Pulse Width Modulated Pixel Sensor

Abstract

This application relates to pulse width modulation (PWM) pixel sensors. A pulse width modulation (PWM) image sensor is described herein. The PWM image sensor may have a stacked configuration. The top die of the PWM image sensor may have a charge-time converter, and the logic die stacked with the top die may include a time-to-digital converter. The PWM image sensor may utilize a variable transfer function to avoid high-light compression and may utilize non-linear time quantization. The threshold voltage, which serves as the input to the charge-time converter, may be further controlled to affect light detection, dynamic range, and other characteristics associated with the PWM image sensor.

Description

[0001] Cross-references to related applications

[0002] This application is a non-provisional application of U.S. Provisional Patent Application No. 63 / 227202, filed July 29, 2021, and claims the benefit of that U.S. Provisional Patent Application under 35 USC119(e), the contents of which are incorporated herein by reference as if fully disclosed herein. Technical Field

[0003] This invention relates generally to image sensors, and more specifically to pulse width modulation image sensors and methods of using them. Background Technology

[0004] Cameras and other image recording devices may use one or more image sensors, such as charge-coupled device (CCD) image sensors or complementary metal-oxide-semiconductor (CMOS) image sensors. A typical CMOS image sensor may include a two-dimensional pixel array, where each pixel may include a photodiode and one or more transistors for activating and reading each pixel.

[0005] In CMOS image sensors, any movement of an object imaged by the CMOS image sensor can blur the image of the object or otherwise cause different types of motion artifacts. CMOS image sensors may struggle to achieve high dynamic range (HDR) in a single shot because high-contrast areas of the image may be depicted as too dark or too bright. CMOS image sensors may also have a limited full-well capacity, which can limit the dynamic range of the output image and result in the inability to capture the full range of light intensities represented in the scene. Summary of the Invention

[0006] This invention provides a simplified description of a series of concepts, which are further described herein. This invention is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0007] According to some embodiments of the disclosed information, a method for operating a pulse width modulation (PWM) image sensor may be provided. The method may include: during a detection period, receiving a number of photons at a PWM pixel of the PWM image sensor; converting the number of photons into a photocurrent as the number of photons is received; during the detection period, accumulating a number of electrons at a sensing node in response to the receiving of the photocurrent at the sensing node; during the detection period and synchronized with the detection period, incrementing a count according to a nonlinear relationship between a trigger time and the photocurrent, the count being synchronized with a time reference; and when the accumulated number of electrons reaches a threshold number of electrons, latching the count value in the memory of a time-to-digital converter (TDC) circuit.

[0008] In some implementations, the count increases at a decreasing rate as the trigger time increases. Furthermore, the time reference t can be... Let be defined, where b is the bit depth of the TDC circuit, n is the counter step number, and t min This is a counter delay. This counter delay can be further... Let N be the elementary charge, where q is the elementary charge and N is the elementary charge. TH For this threshold number of electrons, and I max To generate the maximum photocurrent.

[0009] A method according to the provided disclosure may further include transmitting image information via a bidirectional bus, the image information corresponding to the latched value of the count in the memory of the TDC circuit. The method may also include terminating the duration of the detection cycle in response to the photocurrent. In some embodiments, a higher photocurrent may result in a shorter detection cycle, while a lower photocurrent may result in a longer detection cycle.

[0010] In some embodiments, a method for performing automatic exposure control on a pulse width modulation (PWM) image sensor may be provided. The method may include: retrieving a first value associated with an initial exposure setting of the PWM image sensor from a lookup table of the PWM image sensor, the first value including at least a clock frequency, a detection time, and a threshold electron count for operating the PWM image sensor; dividing the clock frequency by a segmentation factor based at least in part on the threshold electron count and the detection time to obtain an updated clock frequency; using the updated clock frequency to determine whether a transition time is less than or equal to the detection time; and maintaining the initial exposure setting based on the determination that the transition time is less than or equal to the detection time.

[0011] A method according to the provided disclosure may further include: using the PWM image sensor to generate a signal corresponding to light reflected from a scene; using the signal generated by the PWM image sensor to calculate an average brightness value; using the average brightness value to identify a row in the lookup table that includes a second value, the second value including at least a second segmentation coefficient, a second detection time, and a counter delay; and using at least the second segmentation coefficient, the second detection time, and the counter delay to convert the initial exposure setting to an updated exposure setting, thereby changing the operation of the PWM image sensor.

[0012] A method according to the provided disclosure may further include, when the conversion time is greater than the detection time, calculating a second detection time and using the second detection time to change the initial exposure setting of the PWM image sensor to an updated exposure setting.

[0013] In some cases, the threshold number of electrons may be the maximum threshold number of electrons, and the method may further include: reducing the maximum threshold number of electrons when the conversion time is greater than the detection time; and performing a new exposure operation using the reduced maximum threshold number of electrons.

[0014] Calculate the second detection time T DET This second detection time can be achieved through Let F be defined, where b is the bit depth, K is the segmentation coefficient, and F clk This refers to the clock frequency. Determining whether the transition time is less than or equal to the detection time may include evaluation. Where b is the bit depth, K is the segmentation coefficient, and F clk Let T be the clock frequency, and T be... DET This is the detection time. The value of this segmentation coefficient can be based on the maximum detectable photocurrent that can be generated by the photodiode of the PWM image sensor.

[0015] In some embodiments, a pulse width modulation (PWM) image sensor may be provided. The PWM image sensor may include: a top wafer including a charge-time converter (CTC) circuit comprising a photodiode, a reset gate, and a comparator; and a bottom wafer stacked with the top wafer and including a time-to-digital converter (TDC) circuit. In some embodiments, a reset signal may be applied to the reset gate to initiate a detection cycle during which the photodiode may accumulate an electron count, and when the electron count reaches a threshold electron count, the CTC circuit may transmit a write signal from the comparator to the TDC circuit.

[0016] In some implementations, the TDC circuit may be a static random access memory including multiple latches, and the write signal may latch the counts in the multiple latches.

[0017] A threshold voltage can be applied to the comparator, and the number of threshold electrons can correspond to the threshold voltage applied to the comparator. The CTC circuit and the TDC circuit can include pixels, and the PWM image sensor can also include multiple pixels, each of which includes a corresponding CTC circuit and a corresponding TDC circuit pair. Each of the multiple pixels can be arranged in an array and can be read line by line. The top wafer and the bottom wafer can be communicatively coupled via at least one of a vertical transfer gate, a through-silicon via, or a bonding pad. Attached Figure Description

[0018] Reference will now be made to the representative embodiments illustrated in the accompanying drawings. It should be understood that the following description is not intended to limit the embodiments to one or more preferred embodiments. Rather, it is intended to cover alternative forms, modifications, and equivalents that may be included within the substance and scope of the embodiments defined by the appended claims. Where feasible, similar reference numerals have been used to denote similar features.

[0019] Figures 1A to 1C An exemplary pulse width modulation (PWM) image sensor, including a pixel array and its components, as discussed herein, is shown.

[0020] Figure 2 Exemplary pixels and associated electronic components of a PWM image sensor as discussed herein are depicted.

[0021] Figure 3 Timing diagrams of components of an exemplary PWM image sensor as discussed herein are depicted.

[0022] Figure 4 An exemplary relationship between photocurrent and the trigger time of PWM image sensor pixels, as discussed herein, is depicted.

[0023] Figure 5 An exemplary relationship between the output of a time-to-digital converter (TDC) and photocurrent based on a nonlinear sampling rate is depicted, as discussed herein.

[0024] Figures 6A to 6B An exemplary dynamic range extension as discussed herein is described, and the relationship between time and integrated charge is specifically depicted.

[0025] Figure 7 An exemplary TDC transfer function in the time domain, as discussed in this paper, is described, and the relationship between the TDC output code and illuminance is specifically depicted.

[0026] Figure 8 An exemplary relationship between photocurrent and signal-to-noise ratio with respect to the photon noise limit, as discussed herein, is depicted.

[0027] Figures 9A to 9B An example is depicted that controls the number of threshold electrons to extend the dynamic pixel range of a PWM image sensor, as discussed in this paper.

[0028] Figures 10A to 10B Exemplary shutter performance of a PWM image sensor as discussed herein is described.

[0029] Figure 11 An exemplary flowchart of TDC timing control as discussed herein is depicted.

[0030] Figure 12 An exemplary flowchart is depicted for exposure control using maximum photocurrent and threshold number of received electrons, as discussed herein.

[0031] Figure 13 An exemplary flowchart is depicted for exposure control utilizing maximum photocurrent and maximum number of received electrons, as discussed herein.

[0032] Figure 14 An exemplary flowchart depicts exposure control in ramp mode as discussed herein.

[0033] Figures 15A to 15B Exemplary methods and configurations for automatic exposure control, as discussed herein, are described.

[0034] Figure 16 Exemplary methods and configurations for a logarithmic TDC mode for single-shot high dynamic range (HDR) are described as discussed herein.

[0035] The use of crosshairs or shading in the accompanying drawings is generally provided to clarify the boundaries between adjacent elements and to improve the readability of the drawings. Therefore, the presence or absence of crosshairs or shading does not indicate or suggest any preference or requirement for a particular material, material properties, element proportions, element dimensions, commonalities of similar illustrated elements, or any other feature, property, or characteristic of any element shown in the accompanying drawings.

[0036] Additionally, it should be understood that the proportions and dimensions (relative or absolute) of the various features and elements (as well as their sets and groups), and the boundaries, spacing, and positional relationships therebetween, are provided in the accompanying drawings solely to facilitate understanding of the various embodiments described herein, and may therefore be unnecessarily presented or shown for scaling and are not intended to indicate any preference or requirement for the illustrated embodiments to exclude embodiments in conjunction with them. Detailed Implementation

[0037] The provided disclosure relates to pulse width modulation (PWM) image sensors, and more specifically to a PWM image sensor array on a stacked wafer, which aligns the charge-time converter (CTC) and time-to-digital converter (TDC) along the Z-direction to reduce the size of the image sensor pixels in the XY plane. A further aspect of the provided disclosure provides flexible control over the transfer function of the TDC of the PWM image sensor. Other aspects of the provided disclosure are also provided herein.

[0038] Many electronic devices, such as smartphones, tablets, and laptops, include one or more cameras for capturing image and / or video information. For example, a smartphone may have one or more cameras configured to capture high-resolution images and videos. The captured images and videos may be stored in the smartphone's local storage and / or transmitted over a network to other devices for purposes such as sharing photos or videos with other users on social media, and / or for any other purpose using the images or videos.

[0039] As cited above, image sensors can be used in various types of cameras and may include arrays of multiple pixels that convert analog information (e.g., electromagnetic radiation such as light waves) into digital signals for use in and / or display on electronic devices. Through this analog-to-digital conversion process, a digital image corresponding to a scene and / or a real-world object as the object of the image capture operation can be created. A digital image can be created by aggregating digital signals from each pixel associated with the image sensor. Therefore, increasing the number of pixels in a particular image sensor may result in a corresponding increase in the resolution of the resulting image.

[0040] However, electronic devices typically have limited space for internal components, as other typical components in an electronic device (such as microphones, displays, sensors, etc.) may require a portion of the limited space available within the device. Therefore, the pixel space in an image sensor is limited, and the resolution of the image sensor may also be limited by the amount of available space. Furthermore, design considerations (e.g., device thickness or aesthetic appearance) may further limit the amount of available space in a particular electronic device, which may further limit the space available for the pixel array of the image sensor, resulting in a reduction in the resolution of images captured by the associated image sensor and / or camera.

[0041] In addition, certain image sensor architectures (such as some CMOS sensor architectures) may produce images with certain quality defects. For example, conventional sensors may struggle to produce high-quality images in low-light or high-light conditions, when an object is moving relative to the image sensor, when producing images without pixel saturation, and when producing images with high dynamic range.

[0042] The disclosed content relates to a PWM image sensor that may have a reduced pixel size, thereby increasing the number of potential pixels in a specific area, allowing flexible control of the digital domain transfer function, providing single-shot high dynamic range (HDR) imaging, reducing motion blur, and providing additional functionality as described below. The PWM image sensor can utilize PWM control, for example, by controlling a switch that supplies voltage and current to a load during a time-based switching process. The average values ​​of the voltage and current can be modified by changing the switching rate.

[0043] According to the provided disclosure, a PWM image sensor may include a charge-time converter (CTC) and a time-to-digital converter (TDC). The CTC may be located on a top wafer and the TDC may be located on a logic wafer, wherein the logic wafer is positioned below the top wafer along a vertical orientation (e.g., the Z-direction). By utilizing space in the Z-direction, the PWM image sensor can achieve a higher pixel density in the XY plane. Compared to conventional image sensors, stacking the CTC and TDC at the wafer level can further reduce the overall die and / or module size.

[0044] The CTC can be communicatively coupled to the TDC, for example, via one or more traces, and can transmit write signals to the TDC. As discussed herein, the CTC generates a write (WRT) signal when the sensing node (also known as floating diffusion (FD)) accumulates a threshold number of electrons from the photocurrent generated by the CTC's photodiode. Once the threshold number of electrons is reached, the WRT signal is generated and sent to the TDC to latch or otherwise tag image data. In some cases, the threshold number of electrons can be modified manually or automatically to minimize the signal-to-noise ratio (SNR), extend the dynamic range (for all pixels of a PWM sensor or a portion thereof), control exposure settings, etc.

[0045] TDC also allows control of the transfer function in the digital domain during the time-to-digital conversion process. By controlling the transfer function, the dynamic range (DR) of the PWM image sensor can be extended under certain conditions (e.g., low-light or high-light conditions). Various additional functions, such as SNR, exposure settings (manual or automatic), and trigger time selection, can be controlled using a PWM image sensor.

[0046] The following text is for reference only. Figures 1A to 16 These and other implementation schemes are discussed. However, those skilled in the art will readily understand that the detailed descriptions given herein with respect to the accompanying drawings are for illustrative purposes only and should not be construed as limiting.

[0047] Figure 1A An exemplary pulse-width modulation (PWM) image sensor 100 is shown that captures light 112 reflected from an object 110. In Figure 1, the object 110 is depicted as a car, but it should be understood that any object or combination of objects can be the object of the image detection operation of the PWM image sensor 100. For example, the face of a user of an electronic device in which the PWM image sensor 100 is located can be an object detected by the PWM image sensor 100.

[0048] During image detection operations, light 112 is reflected from object 110 and / or the scene and received at the PWM image sensor 100, particularly at one or more photodiodes of the PWM image sensor 100. As described herein, light 112 can be natural light (e.g., generated by the sun), external artificial light (e.g., from an external light bulb), or an artificial light source associated with the PWM image sensor 100 (e.g., a light source from a flash emitting element). An exemplary light source 114 is depicted in Figure 1, and in some embodiments, the exemplary light source may be omitted. In some cases, light source 114 is a light-emitting diode and can be used as a flashlight and / or camera flash. Light source 114 may be operatively coupled to the PWM image sensor 100. In some specific embodiments, light 112 may be light invisible to the human eye, including, for example, infrared or ultraviolet light.

[0049] The PWM image sensor 100 may include multiple layers, such as a top wafer 102 and a bottom logic wafer 104. The top wafer 102 and the bottom logic wafer 104 may be formed of semiconductor materials (such as silicon (Si) or gallium arsenide (GaAs)) and may be bonded together in various ways, such as, but not limited to, direct bonding, plasma-activated bonding, eutectic bonding, hybrid bonding, and any combination thereof. In some cases, the top wafer 102 and the bottom logic wafer 104 are located on opposite sides of a single wafer.

[0050] A charge-time converter (CTC) array 106 may be positioned on the top wafer 102. For example... Figure 1AAs depicted, the CTC array 106 may include a plurality of CTC pixels arranged in a regular and repeating manner, but any arrangement of the CTC pixels may also be used according to the provided disclosure. The CTC array 106 may be formed on the top wafer 102 using any potential techniques or combinations thereof, such as, for example, epitaxial growth, material deposition, etching processes, p-type or n-type doping, soldering, etc. Each CTC pixel of the CTC array 106 may include a photodiode, a reset gate, and a comparator. The photodiode may be any photodiode and may convert electromagnetic energy (e.g., light waves) into current (e.g., photocurrent). The intensity of the photocurrent may depend on the intensity of the incoming electromagnetic energy, such that brighter light results in a stronger photocurrent. The reset gate may include a switch configured to reset the CTC pixel to begin a detection cycle (e.g., by coupling / decoupling the CTC pixel from a voltage source). The comparator may be used to determine when the floating diffusion (FD) voltage reaches a predetermined threshold voltage, as referenced. Figure 3 The discussion focuses on the fact that each CTC pixel may additionally include one or more capacitors, which may be referred to as sensing node / FD capacitors (C). FD In some implementations, CTC is a MOS-PN (PN-type metal-oxide-semiconductor) hybrid device that directly converts light into time at low voltage.

[0051] A time-to-digital converter (TDC) array 108 may be disposed on a bottom logic wafer 104, which may be positioned below or otherwise stacked with the top wafer 102. The top wafer 102 and the bottom logic wafer 104 may be separate wafers, or in some cases, opposite sides of the same wafer. The TDC array 108 may be coupled to the CTC array 106 via one or more communication paths 116, such as, but not limited to, vertical transmission gates, through-silicon vias (TSVs), bonding pads, etc.

[0052] TDC array 108 may include multiple individual TDC pixels. Each TDC pixel may include static random access memory (SRAM) that uses latching circuitry to store data bits. The number of latches in each SRAM can be arbitrary, depending on the desired resolution of each TDC pixel, and each latch may correspond to one data bit. In a non-limiting example, the SRAM of each individual TDC pixel includes five latches, which correspond to five data bits. An external counter may be additionally communicatively coupled to TDC array 108 (e.g., coupled to each TDC pixel), and a bidirectional data bus may transfer data (e.g., image data) to or from the external counter. The external counter may define a sampling rate for sampling photocurrent (e.g., photocurrent generated by a photodiode). Although SRAM has been discussed, any type of memory, such as dynamic random access memory (DRAM), may be used as a TDC pixel of TDC array 108.

[0053] Each TDC pixel can be coupled to a corresponding CTC pixel, such that the number of TDC pixels and CTC pixels are equal. Furthermore, each TDC pixel and CTC pixel pair can form a pixel of the PWM image sensor 100 and can correspond to a pixel on the image generated by the PWM image sensor 100. In this way, the number of TDC / CTC pixel pairs can correspond to the maximum image resolution that the PWM image sensor 100 can generate. In a non-limiting example, if the potential maximum resolution of the image generated by the PWM image sensor 100 is 1792×828, then 1,483,776 TDC / CTC pixel pairs can be provided for the PWM image sensor 100. Although a specific example is provided, the number of TDC / CTC pixel pairs is not limited to any specific value and any number can be provided. Since the TDC pixels and CTC pixels are arranged along the Z-direction, the size of the TDC / CTC pixel pairs in the XY plane can be reduced by stacking the bottom logic wafer 104 and the top wafer 102.

[0054] Figure 1B An exemplary representation of a PWM image sensor 100 is depicted, which includes a CTC array 106 stacked with a TDC array 108. (See attached image.) Figure 1B As depicted, a CTC array 106 may be disposed on a top wafer 102. The CTC array 106 can generate signals (e.g., a WRITE signal) and can transmit these signals to a TDC array 108. The TDC array 108 may be disposed on a bottom logic wafer 104. Figure 1A As depicted, the CTC array 106 and TDC array 108 can be arranged in a stacked configuration.

[0055] Figure 1CAn exemplary CTC circuit 106 is depicted according to various aspects of the provided disclosure. It should be noted that the CTC circuit 106 is merely one example of a CTC circuit, and any number of arrangements may be provided according to the associated disclosure.

[0056] The CTC circuit 106 may include a comparator 107, a photodiode 109, a floating diffusion capacitor 111, and a reset gate 113. Operation of the CTC circuit 106 may begin in response to a RESET signal applied to the reset gate 113. Figure 1C As depicted, when reset gate 113 is closed, a pixel voltage V can be provided. PX It can be applied to the CTC circuit 106, and when the reset gate is open, it can prevent the pixel voltage from reaching the components of the CTC circuit 106.

[0057] A photodiode 109 may be additionally provided, and this photodiode may use light (e.g., photons) to generate an associated photocurrent (e.g., current). The intensity of the photocurrent may depend on the intensity of the light detected by the photodiode 109. The generated photocurrent may cause the floating diffusion capacitor (C) to... FD )111 discharge biases the input of comparator 107. Comparator 107 compares the number of incoming electrons with the threshold number of electrons. This can be achieved by adjusting the threshold voltage V. th The input is fed into comparator 107 to establish the threshold electron count. Threshold voltage V th It can be controllable and variable (e.g., via a controller) to establish different trigger thresholds. Once the threshold number of electrons is reached at comparator 107, a WRITE signal can be generated (e.g., to the TDC circuit).

[0058] Figure 2 Exemplary electronic components of the pixel 200 of a PWM image sensor are depicted. Specifically, in Figure 2 In addition, refer to the following: Figure 3 The timing diagrams depicted are used to discuss the potential operations of pixel 200. While the operation is discussed with reference to a single pixel, it is understood that a large number of individual pixels can be used in PWM image sensors (such as in an array), as discussed regarding... Figure 1A The operation of different pixels can be the same, or it can vary depending on the photocurrent generated by the photodiode and / or other potential settings (analog or digital) of the PWM image sensor.

[0059] Such as about Figure 1AThe pixel 200 of the PWM image sensor discussed may include CTC / TDC pixel pairs. Specifically, a charge-to-time converter (CTC) 206 may be positioned on a first wafer (e.g., top wafer 102), and a time-to-digital converter (TDC) 208 may be positioned on a second wafer (e.g., bottom logic wafer 104) stacked with the first wafer. In some cases, the CTC 206 and TDC 208 are positioned in an alternating arrangement, such as on opposite sides of the same wafer. Additionally or alternatively, it should be noted that the specific arrangement of the CTC 206 and TDC 208 is not particularly limited. Although Figure 1A The CTCs are depicted stacked on top of the TDC, but other arrangements may also be used based on the publicly available information.

[0060] Now refer to Figure 2 and Figure 3 Discuss the operation of pixel 200. As mentioned above, Figure 3 The timing diagrams for the operation of pixels (such as pixel 200) of the PWM image sensor are depicted, and for ease of description, they will be discussed sequentially. Figure 2 and Figure 3 .

[0061] like Figure 2 As depicted, a reset (RST) signal 214 can be applied to the CTC 206. The reset signal 214 can initiate the start of a detection cycle and can clear or reset electrons stored at the sensing node and / or floating diffuser (FD). Reference Figure 3 Reset timing diagram 252 depicts the operation of RST signal 214. As depicted, reset signal 214 can be binary (e.g., 1 and 0 and / or "on" and "off"). For example, the reset signal can be provided to CTC 206 by opening and closing a switch, wherein one end of the switch is connected to CTC 206 (e.g., the sensing node / FD of CTC 206), and the other end of the switch is connected to a fixed voltage (e.g., pixel voltage). Before initiating a detection cycle, the switch can be closed, thereby supplying the pixel voltage to CTC 206 to dissipate any charge that may be present at the sensing node / FD (e.g., from previous image detection operations). At the start of the detection cycle, at time 230, the switch can be opened and the pixel voltage can be stopped. This allows electrons to begin accumulating at the sensing node / FD of CTC 206. After the end of the detection cycle, the switch can be closed (at time 230). Figure 3 Described as occurring between trigger time 232 and the end of frame time 234, a pixel voltage is supplied to dissipate previous charge generated by electron accumulation. When the previous charge is completely dissipated and / or when the switch is moved back to the off position, a subsequent detection cycle can be initiated, thereby stopping the application of pixel voltage to the sensing node / FD of CTC 206.

[0062] At time 230, when the detection cycle begins as a result of the RST signal 214, the accumulation of electrons 217a can begin at the sensing node / FD of CTC 206, as depicted in electron timing curve 256. Electron timing curve 256 depicts the accumulation of electrons during the detection cycle. Voltage timing curve 254 depicts the voltage 216a at the sensing node / FD, which corresponds to the electron count 217a. The electron count 217a can increase at a rate dependent on the light intensity / photocurrent generated by the photodiode of CTC 206 until a threshold electron count 217b is reached. Similarly, the voltage 216a can decrease until a threshold voltage 216b is reached. The time at which voltage 216a reaches the threshold voltage 216b and the time at which electron count 217a reaches the threshold electron count 217b can be referred to as trigger time 232.

[0063] The threshold voltage 216b can be established by supplying a preselected voltage to the CTC 206, such as... Figure 2 As depicted in the diagram. Specifically, a threshold voltage 216b may be applied to the first input of the comparator of the CTC 206. Since the second input of the comparator of the CTC 206 may receive a voltage 216a corresponding to the number 216a of received electrons accumulated at the sensing node / FD of the CTC 206 during the detection period, the comparator may be able to detect when the received voltage 216a matches the threshold voltage 216b, thereby generating a trigger time 232.

[0064] At time 232 (corresponding to the end of the detection period), a write (WRT) signal 218 can be generated at CTC 206 and transmitted to TDC 208. The WRT signal 218 can correspond to the number of electrons 217a accumulated over a specific time period, which corresponds to the brightness of reflected light 212 from the scene and / or object (e.g., object 210) that is the image object. As depicted in write signal curve 258, the WRT signal 218 is initiated after trigger time 232. The WRT signal 218 can correspond to initiating the latch count process in TDC 208. Selection timing curve 260 depicts the read signal 222 being applied to TDC 208, as... Figure 2 and Figure 3 As depicted in the image. Each pixel can be read line by line (corresponding to the CTC / TDC pair).

[0065] The time 234, indicating the end of the frame time, can correspond to the time when all detection and signal analysis processes are completed. For example, image data of one or more pixels of the PWM image sensor can be fully processed at this stage, and subsequent processes can be initiated.

[0066] Data bus 220 may be additionally located between TDC 208 and an external counter (not depicted). The external counter may feed counting data to TDC 208, or may otherwise transmit and / or receive data from TDC 208. Reference Figures 4 to 5 To provide further details about the counting data.

[0067] Now will provide the basis Figures 2 to 3 The exemplary operation is as follows. To initiate the image detection process, a reset signal 214 can be applied to the charge-time converter (CTC) 206. Light 212 reflected from object 210 can be received by CTC 206, and the photodiode of CTC 206 can convert light 212 into current (e.g., photocurrent). This current can supply electrons to the sensing node / floating diffuser where electrons accumulate. Once the accumulated electrons reach a threshold number of electrons (corresponding to a threshold voltage 216 supplied to CTC 206), a write signal 218 corresponding to image information can be sent from CTC 206 to time-to-digital converter (TDC) 208. The write signal 218 can latch a count in TDC 208, and this count can be read by read signal 222. Any number of pixels of the PWM sensor can be read in any order (e.g., line-by-line), and image information of object 210 can be generated. A bidirectional data bus 220 may also be provided between TDC 208 and an external counter. The bidirectional data bus 220 can be bidirectional to save pixel area, thereby further reducing the size of the pixel 200, but alternatively a unidirectional data bus can also be used.

[0068] Figures 4 to 5 The graphs related to the nonlinear counter are described, as shown in the reference to... Figures 2 to 3 The external counters discussed are as follows. Figure 4 A graph 400 is plotted, which shows the relationship between photocurrent (e.g., the current generated by a photodiode in response to receiving light) and trigger time (e.g., the time corresponding to the length of the detection cycle of a specific pixel of a PWM image sensor).

[0069] like Figure 4 As can be seen, the associated trigger time of pixels in the PWM image sensor decreases with increasing photocurrent. This is because high photocurrent leads to a high electron accumulation rate at the sensing node / FD of the CTC. With rapid electron accumulation at the sensing node / FD, the trigger time may occur quickly. Conversely, low photocurrent may result in a low electron accumulation rate, thus prolonging the necessary trigger time. In short, as photocurrent increases, the trigger time decreases, and as photocurrent decreases, the trigger time increases. Figure 4 As depicted, this relationship is not linear, but rather curves between the X and Y axes. The curve can be derived from the equation... Define, where Ttrig For trigger time (e.g., the length of the detection period for a specific pixel), q is the elementary charge (e.g., the charge of a proton), and N is the number of pixels. th The threshold number of electrons required to end the detection cycle, and I ph This refers to the photocurrent generated by the photodiode of the CTC. As used in this document, trigger time may refer to the time it takes for a specific pixel to be triggered, and detection period may refer to the entire period during which the PWM image sensor is active.

[0070] As indicated by graph 400, the relationship between photocurrent and trigger time is non-linear. Therefore, if a linear sampling counter is used, the photocurrent highlights corresponding to the end or beginning of the curve can be compressed. That is, a linear counter with ticks separated by consistent time intervals can compress the photocurrent samples to approximately 0.1 amperes (A) or more. Figure 4 As described in the text. This will result in poor image quality in low light and / or high light conditions (causing highlight compression of pixels corresponding to relatively bright / dark pixels).

[0071] To avoid potential specular compression, one aspect of the provided disclosure offers a nonlinear counter 505 to allow uniform sampling of the photocurrent. In this way, the relationship between the TDC output and the photocurrent can be linear, thereby allowing high-quality specular capture without compression in both low and high photocurrent ranges. Figure 5 The curve 500 depicts the TDC output and photocurrent I based on the nonlinear counter 505. ph The linear relationship between them. The nonlinear counter 505 depicts the nonlinear time between the ticks of the continuous counter, where the high photocurrent is sampled at a faster rate than the low photocurrent based on the steeper photocurrent curve in the high photocurrent range, such as... Figure 4 As depicted in the text.

[0072] In some cases, each tick of the nonlinear counter 505 is spaced by the equation The defined time, where b is the bit depth associated with TDC, n is the counter step number, 2 b –n is the TDC counter code, and t min To detect the maximum possible photocurrent I that the CTC photodiode can generate. max Required counter delay. max The value can be established based on the physical characteristics of the photodiode used in the CTC and / or through software associated with the PWM image sensor. In some cases, t min From the equation Definition, which is similar to a definition Figure 4The equation for the curve differs in that the current is the maximum possible photocurrent that the photodiode in the PWM image sensor can generate, rather than the detected photocurrent. Also note that t... min The value can be changed based on a change in the number of threshold electrons (e.g., by establishing different threshold voltages applied to the CTC).

[0073] Figures 6A to 6B A graph depicting an exemplary dynamic range (DR) extension is shown. Figure 6A It depicts the corresponding number of electrons with a constant threshold (N) th The DR extension curve of 600a, and Figure 6B Describes the corresponding to having a variable N th The DR extension curve is shown in Figure 600b. As used in this document, "trigger time" refers to the moment when a specific pixel of a PWM image sensor has accumulated a threshold number of electrons at the sensing node / FD. The trigger time is initiated at the start of the detection cycle and occurs when a specific pixel is triggered.

[0074] As depicted in curve 600a, corresponding to the constant N th The integrated charge (IC) threshold 602a can be set to a constant value. During the detection period, the time (t) required to reach the IC threshold 602a depends on the value of the photocurrent generated by the photodiode in response to light. For example, as indicated by box "1" in graph 600a, the minimum photocurrent I generated by the photodiode... min Caused trigger time T trig This corresponds to the time when the number of electrons accumulated at the sensing node / FD reaches the IC threshold 602a of a specific pixel. Similarly, as indicated by box "4" in graph 600a, the 4I generated by the photodiode... min Photocurrent causes the second trigger time to be lower than Figure 6A The T depicted in trig The value will be greater because the time required to reach the IC threshold 602a will be faster due to the faster accumulation of electrons at the sensing node / FD. Similarly, the corresponding trigger time will vary based on the value of the photocurrent generated by the photodiode, such as, but not limited to, a range of I... min Up to 7I min The photocurrent. In this way, the specific triggering time is based on the value of the associated current and the number of threshold electrons (e.g., IC threshold 602a).

[0075] In some cases, the IC threshold 602a is the same across multiple pixels in a PWM image sensor. That is, the PWM image sensor can receive different amounts of light at different pixels, causing the corresponding photodiodes associated with those pixels to generate different amounts of photocurrent. Therefore, based on the value of the associated photocurrent, the triggering time of different pixels may differ.

[0076] Figure 6B A graph 600b depicting a variable IC threshold 602b is presented. (As shown...) Figure 6B As described, the variable IC threshold 602b can lead to a reduction in the number of threshold electrons required to end the trigger cycle for a specific pixel of a PWM image sensor. In this way, smaller photocurrents (e.g., I0) below the typical minimum photocurrent can be detected. min / 4), without causing excessively long trigger times. For example, in Figure 6B In the middle, I is described min / 4 of the photocurrent. If the IC threshold is at Figure 6A At the level described in [the document], the IC threshold 602a, i.e., the trigger time required to reach the threshold number of electrons at the sensing node / FD, will increase dramatically. To reduce the trigger time, the IC threshold can be lowered.

[0077] However, lowering the IC threshold on each pixel of a PWM image sensor may result in the loss of valuable image data with little benefit (because the trigger time may already be very short). For example, in highlight conditions (e.g., causing 7I...), min (photocurrent), if the IC threshold setting is different from the setting in the low-light state (e.g., I...) min If the same applies to (4), then a high percentage (e.g., 90+%) of electrons may not be detected. Additionally, unlike in a low-light state, the remaining electrons in a high-light state may be received within a short period.

[0078] The variable IC threshold 602b can be set manually (e.g., via the DR extension knob) or automatically in response to the detected light intensity level. For example, if the electronic device is in a dark environment (e.g., it can be detected by any number of sensors), the variable IC threshold 602b can be lowered.

[0079] The variable IC threshold 602b may vary across different pixels of the PWM image sensor. For example, pixels under high-light conditions (resulting in high photocurrent) may have a relatively high IC threshold, while pixels under low-light conditions (resulting in low photocurrent) may have a relatively low IC threshold. In an additional or alternative embodiment, the IC threshold may be gradually reduced (e.g., at a constant rate) before it is reached. In this way, the entire detection cycle can have a predictable endpoint regardless of the associated photocurrent value and the triggering time of the individual PWM pixels.

[0080] Figure 7 Graph 700 depicts curves with various exemplary TDC transfer functions. The TDC transfer function can refer to an encoded curve based on the output curve of illuminance (measured in lux; lx). Figure 7 In the graph 700 depicted in the figure, the transfer function is drawn for a 5-bit output code (e.g., where the TDC can store 5 information bits).

[0081] Curve 702 depicts the variable threshold number of electrons (N) th The logarithmic TDC transfer function of N. Curve 704 describes the constant N. th The logarithmic TDC transfer function. Curve 706 describes the constant N. th The linearized TDC transfer function. The specific transfer function used can be selected based on the specific application using a PWM image sensor (e.g., depending on camera lighting conditions, video or picture mode, etc.). For example, curves 704 and 706 (representing the TDC transfer function) can be used for highlight conditions where the minimum illuminance is above approximately 300 lx, but any value can be used according to the provided disclosure.

[0082] The TDC transfer function can be expressed in the time domain by the equation t = f(n) * t. min The control is achieved by defining a counter step size, where t min The counter delay is as described above. The function f(n) can reference any linear, logarithmic, and / or piecewise distribution. As a non-restrictive example of such a distribution represented by f(n), the logarithmic function can be derived from the equation... Define, where I max The maximum photocurrent generated by the photodiode and I min Let p be the minimum photocurrent generated by the photodiode. The value p can be obtained from the equation... Define , where n is 0 to 2 b –1 range and where b is the bit depth of the TDC (e.g., in Figure 7The example depicted is number five. According to the provided disclosure, any equation can represent f(n), including (but not limited to) the logarithmic equations shown above, in order to define the transfer function. In the example above, the logarithmic response can reduce or eliminate fixed-mode noise (FPN) caused by threshold variations of the current source (e.g., a current-source transistor).

[0083] Each of the TDC transfer functions described, as well as any other suitable transfer function, can be used in a PWM image sensor. As discussed above, the flexibility of the transfer functions of a PWM image sensor allows it to be specifically tailored to the needs of particular applications, such as those in electronic devices.

[0084] Figure 8 A graph 800 was plotted, which depicts the number of electrons at the first threshold (N). th1 (defined by curve 804) and the second threshold number of electrons (N) th2 The signal-to-noise ratio (SNR) is defined by curve 806. Dashed line 802 can be used to represent a situation where the SNR is largely equivalent to N. phot Photon shot noise N under certain conditions phot .

[0085] SNR can be derived from the equation It means that N phot N represents photon shot noise caused by statistical quantum fluctuations. dark Dark noise represents thermal noise generated by electron movement, and For readout noise, represent the voltage fluctuations of the PWM image sensor (e.g., during the readout process). This is based on the threshold number of electrons (N). th When the potential readout noise and dark noise are much higher, the SNR is primarily defined by photon shot noise. This can result in the SNR being approximately equal to N. th The square root of is represented by line 802 as the photon noise limit.

[0086] As discussed above, curve 804 represents the number of first threshold electrons (N). th1 ), and curve 806 represents the number of second threshold electrons (N) th2 (Refer to N) th1 With the photocurrent I ph As the threshold N increases, the SNR can increase up to the photon noise limit of 802. However, once the photon noise limit of 802 is reached, the SNR remains constant because other aspects of the SNR (e.g., readout noise) become negligible, as the values ​​are masked by photon shot noise. This results in the SNR not increasing once the system reaches the threshold level. At higher threshold levels N... th2 At this point, the SNR may be greater than N. th1The corresponding SNR, but once the photon noise limit 802 is reached, no matter how much the photocurrent I is increased. ph Regardless, the SNR can remain consistent.

[0087] In this way, pixel saturation of any particular pixel in the PWM image sensor can be avoided, and the maximum detectable current (e.g., photocurrent) can be defined by the counter delay of the TDC. Furthermore, with readout noise much lower than photon shot noise, the dynamic range of the PWM pixel can be defined by the maximum trigger time T. trig With minimum trigger time t min The ratio between them is determined by the constant N. th In this case, the ratio is determined by the equation Define , where b is the bit depth of TDC.

[0088] Figures 9A to 9B Depicting the use of N th The curves related to the controlled SNR shaping are shown above. Figure 8 As discussed, assuming that photon shot noise dominates the overall noise in SNR, the SNR value can be approximated as N. th The square root of N. To extend the dynamic range, especially under low-light conditions, a lower N can be selected during the detection period if the threshold number of electrons has not accumulated in the sensing node / FD within a specific time period. th This allows for faster detection cycles, whereas otherwise a particular pixel might require a significant amount of time to accumulate the threshold number of electrons and / or, if the image sensor is in motion relative to the scene and / or object being the image subject, that particular pixel could blur the image.

[0089] refer to Figure 9B The graph 900b depicts the first threshold number of electrons N. th1 The second threshold number of electrons N th2 Within a specific detection period, the photocurrent defined by line 904 can be expected to change with respect to the trigger time T of a specific pixel. trig Reaching N th1 However, in low-light conditions, the photodiode may generate a lower photocurrent, and the trigger time may be reached at a slower rate, as defined in line 906. To avoid an excessively long trigger time, N can be reduced. th Value. As discussed in this paper, N can be reduced by decreasing the threshold voltage applied to CTC. th N can be reduced in other ways or alternatively. th Values, such as reducing the capacitance of the sensing node / FD capacitor or having a double-conversion gain structure.

[0090] Figure 9AThe method described is to reduce N by decreasing the voltage applied to CTC. th Value. As depicted in graph 900a, after a specific time period has elapsed, voltage 902 changes from V. th1 The value decreases to V th2 Value. Therefore, when N is reached th2 value instead of the initial N th1 When the value is reached, the trigger time can end (by T). trig Instructions (see) Figure 9B By reducing N in this way th SNR can be controlled separately because SNR can be approximated as N. th The square root of the value. This allows for SNR shaping when using PWM sensors.

[0091] Figures 10A to 10B The graphs depict different detection periods corresponding to multiple pixels of the PWM image sensor. As discussed above, different pixels can receive different amounts of light, resulting in different values ​​of the corresponding photocurrent. Therefore, different pixels can reach the threshold number of electrons (N) at different times. th In traditional image sensors, this can result in blurry images of fast-moving objects (e.g., the object may be in a different position at the beginning of a detection cycle compared to the end of the detection cycle).

[0092] like Figure 10A As depicted in graph 1000a, for a PWM image sensor, motion blur can be minimized because most pixels can reach N within a short time period. th Value. In the example depicted in graph 1000a, for example, in the value corresponding to T DET 94% of all pixels are triggered within a / 17 trigger time. This can be achieved corresponding to T. DET The remaining 6% of pixels are triggered within the trigger time (e.g., where the associated photocurrent has I). min The value of the pixels (as depicted in line 1002). This control minimizes or eliminates motion blur in the resulting image, particularly for pixels that generate large photocurrents. This is because at least 94% of the pixels constituting the image reach their trigger time within a very small window (e.g., 1 / 17 of the total detection time). Therefore, a sharper image may be produced.

[0093] Figure 10B Graph 1000b depicts the relationship between TDC code (e.g., TDC code in units of least significant bit (1sb)) and time (t). Line 1004 depicts the constant N. thExamples of threshold electron counts are shown, where the number of threshold electrons remains consistent across one or more pixels and / or over the detection period. Line 1006 depicts an example of a ramp Nth, where the number of threshold electrons varies across one or more pixels and / or over the detection period (see example...). Figures 9A to 9B Due to the faster transition curve of line 1004 (representing the constant N), th (In this case), a higher number of pixels can be triggered within a shorter time period. In contrast, the slower transition curve of line 1006 (representing ramp N) th (Situation) may result in a slower trigger time. Therefore, the transition curve corresponding to line 1004 represents a clearer image compared to the transition curve corresponding to line 1006.

[0094] Figure 11 An exemplary structure 1100 for TDC timing control of a PWM image sensor is depicted. For a particular PWM image sensor, the resolution of the time-to-digital conversion (e.g., via TDC) can be determined by... Given, where I max Let b be the maximum photocurrent of the photodiode and b be the bit depth of the TDC. Using this resolution, the minimum time step can be given by the following formula: Where N th Let q be the threshold number of electrons collected as a sensing node / FD, q be the fundamental charge (e.g., the fundamental charge of a proton), and t be the number of electrons collected as a sensing node / FD. min For counter delay. Using minimum time step Δt, clock frequency F clk Alternatively Provided.

[0095] As depicted in exemplary structure 1100, the clock frequency F as defined in the preceding paragraphs clk From the maximum photocurrent I max The clock division factor K is scaled down proportionally; this clock division factor is called the divider 1102. The clock frequency can be proportional to the maximum photocurrent. When the maximum photocurrent is expected (e.g., for the highest amount of measurable light), K can be equal to the value "1".

[0096] The clock frequency divided by the clock division factor can be applied to clock gate 1104. Clock gate 1104 can be used to save power, causing the associated clock circuitry to not operate when not currently in use. The clock can be activated by applying the clock frequency and clock division factor to the clock gate. A counter delay can also be provided to clock gate 1104. Counter 1106 (e.g., as per...) Figures 4 to 5The nonlinear counter discussed can sample the photocurrent generated by the photodiode, and the comparator 1108 (e.g., a digital comparator) can be used to determine when the number of electrons accumulating at the sensing node / FD reaches a threshold number of electrons (e.g., by a threshold voltage input to the comparator 1108).

[0097] like Figure 11 As depicted, the TRIGGER signal can be a non-linear synchronization signal used for the Gray code counter 1110. Counter 1106, comparator 1108, and lookup table 1112 can be used to generate the TRIGGER signal. That is, lookup table 1112 can store a specific range (e.g., 1 to 2). b –1) The clock segment value for each TDC count within the time limit, comparator 1108 can compare the value of counter 1106 with the current value from lookup table 1112, and generate a TRIGGER signal when the two values ​​correspond. Comparator 1108 can also send a reset signal to counter 1106. Divider 1102 can also be used to affect the speed of Gray code counter 1110 and can control the exposure or detection time of the associated PWM image sensor. In some specific implementations, such as in ramp N th In this mode, lookup table 1112 can additionally store N for each TDC count. th value.

[0098] Figure 11 Only one method of TDC timing control is discussed, and any number of TDC timing control methods can be used with respect to the PWM image sensor provided in the disclosure. Figures 12 to 15B Several such examples are provided in the document.

[0099] Figure 12 An exemplary method 1200 for exposure control is described, wherein the initial N th The value is predetermined by the expected SNR value, and Figure 13 An exemplary method 1300 for exposure control is described, in which N is used. th The maximum value (e.g., N) max ).

[0100] As described at operation 1202, the detection period T DET Threshold electron number N th and clock frequency F clk The initial values ​​can be used as the starting point for exposure control. These initial values ​​may correspond to the desired SNR and / or the desired photocurrent range in the object / scene as an image object captured by the PWM image sensor.

[0101] At operation 1204, the value of the clock division factor K can be calculated. (As mentioned above...) Figure 11As discussed, the clock division factor can be used as a scaling factor to scale down the clock frequency from a maximum level (e.g., the level corresponding to the maximum photocurrent). As depicted at operation 1204, this can be derived from the equation... Calculate the value of K, where the ROUND function rounds the calculated value to the nearest integer. The ROUND function may also, alternatively, round up or down.

[0102] At operation 1206, a new clock frequency is determined by dividing the original clock frequency (e.g., as indicated at operation 1202) by the value of K determined at operation 1204. At operation 1208, the determination is performed using the new clock frequency. It is then determined whether the detection period divided by 2 multiplied by a power corresponding to the bit depth of the TDC is greater than or equal to the expression. In other words, determine whether the conversion time is less than or equal to the detection cycle.

[0103] If the answer to the operation at 1208 is negative (“No”), the detection period can be changed to the transition time, as indicated at operation 1210. If the answer to the operation at 1208 is positive (“Yes”), the detection period can remain the same. In the case of operation 1210, the low-light detection time can be extended to reduce the frame rate of the PWM pixels. At operation 1212, K and / or t can be determined and / or stored. min and / or T DET The value (e.g., in a lookup table). As described in this article, Figure 12 The method 1200 described herein typically corresponds to single-shot scene estimation under a logarithmic pattern (e.g., logarithmic transfer function).

[0104] Figure 13 An alternative method 1300 for exposure control is described. As indicated at operation 1302, the maximum “N” value (e.g., the number of threshold electrons accumulated at the sensing node / FD) can be used as the starting point for exposure control. In contrast, Figure 12 The method 1200 described herein uses the expected SNR value (e.g., based on a single shot estimate) with a predetermined N. th value.

[0105] At operation 1302, the detection cycle (T) can be obtained. DET ), clock frequency (F) clk ), maximum number of accumulated electrons (N) max ) and minimum number of electrons accumulated (N min The initial value of ). The clock frequency division factor K can be calculated at operation 1304. Reference Figure 12 The equation depicted at operation 1304 is the same as the equation depicted in operation 1204, except that the maximum threshold number of electrons N is used.max To replace the threshold electron number N estimated based on a single image. th .

[0106] At operation 1306, the calculated clock frequency division factor K can be used to update the clock frequency value, such as F. clk(n) As indicated. In this way, the clock frequency coefficient corresponding to the maximum threshold level of the electron can be calculated. In some cases, the value of "K" is equal to "1" in this initial step.

[0107] At operation 1308, it is determined whether the conversion time is less than or equal to the detection period. If the conversion time is not less than or equal to the detection period (e.g., "No"), the method can proceed to operation 1310, where N is determined. max Is the value greater than N? min Value. If N max Greater than N min (Operate "Yes" at position 1310), then N max It can reduce "1" and can be used as a new N at operation 1302 and thereafter. max Value. In this way, exposure control can be performed incrementally.

[0108] If at operation 1310, the value of Nmax is less than the value of Nmin, then method 1300 can continue (via "No") to operation 1312, where the use of ... Figure 12 The detection period is calculated using the same equation provided at operation 1210. Then, K and t can be obtained and / or stored at operation 1314. min and T DET The value of .

[0109] If, at operation 1308, the detection period is greater than or equal to the conversion time, then method 1300 can immediately proceed to operation 1314, in which K and t can be obtained and / or stored. min and T DET The value of . In this way, method 1300 can be used to reduce the number of threshold electrons to a minimum level (N) before reducing the frame rate (e.g., at operation 1312). min Therefore, method 1300 typically uses a stepwise approach to control a specific exposure value.

[0110] Figure 14 Depicting the slope N th Pattern (e.g., such as) Figures 9A to 9B Additional method 1400 for exposure control under the mode depicted in the diagram. At ramp N th In this mode, N max Value and N minThe value is constant, so there is no need to change the associated value in the lookup table (LUT), such as Figure 13 As depicted in the text.

[0111] At operation 1402, the clock frequency (F) can be obtained. clk ), detection cycle (T) DET ), maximum threshold number of electrons (N) max ) and minimum threshold number of electrons (N) min The initial value of q is used. At operation 1404, the clock frequency division factor K can be calculated using the charge q, clock frequency, maximum threshold electron count, and maximum photocurrent, as discussed above. At operation 1406, the updated clock frequency can be obtained by dividing the original clock frequency by the clock frequency division factor K.

[0112] At operation 1408, the detection cycle (T) is determined. DET Is it greater than or equal to the conversion time, as shown in the equation? As defined in [the document]. If the detection period is greater than or equal to the conversion time, method 1400 can proceed to operation 1412, where K and t can be obtained. min and T DET The value. If the answer to operation 1408 is negative (e.g., "no"), then it can be determined by the equation. The detection period is changed to change the frame rate. After changing the detection period, method 1400 can proceed to operation 1412, as discussed above.

[0113] Figures 15A to 15B An example depicting an automatic exposure process is provided. Figure 15A A method 1500 for an automatic exposure process using a System-on-Chip Lookup Table (SOC LUT) is described. At operation 1502, K and t are input from the address "N" of the LUT associated with the PWM image sensor at i=0. min and T DET The initial value is then obtained. At operation 1504, the window signal value can be obtained from the PWM image sensor, and the average brightness S can be calculated. To calculate S, the equation can be used. Among them I max Proportional to the TDC clock frequency and defined as Where q is the elementary charge, N max F is the maximum number of threshold electrons received at the sensing node / FD. clk Let b be the clock frequency of the TDC, and I be the bit depth of the TDC. ph Let S be the average photocurrent, and K be the segmentation coefficient. The equation for the average brightness S can be simplified to: Where k1 is a constant value.

[0114] At operation 1506, determine if the average brightness is less than or equal to "1". If the average brightness is greater than "1", then at operation 1508, a lookup table (LUT) can be queried to obtain the data for Clog2S and Clog2K. Figures 15A to 15B In the described implementation, this data corresponds to LUT1 and LUT3 (see LUT 1550, e.g.) Figure 15B (As depicted in the text). Although LUT1 and LUT3 are queried at operation 1508, any LUT column can store the necessary information according to the specific LUT settings.

[0115] Once values ​​from LUT1 and LUT3 are received at operation 1508, the counter steps n can be calculated at operation 1510 using the equation n = Clog2S + Clog2K - K2, where n is the counter steps, and C is obtained from the equation... Defined, and K2 is given by the equation K2=Clog2L min -Clog2F c.k +Clog2k1 is defined. As used in this paper, the value of illuminance (L) can be obtained from the equation log... 2[ =log2S - log2K + log2F Klk -log2k1 is exported, and the minimum and maximum values ​​of illuminance can be obtained from... Given, where Ln is the window illuminance.

[0116] At operation 1512, the calculated value of n is used. For example, as depicted at operation 1512, if n is less than or equal to 0, n can be updated to be equal to "0". If n is greater than or equal to N, n can be updated to be equal to "N". At operation 1514, the LUT can be queried to obtain K and t at LUT2, LUT4, and LUT5. min and T trig The table data (see Table 1550, such as...) Figure 15B (As depicted in the text), but the specific columns of these values ​​are provided for illustrative purposes only. At operation 1518, the values ​​obtained at operation 1514 can be output to the PWM image sensor to automatically update the exposure settings.

[0117] If the average brightness value is determined to be less than 1 at operation 1506, operation 1516 can occur. In this case, n can be set to equal N / 2 before querying the LUT at operation 1514.

[0118] At operation 1520, the value of i can be incremented by "1". If at operation 1522 i is less than or equal to 3, the operation can restart at operation 1504. Otherwise, if i is greater than 3, method 1500 can end at operation 1524.

[0119] As mentioned above, Figure 15B An exemplary LUT 1550 is depicted, including columns for average brightness, LUT1, address, and LUT2-LUT5. This LUT 1550 is provided for illustrative purposes only, and any potential LUT layout may be used based on the disclosure provided.

[0120] Figure 16 An exemplary LUT 1600 is depicted, comprising an average luminance S-segment LUT 1602 and an illuminance S-segment LUT 1604. As discussed above, the logarithmic TDC mode allows a PWM image sensor to capture the entire scene dynamic range (e.g., high dynamic range HDR) in a single shot. In this way, the average illuminance of the scene can be measured to help identify the illuminance segment n in the LUT 1600. In some cases, the illuminance segment n can be determined by an equation. The calculation is performed using S, where S is the average measured brightness, b is the bit depth of the TDC, P is the number of stops, and C and K2 are constants.

[0121] By identifying the correct illuminance range n, appropriate scene brightness can be achieved for a specific image, preventing some parts from being too dark or too bright. For example... Figure 16 The text further describes how data can be transmitted from the PWM sensor 1606 to the LUT 1600. Additionally, I... 2 C (internal integrated circuit) can be used to attach the LUT 1600 (specifically, n-segment 1604) to the PWM sensor 1606.

[0122] During any of the exemplary operations, additional processes may occur. For example, to provide color to an image, a Bayer filter array may be provided, and associated processing electronics may determine the color of any particular pixel. Furthermore, software features may be provided to modify the operation of the PWM image sensor. Depending on the operation of the PWM image sensor, any components, structures, filters, methods, processes, etc., may be used.

[0123] Other examples and specific implementations are within the scope and spirit of this disclosure and the appended claims. For example, features that implement the function may also be physically located at various locations, including being distributed such that functional parts are implemented at different physical locations. Furthermore, as used herein, the word "or" used in a series of terms prefixed with "at least one" in the claims indicates a disjoint list, such that, for example, "at least one of A, B, or C" means A or B or C, or AB or AC or BC, or ABC (i.e., A and B and C). Additionally, the term "exemplary" does not imply that the examples described are preferred or better than other examples.

[0124] For illustrative purposes, the foregoing description uses specific names to provide a thorough understanding of the described embodiments. However, it will be apparent to those skilled in the art that specific details are not required to practice the described embodiments. Therefore, the foregoing description of specific embodiments described herein is presented for illustrative and descriptive purposes. They are not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to those skilled in the art that many modifications and variations are possible in light of the teachings above.

Claims

1. A method for operating a pulse width modulation (PWM) image sensor, the method comprising: The number of photons received at the PWM pixel of the PWM image sensor during the detection period; As the number of photons is received, the number of photons is converted into photocurrent; During the detection period, the number of electrons accumulates at the sensing node in response to the sensing node receiving the photocurrent; During and in sync with the detection period, a count is incremented based on the nonlinear relationship between the trigger time and the photocurrent, each value of the count being synchronized with a nonlinear time reference, wherein the count increments at a decreasing rate as the trigger time increases; as well as When the number of accumulated electrons reaches the threshold number of electrons, the count value is latched into the memory of the time-to-digital converter (TDC) circuit.

2. The method according to claim 1, wherein the nonlinear time base t is obtained through... Let be defined, where b is the bit depth of the TDC circuit, n is the counter step number, and t min This is for counter delay.

3. The method of claim 2, wherein the counter delay is further achieved through... Let N be the elementary charge, where q is the elementary charge and N is the elementary charge. TH The threshold number of electrons, and I max To generate the maximum photocurrent.

4. The method of claim 1, further comprising transmitting image information via a bidirectional bus, the image information corresponding to a latched value of the count in the memory of the TDC circuit.

5. The method of claim 1, further comprising terminating the detection period in response to the photocurrent for a duration thereof.

6. The method of claim 5, wherein a higher photocurrent results in a shorter detection period, while a lower photocurrent results in a longer detection period.

7. A pulse width modulation (PWM) image sensor, comprising: Top wafer, the top wafer including charge-time converter (CTC) circuit, the CTC circuit including: Photodiode; Reset gate; and Comparator; and A bottom wafer, stacked with the top wafer, includes a time-to-digital converter (TDC) circuit, wherein: A reset signal is applied to the reset gate to initiate a detection cycle; During the detection period, the photodiode accumulates an electron count, and the TDC circuit increments the count according to a nonlinear relationship between the trigger time and the photocurrent. Each value of the count is synchronized with a nonlinear time reference, wherein the count increments at a decreasing rate as the trigger time increases; and When the number of electrons reaches the threshold number of electrons, the CTC circuit transmits a write signal from the comparator to the TDC circuit.

8. The PWM image sensor according to claim 7, wherein: The TDC circuit is a static random access memory comprising multiple latches; and The write signal latches the count in the plurality of latches.

9. The PWM image sensor according to claim 7, wherein: A threshold voltage is applied to the comparator; and The number of threshold electrons corresponds to the threshold voltage applied to the comparator.

10. The PWM image sensor according to claim 7, wherein: The CTC circuit and the TDC circuit include pixels; and The PWM image sensor also includes multiple pixels, each of which includes a corresponding CTC circuit and a corresponding TDC circuit pair.

11. The PWM image sensor of claim 10, wherein each of the plurality of pixels is arranged in an array and read line by line.

12. The PWM image sensor of claim 7, wherein the top wafer and the bottom wafer are communicatively coupled via at least one of a vertical transmission gate, a through-silicon via, or a bonding pad.

Images