KR20150037018A - CMOS image sensor for preventing flicker noise - Google Patents

Abstract

Disclosed is a CMOS image sensor for preventing flicker noise. The CMOS image sensor includes: a first photo diode which has a first size and is operated for a first exposure time; a first transfer transistor which transfers charges stored in the first photo diode to a floating diffusion region; a second photo diode which has a second size which is relatively larger than the first size; and a second transfer transistor which transfers charges stored in the second photo diode to the floating diffusion region.

Description

Technical Field [0001] The present invention relates to a CMOS image sensor for preventing flicker noise,

The present invention relates to a CMOS image sensor for preventing flicker noise.

Flicker noise is a noise in the form of a stripe pattern caused by a kind of frequency mismatch that occurs when a subject is photographed using an optical imaging device such as a camera in a lighting state of a fluorescent lamp illuminated by a commercial AC power source. Refers to a noise in which a temporal change in brightness occurs in a video signal output due to a difference between a frequency and a vertical synchronization frequency of the camera.

Generally, a CMOS image sensor is composed of a pixel portion for taking an image and a signal processing portion for processing image information generated in the pixel portion.

The pixel portion of the image sensor is composed of several tens to several millions of fine unit pixels. When a pixel is exposed to light in order to take an image, not all the pixels are exposed at the same time. This is because of the feature of the CMOS image sensor that if all the pixels are exposed to light at once, the leakage current increases.

For this reason, in the CMOS image sensor, for example, 800 × 600 image sensor adopts a method of sequentially exposing pixels to light in a row where 800 unit pixels are arranged, that is, a line (rolling shutter method) .

Referring to the operation of the CMOS image sensor, one line is exposed to light and receives light through a photoelectric conversion element (for example, a photodiode) for a predetermined light-condensing time. Next, after a predetermined interval, the next line is exposed and image information is generated for a constant light-condensing time. The image information thus generated is stored in an external memory or the like, and then processed by the signal processing unit and used for image reproduction.

Flicker noise occurs when the line-by-line convergence time is not an integral multiple of the power supply period used for a fluorescent lamp.

In this connection, a brief description will be made with reference to Fig. 1 showing the energy distribution and flicker noise occurrence state for each line.

The energy distribution of the fluorescent light source periodically changes. Assuming that the power source of the light source is 60 Hz, the period T is 1/120 second (8.33 ms). 1, it is assumed that the exposure time (t, condensing time) per line of the image sensor is not an integral multiple of the period T of the power supply of the fluorescent lamp. In the drawing,? Indicates that one line is exposed and the next line is exposed , ≪ / RTI >

Thus, if the exposure time per line is not an integer multiple of the fluorescent lamp cycle, the energy of light entering each line will be different, although the time for each line to converge is the same as t. Such an energy difference affects the brightness of each line, and this difference in brightness causes a stripe bend in the image.

In general, the image sensor has a function called auto exposure (AE, Auto Exposure), which adjusts the exposure time or the gain value when processing image information, thereby adjusting the brightness according to the brightness around the subject This function allows you to take an image that you feel comfortable with.

However, even when an appropriate luminance value is obtained by adjusting the exposure time in the automatic exposure, there is a disadvantage that the above-described flicker noise occurs if the adjusted exposure time is not an integer multiple of the fluorescent lamp cycle.

The present invention provides a CMOS image sensor capable of preventing flicker noise from occurring even when an object is photographed using an optical imaging device in an illumination state by a light source using an AC power source such as a fluorescent lamp.

The present invention relates to a flicker noise preventing method which can maintain a bright image and a high frame rate by using a high sensitivity photodiode and a flicker removing photodiode together in a low illumination environment, And to provide a MOS image sensor.

Other objects of the present invention will become readily apparent from the following description.

[Refer to 10041125, Development of high-resolution image processing function and ECU integrated SoC for SXGA-grade automobile]. This application was developed through the "System Semiconductor Commercialization Technology Development Project", a national research and development project supported by the Ministry of Knowledge Economy.

According to an aspect of the present invention, there is provided a CMOS image sensor, comprising: a first photodiode formed at a first size and operated at a first exposure time; A first transfer transistor for transferring charges accumulated in the first photodiode to a floating diffusion region; A second photodiode having a second size relatively larger than the first size; And a second transfer transistor for transferring the charge accumulated in the second photodiode to the floating diffusion region.

The operation of the second transfer transistor can be stopped in a shooting environment in which flicker noise is generated.

The first exposure time may be set to an integral multiple of the period of the AC power supplied to the light source.

The CMOS image sensor may further include: n third photo-diodes (where n is an arbitrary natural number) formed in the second size; And n third transfer transistors provided to individually correspond to the respective third photodiodes so as to transfer charges accumulated in each of the n third photodiodes to the floating diffusion region, respectively.

Other aspects, features, and advantages will become apparent from the following drawings, claims, and detailed description of the invention.

According to the embodiment of the present invention, flicker noise can be prevented from occurring even when an object is photographed using an optical imaging apparatus in an illumination state by a light source using an AC power source such as a fluorescent lamp.

In addition, in a low-light environment, a high-sensitivity photodiode and a flicker removal photodiode are used together to maintain a bright image and a high frame rate, and exposure control and ISO sensitivity can be easily controlled.

1 is a diagram showing an energy distribution and a flicker noise occurrence state for each line.
2 is a diagram illustrating an equivalent circuit for a unit pixel of a CMOS image sensor according to an embodiment of the present invention.
3 is an operation timing diagram of a CMOS image sensor according to an embodiment of the present invention;
4 illustrates an equivalent circuit for a unit pixel of a CMOS image sensor according to another embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is capable of various modifications and various embodiments, and specific embodiments are illustrated in the drawings and described in detail in the detailed description. It is to be understood, however, that the invention is not to be limited to the specific embodiments, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

The terms first, second, etc. may be used to describe various components, but the components should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from another.

The terminology used in this application is used only to describe a specific embodiment and is not intended to limit the invention. The singular expressions include plural expressions unless the context clearly dictates otherwise. In the present application, the terms "comprises" or "having" and the like are used to specify that there is a feature, a number, a step, an operation, an element, a component or a combination thereof described in the specification, But do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or combinations thereof.

FIG. 2 is a diagram illustrating an equivalent circuit for a unit pixel of a CMOS image sensor according to an embodiment of the present invention. FIG. 3 is a timing chart of operation of a CMOS image sensor according to an embodiment of the present invention. Is an equivalent circuit diagram of a unit pixel of a CMOS image sensor according to another embodiment of the present invention.

2, a unit pixel of the CMOS image sensor is composed of two photodiodes (PDs) 210 and 230 and five NMOS transistors Tx, Rx, Sx, and Dx.

The two photodiodes 210 and 230 may include a first photodiode 210 for preventing flicker noise and a second photodiode 230 for high sensitivity, as described later.

The second photodiode 230 may be formed to have a photodiode region having a larger area than that of the first photodiode 210 in order to secure a bright image and a high frame rate even in a dark low illumination environment.

The second photodiode 230 may experience a phenomenon that the image is saturated even at the minimum ISO sensitivity when the exposure time set for preventing occurrence of flicker noise is applied for high sensitivity, May be set to have a shorter exposure time than the photodiode 210. Of course, it may be set to have the same exposure time as the first photodiode 210 or to have a rather large exposure time if necessary.

Accordingly, in order to prevent the flicker noise, which can not be prevented by the second photodiode 230, from being generated by driving the first photodiode 210, the exposure time of the first photodiode 210 is controlled by the alternating current Can be set to be an integral multiple of the period of the power supply. Also, the brightness of the output may be adjusted by adjusting the ISO sensitivity according to the surrounding brightness environment.

Each of the first photodiode 210 and the second photodiode 230 includes a first transfer transistor Tx for transferring light charges accumulated in each photodiode to a floating diffusion region FD 220 and the second transfer transistor 240, respectively.

As shown in the figure, a reset transistor (Rx) for setting the potential of the floating diffusion region FD to a desired value and discharging charge to reset the floating diffusion region, A source follower transistor Dx acting as a source follower buffer amplifier and a select transistor Sx allowing addressing as a switching function are further included .

The operation of the unit pixel of the CMOS image sensor will be briefly described below.

First, when the reset transistor Rx is turned on, the potential of the floating diffusion region FD becomes the applied voltage VDD. When light is incident on the photodiodes (PD) 210 and 230 from the outside, an electron-hole pair (EHP) is generated and the signal charge is accumulated in the source region of the transfer transistors Tx do.

When the transfer transistors Tx 220 and 240 are turned on, the stored signal charge is transferred to the floating diffusion region FD to change the potential of the floating diffusion region FD and at the same time the gate potential of the source follower transistor Dx changes do. At this time, when the select transistor Sx is turned on by the selection signal, the data data is output to the output terminal Out.

When the reset transistor Rx is turned on again, the potential of the floating diffusion region FD becomes the applied voltage VDD, and this process is repeated to output the video signal.

If the image is photographed in an environment in which flicker noise is generated (that is, when the image is photographed in the illuminated state of the light source to which the AC power is applied), the first transfer transistor 220 operates to generate a first photo And the charge accumulated in the diode 210 is transferred to the floating diffusion (FD) region so as to prevent flicker noise (see Fig. 3 (a)).

The exposure time applied to the first photodiode 210 may be set to have an exposure time at which flicker noise is not generated (for example, an exposure time that is an integer multiple of the AC power supply period supplied to the light source) The brightness of the output through the ISO sensitivity adjustment can be adjusted. When the first photodiode 210 is driven for the purpose of flicker noise removal, the operation of the second photodiode 230 can be stopped.

In addition, in a low-illuminance environment, the second photodiode 230, which is a high-sensitivity photodiode, can be operated independently or can be operated together with the first photodiode 210 for preventing flicker noise for securing a higher sensitivity, (See FIG. 3 (b)) or only with the first transfer transistor 220 (see FIG. 3 (c)).

In addition, the second photodiodes 230 may be connected in parallel in a unit pixel of the image sensor as shown in FIG.

For convenience of description, unit pixels including four types of transistors such as a transfer transistor, a reset transistor, a source follower transistor, and a select transistor are exemplified in the present specification, but the present invention is not limited thereto. For example, it is natural that the present invention can be applied without limitation to a unit pixel including three transistors or a unit pixel including five transistors.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention as defined in the following claims And changes may be made without departing from the spirit and scope of the invention.

210: first photodiode 220: first transfer transistor
230: second photodiode 240: second transfer transistor

Claims (4)

In the CMOS image sensor,
A first photodiode having a first size and operated at a first exposure time;
A first transfer transistor for transferring charges accumulated in the first photodiode to a floating diffusion region;
A second photodiode having a second size relatively larger than the first size; And
And a second transfer transistor for transferring the charge accumulated in the second photodiode to the floating diffusion region.
The method according to claim 1,
The operation of the second transfer transistor is stopped in a shooting environment in which flicker noise is generated.
The method according to claim 1,
Wherein the first exposure time is set to an integral multiple of a period of the AC power supplied to the light source.
The method according to claim 1,
N (n is an arbitrary natural number) third photodiodes formed in the second size; And
Further comprising n third transfer transistors each of which is provided to individually correspond to each third photodiode for transferring the charge accumulated in each of the n third photodiodes to the floating diffusion region, respectively.

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