JPH0258881A - Driving circuit for light emitting element - Google Patents

Abstract

PURPOSE:To prevent the output current fluctuation of a constant current supplying means caused by the fluctuation of a switching means, and enable the stable high speed switching of the current flowing in a light emitting element by arranging a buffer circuit in series with a means for switching the current of the light emitting element and a means for supplying a constant current to the light emitting element. CONSTITUTION:An impedance transforming network as a buffer circuit 54 is arranged in series with a current switching circuit 52 for a light emitting element and a constant current circuit 51. Even if the collector potential of a transistor 4 changes on account of load fluctuation in the buffer circuit, the base potential does not change, since the base impedance of the transistor is low. As a result, the collector current also is stable. Since the emitter potential of the transistor 4 is constant independently of load fluctuation, the influence of load fluctuation does not appear on the base of a transistor 5 even in the case where a capacitor 32 has a fairly large capacitance, so that a filter circuit is not necessary. Since the transistor 4 is used as common base, it can respond to the sharp fluctuation of load, thereby realizing the stabilization of laser current and the high speed switching.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a light emitting element driving device that turns on and off a light emitting element by switching a predetermined current flowing through the light emitting element.

[Conventional technology] FIG. 4 shows an example of a conventional light emitting element drive circuit.

(3% fiJ according to JP-A-62-237-IT, In the same figure, 1 is a laser diode, 2.3.5 is a transistor, 6 is an AND gate of oven collector output, 7 is an operational amplifier, and 9 is a voltage source. , 11-21.41 is resistance, 24
．． 43 is a capacitor, 42 is a choke coil, and 10 is a LASERON signal (Low true) for lighting the laser diode 1. Note that 51 surrounded by dotted lines is a constant current circuit, 52 is a current switch circuit, and 53 is a filter circuit.

First, the operation of the constant current circuit 51 will be explained.

This circuit consists of a voltage generated across a resistor 14 and a voltage source 9.
Since the output of the operational amplifier 7 operates so that the voltages of the transistors are equal, the collector current of the transistor 5, which is the output current of the constant current circuit, is expressed by the following equation.

(voltage value of voltage source 9)÷(resistance value of resistor 14) Next, the operation of the current switch circuit 52 will be explained.

The resistors 16 to 18 are set so that the base potential of the transistor 2 is about +IV when the open collector output AND gate 6 outputs High, and about -1V when the AND gate 6 outputs Low. The base of the transistor 3 is connected to GND through a low resistance 15. That is, when the output of AND gate I-6 is High, transistor 2 is turned on, transistor 3 is turned off, and no current flows through laser diode 1, but when AND gate 6 is Low
When the output is w, the transistor 2 is turned off, the transistor 3 is turned on, and the output current of the constant current circuit 51 flows through the laser diode 1. Resistor 19.20 and capacitor 24
absorbs the spike current generated during current switching and protects the laser diode 1 from tJ[.

The filter circuit 53 is inserted to suppress the output fluctuation and ringing of the constant current circuit caused by the emitter potential fluctuation of the transistor 2.3 caused by the current switch. Fifth
As shown in FIG. 5A, the output transistor 5 of the constant current circuit 51 has a base-collector capacitance 32 due to the characteristics of the transistor. Then, as the collector potential V1 changes, the base potential also changes, and the voltage across the resistor 14 also changes. Since the potential of the resistor 14 is connected to the inverting input of the operational amplifier 7, as shown in FIG.
As shown in b), every time the collector potential vl of the transistor 5 changes, overshoot and vibration occur. This overshoot may destroy the laser diode 1. As a countermeasure to this problem, as shown in FIG. 6(a), the base of the transistor 5 and the
It is conceivable to attach a capacitor 33 between V and V, but if this is done, the responsiveness of the output current to changes in the voltage source 9, which is a human input signal of the constant current circuit, becomes slow as shown in FIG. 6(b). This is especially disadvantageous when changing the voltage source 9 at high speed.

Therefore, using the filter circuit 53, the current switch circuit 5
2, the collector potential of the transistor 5 is gradually changed, overshoot and vibration of the output current of the constant current circuit 51 are suppressed, the output current is stabilized, and the laser diode 1 is prevented from being destroyed. .

[The problem that the invention is trying to solve, however,
In the conventional example described above, since a filter circuit is provided between the constant current circuit and the current switch circuit, there is a problem that the current flowing through the laser diode 1 during current switching becomes dull.

In particular, as shown in FIG. 7, there is a drawback that the rise time from when the current starts flowing through the laser diode 1 until it reaches the specified current value is quite slow.

[Means and effects for solving the problem] According to the present invention, by providing a buffer means in series between the means for switching the current of the light emitting element and the means for supplying a constant current to the light emitting element. This prevents potential fluctuations or current fluctuations of the switching means from being transmitted to the constant current supply means. This realizes stable and high-speed current switching of the light emitting element without impairing the responsiveness of the output current to the input signal of the constant current circuit.

[Example] FIG. 1 shows an example of the present invention. In the figure, the same parts as in the conventional example are given the same reference numerals. 54 is an impedance conversion circuit which is a buffer circuit. 4 is a transistor, 8 is a diode, 22 is a resistor, 2
3 is a capacitor.

The constant current circuit 51 and the current switch circuit 52 operate in the same manner as in the conventional example. The buffer circuit 54 is a common base circuit for the transistor 4, and the base of the transistor 4 is lower than the GND potential by the forward voltage of the diode 8 due to the diode 8 and the resistor 22. The capacitor 23 is provided for the purpose of lowering the AC impedance of the base of the transistor 4. The operation of the buffer circuit 54 will be explained based on FIG. The base-collector capacitance of the transistor 5 is a capacitor 32, and the base-collector capacitance of the transistor 4 is a capacitor 31.

Even if the collector potential of transistor 4 changes due to changes in load, the base of the transistor has low impedance, so the base potential of transistor 4 does not change, and therefore the collector current also remains stable. Further, since the emitter potential of the transistor 4 is always a constant potential regardless of load fluctuations, even if the capacitor 32 has a considerably large value, the base of the transistor 5 is not affected by the load fluctuations. Therefore, a filter circuit is unnecessary. Also,
Since the transistor 4 is a common base circuit, it can cope with rapid load fluctuations. Therefore, laser diode 1
The rise of the current flowing through the current is accelerated.

As explained above, by providing a buffer circuit between the constant current circuit and the current switch circuit, it is possible to prevent overflow caused by potential fluctuations during current switching without impairing the responsiveness of the output current to the input signal of the constant current circuit. Stable and high-speed switching of laser current with suppressed shoot and vibration is achieved.

[Other Embodiments] FIG. 3 shows a second embodiment of the present invention. In this figure, the same parts as in the above embodiment are given the same reference numerals. In the embodiment described above, the output transistor of the constant current circuit was composed of the NPN transistor 5, but in this embodiment, a PNP transistor 35 is used. Therefore, the voltage of the resistor 14 is connected to the positive phase voltage of the operational amplifier 7. In addition, in the buffer circuit, transistor 4
Although the base potential of the resistor 3 was defined using the forward voltage of the diode 8 in the above embodiment, in this embodiment
It is defined by a partial pressure ratio of 6.22. To lower the impedance of the base of transistor 4, resistor 3
6.22 is preferably as small as possible.

In this embodiment as well, since the emitter potential of the transistor 35 is stable, variations in the emitter potential of the transistor 2.3 due to current switching do not affect the constant current circuit. Therefore, high-speed and stable laser current switching is possible.

The circuit shown in FIG. 8 may be used as the current switch circuit. In the circuit of FIG. 8, when the AND gate 6 of the oven collector output has a low output, the transistor 51 is turned on and the laser 1 is turned off, and when the AND gate 6 has a high output, the transistor 51 is turned off and the laser 1 is turned on. It looks like it will. This is because while the forward voltage of the laser diode when it is on is about 2V, the collector-emitter voltage when the transistor 51 is on is sufficiently small, so when the transistor 51 is on, the diode 1 has a sufficient forward voltage. Diode 1 cannot be turned on because no voltage is added.

[Effects of the Invention] As explained above, by providing a buffer circuit in series between the means for switching the current of the light emitting element and the means for supplying constant current to the light emitting element, constant current supply due to fluctuations in the switching means can be achieved. Fluctuations in the output current of the device can be prevented, and high-speed and stable switching of the current flowing through the light-emitting element becomes possible.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a light emitting device driving circuit according to a first embodiment of the present invention, FIG. 2 is a diagram for explaining the operation of the first embodiment, and FIG. 3 is a diagram showing a second embodiment of the present invention. FIG. 4 is a diagram showing a conventional example; FIG. 5 is a diagram illustrating the effect of collector potential fluctuation of the output transistor of a constant current circuit on the output current; FIG. A diagram showing the relationship between the capacitance connected to the base of the output transistor of a constant current circuit and the response. Figure 7 is a diagram showing the laser current waveform with and without a conventional filter circuit. Figure 8 is a diagram of the current switch. FIG. 3 is a diagram showing an example of a circuit. (2) Explanation of the symbols representing the main parts of the drawing: 1 is a laser diode, 2 to 5.35 are transistors, 6 is an AND gate of oven collector output, 7 is an operational amplifier, 8 is a diode, 9 is a constant current circuit Voltage source serving as a signal, 32. 3 is the base-emitter capacitance of the transistor.

Claims (1)

[Claims] A light emitting element driving device comprising means for switching a current of a light emitting element and means for supplying a constant current to the light emitting element, and a buffer means provided in series between the switching means and the supplying means. .