JPH0548149A - Semiconductor light emitting element driving circuit - Google Patents

Abstract

(57) [Abstract] [Purpose] To obtain a semiconductor light emitting element drive circuit in which an optical output waveform is constant regardless of a change in drive current. [Structure] Since the transistor Q12 forms a current mirror circuit with the transistor Q6, the transistor Q12
The current flowing through the transistor Q4 through the transistor Q4 is proportional to the drive current I _L flowing through the transistor Q6. As a result, the rate of change of the current flowing through the transistor Q4 when the transistor Q2 is turned off and then turned on or turned off is irrelevant to the drive current I _L. [Effect] The optical output waveform becomes constant regardless of the change of the drive current I _L.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor light emitting element drive circuit, and more particularly to a semiconductor light emitting element drive circuit capable of improving a light output waveform of the semiconductor light emitting element.

[0002]

2. Description of the Related Art FIG. 3 is a circuit diagram showing a conventional semiconductor light emitting element drive circuit. In the figure, reference numeral 1 is a semiconductor laser diode, and Q1 and Q2 are NPN transistors forming a differential switch circuit 50. The collector of the transistor Q1 is connected to the high potential power source 10 via the resistor 5. The emitter of the transistor Q2 is the transistor Q1.
Semiconductor laser diode 1 for the emitter and collector
Are connected to the respective anodes. The cathode of the semiconductor laser diode 1 is connected to the high potential power supply 10.

Q3 and Q4 are input-stage NPN transistors, and these transistors form a transistor Q.
Controls ON / OFF of 1 and Q2. Transistor Q3
Has a base connected to the first input terminal IN1 and a collector connected to the high-potential power supply 10. The emitter of the transistor Q3 is connected to the low potential power source 20 via the resistor 15 and also to the base of the transistor Q1. The base of the transistor Q4 has a second input terminal I
The collectors are connected to the high potential power source 10 at N2. The emitter of the transistor Q4 is connected to the low potential power source 20 via the resistor 25, and the transistor Q2
It is also connected to the base of.

Q5 and Q6 are NPN transistors forming the current mirror circuit 200. Transistor Q5
Has a base and a collector connected in common, and the common connection point is connected to the high potential power source 10 via the constant current source 100 and the emitter is connected to the low potential power source 20. The transistor Q6 has a base at the base of the transistor Q5, an emitter at the low potential power source 20, and a collector at the transistors Q1 and Q1.
Each is connected to a common emitter connection point of Q2.

Next, the operation will be described. Now, the constant current output from the constant current source 100 is I _S , the transistor Q5,
Assuming that the current mirror ratio of the current mirror circuit 200 formed by Q6 is 1: N, the collector current (driving current) of the transistor Q6 becomes approximately N · I _S (hereinafter referred to as I _L ).

First, the case where the low level voltage V _L is input to the first input terminal IN1 and the high level voltage V _H is input to the second input terminal IN2 will be described. At this time, the base potential V _B1 of the transistor Q1 is

[0007]

[Equation 1]

[0008] On the other hand, the base potential V _B2 of the transistor Q2 is

[0009]

[Equation 2]

[0010] However, V _BE is the forward voltage drop between the base and emitter of the NPN transistor. Here, since V _L <V _H , V _B1 <V _B2 , and the transistor Q1 turns off and the transistor Q2 turns on. As a result, a current substantially equal to I _L flows through the semiconductor laser diode 1 and the semiconductor laser diode 1 is turned on.

Next, the case where the high level voltage V _H is input to the first input terminal IN1 and the low level voltage V _L is input to the second input terminal IN2 will be described. The operation at this time is the reverse of the above operation. That is, the transistor Q1 is turned on, and the collector of the transistor Q1 has almost I
While a current equal to _L flows, the transistor Q2 turns off and the semiconductor laser diode 1 turns off.

In this way, the first and second input terminals IN
ON / OFF of the semiconductor laser diode 1 is controlled by the signal levels to 1 and IN2.

FIG. 4 is a graph showing the relationship between the optical output of the semiconductor laser diode 1 and the drive current I _L. As can be seen from FIG. 4, it is necessary to change the drive current I _L in order to keep the optical output of the semiconductor laser diode 1 constant in response to changes in the operating ambient temperature T _a . For example, in order to keep the optical output of the semiconductor laser diode 1 at P ₀ ,
When the operating ambient temperature T _a is 25 ° C., the drive current I _{L is set} to I ₀
Moreover, when the operating ambient temperature T _a is 60 ° C., the drive current I _L must be I ₁ . Note that I _th0 is T _a = 25.
The threshold current value of the semiconductor laser diode 1 at C °, I _{th 1} is the threshold current value of the semiconductor laser diode 1 at T _a = 60 ° C.

The rate of change of the current flowing through the transistor Q4 when the transistor Q2 is switched will be described below.

A low level voltage V _{L is} applied to the first input terminal IN1.
However, when the high level voltage V _H is input to the second input terminal IN2, the transistor Q2 turns on and the semiconductor laser diode 1 turns on, as described above. If the current amplification factor of the NPN transistor is β, the transistor Q
The base current I _B2 of ₂ is

[0016]

[Equation 3]

From

[0018]

[Equation 4]

It becomes This base current I _B2 is supplied from the transistor Q4. Since the base potential of the transistor Q2 is V _H −V _BE , the potential of the low potential power source 20 is set to 0.
Assuming V, the current I _R25 flowing through the resistor 25 is

[0020]

[Equation 5]

[0021] This current I _{R25 is} also a transistor Q
Supplied from No. 4. Here, R ₂₅ is the resistance value of the resistor 25. Therefore, the output current I _E4 of the transistor Q4 when the transistor Q2 is turned on is given by
Than,

[0022]

[Equation 6]

[0023]

Next, when the high level voltage V _H is input to the first input terminal IN1 and the low level voltage V _L is input to the second input terminal IN2, the transistor Q2 turns off and the base current of the transistor Q2. Is 0. However, the base potential of the transistor Q2 is the V _L -V _BE Since resistor 25 is _{(V L -V BE) / R} 25 becomes a current supplied from the transistor Q4. That is, when the transistor Q2 is off, the output current I _E4 of the transistor Q4 is

[0025]

[Equation 7]

[0026] Comparing equations (6) and (7), the output current I _E4 of the transistor Q4 when the transistor Q2 is on is the drive current I _E of the semiconductor laser diode 1.
_While it depends on _L , the output current I _E4 of the transistor Q4 when the transistor Q2 is off is independent of the drive current I _L of the semiconductor laser diode 1.

Now, description will be made by substituting general numerical values for the symbols shown above. Drive current I _L for obtaining optical output P ₀
Is 50 mA when T _a = 25 ° C. and 1 when T _a = 60 ° C.
It is set to 00 mA. Also, V _L = 2.0 V, V _H = 2.5
V, R ₂₅ = 2 kΩ, V _BE = 0.7 V, β = 100.

The emitter current I _{E4 A} of the transistor Q4 when the transistor Q2 is off is 0.
It becomes 65 mA. On the other hand, the output current I _E4B of the transistor Q4 when the transistor Q2 is turned on at I _L = 50 mA is 1.4 mA from the equation 6, and I _L = 100 mA.
In, the output current I _E4C of the transistor Q4 when the transistor Q2 is turned on is _{1.89 mA} from the equation (6).

Here, the rate of change of the output current of the transistor Q4 when the transistor Q2 changes from off to on is 2.15 (I _E4B / I _E4A ) when I _L = 50 mA, I
_{When L} = 100 mA, it becomes 2.91 (I _E4C / I _E4A ). From this, it can be seen that the output current of the transistor Q4 must be changed more as the drive current I _L increases. As a result, when the drive current I _L is large, the time from when the transistor Q2 is turned off to when it is completely turned on, that is, the time until the collector current of the transistor Q2 reaches from 0 to the maximum value (I _L ) becomes long, and the semiconductor laser The duty of the optical output waveform of the diode 1 changes depending on the value of the drive current I _L as shown in FIGS. FIG 5 (b) is a laser current I _X and the relationship of light output when the I _L = 50 mA, FIG. 5 (c) shows the laser current I _X and the light output relationships when I _L = 100 mA.

[0030]

Since the conventional semiconductor light emitting element drive circuit is configured as described above, when the same light output is obtained, the light output waveform of the semiconductor laser diode 1 varies depending on the magnitude of the drive current I _L. There was a problem that it would change.

The present invention has been made to solve the above problems, and provides a driving circuit for a semiconductor light emitting device capable of obtaining a constant light output waveform regardless of the magnitude of the driving current of the semiconductor light emitting device. The purpose is to get.

[0032]

According to the present invention, a semiconductor light emitting element, a driving current source for outputting a driving current for driving the semiconductor light emitting element, and a semiconductor light emitting element between the semiconductor light emitting element and the driving current source are provided. The present invention is applied to a semiconductor light emitting element drive circuit including a switch circuit that is connected and selectively supplies the drive current to the semiconductor light emitting element, and a switch control circuit that controls switching of the switch circuit.

The semiconductor light emitting element drive circuit according to the present invention is characterized in that current transmission means for changing the current flowing through the switch control circuit according to the drive current is provided.

[0034]

Since the current transmitting means in the present invention changes the current flowing through the switch control circuit according to the drive current, the switch control is performed when the semiconductor light emitting element shifts from off to on or from on to off. The rate of change of the current flowing through the circuit can be made independent of the drive current.

[0035]

1 is a circuit diagram showing an embodiment of a semiconductor light emitting element drive circuit according to the present invention. In the figure, the difference from the conventional circuit shown in FIG. 3 is that the resistors 15 and 25 are eliminated and the NPN transistors Q11 and Q12 are newly provided. The transistors Q11 and Q12 form a current mirror circuit together with the transistors Q5 and Q6. The transistor Q11 has a base connected to the base of the transistor Q6, a collector connected to the emitter of the transistor Q3, and an emitter connected to the low potential power supply 20. The transistor Q12 has a base connected to the base of the transistor Q6, a collector connected to the emitter of the transistor Q4, and an emitter connected to the low potential power source 20. Other configurations are similar to those of the conventional circuit shown in FIG.

Next, the operation will be described. Now, the current mirror ratio between the transistors Q11 and Q12 and the transistor Q6 is 1: M. First, the case where the transistor Q2 is off will be described. Now, the drive current I _L is flowing through the transistor Q6. Therefore, the current flowing through the transistor Q12 becomes _IL / M, and this current is supplied from the transistor Q4. The output current I _E4D of the transistor Q4 at this time is

[0037]

[Equation 8]

[0038]

The output current I _E4E of the transistor Q4 when the transistor Q2 is on is obtained in the same manner as I _E4 shown in the _{equation (} 6). That is,

[0040]

[Equation 9]

It becomes Output current I _E4D when transistor Q2 is off and output current I when transistor Q2 is on
_When the ratio with _E4E (I _E4E / I _E4D ) is calculated from _Eqs. 8 and 9, it becomes 1 + M / (1 + β), which is a constant value irrelevant to the drive current I _L. That is, the rate of change of the current flowing through the transistor Q4 when the transistor Q2 changes from off to on or from on to off is constant regardless of the magnitude of the drive current I _L. As a result, the duty of the light output waveform of the semiconductor light emitting element 1 becomes constant regardless of the magnitude of the drive current I _L , as shown in FIGS. 2B and 2C.

Although the semiconductor laser diode is used as the semiconductor light emitting element in the above embodiment, the same effect can be obtained by using another semiconductor light emitting element such as an LED.

[0043]

As described above, according to the present invention, the current transmitting means for changing the current flowing through the switch control circuit according to the drive current of the drive current source is provided, so that the semiconductor light emitting element is turned on or turned off. It is possible to make the rate of change of the current when transitioning from on to off independent of the drive current. As a result, there is an effect that the optical output waveform becomes constant even if the drive current changes.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a semiconductor light emitting element drive circuit according to the present invention.

FIG. 2 is a diagram for explaining the operation of the circuit shown in FIG.

FIG. 3 is a circuit diagram showing a configuration of a conventional semiconductor light emitting element drive circuit.

FIG. 4 is a graph showing a relationship between a light output of a semiconductor laser diode and a driving current.

FIG. 5 is a diagram for explaining the operation of the circuit shown in FIG.

[Explanation of symbols]

 1 semiconductor laser diode 50 differential switch circuit 100 constant current source 200 current mirror circuit IN1 first input terminal IN2 second input terminal Q3, Q4, Q11, Q12 NPN transistor

Claims (1)

[Claims] 1. A semiconductor light emitting device, a driving current source for outputting a driving current for driving the semiconductor light emitting device, and a semiconductor light emitting device connected to the semiconductor light emitting device and the driving current source. In a semiconductor light emitting element drive circuit including a switch circuit that selectively supplies the drive current and a switch control circuit that controls switching of the switch circuit, a current flowing through the switch control circuit is changed according to the drive current. A semiconductor light emitting element drive circuit comprising a current transfer means for changing the current.