JPH0582875A - Laser diode driving device - Google Patents

Abstract

PURPOSE:To widen the range of driving current so as to obtain the maximum output of a laser diode and eliminate the fogging of print output by inserting a current limiting circuit whose resistance value varies depending on the polarity of the voltage between the output terminal of a buffer amplifier provided at a modulating part and a laser diode terminal. CONSTITUTION:As an input signal S1 changes between positive and negative maximum values, the output of a buffer amplifier 32 changes between + or -10V. The difference between this output voltage and the voltage at a cathode side terminal TCL permits current to flow in a current limiting circuit 33, this being an alternate current IAC. At this time, if the output voltage of the buffer amplifier 32 is higher, the current flows in discharging direction through a resistor 44 and a diode 43. Otherwise, it flows in charging direction through a resistor 42 and a diode 41. Therefore, the maximum value of the alternate current IAC is 48mA in the discharging direction and 48mA in the charging direction. As a result, the laser current ILD which actually flows in a laser diode LD is 2-98mA.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving device for a laser diode used as a light source for a laser printer device or the like.

[0002]

2. Description of the Related Art A conventional laser diode and its driving device will be described with reference to FIG. 1 showing an embodiment of the present invention.

In FIG. 1, a laser diode LD and a pin diode PD that receives the light emission output thereof are enclosed in one container to form a laser diode unit LDU. A cathode side terminal TLC of the laser diode LD and an anode side terminal TPA of the pin diode PD are respectively drawn from the laser diode unit LDU, and the anode side of the laser diode LD and the cathode side of the pin diode PD are commonly connected. It is drawn out as a common terminal TCM for applying a positive voltage.

The drive unit 1 has a DC current I of a constant magnitude.
Constant current unit 11 for flowing DC, modulator 1 for flowing AC current IAC whose current value changes according to input signal S1
2 and a detector 13 for stabilizing the light emission output of the laser diode LD.

The constant current section 11 and the modulation section 12 are connected in parallel with each other, and the sum of the direct current IDC and the alternating current IAC flowing into each is the laser current ILD flowing through the laser diode LD. ..

In the conventional driving device, the modulator 12 is provided with a buffer amplifier 32 whose output voltage changes between positive and negative depending on the input signal S1. The output terminal of the buffer amplifier 32 and the cathode side of the laser diode LD. One resistor R1 (not shown) is connected as a current limiting circuit between the terminal TLC and the terminal TLC.

Therefore, for example, by the constant current unit 11, a DC current ID having a size about half that of the laser current ILD is generated.
C flows, and the alternating current IAC of the modulator 12 is superimposed on this.

[0008]

However, in the above-mentioned conventional driving device, the laser diode LD cannot be sufficiently excited, and the intensity modulation necessary and sufficient to give a sufficient density difference to the photoconductor is performed. I couldn't.

That is, since a voltage of about 5 V is applied to the common terminal TCM of the laser diode LD and the forward voltage drop of the laser diode LD is about 2 V, a positive voltage of about 3 V is applied to the cathode side terminal TLC. Voltage is generated.

Therefore, even though the maximum output voltage of the buffer amplifier 32 is the same in the positive direction and the negative direction, the laser current ILD is deviated to the larger side. For example, the laser current I flowing through the laser diode LD
If the proper range of LD is 10 mA to 100 mA, the direct current IDC of the constant current unit 11 is 50 mA, the maximum output voltage of the buffer amplifier 32 is plus or minus 10 volts, and the resistance value of the resistor R1 is 250 ohms. The maximum value of the AC current IAC is the discharge direction (AC current IAC
In the negative direction of 28 mA (7/250) and in the suction direction (positive direction of the alternating current IAC) 52 mA (13/25)
0).

Therefore, the laser current ILD actually flowing through the laser diode LD is 50 m of the direct current IDC.
When added to A, it becomes 22 mA to 102 mA. Then, since the minimum current does not decrease, the dark output of the laser diode LD does not sufficiently decrease, and black fog may occur in the print output. Moreover, the maximum current becomes too large and the maximum rating of the laser diode LD is exceeded. However, the laser diode LD may be destroyed.

To solve this, a direct current IDC
When the DC current IDC is reduced to 35 mA, for example, the laser current ILD becomes 7 mA to 87 mA, the dynamic range (driving current range) becomes narrow, and the maximum output of the laser diode LD cannot be obtained.

In view of the above problems, the present invention provides a driving device for a laser diode, which can widen the driving current range, obtain the maximum output of the laser diode, and eliminate the fog of print output. It is an object.

[0014]

In order to solve the above-mentioned problems, a device according to the invention of claim 1 has a constant current section for supplying a direct current of a certain magnitude and a current value according to an input signal. In the laser diode drive device, which has a modulation unit for flowing a changing alternating current, and is configured such that the constant current unit and the modulation unit are connected in parallel to flow a laser current to the laser diode, The modulation unit is provided with a buffer amplifier whose output voltage changes according to an input signal between positive and negative, and a resistance value is provided between the output terminal of the buffer amplifier and the terminal of the laser diode depending on whether the voltage is positive or negative. Different current limiting circuits are connected and configured.

In a device according to a second aspect of the present invention, the current limiting circuit has two resistors having different resistance values and two diodes connected in series in reverse polarities with respect to the respective resistors. Composed of and.

[0016]

The current limiting circuit, which has different resistance values depending on whether the voltage is positive or negative, compensates for the asymmetry of the alternating current due to the influence of the voltage at the terminals of the laser diode.

The current limiting circuit is composed of, for example, two resistors and two diodes having different resistance values. As a result, the capacity of the buffer amplifier is efficiently utilized and the driving current range of the laser diode is widened.

[0018]

1 is a circuit diagram of a driving device 1 according to the present invention. The description of the drive device 1 in the related art section will be omitted or simplified here.

The modulator 12 comprises a differential amplifier 31 which receives the input signal S1 and the feedback signal S2 as inputs, a buffer amplifier 32 which operates as a current buffer, a resistor 34 which connects them, and a current limiting circuit 33. ing.

The differential amplifier 31 and the buffer amplifier 32 have a voltage Vc of −12V and a voltage of −12V, respectively.
Vc is supplied as an operating power supply, and the output terminal of the buffer amplifier 32 provides an output of plus or minus 10 V at maximum in accordance with the input signal S1.

The current limiting circuit 33 is constructed by connecting a series connection body of a diode 41 and a resistor 42 and a series connection body of a diode 43 and a resistor 44 in parallel. The diodes 41 and 43 are for passing the current only in one direction, and the resistors 42 and 44 are for limiting the magnitude of the alternating current IAC in the positive and negative directions.

The resistors 42 and 44 have different resistance values, and the resistor 42 has a larger value than the resistor 44. For example, resistor 42 is 250 ohms,
Resistor 44 is 125 ohms.

The detection unit 13 amplifies the detection signal output of the pin diode PD generated at both ends of the resistor 51 by the amplifier 52 with an appropriate amplification factor, and inputs the output to the differential amplifier 31 as the feedback signal S2. ..

A voltage Ve of 5 V is supplied to the common terminal TCM of the laser diode LD. The proper range of the laser current ILD flowing through the laser diode LD is 10 mA to 100 mA, the forward drop voltage by the laser diode LD is 2 V, the forward drop voltage by the diodes 41 and 43 is 1 V, and the direct current IDC by the constant current unit 11 is It is 50 mA.

Next, the driving device 1 constructed as described above.
The operation of will be described. When the input signal S1 changes over the maximum positive and negative values, the output of the buffer amplifier 32 changes between plus and minus 10 volts. Due to the difference between this output voltage and the voltage of the cathode side terminal TLC, a current flows through the current limiting circuit 33, which becomes the alternating current IAC.

At that time, when the output voltage of the buffer amplifier 32 is higher, the resistor 44 and the diode 43 flow in the discharge direction, and in the opposite case, the resistor 42 and the diode 41 flow in the suction direction.

Therefore, the maximum value of the alternating current IAC is
The discharge direction is 48 mA (6/125) and the suction direction is 48 mA (12/250), which are the same value.

Therefore, the laser current ILD actually flowing through the laser diode LD is 50 m of the direct current IDC.
When added to A, it becomes 2 mA to 98 mA. In this case, since the minimum current is sufficiently small as 2 mA, the dark output of the laser diode LD is sufficiently reduced, and black fog does not occur in the print output when used in the printer device. The maximum current is 98 mA, which is very close to and does not exceed the maximum value of the proper current of the laser diode LD, which is 100 mA, so that the maximum output of the laser diode LD can be obtained and the laser diode LD is not destroyed. ..

Further, since the driving current range of the laser diode LD can be widened by making full use of the capability of the buffer amplifier 32, the efficiency is good. In the above-described embodiment, the direct current IDC by the constant current unit 11 is set near the center value of the laser current ILD, so that the driving with good linearity can be performed. But in general,
Considering the characteristics of the photoconductor used in the printer,
Since it is considered that some kind of compensation circuit is often provided in order to obtain the linearity of the entire system, it is not always necessary to improve the linearity of only the driving device 1. Therefore, it is possible to select the DC current IDC to another arbitrary value without considering the linearity. Accordingly
The values of the resistors 42 and 44 may be selected.

In the above-described embodiment, the circuit configurations of the constant current section 11, the modulation section 12, etc. can be variously modified other than those described above.

[0031]

As described above, according to the present invention, it is possible to provide a laser diode drive device which can widen the drive current range, obtain the maximum output of the laser diode, and eliminate the fog of the print output.

According to the second aspect of the invention, the current limiting circuit can be constructed easily and at low cost.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a driving device according to the present invention.

[Explanation of symbols]

 1 Driving Device 11 Constant Current Section 12 Modulation Section 32 Buffer Amplifier 33 Current Limiting Circuit 41, 43 Diode 42, 44 Resistor LD Laser Diode ILD Laser Current IDC DC Current IAC AC Current

Claims (2)

[Claims] 1. A constant current section for flowing a DC current of a constant magnitude, and a modulation section for flowing an AC current whose current value changes according to an input signal. The constant current section and the modulation section are provided. A laser diode driving device configured to flow a laser current to a laser diode by being connected in parallel with a section, the modulating section is provided with a buffer amplifier whose output voltage changes between positive and negative depending on an input signal. A laser diode driving device, characterized in that a current limiting circuit having a different resistance value depending on whether the voltage is positive or negative is connected between the output terminal of the buffer amplifier and the terminal of the laser diode. .. 2. The current limiting circuit comprises two resistors having different resistance values, and two diodes connected in series to the respective resistors in opposite polarities. The driving device for the laser diode according to claim 1.