Patent Application
20240177649
The present disclosure relates to light-emitting displays with pixel control circuits that use temporally variable constant-current control, such as pulse-width modulation, and provide dimming control.
Flat-panel displays are widely used to present images and information in graphic user interfaces controlled by computers. Such displays incorporate an array of light-controlling pixels. Each pixel emits or otherwise controls light. For example, liquid crystal displays control light emitted from a back light with a light-blocking liquid crystal at each pixel, organic light-emitting displays emit light from a stack of organic films, and inorganic light-emitting displays emit light from semiconductor crystals. In binary displays, each pixel controls light to be on at a desired luminance or off at a zero luminance. More commonly, pixels control light over a range of luminances, from off to a maximum designed luminance. The number of distinct luminance levels in a display pixel can be referred to as the gray scale and is defined as a bit depth for a computer-controlled display, for example an eight-bit gray-scale range having 256 different luminance levels or a twelve-bit gray-scale range having 4096 different luminance levels. In general, a greater luminance range is preferred to display images with more shades of light and dark in a color or color combination such as white with reduced contouring.
Portable displays can be used in a wide variety of ambient luminance environments, such as in a dark room or outdoors on a sunny day. The human visual system can adapt to such different ambient luminance environments by increasing or reducing the amount of light admitted to the eye so that a given display at a specific luminance can appear bright when viewed in a dark environment and dim when viewed in a bright environment. To achieve a consistent appearance to the human visual system, a display must have a lesser luminance in a dark environment and a greater luminance in a bright environment. Thus, to be useful in such a wide range of dark and bright environments, a display can benefit from a very wide dynamic range as well as many distinct luminance levels.
Depending on the pixel light-control technology, the luminance of a pixel can be controlled by, for example, driving a pixel over a range of voltages, over a range of currents, or at a constant power (e.g., at a given voltage and current) for a variable amount of time. Pixels that control light with variable time periods can use pulse-width or pulse-density modulation techniques that assign each bit of a multi-bit pixel value to one or more time periods having a total temporal length corresponding to the relative value of the bit in the multi-bit pixel. For example, in a four-bit pixel, the least significant bit can have a temporal period equal to one minimum period and the most significant bit can have a temporal period equal to eight minimum periods. However, in practical implementations, the minimum period can have a value that is limited by the electronic circuits driving the pixels, thereby limiting the luminance range and gray scale of pixels in a display at a given image frame rate.
There is a need, therefore, for pixel control circuits in displays using temporal modulation that provide improved gray-scale bit depth, image frame rates, and dynamic range.
According to some embodiments of the present disclosure, among other embodiments, pixel control circuits in displays can use temporal modulation with a constant power, for example pulse-width or pulse-density modulation with a constant current when turned on, to provide improved gray-scale bit depth, image frame rates, and dynamic range with dimming control that can control the temporal periods of a pulse-width modulation pixel signal.
According to embodiments of the present disclosure, a system with dimming control comprises a variable-frequency-clock signal generator responsive to a luminance signal operable to generate a variable-frequency clock signal, a pixel-control signal generator responsive to the variable-frequency clock signal operable to generate a temporally modulated pixel-control signal, and a pixel comprising a light controller (e.g., a light emitter) responsive to the pixel-control signal. In embodiments, the variable-frequency-clock signal generator is operable to change a frequency or period of the variable-frequency clock signal in response to the luminance signal. The variable-frequency-clock signal generator can be operable to increase the frequency of the variable-frequency clock signal in response to the luminance signal to reduce luminance of the light controller and the variable-frequency-clock signal generator can decrease the frequency of the variable-frequency clock signal in response to the luminance signal to increase luminance of the light controller.
Some embodiments comprise an array of pixels, each of the pixels comprising a light controller responsive to a pixel-control signal from a pixel-control signal generator. Some embodiments comprise a display controller and the display controller comprises the variable-frequency-clock signal generator and the pixel-control signal generator and provides the pixel-control signal to the pixel. Some embodiments comprise a display controller, the display controller comprises the variable-frequency-clock signal generator and provides the variable-frequency clock signal to the pixel and the pixel is operable to receive the variable-frequency clock signal and generate the pixel-control signal in response to the variable-frequency clock signal. Some embodiments comprise a display controller, and the display controller provides the luminance signal to the pixel and the pixel is operable to receive the luminance signal, generate the variable-frequency clock signal in response to the luminance signal, and generate the pixel-control signal in response to the variable-frequency clock signal.
According to some embodiments, the system is operable to display a specified luminance with the light controller in a frame period, the luminance signal is a percent of the frame period, the pixel-control signal has a variable pixel period, and the variable pixel period is equal to the luminance signal times the frame period but no greater than the frame period and no less than a minimum pulse period determined by the system.
According to some embodiments, the pixel comprises multiple light controllers and the system provides each of the light controllers with a different variable pixel period.
In some embodiments, the temporally modulated signal is a constant-current time-modulation signal comprising pulse periods. In some embodiments, the pulse periods correspond to binary-weighted bits that specify a pixel value corresponding to a desired light-emitter luminance.
Some embodiments comprise a frame period. In some embodiments, (i) the binary-weighted bits comprise N bits, (ii) each pulse period has a relative temporal duration corresponding to a relative value of a different bit of the binary-weighted bits, and (iii) a least-significant-bit pulse period has a temporal duration equal to the (frame period times the luminance signal)/(2N−1). In some embodiments, (i) the binary-weighted bits comprise N bits and (ii) each of the pulse periods has a relative temporal duration equal to a (frame period times the luminance signal) divided by the pixel value.
A minimum pulse period can be greater than the least-significant-bit pulse period. Some embodiments comprise a frame period and (i) the binary-weighted bits comprise N bits, (ii) each pulse period has a relative temporal duration equal to the (frame period times the luminance signal) divided by the pixel value. In some embodiments, a minimum pulse period is no greater than the least-significant-bit pulse period and all of the pulse periods have temporal durations that are substantially equal. In some embodiments, a minimum pulse period is no greater than the least-significant-bit pulse period and at least two of the pulse periods have temporal durations that are substantially different and are not a relative power of two.
In some embodiments, the pixel is operable to control the light emitter at a constant current during each of the pulse periods.
According to embodiments of the present disclosure, a display with dimming control comprises a system comprising an array of pixels responsive to the pixel-control signal and a display controller operable to receive or generate the luminance signal for each of the pixels. The display controller can be operable to (i) provide the luminance signal to each of the pixels in the array of pixels, (ii) provide the variable-frequency clock signal to each of the pixels in the array of pixels, or (iii) provide the pixel-control signal to each of the pixels in the array of pixels. In some embodiments, the display controller is operable to (i) provide different luminance signals to one or more pixel groups of pixels in the array of pixels, (ii) provide different variable-frequency clock signals to one or more pixel groups of pixels in the array of pixels, or (iii) provide different pixel-control signals having different pixel periods to one or more pixel groups of pixels in the array of pixels. In some embodiments, the array of pixels comprises rows of pixels and columns of pixels, the variable-frequency signal is provided on row wires to rows of pixels, pixel values are provided on column wires to columns of pixels, and the variable-frequency signal is a pulse-width modulation signal. The variable-frequency signal can have a constant frequency and the variation responsive to the luminance signal can be the frequency. The variable-frequency signal can have a constant pixel period and the variation responsive to the luminance signal can be the temporal duration of the pixel period.
According to embodiments of the present disclosure, a method of operating a pixel with dimming control comprises receiving a first luminance signal, receiving a clock signal, generating a first variable-frequency clock signal responsive to the first luminance signal and the clock signal, receiving a second luminance signal different from the first luminance signal, generating a second variable-frequency clock signal responsive to the second luminance signal and the clock signal. If a luminance corresponding to the first luminance signal is greater than a luminance corresponding to the second luminance signal, a frequency of the first variable-frequency clock signal can be less than a frequency of the second variable-frequency clock signal. If a luminance corresponding to the first luminance signal is less than a luminance corresponding to the second luminance signal, a frequency of the first variable-frequency clock signal can be greater than the frequency of the second variable-frequency clock signal.
In some embodiments of the present disclosure, a method of operating a pixel comprises receiving a luminance signal, generating a variable-frequency clock signal based on the luminance signal, generating a pixel-control signal based on the variable-frequency clock signal, wherein the pixel-control signal is a temporally modulated signal, and driving a light controller using the pixel-control signal. Some methods of the present disclosure comprise changing a frequency of the variable-frequency clock signal in response to the luminance signal. Some methods of the present disclosure comprise increasing a frequency of the variable-frequency clock signal in response to the luminance signal to reduce luminance of the light controller and decreasing a frequency of the variable-frequency clock signal in response to the luminance signal to increase luminance of the light controller.
In some embodiments of the present disclosure, a display controller provides the luminance signal to a variable-frequency-clock signal generator and the variable-frequency-clock signal generator receives the luminance signal and generates the variable-frequency clock signal.
Some methods of the present disclosure comprise displaying a specified luminance with the light controller in a frame period, wherein the luminance signal is a percent of the frame period, the pixel-control signal has a variable pixel period, and the variable pixel period is equal to the luminance signal times the frame period but no greater than the frame period and no less than a minimum pulse period determined by the system. In some embodiments, the temporally modulated signal is a constant-current time-modulation signal comprising pulse periods. In some embodiments, the pulse periods correspond to binary-weighted bits that specify a pixel value corresponding to a desired light-emitter luminance.
In some methods of the present disclosure (i) the binary-weighted bits comprise N bits, (ii) each pulse period has a relative temporal duration corresponding to a relative value of a different bit of the binary-weighted bits, and (iii) a least-significant-bit pulse period has a temporal duration equal to a (frame period times the luminance signal)/(2N−1). In some methods, a minimum pulse period is greater than the least-significant-bit pulse period. In some methods, a minimum pulse period is no greater than the least-significant-bit pulse period and all of the pulse periods have temporal durations that are substantially equal. In some methods, a minimum pulse period is no greater than the least-significant-bit pulse period and at least two of the pulse periods have temporal durations that are substantially different and are not a relative power of two.
In some methods, (i) the binary-weighted bits comprise N bits and (ii) each of the pulse periods has a relative temporal duration equal to a (frame period times the luminance signal) divided by the pixel value. In some methods, the pixel is operable to control the light emitter at a constant current during each of the pulse periods. Certain embodiments of the present disclosure provide a control circuit for temporally modulated pixels in a display that provide improved gray-scale resolution and dimming control. Control circuits disclosed herein are suitable for inorganic micro-light-emitting diodes and can be applied in an array of pixels in a display.
The foregoing and other objects, aspects, features, and advantages of the present disclosure will become more apparent and better understood by referring to the following description taken in conjunction with the accompanying drawings, in which:
Features and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.
Certain embodiments of the present disclosure provide systems and displays comprising one or more temporally modulated pixels with dimming control and no loss of gray-scale resolution at a given image frame rate useful in a display. The pixels can operate at a constant current using pulse-width modulation or pulse-density modulation to control a light controller, for example a light emitter such as an inorganic micro-light-emitting diode. A display can comprise an array of the pixels.
Pixel circuits can have a limited frequency capability, for example a minimum switching period or maximum switching frequency that defines the shortest controllable temporal pulse received or provided by the pixel circuits. This minimum temporal period limits the minimum amount of time that a light controller controlled by the pixel circuit in a pixel can controllably emit light and is referred to herein as a minimum pulse period. This limitation also specifies the maximum frame rate (the minimum frame period) for a display comprising an array of such pixels with a determined temporal control signal, for example the number of bits in a pulse-width-modulation signal. For pixels controlled by temporally modulated signals such as pulse-width-modulation (PWM) signals or pulse-density-modulation (PDM) signals having pulse periods during which light controllers emit light, the smallest pulse period is likewise limited by the shortest controllable temporal pulse and therefore limits the number of different temporally modulated signal values possible in a given period of time (e.g., a PWM signal in an image frame period) and therefore the gray-scale resolution of the pixel. Thus, there is an inherent limit to the image frame rate and gray-scale resolution that can be supported by a pixel circuit defined by the hardware implementation of the pixel and display.
The minimum temporal control period in a pixel circuit might be limited, for example, by the slew rate of an electronic input or output signal, control signal, or driving transistor, by the parasitic resistance, capacitance, or inductance of control signal wires or driving wires, by the pixel circuit's ability to drive or respond to a desired amount of current at a given voltage, or by the pixel circuit's ability to drive or respond to a desired voltage at a given current. For example, if a minimum temporal control period is five hundred nanoseconds and an eight-bit PWM signal is used to control a pixel, the maximum frame rate for a pixel is 255*0.0000005=0.000128 seconds or almost 8000 frames per second. If a twelve-bit signal PWM signal is used with a minimum temporal control period of fifty microseconds, the maximum frame rate for a pixel is about five image frames per second. Contemporary displays can operate at frame rates of up to 480 frames per second (or more) with gray-scale resolutions of twelve bits (4096 levels) or more. In some displays, even greater gray-scale resolutions, for example sixteen or twenty bits, and frame rates, for example 960 frames per second or more can be desired.
The electronic circuits available in some displays can have relatively large and slow transistors (e.g., in thin-film transistor circuits coated on a display substrate). More complex circuits and faster-switching materials can operate at higher frequencies and provide more power at higher voltages but can be more expensive or impractical for a given display. There is, therefore, a need for pixel circuits, in particular digital pixel-control circuits, that can provide improvements in frame rate and gray-scale resolution without requiring expensive and complex control circuits.
Micro-light-emitting diodes (micro-LEDs) provide electrically and optically efficient light output with excellent color saturation and are therefore desirable light emitters in a display. Such micro-LEDs can operate most efficiently at a given constant current and are therefore advantageously operated at the given constant current using temporally modulated signals such as pulse-width modulation or pulse-density modulation. Embodiments of the present disclosure provide efficient and simple systems, circuits, devices, and methods for operating a pixel at a constant current using temporal modulation with dimming control and without loss of gray-scale resolution. Systems, circuits, devices, and methods of the present disclosure can also provide extended dimming control with some reduction in gray scale for pixel luminance, for example where temporally controlled luminance is limited by a minimum temporal pulse length or frame rate.
According to embodiments of the present disclosure and as illustrated in
Light emitter 21 can be a light-emitting diode (LED) such as a micro-light-emitting diode formed in a compound semiconductor. Pixel-control signal generator 17 and variable-frequency-clock signal generator 11 can comprise analog circuit elements, digital circuit elements, or comprise a mixed-signal circuit comprising both analog and digital circuit elements made in a suitable semiconductor such as silicon or a compound semiconductor using photolithographic methods and materials. Either or both pixel-control signal generator 17 and variable-frequency-clock signal generator 11 can be integrated circuits or can be provided in a common integrated circuit. The integrated circuit and light emitter 21 can each be a bare unpackaged die assembled by micro-transfer printing and, in some embodiments, can comprise broken or separated tethers in consequence. In some embodiments, the integrated circuit is a silicon circuit on which light emitter 21 is disposed and to which light emitter 21 is electrically connected, for example using photolithographic methods and materials.
Variable-frequency-clock signal generator 11 can also receive or generate a clock signal 10, for example a clock operating at a desired fixed frequency from which variable-frequency clock signal 14 can be derived in response to luminance signal 12. Variable-frequency clock signal 14 can change frequency in response to luminance signal 12. Pixel-control signal generator 17 can also receive a pixel value 16, for example representing a desired relative luminance of light emitter 21 and can be a digital or analog value. For example, in a digital eight-bit system, pixel value 16 can be a value from 0 to 255 and in a twelve-bit system a value from 0 to 4095. Luminance signal 12 can be a dimming signal that indicates percent values greater than, equal to, or less than 100% and can be any suitable digital or analog signal that represents a desired percent output of a light output from light emitter 21 at a given pixel value 16, for example 150%, 125%, 75%, 50%, 25% or 12.5%. In some embodiments, variable-frequency clock signal 14 increases in frequency in response to luminance signal 12 to reduce light emitter 21 luminance (e.g., luminance signal 12 has a value less than 100%) and variable-frequency clock signal 14 decreases in frequency in response to luminance signal 12 to increase light emitter 21 luminance (e.g., luminance signal 12 has a value greater than 100%).
As illustrated in
As shown in
As shown in
Variable-frequency clock signal 14 can have a variety of forms. In some embodiments, variable-frequency clock signal 14 is a regular signal having a consistent frequency responsive to luminance signal 12. From this frequency, pixel-control signal generator 17 can generate pixel-control signal 18. If pixel-control signal 18 is a pulse-width modulation signal, for example, pixel controller 22 can comprise a counter that generates pulses for each pulse period 42 corresponding to pixel value 16. In some embodiments, variable-frequency clock signal 14 is a pulse-width modulation signal having pulse periods 42 that have relative temporal durations that are successive powers of two. Pixel period 44 of the pulse-width modulation signal is responsive to luminance signal 12. In such embodiments, pixel-control signal generator 17 (e.g., in pixel controller 22) can combine the pulse-width modulation signal with pixel value 16 to provide the appropriate pixel-control signal 18, for example by turning pulse periods 42 of the pulse-width modulation signal on (setting it to a logical value of one to provide a non-zero voltage and current to light controller 21) or off (setting it to a logical value of zero to provide a voltage and current of zero to light controller 21). Thus, variable-frequency-clock signal generator 11 comprises the pulse-width modulation counter and it is not necessary to provide the pulse-width modulation counter in each pixel 20, reducing the total amount of hardware in system 90, for example as shown in
According to embodiments of the present disclosure, pixel values 16 are converted into pixel-control signals 18 that are temporally modulated signals for a pixel period 44 comprising pulse periods 42, shown in
In embodiments of the present disclosure, system 90 can be operable to display a specified luminance in a pixel 20 light emitter 21 during a frame period 40, luminance signal 12 can be a percent of frame period 40, pixel-control signal 18 can have a variable pixel period 44, and variable pixel period 44 is equal to luminance signal 12 times frame period 40. Pixel period 44 can be equal to the sum of pulse periods 42. The actual amount of light output by light emitters 21 during a frame period 40 (integrated light output over time) is determined by pixel value 16, pixel period 44, and the constant current provided to light emitters 21 during pulse periods 42 and light emitters 21 light output in response to the constant current. If pixel period 44 and frame periods 40 are sufficiently short, the integrated light output will appear to the human visual system as a uniform light output during frame period 40. If luminance signal 12 is relatively larger, pixel period 44 is relatively longer and light emitters 21 will emit more light during frame period 40 so that pixel 20 will appear brighter with increased luminance. If luminance signal 12 is relatively smaller, pixel period 44 is relatively shorter and light emitters 21 will emit less light during frame period 40 so that pixel 20 will appear dimmer with reduced luminance.
For example, in the 100% luminance signal 12 case, a pixel value 16 equal to zero corresponds to pulse periods 42 two, four, and eight (pulse periods 42 having a relative temporal duration of two, four, and eight) turned off, a pixel value 16 equal to one corresponds to pulse period 42 two turned on and pulse periods 42 four and eight turned off, a pixel value 16 equal to two corresponds to pulse period 42 four turned on and pulse periods 42 two and eight turned off, a pixel value 16 equal to three corresponds to pulse periods 42 two and four turned on and pulse period 42 eight turned off, a pixel value 16 equal to four corresponds to pulse period 42 eight turned on and pulse periods 42 two and four turned off, a pixel value 16 equal to five corresponds to pulse periods 42 two and eight turned on and pulse period 42 four turned off, a pixel value 16 equal to six corresponds to pulse periods 42 four and eight turned on and pulse period 42 two turned off, and a pixel value 16 equal to seven corresponds to pulse periods 42 two, four, and eight turned on. Pulse periods 42 for the 50% case are similar except that pulse periods 42 are one, two, and four rather than two, four, and eight (e.g., equal to 50% of the 100% pulse periods 42).
As shown in
In
For example, in a software pseudo-program illustration:
In the first two cases of a desirably very dim or a desirably very bright pixel 20 light emitter 21, an alternative mapping of pixel values 16 to achievable pulse periods 42 can be used as illustrated in
According to embodiments of the present disclosure, a system 90 and pixels 20 with dimming control using temporal modulation can reduce or increase luminance output (brightness) without changing frame period 40. In some embodiments, the luminance decreases without changing or decreasing the number of gray levels available for pixel value 16. In some embodiments having a minimum pulse period 43, the number of gray levels decreases at some desired reduced luminance. In some embodiments, the number of gray levels decreases at some desired increased luminance.
In some embodiments, pulse periods 42 have relative temporal durations that are factors of two (e.g., PWM). In some embodiments, pulse periods 42 have relative temporal durations that are substantially equal (e.g., PDM). However, in embodiments wherein light output and pulse periods 42 from system 90 and display 92 is limited by minimum pulse period 43, fewer pulse periods 42 with longer blank periods 46 can be used. Moreover, where blank period 46 are limited in duration, e.g., the time required to turn a light emitter 21 off and then on again has a minimum temporal duration (e.g., a minimum blank period 47) so that light emitters 21 cannot be turned off and then on again as quickly as can be desired, in some embodiments of the present disclosure pulse periods 42 in a modified pulse-density modulation system 90 can have different pulse period 42 temporal durations, for example as shown in frame period 40B for the increased luminance example.
In some embodiments a display 92 comprising an array of pixels 20 and a display controller 30, display controller 30 is operable to generate or receive luminance signal 12, for example from an input signal from a user of display 92 that desires to increase or decrease display 92 luminance or from an input signal provided by an ambient light sensor indicating that optimal luminance for display 92 in the ambient luminance (e.g., bright daylight or dark night) can be achieved by increasing or decreasing luminance signal 12.
In some embodiments, a display 92 comprising an array of pixels 20 comprises a display controller 30 operable to generate luminance signal 12, for example directly from an ambient light sensor, or by analyzing pixel values 16 to determine the desired relative luminance of pixels 20 and determine a range of pixel values 16 with an upper bound, or an upper bound and a lower bound, in an array of pixel values 16 (e.g., a maximum value in the range and optionally a minimum value in the range), for example by analyzing an image for display on display 92. Display controller 30 can provide luminance signal 12 in response to the determined range, upper bound, or lower bound, for example by dividing the upper bound by 2N or 2N−1 where N is the number of bits in pixel values 16. Some embodiments can provide display 92 dimming without loss of pixel value 16 bit-depth (gray-scale resolution) while others, as discussed above can provide display 92 dimming or brightening with some reduction in pixel value 16 bit depth. Such dimming control can be provided very simply, just by modifying a clock signal 10 used to generate pixel-control signal 18.
In some embodiments and as shown in
Thus, in embodiments of the present disclosure, display controller 30 can be operable to (i) provide different luminance signals 12 to two or more pixel groups 24 of pixels 20 in the array of pixels 20, (ii) provide different variable-frequency clock signals 14 to one or more pixel groups 24 of pixels 20 in the array of pixels 20, or (iii) provide different pixel-control signals 18 having different pixel periods 44 to one or more pixel groups 24 of pixels 20 in the array of pixels 20.
According to embodiments of the present disclosure, display controller 30, variable-frequency-clock signal generator 11 and pixel-control signal generator 17 can be constructed using digital, analog, or mixed signal circuits, for example provided in one or more integrated circuits. A simplified and primarily digital implementation is illustrated in
As shown in
One or more of display controller 30, variable-frequency-clock signal generator 11, pixel-control signal generator 17, and pixel controller 22 (circuits) can be digital or mixed-signal circuits provided in one or more integrated circuits (e.g., silicon integrated circuits) and disposed on a display substrate 38 (shown in
Pixels 20 can comprise multiple light controllers 21, for example emitting different colors of light, and each light controller 21 can be responsive to a different pixel value 16. Pixel-control signal generator 17 can apply a common variable-frequency clock signal 14 to generate different pixel-control signals 18 for each light controller 21 in pixel 20.
In some embodiments, a different variable pixel period 44 can be used for each different light controller 21. For example, displays are frequently adjusted to desired white points (e.g., D6500) that specify the combination of light emitted from different light-controller 21 (e.g., red, green, and blue). Different amounts of each color can be applied by system 90 to every pixel value 16 of pixel 20 by adjusting the pixel period 44 applied to each color of light controller 21 without reducing the available gray scale for each color.
Light controllers 21 can be light-emitting diodes (e.g., inorganic light emitting diodes or organic light-emitting diodes) that can switch very rapidly between an on-state and an off-state (e.g., within a few micro-seconds, one micro-second, or less than a micro-second) in response to a digital control signal such as pixel-control signal 18 (e.g., either on at a fixed voltage and constant current emitting light or off and not emitting light at, for example, zero volts). The human visual system averages the light emitted during pulse periods 42 in each frame period 40 (e.g., display image frame) to perceive an average brightness during frame period 40, if the pulses are sufficiently fast and short. In contrast, light emitters in displays driven by a variable voltage or variable current displays are on for the entire display frame but at a brightness dependent on the voltage or current supplied to the light emitters. Light-emitting diodes can have variable efficiency depending on the voltage or current supplied; thus light-emitting diodes driven at a constant current and voltage for variable amounts of time specified by temporal bits P, and according to embodiments of the present disclosure, can be more power efficient by operating at or near peak efficiency during the temporal pulse periods 42.
Display 92 can be a flat-panel display, for example an organic light-emitting diode display, an inorganic light-emitting diode display, or a liquid crystal display. In some embodiments, switching frequencies are limited, for example by electronic devices and connections, or by switching frequencies for the light controllers 21, for example liquid crystal displays that can have liquid crystal switching times in the tens of milliseconds. In such displays, systems 90 and pixels 20 can provide improved image frame rates and gray-scale resolution with dimming control.
Pixel-control signal generator 17 can comprise an effectively binary digital switch fed by a constant-current supply because it does not continuously modulate the amount of current supplied by the constant-current supply but rather operates in a first mode in which light controller 21 is turned off (e.g., at a zero voltage) and no current flows through light controller 21 and a second mode in which the current flows through light emitter 21 at a constant current specified by the constant-current supply and non-zero voltage specified by pixel-control signal 18. Thus, the voltage and current supplied to light controllers 21 is digital and binary (e.g., has two levels including zero).
Certain embodiments of the present disclosure can be applied to, or are, active-matrix displays 92. For example, display control signals from display controller 30 can comprise a row-control signal provided on a row wire 32 and a column-data signal provided on a column wire 34 and electrically connected to an array of pixels 20 arranged in rows and columns on a display substrate 38 in an active-matrix display 92. Each pixel 20 can comprise one or multiple light controllers 21, each of which can comprise, for example, a micro-inorganic-light-emitting diode. Each of multiple light controllers 21 in a pixel 20 can be or include a different inorganic light-emitting diode 21 that emits a different color of light when provided with electrical current at a suitable voltage.
According to some embodiments of the present disclosure, the circuits (e.g., any one or more of display controller 30, variable-frequency-clock signal generator 11, pixel-control signal generator 17, and pixel controller 22) can comprise any of a variety of transistors, for example transistors such as those known in the electronics, integrated circuit, and display industries. Transistors can be thin-film transistors (TFTs), for example amorphous transistors or polysilicon transistors and can be a semiconductor thin-film circuit formed on a substrate, such as a display substrate 38. In some embodiments, transistors are crystalline silicon or compound semiconductor transistors, for example made in an integrated circuit process with a substrate independent from display substrate 38 and can be transfer printed onto a display substrate 38 or onto a pixel module substrate that is transfer printed onto display substrate 38. Such transfer-printed structure can comprise fractured or separated tethers.
According to some embodiments of the present disclosure, light controllers 21 are micro-inorganic-light-emitting diodes (micro-iLEDs) with at least one of a width and a length that is no greater than 500 microns (e.g., no greater than 200 microns, no greater than 100 microns, no greater than 50 microns, no greater than 25 microns, no greater than 15 microns, no greater than 12 microns, no greater than 8 microns, or no greater than 5 microns). Micro-LEDs provide an advantage according to some embodiments of the present disclosure since they are sufficiently small and can be disposed spatially close together so that the different micro-LEDs in a pixel 20 cannot be readily distinguished by the human visual system in a display at a desired viewing distance, improving color mixing of light emitted by pixel 20 and providing apparent improvements in display resolution. Embodiments of the present disclosure can be constructed using micro-transfer printing.
Methods of forming useful micro-transfer printable structures are described, for example, in the paper AMOLED Displays using Transfer-Printed Integrated Circuits, Journal of the SID, 19(4), 2012, and U.S. Pat. No. 8,889,485. For a discussion of micro-transfer printing techniques see, U.S. Pat. Nos. 8,722,458, 7,622,367 and 8,506,867, the disclosures of which are hereby incorporated by reference in their entirety. Micro-transfer printing using compound micro-assembly structures and methods can also be used with the present disclosure, for example, as described in U.S. patent application Ser. No. 14/822,868, filed Aug. 10, 2015, entitled Compound Micro-Assembly Strategies and Devices, the disclosure of which is hereby incorporated by reference in its entirety. In some embodiments, pixels 20 are compound micro-assembled devices.
As is understood by those skilled in the art, the terms “over” and “under”, “above” and “below”, and “top” and “bottom” are relative terms and can be interchanged in reference to different orientations of the layers, elements, and substrates included in the present invention. For example, a first layer on a second layer, in some implementations means a first layer directly on and in contact with a second layer. In other implementations a first layer on a second layer includes a first layer and a second layer with another layer therebetween.
Throughout the description, where apparatus and systems are described as having, including, or comprising specific components, or where processes and methods are described as having, including, or comprising specific steps, it is contemplated that, additionally, there are apparatus, and systems of the disclosed technology that consist essentially of, or consist of, the recited components, and that there are processes and methods according to the disclosed technology that consist essentially of, or consist of, the recited processing steps.
It should be understood that the order of steps or order for performing certain action is immaterial so long as operability is maintained. Moreover, two or more steps or actions in some circumstances can be conducted simultaneously.
Having expressly described certain embodiments, it will now become apparent to one skilled in the art that other embodiments incorporating the concepts of the disclosure may be used. Therefore, the claimed invention should not be limited to the described embodiments, but rather should be limited only by the spirit and scope of the following claims.
