tft lcd driving circuit factory

When compared to the ordinary LCD, TFT LCD gives very sharp and crisp picture/text with shorter response time. TFT LCD displays are used in more and more applications, giving products better visual presentation.

TFT is an abbreviation for "Thin Film Transistor". The colorTFT LCD display has transistors made up of thin films of Amorphous silicon deposited on a glass. It serves as a control valve to provide an appropriate voltage onto liquid crystals for individual sub-pixels. That is why TFT LCD display is also called Active Matrix display.

A TFT LCD has a liquid crystal layer between a glass substrate formed with TFTs and transparent pixel electrodes and another glass substrate with a color filter (RGB) and transparent counter electrodes. Each pixel in an active matrix is paired with a transistor that includes capacitor which gives each sub-pixel the ability to retain its charge, instead of requiring an electrical charge sent each time it needed to be changed. This means that TFT LCD displays are more responsive.

To understand how TFT LCD works, we first need to grasp the concept of field-effect transistor (FET). FET is a type of transistor which uses electric field to control the flow of electrical current. It is a component with three terminals: source, gate, and drain. FETs control the flow of current by the application of a voltage to the gate, which in turn alters the conductivity between the drain and source.

Using FET, we can build a circuit as below. Data Bus sends signal to FET Source, when SEL SIGNAL applies voltage to the Gate, driving voltage is then created on TFT LCD panel. A sub-pixel will be lit up. A TFT LCD display contains thousand or million of such driving circuits.

Topway started TFT LCD manufacturing more than15 years ago. We produce color TFT LCD display from 1.8 to 15+ inches with different resolutions and interfaces. Here is some more readings about how to choose the right TFT LCD.

Actually, the monitors 20 year ago were CRT (Cathode Ray Tube) displays, which requires a large space to run the inner component. And now the screen here in your presence is the LCD (Liquid Crystal Display) screen.

As mentioned above, LCD is the abbreviation of Liquid Crystal Display. It’s a new display technology making use of the optical-electrical characteristic of liquid crystal.

STN LCD: STN is for Super-twisted Nematic. The liquid crystal in STN LCD rotate more angles than that in TN LCD, and have a different electrical feature, allowing STN LCD to display more information. There are many improved version of STN LCD like DSTN LCD (double layer) and CSTN LCD (color). This LCD is used in many early phones, computers and outdoor devices.

TFT LCD: TFT is for Thin Film Transistor. It’s the latest generation of LCD technology and has been applied in all the displaying scenario including electronic devices, motor cars, industrial machines, etc. When you see the word ‘transistor’, you may realize there’s integrated circuits in TFT LCD. That’s correct and the secret that TFT LCD has the advantage of high resolution and full color display.

In a simple way, we can divide TFT LCD into three parts, from bottom to top they are: light system, circuit system and light and color control system.In manufacturing process, we’ll start from inner light and color control system and then stretch out to whole module.

It’s accustomed to divide TFT LCD manufacturing process into three main part: array, cell and module. The former two steps are about the production of light and color control system, which contains TFT, CF (color filter) and LC (liquid crystal), named a cell. And the last step is the assembly of cell, circuit and light system.

First, let me introduce a crucial material, ITO, to you. ITO, abbreviation of Indium tin oxide, has the characteristic of electrical conductivity and optical transparency, as well as can be easily deposited as a thin film. Thus it’s widely used to create circuit on glass.

Now let’s turn to the production of TFT and CF. Here is a common method called PR (photoresist) method. The whole process of PR method will be demonstrated in TFT production.

◇   Use glue to build a boundary for LC on both glass. And on CF glass, apply one more layer of conductive adhesive. This enable LC molecule link to the control circuit.

For TFT-LCD panel manufacturing, gate driver circuit with amorphous silicon thin-film transistor (TFT-ASG circuit) plays an important role. In this paper, we propose two different ASG driver circuit topologies to improve crucial dynamic characteristics and then optimize them with circuit sizing by simulation-based evolutionary method which integrates genetic algorithm and circuit simulator on the unified optimization framework [1]. The first circuit consisting of fourteen a-Si:H TFT devices is designed for the specifications of the rise time

This article will take you through a high level overview of all of the parts of a TFT LCD display.  The vast majority of what I have read on the internet makes this whole issue massively complex.  I’m quite sure that this complexity problem is a real reflection of the serious design and manufacturing complexity in these displays and drivers.  That being said, to get a conceptual understanding is much simpler, and is the point of this article.

A significant amount of my learning about this subject came from a 195 page powerpoint presentation by Dr. Fang-Hsing Wang entitled “Flat Panel Display : Principle and Driving Circuit Design“.  He has graciously allowed me to reproduce a few of his images.  This dude knows way way more about these circuits than I do and I would encourage you to read his work.

The fundamental element in a TFT display is the liquid crystal.  These elements have the property that the crystals will align from horizontal (which blocks the light) to vertical (which lets most of the light through) based on the electric field applied to them.  Basically, you shine light through the liquid crystal, which blocks some or all of the light, the remainder of the white light then goes through a color filter to make red, green, or blue. It works like this:

What does the schematic for one element in a pixel look like?  And where is the T(transistor) in the TFT?  The three letter acronym TFT stands for a thin film transistor that is physically on the top of the LCD matrix right next to each liquid crystal element.  Here is a schematic model for one element in the array.  C-LC represents the capacitance of the liquid crystal.  CS is a storage capacitor that is used to hold the electric field across the liquid crystal when the transistor is OFF.   To apply a voltage across the LC you just turn on the gate and apply the correct voltage to the column commonly known as the source.

You should notice that the “back” terminal of the two capacitors is called “VCOM” and is physically on the other side of the liquid crystal matrix from the TFT.  All of the liquid crystal backsides in the display are connected to the same VCOM.  A bit of painfulness in this system is that the CS capacitor leaks, which means that the LCD changes state which means that each pixel must be updated, properly called refreshed, on a regular basis.

If you have been thinking about this system you might have done a little bit of math and figured out that you are going to need an absolute boatload of source and gate driver signals.  And you would be right!  For example, a 4.3″ screen with 480×272 will require 480x272x3 elements which are probably organized into 480 rows by 816 columns.  This would require a chip with at least 480+816=1296 pins, that is a lot.  It turns out that for small screens <=3.5″ there are chips with enough pins to do the job.  But, for larger screens, it requires multiple chips to do the job.  The “…” in the picture above shows the driver chips being cascaded.  The next thing to know is that “TFT Glass” usually has the driver chip(s) embedded into the screen at the edge (you can see that in the picture from Innolux above).

However, a 3.3v logic 1 is not anywhere high enough to drive the gate so that it can pass the much higher source voltage.  So, you need to level shifter and a buffer to get the “right” voltage.   On page 15, Dr. Wang made a nice picture of this circuit as well.

It turns out that this picture is conceptually correct, but the exact implementation has “a lot going on”.  You can read about the next layer of circuit design in his presentation on pages 15-35.

In its most basic form, the TFT source driver is responsible for taking an 8-bit digital input value representing the value of an individual LCD element and turning it into a voltage, the driving the voltage.  Like this:

The last issue that I will address in TFT LCD drivers is called Gamma Correction or more simply Gamma.  Gamma is an intensity adjustment factor.  For any given digital intensity input, you will need a non-linear translation to a voltage output on the source.  For example a doubling of digital input (so that a pixel appears twice as bright) you will not double but instead will have some non-linear translation of the output voltage.

The good news is that this gamma correction is built into the display drivers.  From my reading, this is sometimes done with digital processing, and sometimes done with an analog circuit.  But in general, it appears to be tuned and programmed into the driver by the panel vendor for these smaller display.

The present invention relates to Liquid Crystal Displays (LCDs), more specifically, to a Thin Film Transistor-Liquid Crystal Display (TFT-LCD) driver circuit and LCD devices. BACKGROUND

Current TFT-LCD technologies have been trending toward higher levels of integration, higher resolution and multi-grayscale capabilities. As such, the TFT-LCD driver circuit device area and power consumption have increased accordingly, leading to higher manufacturing costs. In a traditional TFT-LCD driver circuit, a source drive buffer or latch of the source drive chip occupies a large footprint and consumes a considerable amount of power.

An object according to an embodiment of the present invention is to eliminate the large area occupation and the high power consumption of the source drive buffer in the traditional TFT-LCD driver circuit. Accordingly, a first embodiment discloses a TFT-LCD driver circuit including: a gate driver adapted to control turning on of a Thin Film Transistor (TFT); a grayscale voltage generation circuit capable of providing a grayscale voltage for display points; a timing circuit configured to provide timing signals; a bias circuit configured to provide bias voltage signals; and a source driver operable to charge the display points according to the grayscale voltage. The source driver includes: a source drive latch configured to store data for the display points; a source drive buffer; and a Digital to Analog Converter (DAC) configured to output the grayscale voltage to the source drive buffer according to the stored display data. The source drive buffer includes an Operational Amplifier (OPA), which includes first and second differential amplifiers, The differential amplifiers operate alternatively according to the timing signals and bias voltage signals, and buffer and output voltage signals outputted from the DAC by means of a voltage follower mechanism for charging the display points.

The first differential amplifier includes: a first differential circuit having two N-channel Metal Oxide Semiconductor (NMOS), wherein the gate of the first NMOS is coupled to the output of the DAC and the gate of the second NMOS is coupled to the output of the OPA; a first current mirror source being a loader of the first differential circuit; an end of current source; an output stage which includes a NMOS and a PMOS, the gate of the NMOS and the end of current source being controlled by the bias voltage signals, the gate of the PMOS being coupled to the output of the first current mirror source; and a power down PMOS operable to turning on and off the OPA by timing signals.

The second differential amplifier includes: a second differential circuit having two P-channel Metal Oxide Semiconductor (PMOS), wherein the gate of the first PMOS is coupled to the output of the DAC while the gate of the second PMOS is coupled to the output of the OPA; a second current mirror source being a loader of the second differential circuit; an end of current source; an output stage which includes a PMOS and a NMOS, the gate of the PMOS and the end of current source being controlled by the bias voltage signals, the gate of the NMOS being coupled to the output of the second current mirror source; and a power down NMOS which is applied for turning on and off OPA by timing signals.

Fig. 6 illustrates a diagram of a control circuit for bias voltages PBIASL and NBIASL of the TFT-LCD driver circuit according to an embodiment of the present invention;

Fig. 7 illustrates waveforms of the bias voltages PBIASL and NBIASL of the TFT-LCD driver circuit according to an embodiment of the present invention;

The source drive buffer of the TFT-LCD driver circuit of the presently disclosed embodiments can be formed with two basic differential amplifiers and a CMOS transmission gate. The two differential amplifiers can be turned on alternatively according to timing of control signals, adjust an output voltage through the COMS transmission gate, and charge pixels in a row to the required voltage level until scanning of the pixels in the row is completed.

Fig. 1 illustrates a typical circuit diagram of a TFT panel. The TFT panel provides a plurality of display points forming an n×m matrix with n rows (G1, G2, G3, ..., Gn) and m columns (S1, S2, S3, ...,Sm), wherein each display point represents a Twisted Nematic Liquid Crystal Display (TN-LCD) point and includes a TFT, a parallel plate capacitor (not shown) formed with upper and lower conductive glasses and a storage capacitor, the parallel plate capacitor and storage capacitor having parallel coupling. If a color filter has three basic colors, a basic pixel display unit needs to be provided with three such display points corresponding to red, green and blue, i.e. the three basic primary colors. At a specific time, the gate driver outputs a drive pulse to turn on all TFT"s in a row. At the same time, the source driver charges the display points in the row to the necessary voltage. When the charging of the row is completed, the gate driver turns off the TFT"s in the row, turns on TFT"s in the next row, and charges the display points in the next row.

Fig. 2 illustrates a circuit diagram of a TFT-LCD driver circuit according to the first embodiment of the present invention. For the sake of description, only TFT-components of the LCD driver circuit involved in the invention are shown in Figure 2. The TFT-LCD driver circuit includes gate and source drivers, a grayscale voltage generator with a timing generator (not shown), and a bias voltage generator (not shown).

As shown in Fig. 2, TFTs N1-Nm are provided in the row. A gate of each of the TFTs is turned on or off under the control of the dirve pulse outputted from the gate driver, m sources of the TFTs are respectively coupled to outputs of source drivers, and m drains of the TFTs are respectively coupled to storage capacitors Cs1-Csm. When the row is canned, all the TFTs of the row are turned on by the drive pulse outputted from the gate driver. At the same time, latch data stored within the source drive latch is decoded and converted by Digital to Analog Converters (DACs), to select a grayscale voltage generator to generate the grayscale voltage to be supplied to each display point. The grayscale voltage is transmitted through the source drive buffer and a corresponding one of the transmission gates T1-Tm to charge a display electrode of the respective display point, thereby driving the LCD panel. As shown in Fig. 2, m source drive buffers (within the dashed outline) corresponds to m display points in the row under scan, and each source drive buffer includes an operational amplifier (OPA).

Fig. 3 illustrates the relationship between input and output signals of the OPA of the source drive buffer as shown in Fig. 2. Specifically, the OPA receives the voltage signal PIN from the corresponding DAC, latches the voltage signal PIN and charges the corresponding display electrode through the corresponding one of the transmission gates T1-Tm (as shown in Fig. 2) under the control of timing signals PDP, PDP_N, PDN, PDN_N and bias voltage signals PBIASL and NBIASL. The output OUT of the OPA is short connected to a feedback NIN, causing the output voltage to be fed back to the feedback NIN of the OPA, and the entire OPA is equivalent to a voltage follower. The various timing signals are generated by the timing generators of the TFT-LCD driver circuit.

Fig. 4 is a circuit diagram of the OPA according to an embodiment of the present invention. The OPA includes a first differential amplifier (OPAN) and a second differential amplifier (OPAP), which operate alternatively under the control of timing signals. The input stage of the OPAN includes a first differential circuit 41 having two N-channel Metal Oxide Semiconductors (NMOSs), and the gates of the NMOSs are coupled to the voltage output PIN of the DAC and the output OUT of the source drive buffer. The sources of the two NMOSs can be coupled to each other to form a coupled source pair, and connected with the ground potential VSS of the driver circuit via a first switch Q1. Depending on the bias at the first switch Q1, the drains of the first differential circuit 41 are coupled to the power terminal VDD of the drive circuit via a first current mirror source 42. With the first current mirror source 42 as the load, the output impedance is improved to obtain a higher gain. The output stage of the OPAN is of a simple co-source structure to improve the range of the output waveforms, and is formed with a second switch Q2 and a third switch Q3 coupled in series, and the coupling node between the switches Q2 and Q3 is the output OUT of the OPA. The gates of the second switch Q2 and the first switch Q1 are coupled in series, and turned on under the control of the bias voltage signal NBIASL. The gate of the third switch Q3 is coupled to the source of the first current mirror source 42, and also to the power terminal VDD of the drive circuit via a fourth switch Q4. The gate of the fourth switch Q4 can be turned on under the control of the timing signal PDN_N, to control and ensure that the OPAN is working properly.

Similarly, the input stage of the OPAP includes a second differential circuit 43 formed with two P-channel Metal Oxide Semiconductors (PMOSs), and the gates of the two PMOSs are coupled to the voltage output PIN of the DAC and the output OUT of the source drive buffer. The sources of the two PMOSs are connected with the power terminal VDD of the drive circuit via a fifth switch Q5. Depending on the bias at the fifth switch Q5, the drains of the second differential circuit 43 are coupled to the ground potential VSS of the driver circuit via a second current mirror source 44. With the second current mirror source 44 as the load, the output impedance is improved to obtain a higher gain. The output stage of the OPAP is of a simple co-source structure, and includes sixth and seventh switches Q6, Q7 coupled in series, and the coupling node between the switches Q6 and Q7 is the output OUT of the OPA. The gates of the sixth switch Q6 and the fifth switch Q5 are coupled in series, and are turned on under the control of the bias voltage control signal PBIASL. The gate of the seventh switch Q7 is coupled to the source of the second current mirror source 44, and also to the ground potential VSS of the driver circuit via an eighth switch Q8. The gate of the eighth switch Q8 can be turned on under the control of the timing signal PDP, to control the operation status of the OPAP.

Figs. 6 and 7 respectively illustrate a control circuit diagram and waveforms of the bias voltages PBIASL and NBIASL in the TFT-LCD driver circuit according to an embodiment of the present invention. The generator which generates the bias voltage signals PBIASL and NBIASL serves as a global circuit module which provides the bias voltages for all source drive buffers in the TFT-LCD driver circuit, while the other circuit modules in the TFT-LCD driver circuit are provided with bias voltages by the bias voltage signals PBIAS and NBIAS.

With reference to Figs. 2-8, the working principles of the source drive buffer according to an embodiment of the present invention during the time for scanning a row, i.e. periods tl+t2+t3, are described as follows. The data as stored in the source drive latch are decoded and converted by the DAC, to select the grayscale voltage generator to generate the appropriate grayscale voltage. During period t1, the first switch Q1 and the second switch Q2 are turned on by the bias voltage signal NBIASL, and both the first differential circuit 41 and the first current mirror source 42 start to operate. Because the PDN_N is at a high voltage level, the fourth switch Q4 is off, the OPAN is able to function normally, and the voltage generated by the grayscale voltage generator is buffered and outputted. Further, because the bias voltage signal PBIASL is at a high voltage level, the fifth switch Q5 is off, and because the PDP is at a high voltage level, the eighth switch Q8 is on, thereby pulling down or lowering the gate voltage of the seventh switch Q7 to the low voltage level VSS, turning off the OPAP. That is, the OPAN is in operation during the period t1. During the period t2, the OPAP is operative, the control principle of which is opposite to that during the period t1, and thus will not be elaborated further herein. Because the entire OPA is equivalent to a voltage follower, during the periods t1 and t2, the voltage following approach ensures that the level of the output OUT is equal to the level of the input PIN. It should be appreciated by one skilled in the art that the previously described scenario may be reversed by means of timing control such that the OPAP is in operation during period t1 while the OPAN is in operation during period t2. The operational functions of the amplifiers and corresponding grayscale voltages are similar to those described above and thus will not be elaborated further herein.

In the embodiments described above, the first differential amplifier OPAN is effective when the voltage rises, i.e. dominant when the input voltage changes from a low level to a high level, while the second differential amplifier OPAP is effective when the voltage decreases, i.e. dominant when the input voltage changes from a high level to a low level. Therefore, only one differential amplifier is contributing to the system during the time for scanning one row, thereby causing differentiation between the output waveform and the actual input waveform of the buffer. However, after adjustments by the grayscale voltage, the differentiation will not influence the display on the LCD, because the voltage in the storage capacitor Cs of the LCD panel just before the TFT is turned off is stored, that is, the output of the cache has reached or been close to an ideal value when N1 is changed from the higher voltage to the lower voltage.

Additionally, to expand the range of the common-mode input, mixed use of an NMOS differential pair (first differential amplifier) and a PMOS differential pair (second differential amplifier) may be incorporated as a two-tier operational amplifier. The two operational amplifiers operate in a time division mode through timing control, thereby effectively improving the range of the input voltage. Further, because of the reduced complexity of the source drive buffer, circuitry dimensions can be reduced and less area or die size would be required. Furthermore, power consumption can also be reduced because the two differential operational amplifiers operate in a time division mode.

INT018ATFT is an embedded display driver board based on our 1.8 inch 128 x 160 RGB resolution TFT display module. Mounted on the embedded board is the RAIO RA8872 LCD controller that offers the following features and benefits:

TFT-LCD was invented in 1960 and successfully commercialized as a notebook computer panel in 1991 after continuous improvement, thus entering the TFT-LCD generation.

Simply put, the basic structure of the TFT-LCD panel is a layer of liquid crystal sandwiched between two glass substrates. The front TFT display panel is coated with a color filter, and the back TFT display panel is coated with a thin film transistor (TFT). When a voltage is applied to the transistor, the liquid crystal turns and light passes through the liquid crystal to create a pixel on the front panel. The backlight module is responsible for providing the light source after the TFT-Array panel. Color filters give each pigment a specific color. The combination of each different color pixel gives you an image of the front of the panel.

The TFT panel is composed of millions of TFT devices and ITO (In TI Oxide, a transparent conductive metal) regions arranged like a matrix, and the so-called Array refers to the region of millions of TFT devices arranged neatly, which is the panel display area. The figure below shows the structure of a TFT pixel.

No matter how the design of TFT display board changes or how the manufacturing process is simplified, its structure must have a TFT device and control liquid crystal region (if the light source is penetration-type LCD, the control liquid crystal region is ITO; but for reflective LCD, the metal with high reflection rate is used, such as Al).

The TFT device is a switch, whose function is to control the number of electrons flowing into the ITO region. When the number of electrons flowing into the ITO region reaches the desired value, the TFT device is turned off. At this time, the entire electrons are kept in the ITO region.

The figure above shows the time changes specified at each pixel point. G1 is continuously selected to be turned on by the driver IC from T1 to TN so that the source-driven IC charges TFT pixels on G1 in the order of D1, D2, and Dn. When TN +1, gATE-driven IC is selected G2 again, and source-driven IC is selected sequentially from D1.

Many people don’t understand the differences between generations of TFT-LCD plants, but the principle is quite simple. The main difference between generations of plants is in the size of glass substrates, which are products cut from large glass substrates. Newer plants have larger glass substrates that can be cut to increase productivity and reduce costs, or to produce larger panels (such as TFT display LCD TV panels).

The TFT-LCD industry first emerged in Japan in the 1990s, when a process was designed and built in the country. The first-generation glass substrate is about 30 X 40 cm in size, about the size of a full-size magazine, and can be made into a 15-inch panel. By the time Acer Technology (which was later merged with Unioptronics to become AU Optronics) entered the industry in 1996, the technology had advanced to A 3.5 generation plant (G3.5) with glass substrate size of about 60 X 72 cm.Au Optronics has evolved to a sixth-generation factory (G6) process where the G6 glass substrate measures 150 X 185 cm, the size of a double bed. One G6 glass substrate can cut 30 15-inch panels, compared with the G3.5 which can cut 4 panels and G1 which can only cut one 15-inch panel, the production capacity of the sixth generation factory is enlarged, and the relative cost is reduced. In addition, the large size of the G6 glass substrate can be cut into large-sized panels, which can produce eight 32-inch LCD TV panels, increasing the diversity of panel applications. Therefore, the global TFT LCD manufacturers are all invested in the new generation of plant manufacturing technology.

The TRANSISTor-LCD is an acronym for thin-film TFT Display. Simply put, TFT-LCD panels can be seen as two glass substrates sandwiched between a layer of liquid crystal. The upper glass substrate is connected to a Color Filter, while the lower glass has transistors embedded in it. When the electric field changes through the transistor, the liquid crystal molecules deflect, so as to change the polarization of the light, and the polarizing film is used to determine the light and shade state of the Pixel. In addition, the upper glass is fitted to the color filter, so that each Pixel contains three colors of red, blue and green, which make up the image on the panel.

-The rear module assembly process is the production operation of assembling the glass after the Cell process with other components such as backlight plate, circuit, frame, etc.

The organic light display can be divided into Passive Matrix (PMOLED) and Active Matrix (AMOLED) according to the driving mode. The so-called active driven OLED(AMOLED) can be visualized in the Thin Film Transistor (TFT) as a capacitor that stores signals to provide the ability to visualize the light in a grayscale.

Although the production cost and technical barriers of passive OLED are low, it is limited by the driving mode and the resolution cannot be improved. Therefore, the application product size is limited to about 5″, and the product will be limited to the market of low resolution and small size. For high precision and large picture, the active drive is mainly used. The so-called active drive is capacitive to store the signal, so when the scanning line is swept, the pixel can still maintain its original brightness. In the case of passive drive, only the pixels selected by the scan line are lit. Therefore, in an active-drive mode, OLED does not need to be driven to very high brightness, thus achieving better life performance and high resolution.OLED combined with TFT technology can realize active driving OLED, which can meet the current display market for the smoothness of screen playback, as well as higher and higher resolution requirements, fully display the above superior characteristics of OLED.

The technology to grow The TFT on the glass substrate can be amorphous Silicon (A-SI) manufacturing process and Low-Temperature Poly-Silicon (LTPS). The biggest difference between LTPS TFT and A-SI TFT is the difference between its electrical properties and the complicated manufacturing process. LTPS TFT has a higher carrier mobility rate, which means that TFT can provide more current, but its process is complicated.A-si TFT, on the other hand, although a-Si’s carrier movement rate is not as good as LTPS’s, it has a better competitive advantage in cost due to its simple and mature process.Au Optronics is the only company in the world that has successfully combined OLED with LTPS and A-SI TFT at the same time, making it a leader in active OLED technology.

The LTPS membrane is much more complex than a-SI, yet the LTPS TFT is 100 times more mobile than A-SI TFT. And CMOS program can be carried out directly on a glass substrate. Here are some of the features that p-SI has over A-SI:

2. Vehicle for OLED: High mobility means that the OLED Device can provide a large driving current, so it is more suitable for an active OLED display substrate.

3. Compact module: As part of the drive circuit can be made on the glass substrate, the circuit on the PCB is relatively simple, thus saving the PCB area.

LCD screens are backlit to project images through color filters before they are reflected in our eye Windows. This mode of carrying backlit LCD screens, known as “penetrating” LCD screens, consumes most of the power through backlit devices. The brighter the backlight, the brighter it will appear in front of the screen, but the more power it will consume.

The traditional mechanical instrument lacks the ability to satisfy the market with characters of favorable compatibility, easy upgrading, and fashion. Thus the design of a TFT-LCD (thin film transistor-liquid crystal display) based automobile instrument is carried out. With a 7-inch TFT-LCD and the 32-bit microcontroller MB91F599, the instrument could process various information generated by other electronic control units (ECUs) of a vehicle and display valuable driving parameters on the 7-inch TFT-LCD. The function of aided parking is also provided by the instrument. Basic principles to be obeyed in circuits designing under on-board environment are first pointed out. Then the paper analyzes the signals processed in the automobile

instrument and gives an introduction to the sampling circuits and interfaces related to these signals. Following this is the functional categorizing of the circuit modules, such as video buffer circuit, CAN bus interface circuit, and TFT-LCD drive circuit. Additionally, the external EEPROM stores information of the vehicle for history data query, and the external FLASH enables the display of high quality figures. On the whole, the accomplished automobile instrument meets the requirements of automobile instrument markets with its characters of low cost, favorable compatibility, friendly interfaces, and easy upgrading.

As an essential human-machine interface, the automobile instrument provides the drivers with important information of the vehicle. It is supposed to process various information generated by other ECUs and display important driving parameters in time, only in which way can driving safety be secured. However, the traditional mechanical automobile instrument is incompetent to provide all important information of the vehicle. Besides, the traditional instrument meets great challenge with the development of microelectronic technology, advanced materials, and the transformation of drivers’ aesthetics [1, 2]. Moreover, the parking of the vehicle is also a problem puzzling many new drivers. Given this, traditional instruments should be upgraded in terms of driving safety, cost, and fashion.

The digital instrument has functions of vehicle information displaying, chord alarming, rear video aided parking, LED indicating, step-motor based pointing, and data storage. The instrument adopts dedicated microcontroller MB91F599, a 7-inch LCD, and two step-motors to substitute for the traditional instrument. All the information generated by other ECUs can be acquired via not only the sample circuits but also the CAN bus.

The instrument provides interfaces for different types of signals and the CAN bus. All types of signals (such as square wave signal, switching signal, resistance signal, analog voltage signal, etc.) coming from other ECUs can be acquired either from different types of sampling circuits or from the CAN bus. This makes it suitable for both the outdated application where the information from other ECUs can only be acquired via the sampling circuits and the modern application where the information from other ECUs are transmitted via the CAN bus.

The CAN bus interface and the 7-inch TFT-LCD make it more convenient to upgrade the instrument without changing the hardware. If the software needs to be upgraded, we need not bother to take the instrument down and program the MCU. Instead, we can upgrade the instrument via the vehicle’s CAN network without taking the instrument down, which makes the upgrading more convenient. Most of the information from other ECUs can be transmitted via the CAN bus; so, we do not have to change the hardware circuits if some of the ECUs’ signals are changed in different applications. Besides, since most of the driving parameters are displayed on the TFT-LCD, and the graphical user interface can be designed with great flexibility by programming, only the software needs to be revised to meet different requirements of what kind of driving parameters to display and so forth. These characters, together with the reserved interfaces, enhance the instrument’s compatibility in different applications.

On the one hand, there are some automobile instruments which adopt 8-bit MCUs or 16-bit MCUs which have limited peripherals, so it is difficult for them to meet some requirements such as rearview video and high real-time data processing performance. And many extra components are needed if the designer wants to accomplish some functions such as video input. On the other hand, there are some advanced automobile instruments which adopt high performance MCUs (such as i.MX 53, MPC5121e, and MPC5123) and run Linux on them. They even use larger TFT-LCDs (such as the 12.3-inch TFT-LCD with a resolution of 1280 × 480 pixels) to display driving parameters. These automobile instruments show higher performances than the instrument in this paper. However, they are more expensive than this automobile. This instrument is able to provide almost all the functions of the advanced automobile instrument with a lower cost.

The instrument receives signals from other ECUs via the sampling circuits or the CAN bus interface. It can also receive commands from the driver via the button interface. The signals are then processed by the MCU, after which the MCU may send the vehicle information to the LCD or light the LEDs and so forth, according to the results. Therefore, the automobile instrument can be viewed as a carrier of the information flow. And the design of the system can be viewed from two aspects: the hardware system and the information flow based on it.

From the aspect of hardware system components, the system consists of the MCU MB91F599 and other functional circuits such as sampling circuits and video buffer circuits, as shown in Figure 2.

Overvoltage protection circuits should be placed at the interfaces of power supply and important signals (such as the CAN bus interface) in case of voltage overshoots.3.1.3. Generality

The automobile instrument receives and processes information from other ECUs such as the tachometer, the speedometer, the cooling water temperature gauge, the oil pressure gauge, and the fuel gauge. The signals coming from these ECUs are of different types, according to which different kinds of sampling circuits and interfaces should be designed. Accordingly, a classification of the input signals is first carried out, as shown in Table 1.

Square wave signal is the signal that comes from the tachometer. The engine speed, the velocity of the vehicle, and the mileage are proportional to the frequency of the square wave signal. However, the square wave is not “standard” because it is often corrupted by interferences. Besides, the peak voltage of the square wave is +12 V while the I/O voltage of the microcontroller is . The main task for the circuits is to remove the interferences and convert the +12 V voltage to . As shown in Figure 3, the square wave signal is input from node ②; node ① is connected to one pin of the microcontroller.

The switching signal acts as a trigger signal to trigger some events such as lighting up the backlight and waking up the MCU. It can be categorized into active high and active low according to the ECUs that generate it. Figure 4 offers a complete picture of the sampling circuit of active high signal. The switching signal is input from node ②; node ① is connected to one pin of the microcontroller. Diode clamps the peak voltage of the switching signal (usually +12 V) to the standard I/O voltage of the microcontroller () after resistive subdivision. The sampling circuit of active low signal is similar to Figure 4.

The analog voltage signal reflects the battery voltage and the air pressure. The corresponding circuit adopts the resistive subdivision so as to adjust the ratio of the resistors for putting voltage of the signal below the microcontroller’s maximum I/O voltage. The value of the resistors should be a little larger to lower down the static power consumption of the resistors. It is unnecessary to go into detail of the circuit.

The rearview video contributes a lot to vehicle backing and parking. The signal coming from the rear camera must be regulated before being processed by the microcontroller. The rear camera outputs NTSC video. The MB91F599 integrates a video decoder which supports NTSC/PAL video input, which makes the design of the regulatory circuit simple.

Figure 6 shows RGB with sync in NTSC format. The RGB varies in a positive direction from the “black level” (0 V) to 700 mV. Meanwhile, a sync waveform of −300 mV is attached to the video signal. Since the output video signal of the camera is AC-coupled, a clamp circuit is needed to clamp the RGB and sync to a reference voltage and leave the others to vary. If not clamped, the bias voltage will vary with video content and the brightness information will be lost [5].

The video buffer circuit consists of a clamping circuit (, , ) and an emitter follower (, , ), as shown in Figure 7. Node ① is connected to the NTSC input pin of the microcontroller; node ② is connected to the clamp level output pin of the microcontroller; node ③ is connected to the camera’s signal output. is the coupling capacitor; is the matching resistor to realize the 75 Ω back termination.

Since the FLASH size of the microcontroller is only 1 MB which is limited for the storage of pictures displayed on the LCD, external FLASH is needed to store different kinds of meaningful pictures such as the background of the dial. Two S29GL256N chips with a memory capacity of 256 Mb are chosen for picture data storage for their high performance and low power consumption. The application circuits of the chips are provided in their datasheets, so it is unnecessary to go into the details of them here.

For this design, only the CAN transceiver and its auxiliary circuit are needed since the MB91F599 is integrated with two CAN controllers, which are connected to the high-speed and low-speed CAN bus, respectively. TJA1040 is chosen as the CAN transceiver for its low consumption in standby mode. Besides, it can also be woken up via CAN bus, which is required by some automobile instruments. Detailed circuit is provided in the datasheet of TJA1040, so the repetitious details need not be given here. Note that for high-speed CAN, both ends of the pair of signal wires must be terminated. ISO 11898 requires a cable with a nominal impedance of 120 Ω [19]; therefore, 120 Ω resistors are needed for termination. Here, only the devices on the ends of the cable need 120 Ω termination resistors.

The 7-inch TFT-LCD has a resolution of pixels and supports the 24-bit for three RGB colors. The interface of the 60-pin TFT-LCD can be categorized into data interface, control interface, bias voltage interface, and gamma correction interface.

The data interface supports the parallel data transmitting of 18-bit (6 bits per channel) for three RGB colors. Thus, a range of colors can be generated. The control interface consists of a “horizontal synchronization” which indicates the start of every scan line, a “vertical synchronization” which indicates the start of a new field, and a “pixel clock.” This part is controlled by the graphics display controller which is integrated in the MB91F599. We just need to connect the pins of the LCD to those of the microcontroller correspondingly.

Bias voltages are used to drive the liquid crystal molecules in an alternating form. The compact LCD bias IC TPS65150 provides all bias voltages required by the 7-inch TFT-LCD. The detailed circuit is also provided in the datasheet of TPS65150.

The greatest effect of gamma on the representations of colors is a change in overall brightness. Almost every LCD monitor has an intensity to voltage response curve which is not a linear function. So if the LCD receives a message that a certain pixel should have certain intensity, it will actually display a pixel which has intensity not equal to the certain one. Then the brightness of the picture will be affected. Therefore, gamma correction is needed. Several approaches to gamma correction are discussed in [20–22]. For this specific 7-inch LCD, only the producer knows the relationship between the voltage sent to the LCD and the intensity it produces. The signal can be corrected according to the datasheet of the LCD before it gets to the monitor. According to the datasheet, ten gamma correction voltages are needed. These voltages can be got from a resistive subdivision circuit.

The vehicle electric power system is mainly composed of a generator and a battery [23]. The power voltage of a car is +12 V while that of a bus is +24 V. The power supply of the automobile instrument alternates between the generator and the battery. The generator powers the automobile instrument and charges the battery when working. Note that the battery does not power the instrument when the generator is on. If the generator is not working, the instrument is powered by the battery. Figure 9 shows how the power supply alternates. Node ① is connected to the battery; node ② is connected to the generator; node ③ is connected to other circuits. When the generator is on, and are turned off, which prevents node ③ from getting power from the battery. Then node ③ gets power from the generator via other routes (not shown in the figure). When the generator is off, and are turned on, so node ③ gets power from the battery.

For this instrument, the LED indicators, the backlight, and the chord alarm need to be supplied with a voltage of +12 V; the CAN transceiver, the EEPROM, and the buttons need to be supplied with a voltage of +5 V; the video buffer circuit, the external FLASH, and the data interface of the LCD need to be supplied with a voltage of +3.3 V. Besides, the microcontroller needs to be supplied with voltages of +5 V and +3.3 V simultaneously. Figure 8 offers a detailed block diagram of the power supply for the automobile instrument.

The main task for the program is to calculate the driving parameters of the vehicle and display them on the TFT-LCD. The calculation is triggered by the input signals via the sampling circuits or the CAN bus. The main program flow chart of the system is shown in Figure 10.

The design scheme of a TFT-LCD based automobile instrument is carried out form aspects of both the hardware and the main program flow chart. The MB91F599 simplifies the peripheral circuits with its rich on-chip resources and shows high performance in real-time data processing. The automobile instrument is capable of displaying the velocity of the vehicle, the engine speed, the cooling water temperature, the oil pressure, the fuel volume, the air pressure, and other information on the TFT-LCD, which contributes a lot to driving safety and satisfies drivers’ aesthetics. Besides, the rearview video makes the parking and backing easier and safer for the driver. Moreover, the CAN bus interface and TFT-LCD make it easier for the upgrading of the instrument without changing the hardware, thus saving the cost.

A new method for array testing of TFT-CD panel with the integrated gate driver circuits is presented. As larger size/high resolution TFT-LCD with the peripheral driver circuits has emerged, one of the important problems for manufacturing is array testing on the panel. This paper describes the technology of detecting defective arrays and optimizing the array testing process. For the effective characterization of pixel array, the pixel storage capability is simulated and measured with voltage imaging system. This technology permits full functional testing during the manufacturing process, enabling fabrication of large TFT-LCD panels with the integrated driver circuits.

When compared to the ordinary LCD, TFT LCD gives very sharp and crisp text/graphic with shorter response time. TFT LCD displays are used in more and more applications, giving products better visual presentation.

TFT is an abbreviation for "Thin Film Transistor". The color TFT LCD display has transistors made up of thin films of Amorphous silicon deposited on a glass. It serves as a control valve to provide an appropriate voltage onto liquid crystals for individual sub-pixels. That is why TFT LCD display is also called Active Matrix display.

A TFT LCD has a liquid crystal layer between a glass substrate formed with TFTs and transparent pixel electrodes and another glass substrate with a color filter (RGB) and transparent counter electrodes. Each pixel in an active matrix is paired with a transistor that includes capacitor which gives each sub-pixel the ability to retain its charge, instead of requiring an electrical charge sent each time it needed to be changed. This means that TFT LCD displays are more responsive.

To understand how TFT LCD works, we first need to grasp the concept of field-effect transistor (FET). FET is a type of transistor which uses electric field to control the flow of electrical current. It is a component with three terminals: source, gate, and drain. FETs control the flow of current by the application of a voltage to the gate, which in turn alters the conductivity between the drain and source.

Using FET, we can build a circuit as below. Data Bus sends signal to FET Source, when SEL SIGNAL applies voltage to the Gate, driving voltage is then created on TFT LCD panel. A sub-pixel will be lit up. A TFT LCD display contains thousand or million of such driving circuits.

Topway started TFT LCD manufacturing more than15 years ago. We produce color TFT LCD display from 1.8 to 15+ inches with different resolutions and interfaces. Here is some more readings about how to choose the right TFT LCD.

A thin-film-transistor liquid-crystal display (TFT LCD) is a variant of a liquid-crystal display that uses thin-film-transistor technologyactive matrix LCD, in contrast to passive matrix LCDs or simple, direct-driven (i.e. with segments directly connected to electronics outside the LCD) LCDs with a few segments.

In February 1957, John Wallmark of RCA filed a patent for a thin film MOSFET. Paul K. Weimer, also of RCA implemented Wallmark"s ideas and developed the thin-film transistor (TFT) in 1962, a type of MOSFET distinct from the standard bulk MOSFET. It was made with thin films of cadmium selenide and cadmium sulfide. The idea of a TFT-based liquid-crystal display (LCD) was conceived by Bernard Lechner of RCA Laboratories in 1968. In 1971, Lechner, F. J. Marlowe, E. O. Nester and J. Tults demonstrated a 2-by-18 matrix display driven by a hybrid circuit using the dynamic scattering mode of LCDs.T. Peter Brody, J. A. Asars and G. D. Dixon at Westinghouse Research Laboratories developed a CdSe (cadmium selenide) TFT, which they used to demonstrate the first CdSe thin-film-transistor liquid-crystal display (TFT LCD).active-matrix liquid-crystal display (AM LCD) using CdSe TFTs in 1974, and then Brody coined the term "active matrix" in 1975.high-resolution and high-quality electronic visual display devices use TFT-based active matrix displays.

The circuit layout process of a TFT-LCD is very similar to that of semiconductor products. However, rather than fabricating the transistors from silicon, that is formed into a crystalline silicon wafer, they are made from a thin film of amorphous silicon that is deposited on a glass panel. The silicon layer for TFT-LCDs is typically deposited using the PECVD process.

Polycrystalline silicon is sometimes used in displays requiring higher TFT performance. Examples include small high-resolution displays such as those found in projectors or viewfinders. Amorphous silicon-based TFTs are by far the most common, due to their lower production cost, whereas polycrystalline silicon TFTs are more costly and much more difficult to produce.

The twisted nematic display is one of the oldest and frequently cheapest kind of LCD display technologies available. TN displays benefit from fast pixel response times and less smearing than other LCD display technology, but suffer from poor color reproduction and limited viewing angles, especially in the vertical direction. Colors will shift, potentially to the point of completely inverting, when viewed at an angle that is not perpendicular to the display. Modern, high end consumer products have developed methods to overcome the technology"s shortcomings, such as RTC (Response Time Compensation / Overdrive) technologies. Modern TN displays can look significantly better than older TN displays from decades earlier, but overall TN has inferior viewing angles and poor color in comparison to other technology.

The transmittance of a pixel of an LCD panel typically does not change linearly with the applied voltage,sRGB standard for computer monitors requires a specific nonlinear dependence of the amount of emitted light as a function of the RGB value.

Less expensive PVA panels often use dithering and FRC, whereas super-PVA (S-PVA) panels all use at least 8 bits per color component and do not use color simulation methods.BRAVIA LCD TVs offer 10-bit and xvYCC color support, for example, the Bravia X4500 series. S-PVA also offers fast response times using modern RTC technologies.

TFT dual-transistor pixel or cell technology is a reflective-display technology for use in very-low-power-consumption applications such as electronic shelf labels (ESL), digital watches, or metering. DTP involves adding a secondary transistor gate in the single TFT cell to maintain the display of a pixel during a period of 1s without loss of image or without degrading the TFT transistors over time. By slowing the refresh rate of the standard frequency from 60 Hz to 1 Hz, DTP claims to increase the power efficiency by multiple orders of magnitude.

Due to the very high cost of building TFT factories, there are few major OEM panel vendors for large display panels. The glass panel suppliers are as follows:

External consumer display devices like a TFT LCD feature one or more analog VGA, DVI, HDMI, or DisplayPort interface, with many featuring a selection of these interfaces. Inside external display devices there is a controller board that will convert the video signal using color mapping and image scaling usually employing the discrete cosine transform (DCT) in order to convert any video source like CVBS, VGA, DVI, HDMI, etc. into digital RGB at the native resolution of the display panel. In a laptop the graphics chip will directly produce a signal suitable for connection to the built-in TFT display. A control mechanism for the backlight is usually included on the same controller board.

The low level interface of STN, DSTN, or TFT display panels use either single ended TTL 5 V signal for older displays or TTL 3.3 V for slightly newer displays that transmits the pixel clock, horizontal sync, vertical sync, digital red, digital green, digital blue in parallel. Some models (for example the AT070TN92) also feature input/display enable, horizontal scan direction and vertical scan direction signals.

New and large (>15") TFT displays often use LVDS signaling that transmits the same contents as the parallel interface (Hsync, Vsync, RGB) but will put control and RGB bits into a number of serial transmission lines synchronized to a clock whose rate is equal to the pixel rate. LVDS transmits seven bits per clock per data line, with six bits being data and one bit used to signal if the other six bits need to be inverted in order to maintain DC balance. Low-cost TFT displays often have three data lines and therefore only directly support 18 bits per pixel. Upscale displays have four or five data lines to support 24 bits per pixel (truecolor) or 30 bits per pixel respectively. Panel manufacturers are slowly replacing LVDS with Internal DisplayPort and Embedded DisplayPort, which allow sixfold reduction of the number of differential pairs.

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