Image sensor readout method

Abstract

The present invention provides an image sensor readout method, comprising: in a reset phase, controlling a strobe signal SEL to be set to a low level, and setting a reset signal RX and a transmission signal TX to a high level; when a pixel unit is reset, controlling the transmission signal TX to switch from a high level to a first low level, so that a transmission tube Mtg is disconnected; in an exposure phase, maintaining the strobe signal SEL to be a low level; in a signal reading phase, controlling the strobe signal SEL to be set to a high level, so that the strobe tube Mse l is turned on, and an overflow signal potential VOF stored on a floating node FD is read out; the present application increases the low level of the transmission signal TX, so that saturated electrons on a photodiode PD overflow to a floating node FD, and outputs the overflow signal potential VOF, so that the pixel signal amount finally read out increases by VRST-VOF compared with the traditional method, thereby expanding the dynamic range of the pixel readout signal, thereby eliminating the need to optimize the structure of the photodiode PD, and reducing the processing cost.

Description

Technical Field

[0001] The present invention relates to the technical field of integrated circuits, and in particular to an image sensor readout method. Background Art

[0002] CMOS image sensors (CIS) have been widely used in imaging fields such as video, surveillance, industrial manufacturing, automobiles, and home appliances. With the improvement of application requirements, not only is the CIS design required to be gradually miniaturized, but it is also required to have a higher dynamic range. However, the continuous reduction of pixel units will cause the photosensitivity to continue to decline. To achieve both small area and high dynamic range, the general practice is to improve the photodiode PD performance by optimizing the process, which often leads to an increase in processing costs. In order to solve the above problems, an image sensor readout method is proposed in this application. Summary of the invention

[0003] The object of the present invention is to provide an image sensor readout method, which increases the readout of the overflow signal potential VOF so as to expand the dynamic range of the pixel readout signal, thereby avoiding the optimization of the photodiode PD and reducing the processing cost.

[0004] To achieve the above-mentioned purpose, the present application provides an image sensor readout method, which is applied to an image sensor, comprising:

[0005] In the reset stage, the control selection signal SEL is set to a low level, the reset signal RX and the transmission signal TX are both set to a high level, so that the transmission tube Mtg, the reset tube Mrst and the amplifier tube Msf are all turned on, and the pixel unit is reset;

[0006] When the pixel unit is reset, the transmission signal TX is controlled to switch from a high level to a first low level, so that the transmission tube Mtg is disconnected;

[0007] In the exposure stage, the selection signal SEL is kept at a low level, and the transmission signal TX is kept at a first low level, and the reset signal RX is controlled to switch from a high level to a low level, so that the reset tube Mrst and the transmission tube Mtg are disconnected, the selection tube Msel is turned on, and the photodiode PD starts to expose and accumulate electrons;

[0008] When the photodiode PD starts to be exposed and accumulates electrons, the transmission signal TX is controlled to be pulled up from the first low level to the second low level, so that the photodiode PD is saturated and part of the electrons overflow to the floating node FD;

[0009] In the signal reading stage, the selection signal SEL is controlled to be set to a high level, so that the selection tube Msel is turned on, and the overflow signal potential VOF stored on the floating node FD is read out;

[0010] Then, the reset signal RX is controlled to switch from a low level to a high level, so that the reset tube Mrst is turned on, the floating node FD is reset, and the reset potential VRST is read out;

[0011] Then, the transmission signal TX is controlled to switch from a low level to a high level, so that the transmission tube Mtg is turned on and the exposure integration signal VSIG is read out.

[0012] Optionally, the method further includes:

[0013] The analog-to-digital conversion unit converts the overflow signal potential VOF, the reset potential VRST and the exposure integration signal VSIG into digital quantities, performs a subtraction operation on them, and obtains an actual corresponding digital quantity, and outputs the actual corresponding digital quantity.

[0014] Optionally, the voltage DOUT output by the pixel unit = (VRST-VOF) + (VRST-VSIG) × 2 N / VREF;

[0015] Among them, VREF represents the reference voltage range of the analog-to-digital conversion unit, and N is the bit width of the analog-to-digital conversion unit.

[0016] Optionally, the value range of the first low level is [-1V, -0.5V], and the value range of the second low level is [-0.4V, -0.1V].

[0017] Optionally, an image sensor includes a pixel array, an analog-to-digital conversion unit, a reference signal generator, a timing controller, a row selection decoding driver and an output signal processor, wherein the timing controller is used to control the image sensor to execute the method described above.

[0018] Beneficial effects:

[0019] In the present application, before outputting the reset potential VRST and the exposure integration signal VSIG, the low level of the transmission signal TX is increased so that the saturated electrons on the photodiode PD overflow to the floating node FD, thereby changing the potential of the floating node FD and outputting the overflow signal potential VOF, so that the final readout pixel signal amount increases VRST-VOF compared to the traditional standard four-tube pixel unit circuit, thereby expanding the dynamic range of the pixel readout signal, thereby eliminating the need to optimize the structure of the photodiode PD and reducing processing costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a timing diagram of a standard four-pixel unit circuit in the image sensor readout method of the present invention;

[0021] Figure 2It is a schematic diagram of the circuit structure of a standard four-pixel unit in the image sensor readout method of the present invention;

[0022] Figure 3 This is a timing diagram of a pixel unit circuit in an embodiment of the image sensor readout method of the present invention. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein should be understood by people with general skills in the field to which the present invention belongs. "Including" and similar words used in this article mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0024] This application provides the operation timing of the traditional four-pixel unit as a reference, see Figure 1 As shown, the operation process of the pixel unit is divided into three stages: reset (RESET), exposure (EXP) and read (READ). In the reset stage, the selection signal SEL is low, the transmission signal TX and the reset signal RX are high, the transmission tube Mtg and the reset tube Mrst are both turned on, the floating node FD is reset, and its potential is pulled up to VDD. After that, the selection signal SEL is kept at a low level, so that the reset signal RX and the transmission signal TX are both low, and the exposure stage is entered, and the photodiode PD is sensitive to light and accumulates electrons. Then, the selection signal SEL is set to a high level to enter the reading stage, the reset signal RX becomes a high level to reset the floating node FD, and then the reset signal RX is changed to a low level, and the transmission signal TX remains at a low level. At this time, the amplifier tube Msf is controlled by the potential of the floating node FD, and outputs the reset potential VRST through the output terminal PIX_OUT; after that, the transmission signal TX is set to a high level to transfer the electrons on the photodiode PD to the floating node FD, and then the transmission signal TX is changed to a low level. At this time, the amplifier tube Msf is controlled by the potential of the floating node FD, and outputs the exposure integration signal VSIG through the output terminal PIX_OUT. The reset potential VRST and the exposure integration signal VSIG are converted into digital quantities by the analog-to-digital conversion unit ADC and subtracted to obtain the digital quantity actually corresponding to the photoelectrons on the photodiode PD.

[0025] This application also provides a standard four-pixel unit circuit structure, see Figure 2 As shown, it includes a photodiode PD, a transmission tube Mtg, a reset tube Mrst, an amplifier tube Msf and a selection tube Msel.

[0026] The positive electrode of the photodiode PD is grounded, the negative electrode of the photodiode PD is connected to the source of the transmission tube Mtg, and the gate of the transmission tube Mtg is connected to the output end of the row selection decoding driver to receive the transmission signal TX provided by the row selection decoding driver. The drain of the transmission tube Mtg is connected to the source of the reset tube Mrst and leads to a floating node FD, and the gate of the reset tube Mrst is connected to the output end of the row selection decoding driver to receive the reset signal RX provided by the row selection decoding driver. The drain of the reset tube Mrst is connected to the power supply VDD, the drain of the amplifier tube Msf is connected to the power supply VDD, the gate of the amplifier tube Msf is connected to the floating node FD, the source of the amplifier tube Msf is connected to the drain of the selection tube Msel, and the gate of the selection tube Msel is connected to the output end of the row selection decoding driver to receive the selection signal SEL provided by the row selection decoding driver. The source of the selection tube Msel is the output end PIX_OUT.

[0027] Among them, the photodiode PD is sensitive to light and generates photoelectrons that are proportional to the light intensity. The function of the transmission tube Mtg is to transfer the photoelectrons in the photodiode PD. When the transmission signal TX is at a high level, the transmission tube Mtg is turned on, and the photoelectrons in the photodiode PD are transferred to the floating node FD. The function of the reset tube Mrst is to reset the floating node FD when the reset signal RX is at a high level. When the selection signal SEL is at a high level and the selection tube Msel is turned on, the amplifier tube Msf, the selection tube Msel and the current source to the ground form a path. At this time, the amplifier tube Msf is essentially a source follower, which follows the change in the potential of the floating node FD and is finally output by the output terminal PIX_OUT.

[0028] With the improvement of application requirements, not only is the CIS design required to be gradually miniaturized, but also a higher dynamic range is required. However, the continuous reduction of pixel units will cause the photosensitivity to decline. To achieve both small area and high dynamic range at the same time, the general approach is to improve the photodiode PD performance by optimizing the process, which often leads to an increase in processing costs.

[0029] In view of the problems existing in the prior art, an embodiment of the present invention provides an image sensor readout method, which is applied to an image sensor, referring to Figure 3As shown, including the following:

[0030] In the reset stage, the control selection signal SEL is set to a low level, and the reset signal RX and the transmission signal TX are both set to a high level, so that the transmission tube Mtg, the reset tube Mrst and the amplifier tube Msf are all turned on, and the pixel unit is reset.

[0031] When in use, the selection signal SEL is set to a low level, and the amplifier tube Msf is controlled to be disconnected; the transmission tube Mtg, the reset tube Mrst and the amplifier tube Msf are all turned on, at this time, the pixel unit is reset, and the floating node FD is also reset; wherein, the potentials of the selection signal SEL, the reset signal RX and the transmission signal TX are all controlled by a timing controller.

[0032] When the pixel unit is reset, the transmission signal TX is controlled to switch from a high level to a first low level, so that the transmission tube Mtg is disconnected.

[0033] When in use, the pixel unit is reset in the above steps. When the transmission signal TX switches from a high level to a first low level, the transmission tube Mtg is disconnected, and the disconnection of the transmission tube Mtg can prevent the electrons generated on the photodiode PD from entering the floating node FD.

[0034] In the exposure stage, the selection signal SEL is kept at a low level, and the transmission signal TX is kept at a first low level, and the reset signal RX is controlled to switch from a high level to a low level, so that the reset tube Mrst and the transmission tube Mtg are disconnected, the selection tube Msel is turned on, and the photodiode PD starts to expose and accumulate electrons.

[0035] When in use, the disconnection of the reset tube Mrst and the transmission tube Mtg can prevent the electrons generated on the photodiode PD from escaping. Specifically, the photodiode PD stores the generated electrons, and after the electrons generated on the photodiode PD reach saturation, since the transmission tube Mtg is disconnected, the electrons generated on the photodiode PD will not escape to the floating node FD.

[0036] When the photodiode PD starts to be exposed and accumulate electrons, the transmission signal TX is controlled to be pulled up from the first low level to the second low level, so that the photodiode PD is saturated and part of the electrons overflow to the floating node FD.

[0037] When in use, the transmission signal TX is pulled up from the first low level to the second low level, and the transmission tube Mtg can be controlled to open partially, so that after the electrons generated on the photodiode PD reach saturation, as the electrons continue to be generated, some electrons will overflow to the floating node FD to increase the potential of the floating node FD; specifically, the data of the transmission signal TX being pulled up from the first low level to the second low level changes as follows, the value range of the first low level is [-1V, -0.5V], and the value range of the second low level is [-0.4V, -0.1V]. It can be seen from the above data that the potential of the transmission signal TX after being pulled up is still at a low level, so as to ensure that the photodiode PD will only dissipate the saturated electrons to the floating node FD.

[0038] In the signal reading stage, the selection signal SEL is controlled to be set to a high level, so that the selection tube Msel is turned on, and the overflow signal potential VOF stored on the floating node FD is read out.

[0039] Then, the reset signal RX is controlled to switch from a low level to a high level, so that the reset tube Mrst is turned on, the floating node FD is reset, and the reset potential VRST is read out.

[0040] Then, the transmission signal TX is controlled to switch from a low level to a high level, so that the transmission tube Mtg is turned on and the exposure integration signal VSIG is read out.

[0041] In the signal reading stage, the time axis is divided into the first time period, the second time period, the third time period, the fourth time period and the fifth time period from left to right; specifically, in the first time period, the selection signal SEL is at a high level, then the selection tube Msel is turned on; the transmission signal TX is at a low level, then the transmission tube Mtg is disconnected; the reset signal RX is at a low level, then the reset tube Mrst is disconnected; when in use, due to the disconnection of the transmission tube Mtg and the reset tube Mrst, the selection tube Msel is turned on, the floating node FD is connected to the output terminal PIX_OUT, and at this time the amplifier tube Msf is controlled by the potential of the floating node FD, and outputs the overflow signal potential VOF through the output terminal PIX_OUT.

[0042] In the second time period, the selection signal SEL is at a high level, then the selection tube Msel is turned on; the transmission signal TX is at a low level, then the transmission tube Mtg is turned off; the reset signal RX is at a high level, then the reset tube Mrst is turned on; the setting of this time period is used to reset the pixel unit. Specifically, after detecting the overflow signal potential VOF, by controlling the conduction of the selection tube Msel and the reset tube Mrst, and disconnecting the transmission tube Mtg, the floating node FD is reset and pulled up to VDD.

[0043] In the third time period, the selection signal SEL is at a high level, then the selection tube Msel is turned on; the transmission signal TX is at a low level, then the transmission tube Mtg is turned off; the reset signal RX is at a low level, then the reset tube Mrst is turned off; this time period is set to detect the reset potential VRST; specifically, in the above-mentioned second time period, the floating node FD is reset, and the transmission tube Mtg and the reset tube Mrst are both in a disconnected state. At this time, the amplifier tube Msf is controlled by the potential of the floating node FD, and outputs the reset potential VRST through the output terminal PIX_OUT.

[0044] In the fourth time period, the selection signal SEL is at a high level, then the selection tube Msel is turned on; the reset signal RX is at a low level, then the reset tube Mrst is turned off; the transmission signal TX is at a high level, then the transmission tube Mtg is turned on; the conduction of the transmission tube Mtg will cause the electrons at the photodiode PD to move to the floating node FD.

[0045] In the fifth time period, the selection signal SEL is at a high level, then the selection tube Msel is turned on; the reset signal RX is at a low level, then the reset tube Mrst is turned off; the transmission signal TX is at a low level, then the transmission tube Mtg is turned off; in the fourth time period, there are electrons moving from the photodiode PD at the floating node FD. In this time period, when the transmission tube Mtg and the reset tube Mrst are both turned off, the amplifier tube Msf is controlled by the potential of the floating node FD, and outputs the exposure integration signal VSIG through the output terminal PIX_OUT.

[0046] In this embodiment, the method further includes:

[0047] The analog-to-digital conversion unit converts the overflow signal potential VOF, the reset potential VRST and the exposure integration signal VSIG into digital quantities, performs a subtraction operation on them, and obtains an actual corresponding digital quantity, and outputs the actual corresponding digital quantity.

[0048] The voltage output by the pixel unit DOUT = (VRST-VOF) + (VRST-VSIG) × 2 N / VREF;

[0049] Among them, VREF represents the reference voltage range of the analog-to-digital conversion unit, and N is the bit width of the analog-to-digital conversion unit.

[0050] The present application also provides an image sensor, including a pixel array, an analog-to-digital conversion unit, a reference signal generator, a timing controller, a row selection decoding driver and an output signal processor, wherein the timing controller is used to control the image sensor to execute the method described above.

[0051] In the present application, before outputting the reset potential VRST and the exposure integration signal VSIG, the low level of the transmission signal TX is increased so that the saturated electrons on the photodiode PD overflow to the floating node FD, thereby changing the potential of the floating node FD and outputting the overflow signal potential VOF, so that the final readout pixel signal amount increases VRST-VOF compared to the traditional standard four-tube pixel unit circuit, thereby expanding the dynamic range of the pixel readout signal, thereby eliminating the need to optimize the structure of the photodiode PD and reducing processing costs.

[0052] Although the embodiments of the present invention are described in detail above, it is obvious to those skilled in the art that various modifications and variations can be made to these embodiments. However, it should be understood that such modifications and variations are within the scope and spirit of the present invention as described in the claims. Moreover, the present invention described herein may have other embodiments and may be implemented or realized in a variety of ways.

Claims

1. An image sensor readout method, applied to an image sensor, characterized in that: include: In the reset stage, the control selection signal SEL is set to a low level, the reset signal RX and the transmission signal TX are both set to a high level, so that the transmission tube Mtg, the reset tube Mrst and the amplifier tube Msf are all turned on, and the pixel unit is reset; When the pixel unit is reset, the transmission signal TX is controlled to switch from a high level to a first low level, so that the transmission tube Mtg is disconnected; In the exposure stage, the selection signal SEL is kept at a low level, and the transmission signal TX is kept at a first low level, and the reset signal RX is controlled to switch from a high level to a low level, so that the reset tube Mrst and the transmission tube Mtg are disconnected, the selection tube Msel is turned on, and the photodiode PD starts to expose and accumulate electrons; When the photodiode PD starts to be exposed and accumulates electrons, the transmission signal TX is controlled to be pulled up from the first low level to the second low level, so that the photodiode PD is saturated and part of the electrons overflow to the floating node FD; In the signal reading stage, the selection signal SEL is controlled to be set to a high level, so that the selection tube Msel is turned on, and the overflow signal potential VOF stored on the floating node FD is read out; Then, the reset signal RX is controlled to switch from a low level to a high level, so that the reset tube Mrst is turned on, the floating node FD is reset, and the reset potential VRST is read out; Then the control transmission signal TX is switched from low level to high level, so that the transmission tube Mtg is turned on and the exposure integral signal VSIG is read out; The analog-to-digital conversion unit converts the overflow signal potential VOF, the reset potential VRST and the exposure integration signal VSIG into digital quantities, performs a subtraction operation on them, and obtains an actual corresponding digital quantity, and outputs the actual corresponding digital quantity; The pixel unit output voltage DOUT=(VRST-VOF)+(VRST-VSIG)×2 N / VREF; wherein VREF represents the reference voltage range of the analog-to-digital conversion unit, and N is the bit width of the analog-to-digital conversion unit.

2. The method according to claim 1, characterized in that The value range of the first low level is [-1V, -0.5V], and the value range of the second low level is [-0.4V, -0.1V].

3. An image sensor, characterized in that: The image sensor comprises a pixel array, an analog-to-digital conversion unit, a reference signal generator, a timing controller, a row selection decoding driver and an output signal processor, wherein the timing controller is used to control the image sensor to execute the method described in any one of claims 1 to 2.

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