Abstract:
A charging system that concurrently charges a main system and a battery for powering the main system when the charging system is connected to an adapter. The charging system includes a current sensing resistor, a first current control circuit for regulating the current in a linear battery charger and a second current control circuit for controlling the maximum adapter current. The charging system further includes a first temperature control circuit for regulating the current in the battery charger and a second temperature control circuit for controlling the maximum power dissipation of the charging system. The charging system further includes a linear regulator for providing power to the main system from the adapter and a main system voltage control circuit and a battery voltage control circuit. The charging system apportions the adapter current between the main system and the battery charger giving priority to the main system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     1. Field of the Invention 
     The present invention generally relates to battery chargers and more particularly to battery chargers that support system operation while charging. 
     2. Description of the Related Art 
       FIG. 1  shows a typical battery charger system that supports charging while operating a main system  13 . The system can be a portable electronic device such as a cellular phone handset and the charger can be a unit built-in to the portable device. The charging system  10  includes a rechargeable battery  16 , a charger  14 , a charger controller  15 , and a pair of Schottky diodes  12 ,  17 . An external adapter  11  is connected to the positive side of Schottky diode  12  to provide power to the battery  16  via the charger  14  and to the main system  13 . When charging system  10  is not connected to the adapter  11 , the main system  13  is powered by battery  16  via Schottky diode  17 . Schottky diode  12  prevents battery  16  from activating charger  14 , which would otherwise drain power from the battery. 
     When the charging system  10  is connected to an external adapter  11 , the charging system supplies power to system  13  through diode  12 . The adapter  11  can be an offline AC-DC power supply or a car adapter with a constant DC output voltage. Since the output voltage of the adapter  11  is always higher than the maximum voltage of the battery  16 , the Schottky diode  17  is reverse biased, and the battery  16  is prevented from supplying power to main system  13  when adapter  11  is present. Adapter  11  also supplies power to the charger  14 , which, in turn, provides a controlled charging power to the battery  16  under the control of charger controller  15 . Charger controller  15  sets a proper charging mode for the charger  14 , according to the charging status of the battery and a pre-programmed charging algorithm. 
     The charging system  10 , shown in  FIG. 1 , supports the concurrent operation of the main system  13  and the charging of the battery  16 , when the adapter  11  is connected to the charging system  10 . Though concurrent operation and charging are supported, the main system  13  and the charger  14  both contend for power from the adapter  11 . This contention can cause a serious problem. Specifically, if the combined current for operating the main system  13  and charging the battery  16  exceeds the maximum current that the adapter  11  is capable of supplying, an over-current condition occurs, causing adapter  11  to shutdown by means of an internal protection circuit therein. If the battery is not sufficiently charged, power to the main system  13  would be abruptly lost, thereby shutting the main system  13  down. 
     Thus, it is highly desirable that the main system and the charger do not exceed the maximum adapter current. This implies that the adapter current must be apportioned between the main system and the charger such that current used by one reduces current used by the other and the sum remains equal to the maximum adapter current. 
       FIG. 2  illustrates the voltage and current during a typical charging process for a Lithium Ion battery. There are three time intervals of interest, T 0 -T 1 , T 1 -T 2 , and T 2 -T 3 . Over the time interval T 0 -T 1 , the battery is pre-conditioned with a small and constant charging current of about 100 milliamperes (mA). When the battery voltage reaches about 3.0 volts (V), in period T 1 -T 2 , the charger supplies a constant fast-charge current of about 0.95 amperes (A). The period T 1 -T 2  is known as a constant-current phase. During this constant-current phase, battery voltage gradually increases. Once the battery voltage reaches about 4.2V, at T 2 , a constant-voltage phase T 2 -T 3  is entered. After T 2 , the charger output voltage is maintained at a constant 4.2V and the charging current is gradually decreased, eventually tapering off to below about 100 mA. At T 3 , the battery has re-gained full capacity and the charging process terminates. 
       FIG. 3  shows a prior art adaptive power charging system that prioritizes the system current over the charger current with adapter over-current protection. This charging system monitors the output current of external adapter  21  and dynamically apportions current for operating the main system  24  and current for charging the battery  28  by assigning a higher priority to main system  24  current demand over the charger  27  current demand, unlike the system of FIG.  1 . 
     The charging system of  FIG. 3  includes a charger  27 , an adapter current control circuit, a charger current control circuit, a battery voltage control circuit and a pair of Schottky diodes. The adapter current control circuit includes a current sensing resistor  22 , operational amplifier (opamp)  33 , opamp  34  and an isolating diode  35 . The charger current control circuit includes a current sensing resistor  26 , an opamp  25 , an opamp  31 , and an isolating diode  32 . The battery voltage control circuit includes an opamp  37  and an isolating diode  36 . 
     The current sensing resistor  22  is connected between the adapter  21  and the positive side of first Schottky diode  23 , whose negative side connects to the main system  24 . Opamp  33  is connected differentially across the current sensing resistor  22 . Opamp  34  is connected to the output of opamp  33  and to a first preset voltage. The output of opamp  34  is connected to the positive side of isolating diode  35  whose negative side connects to the control input of charger  27 . 
     Current sensing resistor  26  is connected between the positive side of Schottky diode  23  and the input side of charger  27 . Opamp  25  is connected differentially across current sensing resistor  26 . The output of opamp  25  is connected to the input of opamp  31  whose other input is connected to a second preset voltage. The output of opamp  31  is connected to the positive side of isolating diode  32  whose negative side connects to the control input of charger  27 . 
     Opamp  37  is connected to a third preset voltage and to the battery  28 . The output of opamp  37  is connected to the positive side of isolating diode  36  whose negative side connects to the control input of charger  27 . The output of charger  27  connects to the battery  28  and the second Schottly diode  30  connects the battery  28  to the input of the main system  24 . 
     The adapter  21  supplies a constant DC output voltage with an internal maximum output current limit as protection against overload conditions in the adapter. The current-sensing resistor  22  monitors the total adapter current that flows into the charging system. This total adapter current is the sum of current supplied to the main system  24  through first Schottky diode  23 , and the current supplied to charger  27  through the second current-sensing resistor  26 . 
     The adapter current flows through current sensing resistor  22  is amplified and level shifted by opamp  33 . Assuming that the current sensing resistor has a value of 0.1Ω (ohms), opamp  33  amplifies the voltage drop across the current sensing resistor by a certain gain factor, say a factor of 10. The output of opamp  33  connects to the positive input of opamp  34  and the inverting input of opamp  34  is connected to a preset voltage, for example, 1.5V. The output of opamp  34  is used to control and regulate the charger current via an isolating diode  35 . This adapter current control circuit gives main system  24  a higher priority to receive current from adapter  21 , because the control circuit maintains the adapter current at 1.5 Amps by controlling the charger  27 . If the main system  24  draws 1.0 A from adapter  21  via first Schottky diode  23 , then opamp  34  automatically reduces the current to the charger  27  to no more than 0.5 A, maintaining the adapter current at 1.5 A. If the main system  24  reduces its current demand to 0.5 A, then opamp  34  automatically increases the charging current to 1.0 A. Thus, any current that the main system  24  does not demand is diverted to the charger  27  and the total adapter current is maintained at 1.5 A. 
     It should be noted that the above described operation implies that charger  27  includes a power switch or pass element that reduces its pass-through current when the control terminal of the switch or element is driven high and increases its pass-through current when the control terminal is driven low. A P-MOSFET or PNP power transistor or equivalent device meets this requirement. 
     The charger current control circuit includes a second current sensing resistor  26  (shown as a fixed resistor) that provides programmable constant-current regulation for charger  27 . Opamp  25  amplifies the voltage drop across sensing resistor  26  and the output of opamp  25  connects to the positive input of opamp  31 , whose inverting input is connected to a second preset voltage, an external reference voltage  29 . This external reference voltage is either programmed to a fixed value or adjusted dynamically by a system&#39;s charging control microprocessor. The charger control circuit sets the maximum charging current for charger  27 . The value of the current sensing resistor  26 , the gain of opamp  25  and the second preset voltage  29  determine the maximum current. For example, if the second preset voltage  29  is set at 1.0V, the current sensing resistor is 0.1 ohms, and the gain of the opamp  25  is 10, the charger  27  is limited to a maximum charging current of 1.0 A. This constant current is applied to the battery  28  during the constant current charging phase. 
     The battery voltage control circuit includes opamp  37  which provides constant voltage regulation at 4.2V for charger  27 . Constant voltage is applied to the battery  28  during the constant voltage phase. 
     Diodes  32 ,  35 , and  36  combine provide a logic-OR function for the three opamps  31 ,  34 ,  37 . The opamp that first reaches a current limit or a voltage limit has a higher output voltage to drive the control terminal of battery charger  27 , reducing the charging current. 
     Charger  27  can be a switching converter type or a linear regulator type. As portable electronic devices are getting smaller and adding more features, more and more portable systems are using linear regulator chargers. Linear chargers require no magnetic components such as inductors or transformers and are smaller in size, making integration on a single chip possible. This greatly reduces the number of external components needed to build a charger circuit. 
       FIG. 4  shows a prior art adaptive power charging device that permits concurrent system operation and battery charging and includes over-temperature protection. This system includes a charger current control circuit, a temperature control circuit, and a battery voltage control circuit, a linear charger and a pair of Schottky diodes. 
     The charger  42  is a linear regulator that is controllable to provide a constant-current or a constant-voltage output. Linear charger  42  has a control input that controls the amount of current through the charger. The power input to the charger  42  is connected a current sensing resistor  41  and the power output of the charger  42  is connected to the battery  47 . 
     The charger current control circuit includes the current sensing resistor  41 , and two opanps  48 ,  49 , and an isolating diode  57 . The opamp  48  is connected differentially to the current sensing resistor  41  and the output of opamp  48  is connected to the positive terminal of opamp  49 . The negative terminal of opamp  49  is connected to a preset voltage and the output of opamp  49  is connected to the positive side of the isolating diode  57 , whose negative side is connected to the control input of the linear charger  42 . Sensing resistor  41 , opamp  48  and opamp  49  together set a maximum charging current for the charger  42 . If sensing resistor  41  is 0.1 ohms, the gain of opamp  48  is set at 10, and the reference voltage on the positive input of opamp  49  is set at 1.0 volts, then the maximum charger current is 1.0 A. 
     The battery voltage control circuit includes opamp  46  and isolating diode  59 . Opamp  46  provides a constant-voltage regulation at 4.2V during a constant-voltage charging phase of the battery. Opamp  46  has a positive terminal connected to the battery  47  and a negative terminal connected to a preset voltage for controlling the maximum voltage on the battery  47 . 
     The temperature control circuit includes a temperature sensor  44 , an opamp  45  and an isolating diode  58 . The opamp  45  has a positive terminal that connects to the output of temperature sensor  44  and a negative terminal that is connected to a reference voltage indicative of a preset temperature. The temperature control circuit operates to limit the temperature of the charger system to the preset temperature, when the charger system is packaged in a single unit. It is well-known that a single package unit has a limited power dissipation capability. The parameters that characterize this limited dissipation capability are the maximum operating junction temperature T OP  of devices contained in the package, the maximum operating ambient temperature T A  allowed for the package, and the thermal resistance Θ JA  of the package. Specifically, the power dissipation PD is 
         PD   PACKAGE     =           T   OP     -     T   A         Θ   JA       .         
 
For example, assuming a charger IC with a thermal resistance Θ JA =50° C./W, a maximum charger operating junction temperature of 105° C., and maximum operating ambient temperature is 65° C. The maximum allowable power dissipation of the circuitry in the package is 0.8 Watts.
 
     For the linear charger  42 , the power dissipation is the product of charging current I CH  and the difference between the input voltage to the linear charger  42  and the output voltage (i.e., the battery voltage), PD CHARGER =I CH (V IN −V OUT ). Clearly, the PD of the linear charger  42  cannot exceed the PD limit for the package. As discussed above, the charger current control circuit limits the I CH  current to 1.0 Amp. Therefore, the voltage difference across the linear charger is limited to 0.8 Volts. However, there is no control circuit to guarantee that the battery voltage does not cause a voltage drop greater than 0.8 Volts across the linear charger  42  but the temperature control circuit provides a limit on power dissipation if the voltage drop across the linear charger is too great. For example, if the adapter voltage is 5.0 Volts and the battery voltage is 3.0 Volts at the beginning of the constant-current phase, the voltage drop across the linear charger is 2.0 Volts. This is greater than the 0.8 Volt limit at 1.0 Amps, so the linear charger  42  must be throttled back. If the power dissipation of the package is to be limited to 0.8 Watts, then the linear charger must be limited to 0.4 Amps (0.8/2.0). Thus, as the junction temperature rises towards the preset level (105° C. in this case), opamp  45  output rises in voltage, forcing linear charger  42  to reduce its charging current. In the steady state, the charger system  40  maintains a charging current level that keeps the junction temperature at 105° C. The thermal limit will regulate the charging current to 0.4 A, corresponding to a power dissipation of 0.8 Watts, and therefore, a temperature rise of 40° C. If the actual ambient temperature is lower than 65° C., the charging current may be larger. As a Lithium Ion battery approaches full charge, the voltage of the battery approaches 4.2 Volts and at 65° C. ambient, the thermal limit opamp  45  automatically raises the charging current to 1.0 A, so that total power dissipation is maintained at 0.8 W. 
     In summary, both prior art circuits of FIG.  3  and  FIG. 4  support the concurrent operation of the main system and battery charging. However, the control circuits of  FIG. 3  give priority to main system operation over battery charging but do not provide control of the maximum temperature (or power dissipation) of the charging system when the charger circuit is a linear charger. The control circuits of  FIG. 4 , on the other hand, provide control over the power dissipation of the charging system but do not provide arbitration between the main system current demand and charger current demand. This creates the real possibility that the adapter may shut down because of an over-current condition. 
     Thus, there is a need for a charger system that supports concurrent operation of the main system and battery charging, gives priority to the main system over the charger, prevents over-current conditions in the adapter and over-temperature conditions for the circuitry if contained in a single package. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed towards the above-stated need. A system in accordance with the present invention is a charger system for providing power to a main system and a battery. The charger system includes a current sensing circuit, a linear charger, a linear regulator, first and second current control circuits, a shutdown circuit, a temperature sensor, first and second temperature control circuits, and first and second voltage control circuits. 
     The current sensing circuit includes a current sensing resistor for developing a voltage drop proportional to the current passing through the resistor, and an amplifier connected to the resistor for boosting the voltage drop across the resistor. 
     The linear charger is connected to receive current from an external adapter via the current sensing resistor and to provide, on an output, current to the battery. The linear charger has a first control input that controls the quantity of current provided by the linear charger to the battery. 
     The linear regulator is connected to receive current from an external adapter via the current sensing resistor and to provide, on an output, current to the main system. The linear regulator has a second control input that controls the quantity of current provided by the linear regulator to the main system. 
     The shutdown circuit is configured to prevent the operation of the linear regulator and the linear charger upon receipt of a first or second shutdown input. 
     The first current control circuit is connected to the amplifier output of the current sensing circuit and has an output connected to the first shutdown input and an input for receiving a first preset voltage that determines a signal value on the first shutdown input. 
     The second current control circuit is connected to the amplifier output of the current sensing circuit and has an output connected to the first control input and an input for connecting to a second preset voltage that determines a signal value on the first control input. 
     The temperature sensor is configured to sense the operating temperature of at least the linear regulator and the linear charger and to provide an output indicative of the sensed temperature. 
     The first temperature control circuit has an input connected to receive the temperature sensor output and an input for receiving a voltage indicative of a first preset temperature and an output connected to the first control input to control the current passing through the linear charger. The first temperature control circuit shuts down the linear charger when the sensed temperature exceeds the first preset temperature. 
     The second temperature control circuit has an input connected to receive the temperature sensor output and an input for receiving a voltage indicative of a second preset temperature and an output connected to the second shutdown input to shut down the linear regulator and linear charger when the sensed temperature equals or exceeds the second preset temperature. 
     The first voltage control circuit has a first input connected to the output of the linear regulator and second input for connecting to a third preset voltage and an output connected to the control input of the linear regulator to regulate the voltage provided to the main system. 
     The second voltage control circuit has a first input connected to the output of the linear charger and a second input for connecting to a fourth preset voltage and an output connected to the control input of the linear charger to regulate the voltage provided to the battery. 
     One advantage of the present invention is that the invention provides a comprehensive charger control system and method for a device that may reside in a single package. The comprehensive control system assures an optimal charging current at all times without violating overall adapter current limits or exceeding maximum allowable operating temperatures. 
     Another advantage of the present invention is that the present invention enables more power management functions to be integrated into a single device and increases the overall efficiency of the adapter-charger system. 
     Yet another advantage is that the invention provides a simple yet comprehensive charger control scheme for a basic monolithic linear charger integrated circuit as well as for a linear charger integrated circuit with extended power management functions. 
     Yet another advantage is that control of the adapter current not only assigns higher priority to the main system current and but also protects against system faults such as a short circuit. 
     Yet another advantage is that control of the device temperature not only regulates charging current such that the charging system will not exceed its maximum operating temperature but also protects against main system electrical faults or assembly defects that could cause the main system to overheat the charging system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  shows a typical battery charger system that supports charging while operating a system; 
         FIG. 2  illustrates the voltage and current during a typical charging process for a Lithium Ion battery; 
         FIG. 3  shows a prior art adaptive power charging device that prioritizes the system over the charger with adapter over-current protection; 
         FIG. 4  shows a prior art adaptive power charging device that permits concurrent system operation and battery charging and includes over-temperature protection; 
         FIG. 5  shows a charging circuit with a power MOSFET that replaces the pass diodes of the prior art; and 
         FIG. 6  shows an adaptive power charging devices that prioritizes the system over the charger with adapter current protection and over-temperature protection. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In many portable electronic devices, further integration of power management functions into an adapter-charger system is required. For example, Schottky diodes  12  and  17 , shown in  FIG. 1 , may need to be replaced by power MOSFETs to achieve higher efficiency and reduced heat dissipation. A power MOSFET may be used for either or both of the Schottky diodes  12 ,  17 . 
       FIG. 5  shows a charging circuit with a power MOSFET that replaces a pass diode of the prior art. In  FIG. 5 , power MOSFET  52  replaces the pass-through Schottky diode  12  of FIG.  1 . Power MOSFET  52  supplies the current required for running the system when adapter  51  is connected to a power source and to the charging system  50 . Because power MOSFET  52  resides on the same substrate as linear charger  54 , both contribute heat dissipation to charger system  50 . In other words, the temperature rise of charger system  50  derives from two sources. Furthermore, control circuit  55  now must provide thermal limit protection to prevent linear charger  54  from drawing excessive charging current, or to prevent a main system fault condition from drawing excessive current through power MOSFET  52 . 
     In other portable electronic devices, system design may demand that the adapter output voltage be regulated when the adapter is supplying power to the system. For example, a cellular phone circuit may specify a maximum input voltage of 4.2V, regardless of whether a Lithium Ion battery or an adapter is supplying the power to the phone (the main system  53 ). Since a typical adapter has a 10% tolerance on its nominal output voltage (say 5 Volts), a voltage of 5.5 Volts may be supplied by the adapter. Even with a Schottky diode, the voltage supplied to the main system would exceed the main system&#39;s specifications. Thus, the Schottky diode  12  must be replaced with a linear regulator to guarantee meeting the 4.2 Volt requirement of the main system  53 . It is preferred that the linear regulator use a power MOSFET as the pass device. 
       FIG. 6  shows an adaptive power charging devices that prioritizes the system over the charger and includes adapter over-current protection and system over-temperature protection. The charger system of  FIG. 6  includes current sensing circuit, an adapter current control circuit, a temperature control circuit, a main system voltage control circuit, a battery voltage control circuit, a thermal shutdown circuit, a charger current control circuit, a linear charger  72 , a linear regulator  63 , a shutdown circuit  68  and a temperature sensor  67 . 
     The linear charger  72  has an input connected to receive adapter current via current sensing resistor  62 , an output connected to the battery  76  and a control input for controlling the charging current that passes through the linear charger to the battery. 
     The linear regulator  63  has an input connected to receive adapter current via the current sensing resistor  62 , an output connected to the main system  65  and a control input for controlling the amount of current passing through the linear regulator  63 . 
     The current sensing circuit includes the current sensing resistor  62 , and an opamp  73  connected differentially across the current sensing resistor. In one embodiment, the current sensing resistor has a value of 0.1 ohms and the opamp  73  has a gain of 10. 
     The adapter current control circuit includes a comparator  71 . Opamp  73  output is connected to the positive input of comparator  71 , negative terminal of which is connected to a first preset voltage that determines the maximum current that is allowed to flow from the adapter. If the current sensing resistor is 0.1 ohms, the first preset voltage is 1.0 Volts and the gain of opamp is 10, then the total adapter current is limited to 1 Amp. The output of comparator  71  is connected to one input of a shutdown circuit  68 . If the charger system attempts to draw more than 1.0 Amp from the adapter circuit, the charger system is shut down by shutdown circuit  68 . 
     The temperature control circuit includes opamp  69  and isolating diode  77 . The positive terminal of opamp  69  is connected to the output of the temperature sensor  67  and the negative terminal is connected to a voltage indicative of a preset temperature. The output of opamp  69  is connected to the positive side of isolating diode  77 , whose negative side connects to the control input of linear charger  72 . If the preset temperature is set to 105° C., then the linear charger will throttle down when the temperature of the charging system is 105° C. If the ambient temperature is permitted to be no higher than 65° C., and the thermal resistance of the system is 50° C./W, then the temperature control circuit will limit the power dissipation of the charger system to 0.8 Watts (40° C./50° C./W). If the maximum voltage provided to the battery or the main system is 4.2 Volts and the maximum voltage provided by the adapter  61  is 5.0 Volts, then the maximum adapter current permitted by the temperature control circuit is 1.0 Amps (0.8 Watts/0.8 Volts). 
     The charger current control circuit includes opamp  74  and isolating diode  79 . The positive terminal of opamp  74  is connected to the output of opamp  73  and the negative terminal of opanp  74  is connected to a second preset voltage that determines the maximum adapter current for throttling down the charger. The output of opamp  74  is connected to the positive side of diode  79 , whose negative terminal is connected to the control input of linear charger  72 . If the second preset voltage is set at 0.95 Volts, then the linear charger will be throttled down when the adapter current reaches 0.95 Amps, assuming a current sensing resistor  62  value of 0.1 ohms, and a gain of 10 for opamp  73 . With the second preset voltage at 0.95 Volts, the maximum charger current permitted by the charger current control circuit is 0.95 Amps. 
     The thermal shutdown circuit includes comparator  66 . Temperature sensor  67  is connected to the positive input of comparator  66 , whose negative input is connected to a voltage indicative of a preset shutdown temperature. The output of comparator  66  is connected to another input of shutdown circuit  68 . If the preset shutdown temperature is set at 150° C., the charging system will shutdown when the system reaches 150° C. If the maximum permitted ambient temperature is 65° C. and the thermal resistance of the package is 50° C./W, then the maximum power permitted by the thermal shutdown circuit is 1.7 Watts (85° C./50° C./W). 
     The main system voltage control circuit includes opamp  64 . The positive terminal of opamp  64  is connected to the voltage input for the main system  65  and the negative terminal is set at a third preset voltage. The output of the opamp  64  is connected to the control input of linear regulator  63 . If the third preset voltage is set at 4.2 Volts then the main system voltage control circuit will control the linear regulator  63  to limit the main system input voltage to 4.2 Volts. 
     The battery voltage control circuit includes opamp  75  and isolating diode  78 . The positive input of the opamp  75  is connected to the battery and the negative input is connected to a fourth preset voltage. If the fourth preset voltage is set at 4.2 Volts, then the linear charger will regulate its output voltage at 4.2 V when the battery reaches 4.2 Volts. 
     A Schottky diode  70  connects the battery  76  to the main system  65 . 
     Dual-level Current Limit 
     Thus, as described above, the charger current control circuit is set to limit the charger current to a maximum 0.95 Amps and the adapter current control circuit is set to limit the total adapter current to 1.0 Amps. If the main system  65  requires 0.90 Amps at full load, then 0.05 Amp is available to the charger  72 , because both the charger current control circuit and the adapter current control circuit use the same current sensing resistor. If the main system  65  draws only 0.01 Amps (in a power down or sleep mode), then 0.94 Amps is available to the charger, the total current from the adapter being limited to 0.95 Amps. 
     However, if the main system attempts to draw significantly more than 0.95 Amps (perhaps because of a fault in the main system  65 ), two actions are performed by the circuitry. First, the charger current control circuit shuts down the linear charger  72 , when the 0.95 Amp limit is met or exceeded. This frees up some adapter current to meet the main system  65  demand. Next, if the current demand of the main system is still higher than 0.95 Amps, the adapter current control circuit will limit the adapter current to 1.0 Amps. If the current demand is equal to or higher than 1.0 Amp, the adapter control circuit will shut down the charger system  60 . Charger system  60  thus enters a shutdown mode where both linear regulator  63  and linear charger  72  are turned off. This dual-level current limit scheme uses a single current-sensing resistor  62  and can correctly shut down a linear regulator or a power switch supplying power to system  65 , in the event of system fault causing the over-current condition. 
     Dual-Level Temperature Limit 
     As described above, temperature control circuit throttles down the linear charger  72  when the temperature of the charger system reaches 105° C. and the thermal shutdown circuit shuts down the charger system  60  when the charger system reaches 150° C. Temperature sensor  67  is used by both the temperature control circuit and the thermal shut down circuit. Thus, regardless of whether the heat source is the linear regulator  63  or the linear charger  72  or both, opamp  69  regulates the charging current to maintain a constant junction temperature of 105° C. for devices in charger system  60 . Table 1, set forth below, details a number of conditions, A-F, that are discussed in the text. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 System 
                   
                   
                   
                   
                   
                   
               
               
                 Parameter/ 
               
               
                 Condition 
                 A 
                 B 
                 C 
                 D 
                 E 
                 F 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Adapter 
                 5.00 
                 5.00 
                 5.00 
                 5.00 
                 5.00 
                 5.00 
               
               
                 Voltage 
               
               
                 Adapter 
                 0.71 
                 1.63 
                 0.95 
                 0.95 
                 0.95 
                 0.80 
               
               
                 Current 
               
               
                 Main System 
                 4.20 
                 4.20 
                 4.20 
                 4.20 
                 4.20 
                 4.20 
               
               
                 Voltage 
               
               
                 Main System 
                 0.10 
                 0.10 
                 0.10 
                 0.90 
                 0.25 
                 0.25 
               
               
                 Current 
               
               
                 Battery Voltage 
                 3.00 
                 4.20 
                 4.20 
                 3.00 
                 3.00 
                 3.00 
               
               
                 Battery Current 
                 0.61 
                 1.53 
                 0.85 
                 0.05 
                 0.70 
                 0.55 
               
               
                 Power 
                 0.08 
                 0.08 
                 0.08 
                 0.72 
                 0.20 
                 0.20 
               
               
                 Dissipation - 
               
               
                 Linear 
               
               
                 Regulator 
               
               
                 Power 
                 1.22 
                 1.22 
                 0.68 
                 0.10 
                 1.40 
                 1.10 
               
               
                 Dissipation - 
               
               
                 Linear Charger 
               
               
                 (Actual) 
               
               
                 Power 
                 1.22 
                 1.22 
                 1.22 
                 0.58 
                 1.10 
                 1.10 
               
               
                 Dissipation - 
               
               
                 Linear Charger 
               
               
                 (Temp 
               
               
                 Limited) 
               
               
                 Charger 
                 0.95 
                 0.95 
                 0.95 
                 0.95 
                 0.95 
                 0.95 
               
               
                 Shutdown 
               
               
                 Current 
               
               
                 Maximum 
                 1.00 
                 1.00 
                 1.00 
                 1.00 
                 1.00 
                 1.00 
               
               
                 Adapter 
               
               
                 Current 
               
               
                 Maximum 
                 1.30 
                 1.30 
                 1.30 
                 1.30 
                 1.30 
                 1.30 
               
               
                 package 
               
               
                 dissipation 
               
               
                 Thermal 
                 50 
                 50 
                 50 
                 50 
                 50 
                 50 
               
               
                 Resistance 
               
               
                 Ambient 
                 40 
                 40 
                 40 
                 40 
                 40 
                 40 
               
               
                 Temperature 
               
               
                 Maximum 
                 105 
                 105 
                 105 
                 105 
                 105 
                 105 
               
               
                 operating 
               
               
                 temperature 
               
               
                   
               
             
          
         
       
     
     If the main system  65  is operating at an idle current of 0.1 A, the linear regulator dissipates 0.08 W when adapter voltage is at 5.0V. Assuming the ambient temperature is 40° C., opamp  69  increases the charging current to cause a power dissipation of 1.22 W. If the battery voltage is 3.0 Volts, then the temperature control circuit permits 0.61 Amps to flow through the linear charger  72 . The 0.61 Amps current is lower than the 0.85 Amps permitted by the charger current control circuit under the stated conditions. This is condition A in the table. 
     If the battery voltage is 4.2 Volts, then the temperature control circuit permits 1.53 Amps to flow in the linear charger  72 . This is condition B in the table. This current is greater than the 0.85 Amps permitted by the charger current control circuit, so the current is limited to 0.85 Amps. Thus, under the stated conditions, the charger current control circuit performs a limiting action before the temperature control circuit does. This is condition C in the table. 
     On the other hand, if the main system  65  draws a current of 0.9 A from a 5.0 V adapter, the linear regulator  63  dissipates 0.72 W. Because opamp  74  regulates charger current to 0.05 A under these conditions, the heat dissipated by linear charger  72  is 0.1 Watts, when battery voltage is 3.0V. At an ambient temperature of 40° C., and a thermal resistance of 50° C./W, the maximum dissipation permitted by the temperature control circuit is 1.3 Watts which is greater than the 0.82 Watts dissipated by the linear charger  72  and the linear regulator  63 . Thus, under the stated conditions, no limiting action is performed by the temperature control circuit. This is condition D in the table. 
     When the main system draws a smaller current, a larger portion of adapter current will be available for linear charger  72 . For example, if the main system draws 0.25 A, the remaining 0.70 A is available for charging the battery, as long as thermal limits are not exceeded. If the battery voltage is at 3.0V, linear charger  72  generates too much heat causing temperature control circuit to limit the power dissipation. Specifically, at 0.25 A and a 0.8 Volt drop, linear regulator  63  generates 0.2 Watts, but the linear charger generates 1.4 Watts. At ambient temperature of 40° C., the maximum overall power dissipation permitted by the temperature control circuit is 1.3 W. This is condition E in the table. Therefore, the temperature control circuit become active to reduce the charging current to 0.55 A, rather than 0.7 A that is available. This is condition F in the table. Power dissipation of the linear charger is now 1.1 Watts. 
     Temperature sensor also  67  provides a second level of thermal protection for the charger system  60 . While comparator  71  already provides an over-current protection against system faults such as a short circuit condition, it will not protect charger system  60  from assembly defects such as a cold solder joint which can generate large amounts of heat without exceeding the current limits imposed by the adapter current control circuit or the charger current control circuit. For example, if the charger system  60  is improperly mounted to a system printed circuit board, because of a cold solder joint, the thermal resistance of the charger system  60  could be as high as 200° C./W. Even at moderate power dissipation level, 1.0 W for example, the junction temperature of charger system  60  could increase to a damaging level. Comparator  66  provides protection for charger IC  60  against such assembly defects. In the event of an abnormal thermal resistance, a safe operating power will cause an over-temperature condition. The condition will first cause the temperature control circuit to reduce the charging current to zero. If the over-heating condition persists, the thermal protection circuit will shut down the linear regulator  63  and the linear charger  72 . 
     Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.