2025-02-08
Firstly, what is digital driving?
In recent years, XR display technology has been continuously developing in depth. The silicon-based OLED microdisplay, which uses CMOS circuits to drive OLED pixels on silicon-based panels, has gradually entered the public eye. Especially after the release of the Apple Vision Pro, the combination of silicon-based OLED displays and Pancake optical technology has gradually become the ideal choice for high-end XR device display systems.
We know that the rise and prosperity of silicon-based OLED technology is driven by the demand for miniaturization of display devices. Especially for application scenarios such as VR, MR, AR devices that have strict requirements for volume and weight, highly integrated display technology is needed. Integrating high-density pixel driver circuits on a very small silicon substrate through CMOS technology can help achieve thinner and lighter display devices and improve user comfort.
In the operation mechanism of silicon-based OLED display panels, pixel driving circuits are like a group of behind the scenes workers, each performing their own duties and silently playing a key role, which enables silicon-based OLED screens to display colorful images.
It can be said that the pixel driving circuit controls the brightness and brightness adjustment of each pixel.
Current Steward - Analog Drive
By magnifying the silicon-based OLED screen 50 times under a microscope, a pixel array composed of R, G, and B sub pixels can be seen. To emit light, the pixel needs to be powered by the driving circuit, and the current can be controlled by adjusting the voltage at both ends of the driving tube. By fine-tuning the voltage, different currents can be obtained. When the current flowing is small, the grayscale level of the pixels is low, forming the low grayscale area of the image; When the current flowing is large, the pixel grayscale level is high, forming the high grayscale area of the image. The driving circuit is also responsible for color matching, by precisely controlling the current size of different sub pixels, each sub pixel emits different colors of light, mixing various colors. The ideal pixel driver circuit is like a precise current manager, responsible for providing each sub-pixel with just the right amount of current.
The analog driver was thus generated. Analog driving generally controls the current by adjusting the gate voltage of each pixel driving transistor, just like fine-tuning the voltage knob of a pixel, attempting to make it emit brightness levels ranging from the darkest to the brightest. This adjustment signal is continuous, and theoretically, its value can be any value within a certain range.
Illuminated switch controller - digital drive
But let's not overlook that the primary function of pixel driver circuits is to control the brightness of pixels. Just like installing a switch for each pixel, it can determine when to make the pixel light up and when to turn it off. The application of digital drivers precisely utilizes this function.
The digital driving circuit also supplies power to the pixel unit, but adjusting the brightness of the pixel is not achieved by adjusting the voltage or current, but by maintaining a constant control voltage of 100% and adjusting the proportion of each pixel's lighting and extinguishing time.
If the pixel stays bright for a longer period of time and turns off for a shorter period of time, it will result in bright pixels, forming the high grayscale area of the image; If the pixel is bright for a shorter period of time and extinguished for a longer period of time, low brightness pixels will be obtained, forming the low grayscale area of the image. When mixing colors, the on/off time of sub pixels is also precisely controlled, emitting light according to their respective duty cycles and configuring specific emission ratios to obtain various colors.
We can consider that regardless of the driving method, its essence is to drive pixels to emit specific light. As long as it is an ideal driving circuit, it can accurately control the brightness of pixels.
Digitally driven to achieve a wider range of HDR displays
When we apply theory to practice, we will find that some characteristics of silicon-based OLEDs themselves can affect the adjustment effect of driving methods. Silicon based OLED belongs to miniaturized OLED (Micro OLED), which requires extremely low current during actual light emission, at the nA level. The current in low grayscale areas is even smaller, only at the pA level. In such a situation, the driving tube is extremely sensitive to voltage changes in the region, and even extremely small voltage changes can cause significant fluctuations in brightness.
7.png
In this situation, the superiority of digital driving is highlighted. Digital driving can avoid the above-mentioned problems.
Why do you say that?
Because digital driving does not rely on continuous voltage or current regulation, but controls brightness by adjusting the proportion of pixel lighting and extinguishing time, thus accurately restoring various gray levels in the image. This is a discrete signal, for example, digital driving can divide a brightness cycle into a fixed number of time units, and then achieve different brightness levels by precisely controlling the number of lighting time units. In this way, digital drivers can precisely control the brightness of each pixel and sub-pixel, even in low grayscale situations without exception.
In contrast, analog signals are continuously changing and theoretically can take any value. However, in practical operation, the current difference between some low brightness pixels with adjacent brightness levels is often very subtle. In this case, the analog signal may not be able to accurately grade these pixels, resulting in some gray levels being missing and causing dark and unclear details in the dark parts of the image. When mixing colors, low grayscale sub pixels cannot achieve the correct brightness ratio, resulting in color distortion and an unrealistic image. The on/off time ratio of digital driven sub pixels is precisely controlled by digital signals to maintain the correct light emission ratio, ensure the stability of color gamut, and achieve more realistic display effects.
Digital driving presents smoother and more detailed images across the entire grayscale range.
