JPH0121431Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a light control circuit for a diode having a light emitting function, such as a laser diode.

In order to maintain the optical output of the laser diode constant, this optical output is detected by a photodiode and the current is exchanged.The current output is then converted into voltage and compared with a reference voltage, and the laser diode is adjusted according to this comparison output. An optical control circuit is used for driving.

It is required to control the lighting of the laser diode only when necessary and to turn it off when unnecessary, thereby preventing the characteristic deterioration of the laser diode as much as possible. For this purpose, a method of turning on and off the operating voltage source of the optical control circuit can be considered.

FIG. 1 shows a laser diode 1 manufactured by such a method.
This is a circuit diagram for on/off control, and is currently being proposed by the applicant of the present application. The light emission output of the laser diode 1 is detected by a monitor diode 4, a voltage corresponding to this light emission output is generated by a resistor 5, and this is compared with a divided reference voltage by resistors 7 and 8 by a differential amplifier 6. This comparison output is applied to the base of the drive transistor 2 to keep the drive current of the laser diode 1 constant. Note that 3 is the emitter resistance of the transistor 2.

A stabilizing circuit consisting of a transistor 12, a Zener diode 13, a capacitor 14, and a resistor 15 is provided as a voltage source for the optical control circuit, and a transistor 11 is connected in parallel to both ends of the Zener diode 13. By turning on and off this transistor 11, the capacitor 14
The battery charges and discharges the battery, thereby turning the power supply voltage itself on and off. However, in this configuration, since there is a capacitor in the voltage source, for example, when the power is turned on, the voltage gradually rises (falls) toward the specified value with a predetermined time constant, and in the process, the light control circuit The operation of the differential amplifier (op-amp) etc. that makes up the system becomes unstable. As a result, there is a risk that a so-called surge current will flow through the laser diode, leading to deterioration of the characteristics of the diode.

An object of the present invention is to provide a light control circuit that prevents the risk of a surge current flowing when controlling the lighting of a diode having a light emitting function and eliminates deterioration of diode characteristics.

The light control circuit according to the present invention includes a light receiving element that receives light output from a diode having a light emitting function such as a laser diode, a light receiving voltage generating means that outputs a voltage according to the level of light received by the light receiving element, and a predetermined voltage applied to the light receiving element. a switch element connected in parallel to the capacitor and turned on and off in response to an external control signal, and applying a voltage to the diode according to the voltage difference between the voltage across the capacitor and the output voltage of the photoreceptor voltage generating means. The device is characterized in that the voltage applied to the diode gradually increases during a period in which the voltage across the capacitor gradually changes.

The present invention will be explained below using the drawings.

FIG. 2 is a circuit diagram of an embodiment of the present invention.
Parts equivalent to those in the figures are designated by the same reference numerals. More specifically, the laser diode 1 is driven and controlled by the collector current C of the drive transistor 2. 3
is the emitter resistance of transistor 2. Photodiode 4 that receives the light from laser diode 1
is provided, and the output current of this diode 4 becomes an anti-phase input to the differential amplifier 6. Differential amplifier 6
A divided voltage output of a voltage dividing circuit made up of resistors 7 and 8 is applied to the positive phase input of , and this divided voltage output becomes the reference voltage B. The comparison output from the differential amplifier 6 serves as the base input of the drive transistor 2.
Note that the resistor 9 is a current-voltage conversion resistor. Further, the resistor 5 corresponds to a light receiving voltage generating means, and the differential amplifier 6, the resistors 3 and 9, and the transistor 2 correspond to a driving means.

In order to control the level of the reference voltage B, a parallel connection circuit including a capacitor 10 and a switching transistor 16 as a switch element is provided between the positive phase input of the differential amplifier 6 and the circuit power supply. An external control signal A is applied to the base.

Then, the current flowing through photodiode 4 is i _M
, the current flowing through the resistor 5 is i _r , and the resistor 9 is
If the current of is i _e , then i _M = i _r + i _e , and therefore, i _e = i _M − i _r = i _M −V ₁ /R ₅ . Note that R ₅ is the resistance value of the resistor 5 and V ₁ is the phase input voltage of the amplifier 6. Further, the output voltage V ₀ of the amplifier 6 is expressed as V ₀ = _-ie ·R ₉ +V ₁ . Here, R ₉ is the resistance value of the resistor 9. Therefore, the formula V ₀ =-(i _M −V ₁ /R ₅ )·R ₉ +V ₁ is obtained.

Now, when the light emitting output of the laser diode 1 changes while the transistor 16 is off, this is detected by the diode 4, and a voltage corresponding to the amount of change is obtained at the output (V ₀ ) of the differential amplifier 6.
That is, a voltage change corresponding to this input voltage change appears at the output of the amplifier 6, causing the collector current of the drive transistor 2 to increase or decrease. Since the above circuit is a so-called negative feedback loop,
The light emission output of the laser diode 1 is controlled to a constant value determined by the reference voltage.

Next, the case where the laser diode 1 is on/off controlled by the external control signal A will be explained using the operating waveforms shown in FIG. Figures A to C respectively show the waveforms of signals A to C of the circuit shown in Figure 2, respectively.

First, before time _t1 , control signal A is at ground level G, and transistor 16 is turned on. Therefore, the capacitor 10 is in a discharged state and the positive phase input terminal of the differential amplifier 6 is at the lowest potential V _L of the circuit as shown in FIG. B. Therefore, the output of the differential amplifier 6 is also at the lowest potential V _L , the drive transistor 2 is off, and the laser diode 1 is off.

When the control signal A becomes low level at time t1 to turn on the laser diode ₁ , the transistor 16 is turned off. Therefore, since the capacitor 10 is charged with a predetermined charging time constant, the level at the positive input terminal of the differential amplifier 6, that is, the reference level, changes from the initial value V _L to the final target value V _H as shown in Figure B. gradually rises. When this reference level gradually rises and reaches a certain level (time t ₂ ), the drive transistor first shifts from off to active state, and thereafter, as the reference level rises, the diode drive current gradually increases as shown in Figure C. Then, after a certain period of time, it settles into steady operation.

By doing this, the differential amplifier 6 is always in an active state, so it does not become unstable, and the diode drive current gradually increases as the reference level increases, so there is no generation of surge current. .

To turn off the laser diode, if the control signal A is made to transition to a high level (ground level), the transistor 16 will be turned off and the reference level will instantly drop as shown in Figure B, causing the laser diode 1 to be driven. Since the current disappears, the light can be turned off.

As described above, according to the present invention, it is possible to eliminate the surge current that causes characteristic deterioration when the laser diode is turned on, resulting in an extremely effective circuit.
In addition, since the diode turns on and off in synchronization with changes in the external control signal, it is possible to send out an optical signal that follows the timing of changes in the external control signal.This optical signal can be used, for example, to control other equipment. Can be used as a command signal.

The circuit configuration is not limited to the illustrated example and can be modified in various ways.

[Brief explanation of the drawing]

Figure 1 is a diagram showing an example of a laser diode optical control circuit currently being proposed, Figure 2 is a circuit diagram of an embodiment of the present invention, and Figure 3 is a waveform diagram explaining the operation of the circuit in Figure 2. . Explanation of symbols of main parts: 1...Laser diode, 2...Drive transistor, 4...Photodiode, 6...Differential amplifier, 10...Capacitor, 16...Switching transistor.

Claims (1)

[Scope of utility model registration request] A control circuit that controls the light output of a diode having a light emitting function, the control circuit comprising: a light receiving element that receives the light output of the diode; a light receiving voltage generating means that outputs a voltage according to a level of light received by the light receiving element; and a predetermined voltage. A capacitor to which is applied, a switch element connected in parallel to the capacitor and turned on and off in response to an external control signal, and a voltage corresponding to the difference voltage between the voltage across the capacitor and the output voltage of the light receiving voltage generating means. A light output control circuit comprising a driving means for applying voltage to the diode, the voltage being applied to the diode gradually increases during a period in which the voltage across the capacitor gradually changes.