JPH0671938A - Image forming device - Google Patents

Abstract

PURPOSE:To avoid output fluctuation occurring with fluctuation of reference current and reduce output fluctuation occurring with printing ratio by providing a voltage stabilizing means for stabilizing input voltage to constant current circuits between a power source and the constant current circuits. CONSTITUTION:A voltage stabilizing means such as electrolytic condenser 14 is provided in a stage prior to constant current circuits B1-B64 for stabilizing input voltage. Thus, without improving expensive constant current circuits 8, B1-B64 themselves, problem of fluctuation of input voltage to the constant current circuits occurring due to fluctuation of printing ratio and problem of fluctuation of reference current at the constant current circuits can be solved. To LED arrays L1-L40, switching transistors T1-T40 are connected, respectively. The switching transistors T1-T40 are integrated into 1 chip so as to be compact and properties are unified. The condenser 14 as a voltage stabilizing means is provided in an LED print head. The output stabilized at the condenser 14 and VDD are made to be power source of the LED head.

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 such as an LED head, a thermal head, an EL head, etc., and more particularly to correction of output fluctuation depending on a printing rate.

[0002]

2. Description of the Related Art The inventors have proposed an LED head in which a large number of LED arrays are arranged on a substrate and each LED array is driven by a constant current circuit (for example, JP-A-4-138).
263). Such an LED head is used by being incorporated in a printer, a facsimile, a copying machine or the like, and electric power is supplied to a constant current circuit from a power source of the printer or the like. Further, the LED arrays are driven, for example, one array by time division. For example, a mirror constant current circuit is used for the constant current circuit,
The current flowing through the reference resistor is used as a reference current, and an output current that is substantially equal to the reference current is generated.

Assuming that the light emitting current per LED is 10 mA and the number of LEDs per LED array is 64,
The output of the constant current circuit fluctuates between 0 and 640 mA. Since it is impossible to use a constant current circuit having ideal characteristics in terms of cost, the output of the constant current circuit is affected by fluctuations in input voltage and fluctuations in output current. If the printing rate increases and the output current of the constant current circuit increases, the stability of the power source such as the printer is insufficient, or the line drop between the power source of the printer and the constant current circuit causes a change to the constant current circuit. Input voltage fluctuates. When the input voltage decreases due to the increase in the printing rate, the voltage applied to the reference resistance decreases and the reference current decreases. This is because the input voltage is applied to the reference resistance without being stabilized. Even if the reference current does not decrease, the output current also decreases when the input voltage decreases due to the general characteristics of the constant current circuit. Further, when the printing rate increases, the output current of the constant current circuit further decreases due to an increase in line drops and the like between the constant current circuit and the LED array.

To reduce the light emission current due to the increase in printing rate,
As mentioned above, three mechanisms are involved. The first is that the input voltage to the constant current circuit decreases due to the increase in the output current of the constant current circuit. The second one is
It means that the reference current of the constant current circuit decreases due to the decrease of the input voltage, and the output current decreases in proportion to this. The third is that the constant current circuit cannot sufficiently cope with the increase in the voltage drop on the load side of the constant current circuit.

FIG. 5 shows changes in the light emission current (total light emission current per array) of the LED array in the conventional example. The broken line in the figure shows the ideal output characteristics of the constant current circuit.
Print rate 100% (64 light emitting dots)
Then, the maximum decrease in emission current is about 10 to 20% from the target value. Such a decrease in the light emitting current causes a significant deterioration in print quality.

Although the embodiment has been described by taking the LED head as an example, a thermal head, an EL head or the like has the same problem as long as it is a constant current driving image forming apparatus. In these image forming apparatuses, the LED head is driven by a low voltage and a large current, so that the problem is particularly serious.

As a related prior art, Japanese Utility Model Laid-Open No. 2-54539 proposes to connect a wiring pattern for sensing to a power line and an earth line of an LED head and monitor the voltage. There is. However, this publication does not show that the LED head is driven at a constant voltage and is driven at a constant current.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the problems of fluctuations in the input voltage to the constant current circuit and fluctuations in the reference current of the constant current circuit among the problems of fluctuations in the output of the image forming apparatus due to the printing rate. It is in.

[0009]

According to the image forming apparatus of the present invention, a large number of image element arrays are arranged on a substrate, and the image element arrays are driven by a constant current circuit. A voltage stabilizing unit for stabilizing the input voltage to the constant current circuit is provided between the circuit and the circuit.

[0010]

According to the present invention, the voltage stabilizing means is provided in the preceding stage of the constant current circuit to stabilize the input voltage. As the voltage stabilizing means, for example, a large-capacity capacitor or a literal constant voltage circuit is used, and the input voltage to the constant current circuit is stabilized against load fluctuations due to fluctuations in the printing rate. In this way, among the three problems due to fluctuations in the printing rate without improving the expensive constant current circuit itself, fluctuations in the input voltage to the constant current circuit and fluctuations in the reference current in the constant current circuit. Can be solved.

[0011]

EXAMPLE FIG. 1 shows an example of an LED head.
In the figure, 2 is a control circuit, for example, C of the printer body.
It receives a clock signal CLK, print data DATA, and a reset signal RESET from PU. For example, 4 is a 64-bit shift register, 6 is a 64-bit latch circuit, and A1 to A6
4 is 64 AND circuits. Reference numeral 8 is a constant current circuit for supplying a light emitting current, and 64 individual constant current circuits B1 to
It consists of B64. A data bus 10 is composed of, for example, 64 data lines, and 40 LED arrays L1 to L40 are connected to the data bus 10. Each of the LED arrays L1 to L40 is formed by integrating, for example, 64 LEDs in one chip.

Switching transistors T1 to T40 are connected to the LED arrays L1 to L40, respectively. The switching transistors T1 to T40 are integrated on one chip to be downsized and have uniform characteristics. A block selection circuit 12 receives the clock signal CLK from the control circuit 2,
After counting the clock signal CLK, the decoder is used to decode each of the switching transistors T1 to T40 one by one, for example, 30 μ.
Drive in seconds. As a result, the LED arrays L1 to L40 are driven one by one in a time division manner, for example, in units of 32 μs (of which light emission time is 30 μs).

Reference numeral 14 is a large-capacity capacitor, which is an example of voltage stabilizing means using, for example, an electrolytic capacitor or a tantalum capacitor. The capacitor 14 is preferably provided in the LED printhead and at least in the immediate vicinity of the printhead. The capacitor 14 is connected to a power source such as a printer, and the output stabilized by the capacitor 14 + VDD
Is the power source for the LED head. The capacitance of the capacitor 14 is preferably 200 μF or more, more preferably 2 μF.
000 to 10000 μF. When the capacity of the capacitor 14 is 200 μF, it is assumed that all the light emitting current to the LED array is supplied from the capacitor 14, and the voltage fluctuation to the capacitor 14 when emitting light from one array is 0.1 V.
Then, the electric charge supplied from the capacitor 14 to the LED array is 20 μQ. Maximum of 0.6 at 100% printing rate
When a current of 4 A is supplied for 30 μs, the total amount of charge is 20.
With a μQ and a capacitor 14 of 200 μF or more,
The decrease in input voltage when driving one array at a printing rate of 100% can be suppressed to 0.1 V or less. On the other hand, when the capacitance of the capacitor 14 is 2000 μF or more, when the printing rate is 100% for all of one line, that is, 40 LEs are used.
Even when the D array is driven at a printing rate of 100%, the reduction of the input voltage can be suppressed to about 0.2 V or less.
Despite driving 40 LED arrays, the capacity required for the capacitor 14 increases only about 10 times. The LED arrays L1 to L40 supply current not only from the capacitor 14 but also from an external power supply. Because it is done.

In the embodiment, the LED arrays L1 to L40 are time-divisionally driven as blocks, but static drive may be used. However, in static driving, it is difficult to stabilize the input voltage itself because a constant current circuit of 2560 circuits must be provided and the total emission current is large. Therefore, the stabilization of the input voltage is particularly suitable for time division driving.

FIG. 2 shows the configuration of the constant current circuits B1 to B64. In the figure, 16 is, for example, a mirror constant current circuit, 18
Is the reference resistance, and the output current is determined so as to match the reference current flowing through the reference resistance 18. Since the inexpensive Miller constant current circuit 16 is used, the voltage applied to the reference resistor 18 is not particularly stabilized, and the power supply voltage + VDD is simply divided. Therefore, when the power supply voltage fluctuates, the reference current fluctuates in proportion thereto, and therefore the output current also fluctuates in proportion to fluctuations in the power supply voltage. In addition to this, fluctuations in the input voltage cause fluctuations in the output current as a general characteristic of the Miller constant current circuit.

The operation of the embodiment will be described. When the printing rate changes, the output current of the constant current circuit 8 changes in the range of 0 to 640 mA. The power supply of a printer or the like generally has no output capacity, and the input voltage to the constant current circuit 8 fluctuates due to a line drop or the like between the external power supply and the constant current circuit 8. If a large-capacity capacitor 14 is provided here, when the printing rate increases and the output current increases, the power supply and the capacitor 14
The currents are supplied from both of them, and the fluctuation of the input voltage to the constant current circuit 8 and the fluctuation of the reference current in the reference resistor 18 can be suppressed. For example, if the capacity of the capacitor 14 is 200 μF
Then, the fluctuation of the input voltage during driving one LED array at a printing rate of 100% can be kept to 0.1 V or less. Further, when the capacitance of the capacitor 14 is 2000 μF or more, the fluctuation of the input voltage when printing one line as a whole at a printing rate of 100% can be suppressed to about 0.2 V or less. Therefore, the only remaining problems are the insufficient output capacity of the constant current circuit 8 itself and the voltage drop in the data bus 10 and the switching transistors T1 to T40. Therefore, by simply providing the capacitor 14, it is possible to solve two of the three problems in the fluctuation of the light emission current accompanying the fluctuation of the printing rate.

FIG. 3 shows an embodiment using a constant voltage circuit. In the figure, 22 is a switching transistor, 2
4 is an inductance element, 26 is a flywheel diode, 2
Reference numerals 8 and 30 are resistors, 32 is an amplifier circuit, and 34 is a reference power source using a Zener diode or the like. In this example,
The resistance 30 monitors the fluctuation of the power supply + VDD, the difference from the voltage of the reference power supply 34 is amplified by the amplifier circuit 32, the switching transistor 22 is controlled, and the power supply voltage + VDD is stabilized. An inductance element 24 is used to supply a current even when the switching transistor 22 is off, and a current is supplied from a flywheel diode 26. By doing so, the input voltage to the constant current circuit 8 can be literally stabilized, and since the large-capacity capacitor 14 is not used, it becomes easy to accommodate the constant voltage circuit in the print head.

FIG. 4 shows a third embodiment. In this embodiment, for example, three sets of reference power sources 34, 36, 38 are used,
The counter 40 calculates the printing rate, the decoder 42 decodes it, and drives the switches S1 to S3.
Switch 38. For the switches S1 to S3, switching transistors are used, for example.

The data signal DATA from the printer body or the like is input to the counter 40, and the number of data per 64 clocks is obtained. The value of the counter 40 is input to the decoder 42 by the latch signal LATCH2 synchronized with the latch signal of the latch circuit 6, and immediately after that, the counter 40 is reset by the reset signal RESET2. As a result, the counter 40 is
The number of data signals per unit (number of light emitting dots) is obtained. The decoder 42 switches S1 ...
By operating S3, the reference power sources 34, 36, 3 to the amplifier circuit 32
Switch 8 For example, when the number of LEDs that emit light is 40 or less, the low-voltage reference power supply 34 is used, and the number of LEDs is 41 to 60 and the medium-voltage reference power supply 36 is used.
When the number of D is 61 to 64, the high-voltage reference power supply 38 is used. This not only stabilizes the input voltage to the constant current circuit 8, but also increases it according to the printing rate. For example, a region where the number of LEDs that emit light is 40 or less is a region where the printing rate is low. Here, the input voltage is reduced to reduce the emission energy, and a photoreceptor (not shown) or the like is exposed sharply with sharp light. The number of LEDs that emit light is 41-
With 60, the emission energy of the LED is increased so that the exposure between the emission beams is sufficient. When the number of LEDs that emit light is 61 or more, the printing rate is 10
It is regarded as 0% solid black print, and the emission energy of the LED is increased to enhance the expressiveness to solid black. As a result, it not only solves the problem of light emission current decrease with the increase of printing rate, but also achieves both sharp printing in the low printing rate area and full black expression in the solid black area. Let

Although the embodiment shows the LED head,
It can be similarly applied to other print heads such as a thermal head and an EL head. The structure itself of the LED head is arbitrary and can be freely changed within the range of known technology.

[0021]

According to the present invention, among the output fluctuations of the image forming apparatus due to the fluctuation of the printing rate, the output fluctuations due to the fluctuations of the input voltage to the constant current circuit and the output fluctuations due to the fluctuations of the reference current in the constant current circuit. It solves the fluctuation and reduces the fluctuation of the output due to the printing rate without improving the constant current circuit itself.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an image forming apparatus according to an embodiment.

FIG. 2 is a block diagram of a constant current circuit used in the embodiment.

FIG. 3 is a circuit diagram of a main part of an image forming apparatus according to a second embodiment.

FIG. 4 is a circuit diagram of an image forming apparatus according to a third embodiment.

FIG. 5 is a characteristic diagram of a conventional image forming apparatus.

[Explanation of symbols]

 2 control circuit 4 shift register 6 latch circuit 8 constant current circuit 10 data bus 12 block selection circuit 14 capacitor 16 mirror constant current circuit 18 reference resistance 22 switching transistor 24 inductance element 26 diode 28, 30 resistance 32 amplification circuit 34 reference power supply 36, 38 Reference power supply 40 Counter 42 Decoder A1 to A64 AND circuit B1 to B64 Constant current circuit L1 to L40 LED array T1 to T40 Switching transistor S1 to S3 switch

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location B41J 2/36 H01L 33/00 J 7514-4M

Claims (1)

[Claims] 1. An image forming apparatus in which a large number of image element arrays are arranged on a substrate and the image element arrays are driven by a constant current circuit, wherein the constant current circuit is provided between the power source and the constant current circuit. An image forming apparatus comprising a voltage stabilizing means for stabilizing an input voltage to a current circuit.