JP2000164973A - Drive circuit for semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a laser drive pulse current where vibrations generated on a pulse current is restrained with inductive components of IC pins. SOLUTION: A semiconductor laser drive circuit, which generates a current to be supplied to a semiconductor laser in accordance with a laser modulation signal, is provided with laser drive current generating circuits 101-110 for generating a 1/10 the current of a laser drive current value. The laser drive current generating circuits 101-110 contain delaying means for delaying the laser modulation signal. Differential amplifying circuits, which consists of pairs of transistors whose emitters, are connected with each other, pairs of resistors whose one ends are connected with the respective collectors of pairs of the transistors and other ends of which are connected with a voltage source, and current sources connected with the emitters of the pairs of the transistors are contained. Current weighting is performed on the rising edge and falling edge of the laser drive current, when a pulse current is generated by arbitrarily combining the delaying means or the differential amplifying circuits of the ten laser drive current generating circuits 101-110, and a pulse current is obtained. Thereby the vibrations of output current is restrained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser driving circuit for controlling the driving of a semiconductor laser, for example, suitable for a laser beam printer or a copying machine.

[0002]

2. Description of the Related Art First, a general laser beam printer (hereinafter, referred to as LBP) to which a semiconductor laser driving circuit of the present invention is applied will be described.

Conventionally, an image forming apparatus of an LBP or a digital copying machine for outputting an image using a laser beam has a configuration as shown in FIG.

In FIG. 7, a photodiode 701 monitors a laser beam output from a semiconductor laser 702 as a laser light source. The semiconductor laser 702 is driven by a semiconductor laser driving circuit 703. The light amount control circuit 713 controls the semiconductor laser 70 based on the monitored light amount.
2 is controlled so that the output from the photodiode 701 has a predetermined value.
3 is controlled.

The polygon mirror 704 is a semiconductor laser 70
The laser beam I emitted from the light source 2 is polarized, is fixed to a motor shaft, rotates in the direction of the arrow in FIG. 7, and scans the photosensitive drum 705 with the beam I.
The f-θ lens 706 focuses the polarized laser beam I on the photosensitive drum 705.

A beam detector 707 composed of a light receiving diode detects the information writing start position on the photosensitive drum 705 by the laser beam I, and outputs a horizontal synchronizing signal generating circuit 7.
08 generates a horizontal synchronizing signal Hsync based on the output of the beam detector 707.

A blanking circuit 709 generates an unblanking signal UNBL for turning on the semiconductor laser 702 at the next timing when the beam detector 707 should detect the laser beam I, based on the horizontal synchronizing signal Hsync. 710.

A pixel modulation data generation source 711 generates a pixel clock SCK in synchronization with the horizontal synchronization signal Hsync, and further synchronizes with the pixel clock SCK to generate, for example, 8 bits of pixel modulation data DV representing pixel gradation. Is output.

A pixel modulation circuit 712 generates a pulse-width-modulated signal in synchronization with a pixel clock generated in synchronization with a horizontal synchronization signal Hsync based on pixel modulation data DV generated from a pixel modulation data generation source 71. I do.

A pulse width modulated image signal supplied from the pixel modulation circuit 712 is also input to the OR circuit 710 together with the unblanking signal UNBL. OR circuit 710
Is supplied to the semiconductor laser drive circuit 703,
Thus, the current set by the light amount control circuit 713 is supplied to the semiconductor laser 702.

[0011] By expressing gradations with such a system configuration, a high-definition image output can be obtained.

The semiconductor laser driving circuit 703 used in the above-described LBP is formed in an IC circuit, and the laser driving current output from the semiconductor laser driving circuit 703 passes through a pin (output pin) of an IC (integrated circuit). Supplied to It is a general configuration that the laser drive current is output from the collector of the transistor.

[0013]

However, in the above-described IC circuit, it is necessary to consider the influence of the output pin. In particular, the effect of the inductance is determined by the LC
A circuit is configured to generate overshoot and undershoot in the laser drive current waveform.

As a result, a vibration as shown in FIG. 8 occurs in the laser drive current waveform. The cycle of this vibration is almost

[0015]

2πf = 1 / (LC) ^1/2 (1), and as the rising and falling edges of the laser drive current become steeper, the amplitude of the vibration increases. This vibration gradually attenuates as its energy is consumed in the circuit, but it is difficult to obtain a laser beam of a high-precision light amount using such a distorted drive current. In addition, since the cycle of this oscillation has no correlation with the cycle of the laser drive current, the greater the frequency of the laser drive current, the more serious the problem.

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has as its object to obtain a laser driving pulse current capable of suppressing a vibration generated in a pulse current and having a high-precision laser light without distortion. An object of the present invention is to enable a semiconductor laser driving circuit capable of obtaining light to be easily configured in an IC circuit.

[0017]

According to a first aspect of the present invention, there is provided a semiconductor laser driving circuit for generating a current to be supplied to a semiconductor laser in accordance with an input laser modulation signal. 1 / N of drive current value
(N is a positive integer) N laser driving current generating circuits for generating a current are provided.

Here, preferably, the laser drive current generating circuit includes delay means for delaying the laser modulation signal.

Preferably, the rising and falling edges of the laser driving current are weighted by arbitrarily combining the input and output of the delay means in the laser driving current generating circuit.

Preferably, the laser drive current generating circuit includes a transistor pair having emitters connected to each other, and a resistor pair having one end connected to each collector of the transistor pair and the other end connected to a voltage source. And a differential amplifier circuit comprising a current source connected to the emitter of the transistor pair.

Preferably, the laser drive current generating circuit includes: a transistor pair having emitters connected to each other; a resistor pair having one end connected to the collector of the transistor pair and the other end connected to a voltage source; A differential amplifier circuit including a current source connected to the emitter of the transistor pair.

Preferably, the rising and falling edges of the laser driving current are weighted by arbitrarily combining inputs and outputs of the differential amplifier circuit in the laser driving current generating circuit.

[0023]

Embodiments of the present invention will be described below in detail with reference to the drawings.

In one embodiment of the present invention, the rising and falling edges of the laser driving current are controlled by controlling the rising and falling edges of the laser driving current in order to suppress the vibration described in the section of the prior art. It is configured to prevent the falling edge from becoming steep.

FIG. 1 shows a circuit configuration of a semiconductor laser drive circuit used in one embodiment of the present invention. First, 101 to 11
The drive current generation circuit indicated by 0 will be described. FIG. 2 shows an example of the configuration of each drive current generating circuit.

In FIG. 2, input pulse signals Pin1, Nin
Reference numeral 1 denotes a laser modulation signal, which is a differential signal whose polarity is inverted. VX is an environmentally stable voltage generated from the bandgap voltage Vbg. The input pulse signals Pin1 and Nin1 are input to the bases of transistors Q201 and Q202 whose emitters are connected to each other. A constant current source 1201 is connected to this emitter connection point. The collectors of the transistors Q201 and Q202 are each connected to a resistor R2 having one terminal connected to the voltage VX.
01, R202 and the bases of transistors Q203, Q204.

The emitters of the transistors Q203 and Q204 are connected to the bases of the current sources 1202 and 1203 and the transistors Q205 and Q207, respectively.
3 constitutes a buffer.

Transistor Q205, transistor Q2
07 are connected together, the transistor Q20
6 is connected to the collector. The collector current Ip of the transistor Q205 is connected to the semiconductor laser LD via an IC pin, and the collector current Is of the transistor Q207 is connected to the resistor R1 in FIG. 1 via the IC pin.

The emitter of the transistor Q206 is a resistor R
The laser drive current control signal Vs is input to the base through the ground. This voltage Vs is the output voltage of the light amount control circuit (see 713 in FIG. 7). In the light quantity control circuit, a photodiode PD (7 in FIG. 7) which is usually paired with a semiconductor laser LD (see 702 in FIG. 7) is used.
01), the voltage Vs, which is a laser drive current control signal, is controlled at regular intervals so that the semiconductor laser LD has a desired light emission amount.

When the input pulse signal Pin1 is "H" (high)
When the level is at the level, the transistor Q205 is turned on, and a current generated at the collector of the transistor Q206 by the voltage Vs flows to the semiconductor laser LD as the collector current Ip of the transistor Q205.

On the contrary, when the input pulse signal Pin1 is "L"'
When the transistor Q207 is at the (low) level, the transistor Q207 is turned on, and the current generated at the collector of the transistor Q206 is used as the collector current Is of the transistor Q207.
1, the laser LD stops emitting light.

In this drive current generation circuit, when the transistor Q207 is turned on, a current flows through the resistor R1 as a discard current. By configuring the drive current generation circuit as a differential circuit in this way, the influence of noise is reduced. Is hard to receive.

FIG. 3 shows another example of the circuit configuration of the drive current generating circuit. In the circuit of FIG. 2, a cathode common type laser cannot be used as a bias, but by using the circuit of FIG. 3, it is possible to use both the anode common type and the cathode common type laser.

In FIG. 3, input pulse signals Pin1, Nin
1 is a transistor Q30 whose emitters are connected to each other.
1, Q302. A constant current source 1301 is connected to these emitter connection points. The collectors of the transistors Q301 and Q302 are connected to resistors R301 and R3 each having one terminal connected to the voltage Vs.
02 and the bases of transistors Q303 and Q312. The emitters of the transistors Q303 and Q312 are respectively connected to the diode-connected transistor Q
It is connected to the collector / base connection point of 304, Q313 and the base of transistors Q306, Q310. The emitters of the transistors Q304 and Q313 are further input to the collector-base connection points of the diode-connected transistors Q305 and Q314. Connected to the source.

The collectors of the transistors Q307 and Q311 are both grounded, and the emitter thereof is connected to the transistor Q30.
6, connected to the emitter of Q310. The collectors of the transistors Q306 and Q310 are both connected to a power supply. Transistors Q306, Q307 and Q31
0, Q311 is connected to the transistor Q30
8, input to the base of Q309. Transistor Q3
08, Q309 are connected to each other and a resistor R3
04 is grounded. The collector of the transistor Q308 is connected to the semiconductor laser LD via an IC pin. The collector of the transistor Q309 is connected to the resistor R1 in FIG. 1 via an IC pin.

In the drive current generating circuit of FIG. 3, when the input pulse signal Pin1 is at "H" level (Nin1 is at "L" level), the transistor Q308 is turned on,
Transistor Q309 turns off. When the input pulse signal Pin1 is at L level (Nin1 is at "H" level), the transistor Q309 is turned on and the transistor Q3 is turned on.
08 turns off. By this switching operation, the resistance R
304 is selectively applied to the laser drive pulse current I.
When supplied to the semiconductor laser LD as p, the semiconductor laser LD emits light.

The current value of the laser drive pulse current Ip is determined by the voltage Vs. That is, the resistance value of the resistor R304 is R, and the base-emitter voltage of the transistor is Vb.
Assuming e

[0038]

Ip = (Vs−3 * Vbe) / R (2) This voltage Vs is the output voltage of the light quantity control circuit, although the level is different, as in FIG. Therefore, the voltage Vs is controlled so that the semiconductor laser LD emits a desired amount of light at regular intervals.

Further, since the transistors Q306 and Q307 and the transistors Q310 and Q311 have a push-pull configuration and the impedance at this output point is always low, the falling characteristic of the output pulse current Ip is improved.

When a cathode common laser is connected to the drive current generating circuit, the circuit is configured as shown in FIG. In FIG. 4, the current value Id of the current source for determining the maximum light emission amount of the semiconductor laser LD is set to the output V of the light amount control circuit.
The semiconductor laser LD stops emitting light by controlling with s and subtracting the laser drive pulse current Ip from the drive current generation circuit from this current Id. Therefore, the laser modulation signal Pi
n1 and Ninl are inverted signals when the anode common laser is used.

The ringing of the output current is suppressed by forming a semiconductor laser drive circuit as shown in FIG. 1 using a plurality of drive current generation circuits as shown in FIG. 2 or FIG. In the embodiment of the present invention, ten drive current generation circuits are used. However, the output current value of each drive current generation circuit block is 1/10 of the entire output current value. Here, the drive current generation circuit block is 10
Although it is described that a plurality of blocks are used, the number of blocks is not limited to 10 in an actual circuit configuration, and it is a matter of course that a circuit can be configured using an arbitrary number of blocks.

Next, the operation of the circuit of FIG. 1 will be described.
In FIG. 1, the input signal Pi of each of the drive current generation circuit blocks 101 to 110 depends on the state of the switches 111 to 128.
Except for the first drive current generation circuit 101, n1 and Nin1 use the input signal of the immediately preceding drive current generation circuit or output signals (Pout in FIG. 2 or FIG. 3).
1, Nout1) can be selected.

That is, the transistors Q201 and Q202, the resistors R201 and R202, the current source 1201, or the transistors Q301 and Q302 in FIG. 2 or FIG.
Edge control is performed by using the delay of the comparator constituted by R301 and resistors R302 and 1301 to change the weight of the current at the edge. The amount of delay at this time depends on the IC manufacturing process (integrated circuit manufacturing process), but is about several tens to several hundreds pS (picoseconds). The switches 111 to 119 on the right side of the drawing are turned on when their switch control signals Sd1 to Sd9 are “L”. The switches 120 to 128 on the left side of the drawing select the black circle side contacts in FIG. 4 when the switch control signals Sd1 to Sd9 are “L”.

In the state shown in FIG. 1, the weights of the currents are 1, 2, 4, 2, and 1. The degree of weighting is controlled by signals Sd1 to Sd9. The rise of the pulse current at this time is shown by the curve (a) in FIG.
Shown in At time t0, the input signals Pin1 and Nin1 start rising, and until t1, only 1/10 of the desired output current flows through the drive current generating circuit 101 only.

From time t1 to t2, the driving current generating circuit 1
In addition to 01, current also flows through the drive current generation circuits 102 and 103. Therefore, in this process, the desired output current of 3
A current of / 10 flows through the semiconductor laser LD.

From time t2 to time t3, a current further flows through the drive current generating circuits 104 to 107, and the semiconductor laser LD
Is 7/10 of the desired current, and the current flows through the drive current generating circuits 108 to 109 from time t3 to t4, so that the semiconductor laser LD has a current of 9/10 of the desired current value. Is supplied.

After the elapse of time t4, drive current generating circuit 11
Current flows to 0, and a final output pulse current Ip is obtained.

The curve (b) in FIG. 5 shows the rise of the pulse current when the weighting of the current is 4, 3, 2, and 1, and the curve (c) in FIG. Is shown.

FIG. 6 shows a case where the driving current generating circuits shown in FIG. 3 are used as the driving current generating circuits 101 to 110 of the semiconductor laser driving circuit of FIG. 1 to drive a cathode common type semiconductor laser. 3 shows simulation waveforms of the supply current of the semiconductor laser LD at the time of each weighting. This waveform characteristic is obtained when the current Id flowing into the laser LD is 100 mA and the amplitude of the pulse current is 90 mA.

As can be easily understood from FIG. 6, by generating a pulse current by performing current weighting, a more ideal current waveform can be obtained. In FIG. 6, vibration occurs at the high level of the laser drive current. This is when the transistor Q308 is off in FIG. At this time, the transistor Q308
Formed by the collector capacitance of the pin and the inductive component of the pin
The effect of the circuit is occurring on the pulse current. Conversely, when the transistor Q308 is turned on, the vibration generated in the LC circuit is connected to the resistor R30 via the collector-base junction capacitance.
4, the vibration is considered to be suppressed. Therefore, at this time, the transistor Q30
Vibration does not occur as much as when 8 is off.

In the present embodiment, the switches 111-
Although the weighting can be arbitrarily set using 128 and the switch control data Sd1 to Sd9, the circuit may be configured with a fixed weighting amount in order to reduce the circuit scale.

As described above, in the present embodiment, a plurality of drive current generating circuits are used to delay the laser modulation signal, thereby suppressing the oscillation of the output current. It is sufficient that the laser modulation signal itself can be delayed, and the circuit configuration is not particularly limited to that of the present embodiment.

Further, as described above, the pulse current is weighted at the time of generation of the pulse current to obtain the pulse current. Therefore, it is possible to obtain the pulse current in which the oscillation component of the pulse current generated by the inductive component of the IC pin is suppressed. As a result, it is possible to obtain a high-precision laser beam without distortion.

(Other Embodiments) Even if the present invention is applied to a system constituted by a plurality of devices (for example, a host computer, an interface device, a reader, a printer, etc.), an apparatus comprising one device (For example, a printer, a copying machine, a facsimile machine, etc.).

[0055]

As described above, according to the present invention,
Using a plurality of drive current generation circuits, delaying the laser modulation signal, thereby suppressing the oscillation of the output current,
In addition, since a pulse current is obtained by performing current weighting when a pulse current is generated, an inductive component of the IC pin causes
It is possible to obtain a laser drive pulse current in which oscillations generated in the pulse current are suppressed, and to easily configure a semiconductor laser drive circuit capable of obtaining a high-precision laser beam without distortion in an IC circuit. Play.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a circuit configuration of a semiconductor laser drive circuit according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing an example of a drive current generating circuit used in the semiconductor laser drive circuit of the present invention shown in FIG.

FIG. 3 is a circuit diagram showing another example of a drive current generating circuit used in the semiconductor laser drive circuit of the present invention shown in FIG.

FIG. 4 is a circuit diagram when a cathode common type semiconductor laser is used in one embodiment of the present invention.

5 is a characteristic diagram showing a temporal change of a pulse current edge when the semiconductor laser drive circuit of the present invention of FIG. 1 is used.

FIG. 6 is a waveform diagram showing a simulation waveform of a laser drive current in the semiconductor laser drive circuit of FIG. 1 constituted by the drive current generation circuit of FIG. 3;

FIG. 7 is a system configuration diagram showing a configuration example of a general LBP (laser beam printer) to which the semiconductor laser drive circuit of the present invention is applied.

FIG. 8 is a waveform diagram showing a vibration generated in a pulse current in the LBP of FIG. 7;

[Explanation of symbols]

 101 to 110 Drive current generation circuit 111 to 128 Switch 701 Photodiode 702 Semiconductor laser 703 Semiconductor laser drive circuit 704 Polygon mirror 705 Photosensitive drum 706 f-θ lens 707 Beam detector 708 Horizontal synchronization signal generation circuit 709 Blanking circuit 710 OR circuit 711 Pixel modulation data source 712 Pixel modulation circuit 713 Light intensity control circuit

Claims (6)

[Claims] 1. A semiconductor laser drive circuit for generating a current to be supplied to a semiconductor laser in accordance with an input laser modulation signal, wherein a current of 1 / N (N is a positive integer) of a desired laser drive current value is generated. A semiconductor laser drive circuit comprising N laser drive current generation circuits. 2. The semiconductor laser drive circuit according to claim 1, wherein said laser drive current generation circuit includes delay means for delaying said laser modulation signal. 3. The laser driving current generating circuit according to claim 2, wherein inputs and outputs of said delay means in said laser driving current generating circuit are arbitrarily combined to weight currents on rising and falling edges of the laser driving current. Semiconductor laser drive circuit. 4. The laser driving current generating circuit includes: a transistor pair whose emitters are connected to each other; a resistor pair having one end connected to each collector of the transistor pair and the other end connected to a voltage source; 2. The semiconductor laser drive circuit according to claim 1, further comprising a differential amplifier circuit including a current source connected to an emitter of the transistor pair. 5. The laser drive current generation circuit includes: a transistor pair having emitters connected to each other; a resistor pair having one end connected to the collector of the transistor pair and the other end connected to a voltage source; 2. The semiconductor laser driving circuit according to claim 1, further comprising a differential amplifier circuit including a current source connected to the emitter of the semiconductor laser. 6. The laser driving current generating circuit according to claim 4, wherein the input and output of said differential amplifier circuit are arbitrarily combined to weight the rising and falling edges of the laser driving current. 6. The semiconductor laser drive circuit according to 5.