PATENT DOCUMENT

Publication Number: US-10395594-B1
Application Number: US-201615247661-A
Country: US
Kind Code: B1

Title: Hybrid microdriver and TFT architecture

Abstract:
Methods, systems, and apparatuses for controlling an emission of the light emitting devices are described herein. The light emitting devices may be light emitting diode (LED) devices including μLED devices or organic LED (OLED) devices. Emission control of the LED may be performed using a micro-scale driving circuit (e.g., μDriver) containing drive transistors for constant current driving of the light emitting devices. One embodiment provides for a display driver hardware circuit including emission logic, the emission logic including comparator logic to compare a data voltage from a storage capacitor to a voltage ramp provided via the TFT backplane, the comparator logic to cause the emission logic to generate an emission pulse to an LED device, and wherein the integrated circuit is to switch and drive a plurality of LED devices. In one embodiment each of the plurality of LED devices is a subpixel for a display device.

Claims:
The invention claimed is: 
     
       1. A display driver hardware circuit comprising:
 a thin film transistor (TFT) backplane; and 
 an integrated circuit including emission logic, the emission logic including comparator logic to compare a data voltage from a storage capacitor to a voltage ramp provided via the TFT backplane, the comparator logic to cause the emission logic to generate an emission pulse to an LED device, the emission pulse adjustable from a continuous duty cycle to a non-continuous duty cycle, and wherein the integrated circuit is to switch and drive a plurality of LED devices; 
 wherein the integrated circuit is comprised of crystalline silicon and contained within a chip of an array of chips coupled with the TFT backplane. 
 
     
     
       2. The display driver hardware circuit as in  claim 1 , wherein the TFT backplane includes a low temperature poly-silicon (LTPS) transistor. 
     
     
       3. The display driver hardware circuit as in  claim 1 , wherein the TFT backplane includes an Indium Gallium Zinc Oxide (IGZO) transistor. 
     
     
       4. The display driver hardware circuit as in  claim 1 , wherein the chip has a maximum lateral dimension of 1 to 100 μm. 
     
     
       5. The display driver hardware circuit as in  claim 1 , wherein the comparator logic includes a current comparator coupled to the voltage ramp provided by the TFT backplane. 
     
     
       6. The display driver hardware circuit as in  claim 1 , wherein the comparator logic comprises a transistor coupled to the storage capacitor. 
     
     
       7. The display driver hardware circuit of  claim 1 , wherein the integrated circuit has a maximum lateral dimension of 1 to 100 μm. 
     
     
       8. The display driver hardware circuit as in  claim 7 , wherein the integrated circuit has a maximum lateral dimension of 50 μm. 
     
     
       9. The display driver hardware circuit as in  claim 1 , wherein the emission logic is to supply a constant current to the LED device, the constant current based on an adjustable reference voltage. 
     
     
       10. The display driver hardware circuit as in  claim 9 , wherein the emission logic is to terminate current to the LED device to end the emission pulse based on output of the comparator logic. 
     
     
       11. The display driver hardware circuit as in  claim 1 , wherein each of the plurality of LED devices is a subpixel for a display device. 
     
     
       12. A display driver hardware circuit comprising:
 a thin film transistor (TFT) backplane; and 
 an integrated circuit including emission logic to cause an LED emission pulse, the LED emission pulse adjustable from a continuous duty cycle to a non-continuous duty cycle, wherein the integrated circuit is a crystalline silicon integrated circuit coupled to a ramp signal generator to cause a voltage ramp having a slope based on an analog input voltage from the TFT backplane, wherein a length of the LED emission pulse is related to the slope of the voltage ramp. 
 
     
     
       13. The display driver hardware circuit as in  claim 12 , wherein the integrated circuit additionally includes comparator logic to control the emission logic during the LED emission pulse. 
     
     
       14. The display driver hardware circuit as in  claim 13 , wherein the comparator logic includes a transistor coupled between a data voltage and a ramp voltage. 
     
     
       15. The display driver hardware circuit as in  claim 14 , wherein the comparator logic is to end the LED emission pulse when the ramp voltage is within a threshold of the data voltage. 
     
     
       16. The display driver hardware circuit as in  claim 12 , wherein the ramp signal generator is in a row driver coupled to the display driver hardware circuit. 
     
     
       17. The display driver hardware circuit as in  claim 12 , wherein the ramp signal generator is in a timing control circuit coupled to the display driver hardware circuit. 
     
     
       18. A light emitting assembly comprising:
 an array of light emitting diode (LED) devices; 
 a sample and hold circuit including a thin film transistor (TFT) of a TFT backplane; 
 a ramp signal generator; and 
 an array of microcontroller chips coupled with the TFT backplane, the array of microcontroller chips comprising an array of crystalline silicon integrated circuits to switch and drive the array of LED devices based on a voltage ramp caused by the ramp signal generator, the voltage ramp to determine a pulse length of an emission pulse to an LED device of the array of LED devices, wherein the emission pulse is adjustable from a continuous duty cycle to a non-continuous duty cycle. 
 
     
     
       19. The light emitting assembly as in  claim 18 , wherein a number of the microcontroller chips in the array of microcontroller chips is less than a number of LED devices in the array of LED devices and each microcontroller chip in the array of microcontroller chips is in electrical connection with a plurality of pixels to drive a plurality of LED devices in each pixel. 
     
     
       20. The light emitting assembly as in  claim 18 , wherein each LED device in the array of LED devices has a maximum lateral dimension of 1 to 100 μm. 
     
     
       21. The light emitting assembly as in  claim 18 , wherein at least one microcontroller chip in the array of microcontroller chips has maximum lateral dimension of 1 to 100 μm. 
     
     
       22. The light emitting assembly as in  claim 21 , wherein at least one microcontroller chip in the array of microcontroller chips has maximum lateral dimension of 50 μm. 
     
     
       23. The light emitting assembly as in  claim 18 , wherein the TFT is a low temperature poly-silicon (LTPS) transistor. 
     
     
       24. The light emitting assembly as in  claim 18 , wherein the TFT is an Indium Gallium Zinc Oxide (IGZO) transistor. 
     
     
       25. A display system comprising:
 a thin film transistor (TFT) backplane including an active area; 
 a ramp signal generator; 
 an array of micro driver chips coupled to the TFT backplane in the active area; 
 an array of micro light emitting diode (LED) devices in the active area, the array of micro LED devices electrically connected to the array of micro driver chips, and each micro driver chip controls a plurality of pixels, wherein the array of micro driver chips comprises an array of crystalline silicon integrated circuits to switch and drive the array of micro LED devices; and 
 an emission controller to cause the array of micro driver chips to supply an emission pulse to the array of LED devices, wherein a length of the emission pulse is a function of an analog input data voltage and the emission pulse is adjustable from a continuous duty cycle to a non-continuous duty cycle. 
 
     
     
       26. The display system of  claim 25 , additionally comprising a row of column drivers including a plurality of column drivers and a column of row drivers including a plurality of row drivers. 
     
     
       27. The display system as in  claim 25 , wherein a length of the emission pulse is proportional to a value of the analog input data voltage. 
     
     
       28. The display system as in  claim 25 , wherein the backplane includes a low temperature poly-silicon (LTPS) transistor. 
     
     
       29. The display system as in  claim 25 , wherein the backplane includes an Indium Gallium Zinc Oxide (IGZO) transistor.

Description:
CROSS-REFERENCE 
     This application is a non-provisional application claiming the benefit of U.S. Provisional Application No. 62/220,851 filed on Sep. 18, 2015, which is hereby incorporated herein by reference. 
    
    
     FIELD 
     The disclosure relates generally to a display system, and, more specifically, to display driving circuitry for LED displays. 
     BACKGROUND OF THE DISCLOSURE 
     Display panels are utilized in a wide range of electronic devices. Common types of display panels include active matrix display panels where each pixel may be driven to display a data frame. High-resolution color display panels, such as computer displays, smart phones, and televisions, may use an active matrix display structure. An active matrix display of m×n display (e.g., pixel) elements may be addressed with m row lines and n column lines or a subset thereof. In conventional active matrix display technologies a switching device and storage device is located at every display element of the display. A display element may be a light emitting diode (LED) or other light emitting material. A storage device(s) (e.g., a capacitor or a data register) may be connected to each display (e.g., pixel) element, for example, to load a data signal therein (e.g., corresponding to the emission to be emitted from that display element). The switches in conventional displays are usually implemented through transistors made of deposited thin films, and thus are called thin film transistors (TFTs). A common semiconductor used for TFT integration is amorphous silicon (a-Si), which allows for large-area fabrication in a low temperature process. A main difference between a-Si TFT and a conventional silicon metal-oxide-semiconductor-field-effect-transistor (MOSFET) is lower electron mobility in a-Si due to the presence of electron traps. Another difference includes a larger threshold voltage shift. Low temperature polysilicon (LTPS) represents an alternative material that is used for TFT integration. LTPS TFTs have mobility that is higher than a-Si TFTs, yet lower than MOSFETs. 
     SUMMARY OF THE DESCRIPTION 
     Methods, systems, and apparatuses for controlling an emission of the light emitting devices are described herein. The light emitting devices may be light emitting diode (LED) devices including μLED devices or organic LED (OLED) devices. Emission control of the LED may be performed using a micro-scale driving circuit (e.g., μDriver) containing drive transistors for constant current driving of the light emitting devices. 
     One embodiment provides for a display driver hardware circuit including emission logic, the emission logic including comparator logic to compare a data voltage from a storage capacitor to a voltage ramp provided via the TFT backplane, the comparator logic to cause the emission logic to generate an emission pulse to an LED device, and wherein the integrated circuit is to switch and drive a plurality of LED devices. In one embodiment each of the plurality of LED devices is a subpixel for a display device. In one embodiment the emission logic is to supply a constant current to the LED device, the constant current based on an adjustable reference voltage and is configured to terminate current to the LED device to end the emission pulse based on output of the comparator logic. 
     In one embodiment the TFT backplane includes a low temperature poly-silicon (LTPS) transistor. In one embodiment the TFT backplane includes an Indium Gallium Zinc Oxide (IGZO) transistor. In one embodiment the comparator logic includes a current comparator coupled to the voltage ramp provided by the TFT backplane. In one embodiment the comparator logic comprises a transistor coupled to the storage capacitor. In one embodiment the integrated circuit is comprised of crystalline silicon and has a maximum lateral dimension of 1 to 100 μm. In one embodiment the integrated circuit has a maximum lateral dimension of 50 μm. 
     One embodiment provides fort a display driver hardware circuit comprising a thin film transistor (TFT) backplane, an integrated circuit including emission logic to cause an LED emission pulse, where the LED emission pulse is adjustable from a continuous duty cycle to a non-continuous duty cycle, the integrated circuit is a crystalline silicon integrated circuit coupled a ramp signal generator to cause a voltage ramp having a slope based on an analog input voltage from the TFT backplane, wherein a length of the LED emission pulse is related to the slope of the voltage ramp. The integrated circuit can additionally include comparator logic to control the emission logic during the LED emission pulse. The comparator logic can include a transistor coupled between a data voltage and a ramp voltage. The comparator logic can be configured to end the LED emission pulse when the ramp voltage is within a threshold of the data voltage. The ramp signal generator can be included in a row driver coupled to the display driver hardware circuit or a timing control circuit coupled to the display driver hardware circuit. 
     One embodiment provides for a light emitting assembly comprising an array of light emitting diode (LED) devices, a sample and hold circuit including a thin film transistor (TFT), a ramp signal generator, and an array of microcontrollers to switch and drive the array of LED devices based on a voltage ramp caused by the ramp signal generator, the voltage ramp to determine a pulse length of an emission pulse to an LED device of the array of LED devices. In one embodiment a number of microcontrollers in the array of microcontrollers is less than a number of LED devices in the array of LED devices and each microcontroller in the array of microcontrollers is in electrical connection with a plurality of pixels to drive a plurality of LED devices in each pixel. In one embodiment each LED device in the array of LED devices has a maximum lateral dimension of 1 to 100 μm. In one embodiment at least one microcontroller in the array of microcontrollers has maximum lateral dimension of 1 to 100 μm. In one embodiment at least one microcontroller in the array of microcontrollers has maximum lateral dimension of 50 μm. 
     One embodiment provides for a display system comprising a backplane including an active area, an array of micro driver chips in the active area, an array of micro light emitting diode (LED) devices in the active area, the array of micro LED devices electrically connected to the array of micro driver chips, and each micro driver chip controls a plurality of pixels. The backplane can additionally include an emission controller to cause the array of micro driver chips to supply an emission pulse to the array of LED devices, wherein a length of the emission pulse is a function of an analog input data voltage. In one embodiment the display system additionally comprises a row of column drivers including a plurality of column drivers and a column of row drivers including a plurality of row drivers and/or a length of the emission pulse is proportional to a value of the analog input data voltage. In one embodiment the backplane is a TFT backplane and the array of micro driver chips comprises an array of crystalline silicon integrated circuits to switch and drive the array of micro LED devices. The backplane can include a low temperature poly-silicon (LTPS) transistor and/or an Indium Gallium Zinc Oxide (IGZO) transistor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments are illustrated by way of example and not limitation in the Figures of the accompanying drawings: 
         FIG. 1  is a block diagram of hybrid micro-driver display architecture, according to an embodiment. 
         FIGS. 2A-B  are block diagrams illustrating different views of an additional backplane-driver design, according to an embodiment. 
         FIG. 3  is a block diagram of a basic circuit for a hybrid μDriver, according to one embodiment. 
         FIGS. 4A-B  are circuit diagrams of two exemplary hybrid μDriver circuits. 
         FIG. 5  is an illustration of an exemplary pad layout for a μDriver circuit. 
         FIG. 6A  is an illustration of pulse width modulation (PWM) in accordance with an embodiment. 
         FIG. 6B  is an illustration of PWM determination in a μDriver based on a voltage ramp and an input data voltage, according to embodiments. 
         FIGS. 7A-B  show circuit diagrams for a hybrid analog PWM μLED Driving Circuit that uses a current comparator and an XOR gate as current control logic 
         FIG. 8  is a diagram of a voltage output for a hybrid analog PWM LED driving circuit. 
         FIG. 9  is a diagram of a hybrid analog PWM μLED driving circuit with a reduced power comparator, according to an embodiment. 
         FIG. 10  shows an exemplary integrated circuit. 
         FIG. 11  is shows an exemplary preliminary electrical performance evaluation for an exemplary hybrid analog PWM μLED driving circuit. 
         FIG. 12  is a diagram of an additional hybrid analog PWM μLED driving circuit, according to an embodiment 
         FIG. 13  is an exemplary voltage chart of a multi-segmented voltage ramp input, according to an embodiment. 
         FIG. 14  is a diagram of an additional hybrid analog PWM μLED driving circuit, according to an embodiment. 
         FIG. 15  illustrates an additional exemplary voltage chart of a multi-segmented voltage input ramp. 
         FIG. 16  illustrates the processing of substrates of μDriver and μLEDs into a receiving substrate for a hybrid μDriver and μLED display, according to an embodiment. 
         FIG. 17  is an illustration of a hybrid μDriver display, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In various embodiments, description is made with reference to figures. However, certain embodiments may be practiced without one or more of these specific details, or in combination with other known methods and configurations. In the following description, numerous specific details are set forth, such as specific configurations, dimensions and processes, etc., in order to provide a thorough understanding of the present disclosure. In other instances, well-known techniques and components have not been described in particular detail in order to not unnecessarily obscure the present disclosure. Additionally, concepts described in detail in some figures are not described in detail in other figures. For the sake of brevity of description, the description of identical features that are identified by identical numerals may not be repeated throughout the description. 
     Reference throughout this specification to “one embodiment,” “an embodiment”, or the like means that a particular feature, structure, configuration, or characteristic described in connection with the embodiment is included in at least one embodiment of the disclosure. Thus, the appearances of the phrase “in one embodiment,” “in an embodiment”, or the like in various places throughout this specification are not necessarily referring to the same embodiment of the disclosure. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments. 
     The terms “over,” “to,” “between,” and “on” as used herein may refer to a relative position of one layer with respect to other layers. One layer “over,” or “on” another layer or bonded “to” another layer may be directly in contact with the other layer or may have one or more intervening layers. One layer “between” layers may be directly in contact with the layers or may have one or more intervening layers. 
     The term “ON” as used in this specification in connection with a device state refers to an activated state of the device, and the term “OFF” refers to a de-activated state of the device. The term “ON” as used herein in connection with a signal received by a device refers to a signal that activates the device, and the term “OFF” used in this connection refers to a signal that de-activates the device. A device may be activated by a high voltage or a low voltage, depending on the underlying electronics implementing the device. For example, a PMOS transistor device is activated by a low voltage while a NMOS transistor device is activated by a high voltage. Thus, it should be understood that an “ON” voltage for a PMOS transistor device and a NMOS transistor device correspond to opposite (low vs. high) voltage levels. It is also to be understood that where Vdd and Vss is illustrated or described, it can also indicate one or more Vdd and/or Vss. For example, a digital Vdd for can be used for data input, digital logic, memory devices, etc., while another Vdd is used for driving the LED output block. 
     In accordance with some embodiments, a hybrid LED driving circuit is described which is a hybrid arrangement of microdriver (also referred to as μD or μDriver) chips and a TFT substrate which, in combination, are used to driver a set of light emitting devices such as, but not limited to micro LEDs (also referred to as μLEDs). In an embodiment, a micro LED may be a semiconductor-based material having a maximum lateral dimension of 1 to 300 μm, 1 to 100 μm, 1 to 20 μm, or more specifically 1 to 10 μm, such as 5 μm. The light emitting devices may also be organic LEDs (OLEDs). 
     The hybrid LED driving circuit can use a hybrid of analog and digital driving techniques, in which an analog input voltage is used to control a digital pulse-width-modulation (PWM) driving scheme and may include a set of microdriver (e.g., μDriver) chips, which are integrated circuits containing emission logic to drive the LED devices. A μDriver chip may have a maximum lateral dimension of 1 to 100 μm, and may fit within the pixel layout of the micro LEDs. In accordance with embodiments, the μDriver chips can replace the LED drive transistors for each display element, which are commonly employed as TFT components. The μDriver chips may include digital unit cells, analog unit cells, or hybrid digital and analog unit cells. Additionally, MOSFET processing techniques may be used for fabrication of the μDriver chips on single crystalline silicon, in conjunction with TFT processing techniques on a-Si or LTPS. 
     The hybrid TFT and μDriver circuit can realize the benefits of μDriver circuit technology while reducing the overall size and number of inputs for each μDriver integrated circuit. The hybrid circuit includes a portion of the transistors and capacitors in a TFT layer while including an additional portion of LED switching and driving components in the μDriver integrated circuit, resulting in a reduced size and manufacturing cost of each μDriver circuit relative to including all switching and driving components in the μDriver. This hybrid approach combines traditional analog data driving with digital, constant current emission pulse width modulation (PWM), where the length of the emission pulse is a function of analog data voltage. The use of analog data driving allows the use of traditional SCAN and DATA lines coupled to a TFT substrate, where switching transistors and capacitors on the TFT substrate provide an analog input voltage to the μDriver circuit. 
     Hybrid TFT Micro-Driver Integrated Circuit Display Architecture and Overview 
       FIG. 1  is a block diagram of a hybrid micro-driver display architecture  100 , according to an embodiment. In one embodiment, the hybrid μDriver display architecture  100  includes a data driver (Vdata)  102 , a row driver (Vselect)  104  inputs to control the display, as well as power (Vdd)  106 , and ground (Vss) inputs  107 . A μDriver integrated circuit (IC)  110  and one or more μLEDs  115  are placed on a TFT backplane  108  including switching transistors and capacitors to supply data to the μDriver IC  110 . 
     The μDriver IC  110  includes drive transistors for the one or more μLEDs  115  and can be fabricated separately from the TFT backplane  108  in a crystalline silicon wafer. The μDriver IC  110  can be placed directly onto any active or passive TFT backplane and can interface with any type of LED, including organic LEDs (OLED). The μDriver IC  110  can include a combination of any of the available CMOS types required for implementing the driver (such as CMOS, all NMOS or all PMOS). 
     In this figure, and in the figures to follow, each illustrated LED device (e.g., μLED  115 ) may represent a single LED device, or may represent multiple LED devices arranged in series, in parallel, or a combination of series and parallel. The LED devices can couple to a common ground or may each have a separate ground connection. The exemplary hybrid microdriver display architecture  100  illustrated shows three control inputs and six LED outputs, but embodiments are not so limited. A single μDriver IC  110  can control multiple lighting emitting devices, where each lighting devices has a separate analog input into the μDriver IC  110 . 
     In one embodiment, the μDriver IC  110  couples with one or more red, green, and blue LED devices  115  that emit different colors of light. In a red-green-blue (RGB) sub-pixel arrangement, each pixel includes three sub-pixels that emit red, green and blue lights, respectively. The RGB arrangement is exemplary and that embodiments are not so limited. Additional sub-pixel arrangements include, red-green-blue-yellow (RGBY), red-green-blue-yellow-cyan (RGBYC), or red-green-blue-white (RGBW), or other sub-pixel matrix schemes where the pixels may have a different number of sub-pixels, such as the displays manufactured under the trademark name PenTile®. 
     In one embodiment, the smart-pixel micro-matrix is used in LED lighting solutions, or as an LED backlight for an LCD device. When used as a light source, blue or UV LEDs in combination with a yellow or blue-yellow phosphor may be used to provide a white backlight for LCD displays. In one embodiment, a smart-pixel micro-matrix using one or more blue LED devices, such as an indium gallium nitride (InGaN) LED device, is combined with the yellow luminescence from cerium doped yttrium aluminum garnet (YAG:Ce3+) phosphor. In one embodiment, red, green, and blue phosphors are combined with a near-ultraviolet/ultraviolet (nUV/UV) InGaN LED device to produce white light. The phosphor can be bonded to the surface of the LED device, or a remote phosphor can be used. In addition to white light emission, additional red, green and/or blue LED device can also be used to provide a wider color gamut than otherwise possible with white backlights. 
     In one embodiment, each sub-pixel circuit driver in the μDriver IC  110  is responsible for providing operating current for illumination to each individual LED. Thus, the circuitry for each sub-pixel circuit can be designed specifically for each LED, allowing the switching transistors in the backplane to be implemented by any combination of LTPS (Low Temperature Poly Silicon) and/or Oxide (e.g., IGZO or Indium Gallium Zinc Oxide) TFTs to ensure low leakage devices, while the technology of the μDriver IC  110  is independent of the backplane. The independent backplane and μDriver IC  110  enable the production of low voltage devices having higher mobilities. The higher mobilities of the driving circuit devices provide higher currents to the LEDs, resulting in reduced maximum rail voltages for reduced power consumption while maintaining minimum geometry transistors. The smaller geometry transistors enable the circuit to operate at higher speeds with lower parasitic losses, as the circuit occupies a smaller area. The size of the μDriver IC  110 , in one embodiment is 50 μm wide by 24 μm long. However, the size of each μDriver IC  110  generally depends on the number of sub-pixel circuit drivers the μDriver IC  110  contains. 
       FIGS. 2A-B  are block diagrams illustrating different views of an additional backplane-driver design, according to an embodiment.  FIG. 2A  illustrates an exemplary backplane driver design having a flexible printed circuit (FPC) and a chip on flex (COF) circuit.  FIG. 2B  illustrates a top-down view of the exemplary backplane driver design. 
     As illustrated in  FIG. 2A , the backplane-driver design includes an FPC  202  coupled to an LTPS/Oxide TFT backplane  212 . The FPC  202  can include a COF circuit  204 A, which is an integrated circuit coupled to the FPC  202 . In one embodiment a row driver  206  and an emission driver  208  couple to a TFT backplane  212 , which may be an LTPS/Oxide TFT backplane. The TFT backplane  212  includes a sample and hold circuit having at least one transistor and one capacitor, although other sample and hold circuits may be used. A μDriver IC  110  couples to the TFT backplane  212  and a set of one or more light emitting devices (e.g., R, G, and B LEDs), where multiple light emitting devices can couple to a single μDriver IC  110 . 
       FIG. 2B  illustrates a top-down view of the exemplary backplane driver design, where a the row driver  206  and emission driver  208  are illustrated as coupled to the TFT backplane  212  in conjunction with a data driver  204 B, which may be included in the COF circuit  204 A shown in  FIG. 2A . In one embodiment, the data driver  204 B supplies pixel data values before the lighting elements are signaled for emission by the emission driver  208 . The pixel data values are stored in capacitors selected by the row driver  206 . After each line has been programmed with data, the emission driver  208  is responsible for sending the input to cause the illumination of the lighting elements for a pixel. In the illustrated display architecture, the data driver  204 B controls the gray levels of the pixels and the emission driver  208  controls the brightness. 
     While the backplane driver architecture illustrated uses an active TFT matrix, in one embodiment, a passive matrix is employed, for example, when operational frequencies exceed the operational limits of the TFT backplane due to the low mobilities inherent in some TFT technologies. In a passive TFT matrix architecture, row and emission driving can be realized a chain of μDriver ICs  110  interconnected over a passive TFT backplane. 
       FIG. 3  is a block diagram of a basic circuit for a hybrid microdriver  300 , according to one embodiment. The hybrid microdriver  300  includes a μDriver IC  110  coupled to a TFT backplane  212 . The backplane  212  includes components for a sample and hold circuit, including a switching transistor  308  and a storage capacitor  310 . However, any variant of a sample and hold circuit suitable for use in a TFT backplane may be used. The switching transistor  308  can be any type of insulated-gate field-effect transistor, such an n-type or a p-type semiconductor transistor. In this configuration, the switching transistor  308  has a gate electrode coupled with an input from a scan line  306 , a first source/drain electrode coupled with an input from a data line  304 , and second source/drain electrode coupled with the storage capacitor  310 . In one embodiment, a voltage scan signal enables the storage capacitor  310  to charge, which ultimately provides an input signal used to trigger an emission pulse from one or more LED devices coupled to the μDriver IC  110 . 
       FIGS. 4A-B  are circuit diagrams of two exemplary hybrid microdriver circuits. 
     Each exemplary hybrid microdriver circuit includes a sample and hold circuit  402  on a TFT backplane  412 , including at least a switching transistor  308  and a storage capacitor  310 . The sample and hold circuit  402  couples to data and address inputs that are analogous to the data and row inputs illustrated in other exemplary circuits described herein. The exemplary hybrid microdriver circuits each include a μD chiplet circuit  404 A-B, which are each variants of the μDriver IC  110  of  FIGS. 1-3 . Each μD chiplet circuit  404 A-B includes at least one drive transistor  406 A-B to drive (O)LEDs  410 A-B. The drive transistor  406 A-B and can source large currents using minimal geometry. Simple examples of analog implementations of the proposed embodiment are presented here, where the sample and hold circuit  402  of the TFT backplane couples with each respective μD chiplet circuit  404 A-B to form an analog-only 2T1C architecture version having a single drive transistor  406 B, or a 3T1C architecture version having a drive transistor  406 A and a separate switching transistor  408  as emission switch. 
       FIG. 5  is an illustration of an exemplary pad layout for a microdriver circuit. Each microdriver circuit (e.g., μD chiplet, μDriver IC) includes a minimum number of pads, where the number of pads is defined by the number of sub-pixels controlled by the circuit, as well as the number, power and ground connections. The exemplary pad layout of  FIG. 5  illustrates microdriver circuit configured to couple to a hybrid-backplane, where the circuit includes an emission switch transistor and a driving transistor. The circuit of  FIG. 5  includes a power (Vdd)  503  input and a ground (Vss)  508  input, a green/blue emission control (EMGB)  502 A, and a red emission control (EMR)  502 B. Green and blue LEDs having a similar emission control profile may be used, allowing a shares emission control for those LEDs, while red LEDs have a separate emission control EMR  502 B. 
     The microdriver circuit can drive a total of 12 sub-pixels (e.g., 4 RGB pixels) with 6 sub-pixels  506 A (LED1-6) coupled to a first side of the microdriver circuit and 6 sub-pixels  506 B (LED7-12) coupled to a second side of the microdriver circuit. A first set of control connections  504 A (C1-6) can be used to set a gray level for the first set of sub-pixels  506 A, while a second set of control connections  504 B (C7-12) can be used to set a gray level for the second set of sub-pixels  506 B, where each control line in each set of control lines  504 A-B corresponds to a separate and respective sub-pixel  506 A-B. The control connections  504 A-B are connections to the storage capacitor terminals implemented on a TFT backplane, and the EMGB  502  and EMR  502 B lines can be run in a layer physically underneath the driving and emission transistors. 
     Analog Input with Emission Pulse Width Modulation 
     Some types of light emitting devices, such as the μLEDs described herein, are generally driven at currents in the order of several tens of micro-amperes to achieve highest efficiency and lowest μLED power, which is a relatively high current for such class of devices. In traditional active-matrix TFT displays, LTPS or Oxide (e.g. IGZO or Indium Gallium Zinc Oxide) TFTs are sufficient to drive displays based on liquid crystal or organic LED technology. However, existing TFTs are not optimal for providing the relatively high currents used for μLEDs without employing large size TFTs or utilizing a large amount of power to drive the TFTs. 
     The crystalline silicon MOSFETs used in the uDriver ICs described herein have a mobility at least an order of magnitude higher than the LTPS TFTs used for backplane components and are more suitable to generate the current used to drive μLEDs. Additionally, the μLEDs described herein are more suitably driven using a constant current and modulating brightness using PWM based driving techniques, where emission levels can be determined as a function of the gray level input. 
     While one approach to generate a PWM signal is to provide digital n-bit data to every pixel and generate an emission pulse width as a function of digital data. This approach can utilize digital memory such as SRAM or flops, counters along with transistors as current sources, and emission control switches. However, such digital data driving approaches differ significantly from traditional display designs and may be difficult to integrate into traditional display technology in which analog voltage (e.g. 0-5V) is applied on the data line for gray scale control. Additionally while digital driving techniques may result in a simpler backplane design, including all pixel-driving circuits within the μDriver may result in a large and overall expensive design. To reduce the size and design complexity of the digital μDriver, some μDriver capacitors and switching transistors can be placed on a TFT backplane. The use of analog driving techniques may also simplify the integration of crystalline silicon based μDriver technology into existing displays. 
       FIG. 6A  is an illustration of pulse width modulation (PWM), also referred to as pulse length modulation, in which the pulse width or length sets the emission level, in accordance with an embodiment. A constant current can be used to drive the light emitting elements, where the length in which the current is supplied determines the duty cycle of the light emitting element. As illustrated, a longer pulse width or length corresponds to a higher brightness, with a narrower pulse corresponding to a lower brightness, where pulsing the emission of the light emitting element changes the perceived brightness of the element. Three pulse widths are illustrated, in which a long duration pulse  602  results in a high perceived emission brightness due to a longer emission duty cycle, a medium duration pulse  604  results in a medium perceived emission brightness due to a medium emission duty cycle, and a short duration pulse  606  results in a low perceived emission brightness due to a low emission duty cycle. 
       FIG. 6B  is an illustration of PWM determination in a microdriver based on a voltage ramp and an input data voltage according to embodiments. Embodiments described herein can be configured to generate an LED pulse  610  at a constant current with a specific emission duty cycle based on a comparison between an increasing voltage comparator ramp  608  and an input data voltage  612 . The input data voltage  612  is constant within a pulse period and can be supplied to a microdriver circuit from the storage capacitor of a sample and hold circuit in a TFT backplane. The comparator ramp  608  can be supplied by a circuit external to the microdriver (e.g., row driver, timing controller) or can be ‘locally’ generated within the microdriver circuit. The comparator ramp  608  is an increasing voltage that is compared to the input data voltage. Circuitry within the microdriver uses a comparison between the increasing voltage (Vramp) of the comparator ramp  608  and the input data voltage (Vdata)  612  to determine a length of the LED pulse  610 . In one embodiment, a constant current (ILED) is output to the LED as long as the Vramp of the comparator ramp  608  is less than Vdata  612 . When (Vramp) of the comparator ramp  608  exceeds (Vdata)  612 , the current to the LED is shut off. 
     Multiple implementations of a microdriver circuit will be described to perform the PWM driving techniques of  FIGS. 6A-B . In various embodiments, microdriver circuits can use differing comparison techniques and circuit designs to compare the data voltages with the voltage ramp to generate an emission pulse. The circuitry to generate the ramp signal used to determine the emission pulse width may be located in the row driving circuitry, timing control circuitry, or may be generated within the microdriver integrated circuit. 
     Described herein are several analog hybrid microdriver circuit designs and associated output waveforms. Each design provides for constant current driving of a light-emitting device using pulse width modulation. Designs provided in some embodiments are particularly suited for driving μLED devices as described herein, but may also be used to drive other light emitting devices including OLED devices. In general, the circuits described herein vary primarily in the design of the comparator logic used to control emission pulse length and each design provides various benefits and tradeoffs. In one embodiment, a microdriver circuit includes current comparator logic. In one embodiment a simplified comparator circuit is used to reduce circuit area. In one embodiment, a microdriver circuit having a relaxed comparator offset is used in conjunction with a multi-segmented ramp input. In one embodiment a microdriver circuit includes a relaxed comparator and a locally generated, multi-segmented voltage ramp. 
     In the exemplary circuits of the accompanying figures and as described below, certain specific details such as a number of input and output pads or specific power figures are described. It will be understood that the specific details of each circuit are exemplary of one implementation, and embodiments may vary in these specific details without violating the spirit of the invention described herein. 
     Hybrid Analog PWM μLED Driving Circuit Including Current Comparator Logic 
       FIGS. 7A-B  show circuit diagrams for a hybrid analog PWM μLED Driving Circuit that uses a current comparator and an XOR gate as current control logic. The illustrated μDriver is exemplary of one embodiment, and various implementations are possible, including the other exemplary implementations described herein. One embodiment provides for a PWM LED driving circuit that can be used to control up to 12 subpixels of LED devices, which may be μLED devices. In alternate embodiments, a different number of subpixels may be controlled. 
       FIG. 7A  is a diagram of the hybrid analog PWM LED driving circuit  700  including current comparator logic, according to one embodiment. The PWM LED driving circuit  700  is illustrated as driving a single LED or sub-pixel element. However, multiple circuits may be used to drive multiple sub-pixels for a display. The PWM LED driving circuit  700  includes a TFT backplane  701  having components that provide input a μDriver IC  710 . The TFT backplane  701  may include any combination of LTPS (Low Temperature Poly Silicon) and/or Oxide (e.g., IGZO or Indium Gallium Zinc Oxide) TFTs. In one embodiment the TFT backplane  701  has components including an exemplary sample and hold circuit having a scan (e.g., Vselect) input coupled to a switching transistor  708 , which is coupled to a Vdata input and a backplane storage capacitor  706 . The voltage stored in the storage compactor  706  is supplied to the μDriver IC  710  via a Vdata input pad  709 . A Vdata input pad  709  exists for each subpixel controlled by the μDriver IC  710 . For a 12 subpixel controller, 12 pads may be used as Vdata input pads  709 . 
     In one embodiment the μDriver IC  710  additionally includes a Vramp input pad  704  for a voltage ramp input and a Vstart input pad  702  for a start input voltage. The Vramp, Vstart and Vdata inputs can be used to determine the start time and length of the emission pulse provided to an LED device  720  coupled via a pixel output pad  718 . For a 12-subpixel controller, 12 pads may be used as pixel output pads  718 . In one embodiment the μDriver IC  710  includes a p-channel (e.g., PMOS) transistor as a drive transistor  716  to drive current the LED  720  during the emission pulse. The drive transistor  716  has source electrode coupled to power supply (e.g., VDD) input pad  711  and a gate electrode couple to a reference voltage from a reference voltage input pad  714 . For RGB pixel arrangements, a first reference voltage can be used for red subpixels while a second reference voltage is used for green and blue subpixels, as shown by exemplary inputs EMGB  502 A and EMR  502 B in  FIG. 5 . 
     The drive transistor  716  couples to an emission switch transistor  717  that enables and disables the emission pulse. In one embodiment the gate of the emission switch transistor  717  couples to an XNOR gate  715 . The inputs to the XNOR gate  715  are each provided by separate current comparator circuits  712 A-B. In one embodiment the first comparator circuit  712 A compares a current based on the Vstart input from the Vstart pad  702  with current based on the Vramp input from the Vramp pad  704 . The second comparator circuit  712 B can compare a current based on the Vramp input from the Vramp pad  704  with current based on the Vdata from the Vdata pad  709  for the subpixel. Both comparator circuits  712 A-B couple to the SW pad  713 , which is an enable/disable switch for the comparator circuits  712 A-B, allowing the comparator circuits  712 A-B to be enabled when in use and disabled when not in use, which reduces the overall power consumption of the μDriver IC  710 . 
       FIG. 7B  shows a diagram for an exemplary comparator circuit  730  that may be used as comparators  712 A-B for the μDriver IC  710 . The comparator circuit  730  can include a transistor series including a first transistor  732 , second transistor  733 , and third transistor  734 , where the first transistor  732  is a PMOS transistor coupled to the IC power supply (e.g., Vdd) and gated by the Vramp input. The Vramp input for each comparator  712 A-B couples to the first transistor  732 . For the first comparator  712 A the Vstart input couples to the second transistor  733 . For the second comparator  712 B the Vdata input couples to the second transistor  733 . The third transistor  734  couples the second transistor  733  to ground/Vss and is gated by an SW input  736  which, for each comparator  712 A-B couples to the SW input pad  713  of the μDriver IC  710 . The SW input  736  is a switch input that may be used to enable or disable the comparator circuit  730 , where the comparator circuit  730  is disabled when not in use to save power. The circuit output  735  has a voltage determined by whether the current is greater through the first transistor  732  or the second transistor  733 , where the output  735  is at a low potential when the current in the second transistor is greatest and at a high potential when the current in the first transistor is greatest. 
     During operation, the second transistor  733  is biased as a current source via the Vdata/Vstart voltage, fixing the total current of the comparator. The ramp voltage generator causes Vramp to descend over time at a fixed slope. As the voltage of Vramp descends, the first transistor  732  begins to turn on as the Vramp-Vdd crosses the transistor threshold. Once the current generated by the first transistor  732  is greater than the current generated by the second transistor  733 , the circuit output  735  snaps to high potential (e.g., Vdd). 
       FIG. 8  is a diagram of a voltage output  800  showing operation of the hybrid analog PWM LED driving circuit  700  of  FIG. 7 . The voltage output  800  shows comparator outputs  802 A-B based on a comparison of a ramp voltage  804 , to a start voltage  805  and a stop voltage  806 . Based on the comparator outputs, the LED driving circuit generates an emission pulse (e.g., EM pulse  808 ). An exemplary EM pulse  808  of approximately 300 nanoseconds (ns) is shown. In one embodiment an emission pulse of less than 1 ns may be generated. 
     During operation, a first comparator output  802 A based on a comparison of the start voltage  806  to the ramp voltage  804  causes the emission logic to begin the EM pulse  808 . The second comparator output  802 A based on a comparison of the ramp voltage with the stop voltage  806  causes the emission logic to end the EM pulse  808 . The input data voltage received from the TFT backplane determines the stop voltage  806 . 
     Exemplary Hybrid Analog PWM μLED Driving Circuit with Reduced Power Comparator 
       FIG. 9  is a diagram of a hybrid analog PWM μLED driving circuit  900  with a reduced power comparator, according to an embodiment. The hybrid analog PWM μLED driving circuit  900  provided by one embodiment includes similar components as the hybrid analog PWM μLED driving circuit  700  of  FIG. 7 , while utilizing different comparator logic. The PWM μLED driving circuit  900  includes a TFT backplane  901  coupled to a μDriver IC  910 . In one embodiment the μDriver IC  910  receives a ramp input voltage from a ramp input pad  704 . The ramp input pad  704  couples to the source of a ramp/data NMOS transistor  906  in the μDriver IC  910 . The ramp/data NMOS transistor  906  has a a gate electrode coupled to the data storage capacitor  706  and a drain electrode coupled to the emission switch gate  717  and the drain of a pullup PMOS transistor  904 . The pullup PMOS transistor  904  has a source electrode coupled to the power supply (Vdd) and has a gate electrode coupled to a voltage reference source (Vref) via a Vref pad  902 . The reduced power comparison operation is performed by the ramp/data NMOS transistor  906  instead of the comparator logic  712 A-B shown in the circuit  700  of  FIG. 7 . 
       FIG. 10  shows an integrated circuit  1000  based on the PWM μLED driving circuit  900  of  FIG. 9 . The integrated circuit  1000  shows the logic elements  1006  and contact pads  1004  of the circuit diagram of  FIG. 9 . The illustrated integrated circuit  1000  is a 12 subpixel μLED driving circuit having 12 subpixel regions  1002 A-L, although embodiments are not limited to 12 subpixels, and circuits may be designed and manufactured to control more than 12 or fewer than 12 subpixels. 
     Relative to the circuit  700  of  FIG. 7 , an integrated circuit  1000  based on the circuit  900  of  FIG. 9  can realize significantly reduced circuit area per subpixel, as a tradeoff for lower pulse resolution. In one embodiment, the integrated circuit  1000  can be manufactured having a minimum lateral dimension of 24 μm and a maximum lateral dimension of 50 μm. 
     Returning to  FIG. 7 , the circuit  700  is in emissive operation when the drive transistor  716  provides current to the LED  720  during the emission pulse. The emission pulse becomes active when the ramp input voltage received via the ramp input pad  906  drops below the data voltage stored in the data storage capacitor  706 . The emission pulse ends when the ramp input voltage reaches the data voltage. Operation of the hybrid analog PWM μLED driving circuit  900  of  FIG. 9  is further illustrated by the electrical performance evaluation  1100  of  FIG. 11 . 
       FIG. 11  is shows an exemplary preliminary electrical performance evaluation  1100  of the PWM μLED driving circuit  900  of  FIG. 9 . As illustrated, the input data voltages for various gray levels are stored in a TFT based storage capacitor Cst and are compared against a repeating ramp voltage  1102 , which in one embodiment is supplied to the integrated circuit via a row driver or timing control circuit. For the circuit  900  of  FIG. 9 , progressively lower data voltages  1110 - 1116  result in progressively shorter EM pulses  1120 - 1126 , in which current is driven to the LED for progressively shorted periods  1130 - 1136 , down to a minimum pulse width of approximately 80 ns. 
     As exemplified by the circuits of  FIG. 7  and  FIG. 9 , lower power and smaller silicon area may be achieved by the use of simpler comparator designs at the cost of emission pulse resolution. A comparator having a sufficiently low offset is important for some μDriver circuit designs to achieve a narrow emission pulse used for low gray levels. However, low offset comparator designs consume additional power and circuit area relative to comparator designs having a higher voltage offset. Accordingly, the comparator design plays a key role in creating emission control logic capable of producing a sufficiently narrow emission pulse for low gray level output while minimizing circuit area and power consumption. 
     Embodiments described below provide various designs to relax comparator offset requirements by incorporating low voltage digital logic into the PWM emission control logic of the μDriver IC. Additionally, a multi-segmented and/or non-linear ramp may also be used to further relax offset requirements for the comparator logic. 
     Exemplary Hybrid Analog PWM μLED Driving Circuit Having a Relaxed Comparator Offset 
       FIG. 12  is a diagram of an additional hybrid analog PWM μLED driving circuit  1200 , according to an embodiment. In one embodiment the hybrid analog PWM μLED driving circuit  1200  includes a TFT backplane  1201  having components similar to other circuits described herein and a μDriver IC  1210  including comparator logic  1213  and low voltage digital logic components  1212  to control emission pulse duration. In one embodiment the low voltage digital logic components  1212  include a four transistor ramp segment selector  1222 , a start input coupled to a start input pad  1242 , a 12 transistor JK flip-flop  1232 , and a four transistor XOR logic gate  1252 , although the specific designs of the low voltage digital logic components  1212  can vary. Additionally, the μDriver IC  1210  includes an additional set of input pads  1211  for the low voltage digital logic  1212  that is shared by the emission logic for each subpixel, including a digital power supply (DVdd), clock signal and a digital data input. 
     In one embodiment provides for a power optimization in which the comparator logic  1213  is disabled at the end of an emission pulse. A feedback mechanism may be included such that the comparator logic  1213  is power gated at the end of each emission pulse and reset at the beginning of each frame. Such power optimization can reduce the power consumed by the μDriver circuit  1210  by reducing or eliminating the power consumed by the comparator logic  1213  between emission pulses. 
     In an alternate embodiment the PWM μLED driving circuit  1200  may exclude the JK flip-flop  1232  and substitute control logic coupled to the XOR gate  1252 . In such embodiment, the control signal timing is key to the proper operation of the circuit. 
     In one embodiment, the μDriver IC  1210  includes 34 connector pads to control  12  subpixel elements, including 24 per-subpixel connector pads for LED output and Vdata input. In such embodiment, the μDriver IC  1210  occupies between 75-90 μm 2  of total silicon area, including pad and circuit area. The comparator circuit  1213  can consume between 0 and 10 nA, which is emission dependent. 
     In one embodiment the PWM μLED driving circuit  1200  is operated using a multi-segmented ramp. Multi-segmented digital-to-analog (DAC) conversion may be used such that the encoding for low gray levels that require a finer comparator resolution and shorter pulse widths are grouped within a segment having a lower number of discrete gray levels within the group. Accordingly, higher ramp swings may be used to generate the shorter emission pulses associated with lower gray levels. In such embodiment, the ramp signal may be provided by ramp signal generation logic in row driver or timing control logic that controls pixel output for a display device including the PWM μLED driving circuit  1200 . 
       FIG. 13  is an exemplary voltage chart  1300  of a multi-segmented ramp input provided to a μLED driving circuit such as the PWM μLED driving circuit  1200  of  FIG. 12 , according to an embodiment. A multi-segmented ramp  1302  having a first ramp segment  1306  and a second ramp segment  1308  is shown. However, the multi-segmented ramp  1302  can include any number of individual segments associated with different gray level outputs. Additionally, the multi-segmented ramp  1302  may be a non-linear ramp  1304  comprised of successive rising and falling segments, or may be a linear ramp  1305 . The non-linear ramp  1304  and linear ramp  1305  may each be associated with an uneven distribution of gray levels. For example, where 256 discrete gray levels are supported, 48 gray levels having 48 different pulse widths may be associated with a first segment  1306  while the remaining 208 discrete gray levels may be associated with a second segment  1308 . Accordingly, a greater voltage differential exists between each of the individual lower gray levels, relaxing the voltage offset for the comparator logic when generating short emission pulses, allowing a more accurate translation between analog input voltage levels and emission pulse lengths. 
     Exemplary Hybrid Analog PWM μLED Driving Circuit with a Local Multi-Segmented Ramp 
       FIG. 14  is a diagram of an additional hybrid analog PWM μLED driving circuit  1400 , according to an embodiment. The hybrid analog PWM μLED driving circuit  1400  includes a TFT backplane  1401  similar to other circuits described herein, as well as a μDriver IC  1410  including local ramp generation logic. The local ramp generation logic includes control logic  1412  to control a set of current sources  1410 A-B. The control logic is coupled to a start input pad  1416  and a flip-flip  1414 . Additionally, digital input pads  1402  receive clock and data inputs for the digital logic components of the control logic  1412 . The control logic can be configured to generate a constant ramp or a programmed ramp, which can be linear or non-linear. Additionally, the generated ramp signal may be multi-segmented, such that lower gray levels may be associated with higher resolution ramp signals. 
     The use of digital control logic  1412  to generate a local ramp signal, in addition to the use of a multi-segmented and/or non-linear ramp signal can significantly relax the comparator design requirements. In one embodiment, an inverter  1408  may be used as a comparator. In such embodiment, a static CMOS inverter or another inverter design having little to no static power dissipation may be used. 
     In one embodiment the inverter  1408  couples to an AND gate  1406 . The AND gate  1406  additionally couples to a latch input pad  1404  and the gate electrode of the emission switch transistor  717 . The input via the latch input pad  1404  and the output of the inverter  1408  control the length of the current pulse supplied to the LED  720 . In one embodiment, the current drive assembly and emission switch for the μDriver IC  1410  of  FIG. 14  differs from other circuits described herein in that the emission switch transistor  717  gates the power supply (Vdd) to the current drive transistor  716  instead of gating the current supplied to the LED  720 . 
     In one embodiment the hybrid analog PWM μLED driving circuit  1400  shares a TFT storage capacitor Cst  706  with the ramp generator logic. Vdata input charges Cst  706  and enables an emission pulse. The control logic  1412  uses one of the current sources  1410 A-B to add additional charge to Cst  706  until the charge in Cst  706  reaches a reference voltage, which trips the inverter  1408  and, based on the latch input  1404 , ends the emission pulse. 
       FIG. 15  illustrates an exemplary voltage chart  1500  of a multi-segmented ramp  1502  generated by the μLED driving circuit  1400  of  FIG. 14 . Voltage ramps of varying initial voltages  1504  can be associated with different gray levels. The ramp signal initial voltages can be based at least in part based on the Vdata voltage supplied to the circuit, with higher initial voltages resulting in reduced emission pulse length. The emission pulse for each gray level ends when the ramp voltage begins at its initial voltage and ends when the ramp voltage reaches a pre-determined comparator offset  1503 . Lower initial voltages and/or lower ramp slopes result in longer emission pulses and higher gray levels. Within the time duration allotted to each frame, a smaller number of lower gray levels can associated with an initial period, while a higher number of higher gray levels having a higher pulse duration can be associated with later portions of the frame. For example, lower gray levels can be associated with higher sloped ramps associated with a first segment  1506  within a frame to enable higher emission pulse length resolution, relaxing comparator offset requirements. Higher gray levels can be associated with lower sloped ramps in a second segment  1508  within the frame. The relatively longer emission pulses of the second segment  1508  can reduce the relative resolution required for each gray level. While two segments  1506 ,  1508  are illustrated, any number of segments may be used. Additionally, while linear ramps are illustrated, non-linear ramps as shown in  FIG. 13  may also be used. 
     Hybrid MicroDriver Display System 
       FIG. 16  illustrates the processing of substrates of μDriver and μLEDs into a receiving substrate for a hybrid μDriver and μLED display, according to an embodiment. In one embodiment, separate carrier substrates including one or more μLED substrate(s)  1610  and a μDriver substrate  1620 . One or more transfer assemblies  1600  can be used to pick up and transfer microstructures from the carrier substrates (e.g.,  1610 ,  1620 ) to the receiving display substrate  1630 . 
     In one embodiment, separate transfer assemblies  1600  are used to transfer any combination of μLED colors from the μLED substrate  1610  and μDriver substrate  1620 . In one embodiment the display substrate  1630  is prepared with distribution lines to connect the various the μLED and μC structures. The display substrate can also be prepared with one or more layers of TFT components as described herein. The distribution lines can be coupled to landing pads and an interconnect structure to electrically couple the μLED devices, the μC devices, and the TFT components. The interconnect structure can also be designed to couple the various μC devices to each other to create a μDriver relay to enable communication between the μDriver ICs. The receiving substrate can be a display substrate  1630  of any size ranging from micro displays to large area displays, can be a lighting substrate for LED lighting, or for use as an LED backlight for an LCD display. In one embodiment the μLED and μDriver structures are bonded to the same side of the substrate surface. However, the μDriver and μLED structures may also be bonded to alternate sides of the substrate surface. 
     The μDriver and μLEDs are described herein as coupling to a substrate via connection pads. However, the bonds between the components can be made using various connections such as, but not limited to, pins, conductive pads, conductive bumps, and conductive balls. Metals, metal alloys, solders, conductive polymers, or conductive oxides can be used as the conductive materials forming the pins, pads, bumps, or balls. In an embodiment, heat and/or pressure can be transferred from the array of transfer heads  1605  to facilitate bonding. In an embodiment, conductive contacts on the μC, μLED devices, or other display components (e.g., sensor devices) are thermocompression bonded to conductive pads on the substrate. In this manner, the bonds may function as electrical connections to the μDriver IC and μLED devices. In one embodiment bonding includes indium alloy bonding or gold alloy bonding. Other exemplary bonding methods that may be utilized with embodiments include, but are not limited to, thermal bonding and thermosonic bonding. 
     The specifics of the display substrate  1630  can vary based on the target application. In one embodiment the display substrate  1630  is used to form a microPixel array  1615  for use in a high-resolution display. In one embodiment the microPixel array  1615  can have up to 440 pixels per inch, although other embodiments may be manufactured at higher PPIs. 
     Hybrid MicroDriver Display System 
       FIG. 17  is an illustration of a hybrid micro-driver display, according to an embodiment. In one embodiment, a μDriver and LED substrate  1730  that is prepared with distribution lines to interconnect a micro-matrix of μDriver IC devices and LEDs (e.g., μLEDs, OLEDs, etc. In one embodiment a TFT substrate  1732  including LTPS and/or Oxide transistors and capacitors are deposited or integrated with the μDriver/LED substrate  1730 . An optional sealant  1740  can be used to secure and protect the substrate. In one embodiment, the sealant is transparent, to allow a display or lighting substrate with top emission LED devices to display through the sealant. In one embodiment, the sealant is opaque, for use with bottom emission LED devices. In one embodiment an optional a data driver  1710  and a scan driver  1720  couple with multiple data and scan lines on the display substrate. In one embodiment, each of the smart-pixel devices couple with a refresh and timing controller  1724 . The refresh and timing controller  1724  can address each LED device individually, to enable asynchronous or adaptively synchronous display updates. In one embodiment, an emission controller  1726  can couple with the μDriver/LED substrate  1730  to control the brightness of LEDs, for example, via manipulation of emission control inputs. In one embodiment the emission controller  1726  can couple with one or more optical sensors to allow adaptive adjustment of emission pulse length based on ambient light conditions. In one embodiment the emission controller  1726  can adjust display brightness via manipulation of reference voltages supplied to the μDrivers. 
     A display system may additionally include a receiver to receive display data from outside of the display system. The receiver may be configured to receive data wirelessly, by a wire connection, by an optical interconnect, or any other connection. The receiver may receive display data from a processor via an interface controller. In one embodiment, the processor may be a graphics processing unit (GPU), a general-purpose processor having a GPU located therein, and/or a general-purpose processor with graphics processing capabilities. The display data may be generated in real time by a processor executing one or more instructions in a software program, or retrieved from a system memory. A display system may have any refresh rate, e.g., 50 Hz, 60 Hz, 100 Hz, 120 Hz, 200 Hz, or 240 Hz. 
     Depending on its applications, a display system may include other components. These other components include, but are not limited to, memory, a touch-screen controller, and a battery. In various implementations, the display system may be a television, tablet, phone, laptop, computer monitor, automotive heads-up display, automotive navigation display, kiosk, digital camera, handheld game console, media display, e-book display, or large area signage display. 
     In utilizing the various embodiments of this disclosure, it would become apparent to one skilled in the art that combinations or variations of the above embodiments are possible for controlling emission of a display panel. Although the present disclosure has been described in language specific to structural features and/or methodological acts, it is to be understood that the disclosure defined in the appended claims is not necessarily limited to the specific features or acts described. The specific features and acts disclosed are instead to be understood as particularly graceful implementations of the claimed disclosure and useful for illustrating the present disclosure.

Metadata:
Filing Date: 20160825
Publication Date: 20190827
Grant Date: 20190827
Priority Date: 20150918
Inventors: Charisoulis, Thomas
NAUTA, TORE
SAKARIYA, KAPIL
DROLET, JEAN-JACQUES P.
Assignee: APPLE INC
CPC Classifications: [{"code": "G09G2320/064", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/3275", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2300/0452", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2300/0408", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2330/021", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/3241", "inventive": true, "first": true, "tree": "[]"}, {"code": "G09G3/3258", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2300/0426", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/2018", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2310/0291", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/064", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/2074", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/3275", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2330/021", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2300/0452", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L25/167", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2310/08", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2310/027", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L27/1225", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L29/7869", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L25/167", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2300/0426", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2310/0291", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2320/064", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/2018", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2330/021", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/3258", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G2310/027", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L27/3262", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01L29/78654", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2310/08", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2300/0452", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/3241", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01L27/3251", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/3275", "inventive": true, "first": false, "tree": "[]"}, {"code": "G09G3/2074", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01L29/78672", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G3/2018", "inventive": true, "first": true, "tree": "[]"}, {"code": "H10D86/423", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10D86/60", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10D30/6755", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10D30/6745", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10D30/6744", "inventive": false, "first": false, "tree": "[]"}, {"code": "G09G2300/0426", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10K59/1213", "inventive": false, "first": false, "tree": "[]"}, {"code": "H10K59/127", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 67700740