable signal EN* is used to selectively enable the comparator  60 , so that it can be powered-down during the non-active interval. 
     FIG. 7 shows the high-temperature discharge circuit  24 . A pair of inverters  80  and  82  form a latch  84  whose output controls a switching transistor N 3  arranged in series with a discharge load resistor R 1 . The state of the latch  84  is controlled by pull-up transistors P 1  and P 2  and pull-down transistors N 1  and N 2 , which in turn are controlled by input signals RST*, UNVH*, and OVM 2  as shown. The signal OVM 2  is received from the temperature detection circuit  20  of FIG.  1 . The signal UNVH* is generated by voltage comparison circuitry (not shown), and it indicates whether the battery voltage is less than a predetermined upper limit, such as 4.0 volts. 
     The latch  84  is initialized to the de-asserted state upon assertion of the signal RST*, so that transistor N 3  is turned OFF and no discharge current flows through the load resistor R 1 . When the signal UNVH* is asserted, indicating that the battery voltage is below the upper limit, the latch  84  is maintained in the de-asserted state by action of the pull-up transistor P 1 ; the pull-down transistor N 2  is OFF due to the action of an AND gate  86  having UNVH* as an input. When the battery voltage is above the limit corresponding to the signal UNVH* and the battery temperature is above the threshold corresponding to the signal OVM 2 , the output of the AND gate  86  becomes asserted, which turns on the transistor N 2  and sets the latch  84  to the asserted state. In turn, the transistor N 3  turns on, causing a predetermined discharge current to flow through the discharge load resistor R 1 . This operating condition persists until either the signal RST* is asserted or the battery voltage diminishes to less than the upper limit, as indicated by the assertion of UNVH*. Either of these actions causes the corresponding pull-up transistor P 1  or P 2  to reset the latch  84  to the de-asserted state, stopping the flow of discharge current. 
     A battery protector circuit incorporating several features has been shown It will be apparent to those skilled in the art that other modifications to and variations of the disclosed circuitry are possible without departing from the inventive concepts disclosed herein, and therefore the invention should not be viewed as limited except to the full scope and spirit of the appended claims.