Micro-LED is a current-driven light-emitting device, and its driving mode generally has only two modes: passive addressing drive (PM: Passive Matrix, also known as passive addressing, passive addressing, passive drive, etc.) and active Addressing drive (AM: Active Matrix, also known as active addressing, active addressing, active drive, etc.), this article will also analyze a “semi-active” addressing drive method. These modes have different driving principles and application characteristics, and the following will introduce their principles in detail through the circuit diagram.
What is PM drive mode?
Passive addressing drive mode connects the anode (P-electrode) of each column of LED pixels in the array to the column scan line (Data Current Source), and at the same time connects the cathode (N-electrode) of each row of LED pixels to the row scan Line (Scan Line).
When a specific scan line in column Y and row X is selected, the LED pixel at the intersection point (X, Y) will be lit. In this way, the entire screen can be scanned point by point at high speed to realize the display picture, as shown in Figure 1. This scanning method has a simple structure and is relatively easy to implement.
But the disadvantage is that the connection is complicated (X+Y connection is required), the efficiency is low due to the large parasitic resistance and capacitance, the pixel light-emitting time is short (1 field/XY), which leads to low effective brightness, easy crosstalk between pixels, and The frequency requirement of the scanning signal is high.
Another optimized passive address selection driving method is to add a latch in the column scanning part, and its function is to store the column scanning signals (Y1, Y2… Yn) of all pixels in row X at a certain moment in the latch in advance. device.
When row X is selected, the above-mentioned Y1-Yn signals are loaded on the pixels at the same time. This driving method can reduce the frequency of the column driving signal and increase the brightness and quality of the displayed picture. However, it is still unable to overcome the inherent defects of the passive address selection drive method: the connection is complex, easy to crosstalk, and the pixel strobe signal cannot be preserved. The active address selection drive method provides a good solution for the above difficulties.
What is AM drive mode?
In the active address-selection driving circuit, each Micro-LED pixel has its corresponding independent driving circuit, and the driving current is provided by the driving transistor. The basic active matrix drive circuit is a two-transistor single-capacitor (2T1C: 2 Transistor 1 Capacitor) circuit, as shown in Figure 2.
Figure 2 Active addressing drive mode
At least two transistors are used in each pixel circuit to control the output current, and T1 is a gate transistor, which is used to control the on or off of the pixel circuit. T2 is to drive a transistor, which is connected with a voltage source and provides a stable current for the Micro-LED within a frame.
There is also a storage capacitor C1 in the circuit to store the data signal (Vdata). When the scanning signal pulse of the pixel unit ends, the storage capacitor can still maintain the voltage of the gate of the driving transistor T2, so as to drive the Micro-LED pixel continuously until the end of the Frame.
The 2T1C drive circuit is only a basic pixel circuit structure for active addressing Micro-LEDs, and its structure is relatively simple and easy to implement. However, since its essence is a voltage-controlled current source (VCCS), and the Micro-LED pixel is a current-type device, it will bring certain difficulties in the control of the display grayscale. Discussed in the Transformation and Grayscale section.
Dr. Liu Zhaojun’s research group once proposed a 4T2C current proportional Micro-LED pixel circuit, which uses a current-controlled current source (CCCS) method, which has advantages in realizing gray scale.
What is the “semi-active” addressing drive method?
Another thing that needs to be mentioned is a “semi-active” addressing drive method. This driving method uses a single transistor as the driving circuit of the Micro-LED pixel (as shown in Figure 3), so that crosstalk between pixels can be better avoided.
Comparison of three driving methods
Compared with passive addressing, active addressing has obvious advantages and is more suitable for current-driven light-emitting devices such as Micro-LEDs. The detailed analysis is as follows:
① The driving ability of active site selection is stronger, which can realize the driving of larger area. However, the driving capability of passive address selection is affected by the driving performance of the external integrated circuit, and the driving area and resolution are limited
② Active site selection has better brightness uniformity and contrast. In the passive addressing mode, due to the limited driving capability of the external driving integrated circuit, the brightness of each pixel is affected by the number of lit pixels in this column. Generally speaking, the Micro-LED pixels in the same column share the drive current of one or more output pins of the external drive I
Therefore, when the number of lighted pixels in the two columns is different, the driving current applied to each LED pixel will be different, and the brightness of different columns will vary greatly. This problem will be more seriously reflected in large-area display applications, such as LED TVs and LED large screens. At the same time, as the number of rows and columns increases, this problem will become more sever
③ Active site selection can achieve low power consumption and high efficiency. Large-area display applications require a relatively large pixel density, so the electrode size must be reduced as much as possible, and the voltage required to drive the display will also increase greatly, and a large amount of power will be lost on the row and column scan lines. resulting in inefficienci
.
④ High independent controllability. In passive address selection, higher drive voltage will also bring the second trouble, that is, crosstalk, that is to say, in passive address LED array, the drive current theoretically only passes through the selected LED pixels, but Other surrounding pixels will be affected by the current pulse, which will eventually degrade the display quality. The active address selection method avoids this phenomenon very well see through led wall the pixel circuit composed of a pass transistor and a drive transis
r.
⑤ Higher resolution. Active address selection drives are more suitable for Micro-LED displays with high PPI and high resolu
on.
Although the third “semi-active” drive can better avoid the crosstalk phenomenon between pixels, but because there is no storage capacitor in the pixel circuit, and the driving current signal of each column needs to be modulated separately, it cannot fully achieve the above column. All the advantages of the active addressing drive me
od.
Taking the blue-light Micro-LED epitaxially grown on a sapphire substrate as an example, there are four ways to connect the pixel and the driving transistor T2 as shown in FIG. 4 . However, since the LED epitaxial growth structure is p-type gallium nitride (GaN) on the outermost surface and n-type gallium nitride on the bottom layer, as shown in Figu
From the perspective of manufacturing process, it is more reasonable to connect the output terminal of the driving transistor to the p-electrode of the Micro-LED pixel, namely (a) and (c) in Figure 4. In Figure 4(a), the Micro-LED pixel is connected to the source (Source) of the N-type drive transistor. The inhomogeneity of Micro-LED electrical characteristics caused by the inhomogeneity of epitaxial growth (Epitaxial Growth), manufacturing process, and device aging will directly affect the VGS of the driving transistor, resulting in uneven display im
es.
The Micro-LED pixel in Figure 4(c) is connected to the drain of the P-type drive transistor (Drain), which can avoid the above-mentioned effects, and its current-voltage relationship is shown in Figure 6. Therefore, it is more appropriate to have a P-tube pixel circuit to drive Micro-LED.