JP2546144B2 - Laser diode 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 laser diode drive circuit, and more particularly, to a laser diode drive circuit which enables the drive circuit having an automatic output control circuit configuration including transistors to be integrated into an IC.

[0002]

2. Description of the Related Art Conventionally, as a laser diode drive circuit, a so-called APC (automatic power control: APC) for automatically controlling a laser diode drive current to a predetermined level by controlling a laser diode drive current with a transistor.
A circuit has been proposed. For example, FIG. 3 is a circuit diagram showing an example of a basic circuit thereof, in which a current limiting resistor R13, a driving transistor Tr of the APC circuit 11, and a constant current source I are connected to the laser diode LD, By controlling the base voltage of the driving transistor Tr, its collector current, that is, the bias current supplied from the constant current source I to the laser diode LD is controlled. The laser diode LD monitors the output light,
A photodiode PD that generates a current according to the output light level is integrally assembled. A voltage generating resistor R11 is connected to the photodiode PD, and the voltage V11 at the end of the resistor R11 is applied to the APC of the APC circuit 11. The comparator 12 compares the reference voltage Vr with the reference voltage Vr, outputs a voltage resulting from the difference from the APC comparator 12, and supplies this voltage to the base of the driving transistor Tr.

In the laser diode drive circuit of this APC configuration, when the bias current of the laser diode LD by the drive transistor Tr increases, the light emission output of the laser diode LD increases, so the output light level of the photodiode PD that monitors this increases. Is increased and the output of the APC comparator 12 which compares the resulting voltage V11 with the reference voltage Vr is reduced. Therefore, the base voltage supplied to the base of the driving transistor Tr is reduced, the collector current of the driving transistor Tr is reduced, the bias current of the laser diode LD is reduced, and the light emission output of the laser diode LD is reduced. . Conversely, when the light emission output of the laser diode LD decreases, the output of the photodiode PD decreases, the output of the APC comparator 12 increases, the base voltage of the driving transistor Tr increases, and the bias current of the laser diode LD increases. To be done. By thus controlling the base voltage of the driving transistor Tr by feeding back the light emission output of the laser diode LD, the light emission output of the laser diode LD can be automatically controlled to a desired level.

[0004]

[Problems to be Solved by the Invention] Such a conventional AP
In the laser diode drive circuit having the C circuit configuration, when the laser diode LD is deteriorated or damaged and is brought into a short-circuited state or a state close to this, the output of the photodiode PD becomes almost zero and the APC comparator is used. Since the output of 12 is the minimum, the base voltage of the driving transistor Tr is the maximum, so that the driving transistor T
The maximum current of the constant current source I flows as a collector current in r. Generally, the driving transistor Tr is h
Since the FE (current amplification factor) is made small, the base current of the driving transistor Tr becomes excessive as the collector current increases. Therefore, when the APC circuit 11 including the driving transistor Tr or the laser diode driving circuit is integrated into an IC, an excessive current flows in the circuit connected to the base of the driving transistor Tr, and I
C may be destroyed. An object of the present invention is to provide a laser diode drive circuit which prevents an excessive collector current in a drive transistor and an excessive base current resulting from the excessive collector current and prevents destruction of an IC circuit.

[0005]

A laser diode drive circuit of the present invention includes an automatic output control circuit for controlling a bias current of the laser diode, which is used for driving the laser diode.
The emitter is connected to the base of the transistor by Darlington connection.
Control transistor and laser diode emission output
And a photodiode that detects
From the laser diode output detected by the
Voltage is compared with the first reference voltage and its output is
The first comparator that outputs to the base of the transistor and the control transistor
Voltage detection that detects the collector current of the transistor as a voltage
Resistance and the voltage detected by the voltage detection resistance.
A second comparator for comparing the input voltage with a second reference voltage, and
When the collector voltage of exceeds the second reference voltage,
Limiter circuit that limits the maximum output voltage level of the comparator
With.

[0006]

Next, the present invention will be described with reference to the drawings. FIG. 1 is a block circuit diagram of an embodiment of the present invention.
In the figure, a laser diode LD has a current limiting resistor R3 and a driving transistor T of an APC circuit 1 described later.
The r1 and the constant current source I are connected in series. Further, a photodiode PD which is integrally formed with the laser diode LD and detects the light emission output thereof is provided, and a voltage generating resistor R1 is connected in series to the photodiode PD,
A voltage is generated according to the light emission output of the laser diode LD.

The APC circuit 1 is provided with an APC comparator 2 for comparing the voltage V1 generated by the voltage generating resistor R1 with the first reference voltage Vr1 generated by the first reference voltage circuit 3. , And outputs a voltage according to the comparison result of both voltages. The collector and emitter of the driving transistor Tr1 are connected in series with the laser diode LD, and the control transistor Tr2 is connected to the base of the driving transistor Tr2 in Darlington connection. The collector of the control transistor TR2 is connected to a voltage detection resistor R2 that detects a collector current as a voltage, and the base of the control transistor TR2 is connected to the output end of the APC comparator 2. Further, the collector of the control transistor TR2 compares the voltage V2 detected by the voltage detection resistor R2 with the second reference voltage Vr2 generated by the second reference voltage circuit 4 and responds to the difference. A control comparator 5 that outputs a voltage is provided, and a limiter circuit 6 that limits the maximum value of the output voltage of the APC comparator 2 according to the output of the control comparator 5 is provided.

In the laser diode drive circuit of this APC circuit configuration, when the bias current of the laser diode LD by the drive transistor Tr1 increases, the light emission output of the laser diode LD increases, so the output light of the photodiode PD that monitors this increases. The level increases, and the output of the APC comparator 2 that compares it with the first reference voltage Vr1 is reduced. Therefore, the control transistor Tr2
The base voltage supplied to the base of the control transistor Tr2 is reduced, the emitter voltage of the control transistor Tr2 is reduced, and the base voltage of the drive transistor Tr1 is reduced. As a result, the collector current of the driving transistor Tr1, that is, the bias current of the laser diode LD is reduced, and the light emission output of the laser diode LD is reduced.

On the contrary, when the light emission output of the laser diode LD is lowered, the output of the photodiode PD is lowered and A
The output of the PC comparator 2 is increased and the control transistor T
The base voltage of r2 is increased, the base voltage of the driving transistor Tr1 is further increased, and its collector current, that is, the bias current of the laser diode LD is increased.
In this way, by controlling the base voltage of the driving transistor Tr1 by feeding back the light emission output of the laser diode LD, the light emission output of the laser diode LD can be automatically controlled to a desired level.

Then, the laser diode LD fails,
Alternatively, when the circuit is broken or otherwise brought into a short-circuited state or a state close to this, as shown in the flow chart of FIG. 2, the output of the photodiode PD for detecting the light emission output of the laser diode LD is almost the same. Since the output of the APC comparator 2 that compares the voltage V1 thus obtained with the first reference voltage Vr1 becomes minimum, the driving transistor T
The base voltage of r1 becomes maximum, whereby the maximum current of the constant current source I flows as a collector current in the driving transistor Tr1. At this time, the driving transistor Tr1
Has a small hFE, and the base current of the driving transistor Tr1 increases as the collector current thereof increases, so that the collector current of the control transistor Tr2 also increases at the same time. Therefore, the voltage detection resistor R2
The voltage V2 detected at
The limiter circuit 6 is operated at the time when the detection voltage V2 exceeds the second reference voltage Vr2 by the output of the control comparator 5 that compares with r2 so that the maximum level of the output voltage of the APC comparator 2 is limited. Be operated.

As a result, the output voltage of the APC comparator 2 is forcibly reduced, the base voltage of the control transistor Tr2 is reduced, and the emitter voltage of the control transistor Tr2 is reduced accordingly. The base voltage and its emitter voltage are reduced. As a result, in the driving transistor Tr1, the collector-emitter voltage VCE is increased, and the base current is reduced with respect to the collector current. Therefore, when a failure occurs in the laser diode LD, it is possible to prevent the base current of the driving transistor Tr1 from becoming excessively large even if the bias current increases. Therefore, the laser diode driving circuit including the APC circuit 1 is integrated into an IC. It is possible to prevent the IC from being destroyed.

[0012]

As described above, according to the present invention, since the means for limiting the base current of the driving transistor provided in the APC circuit for controlling the bias current of the laser diode is provided, when the APC circuit is integrated into an IC. However, it is possible to prevent an excessive current from flowing in the base circuit of the driving transistor, and to prevent the destruction of the IC. Also,
A Darlington-connected control transistor is provided at the base of the drive transistor provided in the APC circuit, the collector current of the control transistor is detected as a voltage, and the detected voltage is compared with a reference voltage by a comparison means. Since the base voltage of the control transistor is limited by the limiter means based on the comparison result, the emitter voltage of the drive transistor is limited by limiting the base voltage of the control transistor. -It is possible to expand the voltage between the emitters and limit the maximum value of the base current with respect to the collector current of the driving transistor, thereby preventing IC destruction when the laser diode driving circuit including the APC circuit is integrated. There is an effect that can be done.

[Brief description of drawings]

FIG. 1 is a block circuit diagram of an embodiment of a laser diode drive circuit of the present invention.

FIG. 2 is a flowchart showing a part of the operation of the present invention.

FIG. 3 is a block circuit diagram of an example of a laser diode drive circuit including a conventional APC circuit.

[Explanation of symbols]

 LD laser diode PD photodiode 1 APC circuit 2 APC comparator 3 first reference voltage circuit 4 second reference voltage circuit 5 control comparator 6 limiter circuit Tr1 drive transistor Tr2 control transistor I constant current source

Claims (1)

(57) [Claims] 1. A laser diode drive circuit comprising a laser diode, a constant current source for supplying a bias current to the laser diode, and an automatic output control circuit for controlling the bias current of the laser diode. It includes a driving transistor having a collector-emitter connected between said laser diode and a constant current source, and an automatic power control circuit for controlling the collector current of the driving transistor in response to the light output of the laser diode The automatic output control circuit comprises the drive
Darlington connection to the base of the transistor
Of the controlled transistor and the laser diode
A photodiode that detects the light output and this photodiode
For laser diode emission output detected from iodine
The corresponding voltage is compared with the first reference voltage and its output is
First comparator for outputting to base of control transistor
And the collector current of the control transistor is a voltage
And the voltage detection resistor
The detected collector voltage with the second reference voltage
A second comparator, the collector voltage being a second reference voltage
Maximum level of output voltage of the first comparator when exceeded
And a limiter circuit that limits the laser diode drive circuit.