Abstract:
A method and system, compatible with low-voltage CMOS technology, for controlling the charging of a battery. The method includes monitoring a battery voltage with respect to a threshold voltage. The method further includes coupling a charging control logic supply to ground, generating an active low first control signal, inverting the active low first control signal, and charging the battery at a first rate when the battery voltage is below the threshold voltage. The method further includes coupling the charging control logic supply to the battery voltage, generating an active high second control signal, and charging the battery at a second rate when the battery voltage exceeds the threshold voltage. The first charging rate is slower than the second charging rate. The method further includes supplying battery power to a charger line when the battery voltage exceeds the charger voltage, and suppressing a leakage current.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/694,188, entitled “METHODS AND SYSTEMS FOR BATTERY CHARGING CONTROL BASED ON CMOS TECHNOLOGY,” filed Oct. 28, 2003, which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to battery charging circuits and, more particularly, to battery charging control circuits based on CMOS technology. 
     2. Related Art 
     Most portable electronics require batteries to supply power. When batteries are discharged to a low voltage level, chargers are required to charge the batteries to working condition. Li-Ion batteries need to be charged to about 4.2 V and NiMH/NiCd batteries need to be charged to about 5 V. 
     A battery charging control circuit controls the charging sequence to ensure that the charger safely charges the battery from a deeply discharged state to a fully charged state. There are at least two steps in the charging sequence, a slow charging mode and a fast charging mode. The battery charging control circuit initiates a charging mode according to the threshold voltage of the battery. For example, the threshold voltage of a Li-Ion battery is about 2.7 V. When the battery voltage is below the threshold voltage, the battery charging control circuit initiates the slow charging mode for safety. The slow charging mode current is about 40 mA. Because the voltage level is too low in this mode, the battery should not power external devices or the battery charging control circuit. The charger usually powers the battery charging control circuit in the slow charging mode. When the battery voltage is above the threshold voltage, the battery charging control circuit initiates the fast charging mode. The fast charging mode current is typically around 1 A. In this mode, the battery can power external devices and the battery charging control circuit. 
     A problem with this approach occurs if the battery charging control circuit is implemented with low-voltage CMOS technology. For example, the oxide breakdown voltage for 0.35 Φm CMOS technology is typically 3.3 V. In the slow charging mode, the charger is the only available power source to power the charging control circuit, but the voltage level of the charger can go as high as 13 V, which is substantially higher than the breakdown tolerance of low-voltage CMOS technology. One solution to this problem is to add an external voltage regulator to step down the charger voltage to within the breakdown tolerance of the low-voltage CMOS technology. Another solution is to implement the charging control circuit with special high-voltage CMOS or other technologies. But the problem with these solutions is increased cost and power consumption. 
     What is needed are methods and systems for controlling the charging of a battery that are compatible with low-voltage CMOS technology. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to methods and systems, compatible with relatively low-voltage CMOS technology, for controlling the charging of a battery. In an embodiment, a system for controlling the charging of a battery includes an external charging circuit and a charging control circuit, both coupled between a charger and a battery. The charger has at least two charging modes, a first charging mode that is slower than a second charging mode. The charging control circuit includes a monitor that compares a battery voltage to a threshold voltage and generates a battery status signal, which is received by a charging control logic and a power multiplexer. The charging control logic generates a first charging mode control signal and a second charging mode control signal, which are received by the external charging circuit. 
     When the battery status signal indicates the battery voltage is below the threshold voltage, the power multiplexer couples the charging control logic to ground, and the charging control logic generates an active low first charging mode control signal. An inverter coupled between the charging control circuit and the external charging circuit inverts the first charging mode control signal, which activates the first charging mode of the charger. When the battery status signal indicates the battery voltage exceeds the threshold voltage, the power multiplexer couples the charging control logic to the battery voltage, and the charging control logic generates an active high second charging mode control signal, which activates the second charging mode of the charger. 
     In an embodiment, the system for controlling the charging of a battery includes a diode coupled between the charger and the battery that enables the battery to supply power to the charger line when the battery voltage exceeds the charger voltage. In an embodiment, the external charging circuit includes a MOS device that prevents a leakage current from flowing into the charging control circuit. 
     In another embodiment, a method for controlling the charging of a battery includes monitoring a battery voltage with respect to a threshold voltage. The method further includes coupling a charging control logic supply to ground, generating an active low first control signal, inverting the active low first control signal, and charging the battery at a first rate when the battery voltage is below the threshold voltage. The method further includes coupling the charging control logic supply to the battery voltage, generating an active high second control signal, and charging the battery at a second rate when the battery voltage exceeds the threshold voltage. The first charging rate is slower than the second charging rate. 
     In an embodiment, the method for controlling the charging of a battery further includes supplying battery power to the charger line when the battery voltage exceeds the charger voltage. In an embodiment, the method further includes suppressing a leakage current. 
     Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant arts based on the teachings contained herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number. 
         FIG. 1  illustrates an example environment in which the present invention can be used. 
         FIG. 2  illustrates a block diagram of a non-CMOS battery charging control system. 
         FIG. 3  illustrates a block diagram of a battery charging control system that is compatible with low-voltage CMOS technology, in accordance with an embodiment of the present invention. 
         FIG. 4  illustrates a circuit diagram of a slow charging circuit, in accordance with an embodiment of the present invention, which supports a reverse power mode and suppresses leakage current. 
         FIG. 5  is a process flowchart for controlling the charging of a battery, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Overview 
     The present invention is directed to methods and systems for controlling the charging of a battery. In the detailed description that follows, an example environment in which the present invention can be used is identified and the preferred embodiments of the present invention are presented in detail. While specific features, configurations, and devices are discussed in detail, this description is for illustrative purposes, and persons skilled in the art will recognize that other configurations and devices can be used to achieve the features of the present invention without departing from the scope and spirit thereof. 
     Example Environment 
       FIG. 1  illustrates an example environment  100  in which the present invention can be used. An electronic device  112 , such as a cellular phone, personal digital assistant (PDA), or laptop computer, has a battery  102 , an external charging circuit  106 , and a charging control circuit  108 . Battery  102  discharges when electronic device  112  is used. When battery  102  is discharged to a low voltage level, battery  102  is coupled to a charger  104  for charging. Charging control circuit  108  controls the charging sequence to ensure that battery  102  is charged under safe conditions. External charging circuit  106  switches between a slow charging mode and a fast charging mode under control of charging control circuit  108  until battery  102  is fully charged. 
     Battery Charging Control System 
     In order to describe preferred embodiments of the present invention, it is helpful to contrast the present invention with other approaches. For example,  FIG. 2  illustrates a block diagram of a non-CMOS battery charging control system  200 . System  200  includes charger  104 , an external charging circuit  202 , battery  102 , and a charging control circuit  204 . Charging control circuit  204  receives a battery voltage  203  from battery  102 , and a charger voltage  201  from charger  104 . A battery voltage divider  216  supplies a reduced battery voltage  205  to a battery status monitor  206 , and a battery voltage regulator  214  supplies a regulated battery voltage  221  to an input of a power mulitplexer  210 . A charger voltage divider  220  supplies a reduced charger voltage  215  to a charger status monitor  208 , and a charger voltage regulator  218  supplies a regulated charger voltage  223  to an input of power multiplexer  210 . 
     Battery status monitor  206  determines whether reduced battery voltage  205  is above or below a battery threshold voltage, and generates a battery status signal  207 , which is received by power multiplexer  210  and a charging control logic  212 . In an embodiment, the battery threshold voltage is approximately 2.7 V. Power multiplexer  210  selects one of regulated battery voltage  221  and regulated charger voltage  223  to supply charging control logic  212 . Power multiplexer  210  couples a charging control logic power supply  209  to regulated battery voltage  221  when battery voltage  203  exceeds the battery threshold voltage. Power mulitplexer  210  couples charging control logic power supply  209  to regulated charger voltage  223  when battery voltage  203  is below the battery threshold voltage. 
     Charger status monitor  208  determines whether charger  104  is present and capable of charging battery  102 , and generates a charger status signal  217 , which is received by charging control logic  212 . Charging control logic  212  generates an active high slow charging mode control signal  211  when battery voltage  203  is below the threshold voltage. Slow charging mode control signal  211  activates a slow charging circuit  222 , which generates a slow charging current to safely charge battery  102  until battery voltage  203  exceeds the threshold voltage. When battery voltage  203  exceeds the threshold voltage, charging control logic  212  generates an active high fast charging mode control signal  213 . Fast charging mode control signal  213  activates the fast charging mode of charger  104  until battery  102  is charged. Non-CMOS battery charging control system  200  ensures charger  104  safely charges battery  102  by taking power from charger  104  instead of from battery  102  when battery voltage  203  is below the battery threshold voltage. 
     Battery Charging Control System Using Low-Voltage CMOS Technology 
     A problem with battery charging control system  200  is charging control circuit  204  cannot be implemented with low-voltage CMOS technology. For example, low-voltage CMOS devices in charger voltage regulator  218  could be exposed to charger voltage  201 . Charger voltage  201  could be as high as 13 V, which exceeds the breakdown tolerance of low-voltage CMOS devices. 
       FIG. 3  illustrates a block diagram of a battery charging control system  300 , which is compatible with low-voltage CMOS technology, in accordance with an embodiment of the present invention. In particular, battery charging control system  300  includes charger  104 , an external charging circuit  302 , battery  102 , and a charging control circuit  304 . Charging control circuit  304  receives battery voltage (Vb)  203  from battery  102  and charger voltage (Vc)  201  from charger  104 . A battery voltage regulator  314  and a battery status monitor  306  receive a battery voltage (Vcb)  305 . Battery voltage regulator  314  supplies a regulated battery voltage  321  to an input of a power multiplexer  310 . Another input of power multiplexer  310  is coupled to a ground  318 . 
     Battery status monitor  306  determines whether battery voltage (Vcb)  305  is above or below a battery threshold voltage, and generates a battery status signal (Bp)  307 , which is received by power multiplexer  310  and by a charging control logic  312 . In an embodiment of the present invention, the battery threshold voltage is approximately 2.7 V. Power multiplexer  310  selects one of regulated battery voltage  321  and ground  318  to supply charging control logic  312 . Power multiplexer  310  couples output (Vdd)  309  to regulated battery voltage  321  when battery voltage (Vcb)  305  is above the threshold voltage. Power mulitplexer  310  couples output (Vdd)  309  to ground  318  when battery voltage (Vcb)  305  is below the threshold voltage. Charger status monitor  308  receives a reduced charger voltage (Vcd)  315  from a charger voltage divider  320 , and determines whether charger  104  is present and capable of charging battery  102 . Charger status monitor  308  generates a charger status signal (Cp)  317 , which is received by charging control logic  312 . 
     When battery voltage (Vcb)  305  is below the threshold voltage, battery status signal (Bp)  307  is low, output (Vdd)  309  is grounded, and charging control logic  312  generates an active low slow charging mode control signal (Cs)  311 . In this mode, charging control circuit  304  powers down. An inverter  324  inverts slow charging mode control signal (Cs)  311  to produce inverted slow charging mode control signal (Cs 2 )  319 . In turn, inverted slow charging mode control signal (Cs 2 )  319  activates a slow charging circuit  322 , which generates a slow charging current to safely charge battery  102  until battery voltage (Vcb)  305  exceeds the threshold voltage. 
     When battery voltage (Vcb)  305  exceeds the threshold voltage, battery status signal (Bp)  307  is active high and output (Vdd)  309  is coupled to regulated battery voltage  321 . In this mode, charging control circuit  304  powers up and generates an active high fast charging mode control signal (Cf)  313 . Fast charging mode control signal (Cf)  313  activates a fast charging mode of charger  104  until battery  102  is fully charged. 
     Battery charging control system  300  overcomes the limitations of battery charging control system  200  because charging control logic  312  is isolated from charger voltage  201 , which typically exceeds the breakdown tolerance of low-voltage CMOS devices. Yet battery charging control circuit  304  is capable of activating slow charging circuit  322 , without receiving power from charger  104 , to slowly charge battery  102  when battery voltage (Vcb)  305  is below the threshold voltage. Therefore, charging control logic  312  may be safely implemented with low-voltage CMOS devices. 
     Battery Charging Control System Capable of Reverse Power Mode 
     Supporting a reverse power mode is a desired feature of a battery charging control system. For example, battery charging control system  200 , shown in  FIG. 2 , supports a reverse power mode. A diode  224  is coupled between charger  104  and battery  102 . In an embodiment, diode  224  is a Schottky diode. When battery voltage  203  exceeds charger voltage  201 , diode  224  conducts current in the reverse direction. In this mode, battery  102  supplies power to the charger line  201  and is capable of providing power to other devices. In the example of  FIG. 1 , in the reverse power mode, battery  102  of electronic device  112  could be used to provide power to another electronic device. 
     A potential problem with battery charging control system  200 , shown in  FIG. 2 , is a leakage current that flows in the reverse power mode on the path between battery  102  and charger  104  and into charger voltage regulator  218 . Leakage current is detrimental to charging control circuit  204 . 
     Battery Charging Control System that Suppresses Leakage Current 
       FIG. 4  illustrates a circuit diagram of a slow charging circuit  400 , in accordance with an embodiment of the present invention, which generates a slow charging current (Ic)  403 . Slow charging circuit  400  supports a reverse power mode and substantially prevents leakage current. Slow charging circuit  400  represents current source  322  in  FIG. 3 . 
     In the example of  FIG. 4 , in the slow charging mode, battery voltage  203  is below the battery threshold voltage, charger  104  is present and turned on, and slow charging mode control signal (CHGSS_B)  311  is active low. A resistor (R 3 )  404  coupled to the drain of a MOS device (M 2 )  402  invert slow charging mode control signal (CHGSS_B)  311 . MOS device (M 2 )  402  is turned off and a first bipolar junction transistor (M 1 )  412  is turned on and pulled high through a resistor (R 2 )  410 . A second bipolar transistor (M 3 )  408  is turned on, producing a voltage drop across a resistor (R 1 )  414  and generating slow charging current (Ic)  403 . In an embodiment of the present invention, the voltage drop is approximately 600 mV and slow charging current (Ic)  403  is approximately 40 mA. A node (vx)  401  is pulled low and a PMOS device (M 4 )  416  is turned on and passes slow charging current (Ic)  403 . Diode  326  is turned off. 
     In the reverse power mode, charger  104  is not coupled to slow charging circuit  400 . Diode  326  is turned on and main battery  102  supplies power to the charger line  201 . In an embodiment of the present invention, diode  326  is a Schottky diode. In the reverse power mode, slow charging circuit  400  ensures no leakage current flows into charging control circuit  304 , whether inverted slow charging mode control signal (CHGSS_B)  311  is high (when main battery voltage  203  is below threshold) or low (when main battery voltage  203  exceeds threshold). 
     Method for Controlling the Charging of a Battery Using CMOS Technology 
       FIG. 5  is a process flowchart  500  for controlling the charging of a battery, according to an embodiment of the present invention. If a battery voltage exceeds a charger voltage in step  501 , then the battery supplies the battery voltage to the charger line in a reverse power mode in step  518 . If the battery voltage does not exceed the charger voltage in step  501 , then in step  502 , a monitor determines if the battery voltage exceeds a battery threshold voltage. If the battery voltage is below the battery threshold voltage, a charging control logic power supply line is coupled to ground in step  504 . In an embodiment of the present invention, the charging control logic is implemented with relatively low-voltage CMOS devices. The charging control logic generates an active low slow charging mode control signal in step  506 . In step  507 , an inverter inverts the active low slow charging mode control signal, which causes the external charging circuit to switch to a slow charging mode in step  508 . In step  510 , the charger charges the battery in the slow charging mode and the process resumes monitoring in step  502 . 
     When the battery voltage exceeds the battery threshold voltage in step  502 , the charging control logic power supply line is coupled to the battery in step  512 . In step  514 , the charging control logic generates an active high fast charging mode control signal, which causes the external charging circuit to switch to a fast charging mode. In step  516 , the charger charges the battery in the fast charging mode until the battery is fully charged. For example, a Li-Ion battery is charged to about 4.2 V. 
     CONCLUSION 
     The present invention has been described above with the aid of functional building blocks illustrating the performance of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Any such alternate boundaries are thus within the scope and spirit of the claimed invention. One skilled in the art will recognize that these functional building blocks can be implemented by discrete components, application specific integrated circuits, processors executing appropriate software and the like or any combination thereof. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.