JPS63229792A - Temperature compensated light emitting element drive circuit - Google Patents

Abstract

PURPOSE:To reduce the power consumption of a whole circuit by applying the output signal of a difference detector as a control signal to a light emitting element drive current controller. CONSTITUTION:A switching circuit 20 which closes and opens in response to an input signal is provided, and connected in series with a light emitting element drive current controller 10 connected in series with a light emitting element. A current flowing to the element is detected by a current monitor 30, an environmental temperature is simultaneously detected, the detection signals are added, the difference from a reference value is detected by a difference detector 40, and a difference signal is fed back to the controller 10 to control the energization of the element. That is, when the environmental temperature varies during the lighting of the element, the output of a temperature monitor 35 is varied to control to feed back to cancel the light emitting power of the element, i.e., the variation in the light emitting intensity. Thus, it can prevent a wasteful current consumption.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field 1) The present invention relates to a circuit that drives a light emitting element such as an LED or a semiconductor laser to emit light using a digital signal. Regarding compensation type circuits.

[Prior Art] Various optical communication systems have been developed in the past, but in a light-emitting element drive circuit that converts the digital signal into an optical signal and transmits it by driving the light-emitting element with a digital signal, Temperature compensation may be performed to prevent the light emitting power of the light emitting element from changing due to changes in ambient temperature. As such a conventional circuit, an auto power control circuit shown in FIG. 3 has been used.

In the plS3 diagram, 66 is L used as a light emitting element.
ED, 71 is a photodiode used as a light receiving element. The light emitted from the light emitting element 66 is split into a light branch 68
Here, the output light 69 and the signal light 7 for monitoring are
It is divided into 0 and 0. The signal light 70 for monitoring is transmitted to the light receiving element 7
1, the output power is detected by the output power monitor circuit 72. The output circuit of the output power monitor circuit 72 is given to a control signal generation circuit 74, which compares the detected output power with a reference value, creates a control signal to compensate for the difference, and sends it to the light emitting element drive current control circuit 65. Send and control this.

That is, in the conventional circuit, the intensity of the emitted light is measured,
This was controlled by feedback.

[Problem to be Solved by the Invention 1] Such a conventional light emitting element drive circuit is configured to measure and detect the intensity of emitted light and feed it back, so it requires a light branch and a light receiving element, resulting in a complicated structure and high cost. rise,
This caused problems such as the need for maintenance and inspection. Furthermore, since an optical branch is inserted into the optical path of the emitted light, there is also the drawback that the transmission output is reduced by the amount of insertion loss. Particularly in the case of a digital signal in which the duty ratio of the input signal is not constant, it is very difficult to determine a reference value because the duty ratio of the output light changes accordingly. Therefore, in practice, it could only be used when the duty ratio was constant or when the signal was continuous.

[Means for Solving the Problems] In order to overcome the drawbacks of the conventional light emitting element drive circuit, the present invention provides a system that responds to human input signals as shown in block diagram 7 of FIG. A switching circuit 20 that opens and closes is provided, and this switching circuit 20 is connected in series to the light emitting element drive current control circuit 10 connected in series to the light emitting element, and the current flowing through the light emitting element is detected by the current monitor circuit 30. At the same time, the ambient temperature is detected, and after adding these detection signals, the difference detection circuit 40 detects the difference from the reference value, and this difference signal is fed back to the light emitting element drive circuit 10 to energize the light emitting element. It controls the

That is, according to the present invention, there is provided a light emitting element drive circuit that controls energization to a light emitting element according to an input signal, the light emitting element drive current control circuit controlling energization to the light emitting element, and the light emitting element drive current. A switching circuit that is connected in series to the control circuit and opens and closes according to the input signal, a current monitor circuit that detects the amount of current flowing to the light emitting element, a temperature monitor circuit that detects the ambient temperature, and a current monitor circuit that detects the ambient temperature. An addition circuit that adds the signal and the signal from the temperature monitor circuit, a reference voltage generation circuit that generates a reference voltage, and a difference detection circuit that outputs a signal according to the difference between the signal voltage from the addition circuit and the reference voltage. There is provided a temperature-compensated light emitting element drive circuit characterized in that the output signal of the difference detection circuit is provided as a control signal to the light emitting element drive current control circuit.

[Function j] Since the present invention has the above-described configuration, if there is a change in the ambient temperature while the light emitting element is lit, the output of the temperature monitor circuit changes to compensate for the change in the light emitting power of the light emitting element, that is, the light emitting intensity. This makes it possible to perform feedback control to ensure that Furthermore, when the input signal is at H level, the drive current of the light emitting element is kept constant by feedback control, and when the input signal is at L level, both the switching circuit and the light emitting element drive current control circuit are de-energized. It is possible to prevent unnecessary current consumption.

[Example] The present invention will be explained in more detail with reference to Examples below.

FIG. 2 is a circuit diagram showing an embodiment of the temperature compensated light emitting element driving circuit of the present invention. In FIG. 2, 12 is a light emitting element, such as an LED or a semiconductor laser. Reference numeral 14 denotes a transistor for controlling energization of the light emitting element 12, and its collector-emitter path is connected in series to the light emitting element. In the illustrated embodiment, seven nodes of the light emitting cable 12 are connected to the DC power supply terminal +■, and the cathode is connected to the transistor 1.
4 collector. 16 is transistor 1
4, and transmits an output signal of a differential amplifier 42, which will be described later. The portion consisting of the light emitting element 12, transistor 14, and resistor 16 corresponds to the light emitting element drive current circuit 10 in FIG. 22 is a transistor whose collector-emitter path is connected to the transistor 1
It is connected in series to the collector-emitter path of No. 4.

The base of transistor 22 is connected to input terminal 60. This transistor Z2 corresponds to the switching circuit 20 in FIG.

A series circuit of a diode 36 and a resistor 32 is connected between the emitter of the transistor 22 and ground.

This resistor 32 corresponds to the current monitor circuit 30 in FIG.
A voltage Vm proportional to the current flowing therethrough is generated between both terminals. On the other hand, the diode 36 corresponds to the current monitor circuit 35 in FIG. 1, and generates a voltage Vt across it that is inversely proportional to the ambient temperature. A general silicon diode is used as such a diode. A connection point between the emitter of the transistor 22 and the resistor 32 is connected to an inverting input terminal (-) of an operational amplifier used as the differential amplifier 42. On the other hand, the non-inverting input terminal (+
) is a resistor 5 that constitutes the reference voltage generation circuit 50 in FIG.
A reference DC voltage Vr is provided from a series circuit of 2 and a Zener diode 54. This series circuit is connected between the DC voltage terminal +V and ground as shown.

Next, the operation of the embodiment shown in FIG. 2 will be explained. A digital signal is input to the input terminal 60 from a circuit not shown. Now, if this digital signal is at H level, the transistors 14 and 22 become conductive, and the light emitting element 12
Drive current ■ flows through. Since this drive current flows into the resistor 32 via the collector-emitter path of the transistor 14 and the collector-emitter path of the transistor 22, a voltage drop Vm proportional to the drive current I occurs across the resistor 32. That is, the resistor 32 functions as the current monitor circuit 3° of FIG.

On the other hand, the forward voltage Vt as described above is applied across the diode 36.
is generated, the sum of this voltage V and the voltage Vm generated across the resistor 3Z, Vs = Vt + Vm, is sent to the operational amplifier 42, and the difference between it and the base voltage Vr determined by the Zener diode 54 is detected. be done.

Therefore, the output of the operational amplifier 42 has a differential voltage Vd = Vr - V
s is obtained, and this differential voltage Vd is applied to the base of the transistor 14 via the resistor 16 as a bias.

First, a case where the ambient temperature is constant and the voltage Vt across the diode 36 is constant will be described. Now, assume that the drive current I of the light emitting element 12 increases for some reason. As (2) increases, the voltage drop Vn+ across the resistor 32 increases, so the differential voltage Vd, which is the output of the operational amplifier 4Z, decreases. Therefore, the base bias voltage of transistor 14 is reduced and the drive current is reduced. When (2) decreases, the voltage drop Vm due to the resistor 32 also decreases, so Vs=Vt+
Vm also decreases and the output voltage Vd of the operational amplifier 42 increases. Therefore, the base bias voltage of the transistor 14 increases, and the drive current (2) increases. With such feedback control, during the period when the input signal is at H level,
The drive current I of the light emitting element 12 is always kept constant.

Next, when the input signal becomes L level, the transistor 22 becomes non-conductive, so the drive current I of the light emitting element 12 stops flowing, and the light emitting element 12 turns off. Since (2) becomes O, the voltage drop v11 at the resistor 32 becomes 0, and the forward voltage Vt of the diode 36 also becomes 0, so
Vs=0. Therefore, the output voltage Vd of the operational amplifier 42
is Vd=Vr, and this voltage Vd is applied to the base of the transistor 14 via the resistor 16, so when the input signal becomes H level next time, the transistor 1
4.22 Both can be in a conductive state.

The above explanation was based on a case where the ambient temperature was constant, but when the ambient temperature changes, the light emission intensity of the light emitting element 12 decreases even if the drive current (2) is constant. Such fluctuations in emission intensity are prevented as follows. As mentioned above, the forward voltage Vt of the diode 36 decreases as the ambient temperature rises, so even if the voltage Vm across the resistor 32 is constant, Vs=Vt+Vm
decreases. Therefore, the output voltage Vd of the operational amplifier 42 increases, increasing the light emitting element drive current I and increasing the light emission intensity. On the other hand, when the ambient temperature decreases, the emission intensity increases, but this time, contrary to the above, the drive current I is decreased by the operation of increasing Vt → increasing Vs → decreasing Vd and emitting light. Reduce intensity.

[Effects of the Invention] As described above, in the temperature-compensated light-emitting element drive circuit according to the present invention, fluctuations in the light emission intensity of the light-emitting element due to changes in ambient temperature are effectively prevented by the simple circuit configuration, and the light emission intensity is always constant during light emission. Emission intensity is maintained. Therefore, the optical branching and light receiving elements required in conventional circuits are no longer necessary. Further, constant current control is performed while the light emitting element is on, and it is possible to prevent unnecessary current consumption while the light emitting element is off.

Therefore, when driving a light emitting element with an input signal having a small duty ratio, the present invention has the advantage that the power consumption of the entire circuit can be reduced compared to conventional circuits.

[Brief explanation of the drawing]

! @ Figure 1 is a block diagram showing the principle of the temperature compensated light emitting element driving circuit of the present invention, Figure 2 is a circuit diagram showing an embodiment of the temperature compensated light emitting element driving circuit of the present invention, and Figure 3 is a block diagram showing the principle of the temperature compensated light emitting element driving circuit of the present invention. FIG. 2 is a circuit diagram showing an example of a light emitting element drive circuit using a US auto power control circuit. DESCRIPTION OF SYMBOLS 10... Light emitting element drive current control circuit, 12... Light emitting element, 14... Transistor, 16... Resistor, 20... Switching circuit, 22... Transistor, 30... Constant current monitor Circuit, 35...Temperature monitor circuit, 36...Diode, 32...Resistor, 40...Difference detection circuit, 42...Differential amplifier, 50...Reference voltage generation circuit, 52...・Resistance, 54...Zener diode, 60...Input terminal, ■...Light emitting element drive current, Vm...Voltage drop, Vt...Diode forward voltage, ■"...Reference voltage Vcl ...Differential voltage, 10■ ...DC power supply terminal. Inventors: Oka 1) Hiroshi Shisai, Noritaka Okuno, Mikihiro

Claims (7)

[Claims] (1) A light emitting element drive circuit that controls energization to a light emitting element according to an input signal, the light emitting element drive current control circuit controlling energization to the light emitting element, and the light emitting element drive current control circuit connected in series to the light emitting element drive current control circuit. a switching circuit that is connected to and opens and closes according to the input signal, a current monitor circuit that detects the amount of current flowing to the light emitting element, a temperature monitor circuit that detects the ambient temperature, and a signal from the current monitor circuit and the temperature. Consisting of an adding circuit that adds signals from a monitor circuit, a reference voltage generating circuit that generates a reference voltage, and a difference detection circuit that outputs a signal according to the difference between the signal voltage from the adding circuit and the reference voltage,
A temperature-compensated light-emitting element drive circuit, characterized in that the output signal of the difference detection circuit is provided as a control signal to the light-emitting element drive current control circuit. (2) The temperature compensated light emitting element drive circuit according to claim 1, wherein the light emitting element drive current control circuit is a transistor having a collector-emitter path connected in series to the light emitting element. . (3) The temperature-compensated light-emitting element drive circuit according to claim 2, wherein the switching circuit comprises a transistor connected in series with a series circuit of the light-emitting element and the transistor. (4) The temperature compensated light emitting element drive circuit according to claim 1, wherein the current monitor circuit comprises a resistor connected in series with the light emitting element. (5) The temperature-compensated light-emitting element drive circuit according to claim 1, wherein the temperature monitor circuit comprises a silicon diode connected in series to the light-emitting element. (6) The temperature compensated light emitting element drive circuit according to claim 1, wherein the adding circuit comprises means for connecting the temperature monitor circuit and the current monitor circuit in series. (7) Claims characterized in that the difference detection circuit is composed of a differential amplifier, and its output signal is applied as the control signal to the base of the transistor constituting the light emitting element drive current control circuit. 2. The temperature compensated light emitting element drive circuit according to item 2.