JPH0348468A - Luminous circuit - Google Patents

Abstract

PURPOSE:To adjust a temperature coefficient of a luminous circuit to a temperature coefficient on a light-receiving circuit to set up optional value by optionally adjusting the temperature coefficient of a driving current of a driving circuit adjusting the temperature coefficient of luminous efficiency of a luminous element. CONSTITUTION:By optionally adjusting value of resistors R1 to R3, a temperature coefficient of output voltage Vo of the whole of a light detection circuit, in which a light-emitting circuit A and a light-receiving circuit B are combined, is made zero. That is, assuming that a change portion of output voltage Vo when light is directed to a light-receiving element 3 of the light-receiving circuit to generate a light current I2 is Vo, a formula: Vo=I2'R6 is realized. A light current I2 flowing in the light-receiving element 3 over all flow through feedback resistor R6. In the light detection circuit of a combination of both circuits, when the whole portion of temperature fluctuation is to be made zero, the portion of temperature fluctuation of the current I of the luminous circuit ought to be properly adjusted.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a light emitting circuit, and is used, for example, as a light emitting circuit in a photoelectric smoke detector. [Prior Art] In conventional light emitting circuits, the light emitting elements and drive circuits have temperature coefficients, so the light emitting characteristics of the entire light emitting circuit change as the temperature changes. Figure 3 is a circuit diagram of a conventional light emitting circuit. The current ■ flowing through the light emitting element 1 made of a light emitting diode (LED) is I = (Vce − Vl− − Vsat)/R
−■ becomes. Here, VCC is the power supply voltage, VF is the +e direction voltage of the light emitting diode, and Vsat
is the saturation voltage of transistor Q. From this equation, the temperature characteristics of the current I are expressed by the equation 1 a I I aT , and there are two factors: the temperature change of the current I, which has the characteristics of the formula (■), and the temperature change of the light emission characteristics of the light emitting element 1 alone. The light emitting characteristics of the entire light emitting circuit are determined by these factors.
■As you can see from the equation, the temperature change in current I is Vcc,
Vp and Vsat, which are determined by the configuration of the power source and the structure of the light emitting element itself, and it is difficult to adjust their temperature characteristics to arbitrary values. Therefore, it is very difficult to arbitrarily set the light emitting characteristics of the entire light emitting circuit so as to match the temperature coefficient of the light emitting characteristics of the light emitting element 1. [Problems to be Solved by the Invention] The light emitting circuit as described above is used as an analog photodetecting circuit in combination with a light receiving circuit as shown in FIG. 2, for example. In such a photodetection circuit, if the temperature coefficients on the light emitting side and the light receiving side can be canceled out, it is thought that the overall temperature fluctuation can be reduced to almost zero. The present invention has been made in view of these points, and its purpose is to enable the temperature coefficient of the light emitting circuit to be set to an arbitrary value in accordance with the temperature coefficient of the light receiving circuit side. ．． [Means for Solving the Problems] In order to solve the above problems, in the light emitting circuit according to the present invention, as shown in FIG. It consists of an element 1 and a drive circuit 2 that supplies a drive current to the light emitting element 1, and is provided with means for arbitrarily adjusting the temperature coefficient of the drive current of the drive circuit 2 in accordance with the temperature coefficient of the luminous efficiency of the light emitting element 1. It is characterized by this. [Function] When using the light emitting circuit shown in FIG. 1 in combination with the light receiving circuit shown in FIG. Assuming 700ppet/"C, the temperature coefficient of the light receiving circuit for the amount of light received is 6700ppm/"
'C. Therefore, if the temperature coefficient of the amount of light emission in the light emitting circuit is -6700 ppm/''C, the output voltage Vo of the photodetection circuit will not vary depending on the ambient temperature.A light emitting diode (LED) is used as the light emitting element 1.
, the luminous efficiency (the amount of light emitted with respect to the drive current I1) decreases as the temperature rises, and if the temperature coefficient is, for example, -100000ppm/'C, the drive current by the drive circuit 2 decreases. The temperature coefficient of I, is 3 3 0
It is sufficient to set it to 0 ppm/"C. Specifically, by arbitrarily adjusting the values of the resistors R1 to R shown in FIG.
The temperature coefficient of the output voltage Vo of the entire photodetection circuit, which is a combination of the light emitting circuit shown in FIG. 1 and the light receiving circuit shown in FIG. 2, can be made zero. [Embodiment] Figure 1 is a circuit diagram of an embodiment of the present invention. Luminous signal S
is applied to the gates of PMOS transistor Q and NMOS transistor Q, and its logic value is inverted by inverter N, and the NMOS transistor Q,,Q
．． . is applied to the gate of Light emission signal S is ゜'Hi
gh” level, the PMOS transistor Q, turns off and the NMOS transistor Q8 turns on. As a result, a current mirror circuit consisting of the PMOS transistor Q, Q, the NMOS transistor Q, and the resistor R5 operates. In addition, the output of inverter N is “L os
” level, so NMOS} transistor Q..Q,
. is off. As a result, the NPN transistor QQ
2. Q. ．． Q. and resistances R + , R 2 , R
Current is supplied to a bandgap reference circuit consisting of s. As a result, a voltage VB expressed by the equation is generated at point B of the circuit shown in FIG. Here, V. E, is the forward voltage between the base and emitter of transistor Q, and VT is k
When is the Boltzmann constant, q is the electron charge, and T is the absolute temperature, it is expressed by the following formula. V T = kt/q
...■Here, since the NPN transistor Q5 is operating in a non-saturated state, the voltage between its base and emitter is set to V.
BE, then a voltage of VC:VB-vBE5 is generated at point C, so in the end, the light emitting element 1 has I
1 = Vc/R 4= (Va VBES)/
A constant current of R 4 -■ flows. On the other hand, when the light emission signal S reaches the "Low" level, the transistor Q is turned on and the transistor Q is turned off.
Transistor Q. ,Q,,Q. No current flows through the current mirror circuit formed by the transistors Qs. By turning on Q+o, transistor Q
．． ．． No base current is supplied to the base of Q5, both are in an off state, and no current flows through the light emitting element 1. Here, if the change in the output voltage V0 when the light receiving element 3 of the light receiving circuit shown in FIG. 2 is irradiated with light and the photocurrent I2 is generated is v0, then vo=I2・R6...
■It becomes. In Fig. 2, 4 is an operational amplifier and 5 is a reference voltage source.Since the input impedance of the operational amplifier 4 is very high, all of the photocurrent (2) flowing to the photodetector 3 flows through the feedback resistor R. ,■Equation holds true. From this formula, V. The temperature variation is as follows. Here, the light receiving element 3 is a photodiode,
The resistor R6 is made of a diffused resistor of the integrated circuit, and its temperature fluctuation (1/I2) (θr./θT) and <1/R
i) (θR./θT) each at 3000 ppm/”C
and 3700 ppm/'C, the output voltage v0
The temperature variation (1/v0) (δvo/θT> is 670
0 ppm/''C.In addition, in a photodetection circuit in which the light emitting element 1 shown in Fig. 1 is a general light emitting diode (LED) and the circuits shown in Figs. 1 and 2 are combined, the overall temperature If you want to make the fluctuation part zero, you can make the temperature fluctuation part of the current I1 of the light emitting circuit 3300 pp so/℃.Now, since I1 is expressed by the formula ■, from this, 1 θ
L I1 δT Also, from equation (2), here, the resistance R is a discrete component whose temperature fluctuation can be ignored, and VBE3=VsEs-0 . 7v
, θV BE)/a T−θVBes/θT= 2
+*V/℃, θVT/δT=0.085mV/”C and L
, the values of resistors R1 to R are R+=3KΩ, R2=30KΩ
, R3=IKΩ, then from formula (①) VB = 2.5V... [phase] Also, from formula (①), 3. 8 7a+V/ 'C ・
... Substituting the ■ [phase] formula and the ■ formula into the ■ formula gives ζ 3 3 0 0 ppm/''C. As mentioned above, by appropriately selecting the values of the resistors R1 to R, the first In a photodetection circuit that combines the circuits shown in FIGS. Since the temperature fluctuation can be easily set to any value according to the temperature fluctuation of the light-receiving circuit, it has the effect of easily realizing a photodetector circuit with less overall temperature fluctuation when combined with the light-receiving circuit.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of a light emitting circuit according to an embodiment of the present invention, Fig. 2 is a circuit diagram of a light receiving circuit used in combination with the above light emitting circuit, and Fig. 3 is a circuit diagram of a conventional light emitting circuit. be.

Claims (1)

[Claims] (1) Consisting of a light emitting element that emits light with a brightness that corresponds to the drive current, and a drive circuit that supplies the drive current to the light emitting element, the temperature of the drive current of the drive circuit is adjusted according to the temperature coefficient of the luminous efficiency of the light emitting element. A light emitting circuit characterized in that a means for arbitrarily adjusting a coefficient is provided.