JP2009043500A - Constant current circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a constant current circuit flowing current with a good response to a change of input signals. <P>SOLUTION: The constant current circuit comprises: a light source 1; a switching means 3 connected to the light source 1 for changeover between lighting on/lighting off of the light source 1; a resistor 4 for current detection connected to the switching means 3; a current value adjustment means 2 inputting a first voltage value A pulse-outputted from a control means, and a second voltage value in the resistor 4, adjusting the second voltage value by activating the switching means 3 so that the first and the second voltage values are to be nearly the same, and making a current flowing in the light source 1 nearly constant, and a rectifying element 5 connected to an output side of the current value adjustment means 2 and the resistor 4. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a constant current circuit that allows a desired current to flow through a light source based on an input signal to a current value adjusting means, and is particularly suitable when the input signal changes repeatedly.

As a conventional constant current circuit, for example, one disclosed in Patent Document 1 is known.
In the case of these constant current circuits, for example, a voltage waveform as shown in FIG. 3 can be obtained. As a circuit configuration, a pulse signal (input signal) is inputted as a first voltage value X from the control means, and an operational amplifier (current value adjusting means) used as a differential amplifier is provided. Connected, a light source and a collector as a load are connected, an emitter and a reference potential are connected via a resistor for current detection, and current amplification that controls a current flowing through the light source according to an input from the operational amplifier And a feedback circuit that feeds back the voltage generated by the resistor to the input part of the operational amplifier as a second signal, and a current corresponding to the differential potential of the input of the operational amplifier There was something to make it flow.
JP-A-8-297479

  However, according to the configuration of the constant current circuit, when the input signal (first voltage value) X to the operational amplifier is “L”, the output of the operational amplifier saturates to near the negative power supply voltage of the operational amplifier. There is. Due to this influence, the voltage fluctuation range between “H” and “L” of the output Y of the operational amplifier becomes large. For example, the input signal X changes from “L” to “H” due to the influence of the slew rate characteristic of the operational amplifier. When the output Y of the operational amplifier reaches the threshold value y1, that is, it takes time until the transistor is operated, and there is a problem in the response until the desired voltage waveform Z is obtained. It was. In particular, as shown in FIG. 4, in the case of a pulse with a short “H” time in the input signal X to the operational amplifier, the output Y of the operational amplifier does not reach the threshold value y1, and thus a desired current cannot be output. was there.

  Accordingly, an object of the present invention is to provide a constant current circuit capable of flowing a current with good responsiveness to a change in an input signal, which has been made by paying attention to the above-described problem. .

  The constant current circuit according to the present invention includes a light source, switch means connected to the light source for switching on / off of the light source, and a current detecting resistor connected to the switch means. And the first voltage value pulse-output from the control means and the second voltage value in the resistor, and the switch means so that the first and second voltage values are substantially equal. A current value adjusting means for adjusting the second voltage value by operating to make the current flowing through the light source substantially constant; a rectifying element connected to the output side of the current value adjusting means and the resistor; It is provided with.

  Further, according to claim 2, in the constant current circuit according to claim 1, one end of the illumination means is connected to a collector of the transistor comprising the switch means, and the resistor is connected to the emitter of the transistor. An output portion of an operational amplifier that is the current value adjusting means is connected to the base of the transistor, and a diode that is the rectifier element is connected between the base and emitter of the transistor.

  According to a third aspect of the present invention, there is provided a constant current circuit comprising a current value adjusting means comprising an operational amplifier for inputting a pulse signal as a first voltage value from the control means, and an output section of the current value adjusting means. The base is connected to the light source and the collector is connected to the load, the emitter and the reference potential are connected via a resistor for current detection, and flows to the light source according to the input from the current value adjusting means. Switch means comprising a current amplifying transistor for controlling current; a feedback circuit for feeding back the voltage generated by the resistor to the input portion of the current value adjusting means as a second signal; and a base and emitter of the switch means And a rectifying element made of a diode provided so that the base side is the cathode side.

  According to the present invention, in a constant current circuit that allows a desired current to flow through a light source based on an input signal to a current value adjusting means, it is possible to flow a current with high responsiveness to a change in an input signal.

  Embodiments to which the present invention is applied will be described below with reference to the accompanying drawings.

  An example in which the present invention is applied as a constant current circuit for controlling a current flowing in a light source for illumination used as a backlight of a liquid crystal display panel will be described. FIG. 1 is a diagram showing the electrical configuration of this constant current circuit, which is composed of a light source 1, a current value adjusting means 2, a switch means 3, a resistor 4, and a rectifying element 5.

  The light source 1 serves as a load of a constant current circuit. In this case, a plurality of light emitting diodes for transmitting illumination provided on the opposite side to the display surface of a liquid crystal display panel (not shown) are applied.

  The current value adjusting means 2 inputs a pulse signal from a control means (not shown) as a first voltage value, and inputs a voltage generated in the resistor 4 as a second signal, and the first and second voltage values Are output so as to operate the switch means 3 so that they are substantially equal, the output portion of the current value adjusting means 2 is connected to the base side of the switch means 3. The current value adjusting means 2 can adjust the second voltage value by operating the switch means 3 to make the current flowing through the light source 1 substantially constant. In this case, the current value adjusting means 2 is connected to the base side of the switch means 3 via a resistor provided in consideration of noise.

  The switch means 3 is connected to the light source 1 and switches on / off of the light source 1. The base is connected to the output part of the current value adjusting means 2, the load is connected to the light source 1 and the collector, and the emitter And a reference potential are connected via a resistor 4 for current detection, and a current amplification transistor that controls a current flowing through the light source 1 in accordance with an input from the current value adjusting means 2 can be applied.

  The resistor 4 is provided for current detection between the emitter side of the switch means 3 and the reference potential. The resistor 4 and the switch means 3 have a branch contact connected to the input part of the current value adjusting means 2, and the current value is adjusted using the voltage generated by the resistor 4 as the second signal. A feedback circuit 6 is provided so that it can output to the input part of the means 2.

  The rectifying element 5 can be a diode provided such that the wiring between the output part of the current value adjusting means 2 and the base of the switch means 3 and the feedback circuit 6 are connected, and the feedback circuit 6 side is an anode. . That is, the rectifying element 5 made of a diode is provided between the base and the emitter of the switch means 3 so that the base side is the cathode side.

  Next, an example of a voltage waveform for the pulse signal from the control means in the above-described circuit configuration will be described with reference to FIG.

  The current value adjusting means 2 receives the pulse signal from the control means as the first voltage value A, and clamps the negative voltage by the rectifying element 5 even when the first voltage value X is “L”. This prevents the output Y of the current value adjusting means 2 from being saturated to the negative power supply voltage of the current value adjusting means 2. For this reason, since the voltage fluctuation width between “H” and “L” of the output Y of the current value adjusting means 2 can be kept small, the first voltage value A changes from “L” to “H”. In addition, the light source 1 can be operated by outputting a desired current with high responsiveness. In particular, even when the input signal A to the current value adjusting means 2 is a pulse having a short “H” time, a desired current can be output with good responsiveness.

  Such a constant current circuit includes a light source 1, a switch means 3 connected to the light source 1 for switching on / off of the light source 1, a resistor 4 for current detection connected to the switch means 3, and a pulse output from the control means. The first voltage value A and the second voltage value in the resistor 4 are input, and the switch means 3 is operated so that the first and second voltage values are substantially equal to each other. Current value adjusting means 2 for making the current flowing through the light source 1 substantially constant, and a rectifying element 5 connected to the output side of the current value adjusting means 2 and the resistor 4. .

  Further, a current value adjusting means 2 composed of an operational amplifier that inputs a pulse signal as a first voltage value from the control means, and a base is connected to the output portion of the current value adjusting means 2, and a light source 1 and a collector serving as a load are connected. A switch means comprising a current amplifying transistor for controlling the current flowing through the light source 1 in response to an input from the current value adjusting means 2, the emitter being connected to the reference potential via a current detection resistor 4. 3 and the feedback circuit 6 that feeds back the voltage generated in the resistor 4 to the input portion of the current value adjusting means 2 as a second signal, and the base side between the base and the emitter of the switch means 3 is the cathode side. And a rectifying element 5 made of a diode.

  Therefore, by the action of the rectifying element 5, it is possible to flow a current with high responsiveness to changes in the input signal.

  The constant current circuit of the present invention has been described by way of example in the configuration of the above-described embodiment, but the present invention is not limited to this, and other configurations depart from the gist of the present invention. It goes without saying that various improvements and design changes can be made without departing from the scope.

The block diagram which shows the structure of embodiment of this invention. The figure which shows an example of the voltage waveform A, B, C of embodiment same as the above. The figure which shows an example of the voltage waveform X, Y, Z by a conventional circuit structure. The figure which shows another example of a voltage waveform same as the above.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 2 Current value adjustment means 3 Switch means 4 Resistor 5 Rectifier element A 1st voltage value

Claims (3)

A light source;
Switch means connected to the light source for switching on / off of the light source;
A resistor for current detection connected to the switch means;
The first voltage value pulsed from the control means and the second voltage value in the resistor are input, and the switch means is operated so that the first and second voltage values are substantially equal. Current value adjusting means for adjusting the second voltage value to make the current flowing through the light source substantially constant,
A constant current circuit comprising: a rectifying element connected to an output side of the current value adjusting means and the resistor.   One end of the illumination means is connected to the collector of the transistor comprising the switch means, the resistor is connected to the emitter of the transistor, and the output of the operational amplifier as the current value adjusting means is connected to the base of the transistor, 2. The constant current circuit according to claim 1, wherein a diode which is the rectifier element is connected between a base and an emitter of the transistor. Current value adjusting means comprising an operational amplifier for inputting a pulse signal as a first voltage value from the control means;
A base is connected to the output part of the current value adjusting means, a light source serving as a load and a collector are connected, and an emitter and a reference potential are connected via a resistor for current detection. Switch means comprising a current amplification transistor for controlling the current flowing through the light source in response to an input;
A feedback circuit that feeds back a voltage generated in the resistor to the input unit of the current value adjusting means as a second signal;
And a rectifying element comprising a diode provided between the base and emitter of the switch means so that the base side is the cathode side.

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