JP2000039577A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To automatically control an LD light quantity with simple constitution regardless of fluctuation in a monitor current when the LD light quantity is switched according to pixel density switch. SOLUTION: When the light quantity is switched according to the pixel density switch, a monitor voltage changeover switch SW1 switches variable resistors VR1, VR2, and a reference voltage changeover switch SW2 switches reference voltages Vref1, Vref2. A comparator 12 compares monitor current Im × variable resistor VR1 or VR2 = monitor voltage V2 with the reference voltage Yref1 or Vref2. An error amplifier 16 error amplifies an input voltage V4 according to the comparison result and the reference voltage Vref3, and thus, a collector current of a transistor Tr1 is increased/decreased, and a drive current of an LD2 is increased/decreased, and the light quantity of the LD2 is controlled to a set value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for forming a latent image on a photosensitive member by scanning light emitted from a laser diode (hereinafter abbreviated as "LD"). APC (Automati
c Power Control).

[0002]

2. Description of the Related Art This type of APC circuit is disclosed in
As described in JP-A-226675 and JP-A-8-88429, a monitor current of a diode for monitoring the amount of LD light is converted into a voltage, and this monitor voltage is compared with a reference voltage so that the amount of LD light becomes constant. It is configured to control the drive current of the LD.

[0003]

In this type of image forming apparatus, when the pixel density is switched, the LD light amount may be switched by changing the linear velocity according to the pixel density. For example, when switching between 600 dpi and 1200 dpi, the rotational speed of the polygon motor and the video clock frequency are doubled at 1200 dpi than at 600 dpi, so the linear velocity may be reduced to 1/2, and the LD light amount may be reduced to 1/200. It becomes 2.

However, since the current for monitoring the LD light amount has a large variation, when the LD light amount is switched with the switching of the pixel density, there is a problem that if this monitor current is used as it is, the light amount control becomes difficult.

Accordingly, the present invention provides an image forming apparatus capable of automatically controlling the amount of LD light with a simple configuration regardless of variations in monitor current when switching the amount of LD light in accordance with the switching of pixel density. With the goal.

[0006]

According to a first aspect of the present invention, there is provided an image forming apparatus for forming a latent image on a photosensitive member by scanning an LD light, wherein the monitor current of the LD light is converted to a monitor voltage. A conversion unit, a comparator for comparing the monitor voltage with a reference voltage, and a unit for switching between the monitor voltage and the reference voltage when the LD light amount is switched along with the pixel density switching.

The second means is the first means, wherein the L
It further comprises a capacitance for holding the driving current of D.

The third means is characterized in that the first means further comprises means for switching the value of the capacitance in accordance with the cycle of the scanning synchronization detection signal of the LD light.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram illustrating an LD scanning optical system according to an embodiment of the image forming apparatus according to the present invention, FIG. 2 is a block diagram illustrating a synchronization detection circuit of the image forming apparatus illustrated in FIG. 1, and FIG. FIG. 4 is a timing chart showing a synchronization detection signal of the circuit, and FIG. 4 is a block diagram showing an APC circuit of the image forming apparatus of FIG.

In the LD scanning optical system shown in FIG. 1, LD light modulated according to image data is emitted from an LD 2 mounted on an LD control plate 1, and this LD light is collimated by a collimating lens 3, and then collimated. The polygon scanner 4 is deflected at a constant angular velocity in the main scanning direction. Next, the LD light is corrected to have a constant velocity deflection by the fθ lens 5 and is irradiated onto the photoreceptor 6, whereby a latent image is formed on the photoreceptor 6.

The LD light is received by the synchronization detecting element 7 each time the scanning in the main scanning direction is performed. When the synchronization detection element 7 is L
When the D light is received, a current I flows through the synchronous detection element 7, and
As shown in (1), the comparator 8 outputs a synchronization detection signal DETP which becomes high when R1 × I> Vref. And
Based on the synchronization detection signal DETP, as shown in FIG. 3, a synchronization detection signal XDETP that is low during the section T1 at the beginning of the one-line cycle T2 is generated.
DETP is used as a sample / hold (S / H) signal of the control circuit 13 shown in FIG.

Next, an image forming apparatus according to the present invention will be described in detail with reference to FIG. The LD 2 is provided with a monitor diode (PD) 11 for receiving the light amount. The monitor current Im is applied to one input terminal of the comparator 12 as a monitor voltage V2, and a monitor voltage switch SW1 and a pixel density switch signal. IN1 selectively applies to the line of the variable resistor VR1 or the line of the variable resistor VR2.

The monitor voltage V2 applied to the comparator 12 is V2 = Im * VR1 when the line of the variable resistor VR1 is selected, and V2 when the line of the variable resistor VR2 is selected. = Im * VR2. The reference voltage Vref1 or Vref2 is selectively applied to the other input terminal of the comparator 12 as the reference voltage V1 by the reference voltage changeover switch SW2 and the pixel density changeover signal IN2. The output signal V3 of the comparator 12 goes high when V2> V1, and goes low when V2> V1 is not satisfied.

When the output signal V3 of the comparator 12 becomes high, the control circuit 13 switches the capacitance changeover switch S
Turn on W4. When the switch SW4 is turned on, the charging constant current source 14 is connected to one input terminal (input voltage V4) of the error amplifier 16 via the switch SW4 and to one end of the capacitance changeover switch SW3. Switch SW3 and its switching signal IN
3 selectively connects to the capacitor C1 or C2. Therefore, when the capacitor C1 or C2 is charged, the voltage V applied to one input terminal of the error amplifier 16 is
4 rises.

On the other hand, when the output signal V3 of the comparator 12 becomes low, the control circuit 13 turns on the switch SW5. When the switch SW5 is turned on,
The charge charged in the capacitor C1 or C2 is discharged to the discharging constant current source 15, and the voltage V4 applied to one input terminal of the error amplifier 16 decreases.

The control circuit 13 switches between sampling and holding by a sample / hold (S / H) signal. When the S / H signal is high (image section), the output signal V3 of the comparator 12 is high as described above. , One of the switches SW4 and SW5 is turned on in accordance with the low level. On the other hand, when the S / H signal is low (synchronization detection section), the control circuit 13
Both W4 and SW5 are turned on, whereby the voltage V4 applied to one input terminal of the error amplifier 16 is held at a constant value. In the error amplifier 16, this input voltage V4
The reference voltage Vref3 is amplified by the error, whereby the collector current of the transistor Tr1 increases and decreases, and the drive current of the LD2 increases and decreases, and the light amount of the LD2 is controlled to the set value.

An example of switching between 600 dpi and 1200 dpi will be described. VR1 = 1 kΩ, VR2
= 2 kΩ and VR1 = 1 kΩ is selected in the case of 600 dpi, and VR2 = 2 kΩ is selected in the case of 1200 dpi, so that the input of the comparator 12 is controlled so that V1 = V2. Becomes 1/2 of the case of

Here, when the values of the variable resistors VR1 and VR2 are increased and the monitor current Im becomes too large, the setting accuracy of the LD light quantity deteriorates. Therefore, in this case, if the reference voltages V1 (Vref1, Vref2) of the comparator 12 are reduced, the values of the variable resistors VR1, VR2 do not need to be too large. For example, when Vref1 = 1V and Vref2 = 2V, Vref1 = 1V is selected when the monitor current Im is small, and Vref2 = 2V is selected when the monitor current Im is large.

When the control circuit 13 is in the hold mode, the input voltage V4 of the error amplifier 16 is held by the capacitor C1 or C2.
1, when the capacitance value of C2 is relatively large, the input voltage V4
Is held constant for a relatively long time, while the input voltage V4 is held constant for a relatively short time when the capacitance values of the capacitors C1 and C2 are relatively small. Therefore, when the capacitance values of the capacitors C1 and C2 are relatively small, there is an advantage that the time required for the potential of the input voltage V4 to be charged to the set value is short.

When the synchronization detection signal DETP is used as the S / H signal of the control circuit 13, for example,
If 1 = 1 μF and C2 = 0.01 μF, C1 = 1 μF is selected when the period of the synchronization detection signal XDETP is long, and C2 is selected when the period of the synchronization detection signal XDETP is short.
= 0.01 μF may be selected. If C1 = 1 μF is selected, the time required to reach the set light amount becomes longer, but the time during which the light amount is held constant becomes longer. Therefore, since the APC control is performed so that the monitor current Im becomes constant, the LD light amount can be automatically controlled with a simple configuration when the linear speed is changed according to the pixel density to switch the LD light amount.

[0021]

As described above, according to the first aspect of the present invention, the monitor voltage and the reference voltage to be compared by the comparator are switched when the LD light amount is switched along with the pixel density switching. LD light quantity can be automatically controlled with a simple configuration irrespective of the variation of.

According to the second aspect of the present invention, since the driving current of the LD is held by the capacitance, the LD light amount can be automatically controlled with a simple configuration regardless of the variation of the monitor current.

According to the third aspect of the present invention, the value of the capacitance is switched in accordance with the cycle of the scanning synchronization detection signal of the LD light. LD light quantity can be automatically controlled with a simple configuration irrespective of the variation of.

[Brief description of the drawings]

FIG. 1 is an LD of an embodiment of an image forming apparatus according to the present invention.
FIG. 2 is a configuration diagram illustrating a scanning optical system.

FIG. 2 is a block diagram illustrating a synchronization detection circuit of the image forming apparatus of FIG. 1;

FIG. 3 is a timing chart illustrating a synchronization detection signal of the synchronization detection circuit of FIG. 2;

FIG. 4 is a block diagram illustrating an APC circuit of the image forming apparatus of FIG. 1;

[Explanation of symbols]

 2 LD (laser diode) 11 PD (photodiode) 12 comparator 13 control circuit 14 constant current source for charging 15 constant current source for discharging 16 error amplifier SW1 monitor voltage switch VR1, VR2 variable resistor SW2 reference voltage switch Vref1, Vref2 Reference voltage SW3 to SW5 Capacitance switch C1, C2 Capacitor

Claims (3)

[Claims] 1. An image forming apparatus for forming a latent image on a photosensitive member by scanning light emitted from a laser diode, means for converting a monitor current obtained by monitoring a light amount of the emitted light into a monitor voltage, An image forming apparatus comprising: a comparator that compares the monitor voltage with a reference voltage; and a unit that switches between the monitor voltage and the reference voltage when the amount of light emitted from the laser diode is switched with a change in pixel density. 2. The image forming apparatus according to claim 1, further comprising a capacitor for holding a driving current of the laser diode. 3. The image forming apparatus according to claim 2, further comprising: means for switching the value of the capacitance according to a cycle of a scanning synchronization detection signal of the emitted light.