HK1221780B

HK1221780B - Programmable current source for a time of flight 3d image sensor

Info

Publication number: HK1221780B
Application number: HK16109858.4A
Authority: HK
Inventors: 孙天佳; 王睿; 代铁军
Original assignee: 豪威科技股份有限公司
Priority date: 2014-08-20
Filing date: 2016-08-17
Publication date: 2018-04-27

Abstract

The subject application relates to programmable current source for a time of flight 3D image sensor. A programmable current source for use with a time of flight pixel cell includes a first transistor. A current through the first transistor is responsive to a gate-source voltage of the first transistor. A current control circuit is coupled to the first transistor and coupled to a reference current source to selectively couple a reference current of the reference current source through the first transistor during a sample operation. A sample and hold circuit is coupled to the first transistor to sample a gate-source voltage of the first transistor during the sample operation. The sample and hold circuit is coupled to hold the gate-source voltage during a hold operation after the sample operation substantially equal to the gate-source voltage during the sample operation. A hold current through the first transistor during the hold operation is substantially equal to the reference current.

Description

Programmable current source for time-of-flight 3D image sensor

Technical Field

The present invention relates to an image sensor. In particular, embodiments of the present invention relate to three-dimensional image sensors.

Background

As the popularity of three-dimensional (3D) applications continues to grow in applications such as imaging, movies, games, computers, user interfaces, and the like, interest in 3D cameras is increasing. A typical passive way of generating 3D images is to capture a stereoscopic image or multiple images using multiple cameras. Using stereoscopic images, objects in the images may be triangulated to produce a 3D image. One drawback of this triangulation (triangulation) technique is that it is difficult to generate 3D images using small devices because there must be a minimum separation distance between each camera in order to generate a three-dimensional image. Furthermore, this technique is complex and therefore requires significant computer processing power in order to generate the 3D image in real time.

For applications requiring real-time acquisition of 3D images, active depth imaging systems based on optical time-of-flight measurements are sometimes utilized. Time-of-flight systems typically employ: a light source that directs light at an object; a sensor that directs light reflected from the object; and a processing unit that calculates a distance to an object based on a round trip time of light traveling to and returning from the object. In a typical time-of-flight sensor, a photodiode is often used due to high transfer efficiency from the light detection region to the sensing node. Separate circuitry is coupled to the photodiode in each pixel cell to detect and measure the light reflected from the object.

However, a continuing challenge with acquiring 3D images using time-of-flight systems is the presence of pixel-by-pixel variations in the individual circuits coupled to the photodiodes in each pixel cell. For example, it is not uncommon for there to be a difference of approximately 5% between the pixel cell current mirror outputs across a time-of-flight sensor due to pixel-by-pixel variations that may occur in the sensor. These pixel-by-pixel variations in the current mirror output thus reduce the accuracy and reliability of the time-of-flight sensor.

Disclosure of Invention

In one aspect, the present application relates to a programmable current source for use with a time-of-flight pixel cell. The programmable current source for use with a time-of-flight pixel cell comprises: a first transistor having a gate terminal, a source terminal, and a drain terminal, wherein a current through the first transistor is responsive to a gate-source voltage of the first transistor; a current control circuit coupled to the first transistor and to a reference current source, wherein the current control circuit is coupled to selectively couple a reference current of the reference current source through the first transistor during a sampling operation; and a sample and hold circuit coupled to the first transistor, wherein the sample and hold circuit is coupled to sample a gate-source voltage of the first transistor during the sample operation, wherein the sample and hold circuit is coupled to hold the gate-source voltage during a hold operation subsequent to the sample operation substantially equal to the gate-source voltage during the sample operation, wherein a hold current through the first transistor during the hold operation is substantially equal to the reference current.

In another aspect, the present application relates to a time-of-flight pixel cell. The time-of-flight pixel cell includes: a light sensor for sensing photons reflected from an object; and a pixel support circuit, comprising: a timing control logic coupled to the light sensor to detect when the light sensor senses the photon reflected from the object, wherein the timing control logic is further coupled to receive a timing signal indicative of when a light pulse is emitted from a light source to the object, wherein the timing control logic is coupled to generate a time-of-flight signal indicative of a time-of-flight measurement operation of the time-of-flight pixel cell; a programmable current source coupled to the timing control logic to provide a hold current in response to the time of flight signal coupled to be received from the timing control logic, wherein the programmable current source includes: a first transistor having a gate terminal, a source terminal, and a drain terminal, wherein a current through the first transistor is responsive to a gate-source voltage of the first transistor; a current control circuit coupled to the first transistor and to a reference current source, wherein the current control circuit is coupled to selectively couple a reference current of the reference current source through the first transistor during a sampling operation; and a sample and hold circuit coupled to the first transistor, wherein the sample and hold circuit is coupled to sample a gate-source voltage of the first transistor during the sample operation, wherein the sample and hold circuit is coupled to hold the gate-source voltage during a hold operation subsequent to the sample operation substantially equal to the gate-source voltage during the sample operation, wherein a hold current through the first transistor during the hold operation is substantially equal to the reference current; and a time-of-flight capacitor coupled to the current control circuit to be selectively charged by the holding current in response to the time-of-flight signal, wherein a voltage on the time-of-flight capacitor represents a round-trip distance to the object.

In another aspect, the present application relates to a time of flight sensing system. The time-of-flight sensing system comprises: a light source for emitting light pulses to an object; a reference current source having a reference current; a time-of-flight pixel array having a plurality of time-of-flight pixel cells, wherein each of the time-of-flight pixel cells comprises: a light sensor for sensing photons reflected from the object; a timing control logic coupled to the light sensor to detect when the light sensor senses the photon reflected from the object, wherein the timing control logic is further coupled to receive a timing signal indicative of when a light pulse is emitted from the light source to the object, wherein the timing control logic is coupled to generate a time-of-flight signal indicative of a time-of-flight measurement operation of the time-of-flight pixel array; a programmable current source coupled to the timing control logic to provide a hold current in response to the time of flight signal coupled to be received from the timing control logic, wherein the programmable current source includes: a first transistor having a gate terminal, a source terminal, and a drain terminal, wherein a current through the first transistor is responsive to a gate-source voltage of the first transistor; a current control circuit coupled to the first transistor and to the reference current source, wherein the current control circuit is coupled to selectively couple the reference current of the reference current source through the first transistor during a sampling operation; and a sample and hold circuit coupled to the first transistor, wherein the sample and hold circuit is coupled to sample a gate-source voltage of the first transistor during the sample operation, wherein the sample and hold circuit is coupled to hold the gate-source voltage during a hold operation subsequent to the sample operation substantially equal to the gate-source voltage during the sample operation, wherein a hold current through the first transistor during the hold operation is substantially equal to the reference current; a time-of-flight capacitor coupled to the current control circuit to be selectively charged by the holding current in response to the time-of-flight signal, wherein a voltage on the time-of-flight capacitor represents a round-trip distance to the object; a control circuit coupled to the light source and to the time-of-flight pixel array to synchronize the emission of the light pulses with the timing of the sensing of the photons reflected from the object.

Drawings

Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise indicated.

FIG. 1 is a block diagram showing one example of a time-of-flight sensing system according to the teachings of this disclosure.

FIG. 2 is a block diagram showing an example of a cross section of a time-of-flight sensing system implemented with a pixel die coupled to an Application Specific Integrated Circuit (ASIC) die in a stacked-chip scheme, according to the teachings of this disclosure.

FIG. 3 is a schematic diagram illustrating one example of a time-of-flight pixel according to the teachings of this disclosure.

FIG. 4 is a timing diagram showing an emitted light pulse, respective reflected photons sensed by a photosensor, time-of-flight signals representative of photons reflected from an object, and corresponding voltages accumulated on capacitors in example time-of-flight pixels, according to the teachings of this disclosure.

Figure 5 is a schematic diagram illustrating an example of programmable current sources included in a plurality of time-of-flight pixel cells coupled to a single reference current source, according to the teachings of this disclosure.

FIG. 6 is a block diagram showing a portion of an example time-of-flight sensing system including a time-of-flight pixel array with corresponding readout circuitry, control circuitry, and functional logic, according to the teachings of this disclosure.

Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful and necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.

Detailed Description

Methods and apparatus for programming a current source using a single reference current source in a time-of-flight pixel cell of a 3D time-of-flight sensor are disclosed. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments. However, one skilled in the relevant art will recognize that the technology described herein can be practiced without one or more of the specific details, or with other methods, components, or materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

Several technical terms are used throughout this specification. These terms have their ordinary meaning in the art from which they come, unless explicitly defined herein or the context of their use clearly dictates otherwise. For example, the term "or" is used in an inclusive sense (e.g., as "and/or") unless the context clearly dictates otherwise.

As will be shown, examples of time-of-flight sensing systems including time-of-flight pixel cells are disclosed. In various examples, a time-of-flight pixel cell according to the teachings of this disclosure includes a programmable current source that is programmed using the same single current reference source. Thus, according to the teachings of this disclosure, each of the individual current sources in the pixel support circuitry included in each time-of-flight pixel cell provides substantially equal current to each pixel cell, even if there are pixel-by-pixel variations that may occur between matched transistors across the entire time-of-flight pixel cell array.

To illustrate, FIG. 1 is a block diagram showing one example of a time-of-flight sensing system 100 according to the teachings of this disclosure. As shown, the time of flight sensing system 100 includes a light source 102, the light source 102 emitting a pulse of light, illustrated in fig. 1 as emitted light 104. As shown, the emitted light 104 is directed to an object 106. In one example, the emitted light 104 includes pulses of Infrared (IR) light. It is understood that in other examples, the emitted light 104 may have a wavelength other than infrared, such as visible light, near infrared light, and the like, in accordance with the teachings of this disclosure. The emitted light 104 is then reflected back from the object 106, which is shown in fig. 1 as back reflected light 108. As shown, the reflected light 108 is directed from the object 106 through a lens 110 and then the reflected light 108 is focused onto a time-of-flight pixel array 112. In one example, time-of-flight pixel array 112 includes a plurality of time-of-flight pixel cells arranged in a two-dimensional array. According to the teachings of this disclosure, a synchronization signal 114 is generated by a control circuit 116 coupled to the time-of-flight pixel array 112, the synchronization signal 114 synchronizing the pulses of emitted light 104 with corresponding signals controlling a plurality of pixel cells in the time-of-flight pixel array 112 that sense the reflected light 108.

In the example depicted in fig. 1, it should be noted that time-of-flight pixel array 112 is positioned a focal length f from lens 110_lens. As shown in the example, the light source 102 and the lens 110 are positioned a distance L from the object. In one example, it should be noted that lens 110 may be implemented using a plurality of microlenses integrated into time-of-flight pixel array 112. It should be understood, of course, that FIG. 1 is not to scale and that in one example, the focal length f_lensSubstantially less than the distance L between the lens 110 and the object 106. Therefore, it should be understood that this is for the sake of brevityObject of the invention, distance L and distance L + focal length f according to the teachings of the invention_lensThus, the distance between the light source 102 and the object 106 (and/or the distance between the object 106 and the lens 110) is equal to half of the round trip distance (e.g., D), which is thus equal to 2 × L. in other words, it is assumed that the distance L from the light source 102 to the object 106 plus the distance L from the object 106 back to the lens 110 is equal to the round trip distance D (or 2 × L) in accordance with the teachings of the present invention.

In the depicted example, there is a delay time between the emission of a light pulse of the emitted light 104 and the receipt of the light pulse in the reflected light 108 that results from the amount of time it takes for the light pulse to travel a distance L from the light source 102 to reach the object 106 and then the additional time it takes for the corresponding reflected light pulse 108 to return from the object 106 to travel a distance L to reach the pixel array 112. The delay time between the emitted light 104 and the reflected light 108 represents the time of flight of the light pulse back and forth between the light source 102 and the object 106. Once the time of flight (i.e., TOF) is known, the distance L from the light source 102 to the object 106 can be determined using the following relationships in equations 1 and 2 below:

where c is the speed of light, which is approximately equal to 3x 10⁸m/s and TOF is the amount of time it takes for a light pulse to travel to and return from the object as shown in fig. 1.

FIG. 2 is a block diagram showing an example of a cross section of a time-of-flight sensing system 200 implemented with a pixel die 248 coupled to an Application Specific Integrated Circuit (ASIC) die 250 in a stacked-chip scheme, according to the teachings of this disclosure. It should be appreciated that the time of flight sensing system 200 of FIG. 2 may be one example of the time of flight sensing system 100 of FIG. 1, and thus similarly named and numbered elements referenced below are similarly coupled and function as described above.

In the example depicted in FIG. 2, time-of-flight sensing system 200 includes a pixel die 248, pixel die 248 being coupled to ASIC die 250 in a stacked-chip scheme as illustrated. As shown, time-of-flight sensing system 200 includes a light source 202, light source 202 emitting light pulses 204, light pulses 204 being directed to an object 206. In one example, the emitted light 204 includes pulsed IR light. The emitted light pulse 204 is then reflected back from the object 206, which in the depicted example is shown as reflected light pulse 208 in fig. 2.

In one example, time-of-flight sensing system 200 also includes a pixel die 248, pixel die 248 including a plurality of pixel cells arranged in a time-of-flight pixel array, including pixel cell 218. In the example, each pixel cell 218 includes a photosensor 220, the photosensor 220 including, in the illustrated example, a Single Photon Avalanche Diode (SPAD) that is optically coupled to receive reflected light pulses 208 from the object 206 through the respective microlens 210, as shown. Each photosensor 220 of each pixel cell 218 is coupled to a corresponding pixel support circuit 249, which as shown in the illustrated example, pixel support circuits 249 are disposed in ASIC die 250.

As shown in the depicted example, the pixel support circuitry 249 of each pixel cell 218 is also coupled to a single reference current source 213 included in the ASIC die 250. As will be described in more detail below, the reference current source 213 is coupled to provide a reference current I for each pixel support circuit 249_REF215 to program internal programmable current sources included in each pixel support circuit 249. According to the teachings of this disclosure, since each pixel support circuit 249 usesThe same reference current I_REF215, the current provided by each internal programmable current source included in each pixel support circuit 249 is individually calibrated to be substantially equal, even if there are pixel-by-pixel variations that may occur across all transistors included in all pixel support circuits 249.

In one example, control circuitry 216 is also included in ASIC die 250 and coupled to provide synchronization signal 214 to synchronize pulses of emitted light 204 with corresponding signals that control a plurality of pixel cells 218, which pixel cells 218 sense reflected light 208, in accordance with the teachings of the present disclosure.

Figure 3 is a schematic diagram illustrating one example of a time-of-flight pixel cell 318 according to the teachings of this disclosure. It should be appreciated that pixel cell 318 may be, for example, one of the plurality of pixels included in time-of-flight pixel array 112 of FIG. 1 or one example of one of the plurality of pixel cells 218 included in FIG. 2, and thus like-named and numbered elements referenced below are coupled and function in a similar manner as described above. As shown in the depicted example, pixel cell 318 includes a photosensor 320 and pixel support circuitry 349. Pixel support circuit 349 includes charge control logic 322, programmable current source 326, capacitor C_TOF332. Reset circuit 334, output switch 342, row select switch 343, and amplifier 338. The light sensor 320 senses photons of the reflected light 308 that are reflected from an object (e.g., the object 106 of fig. 1). In one example, the light sensor 320 may include a Single Photon Avalanche Diode (SPAD), as shown in fig. 3.

In the example, the charge control logic 322 is coupled to the light sensor 320 to detect when the light sensor 320 senses photons of the reflected light 308 reflected from the object. According to teachings of this disclosure, charge control logic 322 is further coupled to receive timing signal 315, which in the example may represent when light pulses 204 are emitted from light source 202 to object 206 (as illustrated, for example, in fig. 2), and thus enable pixel cells 318 to be synchronized with light source 202.

As shown in the depicted example, programmable current source 326 is coupled to provide a constant current I in response to a time of flight (TOF) signal 330 coupled to be received from charge control logic 322_H328. In the example, time-of-flight signal 330 is generated by charge control logic 322 and represents the time-of-flight of the round-trip travel of each of light pulses 204 emitted from light source 202 until light sensor 320 senses a respective one of the photons of reflected light 308 reflected from object 206, in accordance with the teachings of this disclosure.

In the example, illustrated as, for example, a time-of-flight capacitor C_TOF332 is coupled to receive a constant current I from the programmable current source 326 in response to the time-of-flight signal 330_H328. As shown in the depicted example, programmable current source 326 is coupled to a single reference current source 313 according to the teachings of this disclosure. In the example, a single reference current source 313 is coupled to provide a reference current 315 (which is utilized by all pixel cells 318 included in a time-of-flight pixel array) to program each respective programmable current source 326 and thus each respective current I_H328 are calibrated to be substantially equal across all pixel cells 318 included in the time-of-flight pixel array.

In the depicted example, programmable current source 326 is coupled to provide a constant current I from the emission of each light pulse 204 from light source 202 until light sensor 320 senses a respective one of the photons of reflected light 308 reflected from object 206_H328 charges a capacitor 332. Therefore, according to the teachings of this disclosure, the accumulation is at capacitor C_TOFVoltage V at 332_TOFRepresenting the round trip distance D to the object 106. In one example, a reset circuit 334 is coupled to capacitor C according to the teachings of this disclosure_TOF332 to generate a voltage V_TOFHas been driven from capacitor C_TOF332 read out and then reset capacitor C in response to reset capacitor signal 336_TOFThe accumulated voltage V on 332_TOF。

As described inIn the example shown, pixel cell 318 also includes an amplifier 338, amplifier 338 being coupled to capacitor C through an output switch 342_TOF332 in the capacitor C_TOF332 read out the accumulated charge on capacitor C after being read in response to the time-of-flight signal 330_TOFVoltage V at 332_TOF. In this example, reset circuit 334 is coupled to be at capacitor C according to the teachings of this disclosure_TOFVoltage V at 332_TOFReset accumulated in the capacitor C after being read out_TOFVoltage V at 332_TOF. In one example, as shown, amplifier 338 is a source follower coupled transistor and output switch 342 is coupled at capacitor C_TOF332 and the gate of the transistor of amplifier 338. In one example, pixel cell 318 also includes a row select switch 343 coupled between the output of amplifier 338 and bit line 340, the output of pixel cell 318 being readable out through row select switch 343, in accordance with the teachings of the present invention.

As illustrated in the example depicted in fig. 3, it should be noted that pixel cell 318 may be implemented in a stacked-chip scheme. For example, as shown in the example, the light sensor 320 may be included in a pixel die 348, while the pixel support circuitry 349 of the pixel unit 318 illustrated in FIG. 3 may be included in a separate ASIC die 350, in accordance with the teachings of the disclosure. In the example, the pixel die 348 and the ASIC die 350 are stacked and coupled together during fabrication to implement a time-of-flight sensing system according to the teachings of this disclosure.

FIG. 4 is a block diagram showing an emitted light pulse from light source 402, a corresponding reflected photon sensed by SPAD 420, a time-of-flight signal 430 output by charge control logic in response to emitted light pulse 402 and the sensed photon from SPAD 420, and a capacitor C accumulated in an example time-of-flight pixel, according to teachings of this disclosure_TOFCorresponding voltage V at 432_TOFTiming diagrams of examples of (1). It is to be appreciated that the light source 402 can, for example, correspond to the light source 102 of FIG. 1 and/or the light source 202 of FIG. 2, and the SPAD 420 can, for example, correspond to the light sensor 320 of FIG. 3, and the time of flight TOF signal 430 can, for example, correspond to the time of flight TOF signal 330 of FIG. 3Should, and the voltage V_TOF432 may be integrated with, for example, capacitor C of fig. 3_TOF332, voltage V_TOFCorrespondingly, and thus, similarly named and numbered elements referenced below are similarly coupled and function as described above. As shown in the example, at time t₁Emits a light pulse which causes the TOF signal 430 to change from a logic low level to a logic high level, which consequently results in a voltage V_TOF432 at time t₁The charging is started.

FIG. 4 also illustrates that SPAD 420 detects at time t₂Is reflected back from the object 106, which causes the TOF signal 430 to change from a logic high level to a logic low level, which consequently results in the voltage V_TOF432 at time t₂The charging is suspended. According to the teachings of this disclosure, the time of flight for a transmitted light pulse to travel back and forth between a light source and a light sensor a round trip distance D is equal to time t as illustrated in FIG. 4₁And t₂The time in between. Thus, the capacitor C_TOFVoltage V at 432_TOFAt time t₁And time t₂The time of flight of the light pulses in between accumulates. According to the teachings of this invention, because of the voltage V_TOF432 due to capacitor C_TOFAt time t₁And time t₂Using a constant current I from a programmable current source 326 during the time of flight of the light pulses in between_H328 charges up at a linear rate, so that the voltage V can be read accordingly_TOF432 to determine time of flight.

Figure 5 is a schematic diagram illustrating an example of multiple programmable current sources included in multiple time-of-flight pixel cells coupled to a single reference current source, according to the teachings of this disclosure. In particular, fig. 5 shows a plurality of programmable current sources including, for example, programmable current source 526A and programmable current source 526B. In the illustrated example, it should be appreciated that each of the programmable current sources are substantially similar to each other, and thus only programmable current source 526A is discussed in detail for simplicity. It should also be noted that programmable current source 526A and programmable current source 526B of fig. 5 may be examples of programmable current source 326 of fig. 3, and thus similarly named and numbered elements referenced below are similarly coupled and function as described above.

As shown in the depicted example, the programmable current source 526A includes a first transistor 552. In the example depicted in fig. 5, the transistor 552 is depicted as a p-channel Metal Oxide Semiconductor Field Effect Transistor (MOSFET), and thus includes a gate terminal, a source terminal, and a drain terminal. It should be appreciated that in other examples, other types of transistors may be used, such as n-channel MOSFETs, Bipolar Junction Transistors (BJTs), or the like. In the depicted example, the drain current I through transistor 552_D556 is responsive to a voltage difference between the gate terminal and the source terminal, which will be illustrated as V in FIG. 5_GS。

In one example, as shown, a current buffer circuit 564 may be coupled to a drain terminal of the transistor 552. Thus, as shown, the drain current I through transistor 552_D556 is also directed through current buffer circuit 564. In one example, as shown, current buffer circuit 564 includes a cascode transistor 566 coupled to the drain terminal of transistor 552.

As shown in FIG. 5, programmable current source 526A also includes a current control circuit 554, current control circuit 554 being coupled to conduct a drain current I from transistor 552_D556. Current control circuit 554 is also coupled to reference current source 513 and to time-of-flight capacitor C_TOF532. It should be appreciated that reference current source 513 and time-of-flight capacitor C in FIG. 5_TOF532 may be the reference current source 313 and the time-of-flight capacitor C of FIG. 3, respectively_TOF332, and thus similarly named and numbered elements referenced below, are similarly coupled and function as described above.

In one example, current control circuit 554 includes switching circuit 568, switching circuit 568 is set to be in the "3" position during a sampling operation. For example, when the switching circuit is in the "3" position, the reference current source 513 is driven by the switching circuit during the sampling operation568 are selectively coupled to force a reference current I_REF515 through transistor 552. Thus, according to the teachings of this disclosure, the drain current I of transistor 552 when switching circuit 568 is in the "3" position during a sampling operation_D556 is forced to be substantially equal to reference current I_REF515。

Continuing with the example depicted in fig. 5, as shown, programmable current source 526A also includes a sample and hold circuit 558 coupled to a transistor 552. Sample and hold circuit 558 is coupled to couple the gate-source voltage V to the transistor during a sample operation_GSSampling is performed. Further, sample and hold circuit 558 is coupled to hold the gate-source voltage V during a hold operation following the sample operation_GS. In other words, according to the teachings of this disclosure, the gate-source voltage V_GSHeld or maintained at the gate-source voltage V corresponding to the previous sample_GSSubstantially equal values.

As will be discussed, according to the teachings of the present invention, because of the drain current I_D556 is forced to be substantially equal to reference current I during the sampling operation_REF515, so the drain current I_D556 will remain substantially equal to reference current I during hold operation_REF515 and a gate-source voltage V_GSIs maintained at V_GSInitially sampled by the same value, wherein the drain current I_D556 is forced to be substantially equal to reference current I_REF515。

In one example, as shown, sample and hold circuit 558 includes a programming capacitor C coupled between the source terminal and the gate terminal of transistor 552_P560, and a switch 562 coupled between the gate terminal and the drain terminal of the transistor 552. In operation, according to the teachings of this disclosure, during a sample operation, switch 562 is coupled to be on or in a "1" position, as shown in fig. 5, and during a hold operation of programmable current source 526A, switch 562 is coupled to be off or in a "2" position, as shown in fig. 5.

In particular, during sampling operations at switch 562With the ON or "1" position and with the transistor 552 operating in saturation during the sampling operation, note that the drain-gate voltage (V) of the transistor 552_DG) And is zero during the sampling operation. Thus, the drain current I_D556 Gate-source Voltage V during sample operation_GSAs a function of (c). Because of the drain current I_D556 is forced to equal reference current I during the sampling operation_REF515 so that the reference current I_REF515 gate-source voltage V to transistor 552 during a sampling operation_GSIs set or programmed to provide a current substantially equal to the reference current I_REF515 drain current I_D556. This gate-source voltage V of transistor 552_GSSampled by sample and hold circuit 558 and held at that voltage by programming capacitor 560. Thus, according to the teachings of this disclosure, at the gate-source voltage V to transistor 552_GSAfter sampling, switch 562 is open or in the "2" position, which remains across programmed capacitor C_PIs sampled by V_GSVoltage and thus the drain current I is maintained_D556 to keep it substantially equal to the reference current I_REF515。

Continuing the example, at a voltage V to the gate-source_GSAfter sampling is done and switch 562 is open or in the "2" position, then switching circuit 568 is coupled to switch from the "3" position to the "4" position during the hold operation, which switches the drain current I_D556 directed to constant programmed holding current I_H528, holding current I_H528 is conducted through switch 570. In such an example, the holding current I is according to the teachings of this disclosure_H528 is coupled to selectively give a time-of-flight capacitor C through a switch 570 in response to a time-of-flight signal TOF 530_TF532 are charged. According to the teachings of this disclosure, since the holding current I_H528 equals the drain current I during the hold operation_D556, thus holding the current I_H528 is substantially equal to the reference current I_REF515。

As shown in the example depicted in FIG. 5, a plurality of programmable current sourcesEach of which includes programmable current source 526A and programmable current source 526B includes a respective current control circuit 554, current control circuit 554 being coupled to single reference current source 513 to selectively couple drain current I of respective transistor 552_D556 is programmed. For example, in operation, after programming of programmable current source 526A is complete, the respective current control circuit 554 of programmable current source 526B may be activated to program programmable current source 526B.

Thus, more than one programmable current source may be programmed with a single reference current source 513 to provide substantially equal holding currents I in accordance with the teachings of this disclosure_H528 to respective time-of-flight capacitors C_TOF532 are charged. For example, in one example, the reference current I of a single reference current source 513 may be utilized in accordance with the teachings of this disclosure_REF515 program all programmable current sources in the time-of-flight pixel array.

It should be noted that in the illustrated example, the reference current I of a single reference current source 513 is utilized_REF515 program one programmable current source at a time. However, in another example, it should be appreciated that a reference current source I of a single reference current source 513 may be utilized in accordance with the teachings of this disclosure_REF515 program one or more other programmable reference current sources coupled to the time-of-flight pixel array, and then can quickly keep the reference current, I, from the original reference current, I, utilizing the "copied" of the other programmable reference current sources_REFThe 515 values are "copied" to the programmable current sources throughout one or more pixel cells of the time-of-flight pixel array. For example, in one example, according to the teachings of this disclosure, a reference current I may be realized_REF515 'copy' operation across the distribution or spread of multiple rows or columns of a time-of-flight pixel array to quickly copy the reference current I_REF515。

To illustrate, FIG. 6 is a block diagram showing a portion of an example time-of-flight sensing system 600 including a time-of-flight pixel array with corresponding readout circuitry, control circuitry, and functional logic, according to the teachings of this disclosure. As shown, the illustrated example of a time-of-flight sensing system 600 includes a time-of-flight pixel array 612, readout circuitry 601, a programmable reference current source array 617, functional logic 605, control circuitry 616, and light source 602 to sense the round-trip distance of object 606, in accordance with the teachings of this disclosure.

In the example illustrated in fig. 6, pixel array 612 is a two-dimensional (2D) array of time-of-flight pixel cells (e.g., pixels P1, P2 …, Pn). In one example, each of the time-of-flight pixel cells P1, P2, …, Pn may be substantially similar to one of the time-of-flight pixels discussed above, for example, in fig. 2-5, and thus similarly named and numbered elements referenced below are similarly coupled and function as described above. As illustrated, each pixel cell is arranged in rows (e.g., rows R1-Ry) and columns (e.g., columns C1-Cx) to acquire time-of-flight information for image object 606 focused onto pixel array 612. Thus, according to the teachings of this disclosure, time-of-flight information may then be used to determine distance or depth information to the object 606.

As shown in the example depicted in fig. 6, with a reference current I_REF615 is coupled to a programmable reference current source array 617. In one example, the programmable reference current source array 617 includes a plurality of programmable reference current sources, each of which may be substantially similar to the example programmable current sources discussed above in fig. 5. For example, in the example depicted in FIG. 6, it should be appreciated that current I has been referenced_REF615 after copying to each of the programmable reference current sources included in the programmable reference current source array 617, each of the "copied" reference current sources in the programmable reference current source array 617 is coupled to program each of the internal programmable current sources included in a respective column of pixel cells of the time-of-flight pixel cell array, as discussed above. According to the teachings of this disclosure, by using the original reference current I_REF615, "via" provided to each internal programmable current source included in the time-of-flight pixel cell arrayDuplicating the "hold reference current I_HIndividually calibrated to be substantially equal even with pixel-by-pixel variations that may occur across all transistors included in the time-of-flight pixel cell array.

To illustrate, the reference current I of the current source 613 is referenced in the column replica loop 672_REF615 to each of the programmable reference current sources included in the programmable reference current source array 617. In the example, each of the programmable reference current sources included in the programmable reference current source array 617 is coupled to a corresponding one of a number of columns of pixel cells in the time-of-flight pixel array 612. According to the teachings of this disclosure, then, as shown, the slave reference current I may be replicated for each column in the row replica loop 674_REF615 copied a copied held reference current I into each of the programmable reference current sources included in the programmable reference current source array 617_HTo quickly replicate all hold reference currents I throughout all rows of pixel cells in pixel array 612_H. In the illustrated example, where time-of-flight pixel array 612 includes x rows and y rows, current I will be referenced during a row copy loop 672_REF615 into each of the x programmable reference current sources in the programmable reference current source array 617. Once programmed, each of the programmable reference current sources included in the programmable reference current source array 617 provides a reference current, which is then copied to each of the y rows of the time-of-flight pixel array 612, according to the teachings of this disclosure.

In other words, during the first x current copy cycles, the reference current I is used_REF615 to copy the reference current to each of the x programmable reference current sources in the programmable reference current source array 617. According to the teachings of this disclosure, each of the replicated reference currents in programmable reference current source array 617 is replicated to a programmable current source in each of the y rows of time-of-flight pixel array 612 during (x +1) to (x + y) current replication cycles.

In one example, the control circuitry 616 controls the light source 602 and uses the synchronization signal 614 to synchronize the light source 602 to emit the light pulses 604 to the object 606. As shown, the reflected light pulses 608 are then reflected back to the pixel array 612. In one example, each of the pixel cells in pixel array 612 senses photons from reflected back light pulse 608 and then responds to photons from a respective C included in the pixel cells in pixel array 612_TOFMeasured V of capacitor_TOFThe corresponding signal of voltage is sensed by sense circuit 601 through bit line 640 as shown. In one example, the sense circuit 601 may include an amplifier to further amplify the signal received through the bit line 640. In one example, the information read out by readout circuitry 601 may then be transferred to functional logic 605. In one example, function logic 605 may determine time-of-flight and distance information for each pixel cell. In one example, function logic 605 may also store time-of-flight information and/or even manipulate time-of-flight information (e.g., crop, rotate, adjust background noise, or the like). In one example, readout circuitry 601 may readout the time-of-flight information along bit lines 640 an entire row at a time (illustrated), or in another example, the time-of-flight information may be readout using various other techniques (not illustrated), such as serial readout or readout of all pixel cells in parallel all at once.

In the illustrated example, the control circuitry 616 is further coupled to the pixel array 612 to control operation of the pixel array 612 and synchronize operation of the pixel array 612 with the light source 602. For example, control circuit 616 may generate timing signal 315, timing signal 315 coupled to be received by charge control logic 322 and output switch 342 and row select switch 343 shown in fig. 3 to determine time of flight information in accordance with the teachings of this disclosure.

In one example, it should be noted that the time of flight sensing system 600 illustrated in fig. 6 may be implemented in a stacked chip scheme. For example, as shown in the example, pixel array 612 may be included in a pixel die, while readout circuitry 601, functional logic 605, and control circuitry 616 may be included in a separate ASIC die, as illustrated in fig. 6, in accordance with the teachings of this disclosure. In such an example, the pixel die and the ASIC die are stacked and coupled together during fabrication to implement a time-of-flight sensing system according to the teachings of this disclosure.

The above description of illustrated embodiments of the invention, including what is described in the abstract of the specification, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.

These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention should be determined entirely by the following claims, which are to be construed in accordance with accepted guidelines for interpretation of the claims.

Claims

1. A programmable current source array having a plurality of programmable current sources, wherein each of the plurality of programmable current sources is used with a respective time-of-flight pixel cell, and each of the plurality of programmable current sources comprises:

a first transistor having a gate terminal, a source terminal, and a drain terminal, wherein a current through the first transistor is responsive to a gate-source voltage of the first transistor;

a current control circuit coupled to the first transistor and to a reference current source, wherein the current control circuit is coupled to selectively couple a reference current of the reference current source through the first transistor during a sampling operation; and

a sample and hold circuit coupled to the first transistor, wherein the sample and hold circuit is coupled to sample a gate-source voltage of the first transistor during the sample operation, wherein the sample and hold circuit is coupled to hold the gate-source voltage during a hold operation subsequent to the sample operation substantially equal to the gate-source voltage during the sample operation, wherein a hold current through the first transistor during the hold operation is substantially equal to the reference current,

wherein the reference current source is a single reference current source coupled to program each of the plurality of programmable current sources during the sampling operation of each of the plurality of programmable current sources.

2. The programmable current source array of claim 1, wherein the sample and hold circuit comprises:

a programming capacitor coupled between the source terminal and the gate terminal of the first transistor; and

a switch coupled between the gate terminal and the drain terminal of the first transistor, wherein the switch is coupled to be on during the sample operation and coupled to be off during the hold operation.

3. The programmable current source array of claim 1, further comprising a current buffer circuit coupled to the drain terminal of the first transistor, wherein the current through the first transistor is conducted through the current buffer circuit.

4. The programmable current source array of claim 3, wherein the current buffer circuit comprises a cascode transistor coupled to the drain terminal of the first transistor.

5. The programmable current source array of claim 1, wherein the current control circuit is further coupled to a time-of-flight capacitor of the time-of-flight pixel cell, wherein the current control circuit is coupled to selectively charge the time-of-flight capacitor using the hold current during a time-of-flight measurement operation of the time-of-flight pixel cell.

6. The programmable current source array of claim 1, wherein each of the plurality of programmable current sources is included in a respective one of a plurality of time-of-flight pixel cells.

7. A time-of-flight pixel cell array having a plurality of time-of-flight pixel cells, wherein each of the plurality of time-of-flight pixel cells comprises:

a light sensor for sensing photons reflected from an object; and

a pixel support circuit, comprising:

a timing control logic coupled to the light sensor to detect when the light sensor senses the photon reflected from the object, wherein the timing control logic is further coupled to receive a timing signal indicative of when a light pulse is emitted from a light source to the object, wherein the timing control logic is coupled to generate a time-of-flight signal indicative of a time-of-flight measurement operation of the time-of-flight pixel cell;

a programmable current source coupled to the timing control logic to provide a hold current in response to the time of flight signal coupled to be received from the timing control logic, wherein the programmable current source includes:

a sample and hold circuit coupled to the first transistor, wherein the sample and hold circuit is coupled to sample a gate-source voltage of the first transistor during the sample operation, wherein the sample and hold circuit is coupled to hold the gate-source voltage during a hold operation subsequent to the sample operation substantially equal to the gate-source voltage during the sample operation, wherein a hold current through the first transistor during the hold operation is substantially equal to the reference current; and

a time-of-flight capacitor coupled to the current control circuit to be selectively charged by the holding current in response to the time-of-flight signal, wherein a voltage on the time-of-flight capacitor represents a round-trip distance to the object, an

Wherein the reference current source is a single reference current source coupled to program each of the programmable current sources in the plurality of time-of-flight pixel cells during the sampling operation of each of the programmable current sources.

8. The array of time-of-flight pixel cells of claim 7, wherein the time-of-flight signal is coupled to represent the time-of-flight for each of the light pulses to be emitted from the light source until the light sensor senses a respective one of the photons reflected from the object.

9. The array of time-of-flight pixel cells of claim 7, wherein, for each of the plurality of time-of-flight pixel cells, the sample and hold circuit comprises:

10. The time-of-flight pixel cell array of claim 7, wherein, for each of the plurality of time-of-flight pixel cells, the programmable current source further comprises a current buffer circuit coupled to the drain terminal of the first transistor, wherein the current through the first transistor is conducted through the current buffer circuit.

11. The time-of-flight pixel cell array of claim 10, wherein, for each of the plurality of time-of-flight pixel cells, the current buffer circuit comprises a cascode coupled transistor coupled to the drain terminal of the first transistor.

12. The time-of-flight pixel cell array of claim 7, wherein, for each of the plurality of time-of-flight pixel cells, the pixel support circuit further comprises an amplifier coupled to the time-of-flight capacitor to readout the voltage on the time-of-flight capacitor after the time-of-flight capacitor is charged by the programmable current source in response to the time-of-flight signal.

13. The array of time-of-flight pixel cells of claim 12, wherein, for each of the plurality of time-of-flight pixel cells, the pixel support circuit further comprises a reset circuit coupled to reset the voltage on the time-of-flight capacitor after the voltage on the time-of-flight capacitor is read out.

14. The array of time-of-flight pixel cells of claim 12, wherein, for each of the plurality of time-of-flight pixel cells, the pixel support circuit further comprises an output switch coupled between the time-of-flight capacitor and a gate of the amplifier.

15. The array of time-of-flight pixel cells of claim 12, wherein, for each of the plurality of time-of-flight pixel cells, the pixel support circuit further comprises a row select switch coupled between an output of the amplifier and a bit line.

16. The time-of-flight pixel cell array of claim 7, wherein, for each of the plurality of time-of-flight pixel cells, the photosensor comprises a single photon avalanche diode, SPAD.

17. The array of time-of-flight pixel cells of claim 7, wherein, for each of the plurality of time-of-flight pixel cells, the photosensor is included in a first die and the pixel support circuitry is included in a second die, and wherein the first die and the second die are stacked and coupled together.

18. A time-of-flight sensing system, comprising:

a light source for emitting light pulses to an object;

a reference current source having a reference current;

a time-of-flight pixel array having a plurality of time-of-flight pixel cells, wherein each of the time-of-flight pixel cells comprises:

a light sensor for sensing photons reflected from the object;

a timing control logic coupled to the light sensor to detect when the light sensor senses the photon reflected from the object, wherein the timing control logic is further coupled to receive a timing signal indicative of when a light pulse is emitted from the light source to the object, wherein the timing control logic is coupled to generate a time-of-flight signal indicative of a time-of-flight measurement operation of the time-of-flight pixel array;

a current control circuit coupled to the first transistor and to the reference current source, wherein the current control circuit is coupled to selectively couple the reference current of the reference current source through the first transistor during a sampling operation; and

a sample and hold circuit coupled to the first transistor, wherein the sample and hold circuit is coupled to sample a gate-source voltage of the first transistor during the sample operation, wherein the sample and hold circuit is coupled to hold the gate-source voltage during a hold operation subsequent to the sample operation substantially equal to the gate-source voltage during the sample operation, wherein a hold current through the first transistor during the hold operation is substantially equal to the reference current;

a time-of-flight capacitor coupled to the current control circuit to be selectively charged by the holding current in response to the time-of-flight signal, wherein a voltage on the time-of-flight capacitor represents a round-trip distance to the object;

a control circuit coupled to the light source and to the time-of-flight pixel array to synchronize the emission of the light pulses with the timing of the sensing of the photons reflected from the object.

19. The time-of-flight sensing system of claim 18, wherein the time-of-flight signal is coupled to represent the time-of-flight for each of the light pulses to be emitted from the light source until the light sensor of a respective one of the plurality of time-of-flight pixel cells senses a respective one of the photons reflected from the object.

20. The time of flight sensing system of claim 18, wherein the sample and hold circuit comprises:

21. The time of flight sensing system of claim 18, in which the programmable current source further comprises a current buffer circuit coupled to the drain terminal of the first transistor, in which the current through the first transistor is conducted through the current buffer circuit.

22. The time of flight sensing system of claim 18, in which the current buffer circuit comprises a cascode transistor coupled to the drain terminal of the first transistor.

23. The time of flight sensing system of claim 18, wherein each of the time of flight pixel cells further comprises an amplifier coupled to the time of flight capacitor to readout the voltage on the time of flight capacitor after the time of flight capacitor is charged by the programmable current source in response to the time of flight signal.

24. The time of flight sensing system of claim 23, wherein each of the time of flight pixel cells further comprises a reset circuit coupled to reset the voltage on the time of flight capacitor after the voltage on the time of flight capacitor is read out.

25. The time of flight sensing system of claim 23, wherein each of the time of flight pixel cells further comprises an output switch coupled between the time of flight capacitor and a gate of the amplifier.

26. The time of flight sensing system of claim 23, wherein each of the time of flight pixel cells further comprises a row select switch coupled between an output of the amplifier and a bit line.

27. The time of flight sensing system of claim 26, further comprising readout circuitry coupled to readout the voltage from each of the time of flight pixel cells through the bit lines.

28. The time of flight sensing system of claim 27, further comprising functional logic coupled to the readout circuitry to store and process time of flight information read out from the time of flight pixel array.

29. The time of flight sensing system of claim 18, in which the light sensor comprises a single photon avalanche diode, SPAD.