Abstract:
A vehicle includes a plurality of switch devices including respective LEDs, which are turned on to illuminate when a lighting switch for headlights and taillights is activated. A current flowing in one of the switch devices is detected and a current supplied from a battery to the switch devices is controlled by a regulator circuit and a single transistor based on the detected current. A voltage corresponding to a battery voltage is superimposed on a current detection voltage, and the regulator circuit immediately stops the current when the battery voltage excessively increases.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based on and incorporates herein by reference Japanese Patent Application No. 2003-301418 filed on Aug. 26, 2003. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to a lighting control apparatus for vehicles, which controls lights of substantially the same luminance provided in a plurality of switch devices installed in a vehicle.  
       BACKGROUND OF THE INVENTION  
       [0003]     In a vehicle, a plurality of switch devices is installed on an instrument panel to be manually operated by a vehicle driver. As shown in  FIG. 3 , each switch device  2  has a light source, which is constructed with a plurality of light emitting diodes (LEDs)  1  connected in series and in parallel as shown in  FIG. 3 . The light source is connected in series with a storage battery  4 , a transistor  51 , a transistor  5  and an emitter resistor  6 . The base of the transistor  5  is connected to a junction of a resistor  7  and a Zener diode  8  through a base resistor  9 .  
         [0004]     When a lighting switch  3  is activated to turn on vehicle lights (headlights and taillights)  60  at night, for instance, a microcomputer  50  responsively turns on the transistor  51  so that electric power is supplied from the storage battery  4  to the LEDs  1  through the transistor  51 , the transistor  5  and the resistor  6 . The base potential of the transistor  5  is regulated at a fixed voltage by the Zener diode  8 , even when the voltage of the battery  4  fluctuates. Thus, the current to the LEDs  1  is substantially unchanged to maintain substantially the same luminance of the LEDs  1  among the switch devices  2 . In this apparatus, the number of electronic switching circuits such as transistors  5  will increase as the number of switch devices  2  increases.  
       SUMMARY OF THE INVENTION  
       [0005]     It is therefore an object of the present invention to provide a light control apparatus for vehicles, which does not require electronic switching circuits for each switch device.  
         [0006]     According to the present invention, a lighting control apparatus for vehicles is connected to a battery and a plurality of switch devices including light components, respectively. The apparatus has a current supply switch, a current detection circuit and a regulator circuit. The current supply switch is connected to all the switch devices to supply a current from the battery to the light components. The current detection circuit is connected to only one of the switch devices for detecting the current flowing in the only one of the switch devices. The regulator circuit regulates the current in accordance with the current detected by the current detecting means. Thus, the construction of the apparatus is simplified by the use of the single current supply switch and the single current detecting circuit, even when the number of the switch devices is increased.  
         [0007]     Further, a voltage indicative of the battery voltage is superimposed on a current detection voltage and the regulator circuit controls the current supply switch based on the sum of the voltages. Thus, the regulator circuit not only feedback controls the current supplied to the switch devices, but also stops the current supply immediately when the battery voltage rises excessively. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:  
         [0009]      FIG. 1  is a circuit diagram showing a light control apparatus for vehicles according to an embodiment of the present invention;  
         [0010]      FIG. 2  is a time chart showing operation of the embodiment; and  
         [0011]      FIG. 3  is a circuit diagram showing a light control apparatus for vehicles according to a related art. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]     Referring first to  FIG. 1 , a plurality of switch devices  2  are connected in parallel with each other and connected in series with a battery  4 . Resistors  11  are connected in series between the switch devices  2  and the ground, respectively, and a switching transistor  43  is connected in common between the battery and the switch devices  2 . One of the switch devices  2  is connected to an input terminal (−IN) of a regulator circuit  14  through a series of resistors  12  and  13 . The resistors  12  and  13  are for detecting a current flowing in the switch device  2 . A capacitor  15  is connected to the resistors  12  and  13 .  
         [0013]     The regulator circuit  14  is a pulse width modulation (PWM) integrated circuit, for instance MB3800 manufactured by FUJITSU SEMICONDUCTOR DEVICES, to produce a PWM output signal from its output terminal (OUT). The regulator circuit  14  includes a reference voltage source  17 , an error amplifier  18 , a PWM comparator  19 , a sawtooth signal generator  20 , an output drive control circuit  21 , a soft start control circuit  22 , and the like.  
         [0014]     The regulator circuit  14  is connected to a voltage source (5 V) of an operating voltage of 5 volts through its terminal (Vcc) to supply it to the reference voltage source  17  and current sources  23 ,  24 . The regulator circuit  14  is connected to the ground through a terminal (OSC) and a parallel circuit of a capacitor  25  and a resistor  26 , which determines the frequency of a sawtooth signal generated by the sawtooth signal generator  20 . The regulator circuit  14  is grounded through a terminal (SCP) and a capacitor  27 , which determines the operation characteristic of the soft start circuit  22 . The regulator circuit  14  is connected to the ground through a terminal (BR/CTL) and a resistor  28 , which maintains the regulator circuit  14  to be continuously active.  
         [0015]     The reference voltage source  17  generates a reference voltage of about 1.25 volts, which is temperature-compensated, from the operating voltage of 5 volts. The error amplifier  18  is connected to the input terminal (−IN) at its inverting terminal (−) and a reference voltage of 0.5 volts at its non-inverting terminal (+). The error amplifier  18  is connected to one of input terminals of the PWM comparator  19  through its output terminal and a resistor  29 .  
         [0016]     The PWM comparator  19  has one inverting terminal (−) and three non-inverting terminals (+). The sawtooth signal of the sawtooth signal generator  20  is applied to the inverting terminal (−) through an offset voltage of 0.1 volt. The sawtooth signal changes its amplitude between 0.1 volt and 0.6 volts, and hence the input voltage applied to the inverting terminal changes between 0.2 volts and 0.7 volt. The PWM comparator  19  receives at its non-inverting terminals an output signal of the soft start control circuit  22 , a stop period setting voltage of 0.6 volts. Thus, the PWM comparator  19  outputs a high level (H) signal when the sawtooth signal voltage is lower than the output voltage of the error amplifier  18 , the soft start setting voltage and the stop period setting voltage.  
         [0017]     The output drive control circuit  21  is constructed in a Totem-pole form to directly drive a transistor  30 . The collector of the transistor  30  is connected the current source  24 . It is also connected to the output terminal (OUT) and to the ground through a resistor  31  of 30 kilo ohms. The emitter of the transistor  30  is grounded. The input terminal (−IN) of the regulator circuit  14  is grounded through a capacitor  32 . The output terminal (FB) of the error amplifier  18  is connected through a resistor  33  to a junction between a resistor  29  and one non-inverting input terminal (+) of the PWM comparator  19 . A parallel circuit of a capacitor  34  and a resistor  35  is connected between the input terminal (−IN) and the output terminal (FB). The input terminal (−IN) is also connected to a junction between resistors  36  and  37 , which divide the voltage of the battery  4  supplied through a transistor  51 . The resistances of the resistors  36  and  37  are determined to apply a voltage of 0.5 volts under normal condition of the battery  4 .  
         [0018]     The output terminal (OUT) of the regulator circuit  14  is connected to the base of a transistor  40  through a series circuit of resistors  38  and  39 . The base of the transistor  40  is grounded through a resistor  41 . The collector of the transistor  40  is connected to the battery  4  through a resistor  42  and to the gate of a P-channel field effect transistor (FET)  43  provided as a switching component. The emitter of the transistor  40  is grounded.  
         [0019]     The source of the FET  43  is connected to the battery  4 , and the anode of the same is connected in common to the anodes of the LEDs  1  of all the switch devices  2 . Thus, when the lighting switch  3  is kept activated to turn on the vehicle head and taillights  60 , the regulator circuit  14  PWM-controls the FET  43  so that luminance of the LEDs  1  may be maintained.  
         [0020]     More specifically, when the lighting switch  3  is activated and the transistor  51  is turned on by the microcomputer  50 , the resistors  36  and  37  apply the divided voltage to the input terminal (−IN) of the regulator circuit  14 . The soft start control circuit  22  operates to gradually increase the ON period of the PWM signal generated by the PWM comparator  19  and applied to the transistor  40  and FET  43 . During the ON period of the PWM signal, the transistor  40  turns on and the FET  43  supplies the battery current to the LEDs  1 .  
         [0021]     The voltage of the battery  4  normally changes as shown in  FIG. 2  (left and central sides), usually under a maximum of 14 volts, depending on electric load conditions. As this voltage increases, the current detection voltage indicative of the current supplied to the LEDs  1  and detected by the resistors  12  and  13  proportionally increases. This current detection voltage is applied to the error amplifier  18  through the input terminal (−IN). The error output voltage of the error amplifier  18  gradually decreases as shown in  FIG. 2  with the increase in the current detection voltage. The PWM comparator  19  compares this error voltage and the sawtooth signal voltage and produces the PWM signal voltage. The ON period (high level period) thus gradually decreases as shown in  FIG. 2  as the error voltage decreases. As a result, the FET  43  shortens its ON period to reduce the current supplied to the LEDs  1 . Thus, the current supplied to the LEDs is feedback-controlled to maintain the luminance of the light source among the switch devices  2 .  
         [0022]     A vehicle engine sometimes cannot be successfully started by a starter motor in extremely cold areas. In this instance, an additional battery may be connected in series with the battery  4  thereby to drive the starter motor with higher voltages. If such an additional battery is used, the input voltage applied to the input terminal (−IN) of the regulator circuit  14  responsively and excessively increases as shown in  FIG. 2  (right side). Therefore, the error voltage produced form the error amplifier  18  immediately decreases and to be below 0.2 volts (minimum voltage of the sawtooth signal voltage), the PWM output signal voltage produced from the PWM comparator  19  remains low (no ON period). Thus, the LEDs  1  are protected from the excessive voltage, before the excessive current supplied to the LEDs  1  is detected and feedback-controlled to reduce the current.  
         [0023]     In the above embodiment, all the switch devices  2  (LEDs  1 ) are controlled by one FET  43  and the current supplied to the switch devices  2  is detected by one current detecting circuit (resistors  12  and  13 ) for the feedback control. Therefore, even if the number of switch devices  2  increases, electronic circuits associated with the switch devices  2  need not be increased in proportion. Further, the total battery voltage is detected by the resistors  36 ,  37  and superimposed with the current detection voltage produced by the resistors  12 ,  13 , the LEDs  1  can be quickly protected from excessive currents.  
         [0024]     The present invention should not be limited to the above embodiment, but may be implemented in many other ways. For instance, the FET  43  may be replaced with a bipolar transistor or an IGBT. The LEDs  1  may be replaced with other light emitting components.