BACKGROUND 
       [0001]    1. Related Applications 
         [0002]    This application is related to a co-pending U.S. patent application filed concurrently herewith whose Attorney Docket No. is US 14856 and entitled “BATTERY CHARGING CONTROL CIRCUIT”, which is incorporated herein in its entirety by reference. 
         [0003]    2. Technical Field 
         [0004]    The present disclosure relates to battery circuits and, particularly, to a battery charging circuit. 
         [0005]    3. General Background 
         [0006]    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. 
         [0007]    Therefore, it is necessary to provide an apparatus and method to overcome the above-identified deficiencies. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    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. 
           [0009]      FIG. 1  is a block diagram of a battery charging control circuit in accordance with an exemplary embodiment. 
           [0010]      FIG. 2  is a circuit diagram of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0011]    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  21  and a branch switch  22 , which are connected in series. The second branch includes a trickle charging unit  30 . The battery charging control circuit  1  also includes a detection switch  23  which is connected between the power source  10  and the branch switch  22 , and a control unit  24  which is connected to the detection switch  23  and the branch switch  22 . The battery charging control circuit  1  further includes a detection unit  50  which 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  21  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, in the trickle mode, the battery charging control circuit  1  charges the rechargeable battery  40  with a relatively small current. 
         [0012]    The constant-current charging unit  21  is configured for charging the rechargeable battery  40  with a relatively large current in the CC mode. The detection switch  23  is configured for being turned on and off alternately according to a voltage of the constant-current charging unit  21 . The control unit  24  is configured for controlling the branch switch  22  to turn off and on alternately depending on the alternately on and off states of the detection switch  23 . When the branch switch  22  is turned off, the constant-current charging unit  21  is disabled and the voltage of the constant-current charging unit  21  drops to zero. As a result, the detection switch  23  is turned off corresponding to the “zero” voltage of the constant-current charging unit  21 . When the detection switch  23  turns off, the control unit  24  controls the branch switch  22  to turn on again and the constant-current charging unit  21  is enabled. In such a way, the branch switch  22  is turned on and off periodically and the charge 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. 
         [0013]    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 states of the battery  40 . The states are determined by a plurality of parameters of the rechargeable battery  40 , the parameters include but are not limited to, temperature of the rechargeable battery  40  and a sufficiency of charged state of the rechargeable battery  40 . The control unit  24  is also configured for controlling the branch switch  22  to turn off upon receiving the detection signals from the detection unit  50 . The control unit  24  is disabled to control the branch switch  22  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. 
         [0014]    Referring to  FIG. 2 , in the exemplary embodiment, the constant-current charging unit  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  22 . The detection switch  23  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  23 . An emitter of the PNP BJT Q 1  is connected to the node A and a base of the PNP BJT Q 1  is connected to the node B. 
         [0015]    The branch switch  22  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 and a gate (shown as node E in the  FIG. 2 ) of the PMOSFET Q 4 . A drain (shown as node F in the  FIG. 2 ) of the PMOSFET Q 4  is connected to an anode of the rechargeable battery  40 . 
         [0016]    The control unit  24  includes a first switch  241  and a second switch  242 . Each of the first switch  241  and the second switch  242  can be an n-channel metal-oxide-semiconductor field-effect transistor (NMOSFET) or a negative-positive-negative (NPN) bipolar junction transistor (BJT). In the exemplary embodiment, NMOSFETs Q 2  and Q 3  are respectively taken as examples to illustrate the first switch  241  and the second switch  242 . A gate (shown as node C in the  FIG. 2 ) of the NMOSFET Q 2  is connected to a collector of the NPN BJT Q 1 . Node C 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 the  FIG. 2 ) of the NMOSFET Q 2  is connected to a gate of the NMOSFET Q 3 . 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 node E of the NMOSFET Q 4  through a resistor R 4 . 
         [0017]    The trickle charging unit  30  includes a resistor R 6 . The resistance value of the resistor R 6  is much larger than a resistance value of the resistor R 1 . 
         [0018]    Because the resistor R 6  has a much larger resistance value than the resistor R 1  does, 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  initially charges the rechargeable battery  40  in the CC mode. The current through the resistor R 1  increases gradually and so does the voltage of the resistor R 1  in proportion to the current increases too. When the voltage of the resistor R 1  is larger than a threshold voltage of the NPN BJT Q 1 , the NPN BJT Q 1  is turned on. 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 then grounded by 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 a voltage drop of the resistor R 5  is removed. There is no voltage drop applied between the source and gate of the PMOSFET Q 4 . Thus, the PMOSFET Q 4  is turned off. 
         [0019]    When the PMOSFET Q 4  is turned off, the current flowing through the resistor R 1  drops and the detection voltage drops too. When the voltage of the resistor R 1  falls below the threshold voltage of the NPN BJT Q 1 , the NPN BJT Q 1  is turned off. The gate of the NMOSFET Q 2  is grounded by the resistor R 2  and the NMOSFET Q 2  is accordingly turned off. The gate of the NMOSFET Q 3  is connected to node B through the resistor R 3  and the NMOSFET Q 3  is accordingly turned on. 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 . 
         [0020]    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. 
         [0021]    In the exemplary embodiment, the detection unit  50  is a full charge detection unit. An output port (not shown) of the full charging detection unit is connected to the gate of the NMOSFET Q 3 . 
         [0022]    If the full charge detection unit detects that the rechargeable battery  40  is nearly fully charged, the full charge detection unit transmits a low voltage signal to the gate of the NMOSFET Q 3  and accordingly turns off the NMOSFET Q 3 . The current path defined by the resistors R 1 , R 4 , R 5  and the NMOSFET Q 3  is cut off and the 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 PMOSFET Q 4  is turned off and the CC mode is terminated, the battery charging control circuit  1  charges the rechargeable battery  40  through the trickle charging unit  30 , 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. 
         [0023]    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.
The present invention provides a charging control circuit for a rechargeable battery. 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 detection 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 detection switch is turned on or off depending on the enable or disable state of the constant-current charging unit. The control unit is connected between the detection switch and the branch switch for controlling the branch switch to turn on or off depending on the off or on state of the detection switch.