Abstract:
Provided is a magnetic sensor capable of identifying an individual having a high offset voltage, which is a cause of an initial defect in magnetic characteristics, and changes over time. A cross-transmission switch is provided in a changeover switch circuit which transmits to an amplifier circuit a differential signal voltage output from a Hall element, and the cross-transmission switch cross-transmits a signal in any one of a first period and a second period during which a current flowing into the Hall element is switched by a control signal from a control circuit, to thereby determine and identify a magnitude of an offset voltage.

Description:
RELATED APPLICATIONS 
     This application is a continuation of PCT/JP2013/060882 filed on Apr. 11, 2013, which claims priority to Japanese Application No. 2012-130161 filed on Jun. 7, 2012. The entire contents of these applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a magnetic sensor using a Hall element on a semiconductor substrate, and more particularly, to a characteristic abnormality detection circuit for the magnetic sensor. 
     2. Description of the Related Art 
     A magnetic sensor is widely used for detecting a movement or a rotation of various movable objects. For example, the magnetic sensor is used to detect opening and closing of a mobile phone, or to detect the rpm of a motor. In recent years, there is used a Hall element formed on a Si substrate, which is inexpensive but has low sensitivity and a high offset voltage. Then, offset voltages of the Hall element and an amplifier are cancelled by signal processing, to thereby realize a magnetic sensor which is inexpensive and has high magnetic detection precision. 
       FIG. 4  is a block diagram of a conventional magnetic sensor. A conventional magnetic sensor  10  includes a Hall element  2 , a changeover switch circuit  30 , an amplifier  4 , a sampling circuit  5 , a reference voltage circuit  60 , a comparator  7 , and an output circuit  8 . The conventional magnetic sensor  10  operates as follows in order to cancel the offset voltage. The changeover switch circuit  30  switches current paths between respective two terminals on a diagonal line of the Hall element  2  in a complementary manner between a first period and a second period. The output voltage is amplified by the amplifier  4 , and is held in a time division manner and averaged by the sampling circuit  5 . The reference voltage circuit  60  outputs a reference voltage Vref. The comparator  7  compares the voltage held in the sampling circuit  5  and the reference voltage Vref, and determines the intensity of the magnetic field. The magnetic sensor  10  outputs the determination result via the output circuit  8 , thereby outputting a detection signal corresponding to the magnetic field. The conventional magnetic sensor  10  operates as described above, and cancels the offset voltages of the Hall element and the amplifier by signal processing. 
     However, when Si is used as a material for the Hall element  2 , the offset voltage is higher by several digits than a magnetic field signal voltage. There has been a problem in that, when the offset voltage is high, the offset voltage is not cancelled mainly due to saturation of the output voltage of the amplifier  4 , and magnetic characteristics significantly deteriorate. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in order to solve the above-mentioned problem, and provides an inexpensive high-precision magnetic sensor with magnetic characteristics. 
     In order to solve the conventional problem, a magnetic sensor according to an exemplary embodiment of the present invention has the following configuration. 
     The magnetic sensor includes a changeover switch circuit which causes a current to flow into a first terminal pair of a Hall element and inputs a first differential signal voltage from a second terminal pair thereof during a first period, and which causes a current to flow into the second terminal pair and inputs a second differential signal voltage from the first terminal pair during a second period. The changeover switch circuit includes series-transmission switches and cross-transmission switches, the series-transmission switches and the cross-transmission switches being provided in series between terminals of the Hall element and output terminals of the changeover switch circuit. The cross-transmission switch is controlled to determine a magnitude of an offset voltage of the Hall element. 
     According to the magnetic sensor of the present invention, only by adding a simple circuit, the voltage based on the offset voltages of the Hall element and an amplifier circuit is evaluated. Therefore, an individual having a high offset voltage is identified by binary decision in an inspection process, and hence there is an effect of improving quality of a product. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a block diagram of a magnetic sensor according to an embodiment of the present invention; 
         FIG. 2  is a detailed circuit diagram of a changeover switch circuit of the magnetic sensor according to the embodiment of the present invention; 
         FIG. 3  is a circuit diagram illustrating one example of a control circuit of the magnetic sensor according to the embodiment of the present invention; and 
         FIG. 4  is a block diagram of a conventional magnetic sensor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following, an embodiment of the present invention is described with reference to the attached drawings. 
       FIG. 1  is a block diagram of a magnetic sensor according to the embodiment of the present invention. 
     A magnetic sensor  1  according to the embodiment of the present invention includes a Hall element  2 , a changeover switch circuit  3 , an amplifier  4 , a sampling circuit  5 , a reference voltage circuit  6 , a comparator  7 , an output circuit  8 , and a control circuit  9 . 
     Via the changeover switch circuit  3 , a current is supplied from a power source terminal to one terminal pair (for example, terminal  21  and terminal  22 ) on a diagonal line of the Hall element  2 , and a differential signal voltage is output from both ends of the other terminal pair thereof (for example, terminal  23  and terminal  24 ). A differential signal voltage of the Hall element  2  is input to the amplifier  4  via the changeover switch circuit  3 . The changeover switch circuit  3  is controlled by control signals  3 A to  3 D output from the control circuit  9 , and switches between a first period and a second period of control of the Hall element  2 . The amplifier  4  amplifies the differential signal voltage of the Hall element  2 , and outputs a differential signal amplified voltage. The reference voltage circuit  6  outputs a reference voltage in accordance with a control signal  6 A of the control circuit  9 . The sampling circuit  5  holds differential signal amplified voltages of the first period and the second period, and outputs an average voltage thereof. The comparator  7  compares a magnitude relation between the average voltage and the reference voltage, and outputs a logic signal. The control circuit  9  outputs respective control signals to the changeover switch circuit  3 , the amplifier  4 , and the reference voltage circuit  6 . The output circuit  8  performs a latch operation and a logical operation based on the logic signal, and outputs a detection signal as a detection signal of the magnetic sensor. 
       FIG. 2  is a detailed circuit diagram of the changeover switch circuit  3  of the magnetic sensor according to the embodiment of the present invention. 
     The changeover switch circuit  3  includes first to fourth current supply switches  311  to  314 , first to fourth series-transmission switches  321  to  324 , and first to fourth cross-transmission switches  331  to  334 . The control signals  3 A to  3 D are output from the control circuit  9 . Terminals  301  and  302  are first and second differential signal voltage output terminals. In this case, a period during which the control signals  3 A and  3 B applied to the first to fourth current supply switches  311  to  314  are respectively “L” and “H”, that is, a period during which a current is supplied to the terminal  21  and the terminal  22  of the Hall element  2 , is set as the first period, and a period during which the control signals  3 A and  3 B are respectively “H” and “L”, that is, a period during which a current is supplied to the terminal  23  and the terminal  24  of the Hall element  2 , is set as the second period. The first to fourth series-transmission switches  321  to  324  are switched in association with the first to fourth current supply switches  311  to  314 . The first to fourth cross-transmission switches  331  to  334  are controlled by the control signals  3 C and  3 D. 
       FIG. 3  is a circuit diagram illustrating one example of the control circuit of the magnetic sensor according to the embodiment of the present invention. The control circuit  9  includes an operating state setting circuit  91 , a NAND circuit  92 , and OR circuits  93 ,  94 , and  95 . A reference CLK signal is input to the control circuit  9 , and the control circuit  9  generates and outputs the control signals  3 A,  3 B,  3 C, and  3 D, and the control signal  6 A. The operating state setting circuit  91  outputs a signal of “H” or “L”, and includes a fuse and a pull-down resistor which are connected in series. For example, the operating state setting circuit  91  may include a non-volatile memory, or may include an external terminal and input a state setting signal from the exterior. Further, as long as a desired signal is generated, other logic circuits are not limited to the above-mentioned circuit configuration. 
     The above-mentioned magnetic sensor  1  operates as follows, and has a function of determining the magnitude of an offset voltage, and a function of determining the intensity of the magnetic field. 
     The magnetic sensor  1  according to the embodiment of the present invention has a first operating state and a second operating state depending on the control signals output from the control circuit  9 . The operating state setting circuit  91  of the control circuit  9  outputs a signal of “H” in the initial state due to the fuse. Therefore, the NAND circuit  92  inverts the reference CLK signal and outputs the inverted signal. In other words, the control signal  3 C is a signal obtained by inverting the reference CLK signal, and the control signal  3 D is the same signal as the reference CLK signal. This state is the first operating state. The operating state setting circuit  91  of the control circuit  9  outputs a signal of “L” due to the pull-down resistor when the fuse is blown. Therefore, output of the NAND circuit  92  is fixed to “H”. In other words, the control signals  3 C and  3 D are respectively fixed to “H” and “L”. This state is the second operating state. In both operating states, the control signal  3 A is the signal obtained by inverting the reference CLK signal, and the control signal  3 B is the same signal as the reference CLK signal. 
     Description is given first of the first operating state. In the first operating state, the magnitude of the offset voltage of the magnetic sensor  1  is determined. 
     In the first operating state, the control circuit  9  outputs the control signals  3 A and  3 C of the signals obtained by inverting the reference CLK signal and the control signals  3 B and  3 D of the same signals as the reference CLK signal. During the first period, the first and fourth current supply switches  311  and  314  are electrically conductive, a current is supplied to the terminal pair of the terminal  21  and the terminal  22  of the Hall element  2 , and the differential signal voltage is output from the terminal pair of the terminal  23  and the terminal  24  thereof. During the second period, the second and third current supply switches  312  and  313  are electrically conductive, a current is supplied to the terminal pair of the terminal  23  and the terminal  24 , and the differential signal voltage is output from the terminal pair of the terminal  21  and the terminal  22 . During the first period, the second cross-transmission switch  332  and the third cross-transmission switch  333  are electrically conductive, and, during the second period, the first cross-transmission switch  331  and the fourth cross-transmission switch  334  are electrically conductive. 
     Therefore, during the first period, the first differential signal voltage output terminal  301  is connected to the terminal  24  of the Hall element  2 , and the second differential signal voltage output terminal  302  is connected to the terminal  23  of the Hall element  2 . During the second period, the first differential signal voltage output terminal  301  is connected to the terminal  21  of the Hall element  2 , and the second differential signal voltage output terminal  302  is connected to the terminal  22  of the Hall element  2 . The output voltages of the first and second differential signal voltage output terminals  301  and  302  are amplified by the amplifier  4 , and the amplified voltage is output. 
     Accordingly, when a differential signal amplified voltage output from the amplifier  4  during the first period is set as Vo1Φ1 and a differential signal amplified voltage output from the amplifier  4  during the second period is set as Vo1Φ2, Vo1Φ1 and Vo1Φ2 are respectively obtained from Equations 1 and 2.
 
 Vo 1Φ1= VDD/ 2+ G ( KH×Bin+Vos )  (1)
 
 Vo 1Φ2= VDD/ 2+ G (− KH×Bin+Vos )  (2)
 
     where VDD represents a power source voltage, G represents a gain of the amplifier  4 , KH represents a magnetoelectric conversion coefficient of the Hall element  2 , Bin represents a magnetic flux density, and Vos represents an offset voltage included in the output voltage of the amplifier  4 . 
     Therefore, an average voltage Vo1, which is obtained by averaging the differential signal amplified voltages Vo1Φ1 and Vo1Φ2 by the sampling circuit  5 , is obtained from Equation 3.
 
 Vo 1= VDD/ 2+ G×Vos   (3)
 
     At that time, in the reference voltage circuit  6 , the reference voltage to be output is switched by the control signal  6 A to a reference voltage Vref2 for determining the offset voltage. Accordingly, the comparator  7  compares the average voltage Vo1 and the reference voltage Vref2 to determine the magnitude of the offset voltage Vos, and the output circuit  8  outputs the result of the determination. 
     In this way, in the first operating state, the magnitude of the offset voltage Vos of the magnetic sensor  1  is determined. Therefore, in the inspection process, an individual having a high offset voltage Vos, which is a defect in the circuit of the magnetic sensor  1 , is sorted out without increasing the circuit scale or using an additional inspection tool. 
     The amplifier  4  may be switched by the control signal of the control circuit  9  to have a gain for determining the offset voltage. By appropriately adjusting the reference voltage of the reference voltage circuit  6  and the gain of the amplifier  4 , the detection operation of the offset voltage may be optimized. 
     Description is given next of the second operating state. The second operating state is a normal operating state, in which the intensity of magnetism is determined. 
     In the second operating state, the control signals  3 C and  3 D are respectively fixed to “H” and “L”, and hence the first cross-transmission switch  331  and the fourth cross-transmission switch  334  are fixed to be electrically conductive. During the first period, the first and fourth current supply switches  311  and  314  are electrically conductive, a current is supplied to the terminal pair of the terminal  21  and the terminal  22  of the Hall element  2 , and the differential signal voltage is output from the terminal pair of the terminal  23  and the terminal  24  thereof During the second period, the second and third current supply switches  312  and  313  are electrically conductive, a current is supplied to the terminal pair of the terminal  23  and the terminal  24 , and the differential signal voltage is output from the terminal pair of the terminal  21  and the terminal  22 . Therefore, during the first period, the first differential signal voltage output terminal  301  is connected to the terminal  23  of the Hall element  2 , and the second differential signal voltage output terminal  302  is connected to the terminal  24  of the Hall element  2 . During the second period, the first differential signal voltage output terminal  301  is connected to the terminal  21  of the Hall element  2 , and the second differential signal voltage output terminal  302  is connected to the terminal  22  of the Hall element  2 . The output voltages of the first and second differential signal voltage output terminals  301  and  302  are amplified by the amplifier  4 , and the amplified voltage is output. 
     When a differential signal amplified voltage output from the amplifier  4  during the first period is set as Vo2Φ1 and a differential signal amplified voltage output from the amplifier  4  during the second period is set as Vo2Φ2, Vo2Φ1 and Vo2Φ2 are respectively obtained by Equations 4 and 5.
 
 Vo 2Φ1= VDD/ 2+ G ×( KH×Bin+Vos )  (4)
 
 Vo 2Φ2= VDD/ 2+ G ×( KH×Bin−Vos )  (5)
 
     Therefore, an average voltage Vo2, which is obtained by averaging the differential signal amplified voltages Vo2Φ1 and Vo2Φ2 by the sampling circuit  5 , is obtained from Equation 6.
 
 Vo 2= VDD/ 2+ G×KH×Bin   (6)
 
     From Equation 6, it is understood that the offset voltage Vos is cancelled in the average voltage Vo2. The comparator  7  compares the average voltage Vo2 obtained from Equation 6 and the reference voltage Vref, and determines the intensity of the magnetic field, thereby outputting the detection signal corresponding to the magnetic field. At that time, the reference voltage circuit  6  is switched to output the normal reference voltage Vref. 
     As described above, in the first operating state, the magnetic sensor according to the embodiment of the present invention evaluates the voltage based on the offset voltages of the Hall element and an amplifier circuit. Therefore, there is identified by binary decision an individual having a high offset voltage liable to generate a magnetic characteristics defect, which is to be an initial defective product. In other words, an individual having a high offset voltage is removed in the inspection process, and hence a high-precision magnetic sensor is provided. 
     In the case of a configuration in which the control circuit  9  inputs a control signal from the exterior, the control signal is input from the exterior, to thereby inspect a product even after product shipment. Therefore, an individual having a higher offset voltage due to change over time is identified.