Welcome to our Knowledge Center, where you can explore and learn more about the technology we offer along with different resources that will help you along in your product decisions for new or current projects.
Our TFT displays are active-matrix LCDs with full RGB color screens. These screens exhibit bright, vivid colors and have the ability to show quick animations, complex graphics and custom fonts.
Standard TFT displays are TN (Twisted Nematic) and offer basic characteristics such as wide operating temperatures ranging from -20°C to +75°C, 12 o’clock or 6 o’clock viewing angles, and support full motion graphics. TN TFTs are the more affordable option for various applications requiring full color graphics.
TFTs are Active-Matrix LCDs that have tiny switching transistors and capacitors. These tiny transistors control each pixel on the display and require very little energy to actively change the orientation of the liquid crystal in the display. This allows for faster control of each Red, Green and Blue sub-pixel cell thus producing clear, fast-moving colorful graphics. The transistors in the TFT are arranged in a matrix on the glass substrate. Each pixel on the display remains off until addressed by applying a charge to the transistor.
Unlike conventional Passive-Matrix displays, in order to activate a specific pixel, the corresponding row is turned on and a charge is sent down the proper column. This is where only the capacitor at the designated pixel receives a charge and is held until the next refresh cycle. Essentially, each transistor acts as an active switch. By incorporating an active switch, this limits the number of scan lines and eliminates cross-talk issues.
Inside a Pixel
The pixels of the TFTs are divided into sub-pixels capable of producing various intensities of Red, Green or Blue. The mixture of color and levels of intensity allows for an accurate depiction of any combination of 16.7 million colors.
IPS (In-Plane Switching) technology acts on the liquid crystals inside an LCD so when voltage is applied, the liquid crystal rotates in parallel (or in-plane) allowing the light to pass through instead of turning upright. This behavior of the crystals greatly improves many viewing aspects of the display.
Compared to regular TN panels, IPS is superior in color, viewing angles and these TFTs can even handle direct sunlight due to their high brightness.
What's Going On?
In the animation to the left, both linear polarizing filters have their axes of transmission in the same direction. To obtain the 90 degree twisted nematic structure of the LC layer between the two glass plates without an applied electric field (OFF state), the inner surfaces of the glass plates are treated to align the bordering LC molecules at a right angle. Because the arrangement of electrodes are in the same plane and on a single glass plate, they generate an electric field essentially parallel to this plate.
The LC molecules have a positive dielectric anisotropy and align themselves with their long axis parallel to an applied electrical field. In the OFF state, entering light becomes linearly polarized by polarizer. The twisted nematic LC layer rotates the polarization axis of the passing light by 90 degrees, so that ideally no light passes through a polarizer. In the ON state, a sufficient voltage is applied between electrodes and a corresponding electrical field is generated that realigns the LC molecules and light can pass through a polarizer.
High Definition Multimedia Interfacing
The HDMI TFT Modules are our high-quality TFT displays paired with a custom designed PCB that supports HDMI interfacing for communication with the display. By utilizing the widely popular HDMI format, these modules do not need a separate controller board or multiple cable conversions to set up. Any controller with HDMI output can be directly connected to the HDMI TFT Modules via standard HDMI cable. After that, interfacing with the display and even a touch panel (connected via USB) can begin right away.
The HDMI you’re familiar with is now the vehicle for data and command transmission to your display. Easy and reliable.
Both touch panel options come with configured USB-HID. Plug the the panel in via USB and drivers will be installed automatically.
Quality TI Hardware
High quality hardware from Texas Instruments is used in the board design to ensure the best performance and longest product life cycle.
On-Board DC Power
An on-board DC power port is integrated to be sure that power supply is not an issue in providing ample power for peak display performance.
We now have IPS HDMI TFT Modules which have all the perks of the IPS technology combined with HDMI.
EVE2 TFT Modules
The EVE2 Modules are Human-Machine Interface (HMI) displays that offer users the highly desired and easy to use serial interface (SPI). The feature-rich EVE2 graphic engine made by FTDI/Bridgetek give these modules a lot of powerful capabilities. High performance video playback, audio synthesization, and 1MB of built-in graphics memory are just a few of the things the EVE2 chip brings to the table. Our EVE2 TFT Modules have two standard 20-pin connector options on every board, IDC & FFC, and also come with backlight drivers that will never sacrifice display brightness.
The Power of the EVE2 Graphic Engine
Multimedia: High resolution graphics, video and audio playback.
SPI: Serial interface for streamlined communication with the display.
RAM: 1MB of built-in graphics memory.
Fonts: Scalable font capabilities.
And More: Visit Bridgetek to learn more about the EVE2 chip.
Sunlight Readable TFTs
Our Sunlight Readable TFT Displays have greatly increased backlight brightness. By using a 3M enhancement film and LED backlights rated up to 1,000 cd/m2, these displays perform well in all environments including direct sunlight. Sunlight Readable TFT Displays are the perfect solution for applications looking for a full color graphic display that can perform even in the brightest conditions.
MVA (Multi-domain Vertical Alignment) displays can offer wide viewing angles, good black depth, fast response times and good color reproduction and depth. Each pixel within a MVA type TFT consists of three sub-pixels (Red, Green and Blue).
Each of these sub-pixels is divided further into two or more sub-pixels, where the liquid crystals are randomly lined up due to the ridged polarized glass. When a charge is applied to the transistor, the crystals twist.
With these crystals being randomly placed, it allows the backlight to shine through in all different directions keeping the intended color saturation retained while giving the display a 150° viewing angle. We offer this technology in our Premium TFT models with the option of a resistive or capacitive touch panel.
LCDs (Liquid Crystal Displays) are passive or active-matrix displays that can demonstrate information as text or in a dot matrix pattern. This type of technology is generally less expensive than VFD or OLED display technology.
We have many different LCD products which can be classified under the following display types: TFT displays, COG displays and standard character or graphic module displays (COB).
Understanding Liquid Crystal Technology
In order to understand how LCDs work, it is important to know how they are made. To start, LCDs are composed of two pieces of polarized glass. On the non-polarized side of the glass, a special polymer is added to create grooves that run in the same direction as the polarizing film. Once this is done, a liquid crystal material is added to the grooved side of one of the polarized glasses. These grooves align the liquid crystal with the glass. The second piece of glass is placed on top with the grooved side in, aligned perpendicular to the first pieces of glass creating a row and column arrangement.
Hows LCDs Work
Where the grooves of the two pieces of polarized glass intersect is a pixel. By blocking the light from passing through the top piece of glass, it creates an area that is darker than its surrounding. This gives the appearance of pixels being turned on or off.
In order to block the light from passing though, the orientation of the liquid crystal has to be changed. To do this, an electric charge is needed. Without an electric charge, the liquid crystal is twisted which changes the angle of the light to match the angle of the top polarized glass. This allows light to pass through.
When an electric charge is applied, the liquid crystal untwists leaving the angle of the light unchanged. This causes the light to be blocked by the top perpendicular piece of polarized glass. The controllers on the display will determine which pixels turn on and off. These controllers are programmed to translate user data into predefined fonts or turn on the appropriately addressed pixels.
TN (Twisted Nematic)
This technology consists of nematic liquid crystal sandwiched between two plates of glass. When power is applied to the electrodes, the liquid crystals twist 90°.
STN (Super Twisted Nematic)
This technology has rapidly become a standard for most monochrome passive-matrix LCDs. It uses the same principle as TN displays but uses less power and is typically less expensive. The liquid crystals in STN technology have a 210° - 270° twist which allows for a broader transition region for gray scaling. STN displays can also be made purely reflective for viewing under direct sunlight.
FSTN (Film Compensated Super Twisted Nematic)
This technology uses a film compensating layer between the STN display and rear polarizer. This additional film increases the overall sharpness and contrast of the display.
Positive Type Displays
Positive displays provide an image with dark pixels on a light background. Ambient light or a backlight can be used for this type of display and is capable of multiple background colors.
Negative Type Displays
Negative displays provide an image with light pixels on a dark background. The backlight must be used for this type of display and is capable of multiple pixel colors.
Transmissive LCDs always require a backlight and provide the highest brightness display. They are best suited for applications where direct sunlight viewing is not involved.
Reflective polarizers are used in high ambient light environments or whenever sufficient power is not available to drive the backlight.
Transflective polarizers have both reflecting and transmitting properties and offer the most versatile viewing characteristics. They can be viewed in direct sunlight and when combined with a backlight in low light conditions.
Super Twisted Nematic (STN) LCDs have a blueish tint to their pixels. This is something to keep in mind when choosing a backlight option for STN type LCDs because of how the contrast is affected with the backlight both off and on.
STN(+) Gray Background
Blue LED Backlight
Orange LED Backlight
Pure Green LED Backlight
White LED Backlight
Yellow-Green LED Backlight
Red LED Backlight
STN(+) Yellow-Green Background
Yellow_green LED Backlight
Pure Green LED Backlight
STN(-) Blue Background
White LED Backlight
Film Compensated Super Twisted Nematic (FSTN) LCDs have acheive a truer black shade in their pixels. This still should be considered when selecting a backlight option and whether usage for both off and on status will be needed in the application.
FSTN(+) Gray Background
Blue LED Backlight
Orange LED Backlight
Pure Green LED Backlight
White LED Backlight
Amber LED Backlight
Red LED Backlight
FSTN(-) Black Background
Blue LED Backlight
Orange LED Backlight
Pure Green LED Backlight
White LED Backight
Amber LED Backlight
Red LED Backlight
RGB Displays have three backlight colors built into one display. This allows them to be programmed to select between three backlight colors Red, Green and Blue or create a custom backlight color by mixing the red, green or blue.
Modules Without a PCB
Unlike typical LCD modules, COG displays are designed without a PCB. Instead, the LCD's driver/controller is directly bonded to the glass of the display. This allows COG displays to be smaller in size, more cost effective and a highly customizable display solution compared to typical LCD modules.
- - 5x8 dot matrix characters or monochrome graphics with custom icons
- - Low power but high brightness
- - No bezel for profiles as thin as 2mm
- - Character COGs have 4 font tables
- - Particularly cost effective
COG LCDs are composed of two pieces of polarized glass. On the non-polarized side of the glass, a special polymer is added to create grooves that run in the same direction as the polarizing film. Once this is done, a liquid crystal material is added to the grooved side of one of the polarized glasses. These grooves align the liquid crystal with the glass. The second piece of glass is placed on top with the grooved side in, aligned perpendicular to the first pieces of glass creating a row and column arrangement. The LCD driver/controller is then mounted directly to the LCD glass by bonding the gold bumps of the IC to the indium tin oxide (ITO) tracks on the glass.
Graphic COB LCDs
Graphic LCD modules come in STN and FSTN types that come in a wide variety of sizes measured in pixel width-by-height dimensions (i.e. 240x128 meaning 240 pixels wide by 128 pixels tall). Our standard LCD graphic displays are passive matrix modules.
They use a dot matrix pattern to display both text and simple bitmap images. They are capable of more complex images than that of a character display, but require much more information to do so since they need to be told what do to with each pixel in their matrix.
Character COB LCDs
Our character LCD modules are monochrome, passive matrix displays with pre-programmed font tables built-in. They come in standard sizes in either STN or FSTN types, from 1 line x 8 characters, to 4 lines x 40 characters that can be compatible replacements for some of our VFD modules.
Character displays display simple text from forms in their built-in font tables. Each display has at least one row with a number of “characters”. Each of these characters is a cell that can be filled with the user’s choice of form from the programmed font table; for example you might want row 1, cell 1 to contain a capital roman “A”.
The OLED (Organic Light Emitting Diode) is a brighter, higher contrast display with faster response times, wider viewing angles, and consumes less power than conventional LCDs or VFDs. OLED displays are self-illuminating due to their organic material, so there's no need for a backlight to achieve maximum visibility in all environments. This also allows OLEDs to be significantly thinner than standard LCD or VFD modules. Our OLED displays come as complete, easy-to-use modules in both character and graphic display types.
Understanding the OLED Structure
OLED displays are self-illuminating due to their organic material, so there’s no need for a backlight to achieve maximum visibility in all environments. OLEDs are made up of a layer of organic material placed between two conductors. These two conductors (an anode and a cathode) are between a glass top plate (seal) and a glass bottom plate (substrate).
When an electric current is applied to the two conductors, the organic material produces a bright, electro-luminescent light. When energy passes from the negatively charged layer (cathode) to the other layer (anode), it stimulates the organic material between the two, which in turn emits light that is visible through the outermost layer of glass.
1. Electrical current flows from the Cathode to the Anode through the organic layers, giving electrons to the emissive layer and removing electrons from the conductive layer.
2. Removing electrons from the conductive layer leaves holes that need to be filled with electrons in the emissive layer.
3. The holes jump to the emissive layer and recombine with the electrons. As the electrons drop into the holes, they release their extra energy as light.
The OLED Pixel
The pixels are created by the arrangement of the cathodes and anodes; which are arranged perpendicular to each other. The electric current applied to the selected strips of anodes and cathodes determine which pixels get turned on and which pixels remain off. The brightness of each pixel is proportional to the amount of applied current.
OLEDs in Full Color
Our 18-bit, full color graphic OLEDs are passive matrix displays that come equipped with all necessary logic and a slim 5mm profile. The full color OLEDs are ideal for consumer or handheld devices to enhance user interface experience. A resisitve or capacitive touch panel can also be applied.
Full Color module displays are displays that have PCB already attached to the glass. They offer mounting holes for easily application and are customizable.
How Full Color OLEDs Work
Color OLEDs consist of a metal cathode with a negative charge, an electron transport layer, organic material, a hole transport layer, and an anode with a positive charge. The cathode and anode are arranged perpendicular to each other, creating a pixel where they intersect. Each pixel of a color OLED is divided into red, green, and blue sub-pixels.
When the controller sends an electric current to a particular pixel, the current passes through the organic material causing the material to emit light. By adjusting the intensity of the electric current in each red, green, and blue sub-pixel, specific colors and gradients can be created.
Monochromatic Graphic OLED Displays
Monochrome graphic OLED displays are easy to use, all-in-one designs. The OLED modules have all the required external logic components embedded, allowing for just one interface supply. Newhaven Display also has Multi-Font feature module options in addition to standard monochrome graphic OLEDs of the same sizes.
Multi-Font Graphic OLEDs feature an additional serial interface IC, providing 37 predefined font tables and support of over 50 languages, which makes them stand out from standard monochrome graphic OLEDs.
The Multi-Font graphic OLED offers users the ability to use multiple languages simultaneously on one display. These unique OLEDs have a built-in IC including over 37 standard font tables and support over 50 languages. This eliminates the need to create your own characters or spend money on additional memory to hold multiple languages.
This Multi-Font IC contains ASCII, Unicode, and ISO8859 tables for flexibility and compatibility. The fonts featured are either fixed width or proportional width and can be read at a clock rate up to 30mHz. When using this product, the user's MPU can communicate directly with this Multi-Font IC to read the font data strings and then output the desired character onto the display. Using just a 4-wire SPI interface, the selected font data is read in vertical structured bytes from the Multi-Font IC and is sent out to the display's controller.
Slim Character OLEDs
Character OLEDs are monochrome passive matrix displays with built-in font tables and screen savers. Each character OLED is self-illuminating and can achieve a higher level of contrast when compared to LCDs because of their ability to obtain richer black levels without backlight interference. These displays are packaged as complete modules including all required logic, while containing a slim, 5mm profile. Character size formats include 2 x lines by 16 characters, 2 x lines by 20 characters and 4 x lines by 20 characters with the flexibility of using them as double height characters. These slim character OLEDs have a superior contrast ratio of 10,000:1 for outstanding clarity and can achieve faster, smoother graphic animations with their 10μsec response time.
2x16 Modular OLEDs
The modular character OLEDs have a modulated design to include an array of options for electrical interfaces and connection ports. They feature all the benefits of other character OLEDs such as a 10,000:1 contrast ratio and double height characters, but they are only offered in a 2 x lines by 16 character format and a blue monochrome color.
The modulated characteristics were designed with all kinds of engineers' project requirements in mind. Choose between three electrical interfaces and five connector options for these character OLED displays. The available interfaces to choose from are Parallel, SPI, and I²C. Connection options are: two dual row pin headers in 2x18 (for Parallel interface only) or 2x8 sizes, two single row through-hole pin outs (in either 1x7 or 1x8 sizes), and an RJ45 port.
Modular Unique Features
- - Six 2x16 Character OLED modules, each with unique connector and interface combination
- - Parallel, SPI, or I²C interface options
- - 2x18/2x8 pin header, 1x7/1x8 through-hole, or RJ45 connector options
- - Built-in screen saver & ultra low-power (<1mA) sleep mode
VFDs use a self-emitting fluorescent light which allows the technology to handle extreme cold and hot temperatures. They are high contrast displays that can demonstrate multi-segment alphanumeric characters, 7-segment numerals or a dot matrix pattern. VFDs displays are typically bright, colored green and will perform well in low lighting or direct sunlight environments.
VFDs are composed of a hot cathode filament, anode segments coated with phosphor and grids within a vacuum sealed glass encasement. The filament is made up of alkaline coated tungsten wires which enable the display to emit light.
How They Work
When heat generated by a power supply is applied to the tungsten wires, the alkaline coating emits electrons. These electrons are controlled and diffused by the thin metal grids within the display. When these electrons come in contact with the phosphor coated plates, they emit light.
Dot Matrix VFDs
Dot matrix VFDs are typically used to display upper and lower case text, numerical information and limited symbols. Typical applications are measurement, scales, medical, vending machines, point-of-sales registers and industrial instrumentations.
Alphanumeric VFD displays are used to display upper and lower case text and numbers. Typical applications are scales, test and measurement, vending machines, and gaming machines.
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How Serial Displays Work
Serial LCD modules have three unique interfaces selectable by soldering jumpers onto the back of the display's PCB. These displays also have an on-board PIC microprocessor that initializes the LCD on power up and translates RS232, SPI or I2C protocol into standard parallel HD44780 protocol. All standard LCD commands are available in each interface protocol.
Like any traditional character LCD, text is displayed on the screen by sending each character's ASCII value to the display's controller. With the addition of the on-board interface translator IC, not only does it eliminate the need for an additional board or logic device, it also allows for easy adjustment of the LCD's contrast and backlight brightness through use of software commands.
LCD serial displays feature 3 unique selectable interfaces:
5V TLL non-inverted
300-115.2K baud rate (9600 default)
100KHz max clock rate
Definable bus address (0x50 default)
Pull-up resistors are built-in
100KHz max clock rate
High level idle clock, active low slave select
Serial VS. Parallel Interface
LCDs, TFTs and OLEDs can offer both serial and parallel modes. A multi-interface LCD board is designed to display information on the LCD using different parallel or serial protocol interfaces. Only one protocol will write to the LCD at a time. Some controller IC's have more than one user-selectable interface option.
No matter which protocol is being used to interface to the LCD, it must be initialized prior to normal use.
There are two ways of transmitting a byte between two digital devices. We can either transmit the byte in PARALLEL or we can transmit the byte in SERIAL form. The drawing to the left illustrates the differences between these two types of communication mechanisms.
In parallel mode, each bit has a single wire devoted to it and all the bits are transmitted at the same time. In serial mode, the bits are transmitted as a series of pulses.
(one bit at a time)
Serial interface consists of an I2C bus, SPI bus, or synchronous serial control and data lines. The biggest advantage of the serial interface displays is they use fewer pins. You save connection pins, board traces and I/O pins. Although a downside is that you usually can't read from the display in serial mode, only write. Also, it's normally slower in serial interface than writing in parallel.
Serial Interface Types
- Synchronous Serial - Serial Data In, Register Select, Reset, & Serial Clock
- Custom - Various configurations - may include Chip Select
- SPI (Serial Peripheral Interface)
- SPI (3 wire) uses Serial Data Out, Serial Data In, and Serial Clock
- SPI (4 wire) adds Chip Select
Timing and Operation May Differ From Usual SPI
- - I2C uses Serial Data Line and Serial Clock
- - LCD controller has certain usable I2C bus addresses
- - Requires pull-up resistors
- - May not function correctly with other devices on same bus
SPI interface operates in full duplex mode, meaning that the devices can communicate with one another simultaneously. For this to work, two data lines are required. With this standard, devices communicate in a master/slave mode, where the master device (host processor) initiates the data and the clock. The LCD/OLED display is the peripheral slave device(s) attached to the data bus. Multiple peripherals are addressed on the same serial data bus. But keep in mind, the display will only listen to the data it sees when the Chip Select line is active (usually low). If the Chip Select line is inactive (usually High), the display listens to the data on the bus, but ignores it. During this state, the SDO line isn't active.
(multiple bits at a time)
Parallel interface consists of 8 data pins and 3 control lines. The control lines used are Enable (E), Register Select (RS), and Read/Write (R/W). RS is used to signal when a command or data is being sent to the display. Enable tells the display that the data or instruction in the register is ready to be interpreted by the display. The Read/Write tells the display whether to write data or read data from the register. 4-bit parallel interface works exactly like the standard 8-bit interface, only each byte of data sent or received is clocked into the display one nibble at a time. After every two clock cycles, the entire byte is combined and executed like an 8-bit command.
In addition to the following common interface connection examples, some displays may include additional control lines.
- - V0 - Contrast Adjustment
- - CS - Chip Select
- - BUSY - Typically a low state indicates busy state
Parallel Interface Types
- 16 Bit Data - Not Common - used on some TFT controllers
- 8 Bit Data - Most Common - used on graphic and character controllers
- 4 Bit Data - A common option on character controllers
- 8080 type - Parallel Data with Write Line and Read Line
- 6800 type - Parallel Data, with Read/Write Line and Enable Line
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Backlight Driving with PWM
Pulse-Width Modulation (PWM)
By using a pulse width modulation scheme, several advantages are realized over a simple DC voltage method. The main advantage is in efficiency. The LEDs are pulsed with high current for a short period of time. For example, consider the NHD-0216K1Z. The nominal LED driving current for this display is 120mA which produces a typical brightness of 50 NIT. If, instead of a DC or constant current, we apply 5 times the current (600mA), for 1/5 of the time, the average current is the same (120mA). See Fig.1 The brightness average of the LED would also be the same if measured by a meter. The only difference is the perceived brightness.
The human eye has a certain degree of persistence. If exposed to a bright light, the eye will remember the light for a short period of time. This allows us to view a motion picture or television screen as a steady image when in fact it is flickering at 24 - 30 times per second. When the LED is flashed "ON" for a short period of time and then turned off, the eye remembers the light at the high brightness level. The result is that the perceived brightness of the backlight is closer to the high pulsed brightness than to the lower DC brightness.
This effect provides several advantages as follows...
If the brightest possible backlight is required, the display can be pulsed at a 1:4 on/off ratio with 5x the typical current. The pulse frequency repetition should be greater than 100Hz but not greater then 1,000Hz, so the flickering is not perceived by the eye.
This technique can also be used to provide a normal looking brightness level to the display but at a lower average current to save power. The average power can be cut by a factor of at least 50% to produce a given perceived brightness level. This may provide a big advantage in battery operated products.
Another use of this method is to facilitate a wide range brightness control for the LED backlight. By adjusting the on/off ratio, a wide range of brightness can be achieved while maintaining a very even backlight appearance. See Fig.2. One can also vary the brightness by adjusting the DC current to the LEDs. But at low current the individual LED emitters start to become visable, resulting in an uneven looking backlight. To implement this technique, the peak current should be set at the specified typical current for the display and the on/off ratio of the pulses varied from near 100% on to near 0% on.
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Voltage Generating Circuits for LCD Contrast Control
Most Liquid Crystal Display modules require a Positive or Negative voltage that is higher than the logic voltage used to power the LCD. This voltage, called VI, VEE or Bias voltage, requires a second power supply. If this power source is not available, the LCD Bias voltage must be generated from an existing voltage, either the logic voltage (+3.0~+5V) or a battery. This application note illustrates circuits for generating either a Positive or Negative LCD Bias voltage from such a voltage source. The LCD Bias voltage is used to directly power the circuits that drive the LCD glass. This voltage sets the contrast level of the LCD. Since any changes in this voltage will cause a visible change in the contrast of the LCD, it must be regulated to more than about 200mV. Any noise or ripple on this signal may cause visible artifacts on the LCD so they must be kept below about 100mV.
Charge Pump Circuits
These simple circuits can be used to generate the bias voltage for character type displays and small graphics types. They have the advantage of being very low in cost but are not regulated and cannot deliver much current. They are also sensitive to variations in the source voltage (Vdd), so it cannot be driven directly from a battery. The driving signal is usually derived from an existing clock signal or generated directly by an I/O pin on a microprocessor. The frequency of the signal can be anywhere from about 1kHz to 50kHz or higher. If the signal is above about 5kHz the simple 1N4148 diodes should be replaced by Schottky diodes such as the 1N5817. The capacitors should also be upgraded to low ESR types. The device generating the signal must be capable of delivering the load current times the multiplication value. In the circuit in Figure 1 driving a small character display the input signal should be able to sink and source at least 4mA.
Figure 1 shows a simple charge pump circuit that generates a Negative 4V from a Positive 5V square wave. It is suitable for driving the VI line on an extended temperature LCD character module
Adjustable Voltage Inverter
It might be desirable to allow the end user of a product to have access to the contrast adjustment. The circuit in Figure 2 utilizes a pot to adjust the contrast voltage from 0V to -12V limits by adding one or two resistors in series with the pot. The total resistance of the pot and any added resistors should not exceed 50k. If the end user is not to have control of the contrast the pot can be substituted with a fixed resistor to set the voltage to the LCD to give the best contrast, which would also eliminate the need to adjust a pot during production. The efficiency is high enough to be used with battery operated equipment and the output can drive most small graphics displays, up to about 240 x 64px resolution.
On/OFF signal. A logic 0 on this pin will turn the display off by removing the VL voltage. If this signal is not needed, tie pin 3 to +5V. Output voltage control. The maximum voltage is set using a single resistor, RVMAX, See Table 1.
Digitally Adjustable Inverter
Some applications require the user to have control of the contrast but do not lend themselves to using a pot to make the adjustment. The circuit in Figure 3 allows a micro controller to adjust the VL voltage in a very simple manner. It also provides an input to shut off negative voltage so the display can be shut down by the micro controller if desired. This shutdown signal can also be used to properly sequence the power to the display during power-up and power-down sequences.
The output can be adjusted from 33% to 100% using the internal 64-step DAC / counter. On powerup or after a RESET command the output voltage is set to mid-range (67% of max voltage.) Each rising edge of the ADJUST signal increments the DAC output. When the DAC reaches 100% output, the next pulse will cause it to wrap-around to the 33% value.
A RESET is accomplished by setting the ADJUST line high and then setting the ON/OFF line low for longer than 400nS
High Voltage Circuits for Larger Panels
Modern 1/4 VGA to full VGA size panels require VEE voltages above 20V. Most monochrome panels require a Negative VEE voltage while most color panels require a Positive voltage. Many of these panels require a Negative VEE voltage while most color panels require a Positive voltage. Many of these panels are used in handheld, battery operated applications and require very efficient conversion using a supply voltage that changes as the charge on the batteries is gradually depleted.
Several semiconductor manufactures have responded to this need with new devices made especially for this application. While the voltage converter can be done "in-house" it is usually not economical to do so because of the complexity of a circuit that has the regulation qualities and the efficiency required.
Figure 5 is a circuit based on the Linear Technologies LT1615 series chips. The circuit shown here generates a Positive voltage for a small 1/4 VGA (320x240) color graphics display that could be used in a palm sized PC running Windows CE. A Negative voltage version LT1617 is also available. The device in this example runs from a pair of AA batteries and must produce a stable output voltage with an input that varies from 2.0V ~ 3.2V.
General Circuit Considerations
All components associated with the circuits in Figure 2, Figure 3 and Figure 5 should be placed physically close to the IC. The decoupling capacitor on the input voltage line should be placed as close to the VIN and GND pins of the IC as possible.
Power Sequencing Considerations
The order in which the power supplies are applied to an LCD, power sequencing must be considered when designing an LCD bias power supply. The power sequencing requirement can be summarized by stating that the VEE (VL) must never be present without VDD also being present. If this condition exists, even for a short period of time, the display may be permanently damaged. The desired power on sequencing for graphics type LCDs with an external controller is shown in Figure 6. For graphics type LCDs with a built in controller you can ignore the "signal" line as this is taken care of in the controller at power on time. For character displays only the VDD and the VEE (VL) lines need to be considered.
All of the circuits described here, except the charge pump in Figure 1, have provisions to shut down the voltage generator with a logic signal. Using this signal the generator IC is kept in the shutdown mode until VDD is stable and the LCD controller has been initialized and has started to scan the display. At this time VEE can be applied to the display safely. The turn off procedure is just the reverse of the turn on procedure.
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Temperature Compensation for LCDs
The optimal contrast setting for LCDs varies with ambient temperature. For most applications this variation in contrast is tolerable over the "normal" temperature range of 0°C to +50°C. Most LCD modules are available with an extended temperature range option which allows the display to operate from -20°C to +70°C. The changes in contrast are NOT usually tolerable over this wide range of temperatures, which means a way of adjusting the contrast voltage as the ambient temperature changes must be provided.
As the temperature decreases the LCD fluid requires a higher operating voltage in order to maintain a given optical contrast. See Figure 1. One way to provide for this is to give the user control of the contrast. This is a simple solution but quite often its not desirable or practical.
The solid line describes Temperature compensated voltage provided by the circuit in Figure 2. The dashed line describes the way in which the LCD operating voltage varies with temperature.
The chart can be used to predict the voltage at VL needed to produce good contrast on the display by adding the "relative voltage" to the contrast voltage of the display at 25°C. If, for instance, a display looks good with -3V on VL at room temperature (25°C) this display will need -2.7V at 50°C.
The controlling microprocessor could measure the ambient temperature and supply the proper voltage to the LCD, but this is complicated and expensive. The most common solution to the temperature compensation problem is to provide a circuit such as that in Figure 2 to adjust the contrast voltage automatically.
This circuit uses a negative temperature coefficient thermistor to sense the ambient temperature. It should be placed as physically close to the LCD module as possible. The PNP transistor is connected as an emitter follower to provide the drive current to the LCDs contrast voltage (VL) input.
The voltage VEE will very depending on the requirements of the LCD. NOTE: VL and VEE are measured in relation to the VDD supplied to the LCD. An extended temperature range character display will require about -7.8V at its VL input at 25°C or about -2.8V relative to ground. The VEE voltage will need to be about 25% higher than the actual voltage required at the VL input of the LCD. During development the VEE should be a variable voltage that can be used to adjust the contrast to an optimal level. The VEE can be made fixed or adjustable for the production units.
This circuit will work for all character modules and graphic modules up to 320 x 240. Modules larger than this are not available with the extended temperature option.
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