JP7304748B2 - Current detection device, current detection method - Google Patents

Description

The present invention relates to technology for detecting current.

For example, when a light-emitting element such as an LED is lit by PWM (Pulse Width Modulation) control and the current flowing at that time is detected, the current is converted into a voltage using a shunt resistor element and amplified, and this voltage is smoothed by a smoothing circuit and input to an analog-to-digital converter, where the value is converted into a current (average current) value. A technique for smoothing a pulse-like voltage and taking it into an analog-to-digital converter is described, for example, in FIG.

By the way, when the current is detected by the above method, if the value becomes very small, close to 0, the influence of errors due to variations in the characteristics of circuit elements such as an amplifier circuit becomes relatively large, and the accuracy of current detection decreases. For example, consider a case where the maximum value of the current to be detected is 1 A and the detection error is ±10 mA. The error rate at the time of detection in this case corresponds to ±1% with respect to the maximum current value. On the other hand, if the current to be detected is 100 mA, the error is ±10 mA, so the error rate is 10%. As described above, when a certain error exists at the time of detection, the smaller the current to be detected, the greater the influence of the error, and the lower the detection accuracy. Also, assuming the same error, and considering the case where the current to be detected is 5 mA, if an error of -10 mA occurs, the current detection value will be -5 mA. However, the detected value is 0A because the analog-to-digital converter does not normally handle voltages of 0V or less. Due to the influence of errors in this way, it is very difficult to detect weak currents with high accuracy.

JP-A-2000-155063

It is an object of a specific aspect of the present invention to provide a technique capable of detecting current over a relatively wide range and improving the detection accuracy of weak current.

[1] A current detection device according to one aspect of the present invention is a device for detecting a current flowing through a PWM-controlled light emitting element,
a shunt resistance element connected to a path of current flowing through the light emitting element;
a first amplifier circuit that amplifies the voltage across the shunt resistor;
a first pull-up circuit that raises the output voltage of the first amplifier circuit by a first predetermined value;
a second amplifier circuit that amplifies the output voltage of the first amplifier circuit;
a cancellation circuit that subtracts the voltage corresponding to the first predetermined value from the input voltage to the second amplifier circuit;
a second pull-up circuit that raises the output voltage of the second amplifier circuit by a second predetermined value;
A first current value indicating the magnitude of the current is calculated based on the output voltage of the first amplifier circuit, and a second current value indicating the magnitude of the current is calculated based on the output voltage of the second amplifier circuit. a control unit that obtains a current value;
is connected between the output terminal of the first amplifier circuit and the control section and between the first amplifier circuit and the second amplifier circuit to smooth the output voltage of the first amplifier circuit; a smoothing circuit for direct current;
including
The control unit obtains the first current value by canceling the first predetermined value during the calculation and obtains the second current value by canceling the second predetermined value, and the current is selecting the first current value as the current detection result in a relatively large first range, and selecting the second current value as the current detection result in a second range in which the current is relatively small;
It is a current detection device.

In this specification, the term "connection" between a circuit element and another circuit element means that these circuit elements are directly connected to each other, and that another circuit element or the like is connected between these circuit elements. It shall be included even if it is connected by intervening.

According to the above configuration, it is possible to detect a current in a relatively wide range and improve the detection accuracy of a weak current.

FIG. 1 is a diagram showing the configuration of a current detection device according to one embodiment. FIG. 2 is a diagram for explaining the relationship between the current (measured current) flowing through the light emitting element and the voltage (detection voltage) input to the microcomputer. FIGS. 3A and 3B are diagrams for explaining an example of measured current and detected voltage during actual current measurement. FIG. 4 is a diagram showing an example of the error rate of measured current. FIG. 5 is a diagram for explaining correction of the measured current.

FIG. 1 is a diagram showing the configuration of a current detection device according to one embodiment. The illustrated current detection device 1 detects a current that flows when a light emitting element (LED) 2 is PWM-controlled by a drive circuit 3 and emits light. This current detection device 1 includes a microcomputer 10, operational amplifiers 11 and 21, resistance elements 12, 13, 14, 15, 16, 22, 23, 24, 25 and 26, capacitance elements 17 and 27, a shunt resistance element 18 and three It includes a reference power supply REF. Although only one light emitting element 2 is used in the illustrated example, any number of light emitting elements may be connected in series, in parallel, or in series-parallel.

A microcomputer (control unit) 10 includes an analog-to-digital converter (AD1) 10a and an analog-to-digital converter (AD2) 10b, which amplify the voltage across a shunt resistance element 18 connected in series with the light emitting element 2 Then, the voltage obtained is taken in and converted into digital data, and the value of the current flowing through the shunt resistance element 18 (that is, the current flowing through the light emitting element 2) is obtained based on the value.

The operational amplifier 11 has a non-inverting input terminal (+) connected to one end (high potential side) of the shunt resistance element 18 via the resistance element 12, and an inverting input terminal (-) connected to the shunt resistance element 18 via the resistance element 13. , and amplifies the voltage across the shunt resistance element 18 .

The operational amplifier 11 has a resistance element 14 connected between the non-inverting input terminal and the reference power supply REF, and the potential of the non-inverting input terminal is pulled up to the voltage of the reference power supply REF (eg +0.5V). When the non-inverting input terminal of the operational amplifier 11 is pulled up to the voltage of the reference power supply REF, the output voltage of the operational amplifier 11 is offset to the positive side by a certain amount of voltage. (or polarity reversal) can be prevented. Note that the resistance element 14 and the reference power supply REF correspond to the "first pull-up circuit".

In addition, the operational amplifier 11 has a resistor element 15 for negative feedback connected between the output terminal and the inverting input terminal. The operational amplifier 11 and the resistance elements 12 to 15 constitute a differential amplifier circuit (first amplifier circuit). The amplification factor of this differential amplifier circuit is R2/R1, where the resistance value of the resistance element 13 is R1 and the resistance value of the resistance element 15 is R2.

The output voltage from the output terminal of the operational amplifier 11 is applied to the resistance element 16 connected between this output terminal and the analog-to-digital converter 10a of the microcomputer 10, the other end of the resistance element 16 (on the side of the analog-to-digital converter 10a), and the reference voltage. The signal is smoothed through a smoothing circuit composed of a capacitive element 17 connected to the potential terminal (ground terminal) and input to the analog-to-digital converter 10a. In this case, the microcomputer 10 subtracts (offsets) the voltage (the voltage of the reference power supply REF) pulled up by the resistance element 14 from the value of the voltage taken in by the analog-to-digital converter 10a. A current flowing through the light emitting element 2 is calculated using the value.

The operational amplifier 21 has a non-inverting input terminal (+) connected to the output terminal of the operational amplifier 11 via the resistance element 16, and an inverting input terminal (-) connected to the reference power supply REF via the resistance element 23. Amplifies the voltage applied between the inverting input terminal and the inverting input terminal.

The operational amplifier 21 has a resistance element 24 connected between the non-inverting input terminal and the reference power supply REF, and the potential of the non-inverting input terminal is pulled up to the voltage of the reference power supply REF (eg +0.5V). This provides the same effect as the operational amplifier 11 described above. Note that the resistance element 24 and the reference power supply REF correspond to the "second pull-up circuit".

Further, the operational amplifier 21 has a resistance element 25 for negative feedback connected between the output terminal and the inverting input terminal. The operational amplifier 21 and the resistance elements 22 to 25 constitute a differential amplifier circuit (second amplifier circuit). The amplification factor of this differential amplifier circuit is r2/r1, where r1 is the resistance value of the resistance element 23 and r2 is the resistance value of the resistance element 25. FIG.

The potential of the inverting input terminal of the operational amplifier 21 is pulled up to the voltage of the reference power supply REF (eg +0.5 V) via the resistance element 23 . As a result, the voltage pulled up from the output voltage of the operational amplifier 11 by the voltage of the reference power supply REF can be canceled. If this cancellation is not performed, the offset voltage (for example, +0.5 V) is amplified by the operational amplifier 21, so that the voltage obtained by multiplying the offset voltage by r2/r1 is always output from the operational amplifier 21. In the case of , the allowable range of the input voltage of the microcomputer 10 is exceeded, and the measured current value swings over to the maximum value. It becomes a circuit part for preventing such an inconvenience. Note that the resistance element 23 and the reference power supply REF correspond to the "cancellation circuit".

The output voltage from the output terminal of the operational amplifier 21 is connected between this output terminal and the analog-to-digital converter 10b of the microcomputer 10 and the other end of the resistance element 26 (on the side of the analog-to-digital converter 10b) and the reference voltage. It is smoothed through a smoothing circuit composed of a capacitive element 27 connected between it and the potential terminal (ground terminal), and is input to the analog-to-digital converter 10b. In this case, the microcomputer 10 subtracts (offsets) the voltage pulled up by the resistance element 24 (the voltage of the reference power supply REF) from the value of the voltage taken in by the analog-to-digital converter 10b, and A current flowing through the light emitting element 2 is calculated using the value.

The voltage input to the analog-to-digital converter 10b is a value obtained by multiplying the voltage amplified by the operational amplifier 11 by the voltage amplified by the operational amplifier 21, and the pulled-up voltage is superimposed thereon. The sensitivity to the voltage across element 18 will be higher. Therefore, the voltage input to this analog-to-digital converter 10b can be used specifically for a weak current. That is, in the microcomputer 10, the value of the current is obtained using the voltage input to the analog-to-digital converter 10a in the range of relatively large current, and the value of current is input to the analog-to-digital converter 10b in the range of relatively small current. If the value of the current is obtained using the voltage, the current can be detected over a relatively wide range, and the detection accuracy of the weak current can be improved.

FIG. 2 is a diagram for explaining the relationship between the current (measured current) flowing through the light emitting element and the voltage (detection voltage) input to the microcomputer. In FIG. 2, the horizontal axis corresponds to the measured current, and the vertical axis corresponds to the detected voltage. For example, assume that the maximum allowable voltage input to the microcomputer 10 is +5V. In this case, if the voltage of the reference power supply REF is +0.5, the minimum value is set to +0.5V in consideration of this. That is, +0.5 V is provided on the lower limit side to prevent overshoot. Also, on the upper limit side, for example, +0.5 V is provided as an overshoot prevention width in consideration of the detection error. From these, by setting the range of +0.5 V to +4.5 V, that is, the allowable range to 4.0 V, it is possible to prevent the detection voltage from swinging out over the entire allowable range. This prevents the voltage from swinging out due to a detection error.

FIGS. 3A and 3B are diagrams for explaining an example of measured current and detected voltage during actual current measurement. For example, assume that the maximum value of the measured current is 500mA. When the measured current is 500 mA, the microcomputer 10 uses the analog-to-digital converter 10a to capture the output voltage of the differential amplifier circuit by the operational amplifier 11, which has standard sensitivity. Obtain the value of the measured current (see FIG. 3(A)). At this time, the value of the output voltage of the differential amplifier circuit by the operational amplifier 21, which has high sensitivity, swings over to the upper limit side when taken into the microcomputer 10, so the microcomputer 10 does not use this output voltage. Similarly, when the measured current is, for example, 200 mA, the value of the measured current is obtained using the output voltage of the differential amplifier circuit by the operational amplifier 11 (see FIG. 3A).

On the other hand, when the measured current is, for example, 50 mA, the microcomputer 10 acquires the output voltage of the differential amplifier circuit by the highly sensitive operational amplifier 21 with the analog-to-digital converter 10b, and uses it to obtain the value of the measured current. (See FIG. 3B).

In this case, since the output voltage of the differential amplifier circuit by the operational amplifier 11 is also within a range in which the detection accuracy does not decrease so much, the microcomputer 10 uses the output voltage of the differential amplifier circuit by the operational amplifier 11 to calculate the value of the measured current. can also be obtained (see FIG. 3(A)). Furthermore, the microcomputer 10 compares the values of the measured currents obtained from the outputs of the differential amplifier circuits by the operational amplifiers 11 and 21, and if the difference is larger than a predetermined standard, a failure occurs in one of the circuits. It may be determined that

Further, for example, when the measured current is 5 mA, the microcomputer 10 acquires the output voltage of the differential amplifier circuit by the highly sensitive operational amplifier 21 with the analog-to-digital converter 10b, and uses it to obtain the value of the measured current ( See FIG. 3(B)).

In this way, the microcomputer 10 obtains the measured current using the output voltage of the differential amplifier circuit by the operational amplifier 11 in the first range (for example, the range of 80 mA to 500 mA in this example) where the measured current is relatively large. Adopted (selected) as the detection result, in the second range where the measured current is relatively small (for example, the range of 0 to 80 mA in this example), the output voltage of the differential amplifier circuit by the operational amplifier 21 is used to obtain the measured current. is adopted (selected) as the detection result. As a result, the entire range of the measured current can be handled, and the detection accuracy can be improved even in a range where the measured current is weak. Here, "adopt (select)" means, for example, that the microcomputer 10 uses it as a detection result of current output to an external device or the like, or that the microcomputer 10 performs other control (for example, brightness control of a light emitting element). ) to use the detection result of the current.

FIG. 4 is a diagram showing an example of the error rate of measured current. As shown by curves a1 and a2 in the figure, the error rate of the measured current based on the output voltage of the differential amplifier circuit by the operational amplifier 11 increases as the value of the measured current decreases, and increases sharply as it approaches zero. On the other hand, as shown by curves b1 and b2 in the figure, the error rate of the measured current based on the output voltage of the differential amplifier circuit by the operational amplifier 21 increases as the value of the measured current decreases and approaches 0. The tendency to increase rapidly is the same, but the absolute value of the error rate is small. Therefore, as described above, the output voltage of the differential amplifier circuit by the operational amplifier 11 is used in the first range L1 where the measured current is relatively large, and the differential voltage by the operational amplifier 21 is used in the second range L2 where the measured current is relatively small. By using the output voltage of the amplifier circuit, it is possible to further improve the detection accuracy in the entire range of the measured current, especially in the weak current range. For example, in the illustrated example, the detection accuracy is within ±8% over the entire range of the measured current.

FIG. 5 is a diagram for explaining correction of the measured current. When the current measuring device of the above-described embodiment is applied to mass-produced products, there may be individual differences in the detection accuracy of the measured current due to variations in parts and the like. In order to absorb such individual differences, it is preferable to perform the correction described below when manufacturing the product. As an example, it is assumed that the maximum value of the measured current is 500 mA and the voltage of the reference power supply REF is +0.5V.

First, zero point correction is performed. Specifically, when no current flows through the light emitting element 2, the voltage value based on the measured current is +0.5 V in an ideal state. However, in reality, the value deviates from +0.5 V due to the influence of variations. In zero-point correction, the microcomputer 10 measures the voltage when no current is actually flowing, and stores the value. Since this value is an individual difference, this stored value is used when actually obtaining the voltage.

Next, full scale correction is performed. Specifically, when the measured current is the maximum value (here, 500 mA), the voltage value based on the measured current is +4.5 V in an ideal state. However, in reality, the value deviates from +4.5V due to the influence of variations. In the full-scale correction, an accurate current of 500 mA is actually applied, the voltage at that time is measured by the microcomputer 10, and the value is stored. Since this value is an individual difference, this stored value is used when actually obtaining the voltage.

As shown in FIG. 5, a straight line connects the voltage values (indicated by triangles in the figure) with respect to the measured current obtained by the 0-point correction and the full-scale correction. Then, the current measurement formula can be expressed by a linear function. Assuming that the measured current in the graph is on the X axis and the detected voltage is on the Y axis, it can be expressed as Y=AX+B (A: gradient of graph, B: 0 point). Next, a correction factor is incorporated into this. For B, the result of the above zero point correction can be substituted as it is. As for A, after the value of B is determined, the value of slope A can be calculated from an equation obtained by applying the results of the current value and the detected voltage during full-scale correction to this equation. Using these A and B, the current calculation formula can be expressed as I=(V−B)/A. V here is the detection voltage. This expresses Ohm's Law, where the I part is the current, the (VB) part is the voltage, and A is the resistance. Since the coefficients (A, B) of this formula are different for each product due to the influence of component variations, the above correction is performed for each product, this formula is retained for each product, and applied when calculating the actual voltage. This makes it possible to absorb individual differences. This equation is prepared for each operational amplifier with standard sensitivity and high sensitivity.

Specific examples of numerical values in each circuit element of the current measuring device will be described below.
The shunt resistance element 18 is set to 1.3Ω, the full-scale current value is set to 500mA, and the voltage of the reference power supply REF is set to +0.5V. Resistive elements 12 and 13 are each 1 kΩ, and resistive elements 14 and 15 are each 6.2 kΩ. Accordingly, since the gain (amplification factor) of the operational amplifier 11 is 6.2 times (6.2k/1k), the voltage when the current of 500mA flows is approximately 4.00V (=1.3Ω×500mA×6.2Ω). 2). Since the reference voltage at 0 A is superimposed, the output voltage of the operational amplifier 11 becomes 4.5 V at this time. In the case of PWM current, this output voltage exhibits PWM behavior, is smoothed by a smoothing circuit, and is taken as a direct current by analog-to-digital converter 10a. A CR circuit may be further provided immediately before the analog-to-digital converter 10a for functioning as a noise filter and for suppressing charge transfer to the internal capacitance of the analog-to-digital converter 10a.

The output voltage of the differential amplifier circuit by the operational amplifier 11 is converted into direct current by the smoothing circuit and input to the operational amplifier 21 . If the resistance elements 22 and 23 are 10 kΩ and the resistance elements 24 and 25 are 75 kΩ, the gain of the operational amplifier 21 is 7.5 times (75 k/10 k). 0.5V is canceled because the reference voltage is superimposed at the inverting input terminal. Therefore, the voltage obtained by subtracting this 0.5 V is amplified by 7.5 times by the operational amplifier 21 .

The current flowing through the shunt resistance element 18 of 1.3Ω passes through the operational amplifiers 11 and 21, and the voltage obtained by multiplying the respective gains becomes the output voltage of the differential amplifier circuit by the operational amplifier 21. FIG. The full-scale current at this time is 4.00V/(1.3*(6.2*7.5))≈66.17mA, where the full-scale voltage is 4.0V. In other words, if the full scale range of the operational amplifier 21 is set to 66.17 mA, the 4.0V range can be used at maximum. Since the reference voltage of 0.5 V is similarly superimposed here as well, the output voltage of the operational amplifier 21 becomes 4.5 V when 66.17 mA flows. This voltage is taken in by the analog-to-digital converter 10b. A CR circuit may be further provided immediately before the analog-to-digital converter 10b for functioning as a noise filter and for suppressing charge transfer to the internal capacitance of the analog-to-digital converter 10b.

In the current detection device according to the numerical example above, the microcomputer 10 obtains the current based on the output voltage of the differential amplifier circuit by the highly sensitive operational amplifier 21 for currents in the range of less than 66.17 mA, and detects this as the result of detection. , and for currents in the range of 66.17 mA or more, the current is obtained based on the output voltage of the differential amplifier circuit by the operational amplifier 11 with standard sensitivity, and this is adopted as the detection result, thereby improving the detection accuracy. be able to.

According to the above-described embodiments, it is possible to detect current in a relatively wide range and improve the detection accuracy of weak current.

It should be noted that the present invention is not limited to the contents of the above-described embodiments, and various modifications can be made within the scope of the gist of the present invention. For example, in the above embodiments, the shunt resistance element is connected in series with the light emitting element, but the shunt resistance element may be connected to one of the branched paths of the current flowing through the light emitting element. Also, in the above-described embodiments, the light-emitting element was exemplified as an example of the electric load, but other electric loads may be used. Further, the driving method of the light emitting element is not limited to PWM control.

1: current detector, 10: microcomputer 10, 10a, 10b: analog-to-digital converter, 11, 21: operational amplifier, 11, 12, 13, 14, 15, 16, 21, 22, 23, 24, 25, 26: Resistance elements 17, 27: capacitance element 18: shunt resistance element REF: reference power supply

Claims (3)

A device for detecting a current flowing through a PWM-controlled light emitting element ,
a shunt resistance element connected to a path of current flowing through the light emitting element ;
a first amplifier circuit that amplifies the voltage across the shunt resistor;
a first pull-up circuit that raises the output voltage of the first amplifier circuit by a first predetermined value;
a second amplifier circuit that amplifies the output voltage of the first amplifier circuit;
a cancellation circuit that subtracts the voltage corresponding to the first predetermined value from the input voltage to the second amplifier circuit;
a second pull-up circuit that raises the output voltage of the second amplifier circuit by a second predetermined value;
A first current value indicating the magnitude of the current is calculated based on the output voltage of the first amplifier circuit, and a second current value indicating the magnitude of the current is calculated based on the output voltage of the second amplifier circuit. a control unit that obtains a current value;
is connected between the output terminal of the first amplifier circuit and the control section and between the first amplifier circuit and the second amplifier circuit to smooth the output voltage of the first amplifier circuit; a smoothing circuit for direct current;
including
The control unit obtains the first current value by canceling the first predetermined value during the calculation and obtains the second current value by canceling the second predetermined value, and the current is selecting the first current value as the current detection result in a relatively large first range, and selecting the second current value as the current detection result in a second range in which the current is relatively small;
Current detection device. The canceling circuit includes a reference power supply and a resistive element connected between the reference power supply and an inverting input terminal of the second amplifier circuit,
The current detection device according to claim 1 . The control unit includes a first analog-to-digital converter that takes in the output voltage of the first amplifier circuit smoothed through the smoothing circuit and converts it into first digital data, and the output voltage of the second amplifier circuit. a second analog-to-digital converter that takes in and converts to second digital data, and performs the calculation using the first digital data and the second digital data;
The current detection device according to claim 1 or 2 .

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