Patent Publication Number: US-11386835-B2

Title: Pixel control architecture for micro-LED micro-display with reduced transistor count

Description:
PRIORITY 
     The present application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional App. No. 63/006,562 (filed Apr. 7, 2020), which is incorporated herein by reference. 
    
    
     BACKGROUND 
     Helmet mounted or head worn micro-displays require high brightness and high resolution in a small area (such as a one-inch square). Some state-of-the-art displays include four subpixels that comprise each pixel. Such displays require separate control circuitry for each addressable display element. 
     To enable the required brightness, a current drive transistor is required for each subpixel and occupies a significant portion of the area available for display control. High-quality graphics requires very fine control of gray scale (brightness levels) for each subpixel. Such control requires a complicated control circuit with many transistors. High transistor count leads to poor process yields and correspondingly high product costs. Simultaneously, the control transistors must be made very small to fit within the available space. Space constraints necessitate the use of very small geometry semiconductor processes with high recurring and non-recurring costs and waste. 
     SUMMARY 
     In one aspect, embodiments of the inventive concepts disclosed herein are directed to a display with subpixel LEDs where two of the subpixel LEDs are controlled via a shared control circuit and switching element. Switching element logic allows one set of brightness control transistors to alternatively control two subpixels. The driving and control elements of a display backplane are organized into pixels units of four driving elements and three control elements. 
     In a further aspect, each pixel comprises two green subpixels controlled via the switching element. Alternatively, each pixel comprises a white subpixel that only illuminates when the colored pixels are off; the green and white subpixels are controlled via the switching element. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and should not restrict the scope of the claims. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments of the inventive concepts disclosed herein and together with the general description, serve to explain the principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous advantages of the embodiments of the inventive concepts disclosed herein may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1  shows a block diagram of a pixel, including subpixels, and control elements according to an exemplary embodiment; 
         FIG. 2  shows a block diagram of a pixel according to an exemplary embodiment; 
         FIG. 3  shows a chart of digital inputs and outputs for a switching element according to an exemplary embodiment; 
         FIG. 4  shows a block diagram of a pixel, including subpixels, and control elements according to an exemplary embodiment; 
         FIG. 5  shows a chart of digital inputs and outputs for a switching element according to an exemplary embodiment; 
     
    
    
     DETAILED DESCRIPTION 
     Before explaining at least one embodiment of the inventive concepts disclosed herein in detail, it is to be understood that the inventive concepts are not limited in their application to the details of construction and the arrangement of the components or steps or methodologies set forth in the following description or illustrated in the drawings. In the following detailed description of embodiments of the instant inventive concepts, numerous specific details are set forth in order to provide a more thorough understanding of the inventive concepts. However, it will be apparent to one of ordinary skill in the art having the benefit of the instant disclosure that the inventive concepts disclosed herein may be practiced without these specific details. In other instances, well-known features may not be described in detail to avoid unnecessarily complicating the instant disclosure. The inventive concepts disclosed herein are capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
     As used herein a letter following a reference numeral is intended to reference an embodiment of the feature or element that may be similar, but not necessarily identical, to a previously described element or feature bearing the same reference numeral (e.g., 1, 1a, 1b). Such shorthand notations are used for purposes of convenience only, and should not be construed to limit the inventive concepts disclosed herein in any way unless expressly stated to the contrary. 
     Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by anyone of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). 
     In addition, use of the “a” or “an” are employed to describe elements and components of embodiments of the instant inventive concepts. This is done merely for convenience and to give a general sense of the inventive concepts, and “a” and “an” are intended to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise. 
     Finally, as used herein any reference to “one embodiment,” or “some embodiments” means that a particular element, feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the inventive concepts disclosed herein. The appearances of the phrase “in some embodiments” in various places in the specification are not necessarily all referring to the same embodiment, and embodiments of the inventive concepts disclosed may include one or more of the features expressly described or inherently present herein, or any combination of sub-combination of two or more such features, along with any other features which may not necessarily be expressly described or inherently present in the instant disclosure. 
     Broadly, embodiments of the inventive concepts disclosed herein are directed to a display with subpixel LEDs where two of the subpixel LEDs are controlled via a shared control circuit and switching element. Switching element logic allows one set of brightness control transistors to alternatively control two subpixels. The driving and control elements of a display backplane are organized into pixels units of four driving elements and three control elements. The architecture of a pixel comprising four subpixels and the corresponding drive elements may be more fully understood with respect to U.S. patent application Ser. No. 16/704,322 “DISPLAY ELEMENT, SYSTEM, AND METHOD” (filed Dec. 5, 2019). 
     Referring to  FIG. 1 , a block diagram of a pixel  100 , including subpixels  102 ,  106 ,  110 ,  112 , and control elements  104 ,  108 ,  114  according to an exemplary embodiment is shown. Each subpixel  102 ,  106 ,  110 ,  112  is driven by an independent current drive transistor. The brightness of each subpixel  102 ,  106 ,  110 ,  112  is controlled via a set of transistors embodying a corresponding control element  104 ,  108 ,  114 . One of the control elements  104 ,  108 ,  114  is configured to control the brightness level of two related subpixels  110 ,  112  in the alternative or in concert. 
     In at least one embodiment, the pixel  100  comprises a first, red subpixel  102  controlled by a first control element  104  and a second, blue subpixel  106  controlled by a second control element  108 . A third, green subpixel  110  and a fourth, white subpixel  112  are controlled by a third control element  114 . A switching element  116  alternatively diverts a control signal to either the third, green subpixel  110  or the fourth, white subpixel  112  based on a set of inputs as more fully described herein. In such embodiment, the fourth, white subpixel  112  is never driven at the same time as the other subpixels  102 ,  106 ,  110 . 
     In at least one embodiment, the pixel  100  comprises a first, red subpixel  102  controlled by a first control element  104  and a second, blue subpixel  106  controlled by a second control element  108 . A third, primary green subpixel  110  and a fourth, secondary green subpixel  112  are controlled by a third control element  114 . A switching element  116  may apply a control signal to the third, primary green subpixel  110  alone, or also to the fourth, secondary green subpixel  112  based on a set of inputs. In such embodiment, the fourth, secondary green subpixel  112 , if driven, is driven at the same brightness as the third, primary green subpixel  110 . 
     In at least one embodiment, where the display is monochrome, each subpixel  102 ,  106 ,  110 ,  112  comprises a green subpixel  102 ,  106 ,  110 ,  112 . The control elements  104 ,  108 ,  114  may set the brightness for each subpixel  102 ,  106 ,  110 ,  112  at substantially the same value. In such embodiment, the switching element  116  may be connected to and apply the same signal to each subpixel  102 ,  106 ,  110 ,  112 . Alternatively, or in addition, a first set of subpixels  102 ,  106  may be controlled via corresponding control elements  104 ,  108  and related subpixels  110 ,  112  are controlled via a combined control element  114  via the switching element  116 . 
     In at least one embodiment, the control elements  104 ,  108 ,  114  and switching element  116  may be embodied in a backplane while the subpixels  102 ,  106 ,  110 ,  112  are embodied in a separate LED plane. Because the switching element  116  may be addressed via inputs to drive either or both of the connected subpixels  110 ,  112 , the same backplane architecture may be utilized for a monochrome LED plane, a red-green-blue-white LED plane, and a red-green-blue-green LED plane. 
     Referring to  FIG. 2 , a block diagram of a pixel  200  according to an exemplary embodiment is shown. The pixel  200  comprises four subpixels  202 ,  204 ,  206 ,  208 . Each subpixel  202 ,  204 ,  206 ,  208  is driven by a corresponding current drive transistor  210 ,  214 ,  218 ,  220 . In at least one embodiment, the current drive transistors  210 ,  214 ,  218 ,  220  may be disposed to maximize available space for control elements  212 ,  216 ,  222 . 
     In at least one embodiment, one of the control elements  212 ,  216 ,  222  may comprise a combined control element  222  configured to control the brightness of two related subpixels  206 ,  208 , either alternatively or in concert. The combined control element  222  may include a switching element/selection logic for determining which of the related subpixels  206 ,  208  to illuminate. 
     In at least one embodiment, the current drive transistors  210 ,  214 ,  218 ,  220  and control elements  212 ,  216 ,  222  may be embodied in a backplane, separate from an LED plane, such that the backplane may be configured to drive subpixels  202 ,  204 ,  206 ,  208  in any LED plane with substantially similar layout, regardless of the composition of the subpixels  202 ,  204 ,  206 ,  208 . 
     Referring to  FIG. 4 , a block diagram of a pixel  400 , including subpixels  402 ,  406 ,  410 ,  412 , and control elements  404 ,  408 ,  414  according to an exemplary embodiment is shown. Each subpixel  402 ,  406 ,  410 ,  412  is driven by an independent current drive transistor, controlled via corresponding control element  404 ,  408 ,  414 . One of the control elements  404 ,  408 ,  414  is configured to control the brightness level of two related subpixels  410 ,  412  in the alternative or in concert. The pixel  400  may comprise a red-blue-green-white subpixel layout, a green monochrome subpixel layout, a red-green-blue-green subpixel layout, or any other subpixel layout wherein at least two subpixels  402 ,  406 ,  410 ,  412  are sufficiently related to allow their brightness values to be set in the alternative or in concert. 
     In at least one embodiment, a switching element  416  comprises selection logic that receives a plurality of inputs to determine which of the two related subpixels  410 ,  412  to illuminate. In at least one embodiment, the inputs may receive a set of bits indicating the type of LED plane (e.g. monochrome or red-green-blue-green) and whether a secondary green subpixel should be driven. 
     In at least one embodiment, the inputs may also comprise one or more input bits of other control elements  404 ,  408 . For example, least significant bits intended for a blue subpixel control element  408  may be received by the switching element  416 . One exemplary chart of inputs and corresponding outputs are shown in  FIG. 3  (output “G” indicating corresponding related subpixel  404 ,  408  is illuminated and output “0” indicating it is not). 
     In one exemplary embodiment, where an input bit indicates a monochrome LED plane (“Mono” equals 1 in  FIG. 3 ), a combined control element  414  will always drive both related pixels  410 ,  412 . Where an input bit indicates a red-green-blue-green LED plane (“RGBG” equals 1 in  FIG. 3 ), the combined control element  414  will illuminate a secondary green subpixel in the related subpixels  410 ,  412  to the same brightness as a primary green subpixel if another bit indicates that the least significant bit of the blue sub pixel control element  408  should be used to determine which of the related subpixels  410 ,  412  to drive (“Video” equals 1 in  FIG. 3 ). It may be appreciated that the least significant bit of a color channel is only an exemplary embodiment; any bit in the video stream may be used. Where none of the Mono, RBGB, or Video inputs indicates those states, the LED plane may be assumed to be a red-green-blue-white LED plane. In that case, because none of the color specific subpixels  402 ,  406 ,  410  would be illuminated at the same time as a white subpixel  412 , a least significant bit of one or more control signals to the non-combined control elements  404 ,  408  may indicate if the combined control element  414  should illuminate the white subpixel  412  (“B 0  equals 1 in  FIG. 3 ). It may be appreciated that the actual values may depend on the architecture of the selection logic in the switching element  416 . For clarity and simplicity,  FIG. 5  shows a similar chart of digital inputs to the switching element  416  wherein the RGBG input is removed. The switching element  416  may still be addressable to illuminate one or both of the related subpixels  410 ,  412 . 
     A display according to the present disclosure may have a backplane with a 25% reduction in the number of control transistors; improving yield up to 25% and reducing recurring cost. The required chip area is also reduced, allowing larger, cheaper semiconductor node size to be used (75 nm or larger as compared to 65 nm), reducing process waste. Furthermore, space and complexity savings may allow for a corresponding increase in brightness control complexity from eight-bit to ten-bit. 
     It should be appreciated that while exemplary embodiments described herein were directed to pixels comprised of four subpixels, other embodiments are envisioned. For example, five or six subpixels are also possible. Any embodiment wherein at least two subpixels are controlled by a single control element is envisioned. Furthermore, multiple sets of related subpixels within a pixel may each be controlled a separate single control element. 
     It is believed that the inventive concepts disclosed herein and many of their attendant advantages will be understood by the foregoing description of embodiments of the inventive concepts disclosed, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components thereof without departing from the broad scope of the inventive concepts disclosed herein or without sacrificing all of their material advantages; and individual features from various embodiments may be combined to arrive at other embodiments. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes. Furthermore, any of the features disclosed in relation to any of the individual embodiments may be incorporated into any other embodiment.