JPH0831642B2 - Driving circuit for semiconductor light emitting device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a drive circuit for a semiconductor light emitting element, and particularly reduces overshoot at the time of rising of a semiconductor light emitting element drive current during a switching operation of the semiconductor light emitting element. It relates to a circuit configuration for.

[Conventional technology]

FIG. 2 is a circuit diagram showing an example of a conventional drive circuit for a semiconductor light emitting element. In the figure, the output of the constant current source 1 is connected to the collector of the current mirror npn transistor 2 and the base of the driving Schottky clamped npn transistor 3. The emitter of the output npn transistor 4 is connected to the emitter of the transistor 2, the base is connected to the base of the transistor 2 and the emitter of the transistor 3, and the collectors of the transistor 3 and the transistor 4 are connected to the high potential power source via the resistor 5 and the resistor 6, respectively. Connected to 7.

Input circuit 9 controlled by on / off control signal 8
Is connected to the common base of the transistors 2 and 4, the cathode of the semiconductor laser diode 10 is connected to the low potential power source 11, and the anode is connected to the common emitter of the transistors 2 and 4.

 Next, the operation will be described.

Now, assuming that the output of the input circuit 9 takes two states, that is, the "L" level state and the high impedance state by the control signal 8, first, when the output of the input circuit 9 is the "L" level state, 2 and the transistor 4 are turned off, and the laser diode 10 is also turned off. At this time, the transistor 3 is on, and its base current is constant current source 1.
Output current (hereinafter referred to as I ₀ ) of the collector current
I _{C (T3)} is the following formula (1), and emitter current I _{E (T3)} is (2)
It is shown by the formula.

I _{C (T3)} = (V _CC -V _{sat (T3)} -V _OL ) / R ₅ ... (1) _{IE (T3)} = _IO + I _{C (T3)} ... (2) However, V _CC is high potential The power supply voltage value, V _{sat (T2)} is the collector-emitter saturation voltage of the transistor 3, V _OL is the “L” level output voltage of the input circuit 9, and R ₅ is the resistance value of the resistor 5. Then, the emitter current I _{E (T3)} flows into the input circuit 9.

On the other hand, when the output of the input circuit 9 is in the high impedance state, the transistors 2 and 4 are turned on and the laser diode
10 is also turned on. At this time, if the base current of the transistor 3 is ignored, I _O flows in the collector of the transistor 2,
The collector of the transistor 4 has a current mirror ratio of N
Then, a current of N · _IO flows. Further, the sum of the base currents of the transistors 2 and 4 is equal to the emitter current I _{E (T3)} of the transistor 3, and I _{E (T3)} = I _{C (T3)} = I _O (1 + N) / β (3) . Where β is the current amplification factor of the transistors 2 and 4.

Therefore, when the output of the input circuit 9 shifts from the "L" level state to the high impedance state, that is, in the transient state in which the laser diode 10 shifts from the off state to the on state, it flows into the input circuit 9 ( The emitter current of the transistor 3 given by the equation (2) becomes the base current of the transistors 2 and 4, and drives the transistors 2 and 4.

However, this base current is larger than the current value (Equation (3)) when the laser diode 10 is constantly turned on, and the difference ΔIE _(T3) is obtained from Equation (2) through Equation (3). By subtracting, ΔIE _(T3) = _IO {β- (1 + N)} / β + IC _(T3) (4).

In the expression (4), β> 1 + N is normally satisfied, so that the expression (4) takes a positive value. Therefore, the emitter currents of the transistors 2 and 4 are larger by βΔIE _(T3) in this transient state than in the steady state. That is, in this transient state, the laser diode 10 is driven with a larger current than in the steady state.

[Problems to be Solved by the Invention]

Since the conventional semiconductor laser diode drive circuit is configured as described above, overshoot occurs in the laser diode drive current in the transient state in which the laser diode shifts from the off state to the on state.

On the other hand, the output of the laser diode is proportional to the drive current when it is in the ON state. Therefore, if the drive current has an overshoot, the laser output also overshoots, and as a result, the laser diode may be destroyed or its life may be shortened. There was a problem.

Further, in JP-A-60-170281, a resistor connected in series with a laser diode is provided to reduce overshoot. However, when such a resistor is provided, the laser diode is normally turned on. There is a problem that unnecessary voltage drop and power consumption occur.

The present invention has been made to solve such a problem, and can reduce the overshoot of the drive current of the semiconductor light emitting element in the transient state in which the semiconductor light emitting element shifts from the off state to the on state. Moreover, it is an object of the present invention to obtain a drive circuit for a semiconductor light emitting element that does not cause unnecessary voltage drop or power consumption during a steady ON period of the light emitting element.

[Means for solving the problem]

In order to achieve the above-mentioned object, according to the present invention, the output of a transistor that receives a current from a power supply and the output of an input circuit that receives an on / off control signal are connected to a common base of a current mirror circuit, Depending on the output state, the output current of a constant current source commonly connected to the base electrode of the transistor and the main electrode of the reference side transistor of the current mirror circuit is switched between the input circuit side and the current mirror circuit side. By turning on the current, the output current from the current mirror circuit to the semiconductor light emitting element is turned on / off.
In a circuit for performing off-drive control, a part of the transient current flowing between the power supply and the transistor when the switching is performed from the input circuit side to the current mirror side bypasses the transistor. A bypass circuit for discharging the electric current is coupled to a current path between the power supply and the transistor.

[Action]

Since a part of the transient current flowing between the transistor and the power supply when the current supply to the light emitting element is started passes through the bypass circuit, a large transient current does not occur in the output of the current mirror circuit through the transistor. As a result, the overshoot current to the light emitting element is reduced.

Further, unlike the case where the resistance element is inserted in series with the light emitting element, unnecessary voltage drop and power consumption do not occur when the light emitting element is constantly turned on.

〔Example〕

 Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the present invention.
11 to 11 are exactly the same as those in the conventional example. The driving transistor 3 is a characteristic element of this embodiment.
Between the collector of and the collector of the output transistor 4,
A capacitor 12 as a bypass circuit is connected.

In the semiconductor light emitting element drive circuit configured as described above, the steady operation of the transistor 3 when the output of the input circuit 9 is at the “L” level, that is, when the current mirror transistors 2 and 4 and the light emitting element 10 are off. Is the same as the conventional example, and the collector potential V _{C (T3)} of the transistor 3 at this time is given by the following equation (5).

V _{C (T3)} = V _OL + V _{sat (T3)} (5) When the output of the input circuit 9 is in a high impedance state, that is, the current mirror transistors 2 and 4 and the light emitting element 10
The steady operation of the transistor 3 when is ON is the same as that of the conventional example, and the collector potential of the transistor 3 at this time is given by the following equation (6) with reference to the above equation (3).

V _{C (T3)} = V _CC −R ₅ · I _{C (T3)} = V _CC −R ₅ · {I _O (1 + N) / β} (6) Equations (5) and (6) As a general numerical example of, if V _OL = 1.0V, V _{sat (T3)} = 0.5V, I _O = 3mA, V _CC = 5V, R ₅ = 1KΩ, N = 9, β = 100, then ( V _{C (T3)} in the equation ₍ 5) is 1.5V, and V _{C (T3)} in the equation (6 ₎ is 4.7V.

Now, for the sake of simplicity, assuming that the resistance value of the resistor 6 is sufficiently small and the voltage drop in the resistor 6 when the transistor 4 is on is small, the collector potential of the transistor 4 is on and off. Always V regardless
Equal to _CC . Therefore, in the transient state in which the light emitting element shifts from the off state to the on state, the collector connection end of the transistor 3 having the capacitance 12 is 1.5 V in the above numerical example.
It must be charged by an amount corresponding to a potential displacement of 4.7V, and this charging current is supplied from the high potential power supply 7 through the resistor 5.

That is, in the conventional example, a part of the transient current supplied from the high potential power supply 7 to the bases of the transistors 2 and 4 through the resistor 5 and the transistor 3 in the transient state in which the light emitting element 10 shifts from OFF to ON is By providing the capacitor 12, the charging current for the capacitor 12 is bypassed. Considering that the change in the base currents of the transistors 2 and 4 is multiplied by β to become the drive current of the light emitting element 10, the excessive drive current in the transient state in which the light emitting element 10 shifts from OFF to ON is It is reduced by β times, and overshoot in the light output of the light emitting element 10 is also reduced.

The case where the voltage drop due to the resistor 5 when the transistor 4 is on cannot be ignored is as follows.
That is, when the collector potential of the transistor 3 increases, the conductivity of the transistor 4 increases, the voltage drop across the resistor 6 also increases, and the collector potential of the transistor 4 decreases. For this, the capacity 12
The potential difference between the both ends of the drive current increases, so that the bypass current further increases and the effect of reducing overshoot in the drive current increases.

On the other hand, when the laser diode 10 changes from the ON state to the OFF state, the capacity 12
Discharges. However, when the discharge is performed, the emitter current of the driving transistor 3 flows into the input circuit 9 side, and the discharge from the capacitor 12 does not burden the laser diode 10.

Further, when the laser diode 10 is in a steady ON state or a steady OFF state, the capacitor 12 does not substantially participate in its driving operation, and there is no voltage drop or power consumption due to the capacitor 12.

Although the capacitor is provided between the collector of the driving transistor and the collector of the output transistor in the above embodiment, it may be provided between the collector of the driving transistor and the high potential power source or between the low potential power source. In addition, an example using a laser diode as a semiconductor light emitting element is shown,
Similar effects can be obtained in other types of semiconductor light emitting devices such as light emitting diodes.

〔The invention's effect〕

As described above, according to the present invention, a part of the transient current flowing between the driving transistor and the power supply is bypassed when the light emitting element shifts from the off state to the on state. The shoot current can be reduced. Therefore, it is possible to prevent the light emitting element from being broken or the life of the light emitting element from being shortened.

Further, unlike a circuit in which a resistance element is inserted in series with a light emitting element, there is no unnecessary voltage drop or power consumption during steady ON.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a circuit diagram showing a conventional semiconductor light emitting element drive circuit. In the figure, 1 is a constant current source, 2 and 4 are npn transistors that form a current mirror circuit, 3 is a Schottky clamp npn transistor that drives the current mirror circuit, 5 and 6 are resistors, 7 is a high potential power source, and 9 is An input circuit, 10 is a semiconductor light emitting element, 11 is a low potential power source, and 12 is a capacitor (bypass circuit). In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. An output of a transistor that receives a current from a power supply and an output of an input circuit that receives an on / off control signal are connected to a common base of a current mirror circuit, and the output of the input circuit is changed according to an output state of the input circuit. The output current of a constant current source, which is commonly connected to the base electrode of the transistor and the main electrode of the reference side transistor of the current mirror circuit, is switched between the input circuit side and the current mirror circuit side, and the current mirror circuit is provided. In a circuit for performing on / off drive control of an output current from a circuit to a semiconductor light emitting element, between the power supply and the transistor when the switching is performed from the input circuit side to the current mirror side. A bypass circuit that causes a part of the transient current to flow by bypassing the transistor is a current between the power supply and the transistor. A drive circuit for a semiconductor light emitting device, characterized in that it is coupled to a path.