JP2008203202A - Sensor threshold circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sensor threshold circuit capable of providing a hysteresis width independent of variation of threshold point. <P>SOLUTION: By generating bias current IB from threshold current IO and hysteresis current IH, the hysteresis width ¾BH¾ is provided using a resistance ratio A and hence determining A keeps the hysteresis width ¾BH¾ constant regardless of a sensor current detecting resistor RS1. When the constant A is determined, the hysteresis width ¾BH¾ is determined as single value, and dispersion, temperature variation, and variation with time are not caused. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a sensor threshold circuit applicable to various sensors, and more particularly to a sensor threshold circuit that determines a threshold for digital output of a sensor output by a product of sensor output impedance and bias current.

  FIG. 5 shows a conventional sensor threshold circuit. This sensor threshold circuit includes a four-terminal sensor 10, a voltage comparator 20, a sensor drive current detection circuit 30, a sensor bias current generation circuit 55, and a bias current switching circuit 380, and is applied from the outside. For example, a sensor output VSO (= VP−VN) is obtained for a sensor input BIN such as a magnetic field. Here, by switching the resistors RO and RR by the bias current switching circuit 380 and changing the bias current IB, a digital output VO having hysteresis characteristics as shown in FIG. 6 can be obtained with respect to the sensor input BIN. A circuit that can be used (see, for example, Patent Document 1).

  FIG. 6 is a diagram showing the relationship between the sensor input BIN and the output voltage VO of the sensor threshold circuit having hysteresis characteristics. When the sensor input BIN is increased, the output voltage VO decreases from VOH to VOL at the threshold BOP. On the other hand, when the sensor input BIN is decreased, the output voltage VO is increased from VOL to VOH at a threshold BRP smaller than the threshold BOP, and a digital output VO having a hysteresis characteristic with a hysteresis width | BH | is obtained.

Next, the operation of the conventional sensor threshold circuit will be described with reference to FIG.
First, the threshold point when the switch SWO of the bias current switching circuit 380 in FIG. 5 is conductive and the switch SWR is open will be described with reference to FIG. FIG. 7 shows the four-terminal sensor 10, the voltage comparator 20, the bias current with the switch SWO of the bias current switching circuit 95 of FIG. 5 turned on and the switch SWR opened for the sake of simplicity. It is a figure which shows the circuit structure which took out IBO.

First, for ease of analysis, the resistance value of the resistor RS that detects the sensor driving current is considered to be very small compared to the resistance values of the sensor resistors R1, R2, R3, and R4, and thereby the driving terminal voltage of the sensor. Assume that VCC2 is equal to VCC. Since the result derived later indicates that the threshold point does not depend on the sensor drive voltage VCC, the resistance value of the sensor drive current detection resistor RS may be an arbitrary value.
At this time, the current IBO generated by the sensor bias current generation circuit 90 is expressed by the following equation (1) using the sensor drive current IS.
IBO = IS × RS / RO (1)
For simplicity, the current mirror ratio 1 / KO is defined as follows.
1 / KO = RS / RO (2)

At this time, as shown in FIG. 7, the current flowing through the sensor resistor R1 is I1, the current flowing through the sensor resistors R3 and R4 is I2, the potential at the connection point of the sensor resistors R1 and R2 is VP, and the sensor resistor When the potential at the connection point of R3 and R4 is VN, the following equations (3a) to (3c) hold.
I1 = (VCC-VP) / R1 (3a)
I2 = VCC / (R3 + R4) (3b)
VP / R2 = I1 + (I1 + I2) / KO (3c)
Solving for VP,
VP = VCC × ((1 + 1 / KO) / R1 + 1 / (KO × (R3 + R4))) / (1 / R2 + (1 + 1 / KO) / R1) (4)
It becomes. Since the voltage comparator 20 switches at a voltage satisfying VP = VN, the following equation (5) is established.
VCC × ((1 + 1 / KO) / R1 + 1 / (KO × (R3 + R4))) / (1 / R2 + (1 + 1 / KO) / R1) = R4 × VCC / (R3 + R4) (5)

In the four-terminal sensor 10, the resistors R1, R2, R3, and R4 lose their balance according to the sensor input BIN applied from the outside, and R1 = R4 = R + ΔR, R2 = R3 = R−ΔR, or R1 = R4 = R−ΔR, R2 = R3 = R + ΔR, and it can be considered that the sensor output voltage VSO (= VP−VN) is generated. From this, when R1 = R4 = R + ΔR and R2 = R3 = R−ΔR,
((1 + 1 / KO) / (R + ΔR) + 1 / (KO × (R−ΔR + R + ΔR))) / (1 / (R−ΔR) + (1 + 1 / KO) / (R + ΔR) = (R + ΔR) / (R−ΔR) + R + ΔR) (6)

ΔR / R that satisfies the above equation (6) is obtained.
ΔR / R = 1 / (2 × KO × (1 + 1 / (2 × KO))
≒ 1 / (2 × KO) ≡BOP (7)
That is, ΔR / R where the above equation (7) holds is the threshold point BOP. Here, the normal sensor output voltage is about several hundred μV to several tens mV, and the sensor drive voltage is about 1 V to 5 V. Accordingly, KO is approximated as a sufficiently large value.

  Similarly, threshold points when the switch SWO of the bias current switching circuit 380 in FIG. 5 is open and the switch SWR is conductive will be described with reference to FIG. FIG. 8 shows a four-terminal sensor 10, a voltage comparator 20, a bias current in a state in which the switch SWO of the bias current switching circuit 95 in FIG. It is a figure which shows the circuit structure which took out IBR.

At this time, the current IBR generated by the sensor bias current generating circuit 55 is expressed by the following equation (8).
IBR = IS × RS / RR (8)
For simplicity, the current mirror ratio 1 / KR is defined as follows.
1 / KR = RS / RR (9)
At this time, as shown in FIG. 8, it is possible to consider the same by setting the current and setting the current mirror ratio 1 / KO at the time of the bias current IBR to 1 / KR, and ΔR / R when VP = VN is established. It is given by the following equation (10).
ΔR / R≈1 / (2 × KR) ≡BRP (10)
That is, ΔR / R that satisfies the above equation (10) is the threshold value BRP.

Next, a hysteresis width | BH | created by switching the switches SWO and SWR of the bias current switching circuit 380 will be considered.
The hysteresis width | BH | can be expressed by the following equation (11).
| BH | = | BOP-BRP | = | 1 / (2 × KO) −1 / (2 × KR) |
= | RS × (1 / RO−1 / RR) / 2 | (11)

FIG. 9 shows the relationship between the threshold values BOP and BRP and the hysteresis width | BH | obtained from the above equations (7), (10), and (11) and the sensor drive current detection resistor RS.
As shown in FIG. 9, in the conventional sensor threshold circuit, when the threshold point is changed by changing the resistor RS of the sensor drive current detection circuit 30, the hysteresis width | BH | changes similarly. I understand. This can also be seen from the above equation (11).
JP 2001-108480 A

However, in the conventional sensor threshold circuit, when the threshold point is changed by changing the resistor RS as described above, the hysteresis width | BH | that changes in the same manner is the output of the sensor output noise. It works to reduce variation. For this reason, when the threshold point is changed, there is a problem in that the influence of variation due to sensor output noise differs depending on the change in the hysteresis width | BH |.
The present invention has been made in view of such a problem, and an object of the present invention is to provide a sensor threshold circuit capable of providing a hysteresis width that does not depend on a change in threshold point.

  To achieve the above object, a sensor threshold circuit according to claim 1 of the present invention binarizes an output voltage of the sensor in a sensor threshold circuit that outputs a digital signal having hysteresis characteristics with respect to an input of the sensor. A voltage comparator, a first sensor drive current detection circuit for detecting the drive current of the sensor, and 1 / K times (K> 0) of the sensor drive current detected by the first sensor drive current detection circuit A threshold current generating circuit for generating a threshold current, a second sensor driving current detecting circuit for detecting a driving current of the sensor, and a sensor driving current detected by the second sensor driving current detecting circuit 1 / A times A hysteresis current generating circuit for generating a hysteresis current of (A> 0), and controlling the hysteresis current based on a signal binarized by the voltage comparator; A bias current based current and the controlled hysteresis current, characterized by comprising a bias current switching circuit that supplies the terminal for outputting the output voltage of the sensor.

A sensor threshold circuit according to a second aspect of the present invention is the sensor threshold circuit according to the first aspect, wherein the bias current switching circuit includes first and second switches, and the hysteresis is switched by switching the first and second switches. A bias current is generated by controlling current and subtracting the threshold current and the hysteresis current.
According to a third aspect of the present invention, there is provided the sensor threshold circuit according to the first aspect, wherein the bias current switching circuit includes first and second switches, and the hysteresis is switched by switching the first and second switches. A bias current is generated by controlling a current and adding the threshold current and the hysteresis current.
A sensor threshold circuit according to a fourth aspect of the present invention is the sensor threshold circuit according to any one of the first to third aspects, wherein the sensor is a four-terminal type sensor, and includes a Hall element, a magnetoresistive element, a strain sensor, a pressure sensor. It is any one of a sensor, a temperature sensor, and an acceleration sensor.

  As described above, according to the present invention, by generating the bias current IB with the threshold current IO and the hysteresis current IH, the hysteresis width | BH | is given by the resistance ratio A. Therefore, if A is determined, the hysteresis width | BH | is kept constant irrespective of the sensor current detection resistor RS1. If the constant A is determined, the hysteresis width | BH | becomes a single value, and there are no variations, temperature fluctuations, and changes with time. Therefore, according to the present invention, it is possible to provide a sensor threshold circuit capable of obtaining the hysteresis width | BH | independent of the sensor drive current detection resistor RS1.

Embodiments of the present invention will be described below with reference to the drawings. However, parts corresponding to each other in all the drawings in this specification are denoted by the same reference numerals, and description of the overlapping parts will be omitted as appropriate.
However, since there are two embodiments as the constituent circuit of the present invention, these will be described as the first and second embodiments, respectively, and then the actions and effects of both will be described together.

(Configuration of the first embodiment)
FIG. 1 is a circuit diagram showing a configuration of a sensor threshold circuit according to the first embodiment of the present invention.
The sensor threshold circuit includes a four-terminal sensor 10, a voltage comparator 20, a sensor drive current detection circuit (first sensor drive current detection circuit) 40, and a sensor drive current detection circuit (second sensor drive current detection circuit). ) 50, a threshold current generation circuit 60, a hysteresis current generation circuit 70, and a bias current switching circuit 180. The 4-terminal sensor 10 is, for example, any one of a Hall element, a magnetoresistive element, a strain sensor, a pressure sensor, a temperature sensor, and an acceleration sensor.

Further, the sensor threshold circuit feeds the sensor bias current IB generated by the bias current switching circuit 180 into the output terminal of the four-terminal sensor 10, and the sensor bias current IB and the impedance ROUT of the output terminal VP of the four-terminal sensor 10 The sensor output VSO (= VP−VN) has a hysteresis characteristic using a voltage drop due to the product IB × ROUT.
In the sensor threshold circuit having such a configuration, the sensor drive current IS is detected using the resistor RS1 of the sensor drive current detection circuit 40, and the threshold current composed of the resistor RS1, the resistor RO, the operational amplifier 61, and the PMOS transistor 62 is detected. The generation circuit 60 generates a threshold current IO that is 1 / K times (K> 0) the sensor drive current IS.

The sensor drive current IS is detected using the resistor RS2 of the sensor drive current detection circuit 50, and the sensor drive current IS is detected by the hysteresis current generation circuit 70 including the resistor RS2, the resistor RH, the operational amplifier 71, and the NMOS transistor 72. 1 / A times (A> 0) of hysteresis current IH.
Further, by switching the bias current switching circuit 180, the bias current IBO is generated using the threshold current IO, and the bias current IBR is generated by subtracting the threshold current IO and the hysteresis current IH.

(Configuration of Second Embodiment)
FIG. 2 is a circuit diagram showing a configuration of a sensor threshold circuit according to the second embodiment of the present invention.
This sensor threshold circuit includes a four-terminal sensor 10, a voltage comparator 20, sensor drive current detection circuits 40 and 50, a threshold current generation circuit 60, a hysteresis current generation circuit 70, and a bias current switching circuit 280. It is prepared for.
Further, the sensor threshold circuit feeds the sensor bias current IB generated by the bias current switching circuit 280 to the output terminal of the four-terminal sensor 10, and the sensor bias current IB and the impedance ROUT of the output terminal VP of the four-terminal sensor 10 This is a sensor threshold circuit having a hysteresis characteristic in the sensor output VSO (= VP−VN) using a voltage drop due to the product IB × ROUT.

In the sensor threshold circuit having such a configuration, the sensor drive current IS is detected using the resistor RS1 of the sensor drive current detection circuit 40, and the threshold current composed of the resistor RS1, the resistor RO, the operational amplifier 61, and the PMOS transistor 62 is detected. The generation circuit 60 generates a threshold current IO that is 1 / K times (K> 0) the sensor drive current IS.
Further, the sensor drive current IS is detected using the resistor RS2 of the sensor drive current detection circuit 50, and the sensor drive current IS is detected by the hysteresis current generation circuit 70 including the resistor RS2, the resistor RH, the operational amplifier 71, and the NMOS transistor 72. 1 / A times (A> 0) of hysteresis current IH.
Further, by switching the bias current switching circuit 280, the bias current IBO is generated by adding the threshold current IO and the hysteresis current IH, and the bias current IBR is generated using the threshold current IO.

(Operations and effects of the first and second embodiments)
The main point of the sensor threshold circuit of the present invention is that the bias current IB given in the prior art is generated by calculating the threshold current IO and the hysteresis current IH.
In order to understand the present invention, first, the hysteresis current IH and the threshold current IO in FIG. 1 are considered. In order to simplify the understanding, the resistance values of the resistors RS1 and RS2 that detect the sensor drive current are considered to be very small compared to the resistance values of the sensor resistors R1, R2, R3, and R4. The sensor drive terminal voltage VCC2 is assumed to be equal to VCC, and the sensor drive terminal voltage GND2 is assumed to be equal to GND. Since the result derived later indicates that the threshold point does not depend on the sensor drive voltage VCC, the resistance value of the sensor drive current detection resistor may be an arbitrary value.

First, the threshold current IO is expressed by the following equation (12) using the sensor drive current IS.
IO = IS × RS1 / RO (12)
For simplicity, the current mirror ratio 1 / K of the threshold current generation circuit is defined as follows.
1 / K = RS1 / RO (13)
The hysteresis current IH is expressed by the following equation (14) using the sensor drive current IS.
IH = IS × RS2 / RH (14)
For simplicity, the current mirror ratio 1 / A of the hysteresis current generating circuit is defined as follows.
1 / A = RS2 / RH (15)

Next, the current is determined as shown in FIG. FIG. 3 shows the four-terminal sensor 10, the bias current switching circuit 180, and the voltage comparator 20 extracted from the sensor threshold circuit shown in FIG. It is a circuit diagram of the structure through which the hysteresis current IH flows.
Consider the threshold point when the first switch SW1 of the bias current switching circuit shown in FIG. 3 is conducting and the second switch SW2 is open.
At this time, the bias current IBO is expressed by the following equation (16).
IBO = IO (16)

The threshold points at this time are the resistor RO or resistor RR of the conventional circuit shown in FIG. 5 as the resistor RO, and the current mirror ratio 1 / KO or 1 / KR of the sensor bias current generating circuit 90 is 1 / K. By replacing, it can be obtained in the same manner as the conventional circuit.
ΔR / R≈1 / (2 × K) ≡BOP (17)
That is, ΔR / R that satisfies the above equation (17) is the threshold value BOP.

Next, a threshold point when the second switch SW2 of the bias current switching circuit 180 is turned on and the first switch SW1 is opened in FIG. 3 will be considered.
At this time, the bias current IBR is expressed by the following equation (18).
IBR = IO-IH (18)
At this time, the following equations (19a) to (19c) hold.
I1 = (VCC-VP) / R1 (19a)
I2 = VCC / (R3 + R4) (19b)
VP / R2 = I1 + ((I1 + I2) / K- (I1 + I2) / A)) (19c)
Solving for VP,
VP = VCC × ((1 + 1 / K−1 / A) / R1 + (1 / K−1 / A) / (R3 + R4))) / (1 / R2 + (1 + 1 / K−1 / A) / R1) ( 20)
It becomes. Since the voltage comparator 20 switches at a voltage satisfying VP = VN, the following equation (21) is established.
VCC × ((1 + 1 / K−1 / A) / R1 + (1 / K−1 / A) / (R3 + R4)) / (1 / R2 + (1 + 1 / K−1 / A) / R1) = R4 × VCC / (R3 + R4) (21)

In the four-terminal sensor 10, the resistors R1, R2, R3, and R4 lose their balance according to the sensor input BIN applied from the outside, and R1 = R4 = R + ΔR, R2 = R3 = R−ΔR, or R1 = R4 = R−ΔR, R2 = R3 = R + ΔR, and it can be considered that the sensor output voltage VSO (= VP−VN) is generated. From this, when R1 = R4 = R + ΔR and R2 = R3 = R−ΔR,
((1 + 1 / K−1 / A) / (R + ΔR) + (1 / K−1 / A) / (R−ΔR + R + ΔR)) / (1 / (R−ΔR) + (1 + 1 / K−1 / A) / (R + ΔR)) = (R + ΔR) / (R−ΔR + R + ΔR) (22)

ΔR / R where the above equation (22) holds is obtained.
ΔR / R = (1 / K−1 / A) / (2 × (1 + 1/2 × (1 / K−1 / A)))
≈ 1 / (2 × K) −1 / (2 × A) ≡BRP (23)
That is, ΔR / R where the above equation (23) holds is the threshold value BRP. Here, approximation was performed using the fact that the constants K and A can be handled as sufficiently large values in the range considered now.

Next, a hysteresis width | BH | created by switching the first switch SW1 and the second switch SW2 of the bias current switching circuit 180 will be considered.
At this time, the hysteresis width | BH | is expressed by the following equation (24).
| BH | = | BOP-BRP | = | 1 / (2 × K) − (1 / (2 × K) −1 / (2 × A)) |
= | 1 / (2 × A) | (24)

Further, it can be seen from the comparison of the above equations (17) and (23) that the threshold point can be moved by 1 / (2 × A) by changing the bias current by the hysteresis current IH. Thus, as shown in FIG. 2, the threshold current IO and the hysteresis current IH are added to generate the bias current IBO, and when the bias current IBR is generated using the threshold current IO, the hysteresis width | BH | It is expressed as
| BH | = | BOP-BRP | = | 1 / (2 × K) + 1 / (2 × A) −1 / (2 × K) |
= | 1 / (2 × A) | (25)

Here, for simplification, the size ratio of the PMOS 282 and the PMOS 283 of the bias current switching circuit is set to 1 time, and the current mirror ratio is set to 1 time. When this mirror ratio is C times (C> 0), it can be considered similarly by treating 1 / A as C / A in the above equation.
FIG. 4 shows the relationship between the threshold values BOP and BRP and the hysteresis width | BH | obtained from the above equations (17), (23), (24), and (25) and the sensor drive current detection resistor RS1.

What is important in the above formulas (24), (25), and FIG. 4 obtained in the first and second embodiments is that the threshold values BOP and BRP are changed using the resistor RS1 of the sensor current detection circuit 40. In this case, the hysteresis width | BH | changes depending on the current mirror ratio coefficient A of the hysteresis current generating circuit 70. Also, from the above equations (24) and (25), it can be seen that the hysteresis width | BH | does not depend on the sensor drive voltage VCC.
Since the above constant A is given by the resistance ratio, if the constant A is determined, the hysteresis width | BH | is determined as one value, and there is no variation, temperature variation, or change with time.
Therefore, according to the first and second sensor threshold circuits, it is possible to provide a hysteresis width that does not depend on a change in threshold point.

It is a circuit diagram which shows the structure of the sensor threshold value circuit in the 1st Embodiment of this invention. It is a circuit diagram which shows the structure of the sensor threshold value circuit in the 2nd Embodiment of this invention. FIG. 2 is a circuit diagram illustrating a circuit configuration in which a four-terminal sensor, a bias current calculation circuit, a voltage comparator, a threshold current IO, and a hysteresis current IH in FIG. 1 are extracted. It is a figure which shows the relationship between the threshold point in 1st and 2nd embodiment, and sensor current detection resistor RS. It is a circuit diagram which shows the structure of the conventional sensor threshold value circuit. It is a figure which shows the relationship between the sensor input of a sensor threshold circuit with a hysteresis characteristic, and an output voltage. FIG. 6 is a circuit diagram illustrating a circuit configuration in which a four-terminal sensor, a bias current IBO, and a voltage comparator are taken out when the first switch of the bias current switching circuit of FIG. 5 is in a conductive state and the second switch SW2 is in an open state. FIG. 6 is a circuit diagram illustrating a circuit configuration in which a four-terminal sensor m bias current IBR and a voltage comparator are extracted when the first switch of the bias current switching circuit of FIG. 5 is in an open state and the second switch SW2 is in a conductive state. It is a figure which shows the relationship between the threshold value point and sensor current detection resistor in the conventional sensor threshold circuit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 4 terminal type sensor 20 Voltage comparator 30 Sensor drive voltage source 40, 50 Sensor drive current detection circuit 60 Threshold current generation circuit 70 Hysteresis current generation circuit 180, 280, 380 Bias current switching circuit 90 Sensor bias current generation circuit 61, 71 , 92 operational amplifier SW1, SW2 switch 62, 282, 283, 91 PMOS transistor 72 NMOS transistor 181, 281, 381 inverter R1, R2, R3, R4, RS, RS1, RS2, RO, RH, RR resistor

Claims (4)

In the sensor threshold circuit that outputs a digital signal with hysteresis characteristics to the sensor input,
A voltage comparator that binarizes the output voltage of the sensor;
A first sensor driving current detection circuit for detecting a driving current of the sensor;
A threshold current generation circuit for generating a threshold current that is 1 / K times (K> 0) the sensor drive current detected by the first sensor drive current detection circuit;
A second sensor drive current detection circuit for detecting a drive current of the sensor;
A hysteresis current generating circuit for generating a hysteresis current 1 / A times (A> 0) the sensor driving current detected by the second sensor driving current detection circuit;
The hysteresis current is controlled based on a signal binarized by the voltage comparator, and a bias current based on the threshold current and the controlled hysteresis current is supplied to a terminal that outputs an output voltage of the sensor. And a bias current switching circuit.   The bias current switching circuit includes first and second switches, controls the hysteresis current by switching the first and second switches, performs subtraction between the threshold current and the hysteresis current, and bias The sensor threshold circuit according to claim 1, wherein a current is generated.   The bias current switching circuit includes first and second switches, controls the hysteresis current by switching the first and second switches, adds the threshold current and the hysteresis current, and biases The sensor threshold circuit according to claim 1, wherein a current is generated.   4. The sensor according to claim 1, wherein the sensor is a four-terminal sensor, and is any one of a Hall element, a magnetoresistive element, a strain sensor, a pressure sensor, a temperature sensor, and an acceleration sensor. The sensor threshold circuit according to claim 1.

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