JP2001223430A - Semiconductor laser driving device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that a light emission amount of a semiconductor laser can not be switched in a device wherein an analog comparator is not used. SOLUTION: In a semiconductor laser driving device which has a circuit 14 which is connected to a photodetector 5 for detecting the light emission amount of a semiconductor laser 4 and converts its output current to a voltage and a logic circuit 13 whereto the output voltage of the circuit 14 is input, compares an output voltage of the circuit 14 to a threshold value of an input terminal of the logic circuit 13, amplifies a current of the semiconductor laser 4 according to the result of the comparison and controls a light emission amount of the semiconductor laser 4 to a prescribed light amount, the current voltage conversion circuit 14 has a plurality of current voltage conversion properties.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a semiconductor laser driving device.

[0002]

2. Description of the Related Art In a conventional semiconductor laser driving device, the light emission amount of a semiconductor laser is switched by switching the reference voltage input to an analog comparator for comparing a reference voltage with an output signal of a photodetector. . FIG. 9 shows an example of a conventional semiconductor laser driving device.

This semiconductor laser driving device comprises a current source 2 for supplying a current corresponding to a terminal voltage of a hold capacitor 3.
A switch circuit 1 for switching according to a semiconductor laser lighting instruction signal; a semiconductor laser 4 for emitting light when a current supplied from the current source 2 through the switch circuit 1 flows; and a semiconductor for receiving light from the semiconductor laser 4 A photodetector 5 for generating a current proportional to the amount of light emitted from the laser 4, a current-voltage conversion circuit 6 for converting an output current of the photodetector 5 into a voltage, an output voltage of the current-voltage conversion circuit 6, and an input reference An analog comparator 7 for comparing the voltage with the voltage, a lighting instruction signal length detector 12 for detecting the signal length of the semiconductor laser lighting instruction signal, and an output signal of the lighting instruction signal length detector 12, A logic IC 8 as a logic circuit for determining whether or not to be added to the switch circuit 9;
Switch circuit 9 that switches by the output signal of 8
A current source 10 for supplying a current for charging the hold capacitor 3 when the switch circuit 9 is closed, and a resistor 11 for discharging the hold capacitor 3 when the switch circuit 9 is opened.

Next, a state in which the semiconductor laser 4 is turned on in a direct current by the semiconductor laser driving device will be described.
The switch circuit 1 is closed by the semiconductor laser lighting instruction signal, and the current source 2 supplies a current proportional to the terminal voltage of the hold capacitor 3. The semiconductor laser 4 receives a current supplied from the current source 2 through the switch circuit 1, and emits light according to the current-light conversion characteristics. The photodetector 5 is composed of a photodiode, receives light emitted by the semiconductor laser 4, and generates a current proportional to the amount of light received. This current is converted into a voltage by the current-voltage conversion circuit 6. The output voltage of the current-voltage conversion circuit 6 is input to an analog comparator 7, and the analog comparator 7 compares the output voltage of the current-voltage conversion circuit 6 with a reference voltage corresponding to a predetermined light amount.

[0005] The comparison result of the analog comparator 7 is applied to the switch circuit 9 through the logic IC 8. If the light emission amount of the semiconductor laser 4 does not reach the predetermined light amount, the switch circuit 9 is closed and the current source 10 The current supplied through the switch circuit 9 flows through the hold capacitor 3 and the resistor 11. Since the current flowing through the resistor 11 is smaller than the current supplied from the current source 10, the charge is stored in the hold capacitor 3 by the current flowing from the current source 10 through the switch circuit 9, and the terminal voltage of the hold capacitor 3 gradually decreases. To rise. When the terminal voltage of the hold capacitor 3 increases, the current generated by the current source 2 increases with the increase of the terminal voltage, and the semiconductor laser 4
The amount of emitted light increases.

When the voltage input from the current-voltage conversion circuit 6 to the analog comparator 7 reaches the reference voltage, the switch circuit 9 opens, and the current source 10
And the current supply to the resistor 11 is stopped. Then,
By discharging from the hold capacitor 3 through the resistor 11, the charge stored in the hold capacitor 3 is gradually discharged, and the terminal voltage of the hold capacitor 3 gradually decreases. When the terminal voltage of the hold capacitor 3 decreases, the current generated by the current source 2 decreases with the decrease of the terminal voltage, and the light emission amount of the semiconductor laser 4 decreases.

Here, when the reference voltage input to the analog comparator 7 is switched to a voltage corresponding to a light amount higher than the predetermined light amount, the result of comparison between the reference voltage of the analog comparator 7 and the output voltage of the current-voltage conversion circuit 6 is obtained. Means that the light emission amount of the semiconductor laser 4 has not reached the light amount corresponding to the high reference voltage, the switch circuit 9 is closed, and the current supplied from the current source 10 flows through the hold capacitor 3 and the resistor 11. Since the current flowing through the resistor 11 is smaller than the current supplied from the current source 10, the charge is stored in the hold capacitor 3 by the current flowing from the current source 10 through the switch circuit 9, and the terminal voltage of the hold capacitor 3 gradually increases. To rise. When the terminal voltage of the hold capacitor 3 increases, the current generated by the current source 2 increases with the increase of the terminal voltage, and the semiconductor laser 4
The amount of emitted light increases.

When the reference voltage input to the analog comparator 7 is switched to a voltage corresponding to a light amount lower than the predetermined light amount, the analog comparator 7
Is compared with the output voltage of the current-voltage conversion circuit 6, the result is that the semiconductor laser 4 emits light with a light amount higher than the light amount corresponding to the low reference voltage, the switch circuit 9 opens, and the current source The current supply from 10 to the hold capacitor 3 and the resistance value 11 is stopped.

Then, the electric charge stored in the hold capacitor 3 is gradually discharged by discharging from the hold capacitor 3 through the resistor 11, and the terminal voltage of the hold capacitor 3 gradually decreases. When the terminal voltage of the hold capacitor 3 decreases, the current generated by the current source 2 decreases with the decrease of the terminal voltage, and the light emission amount of the semiconductor laser 4 decreases. As described above, this conventional semiconductor laser driving device can switch the light emission amount of the semiconductor laser 4 by switching the reference voltage.

Next, a case where the semiconductor laser 4 is switched by the semiconductor laser driving device will be described.
First, the semiconductor laser 4 is turned on as described above by the semiconductor laser lighting instruction signal, and the light emission amount of the semiconductor laser 4 becomes a predetermined light amount. Thereafter, the semiconductor laser lighting instruction signal is switched to turn on / off the switch circuit 1 and the current source 2
The semiconductor laser 4 is switched on / off by turning on / off the current supplied to the semiconductor laser 4 from the switch circuit 1 through the switch circuit 1.

When the switching of the semiconductor laser 4 is performed at a high speed and the photodetector 5 and the current-voltage conversion circuit 6 cannot follow the output signal of the semiconductor laser 4, the light emission amount of the semiconductor laser 4 is actually reduced to a predetermined value. Even if the light amount (set light amount) has been reached, the voltage input from the current-voltage conversion circuit 6 to the analog comparator 7 has a value as if the semiconductor laser 4 emits light with a light amount smaller than a predetermined light amount. As a result, the switch circuit 9 is closed and current is supplied from the current source 10 to the hold capacitor 3 and the resistor 11,
The terminal voltage of the hold capacitor 3 increases, the current from the current source 2 to the semiconductor laser 4 increases, and the light emission amount of the semiconductor laser 4 increases.

In order to prevent this, the charging of the hold capacitor 3 is prohibited by the lighting instruction signal length detector 12 until the photodetector 5 and the current-voltage conversion circuit 6 follow the output signal of the semiconductor laser 4. That is, the lighting instruction signal length detector 12 determines whether or not the lighting instruction of the semiconductor laser 4 is continued until the photodetector 5 and the current-voltage conversion circuit 6 follow the output signal of the semiconductor laser 4. From the continuation length, and when the light-instruction of the semiconductor laser 4 is continued until the photodetector 5 and the current-voltage conversion circuit 6 follow the output signal of the semiconductor laser 4, the charge control permission is given to the logic IC 8. .

The logic IC 8 is an AND gate controlled by an output signal of the lighting instruction signal length detector 12,
When the charge control by the lighting instruction signal length detector 12 is permitted, the comparison result of the analog comparator 7 is added to the switch circuit 9. The lighting instruction signal length detector 12 controls the logic IC 8 to control charging when the lighting instruction of the semiconductor laser 4 is not continued until the photodetector 5 and the current-voltage conversion circuit 6 follow the output signal of the semiconductor laser 4. Without permission, the logic IC 8 does not add the comparison result of the analog comparator 7 to the switch circuit 9 and the charging of the hold capacitor 3 is prohibited.

In this conventional semiconductor laser driving apparatus, in the method of setting the light emission amount of the semiconductor laser 4 to a predetermined light amount, when the output of the semiconductor laser 4 reaches the predetermined light amount, the output voltage of the current-voltage conversion circuit 6 is changed. The current-voltage conversion characteristic is adjusted so that the voltage becomes the same as the reference voltage input to the analog comparator 7. Then, the semiconductor laser 4
When changing or switching the light emission amount of the reference voltage, the set value of the reference voltage input to the analog comparator 7 is changed or switched.

Next, a conventional semiconductor laser driving device which does not use an analog comparator will be described. FIG.
Shows an example of a conventional semiconductor laser driving device which does not use an analog comparator. In this semiconductor laser driving device, in the semiconductor laser driving device shown in FIG. 9 described above, the reference voltage input is lost, and the charge of the analog comparator 7 and the lighting instruction signal length detector 12 is allowed to perform the charge control. The functions of the logic IC 8 added to the logic circuit 9 are combined into an AND gate 13 as a logic IC constituting a logic circuit. The AND gate 13 uses the threshold value inside the input terminal as a reference voltage, compares the output voltage of the current-voltage conversion circuit 6 with the threshold value of the input terminal, and outputs the comparison result. The lighting instruction signal length detector 12 controls the AND gate 13 when the lighting instruction of the semiconductor laser 4 is not continued until the photodetector 5 and the current-voltage conversion circuit 6 follow the output signal of the semiconductor laser 4. Without permission, AND gate 13
Does not add the comparison result to the switch circuit 9 and the charging of the hold capacitor 3 is prohibited.

Since this semiconductor laser driving device does not use an analog comparator, the component cost can be reduced as compared with a conventional semiconductor laser driving device using an analog comparator 7. And gate 1
Since the threshold value inside the input terminal of No. 3 is used as the reference voltage, the reference voltage cannot be changed externally.

[0017]

In the above-described semiconductor laser driving apparatus which does not use an analog comparator, since the reference voltage is a threshold value inside the logic IC 13, the reference voltage is externally applied like a conventional semiconductor laser driving apparatus having an analog comparator. Cannot be changed to switch the light emission amount of the semiconductor laser 4.

An object of the present invention is to solve the above problems and to provide a semiconductor laser driving device which can be realized at low cost without using an analog comparator and which can switch the light emission amount of the semiconductor laser.

[0019]

In order to achieve the above-mentioned object, the invention according to claim 1 comprises a light-emitting semiconductor laser,
A photodetector for detecting a light emission amount of the semiconductor laser and outputting a current signal proportional to the light emission amount, and a current-voltage conversion circuit connected to the photodetector and converting a current output from the photodetector into a voltage And a logic circuit to which the voltage converted by the current-voltage conversion circuit is input, and compares the voltage converted by the current-voltage conversion circuit with a threshold value of the input terminal of the logic circuit. In a semiconductor laser driving device that controls the light emission amount of the semiconductor laser to a predetermined light amount by increasing or decreasing the current of the semiconductor laser, the current-voltage conversion circuit has a plurality of current-voltage conversion characteristics.

According to a second aspect of the present invention, in the semiconductor laser driving device according to the first aspect, the current-to-voltage conversion circuit includes a plurality of resistors having different resistance values and connected in parallel with each other. And a switch circuit for selecting one or more of the plurality of resistors according to a switching instruction signal.

According to a third aspect of the present invention, in the semiconductor laser driving device according to the second aspect, each of the plurality of resistors connected in parallel in the current-to-voltage conversion circuit includes:
Directly connected in series with the photodetector, or in series with the photodetector via a switch circuit that selects one or more of the plurality of resistors according to the current-voltage conversion characteristic switching instruction signal. It is connected to.

According to a fourth aspect of the present invention, in the semiconductor laser driving device according to the third aspect, each of the plurality of resistors connected in parallel in the current-voltage conversion circuit is connected to a power supply.

According to a fifth aspect of the present invention, in the semiconductor laser driving device according to the third aspect, the current-voltage conversion circuit includes an operational amplifier, and each of the plurality of resistors connected in parallel switches the current-voltage conversion characteristics. It is connected in parallel with the operational amplifier via a switch circuit that selects one or more of the plurality of resistors according to an instruction signal.

According to a sixth aspect of the present invention, in the semiconductor laser driving device according to the first aspect, the current-voltage conversion circuit comprises a plurality of resistors having at least different resistance values and connected in series; A switch circuit for short-circuiting both ends of one or more of the plurality of resistors according to a switching instruction signal.

According to a seventh aspect of the present invention, in the semiconductor laser driving device according to the sixth aspect, a resistor at one end of the plurality of serially connected resistors in the current-voltage conversion circuit is the photodetector. Is connected to the vessel.

According to an eighth aspect of the present invention, in the semiconductor laser driving device according to the seventh aspect, the resistor at the other end of the series-connected resistors in the current-voltage conversion circuit is a power supply. It is connected to.

According to a ninth aspect of the present invention, in the semiconductor laser driving device according to the seventh aspect, the current-to-voltage conversion circuit includes an operational amplifier, and the resistor block formed by the plurality of resistors connected in series includes the resistor block. It is connected in parallel with the operational amplifier.

[0028]

FIG. 1 shows a first embodiment of the present invention. The first embodiment is an embodiment of the first aspect of the present invention. In the first embodiment, in the conventional semiconductor laser driving device shown in FIG. 10 described above, a plurality of current-voltage converters for converting the output current of the photodetector 5 into a DC voltage instead of the current-voltage conversion circuit 6 are used. A current-voltage conversion circuit 14 having characteristics is used, and an output voltage of the current-voltage conversion circuit 14 is input to an AND gate 13 as a logic IC constituting a logic circuit.

The AND gate 13 compares the output voltage of the current-voltage conversion circuit 14 with the threshold value of the input terminal and outputs the comparison result to the switch circuit 9. The lighting instruction signal length detector 12 includes the photodetector 5 and the current-voltage conversion circuit 14.
When the lighting instruction of the semiconductor laser 4 is not continued until the signal follows the output signal of the semiconductor laser 4, the AND gate 1
2, the charge control is not permitted, and the AND gate 13 does not add the comparison result to the switch circuit 9, so that the charging of the hold capacitor 3 is prohibited.

Here, a plurality of, for example, five current-voltage conversion characteristics of the current-voltage conversion circuit 14 will be described with reference to FIG. 2, the horizontal axis represents the output current of the photodetector 5, and the vertical axis represents the output voltage of the current-voltage conversion circuit 14.
FIG. 2 shows five current-voltage conversion characteristics of the current-voltage conversion circuit 14 and the input terminal threshold voltage of the AND gate 13. The current-voltage conversion circuit 14 can select any one of the five current-voltage conversion characteristics through a current-voltage conversion characteristic switching instruction signal. Since the output current of the photodetector 5 is proportional to the amount of light emitted from the semiconductor laser 4, the semiconductor laser 4 is switched by switching any one of the current-voltage conversion characteristics of the current-voltage conversion circuit 14 using the current-voltage conversion characteristic switching instruction signal. , The amount of emitted light can be selected in five ways.

Next, the operation of the first embodiment will be described. The photodetector 5 receives the light emitted from the semiconductor laser 4 and converts the amount of light into a current.
4 is converted to a voltage. This voltage is AND gate 1
3 and compared with the threshold value of the input terminal. The comparison result (the difference between the output voltage of the current-voltage conversion circuit 14 and the threshold value) and the charge control stop signal from the lighting instruction signal length detector 12 Is obtained by the AND gate 13, and the output signal of the AND gate 13 is applied to the switch circuit 9.

Therefore, when the semiconductor laser 4 is lit by direct current, the charge control stop signal from the lighting instruction signal length detection terminal 12 becomes high level, and the output current of the photodetector 5 is changed by the current-voltage conversion circuit 14. Is converted into a voltage, and the voltage is compared with a threshold value of the input terminal by the AND gate 13;
The comparison result is applied to the switch circuit 9.

When the semiconductor laser 4 is switched, if the semiconductor laser lighting instruction is not continued until the photodetector 5 and the current-voltage conversion circuit 14 follow the output signal of the semiconductor laser 4, Lighting instruction signal length detection 1
2 becomes low level, the output current of the photodetector 5 is converted into a voltage by the current-voltage conversion circuit 14, and the voltage is compared with the threshold value of the input terminal by the AND gate 13. The comparison result is blocked and the switch circuit 9 remains open.

In order to switch the light emission amount of the semiconductor laser 4 to a plurality of predetermined light amounts, each of the plurality of current-voltage conversion characteristics of the current-voltage conversion circuit 14 must be
The light emission amount of the semiconductor laser 4 is set so as to be a predetermined light amount.

According to the first embodiment, the semiconductor laser 4 emits light, and the photodetector 5 detects the amount of light emitted from the semiconductor laser 4 and outputs a current signal proportional to the amount of light emitted.
A current-voltage conversion circuit 14 connected to the photodetector 5 and converting the current output from the photodetector 5 into a voltage; and a logic circuit to which the voltage converted by the current-voltage conversion circuit 14 is input. An AND gate 13 for comparing the voltage converted by the current-voltage conversion circuit 14 with the threshold value of the input terminal of the logic circuit 13, and increasing or decreasing the current of the semiconductor laser 4 based on the result of the comparison; Is controlled to a predetermined light amount, so that it can be realized at low cost without using an analog comparator. Further, since the current-voltage conversion circuit 14 has a plurality of current-voltage conversion characteristics, the light emission amount of the semiconductor laser can be switched to a plurality of predetermined light emission amounts.

FIG. 3 shows a configuration of a current-voltage conversion circuit according to a second embodiment of the present invention. This second embodiment is an embodiment of the second aspect of the present invention. The second embodiment is different from the first embodiment in that the current-voltage conversion circuit 14
Are four resistors 15 to 18 having different resistance values and connected in parallel, and these four resistors 15 to 18
And four switch circuits 19 to 22 for selecting one or a plurality of resistors by a current-voltage conversion characteristic switching instruction signal (consisting of four instruction signals).
The output current of the photodetector 5 is converted into a voltage by one of the four resistors 15 to 18 selected by the switch circuits 19 to 22.

If one resistor is selected by closing only one of the four switch circuits 19 to 22, the current-voltage conversion circuit 14 determines whether the resistance value of the selected resistor is a current-voltage conversion characteristic. Become. Also, four switch circuits 1
When a plurality of resistors are selected by closing a plurality of switch circuits out of 9 to 22, a combined resistance value of the selected plurality of resistors becomes a current-voltage conversion characteristic.

Here, since the current-voltage conversion circuit 14 has four resistors, there are a total of fifteen combinations of selecting one or a plurality of resistors. If the number of resistors of the current-to-voltage conversion circuit 14 is three, there are seven types of combinations. If the number of resistors of the current-to-voltage conversion circuit 14 is five, the types of the combinations are Is 3
One way. Note that the state where no resistor is selected is not considered here because the combined resistance value of the current-voltage conversion circuit 14 becomes infinite and the present embodiment does not function.

The second embodiment is different from the first embodiment in that the current-voltage conversion circuit 14
Selects at least one of a plurality of resistors 15 to 18 having different resistance values and connected in parallel, and one or more of the plurality of resistors 15 to 18 according to a current-voltage conversion characteristic switching instruction signal. Switch circuits 19-22
It can be realized at low cost without using an analog comparator, and can switch the light emission amount of the semiconductor laser to a plurality of predetermined light emission amounts.

FIG. 4 shows the configuration of a current-voltage conversion circuit according to a third embodiment of the present invention. The third embodiment is an embodiment of the third and fourth aspects of the present invention. In the third embodiment, in the second embodiment, the current-voltage conversion circuit 14 is different from the second embodiment in that the current-voltage conversion circuits 14 have different resistance values and are connected in parallel.
The four resistors 15-18 are connected to the four resistors 15-18.
Switch circuits 1 for selecting one or a plurality of resistors among them by a current-voltage conversion characteristic switching instruction signal
Connected in series with the photodetector 5 through 9 to 22, respectively.
The terminals of the resistors 15 to 18 opposite to the terminals connected to the switch circuits 19 to 22 are connected to a power supply (+ V). Note that one end (anode) of the photodetector 5 is grounded.

The output current of the photodetector 5 has four resistors 1
When one or a plurality of switch circuits among the four switch circuits 19 to 22 out of 5 to 18 are closed, the voltage is converted into a voltage by one or more resistors selected by the selected resistor. Also, four resistors 15 to 18
Is connected to the power supply, the current-voltage conversion circuit 14
The output voltage of the photodetector 5 is determined based on the power supply voltage.
Is a voltage reduced by a value converted into a voltage by a combined resistance value of one or a plurality of resistors selected by the four switch circuits 15 to 18.

The third embodiment is different from the semiconductor laser driving device of the second embodiment in that the current-voltage conversion circuit 14
Each of the plurality of resistors 15 to 18 connected in parallel includes a switch circuit 19 to 22 for selecting one or a plurality of resistors among the plurality of resistors 15 to 18 according to a current-voltage conversion characteristic switching instruction signal. It is connected in series with the photodetector 5 via an intermediary, and can be realized at low cost without using an analog comparator. Further, the light emission amount of the semiconductor laser can be switched to a plurality of predetermined light emission amounts.

In the third embodiment, the four resistors 15 to 18 are connected via four switch circuits 19 to 22 for selecting one or more of the resistors by a current-voltage conversion characteristic switching instruction signal. Although connected in series with the photodetector 5, the four resistors 15 to 18 are connected to the switch circuit 1
It may be directly connected in series with the photodetector 5 without passing through 9 to 22. In that case, the four switch circuits 19 to 22 are connected to portions other than between the photodetector 5 and the four resistors 15 to 18.

In the third embodiment, a plurality of resistors 15 to 18 connected in parallel in the current / voltage conversion circuit 14 are used.
Each of them is connected to a power supply, and a current-voltage conversion characteristic based on a power supply voltage can be switched by a current-voltage conversion characteristic switching instruction signal.

FIG. 5 shows a configuration of a current-voltage conversion circuit according to a fourth embodiment of the present invention. The fourth embodiment is an embodiment of the present invention. The fourth embodiment is different from the second embodiment in that the current-voltage conversion circuit 14
Selects four resistors 15 to 18 having different resistance values and connected in parallel, using one or more of the four resistors 15 to 18 by a current-voltage conversion characteristic switching instruction signal. Four switch circuits 19 to
The four resistors 15 to 18 are connected in series with an operational amplifier (operational amplifier) 23 via four switch circuits 19 to 22, respectively (−input terminal and output terminal). Between). In addition,
One end (anode) of the photodetector 5 is grounded.

The output current of the photodetector 5 has four resistors 1
The operational amplifier 23 in which the gain is set by the combined resistance value of one or more resistors selected by closing one or more of the four switch circuits 19 to 22 from 5 to 18 is selected. Converted to voltage.

The fourth embodiment is different from the semiconductor laser driving device of the second embodiment in that the current-voltage conversion circuit 14
Includes an operational amplifier 23, and each of a plurality of resistors 15 to 18 connected in parallel selects one or more of the plurality of resistors 15 to 18 according to a current-voltage conversion characteristic switching instruction signal. 19 to 22 are connected in parallel with the operational amplifier 23,
This can be realized at low cost without using an analog comparator, and further, the light emission amount of the semiconductor laser can be switched to a plurality of predetermined light emission amounts.

FIG. 11 shows the relationship between the selection and non-selection of the resistors of the current-voltage conversion circuit, the combined resistance value, and the light emission amount of the semiconductor laser 4 in the fifth and sixth embodiments of the present invention. The fifth embodiment and the sixth embodiment are one embodiment of the invention according to claims 4 and 5, and the third embodiment and the fourth embodiment are described below.
In the embodiment of the present invention, the relationship between the resistance values of the resistors 15 to 18 of the current-voltage conversion circuit 14 is as follows.

That is, the resistance value of the resistor 15 is R,
The resistance value of the resistor 16 is １ ／ of the resistance value of the resistor 15 (that is, ＲR), the resistance value of the resistor 17 is １ ／ of the resistance value of the resistor 15 (that is, １ ／ R), The resistance value of 18 is １ ／ of the resistance value of the resistor 15 (that is, １ ／ R). In FIG. 11, ○ indicates a state in which each of the resistors 15 to 18 is selected by the switch circuits 19 to 22, and X indicates a state in which each of the resistors 15 to 18 is not selected by the switch circuits 19 to 22. Po is the semiconductor laser 4 when the combined resistance value of the resistors selected by the switch circuits 19 to 22 is R.
Are shown.

In the fifth and sixth embodiments,
As can be seen from FIG. 11, the resistor 15 is treated as the least significant bit of a binary number, the resistor 18 is treated as the most significant bit of a binary number, and the resistance value of the resistor is counted up sequentially from the least significant bit. By selecting the four resistors 15 to 18 by the four switch circuits 19 to 22, the combined resistance values of the selected resistors are set to 1 / 2R, 1 / 3R, 1
.. / 4R,... 1 / 15R. Thereby, the switch circuits 19 to 22
The combined resistance value of the resistor selected by R is R, 1 / 2R, 1 /
3R, 1 / 4R,... 1 / 15R, the light emission amount of the semiconductor laser 4 becomes 2Po, 3Po, 4Po,
Po,... It goes up by an integral multiple of 15Po.

The fifth and sixth embodiments can be realized at low cost without using an analog comparator, as in the third and fourth embodiments. The light emission amount can be switched to a plurality of predetermined light emission amounts. Further, the light emission amount of the semiconductor laser can be increased by an integral multiple.

FIG. 6 shows the configuration of a current-voltage conversion circuit according to a seventh embodiment of the present invention. The seventh embodiment is an embodiment of the present invention. The seventh embodiment is different from the first embodiment in that the current-voltage conversion circuit 14
Are connected in series with four resistors 24 to 27 having different resistance values and connected in series, and each of the resistors 24 to 27 is connected in parallel to each of the resistors 24 to 27 by a current-voltage conversion characteristic switching instruction signal. It is composed of switch circuits 28 to 31 for short-circuiting both ends of 27.

The current-voltage conversion circuit 14 includes four resistors 2
Out of 4 to 27, the combined resistance value of one or a plurality of resistors whose both ends are not short-circuited by the switch circuits 28 to 31 is the current-voltage conversion characteristic. Here, in the current-voltage conversion circuit 14, since the number of resistors and the number of switch circuits for short-circuiting the resistors are four, there are a total of fifteen combinations of resistors whose both ends are short-circuited. If the number of resistors and the number of switch circuits for short-circuiting the resistors are three, the current-voltage conversion circuit 14 has seven types of combinations of resistors whose both ends are short-circuited. When the number and the number of switch circuits for short-circuiting the resistors are set to five, there are 31 types of combinations of the resistors whose both ends are short-circuited. Note that a state in which all the resistors are short-circuited is not a target here because the combined resistance value becomes 0 and the present embodiment does not function.

The seventh embodiment is different from the semiconductor laser driving device of the first embodiment in that the current-voltage conversion circuit 14
Is at least one of a plurality of resistors 24 to 27 having different resistance values and connected in series, and one or more (three or less) of the plurality of resistors 24 to 27 according to a current-voltage conversion characteristic switching instruction signal. It has switch circuits 28 to 31 for short-circuiting both ends of the resistor, can be realized at low cost without using an analog comparator, and can switch the light emission amount of the semiconductor laser to a plurality of predetermined light emission amounts.

FIG. 7 shows a configuration of a current-voltage conversion circuit according to an eighth embodiment of the present invention. This eighth embodiment is an embodiment of the invention according to claims 7 and 8. The eighth embodiment is different from the seventh embodiment in that the current-voltage conversion circuit 1
Reference numeral 4 denotes a plurality of resistors 24 to 27 having different resistance values and connected in series, and resistors 24 to 27 connected in parallel to the respective resistors 24 to 27 by a current-voltage conversion characteristic switching instruction signal. And a plurality of switch circuits 28 to 31 respectively short-circuiting both ends of each of the resistors 24 to 27. In the resistors 24 to 27, the resistor 27 at one end is connected to the photodetector 5, and the resistor 24 at the other end is Connected to.

The output current of the photodetector 5 is determined by the four resistors 2
The voltage is converted into a voltage by a combined resistance value of one or a plurality of resistors that are not short-circuited at both ends among the switch circuits 4 to 27 by the switch circuits 28 to 31. In addition, one of the four resistors 24 to 27 connected in series, which is not connected to the photodetector 5, is connected to the power supply (+ V) at one end, so that the current-voltage conversion circuit 14 The output voltage is a voltage lower than the power supply voltage by a value obtained by converting the output current of the photodetector 5 into a voltage.

The eighth embodiment is different from the semiconductor laser driving device of the seventh embodiment in that the resistor at one end of the plurality of series-connected resistors 24-27 in the current / voltage conversion circuit 14 is It is connected to the photodetector 5 and can be realized at low cost without using an analog comparator.
Further, the light emission amount of the semiconductor laser can be switched to a plurality of predetermined light emission amounts.

In the eighth embodiment, a plurality of resistors 24 to 27 connected in series in the current / voltage conversion circuit 14 are used.
The resistor at the other end is connected to the power supply, and the current-voltage conversion characteristic based on the power supply voltage can be switched by the current-voltage conversion characteristic switching instruction signal.

FIG. 8 shows the configuration of a current-voltage conversion circuit according to a ninth embodiment of the present invention. The ninth embodiment is an embodiment of the present invention. The ninth embodiment is different from the seventh embodiment in that the current-voltage conversion circuit 14
In the four resistors 24 to 27 having different resistance values and being connected in series, the resistor 27 at one end is connected to the photodetector 5 and the four resistors 24 to 27 connected in series are connected. Is connected in parallel with the operational amplifier 32, and the switch circuit 28 is connected in parallel with each of the four resistors 24-27.
To 31 are connected, and the switch circuits 28 to 31 short-circuit both ends of the resistors 24 to 27 in response to the current-voltage conversion characteristic switching instruction signal. Note that one end (anode) of the photodetector 5 is grounded.

The output current of the photodetector 5 is determined by the four resistors 2
The voltage is converted to a voltage by an operational amplifier 32 whose gain is set by a combined resistance value of one or a plurality of resistors, both ends of which are not short-circuited by the switch circuits 28 to 31.

The ninth embodiment is different from the semiconductor laser driving device of the seventh embodiment in that the current-voltage conversion circuit 14 includes an operational amplifier 32 and a plurality of resistors 2 connected in series.
The resistor blocks 4 to 27 are connected in parallel with the operational amplifier 32 and can be realized at low cost without using an analog comparator. Further, the light emission amount of the semiconductor laser can be switched to a plurality of predetermined light emission amounts. Can be.

[0062]

As described above, according to the first to ninth aspects of the present invention, the semiconductor laser can be realized at low cost without using an analog comparator, and the light emission amount of the semiconductor laser can be switched to a plurality of predetermined light emission amounts. it can.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a plurality of current-voltage conversion characteristics of the current-voltage conversion circuit according to the first embodiment.

FIG. 3 is a circuit diagram showing a configuration of a current-voltage conversion circuit according to a second embodiment of the present invention.

FIG. 4 is a circuit diagram showing a configuration of a current-voltage conversion circuit according to a third embodiment of the present invention.

FIG. 5 is a circuit diagram showing a configuration of a current-voltage conversion circuit according to a fourth embodiment of the present invention.

FIG. 6 is a circuit diagram showing a configuration of a current-voltage conversion circuit according to a seventh embodiment of the present invention.

FIG. 7 is a circuit diagram showing a configuration of a current-voltage conversion circuit according to an eighth embodiment of the present invention.

FIG. 8 is a circuit diagram showing a configuration of a current-voltage conversion circuit according to a ninth embodiment of the present invention.

FIG. 9 is a block diagram showing an example of a conventional semiconductor laser driving device.

FIG. 10 is a block diagram illustrating an example of a conventional semiconductor laser driving device that does not use an analog comparator.

FIG. 11 is a diagram showing a relationship between selection and non-selection of a resistor of a current-voltage conversion circuit, a combined resistance value, and a light emission amount of a semiconductor laser in a fifth embodiment and a sixth embodiment of the present invention.

[Explanation of symbols]

1, 9, 19 to 22, 28 to 31 Switch circuit 2, 10 Current source 3 Hold capacitor 4 Semiconductor laser 5 Photodetector 6 Current / voltage conversion circuit 7 Analog comparator 8 Logic IC 11, 15 to 18, 24 to 27 Resistor DESCRIPTION OF SYMBOLS 12 Lighting instruction signal length detector 13 AND gate 14 Current-voltage conversion circuit having plural current-voltage conversion characteristics 23, 32 Operational amplifier

Claims (9)

[Claims] 1. A semiconductor laser for emitting light, a photodetector for detecting a light emission amount of the semiconductor laser and outputting a current signal proportional to the light emission amount, and a photodetector connected to the photodetector and having an output And a logic circuit to which the voltage converted by the current-voltage conversion circuit is input, and a voltage converted by the current-voltage conversion circuit to an input terminal of the logic circuit. In the semiconductor laser driving device for controlling the light emission amount of the semiconductor laser to a predetermined light amount by increasing or decreasing the current of the semiconductor laser based on the result of the comparison, the current-voltage conversion circuit includes a plurality of current-voltage conversion circuits. A semiconductor laser driving device having characteristics. 2. The semiconductor laser driving device according to claim 1, wherein said current-to-voltage conversion circuit includes a plurality of resistors having different resistance values and connected in parallel with each other, and a current-voltage conversion characteristic switching instruction signal. And a switch circuit for selecting one or more of the resistors. 3. The semiconductor laser driving device according to claim 2, wherein each of the plurality of resistors connected in parallel in the current-voltage conversion circuit is directly connected in series with the photodetector, or A semiconductor laser driving device connected in series with the photodetector via a switch circuit for selecting one or more of the plurality of resistors according to a conversion characteristic switching instruction signal. 4. The semiconductor laser driving device according to claim 3, wherein each of the plurality of resistors connected in parallel in the current-voltage conversion circuit is connected to a power supply. 5. The semiconductor laser driving device according to claim 3, wherein said current-voltage conversion circuit includes an operational amplifier, and each of said plurality of resistors connected in parallel receives said plurality of resistors by said current-voltage conversion characteristic switching instruction signal. A semiconductor laser driving device connected in parallel with the operational amplifier via a switch circuit for selecting one or a plurality of resistors among the resistors. 6. The semiconductor laser driving device according to claim 1, wherein said current-to-voltage conversion circuit includes a plurality of resistors having different resistance values and connected in series, and said plurality of current-to-voltage conversion characteristics switching instruction signals. And a switch circuit for short-circuiting both ends of one or more of the resistors. 7. The semiconductor laser driving device according to claim 6, wherein one of the plurality of series-connected resistors in the current-voltage conversion circuit is connected to the photodetector. A semiconductor laser driving device characterized by the above-mentioned. 8. The semiconductor laser driving device according to claim 7, wherein the resistor at the other end of the series-connected resistors in the current-voltage conversion circuit is connected to a power supply. Characteristic semiconductor laser driving device. 9. The semiconductor laser driving device according to claim 7, wherein said current-voltage conversion circuit includes an operational amplifier, and a resistor block formed by said plurality of series-connected resistors is connected in parallel with said operational amplifier. And a semiconductor laser driving device.