Ramp signal control method of image sensor

Abstract

The invention provides a ramp signal control method for an image sensor, and the method comprises the steps: providing a ramp signal curve comprising at least one linear interval and at least one nonlinear interval through a same ramp signal generation circuit, and carrying out the analog-digital conversion according to the precision and speed requirements of the image sensor for analog-digital conversion, at least part of intervals of the ramp signal curve are selected to output ramp signals, preferably, the higher the precision requirement of analog-to-digital conversion is, the smaller the average slope of the intervals outputting the ramp signals is, and the higher the speed requirement of analog-to-digital conversion is, the larger the average slope of the intervals outputting the ramp signals is. Therefore, compromise of precision and speed is realized according to application flexibility, and meanwhile, the circuit area is saved, and the manufacturing cost is reduced.

Description

Technical Field

[0001] The invention relates to a ramp signal control method for an image sensor. Background Art

[0002] In the field of image sensors, column-level analog-to-digital converters (CLADCs) are widely used in high-pixel and high-frame-rate image sensors due to their low power consumption and small area.

[0003] Some module structures of image sensors using column-level analog-to-digital converters are as follows: Figure 1 The analog-to-digital converter is mainly composed of a comparator and a storage unit or a counter. The basic structure of the comparator is as follows: Figure 2 As shown, the basic working waveform of the comparator is as follows Figure 3 As shown in Figure 2, a ramp signal generator circuit is required in an ADC to provide a comparison reference level for the comparator. The accuracy of the ramp signal directly affects the output accuracy of the ADC. With the increasing demand for image sensor applications, high-speed, high-precision ramp signals are becoming a trend. Therefore, generating high-speed, high-precision ramp signals has become a key research topic in recent years.

[0004] The structure of the ramp signal generating circuit is as follows Figure 4 As shown in the figure, it mainly consists of a control circuit, a row and column decoding circuit, a current source array, and a current-to-voltage conversion circuit. The control circuit controls the output of the decoding circuit to turn on or off each current mirror unit in the current source array, thereby changing the current flowing through the current-to-voltage conversion circuit and ultimately generating a ramp signal VRAMP.

[0005] At present, the linear ramp signal generating circuit with a single slope can no longer meet the application requirements of high-speed and high-precision image sensors. One implementation method of the nonlinear ramp signal generating circuit is as follows: Figure 5 As shown, it includes a bias circuit 1011, a state storage circuit 1012, a current mirror array 102, and a resistor as a current-to-voltage circuit 103. By gradually changing the current of each current mirror unit 1020~102(i-1) in the current source array 102, the change in the magnitude of the current Iout flowing to the current-to-voltage circuit 103 within a unit cycle time is adjusted, and a nonlinear ramp signal with a variable slope can be generated. When using a nonlinear ramp signal, on the one hand, it can greatly shorten the time required for the ramp signal to cover the entire amplitude range, meeting high-speed application scenarios; on the other hand, when the consumption time is the same, the nonlinear ramp signal can achieve higher accuracy requirements, meeting high-precision application scenarios. The waveform diagram of the nonlinear ramp signal and the linear ramp signal is shown in FIG. Figure 6 and Figure 7 shown.

[0006] Therefore, in the ramp signal generating circuit of the prior art, two sets of circuit structures are usually provided for each current mirror unit to respectively realize the provision of linear ramp signal and nonlinear ramp signal, so as to flexibly meet different application requirements. Figure 8 In the illustrated ramp signal generation circuit, each current mirror unit includes a nonlinear ramp current source transistor MP0 and a linear ramp current source transistor MP1. Switch group 1014 is controlled to select one of VG1_N and VG1_L to be connected to bias voltage VG1, while the other is connected to power supply voltage VDD. VG1_N represents the gate voltage of the nonlinear ramp current source transistor MP0, and VG1_L represents the gate voltage of the linear ramp current source transistor MP1. The current source transistor with the gate connected to bias voltage VG1 is turned on, while the current source transistor with the gate connected to power supply voltage VDD is turned off. When the nonlinear ramp current source transistor MP0 is turned on, the output current Iout of the current mirror array 102 varies non-uniformly over time, generating a nonlinear ramp signal. When the linear ramp current source transistor MP1 is turned on, the output current Iout of the current mirror array 102 varies uniformly over time, generating a linear ramp signal. However, since the linear ramp signal and the nonlinear ramp signal are provided through two separate circuit structures, this results in waste in terms of circuit area and manufacturing cost. Summary of the Invention

[0007] The object of the present invention is to provide a ramp signal control method for an image sensor, which can flexibly achieve a compromise between analog-to-digital conversion accuracy and speed according to the application, and can also save circuit area and reduce manufacturing costs.

[0008] Based on the above considerations, the present invention provides a ramp signal control method for an image sensor. The image sensor includes a ramp signal generating circuit, which can provide a ramp signal curve including at least one linear interval and at least one nonlinear interval. The ramp signal generating circuit selects at least a portion of the intervals of the ramp signal curve to output a ramp signal based on the accuracy and speed requirements of analog-to-digital conversion performed by the image sensor.

[0009] Preferably, the higher the accuracy requirement of the image sensor for analog-to-digital conversion, the smaller the average slope of the interval of the output ramp signal; the faster the speed requirement of the image sensor for analog-to-digital conversion, the larger the average slope of the interval of the output ramp signal.

[0010] Preferably, the interval of the output ramp signal includes the linear interval and / or the nonlinear interval.

[0011] Preferably, the ramp signal curve is a curve with an upward slope or a downward slope.

[0012] Preferably, the ramp signal generating circuit comprises a control circuit, a current mirror array and a current-voltage conversion circuit, the control circuit adjusts the output current of the current mirror array by controlling the on-off of the first switch unit inside each current mirror unit of the current mirror array, and the current-voltage conversion circuit converts the output current of the current mirror array into voltage, thereby outputting the ramp signal.

[0013] Preferably, when the output current of the current mirror array changes uniformly with time, it corresponds to the linear interval of the ramp signal curve; when the output current of the current mirror array changes non-uniformly with time, it corresponds to the nonlinear interval of the ramp signal curve.

[0014] Preferably, by controlling the on-off of the first switch unit inside each current mirror unit, at least part of the interval of the ramp signal curve is selected to output the ramp signal.

[0015] Preferably, a second switch unit is arranged at the input end, output end or inside of each current mirror unit, and by controlling the on-off of the second switch unit, at least part of the interval of the ramp signal curve is selected to output the ramp signal.

[0016] The ramp signal control method of the image sensor of the present application provides a ramp signal curve comprising at least one linear interval and at least one nonlinear interval by using the same set of ramp signal generating circuit, and according to the accuracy and speed requirements of the analog-digital conversion of the image sensor, at least part of the interval of the ramp signal curve is selected to output the ramp signal, preferably, the higher the accuracy requirement of the analog-digital conversion, the smaller the average slope of the interval of the output ramp signal, and the faster the speed requirement of the analog-digital conversion, the larger the average slope of the interval of the output ramp signal, thereby flexibly realizing the trade-off between accuracy and speed according to the application, while saving the circuit area and reducing the manufacturing cost. BRIEF DESCRIPTION OF DRAWINGS

[0017] Other features, objects and advantages of the present application will become more apparent from the following detailed description of non-limiting embodiments with reference to the attached drawings.

[0018] Figure 1 Fig. 1 is a schematic diagram of the structure of the prior art image sensor; Figure 2 Fig. 2 is a schematic diagram of the structure of the prior art comparator; Figure 3 Fig. 3 is a working waveform diagram of the prior art comparator; Figure 4 Fig. 4 is a schematic diagram of the structure of the prior art ramp signal generating circuit; Figure 5 Fig. 5 is a circuit diagram of the prior art nonlinear ramp signal generating circuit; Figure 6Waveform diagram of nonlinear ramp signal and linear ramp signal in high-speed application scenario; Figure 7 Waveform diagram of nonlinear ramp signal and linear ramp signal in high-precision application scenario; Figure 8 Circuit diagram of ramp signal generation circuit in prior art; Figure 9 Waveform diagram of ramp signal curve formed according to one embodiment of the present application; Figure 10 Circuit diagram of ramp signal generation circuit according to one embodiment of the present application; Figure 11 Circuit diagram of ramp signal generation circuit according to another embodiment of the present application; Figure 12 Circuit diagram of ramp signal generation circuit according to still another embodiment of the present application.

[0019] In the drawings, like or similar elements are designated with the same or similar reference numerals throughout the several views. DETAILED DESCRIPTION

[0020] To solve the problems in the prior art, the present application provides a ramp signal curve including at least one linear interval and at least one nonlinear interval by using the same set of ramp signal generation circuit, and according to the precision and speed requirements of the analog-to-digital conversion of the image sensor, at least part of the intervals of the ramp signal curve are selected to output the ramp signal, preferably, the higher the precision requirement of the analog-to-digital conversion, the smaller the average slope of the interval of the output ramp signal, and the faster the speed requirement of the analog-to-digital conversion, the larger the average slope of the interval of the output ramp signal, so that the trade-off between the precision and the speed is flexibly realized according to the application, and the effects of saving circuit area and reducing manufacturing cost are achieved.

[0021] In the following detailed description of the preferred embodiments, reference will be made to the accompanying drawings which form a part hereof. The accompanying drawings illustrate specific embodiments in which the present application can be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the present application. Other embodiments can be utilized and structural, or logical changes can be made without departing from the scope of the present application. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present application is defined by the appended claims.

[0022] The present application will be described in detail below with reference to specific embodiments.

[0023] The present application provides a slope signal control method of an image sensor, the image sensor comprising a slope signal generating circuit, the slope signal generating circuit being capable of providing a slope signal curve comprising at least one linear interval and at least one nonlinear interval. For example, Figure 9 A slope signal curve formed according to one embodiment of the present application is shown, wherein interval ① is a linear interval, interval ① is followed by a nonlinear interval, and a curve with a downward slope is taken as an example for illustration, it can be understood by those skilled in the art that a curve with an upward slope can also be applicable to the slope signal control method of the present application, and the specific forms (such as time length, slope, sequence, etc.) of the linear interval and the nonlinear interval can have various changes, and those skilled in the art can set them according to actual needs, and the present application is not limited in this way.

[0024] In practical applications, the slope signal generating circuit can select at least part of the intervals of the slope signal curve to output a slope signal according to the accuracy and speed requirements of the image sensor for analog-to-digital conversion. Preferably, the higher the accuracy requirement of the image sensor for analog-to-digital conversion, the smaller the average slope of the interval of the output slope signal; the faster the speed requirement of the image sensor for analog-to-digital conversion, the larger the average slope of the interval of the output slope signal; thereby flexibly achieving a trade-off between accuracy and speed according to the application, while having the effects of saving circuit area and reducing manufacturing cost.

[0025] Specifically, the start and end positions, duration and accuracy of the interval of the output slope signal can be adjusted as needed, and can include the linear interval and / or the nonlinear interval. For example, in the embodiment shown, Figure 9 In the embodiment shown, the linear n bit slope signal can be output by selecting interval ① of the curve, the n+1 bit slope signal can be output by selecting interval ③ of the curve, the n+2 bit slope signal can be output by selecting interval ④ of the curve, and the n+3 bit slope signal can be output by selecting interval ⑤ of the curve, thereby flexibly achieving a trade-off between accuracy and speed according to the application. In addition to starting from the beginning of the slope signal curve, the slope signal of any bit can also be output by selecting interval ② in the middle part of the curve.

[0026] The slope signal control method of the image sensor of the present application can realize the above-mentioned self-defined interval selection of accuracy and speed by controlling the on-off of the current mirror units in the current mirror array through the control circuit based on the same set of slope generating circuit. For example, to realize interval ②, the current mirror units other than interval ② can be always off, and the current mirror units can be gradually turned off starting from interval ② to realize the selection of the self-defined interval.

[0027] For example, for Figure 5The nonlinear ramp generation circuit shown in the embodiment, the control circuit 101 controls the on-off of the first switch unit (shown as transistors MP3, MP4 here) in each current mirror unit 1020~102(i-1) in the current mirror array 102, adjusts the output current Iout of the current mirror array 102, and then converts the output current Iout of the current mirror array 102 into a voltage through the current-voltage conversion circuit 103, so as to output the ramp signal VRAMP. When the output current Iout of the current mirror array changes uniformly with time, it corresponds to the linear interval of the ramp signal curve; when the output current Iout of the current mirror array changes non-uniformly with time, it corresponds to the nonlinear interval of the ramp signal curve. Therefore, the on-off of the first switch unit MP3, MP4 in each current mirror unit can be directly controlled by the state storage circuit 1012 of the control circuit 101, and the ramp signal can be output in at least part of the interval of the ramp signal curve. In addition, local adjustments can also be made on the basis of the circuit architecture shown in Figure 5 The second switch unit 2040~204(i-1) is arranged at the output end of the current mirror unit 2020~202(i-1); in the embodiment shown in Figure 10 The second switch unit 2040~204(i-1) is arranged in the current mirror unit 2020~202(i-1); in the embodiment shown in Figure 11 The second switch unit 2040~204(i-1) is arranged in the current mirror unit 2020~202(i-1); in the embodiment shown in Figure 12 The second switch unit 2040~204(i-1) is arranged in the current mirror unit 2020~202(i-1); in the embodiment shown in

[0028] In summary, the ramp signal control method of the image sensor of the present application provides a ramp signal curve including at least one linear interval and at least one nonlinear interval through the same set of ramp signal generation circuit, selects at least part of the interval of the ramp signal curve to output the ramp signal according to the accuracy and speed requirements of the analog-to-digital conversion of the image sensor, and preferably, the higher the accuracy requirement of the analog-to-digital conversion, the smaller the average slope of the interval of the output ramp signal, and the faster the speed requirement of the analog-to-digital conversion, the larger the average slope of the interval of the output ramp signal, so as to flexibly realize the trade-off between accuracy and speed according to the application, while saving circuit area and reducing manufacturing cost.

[0029] It will be obvious to a person skilled in the art that the application is not limited to the details of the foregoing exemplary embodiments and can be implemented in other concrete forms without departing from the spirit or general characteristics of the application. Accordingly, the embodiments should be considered in all respects as illustrative and not restrictive. Moreover, it must be stressed that the word "comprising" does not exclude other elements and steps and the word "a" or "an" does not exclude a plurality. A single processor or other unit can fulfil the functions of several units recited in the device claims. The terms first, second and the like do not denote any ordinal number, but are used to distinguish different steps of the method.

Claims

1.A method for controlling a ramp signal of an image sensor, the method comprising: providing a ramp signal curve comprising at least one linear section and at least one non-linear section; and selecting at least a part of the ramp signal curve to output a ramp signal according to a precision and a speed requirement of an analog-to-digital conversion of the image sensor; wherein the higher the precision requirement of the analog-to-digital conversion of the image sensor, the smaller the average slope of the selected section of the ramp signal curve; and the faster the speed requirement of the analog-to-digital conversion of the image sensor, the larger the average slope of the selected section of the ramp signal curve; wherein the selected section of the ramp signal curve comprises the linear section and / or the non-linear section; wherein the ramp signal curve is a curve with a positive slope or a curve with a negative slope; wherein the ramp signal is output by a ramp signal generation circuit comprising a control circuit, a current mirror array and a current-to-voltage conversion circuit, the control circuit adjusts an output current of the current mirror array by controlling a first switch unit in each current mirror unit of the current mirror array, and the current-to-voltage conversion circuit converts the output current of the current mirror array into a voltage to output the ramp signal; wherein the output current of the current mirror array changes uniformly over time corresponding to the linear section of the ramp signal curve, and the output current of the current mirror array changes non-uniformly over time corresponding to the non-linear section of the ramp signal curve; wherein the control circuit selects at least a part of the ramp signal curve to output the ramp signal by controlling the first switch unit in each current mirror unit of the current mirror array; and / or wherein a second switch unit is provided at an input, an output or inside each current mirror unit, and the control circuit selects at least a part of the ramp signal curve to output the ramp signal by controlling the second switch unit. ​ ​ 2. The slope signal control method of an image sensor according to claim 1, wherein ​ 3. The slope signal control method of an image sensor according to claim 1, wherein ​ 4. The slope signal control method of an image sensor according to claim 1, wherein ​ 5. The slope signal control method of an image sensor according to claim 1, wherein ​ 6. The slope signal control method of an image sensor according to claim 5, wherein ​ 7. The slope signal control method of an image sensor according to claim 5, wherein ​ 8. The slope signal control method of an image sensor according to claim 5, wherein ​

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