Abstract:
A load controller includes: an input circuit which detects that a drive instruction signal is less or equal to a first input threshold value; a constant current source activated in accordance with a detection by the input circuit; a PWM signal generating unit that is activated by the constant current source and generates a PWM signal; a comparator that is activated by the constant current source and compares the drive instruction signal with a second input threshold value set to be lower than the first input threshold value; a logic calculation unit that carries out a logic calculation of the PWM signal with a compared result of the comparator; a drive control unit that operates in accordance with an output from the logic calculation unit to generate a PWM drive control signal; and a load driving element that is driven by the PWM drive control signal to control a load.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a load controller, and more particularly to a load controller that PWM-controls a load. 
         [0003]    2. Background Art 
         [0004]    In vehicles, some of devices are required to operate in accordance with a drive instruction by the input of a control signal from an input switch even in a state that an ignition switch is turned off. In such devices, when the input switch is turned off, a current (dark current) needs to be lowered as much as possible, preferably to be set to zero from the viewpoint of a protection of a battery. 
         [0005]    Thus, when a device is to be realized in which a PWM (Pulse Width Modulation) output operation is carried out by inputting a control signal (fixed input) under a state that an ignition switch is turned off, for instance, a load controller is found that is disclosed in JP-A-2001-148294. In this load controller, a load is PWM-controlled by a drive control signal whose frequency and duty ratio are set to a prescribed frequency and a prescribed duty ratio in accordance with a chopping wave generated by a chopping wave generating circuit. 
         [0006]    Further, in a load controller disclosed in JP-A-2004-248093, current cut-off switching elements are added to all control parts for carrying out a drive control of load driving switching elements. 
         [0007]    Further, in the load controller, when the dark current is zero during turning off an input, the accuracy of an operating threshold value voltage of an input part is deteriorated so that a malfunction is liable to occur due to noise or the like. Thus, as the input part of the load controller, such a structure as an input processing circuit disclosed in JP-A-2007-159059 is considered to be adopted. In this input processing circuit, a comparator having a hysteresis applied as input and output characteristics is arranged and a Schmitt circuit is connected to the input. The threshold value of the Schmitt circuit is set to be lower than that of the comparator to control the operation of the comparator by the output of the Schmitt circuit and remove a dark current in a stand-by state. Further, the hysteresis is applied to the comparator as the input and output characteristics to obtain a structure strong to noise. This structure is effective to a malfunction due to a leakage resistance component generated when an input switch is splashed with water or the like. 
         [0008]    In a device disclosed in JP-A-2001-345684, a circuit example is shown that can reduce a dark current even when a gate leakage current of a MOSFET for driving a load is increased. 
         [0009]    However, in the load controller disclosed in JP-A-2001-148294, even when the control input is turned off, an inner circuit always operates. Thus, a problem arises that a large amount of dark current is supplied. 
         [0010]    Further, in the load controller disclosed in JP-A-2004-248093, when the current cut-off switching elements are added to all the control parts in a circuit, a dark current can be suppressed substantially to zero, however, a problem arises that the scale of the circuit is enlarged to enlarge the device. 
         [0011]    Further, when the input processing circuit disclosed in JP-A-2007-159059 tries to be applied to the load controller, a problem arises that the dark current is suppressed only in the input part, but is supplied to other parts. Further, another problem arises that elements forming the circuit such as an inverter for enabling the Schmitt circuit or the comparator are increased to enlarge the scale of the circuit. 
         [0012]    Further, in the load controller disclosed in JP-A-2001-345684, a charge pump is necessary so that the scale of the circuit is enlarged. 
       SUMMARY OF THE INVENTION 
       [0013]    Thus, by considering the above-described problems, it is an object of the present invention to provide a load controller that can reduce a dark current when a load is driven and PWM-controlled by a drive instruction. 
         [0014]    To solve the above-described problems, there is provided a load controller including: an input circuit which detects that a drive instruction signal by an operation of a drive instructing unit is less or equal to a first input threshold value; a constant current source activated in accordance with a detection by the input circuit; a PWM signal generating unit that is activated by the constant current source and generates a PWM signal having a prescribed frequency and a duty ratio; a comparator that is activated by the constant current source and compares the drive instruction signal with a second input threshold value set to be lower than the first input threshold value; a logic calculation unit that carries out a logic calculation of the PWM signal supplied from the PWM signal generating unit with a compared result supplied from the comparator; a drive control unit that operates in accordance with an output from the logic calculation unit to generate a PWM drive control signal; and a load driving element that is driven by the PWM drive control signal supplied from the drive control unit so as to PWM-control a load. 
         [0015]    Preferably, the comparator is a comparator having a hysteresis so that when the drive instruction signal becomes less or equal to the second input threshold value, the second input threshold value becomes a level higher than a level at the time of activation, and when the drive instruction signal becomes more or equal to the second input threshold value at the high level, the second input threshold value returns to the level at the time of activation from the high level. 
         [0016]    According to the above configuration, in a load controller that operates in response to the drive instruction signal of a low level (Lo) to drive a load, the first input threshold value is set by the input circuit having no hysteresis to detect the drive instruction signal. The constant current source is activated in accordance with the detected result. The comparator and the PWM signal generating unit are further activated by the activated constant current source. The PWM drive control signal is generated in accordance with the compared result of the second input threshold value set to be lower than the first input threshold value with the drive instruction signal by the comparator and the PWM signal supplied from the PWM signal generating unit. The load driving element is driven by the generated PWM drive control signal to PWM-control the load. Accordingly, when a drive instruction is not carried out, a dark current can be suppressed substantially zero, a determining accuracy of an input voltage can be improved at the same time. Further, since these structures are simpler than those of a usual device, the device can be made to be more compact and a cost can be lowered. 
         [0017]    According to the above configuration, when the comparator having the hysteresis is used as the comparator, a highly accurate input determination that is strong to noise can be realized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein like reference numerals designate like or corresponding parts throughout the several views, and wherein: 
           [0019]      FIG. 1  is a circuit diagram showing a load controller according to an embodiment of the present invention; 
           [0020]      FIG. 2  is a timing chart of signals of respective parts in the load controller shown in  FIG. 1 ; 
           [0021]      FIG. 3  is a circuit diagram showing a specific circuit example 1 of an input circuit and a constant current source  1  shown in  FIG. 1 ; and 
           [0022]      FIG. 4  is a circuit diagram showing a specific circuit example 2 of the input circuit and the constant current source shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Now, an embodiment of the present invention will be described below by referring to the drawings. 
         [0024]      FIG. 1  is a circuit diagram showing an embodiment of a load controller according to the present invention. In this embodiment, a case will be described that a lamp (for instance, a head lamp, etc.) mounted on a vehicle as a load is PWM-controlled. 
         [0025]    In  FIG. 1 , the load controller includes transistors Q 1  to Q 4 , resistances R 1  to R 8 , a capacitor C 1 , a diode D 1 , an input switch S 1 , a constant current source  1 , a comparator  2 , an oscillator  3 , an AND gate  4 , a P channel MOSFET (refer it to as a PMOSFET, hereinafter)  5 , a lamp  6  mounted on a vehicle as a load and a battery  10 . 
         [0026]    As a switch element having no hysteresis, a pnp-typed transistor Q 1  is provided and the constant current source  1  is driven by the pnp-typed transistor Q 1 . The resistance R 1 , the resistance R 2  and the transistor Q 1  form an input circuit in which a first input threshold value Vth is set. The emitter of the transistor Q 1  and one side of the resistance R 1  are connected to the battery  10  of a source voltage VB (for instance, 12 V). The other side of the resistance R 1  is connected to the base of the transistor Q 1  and one side of the resistance R 2 . The other side of the resistance R 2  is connected to one side of the input switch S 1  as a drive instructing unit and an inverting input terminal of the comparator  2 . The other side of the input switch S 1  is grounded. The collector of the transistor Q 1  is connected to the constant current source  1 . In the device disclosed in JP-A-2007-159059, the first input threshold value has the hysteresis, however, a hysteresis is not provided in the present invention, which is different from the above-described device. 
         [0027]    Then, the comparator  2  and the oscillator  3  driven by the constant current source  1  and an npn-typed transistor Q 2  as a switch element for providing a hysteresis are provided. The base of the transistor Q 2  is connected to the constant current source  1 , a collector thereof is connected to one side of the resistance R 5  and an emitter thereof is grounded. The other side of the resistance R 5  is connected to a non-inverting input terminal of the comparator  2 , one side of the resistance R 4  and one side of the resistance R 3 . The other side of the resistance R 4  is connected to the battery  10 . The other side of the resistance R 3  is connected to an output terminal of the comparator  2 . 
         [0028]    The transistor Q 2  and the resistances R 3 , R 4  and R 5  form a hysteresis applying unit for dividing the source voltage VB of the battery  10  to set a second input threshold value Vref. The second input threshold value Vref is set to be lower than the first input threshold value Vth. 
         [0029]    The comparator  2  includes two fan outs. One of them is connected to the resistance R 3  to function for applying the hysteresis. The other of them is connected to one input terminal of the AND gate  4  as a logic calculation unit of a gate output part. 
         [0030]    The oscillator  3  serves as a PWM signal generating unit and an output thereof is connected to the other input terminal of the AND gate  4 . As the oscillator  3 , an oscillator the same as that disclosed in JP-A-2001-148294 may be used, however, any of oscillators that generate a PWM signal of a prescribed frequency and a duty ratio may be used without a special limitation. To the oscillator  3 , the capacitor C 1  for setting the prescribed frequency and the duty ratio is connected. 
         [0031]    The gate output part includes the AND gate  4  as the logic calculation unit, and the resistances R 6 , R 7 , npn-typed transistors Q 3  and Q 4  and the diode D 1  formed as a drive control unit. The AND gate  4  outputs the AND logic of the output of the comparator  2  and the output of the oscillator  3 . The base of the transistor Q 3  is connected to the output terminal of the AND gate  4 , a collector thereof is connected to one side of the resistance R 6  and one side of the resistance R 7 , and an emitter thereof is grounded. The other side of the resistance R 6  is connected to the battery  10 . The other side of the resistance R 7  is connected to the base of the transistor Q 4  and a cathode of the diode D 1 . An anode of the diode D 1  is connected to the emitter of the transistor Q 4 . The collector of the transistor Q 4  is connected to the battery  10 . 
         [0032]    An output part includes the resistance R 8  and the P channel MOSFET (refer it to as a PMOSFET, hereinafter)  5  as a load driving element. The gate of the PMOSFET  5  is connected to the emitter of the transistor Q 4  and the anode of the diode D 1  through the resistance R 8 . The source of the PMOSFET  5  is connected to the battery  10 , and a drain thereof is connected to one side of the lamp  6  mounted on a vehicle. The other side of the lamp  6  mounted on a vehicle is grounded. The cut-off voltage of the PMOSFET  5  is set to be sufficiently lower than a forward voltage drop Vbe 4  between the base and the emitter of the transistor Q 4  (since Vbe 4  is about −0.7 V, the cut-off voltage is set to a value lower than −0.7 V, for instance, −1.5 V). 
         [0033]    Now, an operation of the load controller shown in  FIG. 1  will be described below by referring to a timing chart of signals of respective parts shown in  FIG. 2 . 
         [0034]    When the input switch S 1  is turned off and an input voltage (a drive instruction signal) is the source voltage VB higher than the first input threshold value Vth of the input circuit, the transistor Q 1  is turned off, the constant current source  1  is stopped and a dark current is not supplied. Assuming that a forward voltage between the base and the emitter of the transistor Q 1  is Vbe, the first input threshold value Vth is given by a below-described equation (1). However, since Vbe changes owing to temperature and is affected by the current amplification factor of the transistor Q 1  in addition thereto, the unevenness of Vbe is large. 
         [0000]        Vth=VB−Vbe *( R 1+ R 2)/ R 1  (1) 
         [0035]    Then, when the input switch S 1  is turned on and an input is grounded so that the input voltage falls to be lower than the first input threshold value Vth, the transistor Q 1  is turned on and the constant current source  1  starts an operation to supply a constant current to parts respectively. Thus, the oscillator  3  is activated to start an oscillating operation. Further, the comparator  2  is also activated by the constant current source  1  and the transistor Q 2  is turned on to input the second input threshold value Vref to the non-inverting input terminal of the comparator  2 . The comparator carries out a comparing operation of the input. 
         [0036]    During the activation of the comparator  2  by turning on the input switch S 1 , while the input voltage is higher than the second input threshold value Vref, the output of the comparator  2  is in a low (Lo) level. Thus, even when the output of the oscillator  3  becomes a high (Hi) level, the output of AND gate  4  of the gate output part is low, so that the base of the transistor Q 3  remains low. 
         [0037]    When the base of the transistor Q 3  is low, the transistor Q 3  is turned off. Thus, a current is supplied to the base of the transistor Q 4  through the resistances R 6  and R 7  to activate the transistor Q 4 , so that the gate voltage VG of the PMOSFET  5  is raised to −0.7 V. Accordingly, the PMOSFET is turned off. At this time, when the PMOSFET  5  is normal and there is no leakage in the gate, the current supplied through the base of the transistor Q 4  from the resistances R 6  and R 7  is zero. Thus, a dark current is not generated in the output part. 
         [0038]    When the leakage is generated in the gate of the PMOSFET  5 , the transistor Q 4  is activated by the current supplied through the base of the transistor Q 4  from the resistances R 6  and R 7  to maintain the gate voltage VG of the PMOSFET  5  to about −0.7 V as a cut-off voltage or higher, so that the PMOSFET  5  is not electrically conducted and the dark current is not generated. 
         [0039]    Then, when the base of the transistor Q 3  is high, the collector voltage of the transistor Q 3  becomes substantially 0 V, so that the gate voltage VG of the PMOSFET  5  is lowered to the cut-off voltage or lower through the resistance R 8 , the diode D 1  and the resistance R 7 . Thus, the PMOSFET  5  is turned on. 
         [0040]    When the input switch S 1  is turned on so that the input voltage further falls from the first input threshold value to be lower than the second input threshold value Vref, the output of the comparator  2  becomes a high level. Accordingly, when a PWM signal of the output of the oscillator  3  is high, the output of the AND gate  4  is also high to turn on the transistor Q 3 . Thus, a PWM drive control signal (VG voltage of  FIG. 2 ) is supplied to the gate of the PMOSFET  5 , and accordingly, the PMOSFET  5  is controlled to be turned on/off. Therefore, as shown in  FIG. 2 , the output voltage of the PMOSFET  5  has a PWM wave form, so that the lamp  6  mounted on a vehicle is PWM controlled to be turned on. 
         [0041]    Assuming that a combined resistance of the resistance R 3  and the resistance R 5  is Rl, the second input threshold value Vref obtained when the output of the comparator  2  is low (that is, during the activation of the comparator  2 ) is Vref-l as a level during an activation expressed by a below-described equation (2). 
         [0000]        V ref- l=Vb*Rl ( R 4+ Rl )  (2) 
         [0042]    Assuming that a combined resistance of the resistance R 3  and the resistance R 4  is Rh, the second input threshold value Vref obtained when the output of the comparator  2  is high becomes a level Vref-h expressed by a below-described equation (3), which is higher than the level Vref-l during the activation. 
         [0000]        V ref- h=Vb*R 5/( Rh+R 5).  (3) 
         [0043]    Since both Vref-l and Vref-h are determined only by the ratio of the resistances, the unevenness of the input voltage can be reduced by such a contrivance as to use the resistances produced in the same production process and a determining accuracy of the input voltage can be improved. Further, while the lamp  6  mounted on a vehicle is turned on, since the second input threshold value Vref in the comparator  2  is Vref-h higher than the level Vref-l during the activation, the erroneous determination of the input voltage due to noise can be eliminated, the lamp  6  mounted on a vehicle that is turned on is not turned off owing to the influence of the noise so that a safety is improved. 
         [0044]    Then, when the input switch S 1  is operated to change a turning on state to a turning off state, the input voltage rises from a state that the input voltage is lower than Vref-h and exceeds the second input threshold value that is the high level Vref-h, the output of the comparator  2  is inverted from a high level to a low level to turn off the PMOSFET  5 . Further, the second input threshold value Vref returns to the level Vref-l during the activation from the high level Vref-h. 
         [0045]    Then, when the input voltage exceeds the high level Vref-h and further rises to be higher than the first input threshold value Vth, the transistor Q 1  is turned off, so that the constant current source  1  is stopped. Thus, the comparator  2  and the oscillator  3  are also stopped to turn off the lamp  6  mounted on a vehicle and suppress the dark current substantially to zero. 
         [0046]      FIG. 3  is a circuit diagram showing a specific circuit example 1 of the input circuit and the constant current source  1 . In  FIG. 3 , a current mirror is formed by a resistance R 9  and pnp-typed transistors Q 5  to Q 8 . 
         [0047]      FIG. 4  is a circuit diagram showing a specific circuit example 2 of the input circuit and the constant current source  1 . In  FIG. 4 , a current mirror is formed by resistances R 9  to R 11  and pnp-typed transistors Q 5  to Q 8 . The resistances R 10  and R 11  and the transistor Q 6  also serve as the input circuit. A first input threshold value Vth in this case is substantially given by a below-described equation (4). 
         [0000]        Vth=VB− 2* Vbe *( R 10+ R 11)/ R 10  (4) 
         [0048]    As described above, according to the present invention, in the device that operates by a low (Lo) input and outputs a PWM pulse by inputting a drive instruction signal (a fixed input), the first input threshold value is set by a combination of the input circuit having no hysteresis and the constant current source  1 . A hysteresis is applied to the second input threshold value by the comparator  2  driven by the constant current source  1  and the hysteresis applying unit to decide the input voltage. The second input threshold value is set to be lower than the first input threshold value. Accordingly, the dark current at the time of turning off an ignition switch can be suppressed substantially to zero and the determining accuracy of the input voltage can be improved at the same time. Further, since the structures thereof are simpler than those of a usual device, the device can be made to be more compact and a cost can be reduced. 
         [0049]    Further, to the load driving element, the PMOSFET  5  whose cut-off voltage is set to be lower than Vbe of the transistor is applied. In the gate drive and output part of the PMOSFET  5 , an emitter follower by the transistor Q 4 , as bias by the resistances R 6  and R 7  and a gate current absorbing function by the diode D 1  are provided. Accordingly, a simple structure can be realized in which a dark current during a normal time can be suppressed substantially to zero, and even when a leakage is generated in the gate of the PMOSFET  5 , the PMOSFET  5  can be prevented from being electrically conducted to increase the dark current. Thus, the device can be made to be compact and the cost can be lowered. 
         [0050]    As described above, the embodiment of the present invention is explained, however, the present invention is not limited thereto and various changes and applications can be made. 
         [0051]    For instance, in the above-described embodiment, the fan outs of the comparator  2  may be reduced to one, and the resistance R 3  may be deleted so that a structure having no hysteresis may be adopted. 
         [0052]    Further, in the above-described embodiment, a part of a circuit enclosed by a dotted line may be formed with an IC (Integrated Circuit).