Decryption Micro-LED three different drive mode
Micro-LED is a current-driven light-emitting device, the drive is generally only two modes: passive location drive (PM: Passive Matrix, also known as passive addressing, passive addressing, passive drive, etc.) and active (AM: ActiveMatrix, also known as active addressing, active addressing, active drivers, etc.), this article also extends the active drive of another "semi-active" driver. These models have different driving principles and application characteristics, the following will be introduced through the circuit diagram to the specific principles.
What is the PM drive mode?
The passive location drive mode connects the P-electrode of the LED pixels in each column of the array to the Data Current Source while connecting the N-electrode of each row of LED pixels to the line scan Line (ScanLine). When a particular Y-th row scan line and the X-th row scan line are gated, the LED pixels at the intersection (X, Y) are lit. The entire screen in this way for high-speed point-by-point scanning to achieve the display dicing, as shown in Figure 1. [1,2].
This scanning method is simple and easy to implement.
But the shortcomings of the connection is complex (requires X + Y root connection), parasitic resistance and capacitance lead to low efficiency, low pixel light emission time (1 field / XY) resulting in low effective brightness, easy to crosstalk between pixels, and The frequency of the scanning signal is high.
Another optimized passive location driver is to add a latch in the column scan section, which is used to store the column scan signals (Y1, Y2 ... Yn) of all pixels at the first time in line X in the latch In the device. When the Xth row is gated, the above Y1-Yn signal is simultaneously loaded on the pixel . This drive can reduce the frequency of the column drive signal, increasing the brightness and quality of the display. But still can not overcome the passive location-driven way of natural defects: wiring complex, easy to crosstalk, pixel strobe signal can not be saved and so on. The active location-driven approach provides a good solution for the above difficulties.
What is AM drive mode?
In the active location drive circuit, each Micro-LED pixel has its own independent drive circuit, the drive current provided by the drive transistor. The basic active matrix drive circuit is a dual transistor single capacitor (2T1C: 2Transistor 1Capacitor) circuit, as shown in Figure 2 .
Figure 2 active location drive mode
Each pixel circuit uses at least two transistors to control the output current, T1 is the gate transistor, used to control the pixel circuit on or off. T2 is driven by a transistor that communicates with the voltage source and provides a stable current for the Micro-LED in a frame time. The circuit also has a storage capacitor C1 to store the data signal (Vdata). When the scanning signal pulse of the pixel unit is finished, the storage capacitor remains the voltage of the gate of the driving transistor T2, thereby continuously driving the current for the Micro-LED pixel until the frame ends.
2T1C drive circuit is only active source Micro-LED a basic pixel circuit structure, its structure is relatively simple and easy to implement. But because of its essence is the voltage control current source (VCCS), and Micro-LED pixels are current-type devices, so the display of gray control will bring some difficulty, which we in the back of the "Micro-LED color And the gray scale "section will be discussed. Dr. Liu Zhaojun has proposed a 4T2C current proportional micro-LED pixel circuit, the use of current control current source (CCCS) way, in the realization of gray scale has an advantage .
What is the "semi-active" location-driven approach
Another need to mention is a "semi-active" location-driven approach . This type of drive using a single transistor as Micro-LED pixel drive circuit (shown in Figure 3), which can be better to avoid the phenomenon of crosstalk between pixels.
Comparison of the three driving methods
Compared with passive location, active location method has obvious advantages, more suitable for Micro-LED current-driven light-emitting devices. Now detailed analysis is as follows:
① active location of the drive ability is stronger, can achieve a larger area of the drive. The drive capability of the passive location is affected by the external integrated circuit drive performance, and the drive area is limited in resolution.
② active location has better brightness uniformity and contrast. In the passive location mode, due to the limited external drive IC drive capability, the brightness of each pixel is affected by the number of pixels in this column. In general, the same column of Micro-LED pixels share the drive current of one or more output pins of the external drive IC.
So, when the number of pixels in the two columns is not the same, the drive current applied to each LED pixel will be different, the brightness of the different columns will be very different. This problem will be more seriously reflected in the large area display applications, such as LED TV and LED large screen and so on. At the same time as the number of rows and columns increased, the problem will become more severe.
③ active location can achieve low power consumption and high efficiency. Large area display applications require relatively large pixel density, so it is necessary to minimize the electrode size, and drive the required voltage will greatly increase the display, a lot of power will be lost in the row and column scan line, Resulting in inefficiency.
④ high independent controllability. Passive location, the higher the drive voltage will also bring the second trouble, that is, crosstalk, that is, in the passive location LED array, the drive current in theory only from the selected LED pixels through, but The other pixels around will be affected by the current pulse, and ultimately will reduce the display quality. The active site selection is a good way to avoid this phenomenon by a pixel circuit consisting of a gate transistor and a driving transistor.
⑤ higher resolution. Active location-driven is more suitable for high-PPI high-resolution Micro-LED display.
While the third "semi-active" drive can better avoid crosstalk between pixels, but because of its pixel circuit does not store capacitors, and each column of the drive current signal needs to be modulated separately, and can not fully meet the above list Out of the active location of the drive all the advantages of the way.
For example, the pixel and the driving transistor T2 are connected in four ways as shown in Fig. 4, for example, in the case of a blue-light Micro-LED epitaxially grown on a sapphire substrate. But since the epitaxial growth structure of the LED is p-type gallium nitride (GaN) on the outermost surface and n-type gallium nitride in the underlying