Abstract:
The present disclosure provides a battery charging control circuit. The charging control circuit includes: a constant-current charging unit and a trickle charging unit. The charging control circuit further includes a branch switch, a control unit, and a detection unit. The branch switch is connected between a power source and the rechargeable battery for enabling or disabling the constant-current charging unit. The control unit is between the constant-current charging unit and the branch switch for controlling the branch switch. The detection unit is used to detect a state of the rechargeable battery. If the detection unit detects the state of the rechargeable battery is correspond to a predetermined state, then the control unit controls the branch switch to disable the constant-current charging unit and enable the trickle charging unit.

Description:
RELATED APPLICATIONS 
     This application is related to a co-pending U.S. patent application Ser. No. 12/411,354 filed concurrently herewith and entitled “BATTERY CHARGING CONTROL CIRCUIT”, which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to battery circuits and, particularly, to a battery charging circuit. 
     2. General Background 
     Generally, battery chargers either use a constant current (CC) mode or a constant voltage (CV) mode to charge a battery. However, in either the CC mode or the CV mode, battery charging is terminated once the battery&#39;s voltage reaches a peak value (e.g., 4.2 V), which may result in the battery not being fully charged. 
     Therefore, it is necessary to provide an apparatus and method to overcome the above-identified deficiencies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The components in the drawings are not necessarily drawn to measuring scale, the emphasis instead being placed upon clearly illustrating the principles of the battery charging control circuit. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a block diagram of a battery charging control circuit in accordance with an exemplary embodiment. 
         FIG. 2  is a circuit diagram of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring to  FIG. 1 , a battery charging control circuit  1  in accordance with an exemplary embodiment, includes a first branch and a second branch which are connected in parallel between a power source  10  and a rechargeable battery  40 . The first branch includes a constant-current charging unit  20  and a branch switch  23 , the second branch includes a trickle charging unit  30 . The constant-current charging unit  20  includes a current detection circuit  21  and a detection switch  22 . The battery charging control circuit  1  also includes a control unit  24 , which is connected to the constant-current charging unit  20  and the branch switch  23 , and includes a detection unit  50 , which is connected to the control unit  24 . The battery charging control circuit  1  firstly charges the rechargeable battery  40  under control of the constant-current charging unit  20  in a constant-current (cc) mode and then under control of the trickle charging unit  30  in a trickle mode. In the cc mode, the battery charging control circuit  1  charges the rechargeable battery  40  with a relatively large current, and in the trickle mode, the battery charging control circuit  1  charges the rechargeable battery  40  with a relatively small current. 
     The constant-current charging unit  20  is configured for charging the rechargeable battery  40  with a relatively large current in the cc mode. The current detection circuit  21  is configured for detecting current of the battery, and providing a detection voltage proportional to the current. The detection switch  22  is turned on or off based on the detection voltage. The control unit  24  is configured for controlling the branch switch  23  to turn off or on depending on the on or off state of the detection switch  22 . The branch switch  23  is configured for enabling or disabling the constant-current charging unit  20  in response to its on or off state. When the branch switch  23  is turned off, the current stops through the first branch, the constant-current charging unit  20  is disabled and the detection voltage drops to zero. As a result, the detection switch  22  is turned off corresponding to the “zero” detection voltage. When the detection switch  22  turns off, the control unit  24  controls the branch switch  23  to turn on again, the current flows through the first branch and the constant-current charging unit  20  is enabled. In such a way, the branch switch  23  is turned on and off periodically and the current flows and stops alternately through the first branch. The battery charging control circuit  1  charges the rechargeable battery  40  in a pulse charging phase of the cc mode. 
     The trickle charging unit  30  is configured for charging the rechargeable battery  40  with a relatively small current in the trickle mode. The detection unit  50  is configured for detecting states of the rechargeable battery  40  and producing detection signals according to the states. In the exemplary embodiment, the states include but are not limited to temperature of the rechargeable battery  40  and a sufficient charged state of the rechargeable battery  40 . The control unit  24  is also configured for controlling the branch switch  23  to turn off upon receiving the detection signal from the detection unit  50 . The control unit  24  is disabled to control the branch switch  23  according to states of the detection switch once the control unit  24  receives the detection signals from the detection unit  50 , then, the first branch is opened and the second branch is employed to charge the rechargeable battery  40 . Accordingly, the cc mode is terminated and the trickle mode is started. 
     Referring to  FIG. 2 , in the exemplary embodiment, the current detection circuit  21  includes a resistor R 1  with a terminal (shown as node A in  FIG. 2 ) connected to the power source  10  and another terminal (shown as node B in  FIG. 2 ) connected to the branch switch  23 . The detection switch  22  can be a p-channel metal-oxide-semiconductor field-effect transistor (PMOSFET) or a positive-negative-positive (PNP) bipolar junction transistor (BJT). In the exemplary embodiment, a PNP BJT Q 1  is taken as an example to illustrate the detection switch  22 . An emitter of the PNP BJT Q 1  is connected to node A and a base of the PNP BJT Q 1  is connected to node B. 
     The branch switch  23  includes a low voltage activated switch Q 4  and a resistor R 5 , the low voltage activated switch Q 4  can be a PMOSFET or a PNP BJT. In the exemplary embodiment, a PMOSFET Q 4  is taken as an example to illustrate the low voltage activated switch Q 4 . The resistor R 5  is connected between a source of the PMOSFET Q 4  and a gate (shown as node E in  FIG. 2 ) of the PMOSFET Q 4 , and a drain (shown as node F in  FIG. 2 ) of the PMOSFET Q 4  is connected to an anode of the rechargeable battery  40 . 
     The control unit  24  includes a first switch  241  and a second switch  242 . The first switch  241  and the second switch  242  can be NMOSFETs or NPN BJTs. In the exemplary embodiment, NMOSFETs Q 2  and Q 3  are taken as an example to illustrate the first switch  241  and the second switch  242  correspondingly. A gate (shown as node C in  FIG. 2 ) of the NMOSFET Q 2  is connected to a collector of the NPN BJT Q 1 , the gate is also connected to ground through a resistor R 2 , a drain of the NMOSFET Q 2  is grounded, a source (shown as node D in  FIG. 2 ) of the NMOSFET Q 2  is connected to a gate of the NMOSFET Q 3 . The node D is also connected to node B through a resistor R 3 . A source of the NMOSFET Q 3  is grounded and a drain of the NMOSFET Q 3  is connected to the gate (node E) of the NMOSEFET Q 4  through a resistor R 4 . 
     The trickle charging unit  30  includes a resistor R 6  which is connected between the power source  10  and the anode of the rechargeable battery  40 . The resistance value of the resistor R 6  is much larger than that of the resistor R 1 . 
     Because the resistor R 6  has a much higher resistance value than the resistor R 1 , when the battery charging control circuit  1  receives the power source  10  and starts charging the rechargeable battery  40 , the resistor R 6  is bypassed by the resistor R 1 , namely the battery charging control circuit  1  charges the rechargeable battery  40  in the cc mode initially. The current of the resistor R 1  is increased gradually and so does the detection voltage in proportion to the current increases too. If the detection voltage is larger than a threshold voltage of the NPN BJT Q 1 , the NPN BJT Q 1  is turned on, then the gate of the NMOSFET Q 2  obtains a high voltage through the NPN BJT Q 1  and the NMOSFET Q 2  is accordingly turned on. The gate of the NMOSFET Q 3  is grounded through the NMOSFET Q 2  and the NMOSFET Q 3  is accordingly turned off. A current path defined by the resistors R 1 , R 4 , R 5  and the NMOSFET Q 3  is cut off and no voltage is applied to the resistor R 5 . There is no voltage drop across the source and gate of the PMOSFET Q 4 . Thus, the PMOSFET Q 4  is turned off. 
     When the PMOSFET Q 4  is turned off, the current of the resistor R 1  drops and the detection voltage drops too. When the detection voltage is lower than the threshold voltage of the NPN BJT Q 1 , the NPN BJT Q 1  is turned off, then the gate of the NMOSFET Q 2  is grounded through the resistor R 2  and the NMOSFET Q 2  is turned off accordingly. The gate of the NMOSFET Q 3  is connected to the node B through the resistor R 3  and the NMOSFET Q 3  is turned on accordingly. The current path defined by the resistors R 1 , R 4 , R 5  and the NMOSFET Q 3  is re-established. A voltage drop across the resistor R 5  turns on the PMOSFET Q 4 , and current flows through the first branch again to charge the rechargeable battery  40 . 
     As described above, in the cc mode, the PMOSFET Q 4  cycles on and off, and the battery charging control circuit  1  charges or pauses charging the rechargeable battery  40  periodically. In other words, the battery charging control circuit  1  charges the recharge battery  40  in the pulse charging phase of the cc mode. 
     In the exemplary embodiment, the detection unit  50  is a full charge detection unit  50 , which is configured for outputting a full-charge signal when the rechargeable battery  40  is nearly fully charged. An output port  501  of the full charge detection unit  50  is connected to the gate of the NMOSFET Q 3 . In the exemplary embodiment, the full-charge signal is a low voltage signal. 
     If the full charge detection unit  50  detects the rechargeable battery  40  is nearly fully charged, the full charge detection unit  50  transmits a low voltage signal to the gate of the NMOSFET Q 3  and accordingly turns off the NMOSFET Q 3 . A current path defined by the resistors R 1 , R 4 , R 5  and the NMOSFET Q 3  is cut off and voltage to the resistor R 5  is removed. There is no voltage drop across the source and gate of the PMOSFET Q 4 . Thus, the NMOSFET Q 4  is turned off and the first branch is cut off, the battery charging control circuit  1  charges the rechargeable battery  40  through the trickle charge unit  30  of the second branch, namely, the battery charging control circuit  1  charges the rechargeable battery  40  in the trickle mode. Because the resistance value of the resistor R 6  of the trickle charging unit  30  is relatively large, the current flowing through the resistor R 6  is small, and the battery charging control circuit  1  charges the rechargeable battery  40  with a small current in the trickle mode. 
     It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the disclosure or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the present disclosure.