JP4698086B2 - Semiconductor laser driving circuit and image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser driving circuit and an image forming apparatus, and more particularly to a semiconductor laser driving circuit and an image forming apparatus for a semiconductor laser used in a laser printer, an optical disk device, a digital copying machine, an optical communication device, and the like. .
[0002]
[Prior art]
Conventional semiconductor laser driving circuits are roughly classified into a non-bias type and a bias type. The no-bias method is a method in which a bias current of a semiconductor laser is set to 0 and a laser diode (hereinafter referred to as “LD”) is driven by a pulse current corresponding to an input signal. In this method, the bias current of the laser is set to the threshold current of the semiconductor laser, and the LD is driven by adding a pulse current corresponding to an input signal to the bias current while constantly supplying the bias current.
[0003]
[Problems to be solved by the invention]
By the way, when a semiconductor laser having a large threshold current is driven in a biasless manner, a certain amount of time is required until a carrier having a concentration capable of laser oscillation is generated even when a drive current corresponding to an input signal is applied to the LD. In short, there is a time delay before light emission. As a result, there is no problem if the input signal is sufficiently larger than the light emission delay time and the light emission delay amount can be ignored, but if you want to drive a semiconductor laser in a laser printer, optical disk device, digital copier, etc. at high speed, Only pulses smaller than the pulse width can be obtained.
[0004]
Therefore, in order to reduce the time delay until laser emission, a biased method in which a semiconductor laser oscillation threshold current is supplied in advance has been proposed. In this biased method, since the oscillation threshold current of the semiconductor laser is supplied in advance, the light emission delay time is eliminated, but even when no light is emitted, light is always emitted in the vicinity of the oscillation threshold (usually 200 μW to 300 μW). Therefore, the extinction ratio becomes small in the case of optical communication, and it becomes a cause of background stains in the case of a laser printer, an optical disk device, a digital copying machine, and the like.
[0005]
In order to solve such a problem, in the field of optical communication, in Japanese Unexamined Patent Publication No. 4-283978, Japanese Unexamined Patent Publication No. 9-83050, etc., basically, a non-bias method is used, and an oscillation threshold is set immediately before light emission. A configuration for passing current has been proposed. Recently, however, systems using a 650-nm red LD, a 400-nm ultraviolet LD, and the like have begun to be put into practical use in laser printers, optical disk devices, digital copying machines, and the like in order to further increase the resolution. These semiconductor lasers have a characteristic that it takes a long time to generate carriers having a concentration capable of laser oscillation, compared to conventional 1.3 μm, 1.5 μm, and 780 nm band LDs. The above method also has a problem that only a pulse width smaller than a desired pulse width can be obtained.
[0006]
The present invention has been made in view of the above problems, and an object thereof is to provide a high-speed and high-precision semiconductor laser driving circuit and an image forming apparatus.
[0007]
[Means for Solving the Problems]
In order to solve the above problems, the present invention employs means for solving the problems having the following characteristics.
[0008]
  According to the first aspect of the present invention, there is provided a bias current that constantly supplies a minute bias current to the semiconductor laser.sourceWhen,SaidThreshold current for supplying a threshold current to a semiconductor lasersourceWhen,Flash commandThe semiconductor laser emits light in response to a signalmodulationA current source,A semiconductor laser driving circuit for driving the semiconductor laser with a current that is a sum of three currents from the bias current source, the modulation current source, and the threshold current source;
Providing a first switch between the semiconductor laser and the modulation current source;
A second switch is provided between the semiconductor laser and the threshold current source;
The first switch is controlled to be turned on / off by a modulation signal that is a delayed light emission command signal,
The second switch is turned on before the first switch is turned on, and is turned off after confirming that the modulation signal is turned off.This is a semiconductor laser driving circuit.
[0009]
  According to the first aspect of the present invention, the bias current that constantly supplies a minute bias current to the semiconductor laser.sourceWhen,SaidThreshold current for supplying a threshold current to a semiconductor lasersourceWhen,Flash commandThe semiconductor laser emits light in response to a signalmodulationA current source,A semiconductor laser driving circuit for driving the semiconductor laser with a current that is a sum of three currents from the bias current source, the modulation current source, and the threshold current source;
Providing a first switch between the semiconductor laser and the modulation current source;
A second switch is provided between the semiconductor laser and the threshold current source;
The first switch is controlled to be turned on / off by a modulation signal that is a delayed light emission command signal,
The second switch is configured to be turned on before the first switch is turned on, and to be turned off after confirming that the modulation signal is turned off.Thus, a high-speed and high-precision semiconductor laser driving circuit can be provided.
[0010]
  The invention described in claim 2 is the semiconductor laser driving circuit according to claim 1, wherein the bias is applied.Current sourceThe current generated in is less than a few mA.
[0011]
  According to the invention described in claim 2, the biasCurrent sourceSince the current generated in the step is several mA or less, a sufficiently high extinction ratio can be secured, and a high-speed and high-precision semiconductor laser driving circuit can be provided.
[0012]
  The invention described in claim 3 is the semiconductor laser drive circuit according to claim 1, whereinThe second switch includes the modulation signal and the light emission command.Depending on the logical OR signal,Turn onIt is characterized by that.
[0013]
  According to the invention described in claim 3, theThe second switch includes the modulation signal and the light emission command.Depending on the logical OR signal,Turn onByThe second switch is turned on before the first switch is turned on.,Flash commandIt is possible to provide an LD driving circuit capable of accurately emitting light from an LD according to a signal.
[0014]
  The invention described in claim 4 is the semiconductor laser driving circuit according to claim 1, whereinmodulationCurrentsourceHas initialization means that operates when power is turned on or reset is released, and the initialization means sets the light amount when the semiconductor laser emits light to a predetermined value.
[0015]
According to a fifth aspect of the present invention, in the semiconductor laser drive circuit according to the fourth aspect, the initialization unit is configured to provide a current or voltage when the light amount of the semiconductor laser is a predetermined value and a light amount of the semiconductor laser. The difference between the current and voltage when the value is smaller than the value is detected, and the amount of light when the semiconductor laser emits light is set to a predetermined value.
[0016]
According to a sixth aspect of the present invention, in the semiconductor laser driving circuit according to the fourth aspect, the initialization unit causes the semiconductor laser to emit an offset light when the light quantity of the semiconductor laser is a predetermined value. In this case, the difference between the current and voltage is detected, and the amount of light when the semiconductor laser emits light is set to a predetermined value.
[0017]
According to a seventh aspect of the present invention, in the semiconductor laser driving circuit according to the fourth aspect, the initialization unit is configured to provide a current or voltage when the light amount of the semiconductor laser is a predetermined value and a light amount of the semiconductor laser. A difference from a current or voltage in the case of 1 / N of the value (N is a natural number of 2 or more) is detected, and the amount of light at the time of light emission of the semiconductor laser is set to a predetermined value. .
[0018]
According to an eighth aspect of the present invention, in the semiconductor laser drive circuit according to any one of the fifth to seventh aspects, the initialization unit includes a timing generation unit, a detection unit that detects the difference, and the semiconductor laser. The current setting unit for setting the amount of light at the time of light emission, the value detected by the detection unit based on the timing generated by the timing generation unit, and the value set by the current setting unit are sequentially compared. It is characterized by comprising a comparison unit to be performed.
[0019]
According to the invention described in claims 4 to 8, the initializing means sets the light amount at the time of light emission of the initial semiconductor laser to a predetermined value, and does not cause overshoot or the like with a simple configuration. A semiconductor laser drive circuit capable of high-speed and high-precision pulse output can be provided.
[0020]
According to a ninth aspect of the present invention, in the semiconductor laser driving circuit according to any one of the first to eighth aspects, a light receiving unit that detects a light output of the semiconductor laser, and the semiconductor laser detected by the light receiving unit. And a current control means for controlling a current supplied to the semiconductor laser based on a light reception signal proportional to the light output of the semiconductor laser.
[0021]
  According to a tenth aspect of the present invention, in the semiconductor laser driving circuit according to the ninth aspect, the current control means generates a control signal by comparing the magnitude of the received light signal with a predetermined value, and the control signal By the threshold currentsourceIt is characterized by controlling.
[0022]
  According to the invention described in claim 9 or 10, the control signal is generated based on the light reception signal and the control signal having a predetermined value, and the threshold current is generated by the control signal.sourceBy controlling the above, it is possible to provide a semiconductor laser driving circuit having a stable output even when there is a change due to temperature.
[0023]
  The invention described in claim 11 is the semiconductor laser drive circuit according to claim 10, wherein the current control means isFirst switchSample the control signal when is in the ON state, and based on the sample value, the threshold currentsourceIt is characterized by controlling.
[0024]
  According to the invention described in claim 11, theFirst switchBy sampling the control signal when the signal is in the ON state and controlling the threshold current generation means based on the sample value, for example, not only the light amount adjustment is performed outside the image writing area, but also within the writing area. However, if the LD is turned on, the semiconductor laser drive circuit having a stable output can be provided even if there is a change due to temperature by performing sampling and controlling each time.
[0025]
According to a twelfth aspect of the present invention, in the semiconductor laser drive circuit according to any one of the first to eleventh aspects, a voltage of an output unit that drives the semiconductor laser is detected, and the semiconductor laser is detected based on the detected value. It has a means to control the voltage of the power supply to supply.
[0026]
According to the twelfth aspect of the invention, the power consumption is provided by detecting the voltage of the output unit for driving the semiconductor laser and controlling the voltage of the power source supplied to the semiconductor laser based on the detected value. It is possible to realize an LD driving circuit that can drive a large number of LDs.
[0027]
According to a thirteenth aspect of the present invention, there is provided a semiconductor laser whose output is modulated by an image modulation signal, scanning means for scanning the rotating photoconductor with the light of the semiconductor laser, and the image modulating signal on the rotating photoconductor. 13. An image forming apparatus for forming a corresponding electrostatic latent image, wherein the semiconductor laser is driven by the semiconductor laser driving circuit according to claim 1.
[0028]
According to the thirteenth aspect of the present invention, since the semiconductor laser driving circuit according to any one of the first to twelfth aspects is used, an image forming apparatus capable of outputting a pulse at higher speed and higher accuracy is realized. Can do.
[0029]
According to a fourteenth aspect of the present invention, in the image forming apparatus according to the thirteenth aspect, the shading correction is performed by changing the full-scale value of the driving current generating unit according to the scanning of the scanning unit. Features.
[0030]
  According to the invention described in claim 14, by performing the shading correction by changing the full scale value of the drive current generating means according to the scanning of the operating means, the shading correction can be surely performed. it can.
  According to a fifteenth aspect of the present invention, in the image forming apparatus according to any one of the first to fourteenth aspects, the threshold current is turned on 1 to 10 ns before the drive current is turned on. To do.
  According to the fifteenth aspect of the present invention, the threshold current is turned on 1 to 10 ns before the drive current is turned on, so that it can be a background stain that does not cause a problem on the image.
  According to a sixteenth aspect of the present invention, in the image forming apparatus according to any one of the first to fifteenth aspects, the threshold current is a few ns after the drive current is turned off.offIt is characterized by becoming.
  According to the invention described in claim 15, the threshold current is a few ns after the drive current is turned off.offAs a result, it is possible to obtain a background stain that does not cause a problem in the image.
[0031]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described with reference to the drawings.
[0032]
In the present invention, focusing on the characteristics of the LD, the semiconductor laser is driven by the sum of three currents of a bias current, an oscillation threshold current, and a light emission current. Here, unlike the biased bias current, the bias current is a very small amount of current. The characteristic of the semiconductor laser is that the impedance is quite large in the unbiased state, and even if a threshold current is passed from this state, it does not immediately reach the level at which the light emission delay time disappears due to the influence of the inductance component in the semiconductor laser, for example, Even if the current is made to flow through the semiconductor laser even at about 1 mA, the impedance of the semiconductor laser is considerably lowered. Therefore, if a threshold current is passed from this state, the light emission delay time can easily be eliminated. It is effective even with current. In this case, the amount of light emitted from the semiconductor laser is at a sufficiently small level, so that the extinction ratio in the case of optical communication, which is a problem with the biased method, is not reduced. It is possible to provide a semiconductor laser driving circuit and an image forming apparatus that do not cause background stains in an apparatus, a digital copying machine, or the like.
(Basic configuration of the present invention)
A basic conceptual diagram (basic configuration) of the present invention is shown in FIG. In FIG. 1, the drive current source that flows to the semiconductor laser 10 is composed of a sum of currents from three current sources, that is, a bias current source 12, a threshold current source 11, and a modulation current source 13. Of these, the bias current source 12 is a current source that supplies a current of about several mA at most at about 1 mA. The threshold current source 11 is a current source that passes a threshold value at which the semiconductor laser 10 emits light. In FIG. 1, since there is the bias current source 12, the current value obtained by subtracting the current value (threshold current-bias current) may be used. The modulation current source 13 is a current source that is modulated in accordance with an input signal, and thereby the light emission of the semiconductor laser 10 is controlled.
[0033]
Here, the bias current source will be described with reference to FIGS. 2 and 3 show actual measurement results of the output P (μW) and the LD drop voltage VLDDOWN when a minute current is passed through a certain LD. 2 and 3, the LD drop voltage VLDDOWN is already generated at about 1.4 V when the LD current ILD is 250 μA, and gradually increases as the ILD increases. Since LD has a DC resistance component, VLDDOWN increases gradually as ILD increases. When ILD is 0, VLDDOWN is 0, whereas when ILD is 250 μA, VLDDOWN is generated at 1.4 V, and the impedance of LD is sufficiently small by only flowing 250 μA through ILD. I understand. Accordingly, it can be predicted that if the threshold current is supplied after 250 μA is supplied, the response characteristic is sufficiently improved. In other words, for example, it can be seen that when a very small bias current of about 1 mA is passed through the LD, the LD drops little and the LD responds at high speed. Further, the light emission output of the LD at this time is 1.26 μW even at 1 mA, and is about 0.1% considering that the normal LD light emission amount is 1 mW or more. It can be seen that there is no level of contamination in printers and digital copying machines. In addition, even in the case of having one PD (photodiode) and many LDs as in an LD array, the other LD emits light of about 1 μW for controlling the light quantity of one LD. It does not matter. In FIGS. 2 and 3, the characteristics of a certain LD are measured and described as an example, but other LDs also exhibit similar characteristics.
(First configuration: generation of control signal of threshold current source)
FIG. 4 shows a first configuration example of the present invention. FIG. 4 shows a configuration example for generating a control signal for the threshold current source. Since the threshold current source 11 of the LD varies greatly depending on the temperature, the threshold current source is controlled at all times or using a sample hold circuit. In the figure, the output voltage of the PD 20 that detects the light output of the semiconductor laser 10 is compared with the light emission control voltage, and the threshold current source 11 is controlled so that the output of the PD 20 matches the light emission control voltage.
[0034]
On the other hand, the bias current source 12 may be a fixed minute current, and the modulation current source 13 is less likely to change due to temperature if it is set by measuring characteristics inherent to the LD at the initial setting. As good.
[0035]
With such a configuration, a high-speed and high-precision semiconductor laser driving circuit can be provided.
(Second configuration)
FIG. 5 shows a second configuration example of the present invention. FIG. 5 shows a configuration in which a switch circuit is connected in series to a threshold current source and a modulation current source. When the threshold ON signal is applied to the switch circuit 31, the current of the threshold current source 11 is supplied to the LD 10. Similarly, when a modulation signal is applied to the switch circuit 32, the current of the modulation current source 13 is supplied to the LD10. Examples of the timing of the modulation signal and the threshold ON signal are shown in FIGS.
[0036]
FIG. 6 shows a light emission command signal (A), a delay pulse (B) of the light emission command signal, a modulation signal (C), a threshold ON signal (D), an LD drive current (E), and an optical waveform (F )It is shown. Although not shown in FIG. 6, the light emission command signal (A) input from the outside is delayed by the delay circuit to become the modulation signal (C), and the logical sum of the light emission command signal (A) and the delay signal (B) is obtained. It becomes a threshold ON signal (D).
[0037]
At this time, the LD drive current (E) is supplied to the LD 10. The LD drive current (E) is the sum of bias current + threshold current + modulation current and 3 currents as shown in FIG. 6 (E). Further, the threshold current is turned on 1 to 10 ns before the modulation current is turned on and turned off simultaneously with the modulation current. The shorter the time difference between the threshold current and the modulation current, the better. However, assuming an actual laser printer or digital copying machine, if the light emission is weak due to the threshold current if it is about 1 dot or less, the light emission Is small, and it is not a problem because it is slightly soiled. In the case of using a red LD or an ultraviolet LD, it has a characteristic that it takes a long time until a carrier having a concentration capable of laser oscillation is generated as compared with an infrared LD. In some cases, a threshold current needs to flow about 10 ns before, and when an LD driver is made into an ASIC (Application Specific IC), if it has a function to control the delay time from the outside, a wide variety of LDs can be used. An ASIC of a corresponding LD driver can be realized.
[0038]
FIG. 7 shows another example of the modulation signal and the threshold-on signal timing different from those in FIG. FIG. 7 shows the light emission command signal (A), the delay pulse (B) of the light emission command signal, the modulation signal (C), the threshold on signal (D), the LD drive current (E), and the LD output as in FIG. An optical waveform (F) is shown.
[0039]
FIG. 7 shows an example of timing for turning off the threshold current after the modulation signal of the threshold on signal is turned off. Normally, even if the modulation signal is turned off at the same time, it is difficult to turn off the threshold value on signal at exactly the same timing. Therefore, the threshold value on signal is turned off after confirming that the modulation signal is turned off. However, the difference is at most a few ns, and even when a laser printer or a digital copying machine is assumed, the background is only a few ns, so there is no problem on the image. By configuring in this way, it is possible to realize an LD driving circuit that can accurately output a pulse without the threshold on signal being turned off prior to the modulation signal.
(Detection of differential quantum efficiency of semiconductor laser and initial setting of modulation current source)
When the power supply voltage is turned on (or when the LD is turned off), the differential quantum efficiency of the LD (the gradient characteristic of the output power with respect to the LD current (differential characteristic)) is detected, and the modulation current is initially set based on the characteristic. . FIG. 8 shows a configuration relating to detection of the differential quantum efficiency of the semiconductor laser and initial setting of the modulation current source at that time. FIG. 8 includes a timing generation unit 51, a differential quantum efficiency detection unit 52, and a D / A (digital / analog conversion) unit 53.
[0040]
The initial setting of the modulation current source is the voltage VLD applied to the LD 10 when the LD 10 is at full power (maximum power)._FULL(The current flowing through the LD at that time is expressed as I_OPAnd the voltage VLD applied to the LD 10 when the threshold current flows._TH(The current flowing through the LD at that time is expressed as I_THThe difference between these voltages is VLD._DAnd the difference between these currents is I_DAnd This VLD_DOr I_DIs used to calculate the current value of the initial modulation current source (I_OP-I_TH). As a result, when the modulation signal is on, the current that is combined with the current of the threshold current source 11 and the LD emits light at full power can be supplied from the modulation current source 13. In addition, since the current supplied from the modulation current source 13 is combined with the current of the threshold current source 11 and has a substantially critical current value at which the LD emits light at full power, the consumption of the modulation current is minimized. be able to. Further, even when combined with the current of the threshold current source 11 and the bias current source, an excessive current is not supplied to the LD, and the life of the LD can be extended.
[0041]
Here, two examples of the setting method will be described.
[0042]
The first method will be described with reference to FIGS. FIG. 9 is a diagram showing the differential quantum efficiency characteristics of the LD, in which the current supplied to the LD increases and the current value I_THThe LD starts to emit light (the voltage applied to the LD at that time is VLD_THThe output power of the LD at that time is P_THIt is. ). Also, the maximum power P determined by the standard etc._OOutput current I_OP(The voltage applied to the LD at that time is VLD._OPIt is. ).
[0043]
10A shows the timing signal LVCO from the timing generator 51, FIG. 10B shows the current supplied to the LD, and FIG. 10C shows the digital signal output from the D / A. Value. Note that the values in FIG. 10C are merely examples, and are not limited thereto.
[0044]
The timing generation unit 51 operates only at the time of initialization, and supplies the timing signal LVCO shown in FIG. 10A to the differential quantum efficiency detection unit 52. The differential quantum efficiency detector 52 generates 10 timings from T = 0 to T = 9 based on the timing signal LVCO. The differential quantum efficiency detection unit 52 performs processing according to each timing, and outputs, for example, an 8-bit value to the D / A unit 53 in accordance with the timing from the timing generation unit 51. For example, values such as 1, 0.5, 0.25, and 0.125 are output from the D / A unit 53 in descending order.
[0045]
The differential quantum efficiency detection unit 52 forcibly turns on the LD (full power lighting) at T = 0, and the LD emits offset light emission (I) at T = 1._THThe LD is turned off (bias current about 1 mA) at T = 9. In addition, at T = 1, I_OP-I_THHold the difference. On the other hand, values such as 1, 0.5, 0.25, 0.125, etc. are output in order from T = 0 to T = 9 to the D / A unit 53 in accordance with the timing from the timing generation unit 51. Here, for example, I_OP-I_THThe value of D / A section 53 corresponds to 1 mA, 0.5 mA, 0.25 mA, 0.125 mA, etc. Thus, the following description will be made assuming that the modulation current source 13 is controlled.
[0046]
At T = 2, an output of 1 is applied to the modulation current source 13 from the D / A section 53, and a current of 1 mA flows from the modulation current source 13. The differential quantum efficiency detector 52 detects this current and compares it with 0.7 mA held. As a result, since 1 mA> 0.7 mA, the differential quantum efficiency detection unit 52 ignores “1” and prepares for the next timing.
[0047]
At T = 3, an output of 0.5 is applied from the D / A unit 53 to the modulation current source 13, and a current of 0.5 mA flows from the modulation current source 13. The differential quantum efficiency detector 52 detects this current and compares it with 0.7 mA held. As a result, since 0.5 mA <0.7 mA, the differential quantum efficiency detection unit 52 sets “0.5” to prepare for the next timing.
[0048]
At T = 4, an output of 0.25 is applied from the D / A unit 53 to the modulation current source 13, and a current of 0.25 mA flows from the modulation current source 13. The differential quantum efficiency detector 52 detects this current, and compares it with 0.75 mA added to “0.5 mA” corresponding to “0.5” set previously, and 0.7 mA held. To do. As a result, since 0.75 mA> 0.7 mA, the differential quantum efficiency detection unit 52 ignores “0.5” and prepares for the next timing.
[0049]
At T = 5, an output of 0.125 is applied from the D / A unit 53 to the modulation current source 13, and a current of 0.125 mA flows from the modulation current source 13. The differential quantum efficiency detector 52 detects this current, and compares it with 0.625 mA added to “0.5 mA” corresponding to “0.5” set previously, and 0.7 mA held. To do. As a result, since 0.625 mA <0.7 mA, the differential quantum efficiency detection unit 52 sets “0.125” to prepare for the next timing.
[0050]
At T = 6, an output of 0.0625 is applied from the D / A unit 53 to the modulation current source 13, and a current of 0.0625 mA flows from the modulation current source 13. The differential quantum efficiency detection unit 52 detects this current, and is held at 0.6875 mA which is the sum of “0.625 mA” corresponding to “0.5” and “0.125” set in advance. Compare with 0.7mA. As a result, since 0.6875 mA <0.7 mA, the differential quantum efficiency detection unit 52 sets “0.0625” to prepare for the next timing.
[0051]
At T = 7, an output of 0.03125 is applied from the D / A unit 53 to the modulation current source 13, and a current of 0.03125 mA flows from the modulation current source 13. The differential quantum efficiency detection unit 52 detects this current and adds up “0.6875 mA” corresponding to “0.5”, “0.125”, and “0.0625” set in advance. Compare 71875 mA with the 0.7 mA held. As a result, since 0.71875 mA> 0.7 mA, the differential quantum efficiency detection unit 52 ignores “0.03125” and thereafter. In this way, the default current value of the modulation current source is set to (I_OP-I_TH). In this example, the differential quantum efficiency detection unit 52 sets the set values of “0.5”, “0.125”, and “0.0625” as the output value of the D / A unit 53, and corresponds to this output value. A current of “0.6875 mA” flows from the modulation current source 13.
[0052]
The above numerical value is an example. Moreover, it can also be set as the numerical value rounded arbitrarily. Further, the example of FIG. 10 is a case where the D / A has an 8-bit configuration, but the required number of timings is changed depending on the number of bits constituting the D / A.
[0053]
In this example, the threshold current I at initialization is_THTherefore, a current source that operates only at the time of initialization for setting so that a desired offset light emission value can be obtained from the external terminal may be provided. Further, the timing signal LVCO may be configured such that its timing can be adjusted by an external terminal.
[0054]
The second method will be described with reference to FIGS. The difference from the first method is that the value held in the differential quantum efficiency detector 52 is (I_OP-I_TH) Instead of (I_OP/ 2-I_TH). Therefore, the differential quantum efficiency detection unit 52 holds (I_OP/ 2-I_TH) And the current corresponding to the value of the D / A section 53 from T = 2 to T = 9. Other than that, it is the same as the first method, and the description is omitted.
[0055]
In the above description, the current value (I_OP-I_TH) Or (I_OP/ 2-I_TH), But this current value (I_OP-I_TH) Or (I_OP/ 2-I_TH) LD voltage (VLD)_FULL-VLD_TH) Or (VLD_FULL/ 2-VLD_TH) May be used to initialize the modulation current source using the LD voltage.
[0056]
Also, in the first method, the LDs are driven and compared from the D / A unit 53 in the descending order of, for example, 1, 0.5, 0.25, 0.125, and the like. As a result, since the initial value is large, there is a case where the drive is performed greatly exceeding the standard of the LD, which may cause damage to the LD or shortening of the life. However, in the second method, (I_OP-I_TH) Instead of (I_OP/ 2-I_TH) Does not occur. However, in the second method, (I_OP/ 2-I_TH) Is compared with the doubled value, the control accuracy is lower than that of the first method.
(Third configuration: generation of control signal of threshold current source)
FIG. 13 shows a configuration example for generating a control signal of a threshold current source different from that in FIG. FIG. 13 shows a configuration example in which the timing for controlling the threshold current is sampled when the modulation signal is on and held when the modulation signal is off. In the case of this configuration, for example, it is possible not only to adjust the light amount only outside the image writing area, but also to perform sampling and control each time the LD is turned on within the writing area.
(Fourth configuration)
FIG. 14 shows a fourth configuration example of the present invention. FIG. 14 shows an example in which a specific configuration example of the timing diagrams shown in FIGS. 6 and 7 is configured by a one-chip ASIC 50. The light emission command signal becomes a modulation signal through the delay unit 54. Further, the light emission command signal (A) and the delay signal (B) become the threshold value on signal (D) through the threshold value signal generation unit 55. Each modulation signal (C) generates a LD drive current (E) as shown in FIGS. 6 and 7 by driving a modulation current switch and a threshold-on signal driving a threshold current switch. Similarly to FIG. 13, the threshold current is sampled when the modulation signal is on and held when the modulation signal is off. The D / A unit for determining the modulation current controls the modulation current source 13 according to the timings shown in FIGS. 9 to 12 so as to obtain a desired light amount from the LD 10.
(Fifth configuration)
FIG. 15 shows a fifth configuration example of the present invention. FIG. 15 shows a configuration example having a shading correction function and an LD power supply (VLD) control function in addition to the same parts as FIG.
[0057]
First, the shading correction function will be described. The differential quantum efficiency of the LD detected at power-on or reset release is set to D / A. When the current or voltage for determining the full scale of the D / A current is input from the external terminal and the full scale is changed, the light emission amount of the LD can be changed. For example, in an LD writing system that performs raster scanning, the energy density at the center is high, so that the light emission amount of the LD is reverse-corrected so that the light emission amount is large at the scanning end and the light emission amount is small at the center. Correction (shading correction). The speed of this correction may be slow as long as the change follows the time during which the LD scans one line. Shading correction is performed by changing the current value of the D / A according to a signal for changing the light emission amount as described above according to the time for scanning one line from the outside.
[0058]
Next, the LD power supply (VLD) control function will be described. The purpose of controlling the VLD is to influence the power consumption of the ASIC because when the LD drive unit is made an ASIC, the drive current of the LD needs to flow a large current of about 100 mA although it depends on the LD. For example, assuming that the drop voltage of the LD is 2 V with a 5 V power supply, the ASIC requires 100 mA, that is, 300 mW at 3 V with only the LD current. When the number of LDs to be driven is 2, 600 mW is required only for the LD current, and when the number of LDs to be driven is 4, 1200 mW is required only for the LD current. If this is the case, it becomes difficult to drive a large number of LDs, so the VLD is controlled. In the previous example, for example, the power consumption of the LD becomes 3V because the LD cathode portion becomes 3V. However, if the LD cathode portion can be controlled to about 1V, the power consumption of the ASIC due to the LD current becomes 1/3. The VLD is detected by detecting the cathode potential of the LD when the threshold ON signal or the modulation signal is ON, and outputting a VLD control signal outside the ASIC so that a desired voltage (for example, 1 V) is obtained. For example, the VLD control signal is input to the base of the power transistor, and if the emitter of the power transistor is connected to the LD power source, a configuration for controlling the VLD can be realized. Since this control speed may be sufficiently slower than the modulation speed of the LD, any power transistor may be used as long as it can supply a sufficient current to the LD. By having such a VLD control function, it is possible to realize an LD drive circuit that can drive a large number of LDs with low power consumption.
[0059]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a semiconductor laser driving circuit and an image forming apparatus that are high-speed and highly accurate.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining a basic conceptual diagram (basic configuration) of the present invention.
FIG. 2 is a diagram for explaining an LED bias current source (part 1);
FIG. 3 is a diagram (part 2) for explaining a bias current source of the LED;
FIG. 4 is a diagram for explaining a first configuration example of the present invention.
FIG. 5 is a diagram for explaining a second configuration example of the present invention.
FIG. 6 is a diagram for explaining an example (part 1) of timings of a modulation signal and a threshold ON signal.
FIG. 7 is a diagram for explaining an example (part 2) of timings of a modulation signal and a threshold-on signal.
FIG. 8 is a diagram for explaining a configuration example for initial setting of a modulation current source;
FIG. 9 is a diagram (No. 1) for describing a first method of initial setting of a modulation current source;
FIG. 10 is a diagram (No. 2) for explaining the first method of the initial setting of the modulation current source;
FIG. 11 is a diagram (No. 1) for describing a second method of initial setting of a modulation current source;
FIG. 12 is a diagram (No. 2) for describing a second method of the initial setting of the modulation current source;
FIG. 13 is a diagram for explaining a configuration example for generating a control signal for a threshold current source;
FIG. 14 is a diagram for explaining a fourth configuration example of the present invention.
FIG. 15 is a diagram for explaining a fifth configuration example of the present invention;
[Explanation of symbols]
10 Laser diode (LD)
11 Threshold current source
12 Bias current source
13 Modulated current source
20 Photodiode (PD)
21 Differential amplifier
31, 32 switch circuit
41 Sample hold circuit
50 ASIC
51 Timing generator
52 Differential quantum efficiency detector
53 D / A section
54 Delay part
55 Threshold signal generator
61 VLD controller
62 VLD detector

Claims (16)

A bias current source for supplying a constant small bias current to the semiconductor laser, and a threshold current source for supplying a threshold current to the semiconductor laser, and a modulation current source for emitting the semiconductor laser in accordance with a light emission command signal, A semiconductor laser driving circuit for driving the semiconductor laser with a current that is a sum of three currents from the bias current source, the modulation current source, and the threshold current source;
Providing a first switch between the semiconductor laser and the modulation current source;
A second switch is provided between the semiconductor laser and the threshold current source;
The first switch is controlled to be turned on / off by a modulation signal that is a delayed light emission command signal,
The ON period of the second switch includes the ON period of the first switch,
2. The semiconductor laser driving circuit according to claim 1, wherein the second switch is turned on before the first switch is turned on, and is turned off after confirming that the modulation signal is turned off . The semiconductor laser driving circuit according to claim 1,
2. A semiconductor laser driving circuit according to claim 1, wherein a current generated in the bias current source is several mA or less. The semiconductor laser driving circuit according to claim 1,
The semiconductor laser driving circuit, wherein the second switch is turned on by a logical sum signal of the modulation signal and the light emission command signal. The semiconductor laser driving circuit according to claim 1,
The modulation current source has initialization means that operates when power is turned on or reset is released,
2. A semiconductor laser driving circuit according to claim 1, wherein the light intensity when the semiconductor laser emits light is set to a predetermined value by the initialization means. The semiconductor laser driving circuit according to claim 4, wherein
The initialization means detects a difference between a current or voltage when the light amount of the semiconductor laser is a predetermined value and a current or voltage when the light amount of the semiconductor laser is smaller than a predetermined value, and emits light from the semiconductor laser. A semiconductor laser driving circuit, wherein the light quantity at the time is set to a predetermined value. The semiconductor laser driving circuit according to claim 4, wherein
The initialization means detects a difference between a current or voltage when the light amount of the semiconductor laser is a predetermined value and a current or voltage when the semiconductor laser emits an offset light, and A semiconductor laser driving circuit characterized in that the amount of light is set to a predetermined value. The semiconductor laser driving circuit according to claim 4, wherein
The initialization means includes a current or voltage when the light amount of the semiconductor laser is a predetermined value, and a current or voltage when the light amount of the semiconductor laser is 1 / N (N is a natural number of 2 or more) of a predetermined value. The semiconductor laser drive circuit is characterized in that the difference between the two is detected and set so that the amount of light emitted from the semiconductor laser becomes a predetermined value. The semiconductor laser driving circuit according to any one of claims 5 to 7,
The initialization unit includes a timing generation unit, a detection unit that detects the difference, a current setting unit that sets a light amount when the semiconductor laser emits light, and the detection unit based on a timing generated by the timing generation unit A semiconductor laser drive circuit comprising: a comparison unit that performs successive comparison so that a value detected by the current setting unit corresponds to a value set by the current setting unit. The semiconductor laser driving circuit according to any one of claims 1 to 8,
A light receiving unit that detects a light output of the semiconductor laser, and a current control unit that controls a current supplied to the semiconductor laser based on a light reception signal proportional to the light output of the semiconductor laser detected by the light receiving unit. A semiconductor laser driving circuit comprising: The semiconductor laser driving circuit according to claim 9, wherein
The current control means generates a control signal by comparing the magnitude of the received light signal with a predetermined value, and controls the threshold current source according to the control signal. The semiconductor laser driving circuit according to claim 10, wherein
The semiconductor laser driving circuit, wherein the current control means samples the control signal when the first switch is in an ON state, and controls the threshold current source based on the sample value. The semiconductor laser driving circuit according to any one of claims 1 to 11,
A semiconductor laser driving circuit comprising: means for detecting a voltage of an output unit for driving the semiconductor laser and controlling a voltage of a power source supplied to the semiconductor laser based on the detected value. A semiconductor laser whose output is modulated by an image modulation signal, scanning means for scanning the rotating photoconductor with the light of the semiconductor laser, and an image for forming an electrostatic latent image corresponding to the image modulation signal on the rotating photoconductor In the forming device,
13. An image forming apparatus, wherein the semiconductor laser is driven by the semiconductor laser driving circuit according to claim 1. The image forming apparatus according to claim 13.
An image forming apparatus, wherein shading correction is performed by changing a full-scale value of the driving current generating unit according to scanning of the scanning unit.   The image forming apparatus according to claim 1, wherein the threshold current is turned on 1 to 10 ns before the drive current is turned on. The threshold current, the image forming apparatus according to any one of claims 1 to 15, characterized in that the off after a few ns off of the drive current.

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