JPS61159822A - Driving circuit of light emitting diode - Google Patents

Abstract

PURPOSE:To decrease the variation of the light emitting quantity of an LED caused by the variation of an ambient temperature, to the extent that it does not exert an influence on the output of a photosensor using this LED as a light source, by inputting the voltage output of a temperature variation detecting circuit and the voltage of a constant-voltage source to the base of a difference voltage driving transistor. CONSTITUTION:An operational amplifying circuit 3 inputs the voltage output of the temperature variation detecting circuit 1 and the constant-voltage source 2, brings its difference voltage to an operational amplification and outputs it. An LED driving transistor 4 increases or decreases a current flowing through an LED by inputting the output of the operational amplifying circuit 3 through a base resistance 5, and maintains the light emitting quantity of the LED by the variation of an ambient temperature, so that it is constant. As for the operational amplifying circuit 3, an inversion amplifying circuit or a non-inversion amplifying circuit is used.

Description

[Detailed Description of the Invention] [Prior Art] The present invention relates to a drive circuit for a light emitting diode used as a light source for an optical sensor such as an optical rotary encoder, an optical position detection means for an electronic commutator motor, or an optical FG signal detection means. It is related to. A conventional circuit of this type will be explained with reference to FIG. Figure 4 (in al.
A%B is a positive and negative DC power supply terminal, R is a resistor, and LED is a light emitting diode. Now, the voltage between AB is vcclLE
The forward voltage of D is VFS, the forward current is IFt, the resistance value is RO
Then, the following formula holds true.

Therefore, when the temperature is at room temperature (26C), Vcc = 5°vy = 1.565v, and Ro = asoΩ, a current of 9.8 mA flows and light is emitted. Figure 4(b) shows the case where multiple LEDs are connected in series, and Figure 4(C) shows the case where multiple LEDs are connected in series and parallel.The basic operation is shown in Figure 4. There is no difference from (a). (Reference: Sei Denda, ed., "Optical, thermal and strained semiconductor elements and their usage," p. 20, published by Kyoritsu Publishing Co., Ltd., July 1, 1970) [Object of the invention] By the way, in the configuration shown in Fig. 4 above, ,
The current I flowing through the LED varies depending on the ambient temperature Ta, as shown in FIG. Furthermore, as shown in FIG.
The amount of light emitted by JD is directly proportional to I. Figure 7 shows the ambient temperature T
9. Shows the relationship between a and the amount of light emitted by the LED. If the amount of light emitted by the LED is 1 at 25C, it is approximately 1 at OC.
．． 45.507:', it is approximately 0.85. In this way, when the current I of the LED changes due to a change in the ambient temperature, the amount of light emitted by the LED changes. This change in the amount of light emitted from the LED affects the output of an optical sensor such as a rotary encoder that uses the LED as a light source, causing S/N deterioration of the output signal and malfunction.

In view of this point, the present invention provides that the LE due to changes in ambient temperature
It is an object of the present invention to provide an LED drive circuit that can reduce changes in the amount of light emitted by D to an extent that does not affect the output of a photosensor that uses this LED as a light source.

[Structure of the Invention] The light emitting diode drive circuit of the present invention includes:
A DC power supply that connects a light emitting diode and its driving transistor in series, a temperature change detection circuit connected to the DC power supply, a constant voltage source, a voltage output of the temperature change detection circuit, and a voltage of the constant voltage source. and an arithmetic amplification circuit consisting of an inverting amplification circuit or a non-inverting amplification circuit which inputs the voltage and outputs the difference voltage, and by inputting the output of the arithmetic amplification circuit to the pace of the driving transistor. The present invention is characterized in that the current flowing through the light emitting diode is controlled.

FIG. 1 shows the configuration of a drive circuit according to the present invention. In the figure, l is a temperature change detection circuit, 2 is a constant voltage source, 8 is an arithmetic amplification circuit consisting of an inverting amplification circuit or a non-inverting amplification circuit,
4 is a transistor for driving the LED, and 5 is its pace resistor. A zero arithmetic amplification circuit 8 inputs the voltage output of the temperature change detection circuit 1 and a constant voltage source 2, and amplifies and outputs the difference voltage. . The LED driving transistor 4 is an arithmetic amplifier circuit 3
By inputting the output of L through the pace resistor 5,
Increase or decrease the current flowing through the ED, and reduce the LE due to changes in ambient temperature.
This is intended to keep the amount of light emitted by D constant. 7th
As shown in the figure, since the amount of light emitted by the LED (relative ratio) has a negative temperature coefficient, the present invention performs temperature compensation by giving the current flowing through the IJD a positive temperature coefficient.

[Embodiment] An embodiment of the present invention will be described with reference to FIG.

FIG. 2 shows a case where an inverting amplification circuit 8a is used as the arithmetic amplification circuit 8. The temperature change detection circuit 1 is constructed by connecting two resistors 6.7 having different temperature coefficients in series. The connection point P (output terminal of the temperature change detection circuit) between these two resistors 6 and 7 is connected to the inverting input terminal of the inverting amplifying circuit 8a via the resistor 8, and the inverting input terminal of the inverting amplifying circuit 8a is A resistor 9 is connected between the input terminal and the output terminal Q.

Next, explain the effect using specific numbers.The resistance value R5 of 0 resistance 5 is changed to 18Ω (temperature coefficient + 800 pprrV/C
), the resistance value of resistance 6 is 10 or more temperature coefficient + z000
pp), and the resistance value R1 of resistor 7 is set to 10kO (temperature coefficient +
800 pplt'), the resistance value R8 of resistor 8 is 50% (temperature coefficient +800ppw/c), the resistance value R0 of resistor 9 is
is 500kQ (temperature coefficient +800pprt?), and current amplification rate Ht ZOO of transistor 4 (VaIIi=0.
7v temperature coefficient -2 mV/C), apply Vcc = 5v (constant) between power supply terminals AB, and apply the voltage of constant voltage source 2 to V, = Vcc/2 = 2.5 v (constant)
shall be. The offset voltage and its temperature coefficient of the inverting amplification circuit 8a and the temperature coefficient of the current amplification rate H of the transistor 4 are ignored. The voltage at point P is vt, 2.5 at 25C
Set to v. Let the voltage at point Q be vs, and transistor 4
If the pace current is Ins and the collector current (current flowing through the LED) is IC, then the following equation holds true.

Using the above four formulas (1) to (4), the ambient temperature is 125C.
125C,? When Ic is calculated for the 8 points of 5tl', the numbers shown in the following table are obtained.

FIG. 8 shows an embodiment of the present invention in which a non-inverting amplification circuit 8b is used as the arithmetic amplification circuit 8, but its structure and operation are substantially the same as the embodiment shown in FIG. 2, which uses an inverting amplification circuit 8a. Since there is no difference, the explanation will be omitted.

[Effects of the Invention] As described above, the present invention can detect temperature changes in the output voltage (V, ) of the temperature change detection circuit 1, temperature changes in the output voltage (■,) of the arithmetic amplification circuit 8, and the base resistance. 5 and the temperature change in VIK of the LED driving transistor 4, the current Ic flowing through the LED increases by +0.0.
It can have a temperature characteristic of 116/l:', L
The temperature characteristic of -0,011/C in the amount of light emitted by the ED can be corrected to a flat rate, which has an excellent effect of keeping the amount of light emitted by the LED constant regardless of changes in the ambient temperature. .

[Brief explanation of the drawing]

Figure 1.2 Block diagram showing the configuration of the LED driving circuit of the present invention Figure 2: Block diagram showing the configuration of an embodiment of the present invention using an inverting amplification circuit as the arithmetic amplification circuit Figure 8: Operation amplification circuit Block diagram showing the configuration of an embodiment of the present invention using a non-inverting amplification circuit in the circuit Figure 4: Conventional LED drive circuit (a) (b) (C)
Figure 5: Graph showing temperature changes in LED voltage and current characteristics Figure 6: Graph showing the relationship between LED current and light emission amount Figure 7: Relationship between temperature and light emission amount of conventional LEDs Graph showing ■... Temperature change detection circuit, 2... Constant voltage source, 8...
・Arithmetic amplification circuit, 3a... Inversion amplification circuit, 8b...
・Non-inverting amplifier circuit, 4...LED driving transistor, 5.6.7.8.9...Resistor, AlB...DC power supply, LED...Light emitting diode Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 LED page voltage VF (V)

Claims (2)

[Claims] (1) A DC power supply that connects a light emitting diode and its driving transistor in series, a temperature change detection circuit connected to the DC power supply, a constant voltage source, a voltage output of the temperature change detection circuit, and the constant voltage source. and an arithmetic amplification circuit consisting of an inverting amplification circuit or a non-inverting amplification circuit which inputs the voltage of A driving circuit for a light emitting diode, characterized in that the current flowing through the light emitting diode is controlled by inputting the current to the light emitting diode. (2) The temperature change detection circuit is configured by connecting two resistors having different temperature coefficients in series, and the connection point of these two resistors is used as its output terminal. Drive circuit for light emitting diode in range (1)