JP2004009492A - Laser drive circuit in image forming apparatus - Google Patents

Abstract

An image forming apparatus that scans a laser beam modulated based on image information is provided.
A laser diode, a photodiode that receives a part of the laser beam and outputs a monitor current,
Means for converting the monitor current into a voltage,
Reference voltage generating means for generating a reference voltage;
Comparing means for comparing the voltage obtained by converting the monitor current output from the light receiving element and the reference voltage output by the reference voltage generating means,
Laser drive control means for setting a drive current of the laser diode based on the voltage of the held capacitor,
The laser control unit has a switching current source 1 that switches with a constant current and a switching current source 2 that switches with a drive current set by the sample and hold unit,
A laser driving circuit in an image forming apparatus, wherein the switching current sources 1 and 2 are connected to a laser diode so as to drive one laser at a time.
[Selection diagram] Fig. 1

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a laser driving circuit of an image forming apparatus that forms an image by scanning a laser beam modulated based on an image data signal on a photosensitive member.
[0002]
[Prior art]
Conventionally, as an image forming apparatus that prints out image data, a laser beam modulated based on image information is scanned and exposed on a photoconductor to form an electrostatic latent image, and a copying process such as development, transfer, and fixing is performed. There is a laser beam printer that obtains and prints a desired image.
[0003]
When the laser is driven to emit light, automatic light amount control (Auto Power Control = APC) is performed in order to reduce a change based on a temperature characteristic of the laser or a change in light emission output due to a change with time. FIG. 1 is a configuration example of a laser drive control circuit 112 that drives a laser diode and performs APC. The laser drive current circuit 7 turns on the laser diode (LD) 1 at a constant current when the Enable signal for permitting laser lighting changes from H to L and the Laser-ON signal for turning on the laser is H. The LD 1 emits light according to the supplied current value, the photodiode (PD) 2 generates a current (I monitor) corresponding to the amount of light emission, and the current flows through the resistor 3 to generate the voltage _VPD . Let it.
[0004]
Comparator 5 compares the voltage _{V PD,} a reference voltage obtained by dividing voltage value Vref 'and the variable resistor 4 (Vref), and sends the result to the sample / hold circuit 6. The comparator outputs H if _VPD > Vref ', and outputs L if _VPD <Vref'.
[0005]
FIG. 6 shows details of the sample / hold circuit. If the output (COMP) of the comparator 5 is L when the S / H signal is H (sample), the switch 63 closes, a charging current flows from the charging current source to the capacitor (CH) 66, and the output (COMP) of the comparator 5 ) Is H, the switch 64 is closed and a discharge current flows from the capacitor (CH) 66 to the discharge current source. When the S / H signal is L (hold), the output of the buffer 61 becomes high impedance and the switches 63 and 64 are both open, so that the charge amount (voltage value) of the capacitor (CH) 66 is maintained. is there.
[0006]
The differential amplifier circuit 12 determines a drive current amount (ΔI _SW ) of the LD 1 according to a deviation between the voltage value of the capacitor (CH) 66 and the voltage value of the reference voltage. Thereby, the light emission intensity of the LD1, that is, the current amount of the PD2 can be corrected to be a target value at the time of sampling, and the corrected light emission intensity can be held at the time of hold.
[0007]
The laser drive current circuit 7 controls the amount of current determined by the differential amplifier circuit 12 to flow to the LD 1 when the Laser-ON signal is H. Note that the light emission intensity can be changed by changing the value of the variable resistor 4.
[0008]
[Problems to be solved by the invention]
FIG. 3 shows the laser drive current circuit 7. The current _{I LD} for driving the LD1 is obtained by adding the current _{I SW} by the current _{I B} and the switching current source 32 bias current source 31. Bias current source supplies for example a constant current circuit such as known current mirror circuit, a constant current I _B which is determined by the value of the resistor 33 to the LD1 when Enable signal of L. Switching current source 32 supplies the _{I SW} in LD1 Laser-ON signal for turning on the laser at the H. That is, the LD1 when Laser-ON signal Enable signal is L is L is supplied by _{I B,} Enable signal _{I B} and _{I SW} is added to the LD1 when Laser-ON signal L is H Is supplied. The switching current source 32 supplies a current amount I _SW determined as a result of performing the APC to the LD 1.
[0009]
FIG. 2 shows the light output-drive current characteristics of the LD1. To obtain a laser light amount _{P O} of the target, the amount of current required to When _{I LD I LD} is composed of _I B _{+ I SW.} The bias current _{I B} as shown in FIG. 2 is set to闘値current Ith following a闘値performing laser light emission, switching the _{I SW} minus _{I B} from _{I LD} in accordance with the Laser-ON signal Performs laser emission.
[0010]
Assuming that the rating of the light emission amount of the LD 1 is Pmax, the current amount at that time is Imax. When the threshold current Ith is exceeded, the amount of light emitted with respect to the drive current sharply increases. Even Therefore there is a difference in the use amount _{P O} rated light amount _Pmax, there is not much difference between the _{I LD} and Imax. Since the switching current I _SW determined by APC is large, if APC runs away due to external noise or the like and causes an error in the value of I _SW , I _LD (= I _B + I _SW ) exceeds Imax and LD 1 May be destroyed.
[0011]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention provides an image forming apparatus that scans a laser beam modulated based on image information to form an image, and includes a laser diode and a laser beam output from the laser diode. A light-receiving element that receives the light and outputs a monitor current according to the light-receiving result; a means for converting the monitor current into a voltage; a reference voltage generating means for generating a reference voltage; and a monitor current output from the light-receiving element. A comparator for comparing the converted voltage with a reference voltage output by the reference voltage generator; a capacitor; and charging or discharging the capacitor for each main scan of the laser beam according to a result of the comparator, thereby performing main scan. Sample and hold means for holding the voltage of the capacitor for a period of time, and a drive voltage for the laser diode based on the held voltage of the capacitor. And a laser drive control means for setting a,
The laser control means has a switching current source 1 for switching with a constant current and a switching current source 2 for switching with a drive current set by the sample and hold means. The switching current sources 1 and 2 simultaneously drive one laser. A laser drive circuit in the image forming apparatus, wherein the laser drive circuit is connected to a laser diode.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described based on preferred embodiments.
[0013]
FIG. 4 is a block diagram illustrating a schematic configuration of an image forming apparatus including a laser drive circuit according to the present embodiment. In FIG. 4, an image signal sent from an external device such as an image scanner or a computer (not shown) is sent to an image writing timing control circuit 101. The image writing timing control circuit 101 responds to magenta (M), cyan (C), yellow (Y), and black (BK) image signals by using a laser lighting signal (Laser-ON signal, C- 2) is generated. The laser drive control circuit 112 modulates and drives the LD 1 according to the laser lighting signal from the image writing timing control circuit 101. The laser light is reflected by the polygon mirror 103 rotating in the direction of the arrow when the polygon motor 106 is driven, f-θ corrected by the f-θ lens 104, and scans the photosensitive drum 105. Thus, an electrostatic latent image is formed on the photosensitive drum 105. The BD sensor 107 is provided in the vicinity of the scanning start position of one line of the laser light, detects the line scanning of the laser light, and uses the image writing timing control circuit 101 to perform a scanning start reference signal (see FIG. An LSYNC signal (B in the figure) and a sample hold signal (S / H signal, D in the figure) are generated. The Laser-ON signal also performs lighting for detecting an LSYNC signal in addition to image formation (C-1 in the figure). Magenta (M), cyan (C), yellow (Y), and black (BK) developing units (not shown) are provided around the photoconductor 105, and four developing units are mounted while the photosensitive drum 105 rotates. The photosensitive drum 105 alternately comes into contact with the photosensitive drum 105 and is developed with toner corresponding to the M, C, Y, and BK electrostatic latent images formed on the photosensitive drum 105. Recording paper 109 fed from a paper cassette (not shown) is wound around a transfer drum 108, and a toner image developed by a developing device is transferred. In the transfer drum 108, there is a sensor 110 for generating an ITOP signal indicating the leading end position of the recording paper 109 on the transfer drum 108, and a flag 111 fixed in the transfer drum 108 by rotation of the transfer drum 108 is detected by the sensor 110. , An ITOP signal (A in the figure) for each color as shown in FIG. 5 is created. The image writing position in the sub-scanning direction is determined based on the ITOP signal, and the image writing position in the main scanning direction is determined based on the LSYNC signal. After the four colors M, C, Y, and BK are sequentially transferred in this manner, the sheet is discharged through a fixing unit (not shown).
[0014]
The laser drive control circuit 112 has substantially the same configuration as that of the conventional example, and FIG. 8 shows a current drive current circuit 7 'different from the present embodiment.
[0015]
The difference between the current driving current circuit 7 'and the conventional example is that the switching current supplied to the LD1 when the Laser-ON signal for lighting the laser is H is divided into two current sources. Switching current _{I SW} as shown in FIG. 8 is composed of _{I SW2} of the switching current source (A) 82 _{I SW1} and the switching current source (B) 83. When the Enable signal is L and the Laser-ON signal is H, the switching current source (A) 82 supplies a constant current I _SW1 determined by the value of the variable resistor 85 to the LD 1. When the signal is L and the Laser-ON signal is H, the switching current source (B) 83 supplies the LD1 with a current amount _ISW2 determined as a result of performing APC on the LD1. When the Enable signal is H or L and the Laser-ON signal is L, both the switching current sources 1 and 2 stop supplying current to LD1. On the other hand, a constant current circuit, such as the bias current source 81 is for example known current mirror circuit, supplies a constant current _{I B} which is determined by the value of the resistor 84 to the LD1 when Enable signal is L regardless of Lsaer-ON signal I do. The roles of the switching current sources (A) and (B) are that the switching current source (A) 82 is for coarse adjustment of the laser light amount, and the switching current source (B) 83 is for fine adjustment of the laser light amount. That is, the variable resistor 85 in advance to adjust slightly smaller amount than the target amount of light, and then fine adjustment so that the target light amount P _O at the variable resistor 4. When the Enable signal is H, all current supply to the LD 1 is stopped regardless of the Lsaer-ON signal.
[0016]
FIG. 7 shows the light output-drive current characteristics of LD1. To obtain a laser light amount _{P O} of the target, the amount of current required to When _{I LD I LD} is composed of _{I B} and _{I SW.} I _SW is composed of I _SW1 and I _SW2 . By setting I _SW2 determined by APC to be smaller than I _SW1 that is a fixed value of current, the range of the amount of current controlled by APC can be reduced. That is, even if the APC runs away due to external noise or the like, since the absolute amount of the error of the switching current I _SW is small, the possibility that I _LD (= I _B + I _SW ) exceeds Imax is reduced. That is, it is possible to prevent an overcurrent to the LD 1 and prevent the laser diode from being destroyed.
[0017]
【The invention's effect】
As described above, with a simple configuration having two switching current sources for the laser diode, one being a fixed current source and the other being a current source adjusted by APC, it prevents overcurrent due to runaway of APC, That is, destruction of the laser diode can be prevented.
[Brief description of the drawings]
FIG. 1 shows the configuration of a laser drive control circuit. FIG. 2 shows the distribution of a conventional drive current by light output-drive current characteristics. FIG. 3 shows the configuration of a conventional laser drive current circuit. FIG. 5 is a timing chart of a laser lighting signal. FIG. 6 is a detail of a sample hold circuit. FIG. 7 is a light output-drive current characteristic, which represents distribution of a drive current in the present embodiment. 8 Configuration of laser drive current circuit in this embodiment

Claims (2)

In an image forming apparatus that forms an image by scanning a laser beam modulated based on image information,
A laser diode,
A light receiving element that receives a part of the laser beam output from the laser diode and outputs a monitor current according to the result of the light reception;
Means for converting the monitor current into a voltage,
Reference voltage generating means for generating a reference voltage;
Comparing means for comparing the voltage obtained by converting the monitor current output from the light receiving element and the reference voltage output by the reference voltage generating means,
A capacitor,
According to the result of the comparison means, charge or discharge the capacitor for each main scan of the laser beam, sample and hold means to hold the voltage of the capacitor during the main scan,
Laser drive control means for setting a drive current of the laser diode based on the voltage of the held capacitor,
The laser control unit has a switching current source 1 that switches with a constant current and a switching current source 2 that switches with a drive current set by the sample and hold unit,
A laser drive circuit in an image forming apparatus, wherein the switching current sources 1 and 2 are connected to a laser diode so as to drive one laser at a time. The laser drive control means of the first aspect also includes a bias current source that supplies a bias current equal to or less than a threshold current of the laser diode to the laser diode.

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