JPH078215Y2 - Light emitting diode print head drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a drive circuit capable of correcting the gradation function of a light emitting diode (LED) print head in an electrophotographic printer.

(Prior Art) Japanese Patent Application No. Sho-6
There is one described in No. 2-146944. FIG. 2 is a block diagram showing an outline of this drive circuit. In FIG.
1 is a pixel memory for storing a pixel signal of an image to be formed by the LED print head, through terminal T ₁ , 2 is a terminal through a terminal T _4, and a gradation of the pixel signal according to a predetermined gradation number Gn It is a gradation memory for inputting and storing a gradation signal for designating.

An AND circuit 3 ANDs a pixel signal read from the pixel memory 1 and a gradation signal read from the gradation memory 2 to obtain a data signal having a pulse width corresponding to the number of gradations Gn. It is outputting. 4a to 4n (n is an integer of 1 or more)
Are drive circuits of the same configuration, and each has a terminal T ₂
From the first clock signal φ _{1 with} a cycle of 4 MHz, terminal T
_A latch signal is input from 3 and a data signal is input from the AND circuit 3, the data signal is latched based on the clock signal φ ₁ and the latch signal, and the drive signals 01 to 0m (m is an integer of 1 or more) via these. ) Is output as a circuit.

5a to 5n are all light emitting diode (LED) arrays having the same configuration, and drive signals 01 to from drive circuits 4a to 4n, respectively.
From 1 chip element in which 0 m is input and 64 dot light emitting diodes LED1 to LEDk (k is a positive integer of 1 or more. In this embodiment, k = 64 is shown) driven by them are arranged. Become.

FIG. 3 shows the details of the drive circuits 4a to 4n.
FIG. 3 is a circuit diagram showing an example.

In FIG. 3, reference numeral 41 is a shift register group, which shifts and stores the data signal from the AND circuit 3 by the clock signal φ ₁ from the terminal T ₂ and outputs it.

Reference numeral 42 is a latch circuit group for latching the data signal of the shift register group 41 by a latch signal.

A driver circuit group 43 is driven by the power supply voltage V _DD and outputs drive signals 01 to 0m in accordance with the data signals latched in the latch circuit group 42.

FIG. 4 shows a driving circuit for the LED print head shown in FIG.
It is a timing chart explaining operation about a pixel. In FIG. 4, 1 to m represent the number of gradations Gn, and the horizontal axis represents time.

The operation of such a configuration will be described. The pixel signal of the image to be printed out by the LED print head is at terminal T ₁
Is input to and stored in the pixel memory 1 via. on the other hand,
The gradation signal is input to the gradation memory 2 via the terminal T ₄ .

Next, the pixel signals are sequentially read from the pixel memory 1 in order to cause the LEDs of the LED arrays 5a to 5n to emit light, and at the same time, the grayscale signals of the grayscale number Gn that determine the grayscales of the pixel signals are output from the grayscale memory 2. Are sequentially read and combined in the AND circuit 3 with the pixel signal of the pixel memory 1 in the form of a logical product.

The output of the AND circuit 3 is used as a data signal for the drive circuits 4a to 4n.
Are sequentially input to the shift register group 41 of the LED array 5a.
Stored in a format corresponding to each of the 5 to 5n light emitting diodes LED1 to LEDk. Next, the latch circuit group 42 latches the data of the shift register group 41 according to the latch signal input from the terminal T ₃ . The data signal of the latch circuit group 42 is output from the driver circuit group 41 as drive signals 01 to 0m,
It is applied to each of the light emitting diodes LED1 to LEDk of the arrays 5a to 5n. Therefore, each of the light emitting diodes LED1 to LEDk emits light for an integral multiple of the reference emission time t fixed by the cycle of the gradation signal and the clock signal φ ₁ . For example, as shown in FIG. 4, the light emitting diode LED1 has a gradation number Gn = m.
× Reference emission time t, and the number of gradations of the light emitting diode LEDk is Gn
= 2 × reference light emission time t, current flows from the constant voltage V _DD, and the light emitting diode emits light.

As described above, each of the light emitting diodes LED1 to LEDk emits light for the number of gradations Gn given by the gradation signal × reference light emission time t with the fixed reference light emission time t as a unit. Here, since any light emitting diode LEDs (s = 1 to k) has a light emission output P, the maximum gradation number Gn _{max is set} to Gn _max = 15, and the exposure when the gradation number Gn is 1,3,15 The quantities E ₁ , E ₃ and E ₁₅ are such that when Gn = 1, E ₁ = P × (1 × t) = Pt Gn = 3, E ₃ = P × (3 × t) = 3Pt = 3E ₁ when _{Gn = 15, E 15 = P} × (15 × t) = 15Pt = 15E 1 = E
_max , and the exposure of any light emitting diode LEDs is 0 to 15
It can take values up to Pt.

A desired gradation printing can be obtained by using the LED print head driven and controlled in this way in an electrophotographic printer described in, for example, Japanese Patent Application No. 60-41203.

By the way, an electrophotographic printer using an LED print head, a photosensitive drum and the like has a γ characteristic having a relationship between the exposure amount E and the print density D as shown in FIG. The γ characteristic is determined based on the light emitting characteristics of the light emitting diode print head in the electrophotographic printer, the sensitivity of the photosensitive drum, the developing conditions, and the like. FIG. 5A shows the case of the ideal γ characteristic, in which the print density D linearly increases in response to the increase of the exposure amount E, and the maximum density D _max at the maximum exposure amount E _max (= E ₁₅ ). I take the. The actual γ characteristics are not ideal as shown in FIG. 5 (a) due to differences in the light emitting characteristics of the LED print head, the sensitivity of the photosensitive drum, the development conditions, etc. When the slope of the γ characteristic is larger than that of the characteristic, for example, as shown by the line in FIG.
May saturate above E ₁₀ . It is given ones exposure E ₁₀ above LED print head of an electrophotographic printer having a γ characteristic as the line of FIG. 5 (b), the exposure amount E ₁₀
With the above, only the same print density D _max can be obtained. Further, as described above, since the exposure charge E is proportional to the number of gradations Gn, the exposure amount E may be replaced with the number of gradations Gn, and the maximum number of gradations Gn _max 10 for obtaining the density of the maximum density D _max. Limited to.

Further, as shown in the line of FIG. 5 (b), contrary to the case of the line of FIG. 5 (b), when the inclination of the γ characteristic is smaller than that of the γ characteristic of FIG. 5 (a), for example, the exposure amount E _{When the} density D starts to be saturated from the exposure amount E of ₁₅ or more, the density D having a linear response is still obtained with the exposure amount E ₁₅ , but the density D does not reach the desired value of the density D _max .

(Problems to be solved by the invention) As described above, in the conventional LED print head drive circuit, even if the same gradation signal is used, the print density is the light emission characteristic of the LED print head, the sensitivity of the photosensitive drum, There is a problem that the γ characteristics differ depending on the development conditions and the like, so that the gradation setting does not match the γ characteristics,
There are problems such as saturation in the region that should be linear and concentration too small, and even if trying to cope with such a difference in γ characteristics, the reference emission time of the light emitting diode is fixed, so the γ characteristic There is a problem in that it is not possible to select the optimum exposure amount in accordance with the above.

The present invention eliminates the problems such as the appropriate number of gradations, the density, and the density shift in each LED in printing as described above, and provides a drive circuit for an LED print head that can obtain good printing. The purpose is to do.

(Means for Solving the Problems) A light emitting diode print head drive circuit according to the present invention is used in an electrophotographic printer having a light emitting diode print head, a photosensitive drum, and the like, and a reference light emission time signal. And, in the drive circuit of the light emitting diode print head that controls the light emitting time of the plurality of light emitting diodes based on the gradation signal of each pixel, set the numerical value selected corresponding to the γ characteristic of the electrophotographic printer. A reference light emission time setting circuit that divides the first clock signal according to the selected numerical value to generate a second clock signal, a reference light emission time signal based on the second clock signal, and a gradation signal of each pixel And a driver circuit group for driving the plurality of light emitting diodes to emit light.

(Operation) According to the drive circuit of the light emitting diode print head of the present invention, it is possible to select an appropriate reference emission time corresponding to the γ characteristic showing the relationship between the exposure amount and the density of the electrophotographic printer. By setting a preset input value of the reference light emission time setting circuit, a cycle of the clock signal for setting the reference light emission time output from the reference light emission time setting circuit is selected, and based on the γ characteristic, To secure the print density with a predetermined number of gradations.

(Embodiment) FIG. 1 is a circuit diagram showing a configuration of a drive circuit showing a first embodiment of the present invention. In FIG. 1, the same reference numerals as those in FIG. 2 indicate the same parts, and 6 is a reference light emission time setting circuit, which inputs a _first clock signal φ ₁ through a terminal T ₂ .
As described below, this clock signal φ ₁ is divided and a _second clock signal φ ₂ for setting the reference light emission time is output. Details of the reference light emission time setting circuit 6 are shown in FIG.

In Figure 6, 61 is a DIP switch, comprising a number of four switches to be set by binary 2 ^{0 ~} 2. Reference numeral 62 is a counter, which guides the numerical value set by the DIP switch 61 to a preset input, counts by this numerical value and the clock signal φ ₁ from the terminal T _2, and counts 0 to 15 pulses of the clock signal φ _1. At this time, it has a function of outputting the clock signal φ ₂ for setting the reference light emission time.

The operation of this embodiment will be described below with reference to FIGS. 1 and 6. Here, the gradation signal is input through the terminal T ₄ ,
For example, the operation of the drive circuit 4a until it is stored in the latch circuit group 42 (see FIG. 3) is the same as that of the above-described conventional technique, and therefore the description thereof is omitted. However, drive circuits 4a-4n
In the description, the clock signal φ _{1 in} FIG. 3 is read as the clock signal φ ₂ . On the other hand, the clock signal φ ₁ input from the terminal T _{2 is} input to the counter 62 of the reference light emission time setting circuit 6 and advances its counting. For example, to turn on only the switch "2 ^1" DIP switch 61, setting a numerical value "0010", the clock signal phi ₂ having a frequency half the frequency of the clock signal phi ₁ is output. For example, if the frequency of the clock signal φ ₁ is 4 MHz, the clock signal φ _{2 of} 2 MHz is output. The clock signal φ ₂ is input to the drive circuits 4a-4n. The latch circuit group 42 in the drive circuits 4a to 4n described above has a reference light emission time.
The drive signals 01 to 0m for causing the light emitting diodes LED1 to LEDk to emit light are output with T ₀ as a unit. The reference emission time T ₀ is determined by the clock signal phi _2, in the case of the clock signal phi ₂ is, for example 2MHz, a 500 ns.

The output from the latch circuit group 42 is each light emitting diode LED1 to LE.
Since it is input to the driver circuit group 41 corresponding to Dk,
The driver circuit group 41 receives the drive signal 01 as described above.
Outputs ~ 0m. And any light emitting diode LEDs
(S = 1 to k) emits light for the number of gradations Gn × reference emission time T ₀ .

FIG. 7 is a γ characteristic diagram when the present embodiment is applied to an electrophotographic printer having the same characteristics as those of FIG. 5 (b) already described, and the line of FIG. 7 (a) is the line of FIG. The case where the γ characteristic is the same as the γ characteristic of the line of (b), that is, the case where the inclination of the γ characteristic is larger than the ideal γ characteristic shown in the above-mentioned FIG. 5 (a), is shown. Shows the case where the γ characteristic is the same as the γ characteristic of the line in FIG. 5 (b), that is, the inclination of the γ characteristic is smaller than the ideal γ characteristic shown in FIG. 5 (a). In this embodiment, the dip switch of the reference light emission time setting circuit 6 is set in order to select the frequency of the clock signal φ _{2 according} to the γ characteristic of the electrophotographic printer. Such a setting is different from that of FIG. 5 (a) in the γ characteristic due to differences in the light emitting characteristics of the LED print head, the sensitivity of the photosensitive drum, the developing conditions, etc., for example, the line of FIG. 7 (a) or Even with the γ characteristic shown by the line in FIG. 7B, the reference light emission time T ₀ can be set arbitrarily, so that the maximum exposure amount E _max and the maximum print density D _max are associated with each other. Make things possible. For example, in the case where the γ characteristic of the electrophotographic printer is as shown in FIG. 5 (a), as shown by the line in FIG. 7 (a), or in the case as shown by the line in FIG. 7 (b). Even if the maximum print density D _max
If the print density is 1.5 and the number of gradations Gn is 15, a print signal of 0.5 is always obtained with a gradation signal of "5" (that is, the exposure amount E ₅ ).

(Effect of the Invention) As described in detail above, according to the present invention, the LED
Even if the γ characteristic changes in various ways due to differences in the print head emission characteristics, the sensitivity of the photosensitive drum, the development conditions, etc., the reference emission time can be selected by the reference emission time setting circuit in response to this change. Since it is configured as described above, it is possible to easily associate the maximum exposure amount with the maximum print density, always print with the predetermined number of gradations according to the exposure amount, that is, in correspondence with the specific gradation information, and between the gradation printings. The print density difference can be adjusted so that it is always constant, and good gradation printing can be obtained.

[Brief description of drawings]

1 is a block diagram showing an embodiment of a drive circuit for an LED print head according to the present invention, FIG. 2 is a block diagram showing an example of a drive circuit for a conventional LED print head, and FIG. 3 is FIG. 4 is a circuit diagram showing details of the drive circuit shown in FIG.
FIG. 5 is a timing chart for explaining the operation of the drive circuit shown in FIG. 2, FIG. 5 is a .gamma. Characteristic diagram of the electrophotographic printer used for explaining the prior art, and FIG. FIG. 7 and FIG. 7 are .gamma. Characteristic diagrams of the electrophotographic printer used for explaining the present invention. 4a ~ 4n ... drive circuit, 5a ~ 5n ... LED array, 6 ... standard light emission time setting circuit, 41 ... driver circuit group, 61 ... dip switch, 62 ... counter, LED1 to LEDk ... light emission diode.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yuji Terauchi 1-7-12 Toranomon, Minato-ku, Tokyo Oki Electric Industry Co., Ltd. (56) Reference JP-A-62-261274 (JP, A)

Claims (2)

[Scope of utility model registration request] 1. An electrophotographic printer having a light emitting diode print head, a photosensitive drum, etc., wherein a plurality of light emitting diodes emit light based on a reference light emitting time signal and a gradation signal of each pixel. In the drive circuit of the light emitting diode print head for controlling the control, the numerical value selected corresponding to the γ characteristic of the electrophotographic printer is set, the first clock signal is divided according to the selected numerical value, and Reference light emission time setting circuit for generating two clock signals, and a driver circuit for driving the plurality of light emitting diodes to emit light based on the reference light emission time signal by the second clock signal and the gradation signal of each pixel. Light emitting diode print characterized by having a group and
Head drive circuit. 2. The drive circuit for a light emitting diode print head according to claim 1, wherein the reference light emission time setting circuit has a plurality of switches for setting a numerical value to be set by a binary number.