JP2005262481A - Light beam emission controller - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the power consumption of a buffer amplifier while maintaining the charging and discharging capability quickly following a significant change when a decoupling capacitor is charged and the decoupling capacitor is discharged by the buffer amplifier to stabilize the voltage value of voltage driving in a driving circuit to emit light beams by voltage driving during a transition period and by current driving during a stable period. <P>SOLUTION: Even when the increase of current consumption due to the expansion (fluctuation control) of the electrostatic capacity of a decoupling capacitor Cd is required by using a variable current source Isnk for charging as a dummy fixed electric current source during discharging and a variable current source Isrc for discharging as the dummy fixed electric current source during charging, to consider a crash of an operational amplifier OP due to the fluctuation of a current source not applied in charging or discharging is not required, the electric current source can be relatively small and current consumption of the operational amplifier OP can be reduced. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention scans a light beam emitted from a light source based on image data, forms an electrostatic latent image on an image carrier, develops it, and transfers the image onto a recording medium to form an image. The present invention relates to a light beam emission control device that is used in a forming apparatus and controls light emission of a light beam emitted from the light source.

  Conventionally, an image forming apparatus irradiates a light beam emitted from a laser as a light source based on image data, scans it with a scanning means such as a polygon mirror (main scanning), and passes through an optical system such as an fθ lens. An electrostatic latent image is recorded on an image carrier (photosensitive drum or the like) that has been charged and exposed in advance, and a developing process is performed in which toner is supplied to make the image visible, and then the toner image is transferred to a predetermined recording medium (recording paper). ) To obtain an image.

  Incidentally, in recent years, a surface emitting laser called VCSEL has been applied as a light source of the image forming apparatus. In this surface emitting laser, it is possible to irradiate a plurality of (for example, 32) light beams from the same surface at the same time. That is, since a plurality of main scans are executed simultaneously, the processing speed of image formation can be dramatically improved.

  Since the surface emitting laser has a higher internal resistance than a general laser, a large current flows when driven by voltage, which is not preferable. Therefore, light is emitted by voltage drive during the transition period, and light is emitted by constant current drive (see Patent Document 1).

  By the way, in order to stabilize the voltage from the voltage source when the laser is voltage-driven and current-driven, a decoupling capacitor Cd is provided, and charging / discharging is performed by a buffer amplifier based on the control voltage (see Patent Document 2). .

  The capacitance (capacitance) of the decoupling capacitor Cd is determined based on the parasitic capacitance in the wiring from the decoupling capacitor Cd to the laser (light emitting element). The larger the capacitance of the decoupling capacitor Cd, the more the laser. The drive voltage is stable.

  However, if the capacity is larger than necessary, charging takes time, and as a result, even if the control voltage is increased to correct the laser drive voltage according to the laser temperature, the laser terminal voltage at the time of laser lighting can follow it. Therefore, there is a problem in image quality that the target image density cannot be obtained because the light amount deviates from the target value.

  For this reason, it is preferable to set the capacity to an extent acceptable for both the above-described drive voltage stability and the rise.

  For example, if the parasitic capacitance is about 10 pF and the allowable fluctuation range of the drive voltage is 1%, the decoupling capacitor Cd is 1000 pF. If it is this level, it will not have a big influence on a start-up and the fluctuation | variation of a drive voltage can be suppressed.

  By the way, the charging is executed when the control voltage is higher than the voltage across the decoupling capacitor Cd. When this difference is large (environmental temperature change, in-plane light amount correction, etc.), the drive capability of the current source for discharging the buffer amplifier is reduced. Need to be charged quickly. Therefore, this can be dealt with by changing the charging current source mounted on the buffer amplifier (see FIG. 9A).

  On the other hand, the discharge is executed when the control voltage is lower than the decoupling capacitor Cd. When this difference is large (environmental temperature change, in-plane light amount correction, etc.), the driving ability of the sink current source of the buffer amplifier is increased and quickly It is necessary to discharge. Therefore, this can be dealt with by making the discharge current source mounted on the buffer amplifier variable without limiting the current (see FIG. 9B).

Here, since it is necessary to improve both the charging and discharging capacities in order to correct the temperature fluctuation and the light quantity accurately at high speed, both current sources (charging current source, discharging current source) can be made variable. (See FIG. 9C).
JP 2003-347663 A JP 2002-335038 A

  However, if both the charging current source and the discharging current source are variable, the current flowing in and out is zero when the control voltage does not fluctuate, so that the discharging current is equal to the discharging current (see FIG. 9D). The value varies depending on the gate oxide film thickness of the MOS transistor, the channel length accuracy, and the like, and varies with the same control voltage. If this current value is too small, the frequency characteristics of the amplifier change and the worst oscillation occurs. For this reason, the transistor is usually designed so that the current value does not affect the frequency characteristics even under the worst conditions. However, since the current is designed so as not to be below a certain level, the current increases when the process swings in the opposite direction.

  For this reason, the current consumption in the buffer amplifier increases, and in particular, in the case of a configuration that simultaneously emits a plurality of light beams (for example, 32 light beams) such as a VCSEL, a separate buffer amplifier corresponds to each light beam. Therefore, as a whole IC composed of a plurality of drive circuits corresponding to a plurality of light beams, the number of light beams is added by a simple calculation, resulting in an increase in power consumption.

  In consideration of the above facts, the present invention provides a decoupling capacitor to a decoupling capacitor by a buffer amplifier for stabilizing the voltage value of voltage drive in a drive circuit that emits a light beam by voltage drive in the transition period and current drive in the stable period. For the purpose of obtaining a light beam emission control device capable of reducing the power consumption of a buffer amplifier while having charging and discharging ability to quickly follow a large control change during charging and discharging from a decoupling capacitor. is there.

  The present invention scans a light beam emitted from a light source based on image data, forms an electrostatic latent image on an image carrier, develops it, and transfers the image onto a recording medium to form an image. A light beam emission control device used in a forming apparatus for controlling light emission of a light beam emitted from the light source, wherein a plurality of control voltages individually holding control voltages for controlling the light emission amount of each light beam to a reference light amount Based on the control voltage held by the holding means, a voltage driving circuit for setting a voltage at the time of voltage driving so as to emit a light beam from the light source with a reference light amount, and a current at the time of current driving are set based on the control voltage held by the holding means A decoupling capacitor that holds a voltage during voltage driving based on a control voltage held in the holding unit, and a decoupling capacitor that holds a voltage during voltage driving based on a control voltage held in the holding unit. It is composed of an amplifier that performs charging or discharging of the pulling capacitor, and both the discharge current source that supplies the charging current and the suction current source that supplies the discharging current in the amplifier are variable. The sink current source is fixed, and the discharge current source is fixed during discharge.

  2. The light beam according to claim 1, wherein, in the present invention, it is set which upper limit current of the discharge or sink current source is limited when the output current from the amplifier is zero. Light emission control device.

  Further, in the present invention, when the light source is turned on and the current output from the amplifier is supplied, the other current source of the current source sets an upper limit current, and the other current source when the output current of the amplifier is zero The current source is set with an upper limit current.

  According to the present invention, on the basis of the control voltage held by the holding means, the driving means performs light emission control by, for example, driving the light beam with voltage in the transient period and driving with current in the stationary period.

  In the voltage drive by the drive means, the decoupling capacitor that holds the voltage drive voltage is charged, and the control voltage held in the hold means is also charged by driving the amplifier. During this charging, the amplifier is optimized to ensure the minimum current required to stabilize the frequency characteristics in any case, so that a large current flows under the standard condition and the power consumption increases.

  By the way, in order to improve the capability of the amplifier, it is necessary to use both the discharge current source during charging and the sink current source during discharge without current limitation. Furthermore, in order to ensure high-speed response, it is necessary to design the transistor and control circuit so that a current exceeding a certain level always flows even if the characteristics of the transistors constituting each of them vary. Will flow.

  Therefore, the current source is current-limited during charging, the current source is current-limited during discharging, and in the case of no load that is neither charged nor discharged, either current source is current-limited. By controlling in this way, current consumption can be suppressed while charging and discharging the decoupling capacitor at high speed.

  As described above, according to the present invention, in a drive circuit that emits a light beam by voltage driving in the transition period and current driving in the stable period, charging the decoupling capacitor by the buffer amplifier for stabilizing the voltage value of voltage driving. In addition, when discharging from the decoupling capacitor, there is an excellent effect that the power consumption of the buffer amplifier can be reduced while providing a charging and discharging capability that quickly follows a large control change.

  FIG. 1 shows a schematic configuration of a scanning optical system unit 12 of an image forming apparatus 10 to which a light beam emission control device of the present invention is applied.

  The scanning optical system unit 12 includes a surface emitting laser (VCSEL) 14 and can emit a plurality of light beams (32 in this embodiment) at the same time.

  A condensing lens 16 is disposed on the downstream side in the optical axis direction of the light beam emitted from the surface emitting laser 14, and the light beam that has passed through the condensing lens 16 is input to a polygon mirror 18 as a scanning optical system. It is supposed to be. The polygon mirror 18 is rotated at a constant speed at a high speed by a driving force of a polygon motor (not shown) in the direction of arrow A in FIG.

  For this reason, it sequentially enters a plurality of reflecting surfaces (eight surfaces in FIG. 1) 18A of the polygon mirror 18, and the reflected light has an fθ lens 20 for correcting the scanning speed on the exposure surface and a lens power in the scanning direction. The image is scanned on the photosensitive drum 24 after passing through an optical system constituted by a cylindrical lens 22 or the like for surface tilt correction.

  The photosensitive drum 24 is rotated (sub-scanned) in a direction orthogonal to the main scanning. By repeating the main scanning while rotating the photosensitive drum 24, a predetermined area of the photosensitive drum 24 is statically moved on the photosensitive drum 24. An electrostatic latent image can be formed.

  The photosensitive drum 24 includes a charging unit, a developing unit, and a transfer unit (not shown) around the photosensitive drum 24, and is controlled to emit light according to image data while being uniformly charged by the charging unit. The light beam is exposed to light and exposed to light, and developed by a developing unit to form a toner image, which is recorded on a recording medium (recording paper) in a transfer unit.

  The surface emitting laser 14 of the scanning optical system unit 12 is driven and controlled by a driving circuit 26 shown in FIG.

  The drive circuit 26 is provided in a 1: 1 relationship with respect to the light emission point (light emitting element 14A) of each light beam.

(Basic configuration of the drive circuit 26)
The basic configuration of the drive circuit 26 will be described below, but the present invention is not limited to the drive circuit 26 and does not affect the present invention.

  As shown in FIG. 2, the drive circuit 26 includes a voltage source composed of a fixed voltage source V, a correction voltage source ΔV and an operational amplifier (buffer function) OP, and a current composed of a fixed current source I and a correction current source ΔI. Source, a bias voltage source Vbias, a decoupling capacitor Cd, and switches SW1 and SW2.

  The correction voltage source ΔV and the correction current source ΔI correct light amount unevenness (smile correction) according to a scanning position generated due to an optical system or the like during main scanning onto the photosensitive drum 24 (see FIG. 1). , Is applied.

  The switch SW1 is switched to either the current source side or the bias voltage source Vbias side. The switch SW2 performs on / off control.

  The output voltage from the voltage source is applied to the light emitting element 14A of the surface emitting laser 14 via the switch SW2 that is turned on and the switch SW1 that is switched to the current source side. The output current from the current source is supplied to the light emitting element 14A of the surface emitting laser 14 via the switch SW1 switched to the current source side.

  The output voltage from the voltage source is designed to be controllable within a predetermined voltage range equal to or higher than the laser oscillation threshold voltage. Further, the output current from the current source is designed to be controllable within a predetermined current value range equal to or greater than the oscillation threshold current of the surface emitting laser 14.

  The decoupling capacitor Cd is charged by the output voltage from the voltage source when the switch SW2 is in the OFF state, and holds the voltage.

  Further, the output voltage of the bias voltage source Vbias is set to a voltage value lower than the laser oscillation threshold voltage with the surface emitting laser 14 in the forward bias state.

  By such decoupling capacitor Cd and bias voltage source Vbias, the surface emitting laser 14 is quickly switched from the OFF state to the ON state (when switching SW2 to the ON state and switching to the current source side of SW1). The surface emitting laser 14 starts or stops light emission.

  The relationship (voltage-current characteristics) between the driving current and the terminal voltage of the surface emitting laser 14 is proportional within a practical range because of its high internal resistance (see FIG. 3). The relationship is proportional to the practical range (substantially linear characteristics as shown in FIG. 3).

  Based on such characteristics, in the drive circuit 26 having the above-described configuration, the fixed current source I is determined such that the light beam amount of the surface emitting laser 14 (light emitting element 14A) becomes the reference light amount, and the correction current The source ΔI is determined to have a value necessary for changing the light beam amount from the surface emitting laser 14 (light emitting element 14A) from the reference light amount to a certain correction light amount.

  Further, on the voltage-current characteristics of the surface emitting laser 14, the values of the fixed voltage source V and the correction voltage source ΔV are determined so as to correspond to the values of the fixed current source I and the correction current source ΔI.

  By simultaneously proportionally controlling the correction current source ΔI and the correction voltage source ΔV according to the characteristics shown in FIG. 3, the light beam light amount can be corrected to an arbitrary light amount between the reference light amount and the correction light amount. It is possible to correct unevenness in the amount of light in the main scanning direction (smile correction).

  Here, the light beam emission control device including the drive circuit 26 has a function of detecting and feeding back the amount of light emitted from the light emitting element 14A and correcting the control voltage supplied to the drive circuit 26 in comparison with the reference value. (APC control function)

  FIG. 4 shows an overall schematic configuration of the light beam emission control circuit including the drive circuit 26 (shown as drivers 1 to N in FIG. 4).

  A photodetector 62 is provided in the vicinity of the plurality of light emitting elements 14A. The photodetector 62 includes a photoelectric conversion element that detects the light amount of the light beam emitted from the light emitting element 14A and converts it into an electric signal. For this reason, the output of the photodetector 62 becomes a current value according to the amount of emitted light.

  The output signal line 64 of the photodetector 62 is connected to the negative input terminal of the differential amplifier 68 via the I / V converter 66.

  A reference value (reference voltage Vref) 70 is input to the positive side input terminal of the differential amplifier 68.

  In the differential amplifier 68, an input reference voltage and a detection voltage are input, and the difference between the two is amplified and output. When the amplification degree is sufficiently large, the detection voltage almost matches the reference voltage. Control is performed.

  This output voltage becomes a control voltage for emitting light from the light emitting element 14A in the drive circuit 26.

  The output terminal of the differential amplifier 68 is connected to the drive circuit 26 and is held in the sample and hold capacitor 84 which is a holding means as the control voltage.

  Based on the control voltage held in the sample hold capacitor 84, the light emitting element 14A emits light.

  For example, when the amount of light emitted from the light emitting element 14A is small, the detection voltage input to the differential amplifier 68 is low, and the reference voltage is higher. The output of the differential amplifier 68 is controlled so as to increase because the reference voltage is higher than the detection voltage. This becomes the control voltage when the light emitting element 14A emits light, and the light amount of the light emitting element 14A increases.

  Conversely, when the amount of light emitted from the light emitting element 14A is large, the detection voltage input to the differential amplifier 60 is high and the reference voltage is lower, so that the output of the differential amplifier 60 is controlled to decrease. The control voltage during the light emission of 14A becomes, and the light amount of the light emitting element 14A decreases.

  Here, the control voltage charged in the sample and hold capacitor 84 shown in FIG. 4 corresponds to the fixed voltage source V and the correction voltage source ΔV shown in FIG. Driven.

  FIG. 5 is a circuit configuration diagram of the operational amplifier OP in FIG. 2. In this operational amplifier OP, the decoupling capacitor Cd connected to the downstream side is charged and discharged as described above.

  The operational amplifier OP is provided with a comparator (comparator) 100 and a first-stage amplifier (amplifier) 102, and a voltage (control voltage) charged in the sampling capacitor 84 is input to each positive side input terminal. (Hereinafter, the voltage of the sampling capacitor 84 is referred to as the control voltage V1), and the voltage (feedback voltage) charged in the decoupling capacitor Cd is input to the negative input terminal (hereinafter referred to as the decoupling capacitor Cd). The voltage is referred to as drive voltage V2.)

  On the downstream side of the first stage amplifier 102, a variable current source Isrc as a driving current source at the time of discharge and a variable current source Isnk as a driving current source at the time of charging are provided in series, and an active signal from the first stage amplifier 102 is provided as an active signal. Based on the reference voltage Vdd, each is driven.

  The variable current source Isrc is connected in series with a current value limiting current source Isrc-1, and the variable current source Isnk is connected in series with a current value limiting current source Isnk-1.

  When the drive voltage V2 is higher than the control voltage V1 (V1 + a ≦ V2), the variable current source Isnk removes the current limitation of Isnk-1 so that the voltage of the decoupling capacitor Cd It functions as a current source for discharging at high speed without limitation, and Isrc operates as a load of Isnk as a constant current source because the comparator 100 sets an upper limit current of Isrc-1.

On the other hand, when the drive voltage V2 is lower than the control voltage V1 (V1 + a> V2 (a is an offset that determines the offset amount)), the variable current source Isrc allows the comparator 100 to release the current limitation of Isrc-1. Therefore, the decoupling capacitor Cd serves as a current source that charges at high speed without limiting the current. At this time, the Isnk sets the upper limit current of Isnk-1 so that the Isrc is a constant current source. Is operating as a load.
Here, the meaning of providing the offset a is as follows. If the comparator has an offset of zero and the drive voltage V2 is close to the control voltage (V1 = V2), the comparator output frequently switches between Isrc-1 and Isnk-1, but the amplifier OP is near V1 = V2. Is in an equilibrium state with negative feedback, and when the comparator switches between Isrc-1 and Isnk-1 with a little noise, the value of the control voltage in the equilibrium state of the transistors constituting each differs, so two equilibrium states are obtained. Appears as noise in the output during transition. Therefore, if the comparator is offset, the discharge current source Isrc is not current limited and the sink current source Isnk is current limited when V1 = V2 as in the case where the voltage V2 of the decoupling capacitor is lower than the control voltage V1. No noise is generated because switching does not occur in this state. If the actual condition where the control voltage must be changed at high speed is the case of temperature correction, all lasers are turned on and if the temperature rises rapidly or if the light quantity distribution is corrected, the in-plane light quantity distribution is large. It is extremely rare when there is a step. Normally, the control voltage can be regarded as being constant within a short scanning distance (for example, about 1 cm). In such a case, when the laser is turned on, only the discharge current source for charging the parasitic capacitance due to the wiring to the laser is output from the output. Outputs current. Therefore, by applying an offset to the comparator, even when the control voltage and drive voltage are almost equal, the discharge current source charges the laser terminal voltage at high speed without limiting the current, and the sink current source is current limited as a load for this. If the current source is operating as a current source, switching of the current limit between Isrc and Isnk is reduced, and noise on the control voltage can be suppressed. Also, when there is a temperature or sudden in-plane correction and the discharge of the decoupling capacitor does not catch up and V1 + a <V2, the current source is switched and decoupling is performed at the maximum current because no negative feedback is applied. Since the capacitor is discharged and V1 = V2 is approached, the equilibrium state is reached, so that the influence on the image quality is limited because it is not a noise but a slight delay in the rise of the laser light quantity.

  That is, according to the comparison result of the comparator 100, when charging is necessary, the upper limit current of the current source Isrc-1 is canceled, and the discharging current source Isrc becomes a pseudo fixed current source. On the other hand, when the discharge is necessary according to the comparison result of the comparator 100, the upper limit current of the current source Isnk-1 is canceled, and the charging current source Isnk becomes a pseudo fixed current source.

  The operation of this embodiment will be described below.

(Image formation process)
First, the photosensitive drum 24 is rotationally driven at a predetermined rotational speed.

  The surface of the photosensitive drum 24 is uniformly charged to a predetermined level by applying a developing bias voltage having a predetermined charging level of the charging unit. The developing bias is configured to superimpose not only a DC voltage but also an AC component on the DC component.

  Next, the surface of each photosensitive drum 24 having a uniform surface potential is irradiated with a light beam by the surface emitting laser 14 (light emitting element 14A) of the scanning optical system unit 12, and electrostatic latent images corresponding to the image data are output. An image is formed.

  That is, the light beam emitted from the light emitting element 14A is deflected by the polygon mirror 18, and this scanning light is main-scanned by the photosensitive drum 24 via the fθ lens 20 and the like. The surface potential of the exposed portion of the photosensitive drum 24 by the light beam is neutralized to a predetermined level.

  Then, the electrostatic latent image formed on the surface of each photosensitive drum 24 is developed by a corresponding developing unit, and the electrostatic latent image on each photosensitive drum 24 is visualized as a toner image.

  Next, each color toner image formed on each photosensitive drum 24 is transferred to a recording medium by a transfer unit. The recording medium is heat-fixed to fix the toner, and the image forming process ends.

(APC control)
Here, at the main scanning interval (before image writing) in the image formation, it is determined whether or not the light amount of the light emitting element 14A maintains the reference value. If not, APC control is executed. . This APC control is executed sequentially, that is, in time series, for the plurality of light emitting elements 14A.

  During APC control, the drive circuit 26 causes the light emitting element 14A to emit light. In the light emitting state of the light emitting element 14A, the amount of emitted light is detected by the photodetector 14A.

  The detected light amount is photoelectrically converted, and a current corresponding to the light amount is input to the negative input end of the differential amplifier 68 via the I / V 66, and a reference value (reference voltage) input to the positive input end. Vref).

  As a result of the comparison by the differential amplifier 68, the control voltage for causing the light emitting element 14A to emit light is corrected according to the difference, and is output from the differential amplifier 68.

  When the light emission amount of the light emitting element 14A is small, the detection voltage input to the differential amplifier 68 is lower than the reference voltage, and therefore the output of the differential amplifier 68 is output with the voltage value increased. As a control voltage during light emission, the light amount of the light emitting element 14A increases.

  On the other hand, when the amount of light emitted from the light emitting element 14A is large, the detection voltage input to the differential amplifier 68 is higher than the reference voltage (Vref) 70, so that the output of the differential amplifier 68 is output with the voltage value reduced. This becomes a control voltage at the time of light emission of the light emitting element 14A, and the light amount of the light emitting element 14A decreases.

  By repeating this, the control voltage converges to a voltage that controls the amount of emitted light to be emitted with the reference voltage, and the light beam emits light with a stable amount of emitted light.

  When the APC control of one light emitting element 14A is completed, the images are selected one after another in time series, and the image forming process is started when the APC control of all the light emitting elements 14A is completed.

  Further, the control voltage after APC control is superimposed with a so-called smile correction for correcting unevenness in the amount of light in the main scanning direction and a correction value resulting from an environmental change such as a temperature change. For this reason, the amount of light emitted by the light beam emitted from the light emitting element 14A during image formation is changed as needed.

Here, the light emission of the light-emitting element 14A is current-driven in the stable period, but in the transition period, the light emission is controlled by voltage drive to improve the rise.
Further, when correcting unevenness in the amount of light in the plane, control voltages are set to ΔV and ΔI in FIG. 2 for in-plane correction.
The correction voltage ΔV and the correction current ΔI are set based on the difference between the control voltage value of the voltage source and the control voltage value of the current source when APC is performed with two light quantities. For example, if the two light amounts are 90% and 110%, the light amount is 90% if the correction is zero, and if the correction is 1, the light amount is 110%. The correction value is determined in advance and held as data, and the correction value is changed in accordance with the scanning position of the laser beam, whereby the in-plane density variation or the like can be corrected by the light amount of the laser beam. Using the present invention, even if the fluctuation in the amount of light in this plane is abrupt, charging / discharging of the decoupling capacitor can be performed at high speed, so that the voltage of the decoupling capacitor can be made to follow the correction and there is no uneven density image Can be formed.

  At this time, the drive voltage for driving the voltage is charged in the decoupling capacitor Cd based on the control voltage, and the correction of the control voltage charged in the sampling capacitor 84 is followed by the in-plane correction or the like. Therefore, it is necessary to change the charging voltage of the decoupling capacitor Cd.

  In order to realize this charging and discharging, the operational amplifier OP is operated, but the capacitance of the decoupling capacitor Cd is determined based on the capacitance parasitic on the wiring, and the wiring to the laser is long. When the parasitic capacitance becomes large due to the above, an electrostatic capacitance that is at least 100 times greater than the parasitic capacitance is required.

  As a result, the operational amplifier OP does not limit the current of the charging drive current source and the discharge drive current source in order to operate to charge and discharge quickly. For this reason, in order to ensure the minimum necessary current for operating the operational amplifier OP, the current consumption tends to increase more than necessary.

  On the other hand, in this embodiment, the operational amplifier OP is equipped with the comparator 100 to determine whether charging or discharging is necessary, and to suppress the maximum value of the current source that is not necessary. I made it.

  That is, the control voltage V 1 charged in the sampling capacitor 84 input to the negative input terminal of the first stage amplifier 102 is also input to the negative input terminal of the comparator 100 and input to the positive input terminal of the first stage amplifier 102. The driving voltage V <b> 2 (feedback) charged in the decoupling capacitor Cd is also input to the negative side input terminal of the comparator 100.

  The first-stage amplifier 102 outputs the difference based on the difference between the control voltage V1 and the drive voltage V2, and when the drive voltage V2 is higher than the control voltage V1 (V1 + a ≦ V2 (a is an offset voltage that determines the offset amount)) A current corresponding to the difference flows by the variable current source Isnk that discharges the voltage of the decoupling capacitor Cd.

  The operational amplifier OP operates by the current from the variable current source Isnk, and discharges the voltage charged in the decoupling capacitor Cd.

  At this time, the comparator compares the control voltage V1 and the drive voltage V2, and naturally determines that the drive voltage V2 is higher than the control voltage V1 (V1 + a ≦ V2 (a is an offset voltage that determines the offset amount)). Then, the current value of the current source Isnk-1 is suppressed to a predetermined value to obtain a constant current source (see FIG. 6A).

  As a result, it is possible to always pass a minimum current that does not affect the frequency characteristics of the operational amplifier OP. Further, since this current value can be defined by design, an increase in current consumption can be suppressed.

  On the other hand, in the first stage amplifier 102, when the drive voltage V2 is lower than the control voltage V1 (V1 + a> V2 (a is an offset voltage that determines the offset amount)), the variable current source Isrc that charges the voltage of the decoupling capacitor Cd. Thus, a current corresponding to the difference flows.

  The operational amplifier OP operates by the current from the variable current source Isnk, and the decoupling capacitor Cd is charged with a voltage.

  At this time, the comparator compares the control voltage V1 and the drive voltage V2, and naturally determines that the drive voltage V2 is lower than the control voltage V1 (V1 + a> V2 (a is an offset voltage that determines the offset amount)). Then, the current value of the current source Isrc-1 is suppressed to a predetermined value to obtain a constant current source (see FIG. 6B).

  As a result, it is possible to always pass a minimum current that does not affect the frequency characteristics of the operational amplifier OP. Further, since this current value can be specified by design, an increase in current consumption can be suppressed ( (See FIG. 7).

  In this way, by making either Isrc or Isnk a constant current, it is possible to accurately control the constant current value while suppressing an increase in current consumption, and it is also possible to suppress deterioration of the frequency characteristics of the amplifier OP.

  FIG. 8 shows a circuit for making the operating current of the operational amplifier OP a pseudo constant current between the variable current sources during charging and discharging.

  The power supply voltage Vdd is connected to the gate G of each of the five series of PMOS transistors 110, 112, 114, 116, and 118.

  The PMOS transistor 110 is a drive source for the first stage amplifier 102. When the PMOS transistor 110 is in an on state, the first stage amplifier 102 configured by two PMOS transistors 120 and 122 and two NMOS transistors 124 and 126 operates.

  The control voltage V1 is input to the gate G of the PMOS transistor 122 constituting the first stage amplifier 102, the drive voltage V2 is input to the gate G of the PMOS transistor 120, and the drain currents of the NMOS transistors 124 and 126 are changed according to the difference. Be controlled.

  The gates G of the NMOS transistors 124 and 126 are connected to the gates G of the NMOS transistors 128 and 130.

  Here, the source S of the NMOS transistor 126 constituting a part of the first-stage amplifier 102 is connected to the gate G of the NMOS transistor 134 serving as the variable current source Isnk, and controls the activation of the variable current source Isnk.

  The source S of the NMOS transistor 124 that constitutes a part of the first stage amplifier 102 and the NMOS transistor 128 that constitutes a current mirror is connected to the current source that is constituted of the PMOS transistor 112 and is further connected to the gate G of the PMOS transistor 136. The variable current source Isrc is controlled. The comparator is configured with the first-stage amplifier 102, the transistor 130, and the transistor 114 as the final stage, and since the first-stage amplifier 102 is shared with the amplifier OP, there is no error due to first-stage offset variation. The offset of the comparator can be adjusted by changing the W / L ratio of the final stage transistors 130 and 114. As described above, even when V1 = V2, in order to obtain the same comparison result as V1> V2, if the W / L of the transistor 128 is increased, the connection point potential of the drains of the transistors 128 and 112 is shifted to the GND side. It can be in the same state as V2 is low. When the final stage of the comparator approaches GND, the current limitation of the current source Isrc-1 configured by the PMOS transistor 138 is released, while Isnk-1 becomes a constant current and is charged without being limited by the amplifier OP to the decoupling capacitor. Is done.

  The source S of the PMOS transistor 136 constituting the variable current source Isrc is connected to the drain D of the PMOS transistor 138 constituting the current source Isrc-1. The drain D of the PMOS transistor 138 is connected to the drain D of the NMOS transistor 134 constituting the variable current source Isnk, and the source S of the NMOS transistor 134 is the drain of the NMOS transistor 140 constituting the current source Isnk-1. Connected to D.

  That is, between the power supply line 142 and the earth line 144, two PMOS transistors 138 (Isrc-1) and 136 (Isrc) and two NMOS transistors 134 (Isnk) and 140 (Isnk-1) are provided. Are connected in series.

  Here, the drive voltage extraction position is between the PMOS transistor 138 (Isrc-1) and the NMOS transistor 134 (Isnk). The take-out line 146 is branched, one is applied for light emission driving of the light emitting element 14A, and the other is fed back and input to the gate G of the PMOS transistor 120 constituting the operational amplifier OP.

1 is a schematic configuration diagram of an image forming apparatus according to the present embodiment. It is a basic drive circuit diagram (including smile correction) for driving a surface emitting laser. It is a laser drive current-laser light quantity (terminal voltage) characteristic view. It is a schematic block diagram of the light beam emission control apparatus provided with the light quantity nonuniformity correction system of the main scanning which concerns on this Embodiment, and a subscanning direction. It is the schematic which shows the structure of the operational amplifier concerning this Embodiment. (A) is an equivalent circuit diagram at the time of discharging in the operational amplifier, (B) is an equivalent circuit diagram at the time of charging in the operational amplifier. It is an output voltage-current consumption characteristic figure in the operational amplifier which concerns on this Embodiment. It is a circuit diagram which shows the Example of the operational amplifier of this invention. (A) is a circuit diagram when the discharging current source is variable, (B) is a circuit diagram when the charging current source is variable, and (C) is when both the charging and discharging current sources are variable. FIG. 9D is an output voltage-current consumption characteristic diagram of an operational amplifier according to a conventional example of the circuit of FIG.

Explanation of symbols

OP Operational Amplifier (Amplifier)
Cd decoupling capacitor 10 image forming apparatus 14 surface emitting laser 14A light emitting element 26 driving circuit (driving means)
62 photodetector 66 I / V
68 Differential amplifier 70 Reference value (reference voltage Vref)
100 comparator 102 first stage amplifier)
110, 112, 114, 116, 118 PMOS transistor 120, 122 PMOS transistor 124, 126 NMOS transistor 128, 130 NMOS transistor 134 NMOS transistor 136 PMOS transistor 138 PMOS transistor 140 NMOS transistor 142 Power line 144 Ground line 146 Extraction line 148 PMOS transistor 150 NMOS transistor 152 Charge / discharge switching unit

Claims (3)

Used in an image forming apparatus that scans a light beam emitted from a light source based on image data, forms an electrostatic latent image on an image carrier, develops it, and transfers the image onto a recording medium to form an image. A light beam emission control device for controlling emission of a light beam emitted from the light source,
A plurality of holding means for individually holding control voltages for controlling the light emission amount of each light beam to a reference light amount;
Based on the control voltage held in the holding means, a voltage driving circuit for setting a voltage at the time of voltage driving so as to emit a light beam from the light source with a reference light amount, and a current driving circuit for setting a current at the time of current driving Drive means comprising
The voltage driving circuit is composed of a decoupling capacitor that holds a voltage at the time of voltage driving based on a control voltage held in the holding means, and an amplifier that performs charging or discharging to the decoupling capacitor,
A light beam emission control device characterized in that any one of a discharge current source and a sink current source constituting an output stage of the amplifier always limits an upper limit current.   2. The light beam emission control device according to claim 1, wherein when the output current from the amplifier is zero, it is set which upper limit current of the discharge or sink current source is limited.   When the light source is turned on and the current output from the amplifier is supplied, the other current source of this current source sets the upper limit current, and the other current source is also the upper limit current even when the output current of the amplifier is zero The light beam emission control device according to claim 2, wherein:

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