KR20070070990A - Image sensor having automatic black level compensation function and method for compensating black level automatically - Google Patents

Abstract

An image sensor having an automatic black level compensation function and a method for automatically compensating for a black level are provided to prevent the characteristic of the image sensor from deterioration irrespective of the environment in which the image sensor is used and effectively remove a black level. An image sensor includes a pixel array(110) and an offset compensator(100). The pixel array includes a first pixel group(111) and a second pixel group(112). The offset compensator compares an offset average value of pixel data read from the first pixel groups for each column line with an initial first offset value to generate an updated second offset value and compensates for pixel data read from the second pixel group by using the second offset value.

Description

Image sensor with automatic black level compensation and automatic black level compensation {IMAGE SENSOR HAVING AUTOMATIC BLACK LEVEL COMPENSATION FUNCTION AND METHOD FOR COMPENSATING BLACK LEVEL AUTOMATICALLY}

1 is a diagram illustrating an offset generated in a general image sensor.

Figure 2 is a block diagram showing the configuration of an image sensor having an automatic black level function according to the prior art.

FIG. 3 is a diagram for explaining the structure of the pixel array unit shown in FIG. 2; FIG.

4 is a waveform diagram illustrating a problem occurring in the image sensor illustrated in FIG. 2.

5 is a block diagram showing a configuration of an image sensor having an automatic black level function according to an embodiment of the present invention.

<Explanation of symbols for the main parts of the drawings>

10, 110: pixel array unit

11, 111: first pixel group

12, 112: second pixel group

13, 113: 3rd pixel group

20, 120: low decoder

30, 130: column decoder

40, 140: analog line buffer

50, 150: PGA

60, 160: ADC

70, 170: line buffer

80, 180: comparison unit

90, 190: timing controller

100: offset compensation unit

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to semiconductor design techniques, and more particularly, to an image sensor and a black level compensation method capable of automatically compensating a black level, that is, a dark current.

Recently, the demand of digital cameras is exploding with the development of video communication using the Internet. Moreover, the demand for small camera modules increases as the popularity of mobile communication terminals such as PDAs equipped with cameras, International Mobile Telecommunications-2000 (IMT-2000), Code Division Multiple Access (CDMA) terminals, etc. increases. Doing.

The camera module basically includes an image sensor. In general, an image sensor refers to a device that converts an optical image into an electrical signal. As such an image sensor, a charge coupled device (hereinafter referred to as a CCD) and a CMOS (Complementary Metal-Oxide-Semiconductor) image sensor are widely used.

CCD has a complicated driving method, high power consumption, complicated process due to the large number of mask processes in the manufacturing process, and it is difficult to realize a signal processing circuit in a chip, making it difficult to make one chip. There are disadvantages. In contrast, CMOS image sensors are receiving more attention recently because of the monolithic integration of control, drive, and signal processing circuitry on a single chip. In addition, CMOS image sensors offer potentially lower cost than conventional CCDs due to low voltage operation and low power consumption, compatibility with peripherals, and the availability of standard CMOS fabrication processes.

Since an image sensor, especially a CMOS image sensor, is made of a semiconductor device, unnecessary current components are generated by a manufacturing process or an external environment. This current is called dark current. When dark current is generated, signal components other than optical factors are included in the image sensor to detect a certain signal level even in a very dark environment, that is, no light. This signal level is called the black level as thermal noise and system noise.

Even though the same light is actually incident on the image sensor, it is output through the analog digital converter which is the final output terminal of the analog signal due to the variation between each pixel and the difference between the offset and the gain amplification value between the analog signal processing paths. The output values have different code values rather than the same digital code value.

As shown in FIG. 1, when the initial value is set to '127 code' in the absence of any light at the initial value of the input, the average value of the total output should be '127 code', This makes them inconsistent with each other. This is because different data values are output to each pixel. If the average value is actually smaller than the input at the initial stage, the average value has a negative offset (-), and in the opposite case, it has a positive offset (+) greater than the initial value.

The proposed circuit to eliminate this phenomenon is the Automatic Black Level Compensation (ABLC) circuit.

FIG. 2 is a block diagram illustrating an image sensor having an ABLC circuit according to the related art, and FIG. 3 is a diagram illustrating a pixel array of the image sensor illustrated in FIG. 2.

2 and 3, the image sensor according to the prior art is arranged in N pixels horizontally, M vertically (N, M is a natural number) to maximize the response to light, to the image coming from the outside The first pixel group 11, that is, a core pixel array unit for sensing information about the pixel array, and one side and the other side in the column direction of the first pixel group 11 are arranged to offset the black level with respect to the constituent pixels. and a pixel array unit 10 including a second pixel group 12 for calculating an offset value and a third pixel group 13 in which light is completely blocked to function as a dummy pixel group. .

Specifically, it is as follows.

An analog line buffer unit which receives and buffers a signal from the second pixel group 12 of the pixel array unit 10 selected for each row line by a row decoder 20 ( 40 and a gain amplifier, for example, a programmable gain amplifier, for amplifying pixel data output from the analog line buffer unit 33 for each column line in response to the control of the column decoder 30. A programmable gain amplifier (hereinafter referred to as PGA) 50, an analog-to-digital converter (hereinafter referred to as an ADC) 60 for converting the output signal of the PGA 50 into a digital signal; A line buffer 70 that stores the pixel data (black level threshold) of the two pixel groups 12 and a frame-specific average value Frame_everage_offset, and an average and initial offset value for each frame stored in the line buffer 70. Comparison unit 80 for comparing (ADC_initial_offset) and the ratio In response to the updated offset value (ADC_update_offset) in accordance with the result of unit 80 it is provided with a timing controller 90 for controlling the PGA (50).

Referring to the automatic black compensation method using the image sensor according to the prior art having such a configuration as follows.

After reading out the pixel data from all the second pixel groups 12 configured in one frame, the average value of each frame is calculated and stored in the line buffer 70. The updated offset value ADC_update_offset is updated by comparing the average value of each frame (Frame_everage_offset) and the initial offset value (ADC_initial_offset) stored in the line buffer 70.

Thereafter, the timing controller 90 controls the gain value of the PGA 50 in response to the updated offset value ADC_update_offset. The PGA 50 amplifies and compensates pixel data read from the first pixel group 11 by using the gain value set by the timing controller 90.

However, in the ABLC method using an image sensor according to the related art, when the gain value is 1 times (A), the offset is not large so that the offset can be removed to some extent, but the actual image is taken. If the gain value is increased by about 6 times in low light, the offset is also increased by 6 times (B). This causes a problem in that the offset becomes wider when the average compensation for each frame is obtained. Further, in the image sensor according to the related art, since the gain value is designed to be adjusted by an integer multiple using the PGA, fine gain value adjustment through the timing controller is virtually impossible.

Accordingly, the present invention has been proposed to solve the above problems of the prior art, and an object thereof is to provide an image sensor capable of preventing deterioration of characteristics regardless of a use environment, that is, a normal environment and a low light environment. .

Another object of the present invention is to provide an automatic black level compensation method of an image sensor that can effectively remove the black level of the image sensor.

According to an aspect of the present invention, there is provided a pixel array unit including first and second pixel groups, an offset average value for each column line of pixel data read from the first pixel group, and And an offset compensator configured to compare an initial first offset value to generate an updated second offset value, and to compensate for pixel data read from the second pixel group using the second offset value.

In addition, according to an aspect of the present invention, there is provided an automatic image sensor including a pixel array including a first pixel group for imaging sensing and a second pixel group for black level compensation. A black level compensation method, comprising: calculating an offset average value for each column line of pixel data read from the second pixel group, and updating the updated second offset value by comparing the average value with an initial first offset value And compensating for the pixel data of the first pixel group using the second offset value.

DETAILED DESCRIPTION Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the technical idea of the present invention. In addition, parts denoted by the same reference numerals throughout the specification represent the same elements performing the same function.

Example

5 is a block diagram illustrating an image sensor having an ABLC circuit according to an embodiment of the present invention.

Referring to FIG. 5, an image sensor according to an exemplary embodiment of the present invention may include an offset average value of each column line of pixel data read from the pixel array unit 110 and the second pixel group 112 of the pixel array unit 110. An offset compensation unit 100 for generating an updated offset value ADC_update_offset by comparing the column_everage_offset) and the initial offset value ADC_initial_offset, and compensating pixel data read from the first pixel group 111 using the updated offset value. ).

The offset compensator 100 detects the second pixel group 112 of the pixel array 110, calculates an average value for each column line, and stores a calculated average value for each column line, and a line buffer 170. Comparator 180 comparing the offset average value (Column_everage_offset) and the initial offset value (ADC_initial_offset) for each column line stored in the line buffer 170 and the updated offset value (ADC_update_offset) in response to the comparison result of the comparator 180 In response, the timing controller 190 controls a code value of the ADC 160.

Specifically, it is as follows.

As illustrated in FIG. 3, the pixel array unit 110 arranges pixels horizontally and horizontally and M vertically (N and M are natural numbers) so as to maximize light response properties. The first pixel group 111 for sensing information and the second pixel group 112 arranged at one side and the other side in the column direction of the first pixel group 111 to calculate a black level offset value for the constituent pixel. ) And a third pixel group 113 that completely blocks light and functions as a dummy pixel group.

The analog line buffer 140 receives and buffers the pixel data of the first pixel group 111 and the second pixel group 112 of the pixel array unit 110 selected by the row decoder 120 for each row line and stores the buffered data. After that, the output signal is sequentially output to the PGA 150 in response to the control signal of the column decoder 130. In addition, the analog line buffer unit 140 performs a correlated double sampling operation on the pixel data read from the pixel array unit 110.

The PGA 150 amplifies and outputs the pixel data sequentially output from the analog line buffer unit 140 to a preset gain value. For example, under low light conditions, the gain value is set higher than that of the normal environment.

The ADC 160 converts the pixel data output from the PGA 150 into a digital signal and outputs the digital signal. At this time, the code value of the pixel data read from the first pixel group 111 is compensated for in response to the code value control signal output from the timing controller 190. For example, when a control signal for compensating '3code' is input from the timing controller 190, when the digital code value of the current pixel data is '117code', '3code' is added to the digital code value of the current pixel data.

The line buffer unit 170 stores the offset average value of each column line of the second pixel group 112 output from the ADC 160.

The comparison unit 180 compares the offset average value Column_everage_offset for each column line stored in the line buffer unit 170 with the initial offset value ADC_initial_offset, and updates and outputs the update offset value ADC_update_offset according to the comparison result. For example, when the initial offset value (ADC_initial_offset) is '120code' and the offset average value per column line is '117code', the update offset value (ADC_update_offset) becomes '+ 3code' and conversely, the initial offset value (ADC_initial_offset) is '120code' When the average value (Column_everage_offset) for each column line becomes '123code', the update offset value (ADC_update_offset) becomes '-3code'.

The timing controller 190 controls the ADC 160 in response to the update offset value ADC_update_offset output from the comparator 180. That is, when the update offset value ADC_update_offset is '+ 3code', the ADC 160 controls to add '+ 3code' to the digital code value of the current pixel data (pixel data of the first pixel group).

Referring to the automatic black compensation method using an image sensor according to an embodiment of the present invention having such a configuration as follows.

After reading out the pixel data from the second pixel group 12, an offset average value for each column line is calculated and stored in the line buffer 170. The updated offset value ADC_update_offset is updated by comparing the offset average value Column_everage_offset for each column line stored in the line buffer 170 with the initial offset value ADC_initial_offset.

Thereafter, the timing controller 190 controls the code value of the ADC 160 in response to the updated offset value ADC_update_offset. The ADC 160 compensates the code value of the pixel data read from the first pixel group 111 by using the code value set by the timing controller 190.

Meanwhile, the above-described initial offset value can be controlled externally and can be arbitrarily set.

Although the technical spirit of the present invention has been described in detail in the preferred embodiments, it should be noted that the above-described embodiments are for the purpose of description and not of limitation. In addition, it will be understood by those skilled in the art that various embodiments are possible within the scope of the technical idea of the present invention.

As described above, according to the present invention, an offset average value of pixel data of a dummy pixel for detecting a black level offset is calculated for each column line, and an offset value to be compensated using the calculated offset average value for each column line is determined. By compensating the actual pixel data with the determined offset value, the deterioration of the device characteristic can be prevented regardless of the use environment. As a result, the desired image can be obtained and the image quality can be improved.

In addition, according to the present invention, the black level can be effectively removed simply by compensating the code value through the ADC, rather than compensating the gain value through the PGA.

Claims (3)

A pixel array unit including first and second pixel groups; And An updated second offset value is generated by comparing an offset average value of each column line of the pixel data read from the first pixel group with an initial first offset value, and reading from the second pixel group by using the second offset value. Offset Compensator for Compensated Pixel Data Image sensor comprising a. The method of claim 1, The offset compensation unit, A line buffer to store the offset average value of each column line; A comparison unit comparing the average value with the first offset value and updating and outputting the second offset value; A timing controller configured to output a code value control signal in response to the second offset value; And An analog-digital converter for compensating pixel data read from the second pixel group in response to the code value control signal Image sensor comprising a. An automatic black level compensation method of an image sensor including a pixel array unit including a first pixel group for imaging sensing and a second pixel group for black level compensation, the method comprising: Calculating an offset average value for each column line of the pixel data read from the second pixel group; Comparing the average value with an initial first offset value to update an updated second offset value; And Compensating pixel data of the first pixel group by using the second offset value Automatic black level compensation method of the image sensor comprising a.

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