JP4928588B2 - D / A converter - Google Patents

Description

  The present invention relates to a D / A converter.

  Conventionally, D / A converters for converting a digital signal into an analog signal have been provided in various forms (see, for example, Patent Document 1). In these D / A converters, an internal circuit is usually overcurrent. A protection function is provided to protect against damage.

  FIG. 5 shows an example of a D / A converter having an overcurrent protection function. The D / A converter A includes a D / A converter 2 that converts input digital data into an analog voltage, and a D / A converter. An operational amplifier OP1 that amplifies the analog voltage from the converter 2, an output transistor Tr1 that generates an output current corresponding to the analog output from the operational amplifier OP1, and a potential difference corresponding to the output current connected to the output transistor Tr1. A current detection resistor R1 to be generated (hereinafter referred to as resistor R1) and an output cutoff transistor that turns on and turns off the output transistor Tr1 when the potential difference generated in the resistor R1 exceeds the on-voltage between the base and the emitter. Tr2 and a CPU 1 that outputs digital data to the D / A converter 2 are provided via output terminals T1 and T2. The connection has been loaded (for example, an inverter, etc.) L, so that the current corresponding to the load resistance.

  Here, when the load resistance of the load L is small, since the output current I flowing through the load L is large, the potential difference generated in the resistor R1 is also large. When this potential difference exceeds the above-mentioned ON voltage, the output cutoff transistor Tr2 is turned on. As a result, the analog output of the operational amplifier OP1 is output to the output cutoff transistor Tr2. Therefore, the output transistor Tr1 Turns off. When the output transistor Tr1 is turned off, the output voltage to the load L is lowered, so that the output current I is also lowered. As a result, the potential difference generated in the resistor R1 becomes smaller than the on-voltage. When the output cutoff transistor Tr2 is turned off, the output transistor Tr1 is turned on again, and the drive signal is output to the load L again. Thereafter, by repeating the above operation, the output voltage is lowered, and it is possible to prevent an excessive load from being applied to the internal power supply circuit and the output transistor Tr1.

JP-A-8-51365 (paragraph [0013] and FIGS. 1 and 2)

  In the D / A converter A shown in FIG. 5 described above, the on-voltage of the output cutoff transistor Tr2 fluctuates due to individual differences or temperature fluctuations, and as a result, detection when the overcurrent protection function operates is detected. Since the current threshold value also varies, it is necessary to consider this threshold value variation when selecting the output transistor Tr1. In addition, when the specification of the allowable current flowing through the output transistor Tr1 is increased, a transistor having a large continuous maximum allowable current (rated current) is selected in consideration of variations in the threshold of the detection current. Since it is necessary to use a large and expensive element for the transistor, the apparatus is increased in size and cost.

  The present invention has been made in view of the above-described problems, and the object of the present invention is to increase the specification of the allowable output current without increasing the size and cost of the apparatus, and for convenience. An object of the present invention is to provide a D / A conversion device with improved performance and reliability.

According to the first aspect of the present invention, there is provided a D / A conversion means for generating an output voltage corresponding to digital data, and an electronic circuit configured to limit an instantaneously flowing output current to a predetermined instantaneous maximum allowable current or less. D / A conversion means, comprising: overcurrent protection means; and second overcurrent protection means configured to limit a continuously flowing output current configured by software to a predetermined continuous maximum allowable current smaller than an instantaneous maximum allowable current. Outputs a D / A converter circuit that converts input digital data into an analog voltage, an amplifier circuit that amplifies the output voltage from the D / A converter circuit, and generates an output current based on the output voltage value of the amplifier circuit. An output circuit that outputs from the terminal, and the first overcurrent protection means limits the output of the output circuit so that the instantaneously flowing output current is less than or equal to the instantaneous maximum allowable current, and the second overcurrent protection Means is characterized that you limit the digital data input to D / A converter circuit so that the output current flowing continuously becomes the continuous maximum allowable current below.

According to a second aspect of the invention, the output circuit includes an output transistor for generating an output current by the output voltage of the amplifying circuit, for current detection for generating a potential difference corresponding to that output current connected to the output transistor A resistor, a voltage detection circuit that detects a potential difference generated in the current detection resistor, and an A / D conversion circuit that converts a detection result of the voltage detection circuit into digital data, and the second overcurrent protection means includes: beyond current calculating means for calculating an output current based on the digital data from the a / D converter circuit, monitors whether exceeds a threshold value calculation result is set in advance of the current calculation unit, computation results to the threshold And a current monitoring means for outputting digital data such that the output current is equal to or less than the maximum continuous allowable current to the D / A conversion circuit.

According to a third aspect of the present invention, the first overcurrent protection means includes an output cutoff transistor that turns on when the potential difference generated in the current detection resistor exceeds the on-voltage between the base and the emitter and turns off the output transistor. It is characterized by providing.

  In the above conventional example, when the specification of the allowable output current of the D / A converter is increased, it is necessary to select an output transistor having a large continuous maximum allowable current in consideration of the threshold of the detection current. According to the invention of claim 1, the second overcurrent protection means configured by software with little variation is used for the continuous maximum allowable current. Even when the specification of the allowable output current is increased, the continuous maximum allowable current need not be increased, and the output circuit can be downsized as compared with the conventional example. In addition, when the output allowable current specification is set to the same value as the conventional example, an output circuit with a smaller continuous maximum allowable current can be used compared to the conventional example, so that downsizing and cost reduction can be realized. There is an effect that can be. Furthermore, although there may be a case where the software cannot cope with an instantaneous overcurrent due to a delay in arithmetic processing or the like, in the present invention, the first overcurrent protection configured by an electronic circuit against the instantaneous overcurrent. Since the means is used, there is an effect that the current can be surely limited without being delayed by arithmetic processing or the like. Also, by increasing the specifications of the allowable output current, the range of devices that can be connected is expanded, improving user convenience, and improving the accuracy of overcurrent detection, which can damage internal circuits due to overcurrent. As a result, it is possible to provide a highly reliable D / A converter.

  According to the invention of claim 2, the same effect as that of claim 1 can be obtained.

  According to the invention of claim 3, the same effect as that of claim 1 can be obtained.

It is a schematic circuit diagram which shows the D / A converter of this embodiment. (A) (b) is a flowchart explaining the operation | movement same as the above. It is explanatory drawing for demonstrating various operation | movement patterns same as the above. (A) is a graph which shows the specification of the D / A converter of a prior art example, (b) is a graph which shows the specification of the D / A converter of this embodiment. It is a schematic circuit diagram which shows the D / A converter of a prior art example.

  Hereinafter, an embodiment of a D / A conversion device according to the present invention will be described with reference to FIGS. The D / A converter according to the present invention is used in, for example, a programmable controller and outputs a drive signal to a load (for example, an analog device such as an inverter) connected via an output terminal.

  FIG. 1 is a schematic circuit diagram of a D / A converter A according to the present embodiment. The D / A converter A is a D / A converter (D / A converter circuit) that converts input digital data into an analog voltage. ) 2, an operational amplifier OP 1 that amplifies the analog voltage from the D / A converter 2, an output transistor (output circuit) Tr 1 that generates an output current I corresponding to the analog output from the operational amplifier OP 1, and an output transistor Tr 1 A resistor R1 (hereinafter referred to as resistor R1) that is connected to generate a potential difference Vr corresponding to the output current and a resistor R1 are connected between the base and the emitter, and a collector is the base of the output transistor Tr1. Is turned on when the potential difference Vr generated in the resistor R1 exceeds the on-voltage between the base and the emitter, and the output transistor Tr1 is turned on. An output cutoff transistor Tr2, an operational amplifier (voltage detection circuit) OP2 that detects the potential difference Vr generated in the resistor R1, and an A / D converter that converts the output of the operational amplifier OP2 into digital data and outputs the digital data to the CPU 1 described later ( (A / D conversion circuit) 3 and a CPU 1 for transmitting digital data to the D / A converter 2 by serial communication based on control information from the outside.

  The output terminal of the D / A converter 2 is connected to the non-inverting input terminal (+ terminal) of the operational amplifier OP1, and the inverting input terminal (− terminal) of the operational amplifier OP1 is connected to the ground GND via the resistor R6. The output terminal of the operational amplifier OP1 is connected to the base of the output transistor Tr1, and the 15V power supply is connected to the collector of the output transistor Tr1. Further, the output terminal of the operational amplifier OP1 is connected to the collector of the output cutoff transistor Tr2, the base of the output cutoff transistor Tr2 is connected to the emitter of the output transistor Tr1, and the emitter of the output cutoff transistor Tr2 is connected through the resistor R7. Are connected to the inverting input terminal of the operational amplifier OP1. One end of the resistor R1 is connected to the emitter of the output transistor Tr1, and the other end of the resistor R1 is connected to one output terminal T1. The other output terminal T2 is connected to the ground GND.

  The inverting input terminal of the operational amplifier OP2 is connected to the other end of the resistor R1 through the resistor R2, and is connected to the output terminal of the operational amplifier OP2 through the resistor R4. Further, the non-inverting input terminal of the operational amplifier OP2 is connected to the one end of the resistor R1 through the resistor R3, and is connected to the ground GND through the resistor R5, thereby forming a so-called differential amplifier circuit. The output terminal of the operational amplifier OP2 is connected to the analog input terminal AN of the A / D converter 3, and the output terminal of the A / D converter 3 is connected to the CPU1. That is, output data from the A / D converter 3 is input to the CPU 1. Note that a 5 V power source is connected to the analog reference voltage terminal AVdd of the A / D converter 3 and a ground GND is connected to the analog reference voltage terminal AVss. The potential difference Vr generated in the resistor R1 is a reference voltage of 5 V. It is converted into a digital value as a ratio to and input to the CPU 1.

  The CPU 1 converts the digital data of K0 to K4095 (K represents a decimal number) into the D / A converter 2 based on the control information from the outside, and the digital data from the A / D converter 3. Based on the current calculation means 13 for calculating the output current I of the present apparatus A and whether or not the calculation result of the current calculation means 13 exceeds a preset threshold value, and if the calculation result exceeds the threshold value, Current monitoring means 12 for limiting digital data written from the data writing means 11 to the D / A converter 2 to a predetermined value (a value such that the output current I is equal to or less than the continuous maximum allowable current) and a storage means 14 are provided. In the storage unit 14, a resistance value of the resistor R1, a threshold value of the detection current, and the like are registered in advance. Here, the data writing unit 11, the current monitoring unit 12, and the current calculation unit 13 are realized by software executed by the CPU 1.

  Here, the D / A converter 2 of this embodiment is a converter having a resolution of 12 bits, and outputs an analog voltage of 0 V to 2.5 V according to the input digital data K0 to K4095. The operational amplifier OP1 is set to a fourfold amplification factor, and as a result, a voltage V of 0V to 10V is output between the output terminals T1 and T2.

  Here, in the D / A converter A of the present embodiment, the potential difference Vr generated in the resistor R1 when the output current I that instantaneously flows through the output transistor Tr1 exceeds a predetermined instantaneous maximum allowable current is the output cutoff. The on-voltage of the transistor Tr2 is set to be exceeded, and the first overcurrent protection means is constituted by the output cutoff transistor Tr2. The current calculation means 13 and the current monitoring means 12 constitute a second overcurrent protection means, and the current monitoring means 12 is configured such that the output current I continuously flowing through the output transistor Tr1 is a predetermined continuous maximum. When the allowable current is exceeded, the digital data written from the data writing unit 11 to the D / A converter 2 is limited to a predetermined value so that the output current I becomes equal to or less than the continuous maximum allowable current.

  FIG. 2 is a flowchart for explaining the operation of the D / A converter A according to the present embodiment. FIG. 2A is output by the second overcurrent protection means (current calculation means 13 and current monitoring means 12). FIG. 2 (b) shows a main routine for limiting (current limiting), and FIG. 2 (b) shows an output limiting operation subroutine. In the main routine, the potential difference Vr generated in the resistor R1 in the operational amplifier OP2 is measured (step S1), and digital data corresponding to the measured value is input to the CPU 1 via the A / D converter 3. Then, in the CPU 1, the current calculation means 13 calculates the output current I from the resistance value of the resistor R1 registered in advance and the measured potential difference Vr (step S2), and then the current monitoring means 12 detects the limit current Il registered in advance. Are compared with the calculated output current I (step S3). As a result, if the output current I is smaller than the limit current Il, the process returns to step S1 to continue monitoring the current. If the output current I is greater than the limit current Il, the output limit shown in FIG. The process proceeds to an operation subroutine (step S4).

  Next, the output limiting operation subroutine will be described. In the following description, the output limiting operation when the digital value D1 = K4095 is written from the data writing means 11 of the CPU 1 to the D / A converter 2 will be described. . The current monitoring unit 12 calculates the analog voltage Vi of the D / A converter 2 from the digital value D1 instructed to the data writing unit 11 in step S5 (step S6). In this example, Vi = 2 × 1.25. X (D1 / 4095) = 2.5V. Next, the current monitoring unit 12 calculates the output voltage V from the calculated analog voltage Vi and the amplification factor of the operational amplifier OP1 (step S7), and in this example, V = Vi × 4 = 10V. Further, the current monitoring unit 12 calculates the load resistance Rz of the load L from the calculated output voltage V and the output current I (for example, 20 mA) calculated in the main routine (step S8), and in this example, Rz = V / I = 500Ω. Subsequently, the current monitoring unit 12 calculates a limit voltage Vl from the calculated load resistance Rz and the limit current Il (for example, 12 mA) (step S9), and in this example, Vl = Il × Rz = 6V. Furthermore, the current monitoring unit 12 converts the calculated limit voltage Vl into a digital value D2 (step S10), and in this example, D2 = Vl ÷ 4 ÷ 2 ÷ 1.25 × 4095 = 2457. Finally, when the converted digital value D2 is written to the D / A converter 2 from the data writing means 11 (step S11), a voltage of 6V is output between the output terminals T1 and T2. At this time, the output current I flowing through the output transistor Tr1 is limited to I = V / Rz = 12 mA.

  Here, for example, when a current exceeding the instantaneous maximum allowable current flows instantaneously in the output transistor Tr1 due to a short circuit or the like, the method using software as described above can effectively limit the current due to a delay in arithmetic processing. Although there is a possibility that it cannot be performed, in this embodiment, such an instantaneous maximum allowable current can be directly limited using the output cutoff transistor Tr2. Hereinafter, the output limiting operation when the output current I exceeds the instantaneous maximum allowable current will be described.

  When the output current I exceeds the instantaneous maximum allowable current, the potential difference Vr generated in the resistor R1 becomes larger than the on-voltage between the base and the emitter of the output cutoff transistor Tr2, and the output cutoff transistor Tr2 is turned on. When the output cutoff transistor Tr2 is turned on, the analog output of the operational amplifier OP1 is output to the output cutoff transistor Tr2 side, so that the output transistor Tr1 is turned off. When the output transistor Tr1 is turned off, the output voltage V to the load L is lowered, so that the output current I is also lowered. As a result, the potential difference Vr generated in the resistor R1 becomes smaller than the above-mentioned on voltage. When the output cutoff transistor Tr2 is turned off, the output transistor Tr1 is turned on again, and the drive signal is output to the load L again. Thereafter, the output voltage V is lowered by repeating the above operation, and an excessive load is prevented from being applied to the internal power supply circuit and the output transistor Tr1.

  Here, in the case of hardware, the electrical characteristics vary due to individual differences, so the detection current threshold varies, but in software there is no variation due to individual differences, etc. It can be made smaller. Further, in order to further reduce the variation in the threshold value of the detection current, it is important to accurately grasp the output current I. For that purpose, it is necessary to accurately grasp the resistance value of the resistor R1. . Therefore, in the present embodiment, a resistor R1 having a high accuracy and a small temperature drift is used, and the resistance value of the resistor R1 is measured by a tester and stored in the storage means 14.

  FIG. 3 shows various operation patterns when the output is limited by software. For example, in the example shown by the solid line a, when the output current I reaches the limit current Il1, it is instantaneously reduced to the threshold current Il2 (Il2 <Il1). That is, in this case, both the time t1 for holding the limit current Il1 and the time t2 required to reduce the limit current Il1 to the threshold current Il2 are both set to zero. Next, in the example shown by the solid line b, when the output current I reaches the limit current Il1, it is instantaneously decreased, so t1 = 0. However, the time t2 (t2> 0) from the limit current Il1 to the threshold current Il2 is set. Over time. Further, in the example shown by the solid line c, when the output current I reaches the limit current Il1, the output current I is held for a certain time (t1> 0) and then instantaneously decreased to the threshold current Il2 (t2 = 0). Finally, in the example shown by the solid line d, when the output current I reaches the limit current Il1, the output current I is held for a certain time (t1> 0) and then gradually decreased to the threshold current Il2 over the time t2 (t2> 0). Yes. That is, when there is an influence on an analog device and its load by abruptly limiting the current, for example, when a step-out occurs due to a rapid deceleration of the motor, as shown by the solid lines b to d above. It is desirable to take a control method.

  Here, since the on-state voltage of the output cutoff transistor Tr2 described above varies due to individual differences or temperature variations, the threshold value of the output transistor Tr1 is the value of the shaded portion e in FIG. As shown in FIG. For this reason, in the conventional D / A converter A that limits the current only by the output cutoff transistor Tr2, if the specification of the output allowable current of the device is to be increased, the continuous maximum allowable current is considered in consideration of the above fluctuation range. A large output transistor Tr1 is selected. As a result, it is necessary to use a large and expensive output transistor, resulting in an increase in size and cost of the device.

  On the other hand, since the D / A converter A according to the present invention uses software with little variation for the continuous maximum allowable current, for example, as shown in FIG. Even when the broken line h) in the figure is changed to 15 mA, the value of the continuous maximum allowable current is not changed, and the output transistor Tr1 can be downsized as compared with the conventional example. In addition, by increasing the specification of the allowable output current, the range of devices that can be connected is expanded, improving the convenience for the user, and further improving the detection accuracy of the overcurrent. Damage to the transistor Tr1, the resistor R1, the power supply IC, etc.) can be prevented, and as a result, a highly reliable D / A converter A can be provided. Note that a two-dot chain line g in FIG. 4B indicates a threshold value for limiting the output current to the continuous maximum allowable current or less. If the output current exceeds this threshold value, the software (current calculation means 13 and Current limiting is performed by the current monitoring means 12).

  In addition, when the specification of the allowable output current is set to the same value as the conventional example, a transistor having a continuous maximum allowable current smaller than that of the conventional example can be used, so that downsizing and cost reduction can be realized. it can. Furthermore, although there may be a case where the software cannot cope with an instantaneous overcurrent due to a delay in arithmetic processing or the like, in the present embodiment, as shown in FIG. Since the output cutoff transistor Tr2 is used, the current can be surely limited without delay due to arithmetic processing or the like. Note that a hatched portion f in FIG. 4B indicates a variation width of the threshold value of the output transistor Tr1 due to a change in the ON voltage of the output cutoff transistor Tr2.

  Here, a solid line j in FIG. 4B indicates an instantaneous overcurrent caused by, for example, a short circuit of the output terminal. In this case, when the output cutoff transistor Tr2 is turned on, the output transistor The current that flows instantaneously through Tr1 is limited to the threshold value f or less. Also, a solid line k in FIG. 4B indicates an overcurrent that continuously flows when an analog device having a large load current is connected. In this case, the current that continuously flows through the output transistor Tr1 It is limited to the threshold value g or less.

  The above-described embodiment is merely an example. If the instantaneous maximum allowable current is limited by an electronic circuit and the continuous maximum allowable current is limited by software, other It may be configured as follows.

1 CPU
2 D / A converter (D / A conversion circuit)
AD / A converter OP1 operational amplifier (amplifier circuit)
Tr1 output transistor (output circuit)
Tr2 output cutoff transistor (first overcurrent protection means)

Claims (3)

D / A conversion means for generating an output voltage corresponding to digital data, first overcurrent protection means configured by an electronic circuit to limit an instantaneously flowing output current to a predetermined instantaneous maximum allowable current or less, software And a second overcurrent protection means for limiting the continuously flowing output current to a predetermined continuous maximum allowable current smaller than the instantaneous maximum allowable current .
The D / A conversion means includes a D / A conversion circuit that converts input digital data into an analog voltage, an amplification circuit that amplifies an output voltage from the D / A conversion circuit, and an output voltage value of the amplification circuit And an output circuit that generates an output current and outputs it from the output terminal,
The first overcurrent protection means limits the output of the output circuit so that an instantaneously flowing output current is less than or equal to the instantaneous maximum allowable current,
Said second overcurrent protection means, D characterized that you limit the digital data output current continuously flows is input to the D / A converter circuit so as not to exceed the maximum continuous allowable current / A conversion device. The output circuit includes an output transistor that generates an output current according to an output voltage value of the amplifier circuit,
A current detection resistor connected to the output transistor for generating a potential difference according to an output current, a voltage detection circuit for detecting a potential difference generated in the current detection resistor, and a detection result of the voltage detection circuit as digital data And an A / D conversion circuit for converting to
The second overcurrent protection unit includes: a current calculation unit that calculates the output current based on digital data from the A / D conversion circuit; and whether a calculation result of the current calculation unit exceeds a preset threshold value And current monitoring means for outputting digital data to the D / A conversion circuit such that when the calculation result exceeds the threshold value, the output current is equal to or less than the continuous maximum allowable current. The D / A converter according to claim 1. The first overcurrent protection means includes an output cutoff transistor that turns on and turns off the output transistor when a potential difference generated in the current detection resistor exceeds an on-voltage between a base and an emitter. A D / A converter according to claim 2 .

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