JPH04359482A - Led array head and its driver circuit - Google Patents

Abstract

PURPOSE:To compensate brightness effectively even at a side where a printing density is low. CONSTITUTION:A driver circuit 10 includes a brightness compensation circuit 12, a gradation circuit 14, and a driver transistor portion. The brightness compensation circuit 12 selects a size of the driver transistor according to a brightness compensation data for maintaining an amount of emission of each LED element 16 to be constant corresponding to scattering of the brightness of each LED element 16. The brightness compensation data is written once into the brightness compensation circuit 12 at the start of printing operation and is retained. The gradation circuit 14 determines ON time of each LED element 16 according to the gradation data which is given as printing data. The gradation data is sent to the gradation circuit 14 for each line.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is applicable to optical printers, scanners,
The present invention relates to an LED array head and its driver circuit used in copying machines and the like, and more particularly to an LED array head and its driver circuit equipped with a brightness correction means for correcting the brightness of each LED element.

0002

2. Description of the Related Art To explain an LED array head for an optical printer as an example, an A4 size one with a printing density of 300 dots/inch has approximately 2,500 LED elements.
The same number of driver circuits are required. Due to constraints in the manufacturing process, simply assembling the product will result in a brightness error of approximately ±10% within the head due to variations in the amount of light emitted between LED elements and variations in the characteristics of the driver circuit, resulting in a decrease in print quality. cause it to happen.

On the other hand, in optical printers, there is a demand for gradation for each printed dot. For example, 16 gradations/
In a dot optical printer, the gradation difference is 1/16 = approximately 6%, which is lower than the variation in brightness within a simply assembled head. Therefore, in an LED array head that performs gradation, brightness correction for each LED element is essential.

[0004] As an LED array head that performs brightness correction and gradation adjustment, there is one that controls the ON time of an LED element (see, for example, Japanese Patent Laid-Open No. 2-43634, US Pat. No. 4,455,562, etc.). As shown in the cited example, in conventional driver circuits that perform brightness correction and gradation adjustment, the amount of light emitted by each LED element is kept constant in response to variations in the brightness of each LED element, as shown in Figure 4. The mixer circuit 2 mixes the brightness correction data for the 3D printing and the gradation data given as the print data to create drive data for each LED element, and the driver circuit 4 uses the drive data to determine the on-time of each LED element 6. Control.

[0005]

[Problems to be Solved by the Invention] In the driver circuit of the type shown in FIG.
Brightness correction on the bright side of the light emitted by the ED element can be performed with sufficient accuracy. However, the accuracy of brightness correction on the dark side becomes extremely poor, and there is no correction at all near the darkest level except for 0. Suppose that the darkest level (1
If the brightness is to be corrected within ±3% (=approximately 1/32) at a brightness of /16), the driver circuit 4 would need to be able to perform 512 types of gradation, which would require a large circuit scale.

[0006] It is necessary to create new data by mixing gradation data and brightness correction data for each LED element, which is also inconvenient. Furthermore, in an optical printer, there is a so-called gamma characteristic between the output power representing the brightness of the LED element and the print density, as shown in FIG. 5, in which the rate of change in print density decreases as the output power increases. Therefore, since variations in brightness on the side where the LED output power is small (dark side) have a large effect, it is an absolute requirement that brightness correction on the dark side work effectively. As a result, print quality is poor in conventional driver circuits that do not adequately compensate for dark areas.

[0007] The present invention provides an LED driver circuit that can effectively correct brightness even on the lower gradation side, that is, on the lower printing density side in a printer, for example, and an LED array head equipped with such a driver circuit. The purpose is to

[0008]

[Means for Solving the Problems] The driver circuit of the present invention includes a driver transistor section that includes a plurality of driver transistors of different sizes per LED element, and a driver transistor section that includes a plurality of driver transistors of different sizes for each LED element, and a
The device also includes a brightness correction circuit that selects the driver transistor based on brightness correction data for making the amount of light emitted by the device constant.

In addition to the above, the driver circuit of the present invention also includes a gradation circuit that determines the ON time of each LED element based on gradation data given as print data. The LED array head of the present invention includes LED
An LED array in which elements are arranged in a straight line is provided with the driver circuit of the present invention as a driver circuit.

0010

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 schematically shows an embodiment including both a luminance correction circuit and a gradation circuit. Reference numeral 10 represents a driver circuit, and the driver circuit 10 includes a driver transistor section in addition to a luminance correction circuit 12 and a gradation circuit 14. The brightness correction circuit 12 selects the size of the driver transistor based on brightness correction data for making the amount of light emitted from each LED element 16 constant in response to variations in brightness of each LED element 16. The brightness correction data is written into the brightness correction circuit 12 only once at the beginning of a printing operation and is held. The gradation circuit 14 determines the ON time of each LED element 16 based on gradation data given as print data. The gradation data is sent to the gradation circuit 14 for each line.
sent to.

FIG. 2 specifically shows one embodiment, and shows a driver circuit for one LED element. In this example, brightness correction data (KDATAIN
), 2-bit data is used as gradation data, and 4-bit gradation data (D
ATAIN) is used. In the driver transistor section 18 that controls the current flowing through the LED element 16, three driver transistors 18a, 18b, and 18c of different sizes per one LED element are connected in parallel. These driver transistors 18a, 18b,
The size of the driver transistor 18c is set at a ratio of 17:2:1, and if the current value when all are turned on is 100, the driver transistor 18a is 85%, the driver transistor 18b is 10%, and the driver transistor 18
c can flow a current of 5%. By selecting the driver transistors 18a to 18c and combining those that turn on, the brightness of the LED element 16 can be corrected in 5% increments.

In order to select and turn on the driver transistors 18a to 18c, the brightness correction circuit 12 includes a shift register 20 that sends brightness correction data using a clock signal KCLK, and a shift register 20 that sends brightness correction data using a clock signal KCLK. A NAND gate 22, an inverter 24, and their NAND gates 22, inverters 24, and NAND gates 22, inverters 24, and their
A gate circuit 26 is provided that operates each of the driver transistors 18a to 18c using the outputs of the AND gate 22 and the inverter 24. driver transistor 1
8a is always turned on if the output of the gradation circuit 14 is at a high level, regardless of the brightness correction data. A current mirror circuit 28 is provided to flow a predetermined current value depending on the size of each driver transistor 18a to 18c.
is provided. In the brightness correction circuit 12, ±0%, -5% is determined by selecting driver transistors 18a to 18c.
, -10%, -15% brightness correction can be performed.

The gradation circuit 14 includes a shift register 30 that sends 4-bit gradation data per LED element using a clock signal DCLK, and a shift register 30 that sends gradation data of one line to the shift register 30. A latch 32 that holds 30 data, and enable signals EN1, EN2, EN with pulse widths corresponding to each bit of gradation data.
A multiplexer 34 is provided which selects 4 and EN8 in accordance with the tone data held in the latch 32.

Enable signals EN1, EN2, EN4,
As shown in FIG. 3, EN8 uses a pulse signal having a pulse width corresponding to 4-bit data. This gradation circuit 14 uses 4-bit gradation data to turn on the driver transistor at 0/15, 1/15,
The gradation is performed by controlling to any of the 16 gradations of 2/15, . . . 15/15 and changing the amount of LED light. By combining the brightness correction circuit 12 and the gradation circuit 14,
Even when the ON time is, for example, a gradation of 1/15, the amount of light after correcting variations in brightness of the LED elements is corrected in four steps in 5% increments according to the brightness correction data.

Next, the operation of this embodiment will be explained. The variations in brightness (including differences in luminous efficiency of LED elements and differences in driver circuits) for each LED element 16 are measured in advance, so that when the same gradation data is given to each LED element, the same amount of light is emitted. Brightness correction data KDA
TAIN has been created in advance. Brightness correction data is RO
It is stored in M etc. At the beginning of a printing operation, the brightness correction data is sent from a ROM or the like to the brightness correction circuit 12 and held in the shift register 20.

When the printing operation is started, gradation data DATAIN is sent as printing data, and is sent to the shift register 30 of the gradation circuit 14 for each printing line. The gradation data sent to the shift register 30 is held in the latch 32, and the enable signal EN1 is output according to the gradation data.
, EN2, EN4, and EN8 are selected by the multiplexer 34, and a pulse signal (including 0) corresponding to the gradation data is sent to the brightness correction circuit 12 as an output of the gradation circuit 14. In the luminance correction circuit 12, the driver transistor 18a is turned on while the output of the gradation circuit 14 is on, and driver transistors 18b and 18c are selected according to the output of the gradation circuit 14 and the logic of the luminance correction data, and the selected driver transistor LED by 18a~18c
The luminance of the element 16 is corrected and the element 16 lights up.

[0017]

[Effects of the Invention] Since the present invention performs brightness correction and gradation independently, it is necessary to combine the brightness correction data and gradation data and convert them into new data before inputting the data to the driver circuit. There is no. Therefore, it becomes possible to store luminance correction data within the driver circuit. Furthermore, since the luminance correction and the gradation appear as an effect in the form of a product, the luminance correction can be performed accurately even in the dark gradation, and the printing quality in the dark gradation can be improved.

[0018] Luminance correction and gradation can be weighted separately, which facilitates data compression. Since brightness correction and gradation are processed separately, for example, on the brightness correction side, 2-bit brightness correction data can be adjusted to 95% or 100%, depending on the range of brightness variations of LED elements.
, 105%, and 110% in 5% increments. When the range of variation is large, correction can be made in 10% increments by setting the driver transistor size to 7:2:1, for example.

Furthermore, on the gradation side, by manipulating the input clock width according to the gradation data, even 4-bit gradation data can be adjusted to correspond to the gamma characteristics of the print density, such as 0, 5, 10. ,15,20,25,30,35,40
, 45, 50, 60, 70, 80, 90, and 100%, it is also possible to perform gamma correction such that the gradation becomes rougher as the output power of the LED element becomes larger.

[Brief explanation of drawings]

FIG. 1 is a block diagram schematically illustrating the invention.

FIG. 2 is a circuit diagram showing a specific example per one LED element.

FIG. 3 is a waveform diagram showing an enable signal used in the same embodiment.

FIG. 4 is a block diagram schematically showing a conventional driver circuit.

FIG. 5 is a diagram showing print density characteristics in an optical printer.

[Explanation of symbols]

10 Driver circuit 12 Brightness correction circuit 14 Gradation circuit 18 Driver transistor section 16 LED element

Claims (3)

[Claims] 1. A driver transistor section including a plurality of driver transistors of different sizes per LED element, and a brightness for making the amount of light emitted by each LED element constant in response to variations in brightness of each LED element. A driver circuit for an LED array head, comprising: a brightness correction circuit that selects the driver transistor based on correction data. 2. A driver transistor section including a plurality of driver transistors of different sizes per LED element, and a brightness for making the amount of light emitted from each LED element constant in response to variations in brightness of each LED element. A driver circuit for an LED array head, comprising: a brightness correction circuit that selects the driver transistor based on correction data; and a gradation circuit that determines the on time of each LED element based on gradation data given as print data. [Claim 3] LE in which LED elements are arranged on a straight line
A D array, a driver transistor section that includes multiple driver transistors of different sizes per LED element, and brightness correction to make the amount of light emitted by each LED constant in response to variations in brightness of each LED element. An LED array head comprising: a brightness correction circuit that selects the driver transistor based on data; and a gradation circuit that determines the on time of each LED element based on gradation data given as print data.