CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims priority to Chinese Patent Application No. 202111383860.4, filed on Nov. 22, 2021, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a display apparatus.

BACKGROUND

Micro-LED (Light-emitting Diode) technology refers to the integration of high-density, small-sized LED arrays on one substrate. Depending on the display apparatuses, the size of one single pixel ranges from several micrometers to several hundreds of micrometers. The LED in one pixel is smaller than the pixel since the driving circuit occupies a certain area. To drive such a high-density Micro-LED display, a relatively mature method is to attach or electrically connect the LEDs to a CMOS (Complementary Metal Oxide Semiconductor) integrated circuit on a silicon substrate. The Micro-LEDs such as gallium nitride (GaN) crystals are fabricated on a sapphire substrate, have high conversion efficiency, high brightness, long lifetime, ultra-fast response speed, etc., and hence are considered the most ideal light-emitting sources for various displays in the future. Recent years, especially with the gradual maturity of the massive transfer technology of transferring millions of micro-LED devices from a growth substrate (sapphire) to a target substrate (glass substrate or silicon substrate) for electrical connection and mechanical support, it has become feasible to use Micro-LEDs in TV displays, mobile phone displays, and even micro-displays in AR (Augmented Reality) glasses. Compared with the currently widely used organic LED (OLED) display technology, Micro-LED has excellent luminous brightness and long lifespan, and the industry even thinks that Micro-LED technology may eventually replace OLED display technology.

However, the current Micro-LED technology still faces many technical and manufacturing challenges, such as the needs to improve electro-optical conversion efficiency, the full colorization, manufacturing yield and chromaticity uniformity. The conversion efficiency of the GaN LED die, defined as the luminous flux per injected current, is not a constant, but the relationship of the luminous flux in the light emitting area with the injected current is a non-linear function. When the injected current is very small, the conversion efficiency is low, and it gradually increases with the injected current before it reaches a peak value at a certain injected current, then it decreases. During this current changing process, the light spectrum of the GaN LED will exhibit a drift. Therefore, in order to reduce the power consumption and maintain a uniform chromaticity, it is necessary to drive each Micro-LED die of the display apparatus with a fixed current range that keeps the conversion efficiency at a relatively high level. In order to modifying the brightness or color of the pixels, a mature solution is to apply pulse-width modulate (PWM) on each LED die through the drive chip, that is, to realize digital modulation of the brightness by modifying the duty cycle of the current pulse. Although the luminous intensity of the LED driven by each pulse is same, the average brightness perceived by the human eye is still changing with modification of the duration of the light pulse from the LED.

However, due to manufacture process variations or inhomogeneities in the OLED raw materials, the brightness and chromaticity of every Micro-LED will still be different even the Micro-LEDs are driven by the same current. A full-color RGB array has at least hundreds of thousands of LEDs emitting red, green and blue respectively, their brightness difference and chromaticity difference will be directly shown up on the display screen. Conventional approach for this problem is first dicing the epitaxially grown GaN into individual LED dies and sorting them according to their brightness and chromaticity, and then transferring those LED dies whose brightness and chromaticity are similar to one display array. This approach, however, will discard LED dies with luminescence characteristics deviating from those being selected, and then reduce manufacturing yield and increase manufacturing costs. Even for the case of monochromatic LED dies, for example, in the case where blue LED dies are combined with red and green quantum dot light-emitting materials respectively to create full color display, the differences in the brightness and the characteristics of the current in the blue LED dies will still cause both brightness and chromaticity variations across the display.

Applications of the Micro-LED in the AR glasses with extremely high pixel density are facing another technical hurdle and which is becoming more and more obvious. When the LED die is driven by an optimum voltage that is supposed to achieve a better electro-optical conversion efficiency, the optimum voltage may exceed ideal driving voltage range of the transistors in CMOS circuit. With the increase of pixel density, the dimensions of the CMOS transistors driving the LED array and their driving voltages are supposed to be scaled down according to certain rules in semiconductor integrated circuit. This is to suppress various parasitic effects generated under higher driving voltages in silicon substrate, such as the Latch-up effect in CMOS circuits, and the vertical and horizontal leakage currents in substrate. However, since the brightness of the LED die is strongly dependent on the current passed through or the driving voltage applied, the driving voltage of the LED die cannot be simply reduced just for the benefit of reliability of CMOS circuit.

SUMMARY

In order to overcome the technical hurdles described above, the present disclosure provides a display apparatus including a pixel array composed of a plurality of pixels, wherein, each pixel includes a pulse-generating unit, a first switching unit, a light-emitting unit and a second switching unit, and the later three units are connected in series. The first switching unit and the second switching unit are turned on or turned off synchronously. When they are turned on simultaneously, the anode voltage and the cathode voltage are simultaneously applied to the light-emitting unit to excite light. The total driving voltage, which is equal to a difference between the anode voltage and the cathode voltage, is shared by the two switching units and the light-emitting unit. Even the total driving voltage is higher than an ideal operational voltage in a CMOS transistor, less than half of the total driving voltage is actually applied to each switching unit, which greatly reduces the operational risks caused by the parasitic effects. The pulse-generating unit produces two synchronized but opposite PWM pulses, that the first pulse is for controlling the on-off state of the first switching unit, the second pulse is for controlling the on-off state of the second switching unit. Since the first pulse and the second pulse are opposite in phase, noise spikes induced by the rising and falling edges of the pulses in the pixel array are canceled in situ by themselves, resulting in less electronic noise.

The first switching unit and the second switching unit may use different conduction types of MOS (Metal-Oxide-Semiconductor) transistors. For example, the first switching unit may use a PMOS (P-channel MOS) transistor or an NMOS (N-channel MOS) transistor, while the second switching unit may use an NMOS transistor or a PMOS transistor. The pulse-generating unit may include a voltage comparator and a first inverter. The voltage comparator may have a first input terminal and a second input terminal, linking to a data voltage and a sawtooth voltage, respectively. An output port of the voltage comparator is connected to an input terminal of the first inverter. The first inverter produces an inverted PWM pulse with certain time delay for the second switching unit, comparing with the output pulse from the voltage comparator. A first buffer unit, or a voltage follower or a simply time-delay unit can be provided between the output port of the voltage comparator and the first switching unit, to ensure that the phase difference between the first and the second PWM pulses is approximately equal to 180°.

In addition, the voltage comparators in different rows of the pixel array can have sawtooth pulses with opposite phases, for example, voltage comparators in pixels on odd number rows and in pixels on even number rows have sawtooth pulses as input with opposite phase. This scheme can be applied to odd number columns and even number columns as well. The high frequency noises induced by the sawtooth voltages on the bus bars of the pixel array can be canceled by themselves in situ, resulting in less noise interference to the analog signal during the data writing cycle.

The light-emitting unit in each pixel may include one or multiple light-emitting elements connected in series or in parallel. The light-emitting elements having similar chromaticity should be put into one pixel to ensure consistency of chromaticity and white balance of the display image. The intentionally picking up multiple LED dies with different brightness and combining them into one light-emitting unit in a pixel and making sure all pixels in the pixel array having similar brightness in average, will certainly and significantly improve utilization efficiency of the Micro-LED. Regarding the implementation of this approach, two or more Micro-LED dies may be selected from the current—brightness distribution curve, and connect them in series as the light-emitting unit in a pixel. Alternatively, two or more Micro-LED dies may be selected from a voltage-brightness distribution curve and connect them in parallel as the light-emitting unit in a pixel. Picking and combining Micro-LED dies with different brightness in pixel not only significantly improves the manufacturing yield but also results in a better uniformity of the display apparatus.

It should be readily understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not intended as a limitation to the scope of the present disclosure.

