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

In a display with subpixel LEDs, each pixel includes two subpixel LEDs 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. Each pixel may comprise two green subpixels controlled via the switching element. Alternatively, each pixel may comprise a white subpixel that only illuminates when the colored pixels are off; the green and white subpixels are controlled via the switching element.

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.

DETAILED DESCRIPTION

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 toFIG. 1, a block diagram of a pixel100, including subpixels102,106,110,112, and control elements104,108,114according to an exemplary embodiment is shown. Each subpixel102,106,110,112is driven by an independent current drive transistor. The brightness of each subpixel102,106,110,112is controlled via a set of transistors embodying a corresponding control element104,108,114. One of the control elements104,108,114is configured to control the brightness level of two related subpixels110,112in the alternative or in concert.

In at least one embodiment, the pixel100comprises a first, red subpixel102controlled by a first control element104and a second, blue subpixel106controlled by a second control element108. A third, green subpixel110and a fourth, white subpixel112are controlled by a third control element114. A switching element116alternatively diverts a control signal to either the third, green subpixel110or the fourth, white subpixel112based on a set of inputs as more fully described herein. In such embodiment, the fourth, white subpixel112is never driven at the same time as the other subpixels102,106,110.

In at least one embodiment, the pixel100comprises a first, red subpixel102controlled by a first control element104and a second, blue subpixel106controlled by a second control element108. A third, primary green subpixel110and a fourth, secondary green subpixel112are controlled by a third control element114. A switching element116may apply a control signal to the third, primary green subpixel110alone, or also to the fourth, secondary green subpixel112based on a set of inputs. In such embodiment, the fourth, secondary green subpixel112, if driven, is driven at the same brightness as the third, primary green subpixel110.

In at least one embodiment, where the display is monochrome, each subpixel102,106,110,112comprises a green subpixel102,106,110,112. The control elements104,108,114may set the brightness for each subpixel102,106,110,112at substantially the same value. In such embodiment, the switching element116may be connected to and apply the same signal to each subpixel102,106,110,112. Alternatively, or in addition, a first set of subpixels102,106may be controlled via corresponding control elements104,108and related subpixels110,112are controlled via a combined control element114via the switching element116.

In at least one embodiment, the control elements104,108,114and switching element116may be embodied in a backplane while the subpixels102,106,110,112are embodied in a separate LED plane. Because the switching element116may be addressed via inputs to drive either or both of the connected subpixels110,112, 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 toFIG. 2, a block diagram of a pixel200according to an exemplary embodiment is shown. The pixel200comprises four subpixels202,204,206,208. Each subpixel202,204,206,208is driven by a corresponding current drive transistor210,214,218,220. In at least one embodiment, the current drive transistors210,214,218,220may be disposed to maximize available space for control elements212,216,222.

In at least one embodiment, one of the control elements212,216,222may comprise a combined control element222configured to control the brightness of two related subpixels206,208, either alternatively or in concert. The combined control element222may include a switching element/selection logic for determining which of the related subpixels206,208to illuminate.

In at least one embodiment, the current drive transistors210,214,218,220and control elements212,216,222may be embodied in a backplane, separate from an LED plane, such that the backplane may be configured to drive subpixels202,204,206,208in any LED plane with substantially similar layout, regardless of the composition of the subpixels202,204,206,208.

Referring toFIG. 4, a block diagram of a pixel400, including subpixels402,406,410,412, and control elements404,408,414according to an exemplary embodiment is shown. Each subpixel402,406,410,412is driven by an independent current drive transistor, controlled via corresponding control element404,408,414. One of the control elements404,408,414is configured to control the brightness level of two related subpixels410,412in the alternative or in concert. The pixel400may 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 subpixels402,406,410,412are sufficiently related to allow their brightness values to be set in the alternative or in concert.

In at least one embodiment, a switching element416comprises selection logic that receives a plurality of inputs to determine which of the two related subpixels410,412to 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 elements404,408. For example, least significant bits intended for a blue subpixel control element408may be received by the switching element416. One exemplary chart of inputs and corresponding outputs are shown inFIG. 3(output “G” indicating corresponding related subpixel404,408is illuminated and output “0” indicating it is not).

In one exemplary embodiment, where an input bit indicates a monochrome LED plane (“Mono” equals 1 inFIG. 3), a combined control element414will always drive both related pixels410,412. Where an input bit indicates a red-green-blue-green LED plane (“RGBG” equals 1 inFIG. 3), the combined control element414will illuminate a secondary green subpixel in the related subpixels410,412to the same brightness as a primary green subpixel if another bit indicates that the least significant bit of the blue sub pixel control element408should be used to determine which of the related subpixels410,412to drive (“Video” equals 1 inFIG. 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 subpixels402,406,410would be illuminated at the same time as a white subpixel412, a least significant bit of one or more control signals to the non-combined control elements404,408may indicate if the combined control element414should illuminate the white subpixel412(“B0equals 1 inFIG. 3). It may be appreciated that the actual values may depend on the architecture of the selection logic in the switching element416. For clarity and simplicity,FIG. 5shows a similar chart of digital inputs to the switching element416wherein the RGBG input is removed. The switching element416may still be addressable to illuminate one or both of the related subpixels410,412.

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.