JP2710487B2 - Semiconductor light emitting element drive circuit - Google Patents

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 device driving circuit, and more particularly to a semiconductor light emitting device driving circuit capable of improving an optical output waveform of a semiconductor light emitting device.

[0002]

2. Description of the Related Art FIG. 3 is a circuit diagram showing a conventional semiconductor light emitting element driving circuit. In the figure, 1 is a semiconductor laser diode, and Q1 and Q2 are NPN transistors constituting the differential switch circuit 50. The transistor Q1 has a collector connected to the high potential power supply 10 via the resistor 5. The transistor Q2 has an emitter of the transistor Q1.
Semiconductor laser diode 1 as emitter and collector
Are connected to the respective cathodes . The anode of the semiconductor laser diode 1 is connected to a high potential power supply 10.

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

[0004] Q5 and Q6 are NPN transistors constituting the current mirror circuit 200. Transistor Q5
The base and the collector are connected in common, and this common connection point is connected to the high potential power supply 10 via the constant current source 100, and the emitter is connected to the low potential power supply 20. The transistor Q6 has a base at the base of the transistor Q5, an emitter at the low potential power supply 20, and a collector at the transistor Q1,
It is connected to the 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 constituted by Q6 is 1: N, the collector current (drive current) of the transistor Q6 is approximately N · I _S (hereinafter referred to as _IL ).

First, a case where the low level voltage _VL is input to the first input terminal IN1 and the high level voltage _VH 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]

## EQU1 ## V _B1 = V _L -V _BE

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

[0009]

## EQU2 ## V _B2 = V _H -V _BE

## EQU1 ## Here, V _BE is a forward voltage drop between the base and the emitter of the NPN transistor. Here, since _VL < _VH , _VB1 < _VB2 , and the transistor Q1 is turned off and the transistor Q2 is turned on. As a result, the semiconductor laser diode 1 to flow a current substantially equal to I _L in 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.

As described above, the first and second input terminals IN
The on / off of the semiconductor laser diode 1 is controlled by the signal level to IN1 and IN2.

[0013] FIG. 4 is a graph showing the relationship between the semiconductor laser diode 1 of light output and the drive current I _L. As it can be seen from Figure 4, 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, it is necessary to change the drive current I _L. For example, to keep the optical output of the semiconductor laser diode 1 at P ₀ ,
I ₀ the drive current I _L when the operating ambient temperature T _a is 25 ° C.
, The operating ambient temperature T _a is at the time of 60 ° C. should the drive current I _L to I _1. In addition, I _th0 is T _a = 25
Threshold current value of semiconductor laser diode 1 at ° C., I _{th 1} is a threshold current value of semiconductor laser diode 1 in T _a = 60 ℃.

The rate of change of the current flowing through transistor Q4 during switching of transistor Q2 will be described below.

The 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. Assuming that the current amplification factor of the NPN transistor is β, the transistor Q
2, the base current I _B2 is

[0016]

## EQU3 ## I _L = I _B2 · (1 + β)

[0017]

[0018]

## EQU4 ## I _B2 = I _L / (1 + β)

## EQU1 ## This base current _IB2 is supplied from the transistor Q4. Further, since the base potential of the transistor Q2 is V _H −V _BE , the potential of the low potential power
If V, the current I _R25 flowing through the resistor 25 is

[0020]

[Number 5] _{I R25 = (V H -V BE} ) / R 25

## EQU1 ## This current I _{R25 is} also the transistor Q
Supplied from 4. Here, R ₂₅ is the resistance value of the resistor 25. Thus, the number and the output current I _E4 number 4 of the transistors Q4 when the transistor Q2 is turned on 5
Than,

[0022]

I _E4 = I _L / (1 + β) + (V _H −V _BE ) / R ₂₅

## EQU1 ##

Next, when the high level voltage V _H is input to the first input terminal IN1 and the low level voltage _VL is input to the second input terminal IN2, the transistor Q2 is turned off, and the base current of the transistor Q2 is turned off. Becomes 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, the output current I _E4 of the transistor Q4 when the transistor Q2 is off,

[0025]

[Equation 7] _{I E4 = (V L -V BE} ) / R 25

## EQU1 ## Comparing Equations 6 and 7, the output current I _E4 of the transistor Q4 when the transistor Q2 is on is equal to the driving current I _E of the semiconductor laser diode 1.
While dependent _{L, and} the transistor Q2 is the output current I _E4 of the transistor Q4 when turned off is found to be independent of the drive current I _L of the semiconductor laser diode 1.

Now, description will be made by substituting general numerical values into the above-mentioned symbols. Drive current I _L for obtaining optical output P ₀
When the T _a = 25 ℃ 50mA when, for T _a = 60 ℃ 1
00 mA. V _L = 2.0 V, V _H = 2.5
V, R ₂₅ = 2 kΩ, V _BE = 0.7 V, β = 100.

When the transistor Q2 is off, the emitter current I _{E4 A} of the transistor Q4 is equal to 0
65 mA. On the other hand, the output current I _E4B of the transistor Q4 when the transistor Q2 is on when I _L = 50 mA is 1.4 mA and I _L = 100 mA from the equation (6).
, The output current _IE4C of the transistor Q4 when the transistor Q2 is 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, and I
_{When L} = 100 mA, it becomes 2.91 (I _E4C / I _E4A ). From this, it can be seen that must greatly change the output current of the driving current I _L as increases transistor Q4. As a result, when the driving current I _L is large, the time from when the transistor Q2 is completely turned on to when it is off, that is, the time when the collector current of the transistor Q2 reaches from 0 to the maximum value (I _L ) becomes longer. duty of the diode 1 of the optical output waveform FIG. 5 (b), will vary depending on the value of the drive current I _L as shown in (c). FIG. 5B shows the relationship between the laser current _IX and the optical output when I _L = 50 mA, and FIG. 5C shows the relationship between the laser current _IX and the optical output when I _L = 100 mA.

[0030]

Since THE INVENTION Problems to be Solved] Conventional semiconductor light emitting element driving circuit is constructed as described above, when obtaining the same light output, the optical output waveform of the semiconductor laser diode 1 is the magnitude of the drive current I _L There was a problem that it would change.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and a drive circuit for a semiconductor light emitting device capable of obtaining a constant light output waveform regardless of the magnitude of the drive current for the semiconductor light emitting device is provided. The purpose is to gain.

[0032]

The present invention provides a semiconductor light emitting device, a driving current source for outputting a driving current for driving the semiconductor light emitting device, and a driving current source between the semiconductor light emitting device and the driving current source. connected, is applied to the semiconductor selectively supplying switch circuit the drive current to the light emitting element, a semiconductor light emitting element driving circuit and a switch control circuit for controlling the switch Chin grayed of the switching circuit.

In the semiconductor light emitting device driving circuit according to the present invention ,
In this case, the current flowing through the switch control circuit
A current mirror circuit composed of a control circuit and a drive current source
It is controlled according to the drive current by the road.
I do.

[0034]

According to the present invention, the current mirror circuit is used.
And the current flowing through the switch control circuit depends on the drive current.
Since the control is performed , the rate of change of the current flowing through the switch control circuit when the semiconductor light emitting element shifts from off to on or from on to off does not depend on the drive current.

[0035]

FIG. 1 is a circuit diagram showing an embodiment of a semiconductor light emitting element driving 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 NPN transistors Q11 and Q12 are newly provided. The transistors Q11 and Q12 form a current mirror circuit 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 supply 20. Other configurations are the same as those of the conventional circuit shown in FIG.

Next, the operation will be described. Now, it is assumed that 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 transistor Q6 is driving current I _L flows. Therefore, the current flowing through the transistor Q12 becomes I _L / M, and this current is supplied from the transistor Q4. The output current _IE4D of the transistor Q4 at this time is

[0037]

[ _Expression 8] I _E4D = I _L / M

## EQU4 ##

The transistor Q2 is the output current I _E4E transistor Q4 when the ON is determined in the same manner as I _E4 shown in Equation 6. That is,

[0040]

I _E4E = I _L / (1 + β) + I _L / M

## EQU1 ## The output current I _E4D when the transistor Q2 is off and the output current I _E when the transistor Q2 is on
The ratio of the _E4E (I _E4E / I _E4D) the number 8, and obtained from a number 9, the independent constant value 1 + M / (1 + β ) next to the drive current I _L. In other words, the rate of change of current flowing through the transistor Q4 when the transistor Q2 is changed from on to off or from off is constant regardless of the magnitude of the drive current I _L. As a result, the duty of the optical output waveform of the semiconductor light emitting element 1 FIG. 2 (b), Ri Do constant regardless of the magnitude of the drive current I _L as shown in (c), or
First, the effect of variations in drive current characteristics between semiconductor light emitting devices
I do not receive it.

In the above embodiment, the case where a semiconductor laser diode is used as a semiconductor light emitting element has been described. However, similar effects can be obtained by using another semiconductor light emitting element, for example, an LED.

[0043]

According to this invention, as described above, according to the present invention, Karen
Current flowing through the switch control circuit by the trimmer circuit
Is controlled in accordance with the drive current of the drive current source, so that the rate of change of the current when the semiconductor light emitting element shifts from off to on or from on to off does not depend on the drive current. As a result, there is an effect that the optical output waveform becomes constant even if the drive current changes. Moreover, such effects
The result is the effect of variations in drive current characteristics between semiconductor light emitting devices
I do not receive it.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one 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 driving circuit.

FIG. 4 is a graph showing a relationship between an optical 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. 3;

[Explanation of symbols]

 Reference Signs List 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)

(57) [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 between the semiconductor light emitting device and the driving current source. selectively supplying switch circuit the driving current, in the semiconductor light emitting element driving circuit and a switch control circuit for controlling the switch Chin grayed of the switching circuit, a current flowing through the switch control circuit, said switch system
Current mirror composed of a control circuit and the driving current source
That the circuit is controlled in accordance with the drive current.
Characteristic semiconductor light emitting element driving circuit.