JPH02205380A - Light emitting element driving circuit - Google Patents

Abstract

PURPOSE:To prevent output variation of a light emitting element by setting a current controller for controlling the level of a driving current of the light emitting element in a non-feedback type. CONSTITUTION:A laser diode 31, a photodiode 32, a CPU 33, a current controller 34, and a differential current switch 35 are provided. A bias voltage is applied to the base of a transistor 42 of the switch 35 through voltage dividing resistors 44, 45. The controller 34 is composed of 8 constant-current diodes 51-58 and 8 transistors 61-68. That is, the controller 34 for controlling the level of the driving current of the light emitting element 31 is formed in a non-feedback type. Thus, the output current of the controller 34 is not varied upon switching of the switch 35 to prevent the output variation of the light emitting element.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a light emitting element driving circuit for driving a light emitting element such as a semiconductor laser used as an image writing means of a laser beam printer or the like.

[Prior Art] As this type of light emitting element driving circuit, the one shown in FIG. 5 has been known (see Japanese Patent Laid-Open No. 62-237473).

This light-emitting element drive circuit includes a laser diode 1 as a light-emitting element, a photodiode 2 that detects the amount of light emitted from the back beam of the laser diode 1, and a photodiode 2 that detects the amount of light emitted from the laser diode 1 based on the detected value of the photodiode 2. and a D/A converter 4 that converts the correction data into an analog signal.

A current control circuit 5 controls the drive current of the laser diode 1 to a predetermined level based on the output level of the D/A converter 4, and a current control circuit 5 controls the drive current of the laser diode 1 based on the video signal etc. sent from the CPU 3. It has a differential type current switch 6 that switches on and off.

The CPU 3 A/D converts the analog detection signal obtained from the photodiode 2 via the operational amplifier, selects a specified one from among multiple reference data prepared in the memory, and combines this reference data with the above. The detected data after A/D conversion is compared, and the above correction data is generated from this difference and is supplied to the D/A converter 4.

The D/A converter 4 converts the correction data into an analog signal and supplies it to the current control circuit 5 as an output voltage.

The current control circuit 5 includes an operational amplifier 8, a transistor 9, a feedback resistor 10, etc., and controls the drive current to a predetermined value based on the output voltage and feedback voltage of the D/A converter 4.

The differential current switch 6 includes two transistors 11.1
2 and resistors 13 to 16, transistors 11 and 12 alternately open and close based on the video signal and scan ready signal obtained through the operational amplifier 17, and the drive current of the laser diode I is interrupted to correspond to the video signal. This is to obtain a modulated beam. Note that a parallel circuit of a resistor 18 and a coil 19 is inserted between the differential current switch 6 and the current control circuit 5 in order to reduce the vibration of the output current generated on the current control circuit 5 side on the laser diode 1 side. has been done.

Furthermore, this light emitting element drive circuit is provided with a resistor 20 for preventing overcurrent of the laser diode 1, a resistor 21 and a capacitor 22 for ensuring frequency characteristics, and the like.

[Problems to be Solved by the Invention] However, in the light emitting element drive circuit described above, the current control circuit 5 includes the operational amplifier 8, the transistor 9, and the feedback resistor 10.
Since the circuit is configured as a feedback type with there were. In particular, this variation in output current appears as density unevenness on an image in a laser beam printer or the like. Further, in some cases, due to this variation in output current, the amount of light from the laser diode exceeds the maximum rated value, which may lead to breakage of the end face of the laser diode.

The present invention provides the following features for the switching operation of the current switch:
It is an object of the present invention to provide a light emitting element drive circuit that does not cause fluctuations in the output current of a current control circuit and can reliably prevent fluctuations in the output of a light emitting element.

[Means for Solving the Problems] The present invention includes a current switch that intermittents the drive current of a light emitting element, and a current control circuit that is connected to the current switch and controls the drive current of the light emitting element to a predetermined level. The present invention is characterized in that the current control circuit is configured as a non-feedback type.

[Function] In the present invention, since the current control circuit that controls the level of the drive current of one light emitting element is of a non-feedback type, the output current of the current control circuit does not fluctuate in response to the switching operation of the current switch. Therefore, fluctuations in the output of the light emitting element can be reliably prevented.

[Example] FIG. 1 is a circuit diagram showing an embodiment of the present invention.

This light emitting element drive circuit includes a laser diode 31, a photodiode 32, a CPU 33, and a current control circuit 3.
4 and a differential current switch 35.

Of these, laser diode 31, photodiode 32
and differential current switch 35, peripheral operational amplifiers 36, 37, resistors 38, 39, capacitor 40, etc.
The configuration is almost the same as that shown in FIG. 5 above. However, in this example.

A bias voltage is applied to the base of the transistor 42 of the differential current switch 35 via voltage dividing resistors 44 and 45.

The current control circuit 34 includes eight constant current diodes 51 to 5.
8 and eight transistors 61 to 68.

The current value of each constant current diode 51 to 58 is given by I [mAl, the maximum rated current value of the laser diode 31 is
In order, I/2 [mA1. I/4 [Wisteria Al, I/8 [m
Al, I/16 [s+A], I/32 [mAl
, I/64[mAl, I/128[layer Al, I/
256 mAl, and by selecting a combination of these constant current diodes 51 to 58, a desired laser driving current can be obtained. In addition, the current value of each constant current diode 51 to 58 is as shown in FIG.
A resistor 71 is connected in parallel to each constant current diode 51 to 58.
By connecting the resistors 78 to 78 and appropriately setting the respective resistance values, the desired value can be adjusted.

Each of the constant current diodes 51 to 58 is connected to the collector of the corresponding transistor 61 to 68 to form eight series circuits. Each transistor 41
．． 42 emitters in series.

Further, a drive current control signal is input from the CPU 33 to the bases of the transistors 61 to 68 of the current control circuit 34 . That is, the transistors 61 to 68 are selectively turned on by this drive current control signal, and a laser drive current resulting from the combination of the constant current diodes 51 to 58 is output.

On the other hand, the CPU 33 A/D converts the analog detection signal obtained from the photodiode 32 via the operational amplifier 36, selects a specified one from a plurality of reference data prepared in the memory, and converts the reference data into The data is compared with the detected data after the A/D conversion, and a combination of current values of the constant current diodes 51 to 58 is determined from the difference. The transistors 61 to 68 correspond to the constant current diodes 51 to 58 corresponding to this combination.
In order to turn on the transistors, only the corresponding terminal among the eight terminals connected to the bases of the transistors 61 to 68 outputs a drive current control signal. That is, the signal sent from the CPU 33 to the current control circuit 34 is a drive current control signal when viewed individually for each terminal, but when viewed as a whole, it is 8-bit code data, and this code data is divided into eight constants. Since one laser drive current is obtained by selecting the current diodes 51 to 58, the current control circuit 34 of this embodiment has a structure that also functions as a conventional D/A converter.

In the manner described above, a desired laser drive current is obtained, and this laser drive current is stabilized with an accuracy of up to 256 [mAl] and controlled to a desired light emission output level.

In the above configuration, since the current control circuit 34 is configured as a non-feedback type using constant current diodes 51 to 58,
The constant current output does not oscillate due to the switching operation of the differential current switch 35, and a stable laser drive current can be supplied.

Furthermore, since the current control circuit 34 of this embodiment has a configuration that also functions as a conventional D/A converter as described above, there is no need to provide a dedicated D/A converter as in the conventional case. Since the output is sent to transistors 61 to 68 without D/A conversion, high-speed D/A is possible.
There is an advantage that a conversion operation can be obtained.

FIG. 3 is a circuit diagram showing another embodiment of the present invention.

This light emitting element drive circuit is a modification of the above-mentioned current control circuit, and the other configurations are the same as those shown in FIG. 1, so the same reference numerals are given to them and the explanation will be omitted.

The current control circuit 34' in this embodiment includes eight resistors 81 to 88 connected in series to the differential current switch 35, and eight resistors 81 to 88 connected in parallel to these resistors 81 to 88. It has eight photocouplers 91 to 98 that bypass the photocouplers 91 to 88. Each of the photocouplers 91 to 98 is composed of a phototransistor 46 and a light emitting diode 47, and the light emitting diode 47 is
Each is connected to the CPU 33 via a resistor 48 for overcurrent limiting.

In this current control circuit 34', each of the photocouplers 91 to 9 is controlled by a drive current control signal sent from the CPU 33.
By selectively turning on resistors 8 and changing the combination of bypassing each resistor 81 to 88, the overall resistance value is changed to obtain a desired laser drive current.

In this light emitting element drive circuit, the voltage value applied to the current control circuit 34' side is set larger than the voltage value on the differential type current switch 35 side.
Stable current control can be performed without being affected by voltage fluctuations on the 5 side.

When the current control circuit 34' is configured by the combination of the resistors 81 to 8B in this way, -VccllEI
Although there is a drawback that the voltage of i increases.

High-precision resistors 81 to 88 can be used,
This has the advantage of saving time and effort for adjustment.

Moreover, FIG. 4 is a circuit diagram showing another example of the differential type current switch 35'.

This differential current switch 35' connects and connects the laser diode 31 using one transistor 43. That is, by turning on and off the transistor 43 based on the video signal etc. input from the operational amplifier 37, the bypass path of the laser diode 31, which the transistor 43 constitutes, is opened and closed, and the laser diode 31 blinks. become.

Note that the present invention is not limited to the above-mentioned embodiments, and for example, the current control circuit may have a configuration in which a plurality of resistors are connected in parallel and a combination thereof is selected.

[Effects of the Invention] According to the present invention, since the current control circuit that controls the drive current level of one light emitting element is of a non-feedback type, the output current of the current control circuit is This has the effect of reliably preventing fluctuations in the output of the light emitting element without causing fluctuations.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing one embodiment of the present invention. FIG. 2 is a circuit diagram showing a configuration for adjusting the current value of a constant current diode in the same embodiment. FIG. 3 is a circuit diagram showing another embodiment of the present invention. FIG. 4 is a circuit diagram showing another example of a differential current switch. FIG. 5 is a circuit diagram showing an example of a conventional light emitting element drive circuit. 81-88...Resistance, 91-98...Photocabra.

Claims (3)

[Claims] (1) A current switch that switches on and off the drive current of the light emitting element;
a current control circuit connected to the current switch to direct current and controlling the drive current of the light emitting element to a predetermined level;
A light emitting element drive circuit characterized in that the current control circuit is configured as a non-feedback type. (2) In claim (1), the non-feedback type current control circuit has a plurality of constant current diodes, and the light emitting device is characterized in that the current is controlled by selecting a combination of these constant current diodes. Element drive circuit. (3) The light emitting element drive circuit according to claim (1), wherein the non-feedback type current control circuit has a plurality of resistors and controls the current by selecting a combination of these resistors.