introduction

Due to design and process reasons, some panels have problems such as the properties of liquid crystal molecules being easily damaged. Therefore, when designing the drive circuit for driving the LCD panel, special functional circuits need to be added to control the deflection direction of the liquid crystal molecules. This article introduces a method for controlling the polarity conversion of the pixel voltage of a liquid crystal display, which overcomes the problem in the existing technology that the characteristics of the liquid crystal molecules are easily destroyed due to the single polarity conversion law of the polarity conversion signal.

1 System overall block diagram

The basic unit of this design system consists of a timing controller (T-CON), a micro control unit (MCU), a polarity protection circuit, and an LCD panel. The system block diagram is shown in Figure 1. The timing controller is the core device for driving the LCD panel. Its main function is to provide the necessary timing control signals for the gate driver and source driver in the TFT-LCD panel. It will receive the LVDS (LowVoltageDifferentialSignaling, low-voltage differential signal) signal sent from the front end and convert it into a MINI-LVDS signal, and drive the LCD panel by outputting the corresponding timing control signal so that each pixel displays the corresponding pixel voltage. The microcontroller plays a counting control role in this system. It realizes the timing control of the pOL flip signal flip by counting the corresponding control signals sent from the T-CON. The main function of the polarity protection circuit in this design is to consider that if the MCU does not work properly and the pOL flip signal is not normally output to the LCD panel, it controls the STV signal so that the signal driving the LCD panel is not output, so as to achieve Prevent the screen from polarizing in a short period of time. The system also includes a power management chip, whose main function is to provide the power supply voltage required for normal operation of T-CON, MCU and protection circuit. The chips used in the power management unit are DC/DC chips and LDO (LowDropoutRegulator, low voltage dropout linear stabilizer). voltage converter), the DC/DC chip converts the input 12V voltage into 3.3V through the BUCK circuit (step-down conversion circuit), and the LDO linearly converts the 3.3V voltage into 1.8V voltage.

2 Overall design

2.1 Introduction to timing controller and output waveform

The four main control signals output by the timing controller are STV, CpV, Tp, and pOL signals, as shown in Figure 2. The STV signal is the starting signal of a frame of image; the CpV signal is the clock signal output by T-CON to the gate driver. It is sequentially output to the thin film transistor (thinfilm transistor, TFT) of each row through the shift register to control the TFT. On and off; the Tp signal is the data source row latch signal output by T-CON to the source driver. When the TFT of a certain row is turned on, the source driver converts the input digital signal into an analog signal and outputs it to the source terminal of the TFT. Data is latched on the rising edge of Tp and data is output on the falling edge; the pOL signal is a polarity reversal signal that controls the pixel voltage. This design uses ZINVERSION's polarity reversal method. Due to the special panel architecture (that is, two adjacent columns with the same polarity (The polar pixels are all connected together), so that its polarity reversal frequency is equal to the frame rate, which can greatly reduce the frequency of the pOL signal, and also correspondingly reduces the power consumption and temperature of the source driver. Before a frame of image arrives, the pOL signal will control the polarity of the pixel voltage of this frame of image according to the set polarity flip method. Taking a 60Hz 1,366×768 resolution LCD panel as an example, Figure 2 is the timing relationship diagram of these control signals. It can be seen from the figure that the pOL flip signal occurs in the BLANK area (invalid data area) of the previous frame. , it has been flipped before the STV of the next frame comes, and there is 23.6μs from the rising edge of STV, which means that the polarity of the pixel voltage of a certain frame has been set at the beginning of this frame. When data is to be transmitted in this frame of image, first the STV signal comes to a high level with a pulse width of 21μs, and then the CpV starts to act after a delay of 4.8μs, turning on all the TFTs in the first row, and then delaying the next 4.4μs. A Tp starts to act. The rising edge of Tp latches the data. The high level time of Tp is 2μs. At the falling edge, the first row of data is output to the source terminal of the TFT to display the first row of data. According to this timing relationship, 768 Tp signals are required when all the data of this frame is displayed. The number of Tp signals set on the front end of the 60Hz panel is 789. The period from 769 to 789 Tp is the BLANK area. The Tp signals during this time are all It is an invalid Tp, which means that no data DATA is sent during this period.

2.2MCU and inversion mechanism

This design uses an 8-bit C-MOS flash memory microcontroller. The microcontroller has 5 I/O ports, namely Gp0, Gp1, Gp2, Gp4, Gp5, and an interface Gp3 which is only used for input. It can be realized through microcontroller programming. Inversion of the pOL signal.

The MCU pins are defined as 1-VDD, 2-Tp, 3-pOL_IN, 4-STV, 5-default low level, 6-pOL_OUT, 7-28s flip trigger level, 8-VSS, and the signal input terminals are connected respectively. A 100Ω resistor is used as a protection resistor for the MCUI/O port.

The Tp, pOL_IN, and STV signals sent out by T-CON are used as input signals and input to pins 2, 3, and 4 of the MCU respectively as counting signals. When the MCU is working normally, the output of pin 5 is set to low level, and pin 6 is The inverted pOL output signal. In order to trigger the pOL flip signal every 28s, a trigger level is set. When the 28s reverses, the level will flip from high to bottom or from low to high, making it easy to trigger. . The mechanism to achieve 28s flip is realized by MCU counting the number of STV and Tp. At 60Hz, 1s is 60 frames, and one frame of image has one STV, so there are 28×60=1,680 STV signals in 28s. , the counting mechanism is to count 1,679 STVs through the MCU, and then flip the pOL after counting 780 Tp. Due to the instruction cycle of the MCU, it is necessary to limit the time when the pOL inversion ends to fall after the end of the valid data source row latch signal of the current image frame and before the start signal of the next image frame.

1 is the trigger level, 2 is the signal after pOL flip, 3 is the STV signal, and 4 is the Tp signal. It can be seen from the figure that the end moment of the pOL flip pulse does occur before the STV signal of the next frame. This flip pulse The width is about 150μs. The MCU counts from detecting the 1,679th STV and 780th Tp signal to the pOL signal inversion. Since the execution of the MCU addressing statement requires 3 Tp time, there are 3 Tp before the flip pulse comes. Tp time. It can be seen from the figure that the level of pOL before and after the flip is the same. Under normal circumstances, the pOL signal of the next frame should be high level, but after the inversion in the figure, pOL is still low level, which means that pOL has passed After a level flip, the pOL_OUT signal and pOL_IN signal are output in reverse, realizing the function of inverting pOL_OUT and pOL_IN every 28 seconds.

2.3pOL_IN and pOL_OUT waveforms

As shown in Figure 5, 1 is the pOL_IN signal, 2 is the pOL_OUT signal, and 3 is the flip trigger level. The conventional pOL signal is a standard high and low level square wave signal. Each high and low level controls the pixel voltage of a frame of image respectively. Polarity, the high-level and low-level image frame pixel voltages have different polarities. As can be seen from the figure, the MCU inverts the input pOL_IN signal. The pOL_OUT signal and the pOL_IN signal are synchronized in the 28s before the flip pulse. The pOL_OUT and pOL_IN are reversed in the 28s after the flip pulse. That is to say, every 28 seconds, the MCU reversely outputs the pOL_IN signal. This is to prevent the single conversion law of the pOL polarity conversion signal from destroying the characteristics of the liquid crystal molecules and causing polarization.

2.4 Working principle of protection circuit

The circuit structure diagram of the polarity protection unit is shown in Figure 3. Pin 5 of the microcontroller is electrically connected to the base terminal of the NpN transistor through a resistor R6 with a resistance of 4.7KΩ, and is connected to the base terminal of the NpN transistor through a resistor R5 with a resistance of 4.7KΩ. The voltage terminal (3.3V) is electrically connected. The collector terminal of the NpN transistor is electrically connected to the voltage terminal (3.3V) through a resistor R7 with a resistance of 4.7KΩ, and the STV signal output by the timing controller is input to the collector terminal of the NpN transistor.

The emitter of the NpN transistor is connected to ground. The working principle of the protection circuit is to use an NpN transistor to control the starting signal STV signal of the image to protect the LCD screen. When the MCU is working normally, the programming output pin 5 is low level, the transistor is cut off, and the STV signal is just Adding a pull-up resistor to 3.3V increases the drive current of the STV signal without affecting the normal output of the signal image. When the MCU does not work normally, pin 5 defaults to a high-impedance state. The base of the transistor is connected to two 4.7KΩ resistors to 3.3V. If the base voltage of the transistor is greater than 0.7V, it conducts normally and is connected to the STV signal at the collector terminal. It is forced to the ground, so that the picture has no output, which can protect the LCD screen from polarization in a short period of time because there is no output of pOL signal.

3Conclusion

Using MCU to build a flip circuit realizes the function of controlling the voltage polarity of liquid crystal molecule pixels. This design has the advantages of simple system, low cost, low loss and high efficiency. It has successfully solved the polarization problem caused by a single flip of the polarity control signal. .

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