Abstract:
A CMOS image sensor includes a photodiode, a switch configured to transfer a signal sensed by the photodiode to a sensing node, and a comparator electrically and directly connected to the sensing node and configured to compare the sensed signal of the sensing node and a ramp signal. Reset offset of the comparator is maintained at a constant offset voltage level during an initialization mode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2009-0078192, filed on Aug. 24, 2009, the subject matter of which is hereby incorporated by reference. 
       BACKGROUND 
       [0002]    The present disclosure relates to Complementary Metal Oxide Semiconductor (CMOS) image sensors and, more specifically, to CMOS image sensors incorporating a comparator configured to reduce reset offset between pixels. 
         [0003]    CMOS image sensors are widely used in digital cameras to convert optical signals into corresponding electrical signals. This conversion occurs in so-called “pixels” of the CMOS image sensor. Each image pixel is associated with a photodiode and read-out circuit, wherein the photodiode generates an electrical charge in relation to absorbed incident light. The charge generated by the photodiode is then converted into an analog voltage and transferred to the read-out circuit. The read-out circuit converts the analog voltage into a voltage waveform indicative of a digital value using an analog-to-digital (A/D) conversion process. 
         [0004]    In certain devices, the A/D conversion is performed by comparing the analog voltage of a pixel to a reference ramp voltage using a comparator circuit. A counted value is generated over a period of time that it takes for the increasing/decreasing ramp voltage to reach to the same level as the analog voltage. This time-wise counted value may be used as a digital data value equivalent (or representation) of the analog voltage. 
         [0005]    Generally, each pixel of a CMOS image sensor is implemented by a structure including four (4) N-type MOS transistors (NMOS). A first NMOS transistor is used to initialize the pixel. A second NMOS transistor is used to transfer image information (e.g., electrical charge) from the pixel. A third NMOS transistor is used to select the pixel, and a fourth NMOS transistor in a source follower configuration is used as a buffer for transferring the image information from the pixel. 
         [0006]    However, when a pixel transfers a sensing signal (e.g., a floating diffusion) to the read-out circuit through the NMOS source follower, the corresponding response signal is limited in its dynamic range and noise increases. 
       SUMMARY 
       [0007]    Embodiments of the inventive concept provide improved CMOS image sensors. In certain embodiments of the inventive concept, the CMOS image sensor may include a photodiode, a switch configured to transfer a signal sensed by the photodiode to a sensing node, and a comparator electrically and directly connected to the sensing node and configured to compare the sensed signal of the sensing node and a ramp signal. Reset offset of the comparator is maintained at a constant offset voltage level during an initialization mode. 
         [0008]    The present disclosure also provides a method for detecting an image signal of a CMOS image sensor. In some embodiments of the inventive concept, the method may include initializing a sensing node and setting reset offset corresponding to the initialized sensing node to a predetermined voltage level and converting the predetermined voltage level into a digital signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The inventive concept will become more apparent in view of the attached drawings and accompanying detailed description. The embodiments depicted therein are provided by way of example, not by way of limitation, wherein like reference numerals refer to the same or similar elements. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating aspects of the inventive concept. 
           [0010]      FIG. 1  is a block diagram of a CMOS image sensor according to an exemplary embodiment of the inventive concept. 
           [0011]      FIG. 2  is a circuit diagram of a pixel in a pixel array and a comparator shown in  FIG. 1 . 
           [0012]      FIG. 3  is a graphic diagram comparing a ramp signal shown in  FIG. 2  with a sensing signal. 
           [0013]      FIG. 4  is a timing diagram illustrating operations of the pixel and the comparator shown in  FIG. 2 . 
           [0014]      FIG. 5  is a block diagram of a digital camera system including a CMOS image sensor. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0015]    Certain embodiments of the inventive concept will now be described is some additional detail with reference to the accompanying drawings. However, the inventive concept may be embodied in many different forms and should not be construed as being limited only to the illustrated embodiments. Rather, these embodiments are provided as examples to convey the making and use of the inventive concept to one of ordinary skill in the art. Accordingly, known processes, elements, and techniques are not described with respect to some of the embodiments of the inventive concept. Throughout the drawings and written description, like reference numerals will be used to refer to like or similar elements unless otherwise specified. 
         [0016]    Figure ( FIG. 1  is a block diagram of a CMOS image sensor  100  according to an embodiment of the inventive concept. As illustrated, the CMOS image sensor  100  includes a timing controller  10 , a pixel array  20 , an analog-to-digital converter (ADC)  30 , and a buffer  40 . 
         [0017]    The timing controller  10  controls the pixel array  20  according to certain control signals, e.g., RX, TX, and SEL. The timing controller  10  controls the ADC  30  according to additional control signals, e.g., RON, CLK, and RST, and the timing controller  10  controls the buffer  40  according to, e.g., a control signal R_AD. Those skilled in the art will recognize that these well understood control signals are merely representative of a broad class of control signals that might be used to effect respective control and interoperability of the exemplary circuit blocks shown in  FIG. 1 . 
         [0018]    In operation, the pixel array  20  senses the optical signals associated with an external image and transfers a corresponding sensing signal (floating diffusion) to the ADC  30 . One example of a possible constitution of the individual pixels forming the pixel array  20  will be described in some additional detail hereafter with reference to  FIG. 2 . 
         [0019]    In the illustrated embodiment of  FIG. 1 , the ADC  30  comprises a comparator  31 , a counter  32 , and a ramp signal generator  33 . 
         [0020]    In view of this configuration, the pixel array  20  transfers the sensing signal (floating diffusion) V FD  to the comparator  31  in response to the control signals RX, TX, and SEL. The comparator  31  receives the sensing signal V FD  and a ramp signal V RAMP  from the ramp signal generator  33 . The ramp signal V RAMP  is characterized by a voltage level that rises and/or falls over a defined time period. In the illustrated embodiment of  FIG. 1 , the ramp signal V RAMP  is assumed to fall during a defined time period. 
         [0021]    The counter  32  begins counting from the point at which the comparator  31  compares the sensing signal V FD  with the ramp signal V RAMP  in response to control signals CLK and RST. That is, the comparator  31  compares the sensing signal V FD  with the ramp signal V RAMP  and then transfers a resulting comparison signal V LATCH  corresponding to a voltage difference between the sensing signal V FD  and the ramp signal V RAMP  to the timing controller  10 . The counter  32  stops counting in response to the control signal CLK provided from the timing controller  10 . A counted value stored in the counter  32  is digital data corresponding to the sensing signal V FD . The analog signal converted into the digital data is stored in the buffer  40 . The timing controller  10  transfers a control signal R_AD, and receives a data signal R_D. 
         [0022]    Possible circuit structures for the pixel array  20  and comparator  31  will now be described with reference to  FIG. 2 . 
         [0023]      FIG. 2  is a circuit diagram further illustrating one possible circuit structure for both the pixel array  20  and comparator  31  of  FIG. 1 . Thus, the pixel circuit shown in  FIG. 2  implements one pixel of the N by M pixel array  20 . 
         [0024]    Referring collectively to  FIGS. 1 and 2 , the exemplary circuit structure of pixel  20  of the N by M pixel array comprises four (4) NMOS transistors M RX , M TX , M SF1 , and M SEL1  and a photodiode PD. 
         [0025]    The transistor M RX  is coupled between a power supply voltage V DD  and a sensing node FD and controlled by a control signal RX. The transistor M RX  initializes the pixel  20  in response to the control signal RX. 
         [0026]    The transistor M TX  is coupled between the sensing node FD and the photodiode PD and controlled by a control signal TX. The transistor M TX  transfers a sensing signal V FD  to the sensing node FD in response to the control signal TX. 
         [0027]    The transistor M SF1  is coupled between the power supply voltage V DD  and the transistor M SEL  and controlled by the sensing node FD. The transistor M SF1  and a current source Ib constitute a source follower. The source follower functions as a buffer. That is, if a voltage of the sensing node FD is a power supply voltage V DD , a sensing signal V FD  transferred to a comparator  31  corresponds to a difference between the power supply voltage V DD  and a threshold voltage of the transistor M SF1 . Thus, a dynamic range of the sensing node FD input to a gate of the transistor M SF1  is limited. 
         [0028]    Because the sensing signal V FD  passing the sensing node FD is transferred to the comparator  31  through the transistors M SF1  and M SEL1 , it may contain noise induced by the transistors M SF1  and M SEL1 . 
         [0029]    The transistor M SEL1  is coupled between the transistor M SF1  and the current source and controlled by the control signal SEL. The transistor M SEL1  selects one of pixels in response of the control signal SEL. 
         [0030]    The timing controller  10  activates the control signal RX to initialize the sensing node FD. The pixel  20  outputs the initialized sensing signal V DD  to the comparator  31 . The comparator  31  compares the sensing signal V FD  with a ramp signal V RAMP . The comparator  31  will be described below in detail with reference to  FIG. 3 . 
         [0031]      FIG. 3  is a graph depicting a decreasing ramp signal and a related sensing signal according to the description given above in relation to  FIG. 2 . In  FIG. 3 , the x-axis represents time and the y-axis represents voltage level. 
         [0032]    Referring to  FIGS. 1 ,  2  and  3 , the ramp signal generator  33  generates a ramp signal V RAMP  in response to a control signal RON from the timing controller  10 . The ramp signal V RAMP  according to the illustrated embodiment of the inventive concept decreases at a regular rate over a defined period of time. 
         [0033]    When the comparator  31  compares the sensing signal V FD  with the ramp signal V RAMP , the timing controller  10  activates the control signal CLK to activate the counter  32 . 
         [0034]    Before a time point t latch , a voltage level of the sensing signal V FD  is lower than that of the ramp signal V RAMP . However, after the time point t latch  the voltage level of the sensing signal V RAMP  is lower than that of the ramp signal V FD . At this point, the comparator  31  activates a comparison signal V LATCH . The timing controller  10  does not generate the control signal CLK when the comparison signal V LATCH  is activated. Thus, the operation of the counter  32  is stopped. A counted value obtained when the counter  32  is stopped in the digital data equivalent to the level of the sensing signal V FD . 
         [0035]    Continuing with  FIGS. 1 and 2 , the timing controller  10  activates the control signal TX to transfer the sensing signal V FD  transferred from the photodiode PD to the sensing node FD. The pixel  20  transfers the sensing signal V DD  corresponding to external image information to the comparator  31 . 
         [0036]    The comparator  31  performs a digital double sampling (DDS) to accurately convert the analog signal provided by the pixel  20  into a corresponding digital signal. A difference between digital data Dsig and digital data Drst (Dsig-Drst) is obtained by performing the DDS. The digital data Drst is obtained by digitally converting an analog signal when a pixel is initialized to convert an accurate analog signal from the pixel into digital data. The digital data Dsig is obtained by digitally converting an analog signal corresponding to an external image signal from a pixel receiving the image signal. 
         [0037]    The comparator  31  of  FIG. 2  comprises a capacitor C off , transistors M p , M off , M SF2 , M SEL2 , and M C2 , and current sources I b  and I b2 . 
         [0038]    The transistor M p  is coupled between the power supply voltage V DD  and a source of the transistor M off  and controlled by a drain of the transistor M off . The transistor M off  is coupled between the capacitor C off  and a drain of the transistor M p  and controlled by a control signal RSTn. When the control signal RSTn is activated, the transistor M p  is diode-connected. That is, a drain and a source are connected to the transistor M p . 
         [0039]    The capacitor C oif  is connected to a drain of the transistor M off . That is, the capacitor C off  stores a regular voltage level (i.e., reset offset) formed by the diode-connected transistor M p . 
         [0040]    For instance, when a control signal RST is activated, the sensing node FD is initialized. In addition, the control signal RSTn is activated to remove deviation of reset offset between the pixels  20 . The capacitor C off  stores the sum of a threshold voltage Δ T  of the diode-connected transistor M p  and a saturation voltage Δ SAT  of the transistor M p . An output voltage V C  of the comparator  31  is also determined as V T +Δ SAT . That is, the voltage level of the initialized sensing node FD is always determined as V T +Δ SAT . Accordingly, the deviation of reset offset between pixels may be prevented. 
         [0041]    The transistor M SF2  is coupled between the drain of the transistor M p  and a drain of the transistor M SEL2  and controlled by the ramp signal V RAMP . The transistor M SF2  matches the transistor M SF1 . 
         [0042]    The transistor M SEL2  is coupled between the current source I b  and a source of the transistor M SF2  and controlled by the power supply voltage V DD . That is, the transistor M SEL2  is added to match the transistor M SEL1 . 
         [0043]    The transistor M C2  is coupled between the power supply voltage and the current source I b2  and controlled by an output voltage V C1  of the comparator  31 . The transistor M C2  buffers the output voltage V C1  of the comparator  31 . 
         [0044]    Generally, reset offset of the comparator  31  is set by a voltage level of an initialized sensing node FD. According to a digital double sampling (DDS) method, reset offset of a comparator may be removed but the size of a maximum input signal may decrease as much as the size of the reset offset. In addition, when the size of the reset offset increases, an output of the comparator may be biased toward a power supply voltage VDD or a ground voltage VSS. Accordingly, the reset offset of the comparator  31  is set to a predetermined low voltage level. 
         [0045]    A more detailed description of the operations of the pixel  20  and comparator  31  shown in  FIG. 2  will now be given with reference to the timing diagram of  FIG. 4 . 
         [0046]    Referring to  FIGS. 1 ,  2 ,  3  and  4 , the operation of the CMOS image sensor according to certain embodiments of the inventive concept may be divided into an initialization mode and an image input mode. During the initialization mode, a pixel is initialized (T 1 ) and a voltage corresponding to the initialized pixel is analog-to-digital converted (T 2 ). During the image input mode, external image information is transferred to the pixel (T 3 ) and a voltage corresponding to the external image information is analog-to-digital converted (T 4 ). 
         [0047]    During the time periods T 1  and T 2 , the comparator  31  compares a sensing signal V FD  having a predetermined voltage level with a ramp signal V RAMP  and transfers a comparison signal V LATCH  to the timing controller  10 . The timing controller  10  receives the comparison signal V LATCH  to generate control signals CLK and RST. The counter  32  converts the sensing signal V FD  having a predetermined voltage level into a digital signal Drst in response to the control signals CLK and RST. The conversion of an analog signal into digital data by initialization of the pixel array  20  is determined as the digital data Drst. 
         [0048]    During the time period T 3 , the photodiode PD senses the external image signal. When the control signal TX is activated, the sensed signal V FD  is transferred to the sensing node FD. 
         [0049]    During the time period T 4 , the comparator  31  compares the sensed signal V FD  with the ramp signal V RAM P and transfers eth comparison signal V LATCH  to the timing controller  10 . The timing controller  10  receives the comparison signal V LATCH  to generate control signals CLK and RST. The counter  32  converts a sensing signal V FD  having a predetermined voltage level into a digital signal Dsig in response to the control signals CLK and RST. The conversion of an analog signal into a digital signal by external image information is determined as digital data Dsig. That is, digital double sampling is a procedure for obtaining a value between digital data Drst and digital data Dsig. 
         [0050]      FIG. 5  is a block diagram of an exemplary digital camera system including a CMOS image sensor. 
         [0051]    Referring to  FIG. 5 , the digital camera system includes a CMOS image sensor  100 , a processor  200 , a memory  300 , a display  400 , and a bus  500 . As illustrated in  FIG. 1 , the CMOS image sensor  100  captures external image information in response to the control by the processor  200 . The CMOS image sensor  100  may include the same structure as shown in  FIGS. 1 and 2 . 
         [0052]    The processor  200  stores the captured image information in the memory  300  through the bus  500 . The processor  200  also outputs the image information to the display  400  from the memory  300 . 
         [0053]    As explained so far, according to the inventive concept, reset offset deviation between pixels is prevented. In addition, a sensing signal is directly input to a comparator to expand a dynamic range of the sensing signal and reduce noise. 
         [0054]    While the inventive concept has been described with reference to exemplary embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the inventive concept. Therefore, it should be understood that the above embodiments are not limiting, but illustrative. Thus, the scope of the inventive concept is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing description.