Abstract:
In an output short-circuiting protecting circuit, a current flows in advance in a current sense resistor that is connected to a current source circuit and monitors the current. When a desired short-circuiting current flows, a voltage for operating the protecting circuit is developed in the current sense resistor, and the current is adjusted to an arbitrary short-circuiting current.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a voltage control circuit in a monolithic integrated circuit form, in particular, to an output voltage protecting circuit.  
           [0003]    2. Description of the Related Art  
           [0004]    There has been known an output short-circuiting protecting circuit for a voltage control circuit disclosed in Japanese Patent Examined Publication No. Hei 7-74976. A circuit diagram of the output short-circuiting protecting circuit for the conventional voltage control circuit is shown in FIG. 2. A voltage Vin inputted from an input terminal  201  is outputted to an output terminal  203  through a control MOS transistor  202 . The output terminal  203  is connected with resistors  204  and  205 , and a voltage at a node between the resistors  204  and  205  is inputted to a positive input terminal side of an amplifier  206 . On the other hand, a negative input terminal side of the amplifier  206  is inputted with a reference voltage Vref from the power supply  207 . Also, an output terminal of the amplifier  206  is connected to a gate of the control transistor  202 .  
           [0005]    A circuit in which a transistor  213  for monitoring a current and a resistor  208  are connected in series is inserted in parallel with the control transistor  202 , and a gate voltage of the transistor  209  is supplied from a node of the transistor  213  and the resistor  208 . A resistor  210  is inserted between the transistor  209  and the input terminal  201  to constitute an inverter circuit. The output voltage of the node  212  of the inverter circuit is inputted to a transistor  211  inserted between the gate and source of the control transistor  202 . Also, the gate voltage of the transistor  213  is supplied from an amplifier  206  as in the control transistor  202 .  
           [0006]    An output current that can be extracted from the output terminal  203  by employing the above-described circuit structure and an output voltage characteristic at that time exhibit the characteristic shown in FIG. 5. In this example, Is is an output retaining current, Im is a maximum current.  
           [0007]    However, the conventional output short-circuiting protecting circuit suffers from such a defect that it is difficult to adjust the output retaining current Is to an arbitrary value. This is because a resistance, a threshold value of the transistor, etc., vary from values estimated at the time of designing due to a variation of the manufacture process, a variation of the substrate density, a variation of characteristics of an element on a substrate, etc.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to eliminate the drawbacks with the conventional device.  
           [0009]    In the present invention, a current source circuit is newly added to allow a current to flow in advance in a current sense resistor that monitors the current. Then, when a desired short-circuiting current flows in the current sense resistor, a voltage for operating the protecting circuit develops in the current sense resistor to adjust the short-circuiting current to an arbitrary short-circuiting current.  
           [0010]    The present invention uses a circuit in which a resistor is disposed in a current path, and a voltage drop developed by the resistor is detected to conduct a current limit. In this example, a back gate of an MOS transistor is used.  
           [0011]    Further, there is used a voltage control circuit comprised of: a current monitor circuit connected in series to a transistor and a resistor; an output voltage control circuit that connects the current monitor circuit to an input terminal and an output terminal in parallel; and a current source circuit connected to the resistor. Still further, there is used a voltage control circuit comprised of: a current monitor circuit connected in series to a transistor and a resistor; an output voltage control circuit that connects the current monitor circuit to an input terminal and an output terminal in parallel; an output short-circuiting protecting circuit of the output voltage control circuit; and a current source circuit connected to the resistor, in which the current source circuit applies a voltage for operating the output short-circuiting protecting circuit to the resistor.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    These and other objects and advantages of this invention will become more fully apparent from the following detailed description taken with the accompanying drawings in which:  
         [0013]    [0013]FIG. 1 is a circuit diagram showing a first embodiment of the present invention;  
         [0014]    [0014]FIG. 2 is a conventional voltage control circuit;  
         [0015]    [0015]FIG. 3 is a circuit block diagram showing a measuring device in accordance with a second embodiment of the present invention;  
         [0016]    [0016]FIG. 4 is a circuit block diagram showing a measuring device in accordance with a third embodiment of the present invention; and  
         [0017]    [0017]FIG. 5 is an output voltage characteristic of a conventional voltage control circuit. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    Now, a description will be given of preferred embodiments of the present invention with reference to the accompanying drawings.  
         [0019]    (First Embodiment)  
         [0020]    [0020]FIG. 1 is a circuit diagram showing a first embodiment of the present invention. The same parts as those in FIG. 2 will be omitted from their description. A current source  101  is connected to a node of a transistor  213  and a resistor  208 . The current source  101  has a function of adjusting a current value by a design constant, a fuse trimming, a laser trimming or other methods.  
         [0021]    When the output terminal  3  is short-circuited to a ground potential, a retaining current Is flows. The Is can be obtained by the following expression. 
           Is=N× ( V   TN   /R   1 − I   A )  (1) 
         [0022]    where V TN  is a threshold voltage of the transistor  209 , R 1  is a resistance of the resistor  208 , N is a current mirror ratio of the transistor  202  and the transistor  213 , and I A  is a current that flows in a node of the transistor  213  and the resistor  208  from the current source  101  or flows out of the node.  
         [0023]    As is apparent from the expression (1), Is can be set to an arbitrary value by adjusting I A . For example, in the case where Is is set to 30 mA, assuming that V TN =0.5 V, R 1 =500 Ω, and the mirror ratio of the transistor  202  and the transistor  213  is 100, 
         30×0.001=100×(0.5/500− I   A )  (2) 
         [0024]    From the expression (2), 
           I   A =0.0007  A =0.7 mA 
         [0025]    Therefore, when a current of 0.7 mA flows from the current source  101 , the retaining current Is can be adjusted to 30 mA.  
         [0026]    (Second Embodiment)  
         [0027]    [0027]FIG. 3 is a circuit diagram showing a second embodiment of the present invention. Duplex portions of FIG. 1 will be omitted from description. A gate and source of a depletion type transistor  301  are grounded. A transistor  302  has a source and a bulk connected to an input terminal  201  and a gate and a drain connected to the transistor  301 . A transistor  303  is connected to a node of the input terminal  201 , a transistor  213  and a resistor  208 . When a voltage is inputted to the input terminal and the drain voltage of the transistor  301  becomes a voltage of a threshold value or more, the transistor  301  functions as a constant current circuit. Since the transistor  302  and the transistor  301  have a common path along which the current flows, the equal current flows in the transistors  302  and  301 . Because the transistor  302  and the transistor  303  have a common gate, a current I A  proportional to a current that flows in the transistor  301  flows in the transistors  302  and  303 . The proportion constant of the current is determined by the respective sizes of the transistors  302  and  303 . Assuming that the channel lengths of the transistors  302  and  303  are L 1  and L 2 , respectively, the channel widths thereof are W 1  and W 2 , respectively, and a current that flows in the depletion type transistor  301  is Idep, I A  is represented by the following expression. 
           I   A =( W   2 / L   2 )/( W   1 / L   2 )× Idep   
         [0028]    Therefore, I A  can be adjusted by appropriately setting the sizes of the transistors  302  and  303 . As described in the above first embodiment, the retaining current Is can be set to an arbitrary value by adjusting I A , and it is apparent that Is can be adjusted to an arbitrary value in the circuit shown in FIG. 3.  
         [0029]    (Third Embodiment)  
         [0030]    [0030]FIG. 4 is a circuit diagram showing a third embodiment of the present invention. The duplex portions of FIGS.  1  to  3  will be omitted from description. A depletion type transistor  404  has a gate and a source connected to an output terminal  203 , and a bulk grounded. A transistor  402  has a source and a bulk connected to an input terminal  201  and a gate and a drain connected to a transistor  404 . A transistor  403  is connected to a node of the input terminal  201 , a transistor  213  and a resistor  208 . When a voltage is inputted to the input terminal and the drain voltage of the transistor  404  becomes a voltage of a threshold value or more, the transistor  404  functions as a constant current circuit. Since the transistor  402  and the transistor  404  have a common path along which the current flows, the equal current flows in the transistors  402  and  404 . Because the transistor  402  and the transistor  403  have a common gate, a current I A  proportional to a current that flows in the transistor  404  flows in the transistors  402  and  403 . The proportion constant of the current is determined by there spective sizes of the transistors  402  and  403 . Assuming that the channel lengths of the transistors  402  and  403  are L 1  and L 2 , respectively, the channel widths thereof are W 1  and W 2 , respectively, and a current that flows in the depletion type transistor  404  is Idep, I A  is represented by the following expression. 
           I   A =( W   2 / L   2 )/( W   1 / L   2 )× Idep   
         [0031]    Therefore, I A  can be adjusted by appropriately setting the sizes of the transistors  402  and  403 . As described in the above first embodiment, the retaining current Is can be set to an arbitrary value by adjusting I A,  and it is apparent that Is can be adjusted to an arbitrary value in the circuit shown in FIG. 3.  
         [0032]    As was described above, the measuring circuit according to the present invention has the following advantage. Since a current source is added to a conventional power source protecting circuit, and a current value from the current source is set to an appropriate value, a retaining current Is can be set to an arbitrary value.  
         [0033]    The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.