WO2018151499A1

WO2018151499A1 - Method for driving pixels and cmos image sensor using same

Info

Publication number: WO2018151499A1
Application number: PCT/KR2018/001868
Authority: WO
Inventors: 한상만; 하승재; 문보현
Original assignee: (주)픽셀플러스
Priority date: 2017-02-20
Filing date: 2018-02-13
Publication date: 2018-08-23
Also published as: KR20180096839A; KR101895982B1

Abstract

The present invention relates to a CMOS image sensor, and the present invention comprises: a pixel unit in which a plurality of unit pixels generating electric charge in accordance with the quantity of incident light are disposed in the form of a matrix; and a pixel driving unit which consecutively performs light exposure control on the pixel unit with a predetermined time lag by rows, and which divides a first light exposure period, which is part of the entire light exposure period for the unit pixels, into a plurality of divided light exposure periods. Accordingly, the present invention can mitigate image distortion by a flicker of a light source, such as an LED and a fluorescent light, by dividing part of the entire light exposure period into a plurality of divided light exposure periods by factoring in a flicker frequency of the light source.

Description

Pixel Driving Method and CMOS Image Sensor Using the Same

The present invention relates to an image sensor, and to a complementary metal-oxide-semiconductor (CMOS) image sensor for converting an external optical signal into an electrical image signal and a method of driving pixels included in the CMOS image sensor.

In general, an image sensor is a device that converts an external optical image signal into an electrical image signal.

In particular, a complementary metal-oxide-semiconductor (CMOS) image sensor is an image sensor manufactured using CMOS fabrication technology. In the CMOS image sensor, the pixel unit converts the light signal radiated from the corresponding part of the object into electrons using a photodiode, stores the converted light signal, and converts the amount of charge appearing in proportion to the accumulated number of electrons into a voltage signal.

CMOS image sensor generally includes a photoelectric conversion element that generates charges according to the amount of incident light and accumulates therein, a transfer transistor that transfers charges accumulated in the photoelectric conversion element, a floating diffusion region (FD) that converts the transferred charges into a voltage, A plurality of pixels including a reset transistor for resetting charge transferred to the floating diffusion region FD, an amplifying transistor for reading a voltage-converted signal in the floating diffusion region, and a selection transistor for selecting a line for reading a signal among XY pixel addresses Consists of

Typical CMOS image sensors require wide dynamic range performance for automotive or surveillance imaging devices. In addition, digital cameras and digital video cameras are also required to have a wide dynamic range.

As a method of improving the performance of a general dynamic range, the method of combining the image of a short exposure time and the image of a long exposure time, and producing a wide image of a dynamic range is known.

However, in order to increase the dynamic range, when a short exposure time is taken from a light source such as an LED or a fluorescent lamp flashing in synchronization with a commercial power source, a light source that appears to be lit in the human eye is not lit in a still image. There is a problem that the image appears, or the distortion of the image appears in the form of a severe flicker in the video.

An object of the present invention is to use a pixel having a separate charge holding means in addition to the floating diffusion region, and by changing the driving timing for exposure control of the pixel, while maintaining the performance of a wide dynamic range by the flicker of light sources such as LEDs and fluorescent lamps. The present invention provides a pixel driving method capable of alleviating image distortion and a CMOS image sensor using the same.

According to an aspect of the present invention, a CMOS image sensor includes: a pixel unit in which a plurality of unit pixels generating charges corresponding to an incident light amount are arranged in a matrix form; And a pixel driver configured to sequentially perform exposure control with a predetermined time difference on a row-by-row basis, and to divide the first exposure period, which is a part of the entire exposure period for the unit pixel, into a plurality of divided exposure periods. .

The repetition period and the number of repetitions of the divided exposure period may be set by the flicker frequency of the light source. In addition, the repetition period of the divided exposure period may be set to a time shorter than "1 / n" times less than the flicker period of the light source, where "n" may be a value of 2 or more.

The unit pixel may include a photoelectric conversion element that generates charges and accumulates therein corresponding to an incident light amount, a charge holding unit holding charges transferred from the photoelectric conversion elements, and a floating storing electric charges transferred from the charge holding units. And a diffusion region, wherein the pixel driver sequentially transfers the charges accumulated in the photoelectric conversion element to the charge holding part during the divided exposure periods after the initialization of the unit pixel. A second transfer for transferring the charge stored in the charge holding unit as first signal charge to the floating diffusion region after a transfer operation and a predetermined period of time after completion of the first transfer operation. An operation and all the accumulation in the photoelectric conversion element during the second exposure period after the completion of the second exposure period after the completion of the first transfer operation. A third transfer operation for transferring charge to the charge holding portion as a second signal charge, and a fourth transfer for transferring the second signal charges transferred by the third transfer operation to the floating diffusion region. Characterized in performing the operation.

The first transfer operation may include an operation of transferring the charge accumulated in the photoelectric conversion element to the charge holding unit and resetting the charge accumulated in the photoelectric conversion element for a period after a predetermined period elapses.

The ratio of the first exposure period and the second exposure period may be determined by a reduction ratio of the light sensitivity of the present CMOS image sensor itself.

The unit pixel further includes a first transfer switch for transferring the charge accumulated in the photoelectric conversion element to the charge holding unit, and an emission transistor for discharging the charge accumulated in the photoelectric conversion element to the outside, and the division exposure The period may be set by a period from the turn-off time of the discharge transistor to the turn-off time of the first transmission transistor.

Another aspect of the present invention for achieving the above object is a photoelectric conversion element for generating a charge to accumulate therein corresponding to the amount of incident light, a charge holding unit for holding a charge transferred from the photoelectric conversion element, and the charge A pixel driving method in which a plurality of unit pixels including a floating diffusion region storing charges transferred from a holding unit is arranged in a matrix form and sequentially performs exposure control with a predetermined time difference for each row.

The pixel driving method may further include: dividing a first exposure period, which is a part of an entire exposure period of the unit pixel, into a plurality of divided exposure periods; A first transfer step of sequentially transferring charges accumulated in the photoelectric conversion element to the charge holding unit for the divided exposure periods during the respective divided exposure periods after initialization of the unit pixel; A second transfer step of transferring, by the first transfer operation, a charge stored in the charge holding unit as first signal charge to the floating diffusion region after a predetermined period of time after completion of the first transfer step; A third transfer step of transferring the charge accumulated in the photoelectric conversion element during the second exposure period to the charge holding part as a second signal charge after the completion of the second exposure period after completion of the first transfer step; And a fourth transfer step of transferring the second signal charges transferred by the third transfer step and stored in the charge holding unit to the floating diffusion region.

As described above, the present invention can alleviate the distortion of the image caused by the flicker of the light source by dividing a part of the entire exposure period into a plurality of divided exposure periods in consideration of the flicker frequencies of the light sources such as the LED and the fluorescent lamp.

In addition, the present invention sets the ratio between the first exposure period for alleviating distortion of the image by the flicker of the light source and the second exposure period for sensing light without considering the flicker of the light source in consideration of the reduction rate of the light sensitivity of the image sensor. As a result, the distortion of the image due to the flicker of the light source can be alleviated and the loss of light sensitivity can be minimized.

1 is a block diagram of a CMOS image sensor according to an embodiment of the present invention.

2 is a block diagram of a pixel driver according to an exemplary embodiment of the present invention.

3 is a circuit diagram illustrating a configuration of a unit pixel according to an exemplary embodiment of the present invention.

4 is a timing diagram illustrating a pixel driving method according to an embodiment of the present invention.

5 to 9 are potential diagrams for explaining the potential corresponding to the timing diagram of FIG. 4.

Hereinafter, a CMOS image sensor according to an exemplary embodiment of the present invention will be described with reference to FIGS. 1 to 9.

As shown in FIG. 1, the CMOS image sensor 1 according to the present exemplary embodiment includes a pixel unit 100, a pixel driver 200, and a signal processor 300 formed on a semiconductor substrate (not shown).

In the pixel unit 100, a plurality of unit pixels having photoelectric conversion elements that generate charges and accumulate therein corresponding to the amount of incident light are arranged in a matrix in two dimensions.

Hereinafter, the unit pixel 110 will be described with reference to FIG. 2. As shown in FIG. 2, the unit pixel 110 includes the photoelectric conversion element 111, the first transfer transistor 112, the charge holding unit 113, the second transfer transistor 114, and the floating diffusion region FD. Diffusion) 115, a reset transistor 116, an amplifying transistor 117, a selection transistor 118, and an exhaust transistor 119.

The photoelectric conversion element 111 may be generally provided as a photodiode PD. The photodiode PD generates a charge corresponding to the incident light amount and accumulates therein. For example, the photodiode PD may be formed by forming a P-type layer on the substrate surface side and embedding the N-type buried layer with respect to the P-type well layer formed on the N-type substrate.

The first transfer transistor 112 is connected between the photoelectric conversion element 111 and the charge holding unit 113 and receives the first transfer control signal TX1 through the gate terminal. When the first transfer transistor 112 is turned on by the high potential of the first transfer control signal TX1, the charge accumulated in the photoelectric conversion element 111 is transferred to the charge holding unit 113. The high potential level of the first transmission control signal TX1 applied to turn on the first transmission transistor 112 may be set to the power supply voltage VDD. As an example, the first transfer transistor 112 may be implemented as an NMOS transistor.

The charge holding unit 113 is disposed between the photoelectric conversion element 111 and the floating diffusion region 115 and stores charges transferred from the photoelectric conversion element 111 when the first transfer transistor 112 is turned on. The charge stored in the charge holding unit 113 is transferred to the floating diffusion region 115 by the turn-on of the second transfer transistor 114.

The second transfer transistor 114 is connected between the charge holding unit 113 and the floating diffusion region 115, and the second transfer control signal TX2 is applied through the gate terminal. When the second transfer transistor 114 is turned on by the high potential of the second transfer control signal TX2, the charge accumulated in the charge holding unit 113 is transferred to the floating diffusion region 115. The high potential level of the second transmission control signal TX2 supplied for turning on the second transmission transistor 114 may be set to the power supply voltage VDD. As an example, the second transfer transistor 114 may be implemented as an NMOS transistor.

The floating diffusion region 115 stores the charges transferred from the charge holding part 113 when the second transfer transistor 114 is turned on. The floating diffusion region 115 serves as a charge amount sensing node for sensing the signal charge amount of a unit pixel.

The reset transistor 116 is connected between the VDD of the power supply voltage terminal and the floating diffusion region 115, and the reset control signal RX is applied through the gate terminal. When the reset transistor 116 is turned on by the high potential of the reset control signal RX, the charge stored in the floating diffusion region 115 is discharged through the VDD of the power supply voltage terminal. In this regard, the reset transistor 116 may reset the charge stored in the floating diffusion region 115.

The amplifying transistor 117 is connected between the power supply voltage terminal VDD and the selection transistor 118 and a gate terminal is connected to the floating diffusion region 115. As a result, the amplifying transistor 117 serves as a source follower for amplifying a current corresponding to the charge stored in the floating diffusion region 115. The amplifying transistor 117 converts the charge stored in the floating diffusion region 115 into an electrical signal and outputs the electric charge to the outside of the unit pixel 110 through the selection transistor 118.

The selection transistor 118 is connected between the amplifying transistor 117 and the vertical signal line, and the selection control signal LS is applied through the gate terminal. When the selection transistor 118 is turned on by the selection control signal LS, the signal amplified by the amplifying transistor 117 is transmitted to the outside of the unit pixel 110 through the vertical signal line.

The discharge transistor 119 is disposed adjacent to the photoelectric conversion element 111 and serves to discharge the accumulated charge of the photoelectric conversion element 111. The discharge transistor 119 discharges the charge of the photoelectric conversion element 111 when the high potential of the discharge control signal TXD is applied to the gate electrode at the start of exposure. The discharge transistor 119 can prevent the photoelectric conversion element 111 from saturating during the readout period after the end of the exposure so that the charge overflows.

In the present embodiment, the pixel structure including the emission transistor 119 is adopted, but as another embodiment, the first transmission control signal TX1 and the second transmission control signal TX2 are not introduced into the configuration of the emission transistor 119. And the reset control signal RX are all applied at the high potential level to turn on the first transmission transistor 112, the second transmission transistor 114, and the reset transistor 116 to perform the same function as the emission transistor 119. Can be.

The pixel driver 200 performs various functions of controlling the operation of the pixel unit 100 and processing and transmitting an electrical signal output from the pixel unit 100 to the signal processor 300.

One of the main features of the present embodiment of the pixel driver 200 relates to exposure control of the pixel unit 100. The pixel driver 200 sequentially performs exposure control with a predetermined time difference for the pixel unit 100 row by row. Perform. A detailed operation relating to the exposure control of the pixel driver 200 will be described later.

Hereinafter, the pixel driver 200 will be described with reference to FIG. 3. As shown in FIG. 3, the pixel driver 200 includes a vertical driving module 210, a column processing module 230, a horizontal driving module 250, and a control module 260.

The vertical driving module 210 is configured by a shift register or an address decoder, and drives the unit pixel 110 of the pixel unit 100. The vertical driving module 210 sequentially drives the unit pixels 110 of the pixel unit 100 in order to read signals from the unit pixels 110.

In addition, the vertical driving module 210 performs an electronic shutter operation of resetting unnecessary charges from the photoelectric conversion element 111 of the unit pixel 110 of the row to be read. The vertical driving module 210 supplies a signal output from each unit pixel 110 to the column processing module 230.

The column processing module 230 performs a correlated double sampling (CDS) process, an analog-to-digital conversion, and a digitally converted signal on a signal input from each unit pixel 110 of the selected row. do. The column processing module 230 outputs the converted digital signal to the signal processor 300.

The column processing module 230 may classify and read an electrical signal output to the outside of the unit pixel 110 by turning on the amplifying transistor 117 into a reset level or a signal level. For this read operation, the selection transistor 118 should be turned on by the selection control signal LS.

The horizontal driving module 250 is configured by a shift register, an address decoder, or the like, and sequentially selects the unit pixels 110 corresponding to the pixel columns of the column processing module 230. By the selection and scanning by the horizontal driving module 250, the pixel signals processed by the column processing module 230 are sequentially output.

The control module 260 is configured by a timing generator for generating various timing signals, and includes a vertical drive module 210, a column processing module 230, and a horizontal drive module based on various timing signals generated by the timing generator. 250) and the like. The control module 260 performs exposure control on the pixel unit 100 through the vertical driving module 210.

Hereinafter, a driving method of a pixel according to an exemplary embodiment of the present invention will be described with reference to FIGS. 4 through 12.

The pixel driver 200 sequentially performs the exposure control with the pixel unit 100 having a predetermined time difference for each row. In addition, the pixel driver 200 divides the entire exposure period of the unit pixel into a first exposure period T1 and a second exposure period T2. The first exposure period T1 is a period set to prevent image distortion by the LED flicker and includes the divided exposure periods Td which are divided into a plurality of parts in consideration of the flicker frequency of the light source such as the LED and the fluorescent lamp.

The second exposure period T2 is a period for sensing light without considering flicker of the light source, and the ratio of the first exposure period T1 and the second exposure period T2 is the CMOS image sensor according to the present embodiment. 1) can be set by the reduction rate of its own light sensitivity.

The reduction ratio of the light sensitivity may be expressed by a formula such as "second exposure period T2 / total exposure period". When the entire exposure period is set, the second exposure period T2 is set by a predetermined reduction ratio of light sensitivity.

For example, as the second exposure period T2 is smaller, the light sensitivity is decreased. When the first exposure period T1 is excessively increased to alleviate the LED flicker, the light sensitivity of the image sensor is reduced.

The CMOS image sensor 1 according to the present exemplary embodiment performs the operation of the flicker reduction mode during the first exposure period T1 and the operation of the normal mode during the second exposure period T2.

In FIG. 4, for convenience, a timing diagram is illustrated for only two of the plurality of rows.

As shown in the timing diagram of FIG. 4 and the potential diagram "(1)" of FIG. 5, the pixel driver 200 may be configured to have the reset control signal RX maintained at the high potential level and thus the reset transistor 116 is turned on. 2 The second transmission transistor 114 and the discharge transistor 119 are turned on by applying the transmission control signal TX2 and the emission control signal TXD to the high potential level. As a result, the electric charge remaining in the photoelectric conversion element 111, the charge holding part 113, and the floating diffusion region 115 may be reset, and thus the unit pixel 110 may be initialized before the start of row-by-row exposure.

As shown in "(2)" of the timing diagram of FIG. 4 and the potential diagram of FIG. 5, the pixel driver 200 performs an emission control signal TXD and a first transmission control signal TX1 after initialization of the unit pixel 110. FIG. The emission transistor 119, the first transmission transistor 112, and the second transmission transistor 114 are turned off by applying the second transmission control signal TX2 at a low potential level.

As shown in "(3)" of the timing diagram of FIG. 4 and the potential diagram of FIG. 5, the pixel driver 200 applies the first transmission control signal TX1 at a high potential level to apply the first transmission transistor 112. By turning on, the charge accumulated in the photoelectric conversion element 111 is transferred to the charge holding unit 113 from the turn-off time of the discharge transistor 119.

Accurately, charge accumulation by the photoelectric conversion element 111 continues even in the turn-on state of the first transfer transistor 112. Therefore, the charge transferred to the charge holding part 113 by the turn-on of the first transfer transistor 112 corresponds to the charge accumulated in the photoelectric conversion element 111 during the divided exposure period Td as shown in FIG. 4.

In other words, the divided exposure period Td is set by the period from the turn-off time of the discharge transistor 119 to the turn-off time of the first transmission transistor 112. In FIG. 4, the divided exposure periods Td are assumed to be two for convenience, but in practice, two or more divided exposure periods Td are included.

The repetition period and number of the plurality of divided exposure periods Td included in the first exposure period T1 are set in consideration of the flicker frequency of the light source in order to prevent distortion of the image by the flicker of the light source.

The repetition period of the divided exposure period Td may be determined to be shorter by "1 / n" times or less than the flicker period (1 / frequency) of the light source. "n" can be set to a value of two or more.

For example, the flicker period of the LED light source among the light sources is not usually set to "1/80" to "1/480" second, and may be shortened to "1/2000" second for a special LED. Therefore, the repetition period of the divided exposure period Td can be set up to several ms when the minimum is shortened.

Both the first exposure period T1 and the second exposure period T2 may be set longer than the maximum flicker period "1/80" second of the LED light source.

As shown in the timing diagram of FIG. 4 and the potential diagram of FIG. 5, the pixel driver 200 turns off the first transfer transistor 112. Charge is accumulated in the photoelectric conversion element 111 from the turn-off time of the first transfer transistor 112, and the accumulation of the charge may be performed for an exposure period longer than the first transfer split exposure period Td. For example, charges may accumulate in the photoelectric conversion element 111 for several to several tens of times of the divided exposure period Td from the turn-off of the first transfer transistor 112.

As shown in "(5)" of the timing diagram of FIG. 4 and the potential diagram of FIG. 6, the pixel driver 200 turns on the emission transistor 119 to the photoelectric conversion element 111 in the "(4)" state. Discharge the accumulated charge.

The timing diagram of FIG. 4 and the operations shown by "(6)" to "(9)" in FIGS. 6 and 7 are the same as the operations shown in "(2)" to "(5)" described above. This repetitive operation is repeatedly performed as many as the divided exposure period Td.

If the number of divided exposure periods Td is "N", the wide dynamic range ratio (WDR ratio) can be determined by the formula "T2 / (N x Td)".

The pixel driver 200 sequentially stores the charges accumulated in the photoelectric conversion element 111 during the plurality of divided exposure periods Td by the number of divided exposure periods Td, such as "(3)" and "(7)". The first transfer operation for transferring to the charge holding unit 113 is performed.

The first transfer operation may include an operation of resetting charges accumulated in the photoelectric conversion element 111 during the "(4)" and "(8)" periods, such as "(5)" and "(9)". . The timing diagram shown in FIG. 4 is based on the premise that such a reset operation is included in the first transmission operation.

As shown in "(10)" and "(11)" of the timing diagram of FIG. 4 and the potential diagram of FIG. 7, the pixel driver 200 turns off the discharge transistor 119. FIG. Charge is accumulated in the photoelectric conversion element 111 from the turn-off time of the discharge transistor 119. Charges stored in the charge holding unit 113 in the "(10)" and "(11)" states are first signal charges which are later transferred to the floating diffusion region for reading.

The charge stored in the charge holding unit 113 in the "(10)" and "(11)" states of FIG. 7 is shown to have a large amount of charge unlike the charge stored in the charge holding unit 113 in the "(9)" state described above. One is to indicate that the charge actually used as the first signal charge is the charge accumulated by two or more divided exposure periods (Td).

As shown in the timing diagram of FIG. 4 and the potential diagram of FIG. 7, the pixel driver 200 resets the floating diffusion region 115 while the reset control signal RX maintains the high potential level. The read mode is started by turning on the selection transistor 118 by applying the selection control signal LS to the high potential level.

As shown in "(13)" of the timing diagram of FIG. 4 and the potential diagram of FIG. 8, the pixel driver 200 applies the selection control signal LS at a high potential level and immediately resets the reset control signal RX. Switching to the potential level, the charge value of the floating diffusion region 115 reset at " (12) " is read at the reset level of the flicker reduction mode.

The reading of the reset level is performed when the reset level sampling control signal R_SH is at the high potential level. The amplifying transistor 117 converts the electric charge stored in the reset floating diffusion region 115 into an electrical signal. The electrical signal converted by the amplifying transistor 117 is read by the column processing module 230 of the pixel driver 200 via the selection transistor 118.

The pixel driver 200 maintains the reset control signal RX at a high potential level until just before the reset level is read, thereby overflowing while the charges are accumulated in the photoelectric conversion element 111, so that the floating diffusion region 115 is formed. The dark signal generated from the charge transmitted to the second transmission transistor 114 and the floating diffusion region 115 may be removed.

As shown in "(14)" of the timing diagram of FIG. 4 and the potential diagram of FIG. 8, the pixel driver 200 turns on the second transfer transistor 114 after reading the reset level by "(13)." A second transmission operation for transmitting the first signal charge stored in the unit 113 to the floating diffusion region 115 is performed. The turn-on of the second transmission transistor 114 is performed when the second transmission control signal TX2 is at the high potential level.

As shown in the timing diagram of FIG. 4 and the potential diagram of FIG. 8, the pixel driver 200 turns off the second transfer transistor 114 after the second transfer operation by “14”. The first signal charge stored in the floating diffusion region 115 is read at the signal level of the flicker reduction mode.

The reading of the signal level is performed when the signal level sampling control signal S_SH is at the high potential level. The amplifying transistor 117 converts the first signal charge stored in the floating diffusion region 115 into an electrical signal. The electrical signal converted by the amplifying transistor 117 is read by the column processing module 230 of the pixel driver 200 via the selection transistor 118.

That is, the column processing module 230 reads an electrical signal input from each unit pixel 110 at a reset level and a signal level to perform correlation double sampling processing and analog-digital conversion.

As shown in the timing diagram of FIG. 4 and the

potentials

16 and 17 of the 8 and 9 potential diagrams, the pixel driver 200 turns on the reset transistor 116 to open the floating diffusion region 115. After reset, the first transfer transistor 112 is turned on and the second signal charge accumulated in the photoelectric conversion element 111 after the elapse of the second exposure period T2 after the completion of the above-described first transfer operation is maintained. To perform a third transmission operation.

The second signal charge is the charge accumulated in the photoelectric conversion element 111 during the second exposure period T2 after the above-described first transfer operation is completed. The second signal charge is from the time when the discharge transistor 119 is turned off by " (9) " of FIG. 7, from the time when the first transmission transistor 112 is turned off by " (17) " 2 is charge accumulated in the photoelectric conversion element 111 during the exposure period T2.

The pixel driver 200 applies the reset level sampling control signal R_SH to the high potential level to change the charge value stored in the floating diffusion region 115 reset by "16" in FIG. 8 to the reset level in the normal mode. Read it.

As shown in "(18)" and "(19)" of the timing diagram of FIG. 4 and the potential diagram of FIG. 9, the pixel driver 200 turns off the first transfer transistor 112 and the second transfer transistor 114. ) Is turned on to transmit the second signal charge transmitted by " (17) " to the floating diffusion region 115.

As shown in "(20)" of the timing diagram of FIG. 4 and the potential diagram of FIG. 9, the pixel driver 200 turns off the second transfer transistor 114 and sets the signal level sampling control signal S_SH to a high potential level. The second signal charges stored in the floating diffusion region 115 are read at the signal level in the normal mode by applying to.

Meanwhile, the CMOS image sensor 1 according to the present exemplary embodiment may operate by selecting only one of the first exposure period T1 and the second exposure period T2. That is, the CMOS image sensor 1 according to the present embodiment may operate not only in two modes but also in only one of the flicker reduction mode and the normal mode.

As described above, the CMOS image sensor 1 according to the present exemplary embodiment divides a part of the entire exposure period into a plurality of divided exposure periods in consideration of the flicker frequency of the light source such as the LED and the fluorescent lamp to alleviate the distortion of the image due to the flicker of the light source. You can.

In addition, the CMOS image sensor 1 according to the present embodiment images the ratio of the first exposure period for alleviating the distortion of the image by the flicker of the light source and the second exposure period for sensing the light without considering the flicker of the light source. By setting in consideration of the reduction rate of the photosensitivity of the sensor, it is possible to mitigate the distortion of the image due to the flicker of the light source while minimizing the loss of photosensitivity.

It can be widely used in various kinds of CMOS image sensors.

Claims

A pixel unit in which a plurality of unit pixels generating charges in correspondence with incident light amount are arranged in a matrix form; And

And a pixel driver configured to sequentially perform exposure control with a predetermined time difference for each pixel unit, and divide the first exposure period, which is a part of the entire exposure period for the unit pixel, into a plurality of divided exposure periods. Image sensor.
The method of claim 1,

The repetition period and the number of repetitions of the divided exposure period are set by the flicker frequency of the light source.
The method of claim 2,

The repetition period of the divided exposure period is set to a time shorter than " 1 / n " times less than the flicker period of the light source, and " n "
The method of claim 1,

The unit pixel may include a photoelectric conversion element that generates charges and accumulates therein corresponding to an incident light amount, a charge holding unit holding charges transferred from the photoelectric conversion elements, and a floating storing electric charges transferred from the charge holding units. Having a diffusion region,

A first transfer operation of sequentially transferring charges accumulated in the photoelectric conversion element to the charge holding unit for the divided exposure periods during the respective divided exposure periods after initialization of the unit pixel; A second transfer operation transferred by the first transfer operation after a predetermined period of time after completion of the first transfer operation to transfer charges stored in the charge holding unit to the floating diffusion region as first signal charges; A third transfer operation for transferring charge accumulated in the photoelectric conversion element to the charge holding unit as a second signal charge after the second exposure period has elapsed after completion of the transfer operation; and by the third transfer operation. And a fourth transfer operation for transferring the second signal charge stored in the charge holding unit to the floating diffusion region. Image sensor.
The method of claim 4, wherein

The first transfer operation includes transferring the charge accumulated in the photoelectric conversion element to the charge holding unit and resetting the charge accumulated in the photoelectric conversion element for a period after a predetermined period elapses. Image sensor.
The method of claim 4, wherein

And the ratio of the first exposure period and the second exposure period is determined by a predetermined reduction rate of light sensitivity of the present CMOS image sensor itself.
The method of claim 4, wherein

The unit pixel further includes a first transfer switch for transferring the charge accumulated in the photoelectric conversion element to the charge holding unit, and an emission transistor for discharging the charge accumulated in the photoelectric conversion element to the outside,

And wherein the divided exposure period is set by a period from the turn-off time of the emission transistor to the turn-off time of the first transfer transistor.
A photoelectric conversion element that generates charge and accumulates therein in response to an incident light amount, a charge holding portion holding charges transferred from the photoelectric conversion elements, and a floating diffusion region storing charges transferred from the charge holding portions In the driving method of the pixel to sequentially perform the exposure control with a predetermined time difference for each pixel unit arranged a plurality of unit pixels in a matrix form,

Dividing a first exposure period that is a part of an entire exposure period for the unit pixel into a plurality of divided exposure periods;

A first transfer step of sequentially transferring charges accumulated in the photoelectric conversion element to the charge holding unit for the divided exposure periods during the respective divided exposure periods after initialization of the unit pixel;

A second transfer step of transferring, by the first transfer operation, a charge stored in the charge holding unit as first signal charge to the floating diffusion region after a predetermined period of time after completion of the first transfer step;

A third transfer step of transferring the charge accumulated in the photoelectric conversion element during the second exposure period to the charge holding part as a second signal charge after the completion of the second exposure period after completion of the first transfer step; And

And a fourth transfer step of transferring the second signal charges transferred by the third transfer step and stored in the charge holding unit to the floating diffusion region.