JP2000238331A - Light source-driving apparatus and image-forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the situation of breaking a light source because of turning a variable resistor for light quantity adjustment too much in an APC circuit for adjusting a quantity of light of a laser diode to a desired value. SOLUTION: The apparatus consists of a PD 2 for receiving light beams projected from an LD 1 and generating a monitor current corresponding to a quantity of light of the light beams, and a first variable resistor 21 and a second variable resistor 22 each connected in series to the PD 2. The apparatus includes a converting means for converting the monitor current generated from the PD 2 to a monitor voltage and a means for generating a driving current for the LD 1 in accordance with the monitor voltage. The first variable resistor 21 sets an upper limit value of a target quantity of light of a light source and the second variable resistor 22 adjusts the target quantity of light of the light source.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a driving circuit for a light source used in an image forming apparatus for forming an image by scanning a photosensitive member with a light beam modulated based on an image data signal.

[0002]

2. Description of the Related 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 photoreceptor to form an electrostatic latent image.
There is a laser beam printer that obtains a copying process such as development, transfer, fixing and prints a desired image.

When the laser is driven to emit light, an automatic light amount control (Auto Pow) is used in order to reduce a change based on the temperature characteristic of the laser and a change in the light emission output due to a temporal change.
erControl = APC). FIG. 2 is a configuration example of a laser drive circuit that drives a laser diode as a light source and performs APC. Current switching 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 is H. The LD 1 emits light according to the supplied current value, and the photodiode (PD) 2
Receives a light beam (back beam) emitted from the rear of the LD 1 to generate a current corresponding to the amount of emitted light, and the current flows through the resistor 3 to generate a voltage.

The comparator 5 generates a voltage V generated by a current generated by the PD 2 flowing through the resistor 3.
_{The PD} is compared with a voltage value Vref ′ obtained by dividing the first reference voltage (Vref) by the variable resistor 4, and the result is sent to the sample and hold circuit 6. The comparator is: V _LD > V
H is output if ref ', and L if V _LD <Vref'.

FIG. 6 shows details of the sample and hold circuit. If the output (COMP) of the comparator 5 is L when the S / H signal is H (sample), the switch 63 is closed, 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 closes and a discharge current flows from the capacitor (CH) 66 to the discharge current source. S / H signal is L
At the time of (hold), the output of the buffer 61 becomes high impedance and the switches 63 and 64 are both open, so that the charge (voltage value) of the capacitor (CH) 66
Remains held.

The differential amplifier circuit 12 uses this capacitor (C
H) The drive current amount of the LD 1 is determined according to the deviation between the voltage value of 66 and the voltage value of the second reference voltage. Thus, the light emission intensity of the LD 1, that is, the current amount of the PD 2 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 holding.

The current switching circuit 7 is a differential amplifier 1
2 is controlled so as to flow to the LD 1 when the Laser-ON signal is H. Note that the light emission intensity can be converted by changing the value of the variable resistor 4.

[0008]

Since a laser diode may be destroyed by an overcurrent, a laser drive current limiter circuit is usually provided to prevent an overcurrent. FIG. 3 shows an example of a laser drive circuit to which a laser drive current limiter circuit is added. Those having the same numbers as those of the laser drive circuit in FIG. 2 perform the same operations. The drive current of the LD 1 flows through the resistor 8 and generates a voltage according to the amount of current. Comparator 10 and the voltage V _LD of the resistor 8, the voltage value reference voltage (Vref) divided by the variable resistor 9 Vre
The comparator 10 outputs H to the OR gate 11 when VLD> Vref '. The OR gate 11 also receives an output of the comparator 10 and an Enable signal for permitting laser lighting, and the current switching circuit 7 permits laser lighting only when both inputs are L. That is, if a steady current flows through LD1, V
_{Since LD} <Vref ′, and the output of the comparator 10 becomes L, laser lighting is possible. When V _LD > Vref ′ due to an overcurrent flowing through LD1, the comparator 10
Becomes H, and the current switching circuit 7 cannot turn on the laser. Note that by changing the value of the variable resistor 9, the amount of current that can flow through the LD 1 can be limited, thereby setting the upper limit of the drive current.

However, the driving current of a laser diode usually changes due to a change over time or a change in temperature, even if the amount of emitted light is constant. There is a possibility that the drive current reaches the upper limit value of the drive current set by the variable resistor 9 due to a temperature change or a change over time, so that the laser cannot be turned on. For this reason, it is conceivable that the upper limit value of the drive current is set to a relatively large value by the variable resistor 9. However, in consideration of variations in various characteristics of the laser, depending on the laser, the drive current may break before reaching the upper limit value. Could be As described above, if the upper limit of the drive current is set to a lower value in consideration of the variation in the characteristics of the laser, temperature change and aging change become a problem, and conversely, the influence of temperature change and aging change is considered. If the upper limit of the drive current is set to a higher value, variations in individual characteristics of the laser become a problem, and there is a problem that it is difficult to set an adjustment value of the upper limit.

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above problems, and has as its object to prevent a situation in which a light source is destroyed due to erroneously turning a variable resistor for adjusting a light amount.

[0011]

An image forming apparatus according to the present invention receives a light beam emitted from a light source and generates a monitor current in accordance with the light amount of the light beam; Converting means for converting a monitor current generated from the light receiving means into a monitor voltage, comprising: a first variable resistor and a second variable resistor each connected in series; and the light source according to the monitor voltage. Means for generating the drive current of the above, wherein the upper limit of the target light amount of the light source is set by the first variable resistor, and the target light amount of the light source is adjusted by the second variable resistor. And

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, based on a preferred embodiment,
The present invention will be described.

FIG. 4 is a block diagram showing a schematic configuration of an image forming apparatus having a laser drive circuit according to the present embodiment. 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 the magenta (M), cyan (C), yellow (Y), and black (BK) image signals by using a laser lighting signal (Laser) as shown in FIG.
-Generate an ON signal, C-2) in the figure. The laser drive control circuit 112 modulates and drives the laser diode 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, the photosensitive drum 1
05, an electrostatic latent image is formed. 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 the image writing timing control circuit 101 causes 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. La
The ser-ON signal also performs lighting for detecting the LSYNC signal in addition to the 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 provided while the photosensitive drum 105 rotates four times. Alternately contact the photosensitive drum 105, and the photosensitive drum 10
5 is developed with toner corresponding to the electrostatic latent images of M, C, Y, and BK formed on the surface 5. 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. The recording paper 10 on the transfer drum 108 is provided in the transfer drum 108.
Sensor 1 for generating an ITOP signal indicating the tip position of the sensor 9
10, the transfer drum 108 rotates and the transfer drum 10
When the flag 111 fixed in 8 passes through the sensor 110, an ITOP signal (A in the figure) for each color as shown in FIG. 5 is generated. 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. Thus, M,
After the four colors C, Y, and BK are sequentially transferred, the sheet is discharged through a fixing unit (not shown).

FIG. 1 shows a detailed configuration of the laser drive control circuit. Note that the components having the same numbers as those of the laser driving circuit in FIG.
The difference from the conventional example is that the current flowing to PD2 flows to both variable resistors 21 and 22 instead of the resistor, and the first reference voltage Vref is directly input to the inverting terminal (-) of the comparator 5. It is. The voltage value generated by the current flowing through the PD 2 changes depending on the total resistance value of the variable resistors 21 and 22. Since the drive current of the LD 1 is controlled so that this voltage value and Vref become equal, the light emission amount of the laser diode can be changed by the total resistance value of the variable resistors 21 and 22.

The role of the two variable resistors 21 and 22 will be described. The variable resistor 21 is for setting the upper limit of the laser drive current, and the variable resistor 22 is for adjusting the laser drive current to a desired value. At the time of sampling, the voltage of the non-inverting terminal (+) of the comparator 5 is equal to the reference voltage Vref.
Is controlled so as to be always constant, so that when the total resistance value of the variable resistors 21 and 22 is large, the light emission amount of the LD1 is reduced so that the current amount of the PD2 is reduced.
When the total resistance value is small, the light emission amount of LD1 is increased so that the current amount of PD2 is increased.

First, the adjustment variable resistor 22 is reduced to 0Ω, and in this state, the amount of emitted light is set by the variable resistor 21 for setting the upper limit so as not to exceed the rating of the laser diode. This is the upper limit of the target light quantity. Next, when the resistance value is increased from 0Ω by the variable resistor 22 for adjustment, the amount of emitted light gradually decreases. When the amount of emitted light reaches a desired value, the adjustment of the variable resistor 22 is completed and fixed. As a result, even if the laser light emission amount changes due to a change with time of the laser or the like, the light amount changes only within a range that does not exceed the rating as long as adjustment is performed only by the variable resistor 22 for adjustment. Therefore, it is possible to prevent the laser diode from being broken even if the variable resistor 22 is turned by mistake.

As described above, by using two variable resistors for setting and adjusting the upper limit value of the amount of light emitted from the laser diode, the variable resistor for setting the upper limit value is set once and this resistance value is fixed. The laser can be prevented from being destroyed even if the adjusting variable resistor is turned by mistake.

[0018]

As described above, according to the present invention, a variable resistor for adjusting the light quantity can be provided with a simple configuration using two variable resistors for setting the upper limit value of the light emission quantity of the light source and for adjusting. Even if it is turned by mistake, the light source can be prevented from being destroyed.

[Brief description of the drawings]

FIG. 1 is a configuration of a laser driving circuit according to an embodiment.

FIG. 2 is a configuration of a conventional laser driving circuit.

FIG. 3 is a configuration of a conventional laser limiter circuit.

FIG. 4 is a configuration of an image forming apparatus according to the present exemplary embodiment.

FIG. 5 is a timing chart of a laser lighting signal.

FIG. 6 shows details of a sample hold circuit.

[Explanation of symbols]

 Reference Signs List 1 LD 2 PD 5 Comparator 6 Sample / hold circuit 12 Differential amplifier circuit 21 Variable resistor 22 Variable resistor

Claims (5)

[Claims] 1. A light receiving means for receiving a light beam emitted from a light source and generating a monitor current according to the light amount of the light beam, and a first variable resistor connected in series with each of the light receiving elements. And a second variable resistor, comprising: conversion means for converting a monitor current generated from the light receiving means into a monitor voltage; and means for generating a drive current for the light source in accordance with the monitor voltage, An apparatus for driving a light source, wherein an upper limit value of a target light amount of a light source is set by the first variable resistor, and a target light amount of the light source is adjusted by the second variable resistor. 2. A driving device for driving the light source according to a voltage of the capacitor, comprising comparing means for comparing the monitor voltage output from the conversion means with a first reference voltage to control charging and discharging of the capacitor. The driving device for a light source according to claim 1, wherein the driving device generates an electric current. 3. The light source driving device according to claim 2, wherein a driving current for the light source is generated according to a deviation between the voltage of the capacitor and a second reference voltage. 4. The light source driving device according to claim 1, wherein the light source is a laser diode. 5. A light beam emitted from a light source,
An image forming apparatus for forming an image on a photoreceptor, comprising: a light receiving unit that receives a light beam emitted from the light source and generates a monitor current according to a light amount of the light beam; A first variable resistor and a second variable resistor that are connected in series, a converting unit that converts a monitor current generated from the light receiving unit into a monitor voltage, and a conversion unit that controls the light source according to the monitor voltage. Means for generating a drive current, wherein an upper limit value of a target light amount of the light source is set by the first variable resistor, and a target light amount of the light source is adjusted by the second variable resistor. Characteristic image forming apparatus.