Abstract:
A cellular telephone charging circuit using a dry battery and capable of displaying a charging operation by a light-emitting diode (LED) when the charging operation is actually carried out is provided. The cellular telephone charging circuit using the dry battery as a power supply and including a boost switching regulator that supplies a predetermined charging power to a charging terminal of a cellular telephone, includes: the LED for displaying the charging operation; a control circuit that generates an ON and OFF control signal for driving a boost chopper included in the boost switching regulator; a chopper ON-period detection circuit that detects an ON-period of the boost chopper; and an LED lighting circuit that drives the LED when the ON-period detected by the chopper ON-period detection circuit reaches a predetermined value.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a cellular telephone charging circuit for charging (normally quickly charging) a battery built in the cellular telephone by using a dry battery. More specifically, the present invention relate to a cellular telephone charging circuit capable of displaying a charging operation using a light-emitting diode (LED) only when a battery built in the cellular telephone is actually charged.  
         [0003]     2. Description of the Related Art  
         [0004]     Many conventional cellular telephone charging circuits using dry batteries are not provided with means (LED&#39;s) for displaying respective charging operations. Some cellular telephone chargers include LED&#39;s for displaying respective charging operations and fully charged states.  
         [0005]      FIG. 6  is a block diagram that depicts an example of a conventional cellular telephone charger  5  using a dry battery and including an LED for displaying a charging operation. In the charger  5  shown in  FIG. 6 , a dry battery BC (at a power supply voltage E) is connected to input terminals a 1  and a 2  of a power supply circuit (DC/DC converter)  51 . A charging current-limiting resistor R 1  is connected between the power supply circuit  51  and one output terminal b 1 . A display circuit constituted by an LED current-limiting resistor R 2  and an LED D 1  is connected between output terminals b 1  and b 2 . A cellular telephone built-in battery  52  is connected to the output terminals b 1  and b 2  and the LED D 1  is turned on during a charging operation.  
         [0006]      FIG. 7  is a block diagram that depicts an example of a cellular telephone charger  6  that charges a battery built in a cellular telephone and that includes an LED for displaying a charging operation. In the charger  6  shown in  FIG. 7 , a commercial AC power supply is connected to input terminals a 1  and a 2  of a power supply circuit (an AC/DC inverter)  61 . A charging current-limiting resistor R 1  is connected between the power supply circuit  61  and one output terminal b 1 . A comparison circuit  63  constituted by a comparator CMP, input-side resistors R 3 , R 4 , R 5 , and R 6 , and a feedback resistor R 7  is connected in rear of the power supply circuit  61 . A display circuit constituted by an LED current-limiting resistor R 2  and an LED D 1  is connected between one terminal of the power supply circuit  61 , which terminal is connected to the charging current-limiting resistor R 1  and an output terminal of the comparator CMP. A cellular telephone built-in battery  62  is connected to output terminals b 1  and b 2  and the LED D 1  is turned on during a charging operation.  
         [0007]     In the charger  5  shown in  FIG. 6 , when the power supply circuit  51  is driven, a power is supplied to the cellular telephone built-in battery  52  and the LED D 1  is turned on by a charging voltage between the output terminals b 1  and b 2 . Due to this, the LED D 1  is turned on whenever the power supply circuit  51  is driven irrespectively of whether the cellular telephone built-in battery  52  is charged. A user, therefore, determines that the charging operation is carried out even in a non-chargeable state.  
         [0008]     Specifically, the LED D 1  is turned on even if (1) the cellular telephone built-in battery  52  rejects being charged, (2) the cellular telephone built-in battery  52  is fully charged, or (3) a capacity of the battery cell BC of the charger  5  is reduced, the battery cell BC is incapable of charging the battery, but the battery cell BC outputs a voltage sufficient to turn on the LED D 1 .  
         [0009]     An ordinary LED is turned on whenever a voltage equal to or higher than a forward voltage Vf is applied between an anode and a cathode. The forward voltage Vf for the LED depends on an emission color, a current, a manufacturing method, an emission material, or the like, and the forward voltage Vf for an ordinary red LED is about 1.5 to 2.0 volts (V). The LED is, therefore, driven even if the capacity of the dry cell of the charger  5  is reduced (case (3) stated above).  
         [0010]     In the charger  6  shown in  FIG. 7 , the LED can be turned on when a current equal to or higher than a certain current is carried to the cellular telephone built-in battery  62 .  
         [0011]     Namely, in the charger  6 , if a voltage difference Vd of the charging current-limiting resistor R 1  is detected and the voltage difference Vd is equal to or higher than a certain value (R 1 ×i=Vd, where symbol i denotes a charging current), then the comparator CMP is actuated, a current is applied to the LED D 1 , and the LED D 1  is thereby turned on.  
         [0012]     This conventional charger  6  has, however, many components and each component is expensive. Due to this, although the charger  6  can be used as a charger for charging a cellular telephone built-in battery, it cannot be used as a cellular telephone charger using a dry battery and sold at a low price.  
       SUMMARY OF THE INVENTION  
       [0013]     It is an object of the present invention to provide a cellular telephone charging circuit using a dry battery and capable of displaying a charging operation only when the charging operation is actually carried out, with a simple circuit configuration.  
         [0014]     According to the present invention, there is provided a cellular telephone charging circuit using a dry battery as a power supply and including a boost switching regulator that supplies a predetermined charging power to a charging terminal of a cellular telephone, comprising: a light-emitting diode for displaying a charging operation; a control circuit that generates an ON and OFF control signal for driving a boost chopper included in the boost switching regulator; a chopper ON-period detection circuit that detects an ON-period of the boost chopper; and a light-emitting diode lighting circuit that drives the light-emitting device when the ON-period detected by the chopper ON-period detection circuit reaches a predetermined value.  
         [0015]     The cellular telephone charging circuit using the dry battery according to the present invention is mainly applied to a cellular telephone quick charger.  
         [0016]     The cellular telephone charging circuit using the dry battery according to the present invention can be constitute so that the chopper ON-period detection circuit is a CR integrating circuit that inputs the ON and OFF control signal, and so that the light-emitting diode lighting circuit drives the light-emitting diode when a voltage of the CR integrating circuit reaches a predetermined voltage. In this case, the light-emitting diode lighting circuit includes a resistor and a transistor switch between a positive terminal and a negative terminal of the battery, the light-emitting device is connected between the transistor switch and a ground, and the voltage output from the CR integrating circuit is input to a control terminal of the transistor switch.  
         [0017]     The present invention can provide the cellular telephone charging circuit using the dry battery, and capable of displaying a charging operation only when the charging operation is actually carried out, with a simple circuit configuration, or low production cost. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a functional block diagram of a cellular telephone charging circuit using a dry battery according to the present invention;  
         [0019]      FIG. 2  is a specific circuit diagram of the cellular telephone charging circuit using the dry battery shown in  FIG. 1 ;  
         [0020]      FIG. 3  is a circuit diagram that depicts a chopper ON-period detection circuit, an LED lighting circuit, and an LED extracted from the circuit shown in  FIG. 2 ;  
         [0021]      FIG. 4  is a time chart that depicts a control signal, a terminal voltage of a capacitor, and transistor ON and OFF states in the circuit shown in  FIGS. 2 and 3 ;  
         [0022]      FIG. 5  is a front view of a cellular telephone charger mounting therein the cellular telephone charging circuit using the dry battery according to the present invention;  
         [0023]      FIG. 6  is a block diagram that depicts one example of a conventional cellular telephone charger using a dry battery and including an LED for displaying a charging operation; and  
         [0024]      FIG. 7  is a block diagram that depicts one example of a conventional cellular telephone built-in battery charger and including an LED for displaying a charging operation. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]      FIG. 1  is a functional block diagram of a cellular telephone charging circuit  100  using a dry battery according to the present invention. In  FIG. 1 , the cellular telephone charging circuit  100  using a dry battery includes a boost chopper  101 , a control circuit  102 , a chopper ON-period detection circuit  103 , an LED lighting circuit  104 , and a charging operation display LED  105 . The boost chopper  101  is constituted by an inductor and a transistor switch, as will be described later.  
         [0026]     In an actual charging circuit, the boost chopper  101  and an output circuit  106  constitute a boost switching regulator as shown in  FIG. 2 .  
         [0027]     The control circuit  102  can generate an on and off control signal, e.g., a pulse width modulation (PWM) control signal or a pulse frequency modulation (PFM) control signal, for driving the boost chopper  101 . The chopper ON-period detection circuit  103  can detect an ON period of the boost chopper  101 . The LED lighting circuit  104  can drive the LED  105  when the ON period detected by the chopper ON-period detection circuit  103  reaches a predetermined value. The LED  105  is, for example, red light-emitting diode (red LED).  
         [0028]     In the cellular telephone charging circuit  100  using the dry battery  200  shown in  FIG. 1 , when a power SI is supplied (from the dry battery  200 ), the control circuit  102  controls the boost chopper  101  by a predetermined signal S 3 . Under control of the control circuit  102 , the power S 1  from the dry battery  200  is supplied to the boost chopper  101 , and the boost chopper  101  charges a cellular telephone built-in battery  300  at a predetermined charging voltage and a predetermined charging current denoted by reference symbol S 2 .  
         [0029]     The control signal S 3  from the control circuit  102  is also input to the chopper ON-period detection circuit  103 , and the copper ON-period detection circuit  103  can thereby detect an ON period of the boost chopper  101 . Specifically, the chopper ON-period detection circuit  103  can be constituted by a CR integrating circuit, as will be described later. If the chopper ON-period detection circuit  103  is the CR integrating circuit, for example, the LED lighting circuit  104  turns on the LED  105  when an output voltage (a charging voltage of a capacitor constituting the CR integrating circuit) reaches a predetermined voltage (in response to a detection signal S 4  transmitted from the LED lighting circuit  104 ). In addition, the LED  105  is turned on by a power S 5  from the dry battery  200 .  
         [0030]     A lithium-ion battery is normally used as the cellular telephone built-in battery  300 . A rated output voltage of the lithium-ion battery is normally about 3.6 to 3.7 V. It is necessary to charge the lithium-ion battery at a voltage equal to or higher than the rated output voltage and a current equal to or higher than 80 milliamperes (mA). Normally, the lithium-ion battery is charged at a voltage equal to or higher than 4.5 V and lower than 5.7 V and a current of 300 to 600 mA.  
         [0031]      FIG. 2  is a specific circuit diagram of the cellular telephone charging circuit  100  using the dry battery  200  shown in  FIG. 1 . In the cellular telephone charging circuit  100  using the dry battery  200  shown in  FIG. 2 , the dry battery  200  is connected to input terminals a 1  and a 2 . It is assumed herein that the dry battery  200  is two battery cells each at a voltage of 1.5 V, connected in series, and having a voltage of 3 V in all.  
         [0032]     The boost chopper  101  is constituted by an inductor L, a transistor switch (a field effect transistor (FET) in this embodiment) Q 1 , and a diode (a Schottky barrier diode) SBD. The control circuit  102  is constituted by an integrated circuit (IC) capable of outputting the PWM or PFM control signal to an output terminal of the transistor switch Q 1 .  
         [0033]     The chopper ON-period detection circuit  103  is constituted by an integrating circuit composed by a resistor R 2  and a capacitor C 2 . The LED lighting circuit  104  is constituted by an LED current-limiting resistor R 3  and a transistor switch (a bipolar transistor in this embodiment) Q 2 . The LED  105  is a red LED.  
         [0034]      FIG. 2  also shows that the output circuit  106  composed by an output current-limiting resistor R 1  and a smoothing capacitor C 1  is connected to an output stage (between the output terminals b 1  and b 2 ) of the cellular telephone charging circuit  100  using the dry battery  200 .  
         [0035]     In  FIG. 2 , the boost chopper  101  and the output circuit  106  constitute the boost switching regulator.  
         [0036]     In the cellular telephone charging circuit  100  using the dry battery  200  shown in  FIG. 2 , when the transistor switch Q 1  is turned on, energy is accumulated in the inductor L. When the transistor switch Q 1  is turned off, the energy accumulated in the inductor L is supplied to the output circuit  106  through the Schottky barrier diode SBD and a charging current i is supplied to the cellular telephone built-in battery  300 .  
         [0037]     Therefore, if the ON period of the transistor switch Q 1  (a period for energizing the inductor L) is longer, the energy accumulated in the inductor L is higher and the charging current i is higher.  
         [0038]     In this embodiment, if the charging current i exceeds a predetermined value, the ON period of the transistor switch Q 1  is equal to or longer than a certain time and the LED  105  is turned on, accordingly.  
         [0039]      FIG. 3  is a circuit diagram that depicts the chopper ON-period detection circuit  103 , the LED lighting circuit  104 , and the LED  105  extracted from the circuit shown in  FIG. 2 . Referring to  FIG. 3 , a control signal CTRL (appearing at a point A) of the transistor switch Q 1  is integrated by the chopper ON-period (integrating circuit)  103  composed by the resistor R 2  and the capacitor C 2 . When a voltage at a point B (a voltage of the capacitor C 2 ) reaches a predetermined voltage Vsh (=Vbe+Vf), the transistor switch Q 2  is turned on to carry a current to the LED  105 , thereby turning on the LED  105 .  
         [0040]     The voltage Vbe is an emitter-base voltage of the transistor switch Q 2 , and the voltage Vf is a forward voltage of the LED  105 .  
         [0041]      FIG. 4  is a time chart that depicts the control signal CTRL, a terminal voltage VC 2  of the capacitor C 2 , and ON and OFF states of the transistor switch Q 2 .  
         [0042]     In this embodiment, the LED  105  is connected to an emitter side of the transistor switch Q 2 . It is thereby possible to increase the predetermined voltage, i.e., threshold voltage Vsh, at which the transistor switch Q 2  is turned on, by as much as the forward voltage Vf (e.g., about 1.7 V).  
         [0043]     Further, when a waveform at the point A is at an L level, charges accumulated in the capacitor C 2  can be promptly emitted (that is, a difference between the Vsh and an L-level potential can be set large). It is thereby possible to ensure turning off the LED  105  when an H-level time of the waveform at the point A is short and the charging current i is not carried to the cellular telephone built-in battery  300 .  
         [0044]     When the dry battery  200  is consumed and the battery voltage E is lower, the H-level time at the point A is longer even at the equal charging current i. In this embodiment, the LED  105  is connected to the emitter of the transistor switch Q 2  and directly driven by the battery voltage E.  
         [0045]     Therefore, to turn on the LED  105 , the following condition needs to be satisfied. 
 
 Vf&lt;E−Vcc− If× R   3 , that is, 
 
         [0046]     E&gt;Vf+Vce−If×R 3 , where symbol Vce denotes an emitter-collector voltage of the transistor switch Q 2  and If denotes the current carried to the LED  105 .  
         [0047]     If it is assumed that Vf is 1.7 V, Vce is 0.2 V, and If×R 3  is 0.1 V, the condition is represented by: 
 
 E&gt; 1.7+0.2+0.1=2.0   [V]
 
         [0048]     According to this embodiment, therefore, the LED  105  is turned of f in a battery consumed state (the battery voltage lower than 2.0 V).  
         [0049]      FIG. 5  is a front view of a cellular telephone charger  400  mounting therein the cellular telephone charging circuit  100  using the dry battery  200 . In the cellular telephone charger  400  mounting therein the cellular telephone charging circuit  100  using the dry battery  200  shown in  FIG. 5 , when a terminal portion  401  (corresponding to the output terminals b 1  and b 2 ) is connected to a charging terminal of a cellular telephone (not shown), the charging current is carried from the dry battery  200 . In addition, the LED  105  is turned on only when an appropriate charging operation is carried out.