Abstract:
A semiconductor device has a transistor that controls, according to the resistance of the load externally connected to the output terminal thereof, the current fed to the base of an output-stage transistor for driving the load to turn it on. In this circuit configuration, even if circuit constants are so determined as to permit the semiconductor device to drive the heaviest permissible load, only a reduced amount of extra current is fed to the base of the output-stage transistor when it is turned on with a light load connected to the semiconductor device.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a semiconductor device.  
           [0003]    2. Description of the Prior Art  
           [0004]    In a conventional semiconductor device, circuit constants are set in such a way that the semiconductor device can drive the heaviest load (in other words, the load having the lowest resistance) within the range of loads for which it is designed to drive. For example, as FIG. 3 shows, in the case of an output-stage transistor  3  for driving an external load  100 , the current I B  that is fed to the base of the transistor  3  to turn it on is set in such a way that the collector current I C  of that transistor  3  fulfills 
             I   C =( V   CC   −V   LOW )/ R   LMIN   
           [0005]    where V CC  represents the supply voltage, V LOW  represents the voltage desired at the output terminal OUT when the output-stage transistor  3  is turned on, and R LMIN  represents the minimum value of the resistance of the load that can be connected as the load  100 . Once set, this current I B  that is fed to the base of the output-stage transistor  3  to turn it on is kept fixed.  
           [0006]    Thus, in a conventional semiconductor device, the lighter the load (in other words, the higher the resistance of the load), the larger the amount of extra current that is fed to the base of the output-stage transistor when it is turned on, and thus the higher unnecessary current consumption. To reduce this unnecessary current consumption, it is necessary to narrow the range of resistance of loads for which the semiconductor device is designed to drive.  
           [0007]    Moreover, since the output-stage transistor is turned on with a fixed base current irrespective of the resistance of the load, the output voltage obtained when the output-stage transistor is turned on varies greatly according to the resistance of the load.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to provide a semiconductor device that operates with reduced unnecessary current consumption, and with reduced variation in the output voltage resulting from variation in the resistance of the load, but without narrowing the range of resistance of loads for which the semiconductor device is designed to drive.  
           [0009]    To achieve the above object, according to one aspect of the present invention, a semiconductor device is provided with a transistor that controls, according to the resistance of the load externally connected to the output terminal thereof, the current fed to the base of an output-stage transistor for driving the load to turn it on.  
           [0010]    In this circuit configuration, even if circuit constants are so determined as to permit the semiconductor device to drive the heaviest permissible load, only a reduced amount of extra current is fed to the base of the output-stage transistor when it is turned on with a light load connected to the semiconductor device. In this way, it is possible to reduce unnecessary current consumption, without narrowing the range of resistance of loads for which the semiconductor device is designed to drive. Moreover, the lighter the load, the lower the current that is fed to the base of the output-stage transistor to turn it on. This helps reduce variation in the output voltage resulting from variation in the resistance of the load.  
           [0011]    According to another aspect of the present invention, a semiconductor device is provided with: a constant-current source; a current amplifier circuit that is connected to the output side of the constant-current source; a first transistor that receives at the base thereof the current output from the current amplifier circuit, has the emitter thereof connected to a reference potential, and has the collector thereof connected to an output terminal; and a second transistor, of the opposite conductivity type to the first transistor, that has the base thereof connected to the output terminal, has the emitter thereof connected to the output side of the constant-current source, and has the collector thereof connected to the reference potential.  
           [0012]    In this circuit configuration, the second transistor operates in such a way that, the lighter the load, the lower the input current to the current amplifier circuit. As a result, only a reduced amount of extra current is fed to the base of the first transistor when it is turned on. In this way, it is possible to reduce unnecessary current consumption without narrowing the range of resistance of loads for which the semiconductor device is designed to drive. Moreover, the lighter the load, the lower the current that is fed to the base of the output-stage transistor to turn it on. This helps reduce variation in the output voltage resulting from variation in the resistance of the load.  
           [0013]    In this semiconductor device, it is also possible to additionally provide a third transistor that has the base thereof connected to the output side of the constant-current source, has the emitter thereof connected to the reference potential, and has the collector thereof connected to the output terminal, with the second transistor parasitic on the third transistor. This helps increase the withstand voltage against electrostatic destruction. Alternatively, it is also possible to additionally provide a supply voltage detection circuit that chooses whether to short-circuit the output side of the constant-current source to the reference potential or not according to the supply voltage. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    This and other objects and features of the present invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanying drawings in which:  
         [0015]    [0015]FIG. 1 is a circuit diagram of a semiconductor device embodying the invention;  
         [0016]    [0016]FIG. 2 is a sectional view of the transistor provided in a semiconductor device embodying the invention; and  
         [0017]    [0017]FIG. 3 is a diagram illustrating an example of prior art.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a circuit diagram of a semiconductor device embodying the invention. In this figure, reference numeral  1  represents a supply voltage detection circuit, reference numeral  2  represents a current amplifier circuit, reference numerals  3  and  4  represent NPN-type transistors, reference numerals  11  and  12  represent resistors, reference numeral  13  represents a comparator, reference numeral  14  represents a constant-voltage source, reference numeral  15  represents an NPN-type transistor, reference numeral  16  represents a constant-current source, reference numeral  21  represents an NPN-type transistor, reference numerals  22  and  23  represent PNP-type transistors, reference numeral  24  represents a resistor, and reference numeral  100  represents an externally connected load.  
         [0019]    The supply voltage detection circuit  1  is composed of the resistors  11  and  12 , comparator  13 , constant-voltage source  14 , transistor  15 , and constant-current source  16 . The resistors  11  and  12  are connected in series between a supply voltage V CC  and ground GND. The comparator  13  has its non-inverting input terminal (+) connected to the node between the resistors  11  and  12 , and receives at its inverting input terminal (−) a constant voltage V ref  generated by the constant-voltage source  14 . The transistor  15  has its base connected to the output terminal of the comparator  13 , has its emitter connected to ground GND, and has its collector connected to the constant-current source  16 .  
         [0020]    The current amplifier circuit  2  is composed of the transistors  21 ,  22 , and  23 , and resistor  24 . The transistor  21  has its base connected to the collector of the transistor  15 , has its emitter connected to ground GND, and has its collector connected to the collector of the transistor  22 . The transistor  22  has its base and collector connected together, and has its emitter connected to the supply voltage V CC . The transistor  23  has its base connected to the base of the transistor  22 , has its emitter connected to the supply voltage V CC , and has its collector connected through the resistor  24  to ground GND.  
         [0021]    The transistor  3  has its base connected to the node between the collector of the transistor  23  and the resistor  24 , has its emitter connected to ground GND, and has its collector connected to an output terminal OUT. Between the supply voltage V CC  and the output terminal OUT, the load  100  is connected externally.  
         [0022]    The transistor  4  has its base connected to the collector of the transistor  15 , has its emitter connected to ground GND, and has its collector connected to the output terminal OUT. FIG. 2 shows a sectional view of the transistor  4 . As this figure shows, the transistor  4  is so structured that a PNP-type transistor  5  is parasitic on it, with the base region  43 , collector region  42 , and substrate (P-type semiconductor substrate)  41  of the transistor  4  acting respectively as the emitter, base, and collector of the parasitic transistor  5 . Another PNP-type transistor is parasitic on the transistor  3  also, but the effect of this parasitic transistor is minimized by reducing its current amplification factor (β) through appropriate designing of the layout or by another means.  
         [0023]    In the circuit configuration described above, when the supply voltage V CC  becomes higher than a predetermined level, the comparator  13  outputs a high-level voltage, and thereby turns the transistor  15  on. As a result, all the constant current I ref  output from the constant-current source  16  flows into the collector of the transistor  15 , and no current is fed to the bases of the transistors  21  and  4 . Accordingly, the transistors  3  and  4  are turned off, and thus no current flows through the load  100 . This turns the voltage at the output terminal OUT (hereinafter referred to as the “output voltage”) to a high level (the level of the supply voltage).  
         [0024]    By contrast, when the supply voltage V CC  becomes lower than the predetermined level, the comparator  13  outputs a low-level voltage, and thereby turns the transistor  15  off. As a result, the constant current I ref  output from the constant-current source  16  flows through the transistors  21  and  4  also. Accordingly, the transistors  3  and  4  are turned on to operate in a saturated region, and thus the output voltage becomes equal to a low level (nearly equal to the ground level). That is, the circuit shown in FIG. 1 functions as a reset circuit, or supply voltage detection circuit, that outputs a signal according to the supply voltage.  
         [0025]    Here, let the base current of the transistor  21  be I B21 , the current amplification factor of the transistor  21  be h FE21 , the ratio of the emitter area of the transistor  23  to that of the transistor  22  be n, the base-emitter voltage of the transistor  3  be V BE3 , and the resistance of the resistor  24  be R, then the base current I B3  of the transistor  3  is given by 
         
       I 
       B3 
       =n·h 
       FE21 
       ·I 
       B21 
       −V 
       BE3 
       /R 
     
         [0026]    Moreover, in this embodiment, circuit constants are set in such a way that the desired output voltage is obtained when the transistors  3  and  4  are turned on with the heaviest permissible load connected as the load  100 .  
         [0027]    In the circuit configuration described above, the lighter the load  100 , the lower the current that flows through the transistor  3  when the transistors  3  and  4  are turned on, and thus the lower the output voltage. As the output voltage decreases, the emitter-base voltage of the transistor  5  increases, and accordingly the current that flows from the constant-current source  16  into the emitter of the transistor  5  increases.  
         [0028]    In this way, the transistor  5  operates in such a way that, the lighter the load  100 , the lower the input current to the current amplifier circuit  2  (i.e. the base current of the transistor  21 ). This helps reduce the amount of extra current that is fed to the base of the transistor  3  when it is turned on, and thereby reduce unnecessary current consumption.  
         [0029]    Moreover, the lighter the load  100 , the higher the current that flows from the constant-current source  16  into the emitter of the transistor  5 , and thus the lower the input current to the current amplifier circuit  2 . This helps reduce current consumption.  
         [0030]    Moreover, the lighter the load  100 , the lower the currents that need to be fed to the bases of the transistors  3  and  4  to turn them on. This helps reduce variation in the output voltage obtained when the transistors  3  and  4  are turned on in the face of variation in the resistance of the load  100 .  
         [0031]    In this embodiment, the use, as the transistor  5 , of the transistor parasitic on the transistor  4  helps increase the withstand voltage against electrostatic destruction. It is also possible to omit the supply voltage detection circuit  1  so that the output current is controlled according to the resistance of the load alone, irrespective of the supply voltage.