JP2000013193A - Constant current circuit and pulse width conversion circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pulse width conversion circuit which can obtain a signal having highly stable pulse width even if it is made into IC by realizing a stable constant current source circuit against a temperature change. SOLUTION: A constant current circuit is provided with a current mirror-type constant current circuit having a terminal to which an external reference resistance deciding the current value of the constant current circuit is connected and a voltage feedback circuit which voltage-feeds back the current mirror-type constant current circuit so that the voltage of the terminal becomes equal to reference voltage. Another feature of the circuit resides in a ramp waveform generation circuit 10 formed of the consent current circuit, a capacitor and a switch element and in a pulse width conversion circuit which is provided with a comparator 11 and which incorporates IC. The current circuit can obtain stable current against a temperature change and the operation of the pule width conversion circuit is stabilized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a constant current circuit and a pulse width conversion circuit, and more particularly to a constant current circuit that is stable against temperature changes and a pulse width conversion circuit using the constant current circuit.

[0002]

2. Description of the Related Art Conventionally, for example, when a transmission signal from a transmitter is received by a receiver, and a circuit in the receiver is operated based on the received signal, the receiver side satisfies the requirements of the circuit based on the received signal. It may be required to convert to a satisfying signal. In such a case, a pulse width conversion circuit is provided on the receiver side. This pulse width conversion circuit converts a transmission signal having an unspecified pulse width into a pulse having a constant width which meets the specifications of the receiver.

As the pulse width conversion circuit, for example, a mono-multi circuit is used. This mono-multi circuit generates a ramp waveform based on an input signal,
Predetermined reference voltage value (threshold level)
Are compared by a comparator to generate a signal of a predetermined width. Further, the pulse width of the output signal can be controlled by controlling the threshold level.

The ramp waveform generating circuit is composed of, for example, a constant current source and a capacitor, and generates a ramp waveform by charging a capacitor discharged by an input signal with a constant current source. In this ramp waveform generation circuit, the stability of the pulse width of the output signal is determined by the stability of the constant current source. In order to obtain a stable output on the receiving circuit side, it is desirable that the pulse width of the control signal generated by the pulse width conversion circuit does not change even if environmental conditions, for example, temperature change. For this purpose, a stable constant current source whose current value hardly changes even when the temperature changes is required.

[0005]

In recent years, high-density mounting technology has been advanced, and components have been rapidly reduced in size. For this reason, the above-mentioned pulse width conversion circuit is also integrated, and is often incorporated as a negative part of an IC. However, when the conventional pulse width conversion circuit as described above is integrated into an IC as it is, the circuit elements are configured in the IC. Therefore, a current source that is stable against temperature changes due to the temperature characteristics of the elements in the IC. Is difficult to achieve.

An object of the present invention is to solve the above-mentioned problems of the prior art and to realize a constant current source circuit that is stable against a temperature change, thereby forming a pulse width conversion circuit in an IC. Another object of the present invention is to provide a pulse width conversion circuit capable of obtaining a signal having a highly stable pulse width.

[0007]

According to the present invention, there is provided a constant current circuit comprising:
A current mirror type constant current circuit having a terminal to which an external reference resistor for determining the current value of the constant current circuit is connected; and a current mirror type constant current circuit so that the voltage of the terminal becomes equal to the reference voltage. And a voltage feedback circuit for performing feedback. There is also a feature in a built-in IC type pulse width conversion circuit provided with the constant current circuit.

According to the present invention, voltage feedback is applied to the current mirror type constant current circuit and a reference resistor is externally provided, so that the current value of the constant current circuit is affected only by the temperature characteristics of the reference resistor. . Then, by selecting and connecting a resistor having a good temperature coefficient, it is possible to greatly reduce the temperature change of the resistance value, and it is possible to obtain a stable current with respect to the temperature change.

[0009]

Embodiments of the present invention will be described below in detail. FIG. 1 is a block diagram showing a mono-multi circuit which is an example of a pulse width conversion circuit. This mono-multi circuit generates a ramp waveform by a ramp waveform generation circuit 10 based on an input signal, and compares the ramp waveform with a predetermined voltage value (threshold level) of a reference voltage source 12 by a comparator 11. To generate an output pulse signal having a predetermined width. Further, the pulse width of the output signal can be controlled by controlling the threshold level. The desired control signal is generated by using a logic IC (exclusive NOR) 13 and calculating the difference between the output signal and the input signal. In this method,
If the pulse width of the output signal is stable, a control signal having a constant pulse width can be obtained regardless of the pulse width of the input signal.

FIG. 2 is a block diagram showing a general circuit configuration of the ramp waveform generating circuit. The ramp waveform generating circuit includes a constant current source 20 and a capacitor 21, and the constant current source 20 charges the capacitor 21 to generate a ramp waveform. The switch element 22 is turned on only while the input pulse is “L”, for example.

FIG. 3 is a waveform diagram showing waveforms at various parts of the mono-multi circuit of FIG. When a negative logic pulse is input as an input signal, the switch element 22 is first turned on during the period in which the input pulse is “L”, and the electric charge of the capacitor 21 is discharged. Next, in synchronization with the rising timing of the input pulse, the switch element 22 is turned off, and charging of the capacitor 21 by the constant current source 20 starts. By this operation, a voltage output proportional to the product of the current value of the current source 20 and time is obtained, and this exhibits a ramp waveform.

The ramp waveform is compared with a voltage value (threshold level) of a reference voltage source 12 by a comparator 11 to generate an output pulse signal having a predetermined width. FIG.
In the ramp waveform generation circuit shown in FIG. 1, the stability of the pulse width of the output signal is determined by the stability of the constant current source 20.

FIG. 4 is a circuit diagram showing a circuit configuration of a general current source used in the ramp waveform generating circuit. The constant current source circuit of FIG. 4 basically includes transistors Q2, Q3,
This is a circuit called a current mirror circuit including Q4 and Q5 and resistors R2 and R3. The current value I of the current source is represented by a resistor R
It is determined by the potential at points 2 and x. Further, the potential at the point x is determined by the voltage value of the bias power supply V2 and the base-emitter voltage VBE of the transistors Q2, Q4, Q5. By connecting the transistor Q1 and the resistor R1 having the same characteristics as those of the transistor Q2, a current source having a current value I ′ determined by the potential at the point x and the resistance value of the resistor R1 can be obtained. Note that the capacitor C and the transistor Q10 correspond to the capacitor 21 and the switch element 22 in FIG. 2, respectively.

Here, even if the bias power supply V2 is stable with respect to temperature, the base-emitter voltage VBE of the transistor has a temperature dependency, so that when the temperature is changed, the potential at the point x changes. I will. Further, the circuit is
In the case of the configuration in C, if the temperature is changed due to the temperature characteristics of the elements in the IC, the resistance value of the resistor R2 also changes. Therefore, the current value I of the current source is
It cannot be said that it is stable against temperature changes, and in such a pulse width conversion circuit using a current source,
A sufficiently stable pulse width cannot be realized.

FIG. 5 is a circuit diagram showing a circuit configuration of the pulse width conversion circuit of the present invention. This circuit is different from the constant current circuit shown in FIG. 4 in that transistors Q6, Q7, Q8, Q
9. A voltage feedback circuit including a current source J is added. Further, the connection position of the bias power supply V2 is changed. Further, an external reference resistor R is also added.

In the constant current circuit shown in FIG. 5, since the voltage feedback is applied, the constant current circuit operates so that the potential at the point z is always the same as the potential at the point y. The current value I of the current source is intentionally determined by the potential at the point z and the resistance value of the reference resistor R connected thereto. By connecting a transistor Q1 and a resistor R1 having the same characteristics as Q2 to this constant current circuit, a current source having a current value I 'determined by the current value I and the resistance value of the resistor R1 can be obtained. Here, if the bias power supply V2 is stable with respect to the temperature, voltage feedback is applied, so that the potential at the point z is stable without depending on the temperature. Therefore, the current value I
Is affected only by the temperature characteristics of the reference resistor R.

Generally, a resistance element in an IC has a temperature coefficient of about 250 ppm / ° C. with respect to a resistance value,
This effect cannot be avoided if the reference resistor R is realized by an element in the IC. Therefore, in the circuit of the present invention, in order to minimize this effect, the reference resistance R is set to IC
It has a circuit configuration that can be connected as an external component instead of inside.

As a result, it is possible to select and connect parts having a good temperature coefficient, and it is possible to suppress the temperature change of the resistance value to about 1/5 as compared with the case where the resistance element in the IC is used. become. Therefore, a stable current can be obtained as a current value I with respect to a temperature change.

As a specific numerical example, when a general current source circuit shown in FIG. 4 is formed in an IC, for example,
When the desired control signal pulse width is 800 ns, the stability of the pulse width in the pulse width conversion circuit is from −25 ° C. to +
For a temperature change of 75 ° C., it is about 800 ns ± 50%. On the other hand, in the pulse width conversion circuit of the present invention, when a chip resistor having a temperature coefficient of 50 ppm / ° C. is used as the reference resistor R, the resistance is 8 under the same conditions as above.
It is possible to achieve 5% of 00 ns soil and obtain 10 times the stability.

Although the embodiment of the present invention has been disclosed above, the constant current circuit of the present invention is applicable not only to the pulse width conversion circuit but also to any circuit requiring a stable current with respect to temperature change. is there.

[0021]

As described above, in the pulse width conversion circuit of the present invention, voltage feedback is applied to the constant current circuit which controls the stability of the pulse width, and the current value of the constant current circuit is further reduced. The circuit configuration using the determined reference resistor as an external component has an effect that it is possible to realize a pulse width conversion circuit with a built-in IC having a stability about 10 times as large as that of a conventional circuit against temperature changes. Therefore, if the pulse width conversion circuit of the present invention is applied to, for example, a video camera or the like, even when the video camera is operated in synchronization with a transmission signal from a transmitter, an operation having high stability against a temperature change can be performed. Can be guaranteed.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a mono-multi circuit as an example of a pulse width conversion circuit.

FIG. 2 is a block diagram showing a general circuit configuration of a ramp waveform generation circuit.

FIG. 3 is a waveform chart showing waveforms at various parts of the mono-multi circuit of FIG. 1;

FIG. 4 is a circuit diagram showing a circuit configuration of a general current source.

FIG. 5 is a circuit diagram showing a circuit configuration of a pulse width conversion circuit of the present invention.

[Explanation of symbols]

10: ramp waveform generation circuit, 11: comparator, 12
... Reference voltage source, 13 ... Logic IC (Exclusive NO
R), 20: constant current source, 21: capacitor, 22: switch, Q1 to Q10: transistor, R, R1 to R3: resistor, C: capacitor, V1, V2: voltage source, J: current source

Claims (2)

[Claims] 1. A current mirror type constant current circuit having a terminal to which an external reference resistor for determining a current value of a constant current circuit is connected, and said current mirror so that a voltage at said terminal becomes equal to a reference voltage. A constant current circuit comprising: a type constant current circuit; and a voltage feedback circuit for applying voltage feedback to the constant current circuit. 2. A current mirror type constant current circuit having a terminal to which an external reference resistor for determining a current value of a constant current circuit is connected, and said current mirror type constant current circuit having a terminal voltage equal to a reference voltage. A voltage feedback circuit for applying voltage feedback to the constant current circuit; a capacitor connected in series with the current mirror type constant current circuit; discharge means for discharging the charge of the capacitor by a signal from the outside; and both ends of the capacitor. A pulse width conversion circuit with a built-in IC, comprising: comparison means for comparing a voltage with a reference voltage.