Patent Application
20240371320
The present disclosure relates to the field of driver circuits for light emitting elements of a light emitting module, chips for such driver circuits and light emitting modules.
It is a current tendency to progress towards light emitting elements having a smaller size to improve today's displays resolution. It has become a new trend to build displays with for example micro-LEDs (μLEDs).
The use of micro-LEDs (μLEDs) in LED display technology brings about new challenges to be solved. μLEDs have, as indicated in their name, a micrometer-size scale. Accordingly, they also require micrometer-size scale contacting methods.
Currently, a lot of novel systems are under investigation as traditional contacting methods, such as soldering, gluing, are not possible with μLEDs, especially since these materials need to be precisely applied on the target by either reflow stencil (limited in aperture and positioning tolerances) or XY dispensing equipment (limited in volume and XY positioning accuracy during dispensing).
Furthermore, due to the small contact pads, the μLED architecture cannot be based on PCB anymore. In fact, the use of PCB restricts the dimensions of the wires to a size which is even larger than the LEDs themselves. The industrial manufacturing process has limitations. The substrates need to be lithographically defined (conf. LCD, OLED, silicon chips, etc.) and comprises single-sided contacts and processing due to the process technology (e.g., TFT LTPS technology).
Currently, most LED displays are driven by passive matrix drivers, located on the backside of the LED panels, to keep the distance between the chip and the LED as small as possible and thereby avoid performance reduction due to parasitic effects. PM has the advantage that the PWM is generated in driver chips with time-multiplexed driving of groups of LEDs, sharing the same current source.
Since the technology of μLEDs will be preferably TFT single side based, this is not possible anymore. AM has the advantage of integrating active components in the display substrate and sample—and—hold the incoming data by means of a scanline to the individual pixels. To generate PWM driven constant current, extra TFTs are needed in the pixels, which usually do not fit the available area in the active matrix.
There is thus a need for improvement in the art.
Aspects of the present disclosure relate to a driver circuit for driving a matrix of N×M pixels of a light emitting module, wherein each pixel is composed of at least three types of light emitting elements. The light emitting elements being driven by a modulation control signal of period T configured to switch the light emitting elements on and off at most during 1/Nth of the period T, wherein the period T corresponds to the duration of a frame. The driver circuit is embedded in a TFT layer such that a switch is embedded in each light emitting element. The driver circuit is also configured to cooperate with N multiplexers provided in an external driver circuit, each multiplexer being configured to drive one line of M pixels of the matrix, and the combination of multiplexers is configured to address all the pixels of the N×M matrix during one period of the control signal. Each pixel of a group of M pixels is shown consecutively during at most 1/Nth of the period T. The driver circuit is also configured to cooperate with at least one external current source per type of light emitting element to drive the N×M matrix. Each external current source is mirrored M times on the TFT layer of the driver circuit and is arranged in series with a switch for generating the control signal provided for each of the M columns of the matrix.
A matrix of N×M pixels shares common functionalities, which allows to gain space for each pixel. The numbers of N and M pixels can be decided based on a trade-off, depending on the type of application and the requirements of space, data to transfer, etc. In fact, N determines the number of multiplexers for the time division multiplexing. This will also have an impact on visual artefacts which may appear on the display. M determines the maximum intensity of the current to provide the desired maximum signal.
A current source is provided for each type of sub-pixel, or for each color, as each color is to be driven independently of the others to be able to achieve the desired white point of the display. The functionalities provided by the TFT can be shared among multiple pixels.
The current sources and the PWM transistor are shared, instead of being individual as in a classic active matrix driving scheme. It is also an advantage that some functionalities remain on the active matrix, i.e., the current mirrors and the PWM switch.
As TFT is not an ideal conductor, it is preferable to implement the multiplexers on an external driver circuit.
The control signal is preferably a pulse width modulation signal, and the control signal switch is a pulse width modulation switch.
Pulse with modulation permits to reduce the average power delivered by an electrical signal, reduce power losses while being able to provide a signal with sufficient bit depth.
Other types of signals can also be implemented, such as Pulse Amplitude Modulation, PAM. The control signal switch is then adapted to PAM driving.
Preferably, the three types of light emitting elements emit a different color, wherein the different colors are at least red, green, and blue. The desired white point can thereby be achieved. In addition, it is possible to provide an additional light emitting element per pixel, for example red, green or blue, such that each pixel comprises two distinct light emitting elements and two identical ones. This can be beneficial for implementing for example a Bayern pattern. An additional current source would then be provided for the additional light emitting element per pixel.
The multiplexers are preferably provided in a PCB arranged under of the matrix. They can be connected by side connections, such as a side flexible PCB for example.
The multiplexers can be transistors provided by a low Rds (on) FET.
In fact, TFT is not an ideal conductive material. Such an implementation of transistors outside of the TFT has the advantage that there is no voltage drop on the TFT, and thereby the appearance of hot spots is avoided. In addition, this implementation results in very low power losses, and heat dissipation.
If it were implemented on the TFT, all the currents would go through the cathode side of the diodes, through one transistor and since the transistor is not an ideal transistor, this would result in a high voltage drop over the switching transistor and result in heat generation on the TFT, which may further result in the appearance of hot spots.
Each multiplexer may also be configured to drive a line of M pixels.
Such a configuration has the advantage of being simpler to implement.
The lines of the multiplexers for addressing the pixels can be such that one line for addressing the pixels comprises pixels of different rows.
This provides the possibility of avoiding visual artefacts by making light emitting elements of different rows light up simultaneously.
The lines of the multiplexers for addressing the pixels may also be such that at least two pixels in the same column are addressed by the same line.
This provides the possibility of further reducing visual artefacts by increasing the possibility to randomize the order in which light emitting elements light up during a cycle.
The order in which the multiplexers are addressed may advantageously not be linear. Such an addressing scheme may also further reduce visual artefacts.
Advantageously, the pixels are addressed several times during one frame.
This enables the reduction of visual artifacts.
Preferably, the colors of sub-pixels composing a pixel are at least red, green, blue, and may further comprise any one of red, green, blue, white, yellow, cyan, magenta or any other color.
This driving scheme is possible with any type of display. A current source may be provided for each type of sub-pixel, such that the driving can be adapted.
Preferably, the light emitting elements are any one of LEDs, OLED, and variations thereof, QD-LED, EL-QLED, AMOLED, mini-LED, micro-LED. The invention is not limited to a particular type of light emitting element, and the advantages provided can be beneficial to any type of light emitting element.
Advantageously, the light emitting elements are provided with quantum dots to generate the different colors of emission. Quantum dots have the advantage of improving the brightness of a display and can also improve the color points.
In another aspect there is provided a first chip configured to cooperate with the driver circuit defined above. The first chip preferably comprises N multiplexers, each multiplexer being configured to drive one line of M pixels, and the combination of multiplexers being configured to address all the pixels of the N×M matrix during one period of the signal, such as Pulse Width Modulated signal.
If a display module comprises a total of G light emitting modules, the number of lines P per display module will be divided in G groups of N lines such that G*N=P, each group having a unique set of current sources for red (R), green (G) and blue (B).
In another aspect there is provided a second chip configured to cooperate with the driver circuit defined above, wherein the second chip may further comprise one current source per color, or sub-pixel, to drive the N×M matrix. The current source is mirrored M times on the TFT layer and arranged in series with a switch, such as a PWM switch, provided in each of the M columns of the matrix.
In another aspect there is also provided a chip configured to cooperate with the driver circuit and comprising the first and second chips defined above.
In another aspect there is also provided a light emitting module comprising a matrix of N×M pixels, at least one driver circuit associated to said matrix as defined above, and for each driver circuit and associated matrix, a first and second chip or the combined chip, as defined above.
There is also provided a display module comprising at least one light emitting module.
There is also provided a tiled display comprising at least one display module.
The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, may be best understood by reference to the following detailed description of the invention, which describes an exemplary embodiment of the invention, taken in conjunction with the accompanying drawings, in which:
Active matrix is a type of addressing scheme used in flat panel displays. In this method of switching individual elements (pixels), each pixel is attached to a transistor and capacitor actively maintaining the pixel state while other pixels are being addressed.
Active-matrix circuits are commonly constructed with thin-film transistors (TFTs) in a semiconductor layer formed over a display substrate and employing a separate TFT circuit to control each light-emitting pixel in the display. The semiconductor layer is typically amorphous silicon, poly-crystalline silicon and is distributed over the entire flat-panel display substrate.
A display sub-pixel can be controlled by one control element, and each control element includes at least one transistor. For example, in a simple active-matrix light-emitting diode display, each control element includes two transistors (a select transistor and a power transistor) and one capacitor for storing a charge specifying the luminance of the sub-pixel. Each LED element employs an independent control electrode connected to the power transistor and a common electrode. Control of the light-emitting elements in an active matrix known to the art is usually provided through a data signal line, a select signal line, a power or supply connection (referred to as e.g., VDD) and a ground connection.
Throughout the description of the present invention, active matrix refers to a functionality implemented in the TFT layer on the glass.
A display screen can be composed of light emitting pixel structures referred to as “display pixels” or “pixels” where the amount of display pixels determines the “display resolution”, sometimes referred to as the “native display resolution” or the “native pixel resolution”. A measure of the display resolution can be the total number of display pixels in a display, for example 1920×1080 pixels. Each display pixel can emit light in all colors of the display color gamut (i.e. the set of colors the display is able to provide).
Each display pixel can be composed of light emitting structures referred to as “sub-pixels”, often being able to emit the colors red (R), green (G) or blue (B) (but also white, yellow or other colors are possible). A display pixel can be composed of at least three sub-pixels: One red, one green and one blue sub-pixel. Additionally, the display pixel can comprise other sub-pixels in any of the aforementioned colors (to further increase the color gamut). Depending on the types of sub-pixels, the display pixel can then be referred to as a RGB-, RGGB-, RRGB-pixel, etc. While a single display pixel can generate all colors of the display color gamut, a single sub-pixel cannot.
The light emission of a single sub-pixel can be controlled individually so that each display pixel can emit the brightness and color required to form the requested image. The distinction between display pixels and sub-pixels will be used consistently in this text.
A light emitting module is an opto-mechanical-electronic carrier of a certain size which carries light emitting elements directed towards a viewer and possible light emitting elements driving and control electronics. These light emitting elements are driven to create an image, either static or dynamic (video). In the following the light emitting module will be called an “LED module”, although the invention is not restricted to LEDs. Several LED modules or OLED modules can be positioned next to each other to form a display module. Several display modules can be tiled together to form a larger tiled display.
A small LED module which is an atomic element, i.e., indivisible, can be called a “stamp”. The light emitting module can have any size and shape. It can be rectangular or square, hexagonal, triangular, any shape, if it fits in a pick and place robot used to place it on a display module. It can also comprise one pixel, which comprises a red, green, and blue LED.
The light emitting module comprises at least one backplane. The top surface of the backplane comprises the light emitting elements and associated conducting tracks which connect the various light emitting elements to various electronic components (like e.g., current drivers, power supply contacts etc.). The backplane can be a PCB, TFT on glass, TFT on PI, etc.
The following patent applications, from the same applicant, provide definitions of LED displays and related terms. These are hereby incorporated by reference for the definitions of those terms.
Pulse-width modulation uses a rectangular pulse wave whose pulse width is modulated resulting in the variation of the average value of the waveform. The square wave has a period T, a lower limit 10 (typically 0 in our case), a higher limit 11 and a duty cycle D. The duration of one pulse P (the time during which the signal is at its higher limit) is D/100*T (if D is expressed in %). For instance, if D=50%, the duration of the pulse is ½ T. A more complete definition can be found in WO2019185935A1 from the same applicant.
PWM switch is a device configured to interrupt the driving source of a light emitting device in a time-modulated (PWM) mode.
Terminology used for describing particular embodiments is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term “and/or” includes any and all combinations of one or more of the associated listed items. It will be understood that the terms “comprises” and/or “comprising” specify the presence of stated features but do not preclude the presence or addition of one or more other features. It will be further understood that when a particular step of a method is referred to as subsequent to another step, it can directly follow said other step or one or more intermediate steps may be carried out before carrying out the particular step, unless specified otherwise. Likewise it will be understood that when a connection between structures or components is described, this connection may be established directly or through intermediate structures or components unless specified otherwise.
The present invention will be described with respect to particular embodiments and with reference to certain drawings, but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Where the term “comprising” is used in the present description and claims, it does not exclude other elements or steps.
Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.
The terms “about” or “approximate” and the like are synonymous and are used to indicate that the value modified by the term has an understood range associated with it, where the range can be +20%, +15%, +10%, +5%, or +1%. The term “substantially” is used to indicate that a result (e.g., measurement value) is close to a targeted value, where close can mean, for example, the result is within 80% of the value, within 90% of the value, within 95% of the value, or within 99% of the value.
As mini and micro-LEDs are determining the display trends of the future, there will be a high need for driving the LED matrix, either by Passive Matrix (PM), well known in current LED displays and driven by dedicated chips generating the Pulse Width Modulation (PWM) and Active Matrix (AM), well known in the LCD industry to be deployed on large area glass but using Analog drive, not suited for driving direct view LED displays.
Both systems provide advantages and disadvantages, but none offers a single solution for driving μLEDs.
PM has the advantage that the PWM is generated in driver chips with time-multiplexed driving of groups of LEDs, sharing the same current source.
AM has the advantage of integrating active components in the display substrate and sample—and—hold the incoming data by means of a scanline to the individual pixels. To generate PWM driven constant current, extra TFTs are needed in the pixels, which usually do not fit the available area in the active matrix.
In AM, the switching and the current source is usually implemented for each sub-pixel. However, this is not possible for micro-LEDs as the dimensions of the contact pads are drastically reduced.
The present invention provides means to combine the functionalities of AM and PM for driving light emitting elements, such as LEDs, and is particularly advantageous for driving mini—and micro-LEDs. It is possible to benefit from using shared current sources among the pixels and sample/hold the PWM value in the pixels. Sharing common resources, such as the current sources and the switching can provide the required space. This is made possible with the use of multiplexers, to be able to address each pixel individually. A current source for every individual pixel and a switching transistor for every individual pixel may not be needed anymore.
In the present invention, the active matrix is implemented on a TFT layer. The display pixels are composed of at least three sub-pixels provided by light emitting elements of different colors, the pixels being arranged in an N×M matrix. This active matrix may only represent a portion of a light emitting module, as described above.
Thus, several active matrices as described in the present specification may be implemented on a light emitting module.
The pixels of each N×M matrix on the TFT layer of the light emitting module will therefore share common resources and enable to reduce the number of contacts on the TFT layer. This will result in more space for implementing micro and mini-LEDs for example. This technology may also be beneficial to other types of light emitting elements.
Pulse width modulation (PWM) driving can be used to achieve the gray scales in this passive matrix driving. A current source 230 is provided next to a PWM switch 220, in each column of the passive matrix. Each row comprises a time-multiplex switch, 2010 (Mux 0), 2011 (Mux 1), 2012 (Mux 2), 2013 (Mux N−1).
Since the active time of the LEDs is reduced proportionally to the number of multiplexers, the momentary light-output needs to be increased accordingly to end up with the same brightness in the given timeframe. This is achieved by increasing the pulsed current proportionally to the number of multiplexers.
The inventors have imagined implementing this driving scheme in an Active Matrix. To achieve good results, the following considerations need to be considered when implementing PM driving on an Active Matrix:
The driver circuit 3010 of the active matrix implemented on the TFT layer 3000 is configured to cooperate with N multiplexers 3110, 3111, 3112, 3113 provided for example in an external driver circuit 3100, each multiplexer being configured to drive for example one line of M pixels, and the combination of multiplexers being configured to address all the pixels of the N×M matrix during one period of the Pulse Width Modulated signal. In this example, N=M=4.
The driver circuit 3010 is further configured to cooperate with at least one external current source 3230 per color to drive the N×M matrix, said current source 3220 being mirrored M times 3230, 3231, 3232, 3233 on the active matrix and arranged in series with a PWM switch 3240, 3241, 3242, 3243 provided in each of the M columns of the active matrix.
In
In addition, the current sources are also provided by an external circuit 3100, which may be the same as the one comprising the multiplexers or a different one. They can thus be provided by a second backplane, or also implemented on a PCB provided in the back and connected to the active matrix by side contacts. The current sources may also be implemented on a chip.
It is also possible to combine the implementation of the external multiplexers and external current sources in a same chip. This chip could then be implemented in the second backplane and connections to the N×M pixels made via through holes in the PI, provided under the TFT, or by side contacting.
With this driving scheme, instead of providing one current source per pixel, as is the case in AM driving, one current source is shared among several (N) pixels.
In addition, current mirroring is implemented on the TFT with a current mirror transistor per type of light emitting element (color) or per sub-pixel. Copying the incoming reference current to each light emitting element column is done by the pulse width modulation switch 3240, 3241, 3242, 3243, thereby interrupting the current during the dutycyle of each of the N time-multiplexers 3110, 3111, 3112, 3113. Sharing current sources has the advantage of reducing the panel-overhead.
Current mirrors are a technique commonly used in semiconductors, either in chips or TFT backplanes. The principle is that a current reference is ‘mirrored’ to the application to provide a multitude of the same current-source available to feed different circuitry.
Besides mirroring, it is also possible to reduce or increase the current in a determined ratio, defined by design-parameter (so called W/L ratio in TFT terms). The advantage is that multiple ‘slave’ MOSFETs can be connected in parallel to the same reference voltage “Vref”, resulting in a multitude of current sources, copied from the same source. By changing the source, the base current can be easily set for a complete panel.
In addition, since each column of light emitting elements needs a current source, an external connection to a source for each column is not needed anymore, but a copy in each column to the same source. It is thus desirable to keep the reference voltage Vref stable over the entire panel. This Vref can be referenced as GND (current sink) or to Vdd (current source). It is thus desirable to keep the voltage drop minimal over the panel to keep the current constant over the panel.
The current to be fed in this AM/PM combined mode corresponds to the current needed to achieve the desired brightness in AM mode, multiplied by N (number of time-multiplexers), as each light emitting element in only on 1/Nth of the time. However, as the current is provided by an external circuit, the resistance Rds (on) is lower than in a TFT. Heating may thus be avoided and the appearance of hot spots throughout the panel are also avoided.
By combining PWM and time-multiplexing, a group of N×M pixels is driven together and share common driving functionalities. Thus, if each pixel comprises three sub-pixels, each group of N×M pixels is driven by a set of M×3 PWM inputs to the switches and N inputs to the mux-switches which results in a total of (M×3)+N contacts to be made on the panel.
Typically, PM drivers can drive 16 RGB pixels with 16 time-multiplexers (muxes), resulting in 16×16=256 pixel. The number of connections for every chip comprises then 3×16+16=64 contacts.
With this AM/PM approach, the number of contacts is not limited. Since all the ‘analog’ functions are incorporated in the light emitting module (e.g. current sources & PWM switch), only the digital functions remain in the light emitting module and the module can be driven by digital drivers (i.e. programmable logic) and does not require the use of mixed signal chips. The multiplexers have the width of a panel wide, contrary to PM which are limited to a part of a line.
The advantages of this combined driving scheme are the following:
While this solution provides many advantages, there are also some drawbacks. For example, time multiplexing may cause visual artifacts since only 1/Nth of the pixels are simultaneously on at one moment in time. These visual artifacts may however be reduced by changing the arrangements of the driving lines of the multiplexers. For example, one line of a multiplexer could address pixels which are arranged in different columns also in different rows, or address only pixels of certain columns and certain rows, etc.
Other addressing schemes are of course possible. Any random order in addressing the pixels in the rows or in the columns may be provided.
Each multiplexer of the N multiplexers may, instead of being on following the order of the rows, be on following a random order or a special sequence.
The light emitting elements of a same color or type are arranged in a same column and are connected to a same Pulse Width modulation switch such that light emitting elements R1, R2, R3 and R4 receive signal PWM1. R1, R2, R3 and R4 are all connected to a different multiplexer and all four light emitting elements are thus not active simultaneously.
In
As shown in
A second control on the LED provided by the PWM switch is used during this time to generate the PWM signal. Thus, there is only a part of the LEDs which are active at one moment in time which could result in visual artefacts. By increasing the cycle time (=N*(Ton+Toff)) this effect can be minimized.
On top of that, the active LEDs per MUX do not have to be arranged in a single line but these can be rearranged differently over the panel, as already discussed above. It will of course make the connections more complex since the MUX preferably follows a row/column arrangement wherein at each crossing a LED can be connected.
Reduce the number of driving signals to the pixels.
Each pixel is usually composed of three sub-pixels, a red, green, and blue sub-pixel. These are usually embedded by three LEDs of a different color for example, or with quantum dots.
The colors of sub-pixels composing a pixel are at least red, green, blue, and may further comprise any one of red, green, blue, white, yellow, cyan, magenta or any other color. However, a pixel may also comprise additional sub-pixels, such as an additional red, green, or blue sub-pixel, or even an additional yellow, or white pixel, or even, cyan or magenta.
Usually, the current source may be provided for each sub-pixel composing a pixel. Thus, if a pixel is composed of four sub-pixels, e.g., red, green, green, blue, then four current sources may be provided for each sub-pixel and may be mirrored M times in the N×M active matrix group of pixels.
While the invention has been illustrated and described mostly in reference to LEDs or micro LEDs, the invention is not limited thereto, and may also be advantageous for other the types of light emitting elements, such as of LEDs, OLED, and variations thereof, QD-LED, EL-QLED, AMOLED, mini-LED, micro-LED.
The driver circuit described in the present disclosure may also be used for driving other elements or components than light emitting elements. In fact, this driving scheme provides many advantages which are applicable in other fields. For example, instead of the light emitting elements, other types of sensors, such as for example photometers can be used.
It will be appreciated that various modifications may be made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of the driver circuit and the various chips and component parts thereof and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment.
| Number | Date | Country | Kind |
|---|---|---|---|
| LU500366 | Jun 2021 | LU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/068038 | 6/30/2022 | WO |
