Technological Advances Increase Vacuum Fluorescent Display Options, Decrease Prices

Because of the recent explosion in availability of new color LCD modules, in response to international demand for better notebook computer displays, the vacuum fluorescent option is often overlooked by engineers designing with flat panel displays. The inherent readability advantages of vacuum fluorescent display (VFD) modules, however, together with recent technological advances that have increased selection and lowered prices in many cases, make VFDs the best flat panel display solution for many applications. Vacuum fluorescent displays were introduced in 1966 by Dr. Tadashi Nakamura of Noritake. The original technology consisted of a basic triode design with a self-emitting light source. Early applications included desktop calculators and other basic office equipment because of the brightness, size and economical price of the first seven-segment products. Today VFDs are used in a wide range of industrial, automotive, medical, as well as audio and other consumer electronics.

The advantages of vacuum fluorescent technology are fundamental. VFDs are brighter and easier to read from any angle than other displays, including LCDs. General visibility data are:

Recent Innovations

Noritake engineers built a 16 x 16 dot phosphor matrix on top of a 5.4 x 6 mm semiconductor chip that integrates memory functions and display driver circuits. These chips, arranged in a single-or double-stage format, make possible highly precise graphic displays.

High precision means that complex characters appear vivid on the display. Also, because the display uses a self-emitting design, no back light is required. New technology has given CLVFDs the additional benefits of low voltage with high brightness, as detailed above; long life (20,000 hours); and low noise. In addition, brightness can be controlled by blanking signals so the appropriate brightness for a given application can be attained. The basic structure of the active matrix display is the same as that of conventional VFDs. It consists of three electrodes: cathode, grid and anode. However the structure of the anode in the new tube is completely different from that in conventional tubes. In conventional VFDs, a complex design of circuit wiring with insulation and phosphor layers is fabricated on the glass plate. Their complex anode design requires many lead pins to be pulled out of conventional VFDs.

With the new active matrix technology, semiconductor chips on which the phosphor matrixes are fabricated are arranged on the glass plate, and the chips and outer leads are connected by wire bonding. This technology reduces the number of lead wires pulled out of the VFD. Therefore, mounting this device on printed circuit boards is easy compared to conventional graphic VFDs.

Because these graphic displays incorporate semiconductor driver ICs with embedded memory, static display mode becomes available even for graphic display applications. Therefore the grid and anode voltage can be set between 12 V and 18 V. Thus no special power source is required. Conventional tubes require drivers and multi-functional controllers. CLVFDs do not because they contain internal semiconductor chips. They can easily be controlled using a generally available CPU. The interface adopts a synchronized serial format and consists of data clock, latch and enabling logics. CLVFDs of five sizes are available as standard products. Although these products feature different dot structures, the grids, anode voltages interfaces and timing are the same for all sizes. (See Table 1.)

The introduction of active matrix technologies has produced a number of benefits, including (in addition to lower noise, longer life and higher brightness mentioned above) fewer soldering points on PCBs, improved mounting workability and more stable operation under a wide range of operating temperatures.

Chip-in-Glass (CIG, BD series). New slim chips, hidden in the glass envelope, enable chip-in-glass displays to be smaller, more functional and less expensive. In addition, the results of Noritake's CIG technology are displays that are brighter, longer lasting and require less power than conventional VFDs. Vacuum fluorescent displays, popular as high visibility display elements, are experiencing new technical demands. Engineers, faced with an ever increasing variety of electronic devices, demand smaller, lighter VFDs that consume less energy. They also require displays of longer service life and enhanced luminosity.

To meet these demands Noritake has developed its BD series of-chip-in-glass VFDs. Despite their use of CIG formats, the models in the BD series feature package sizes equivalent to those of conventional VFDs. In short, Noritake engineers have produced an even more compact product than before, because the BD series does not require the chip mounting space required in conventional CIGs. Incorporating dense drivers, the BD series also expands the possibility of using static drives to display patterns a difficult task for conventional VFDs.

CIG devices normally require extra space for mounting drivers. As a result, the VFD main unit has larger external dimensions than regular VFDs. To solve this problem, Noritake developed a slim driver chip exclusively for the BD series, allowing the driver to be installed under a frame supporting the filament. This technique makes Noritake BD series CIGs more compact than conventional CIGs. BD series VFDs are equal in size or smaller than conventional VFDs. The number of lead pins limits the degree to which conventional VFDs can be reduced in size. Noritake's BD series uses fewer lead pins, and can therefore achieve smaller sizes.

To construct the display, engineers affix the built-in driver to the glass substrate under the metal frame. This frame supports the filament. Wire bonding connects it to the conductive pad that attaches to the glass plate. Input signals and power supplies reach the drivers through conductor traces on the glass plate. Output from the drivers travels to each segment or grid through conductor traces on the glass plate as well. Depending on the uses and display patterns, the BD series of VFDs incorporates one to four slim driver chips, each offering 96 or 128 bits and high voltage output ports. The driver incorporates a level shifter, consisting of CMOS field effect transistors (FETs), a latch and shift register circuits. This is consistent with conventional VFD drivers. Additionally, the driver incorporates a general synchronous serial data input for the interface. When installing multiple drivers, it is most common to use serial connections between the drivers.

It is difficult for conventional VFDs to use static drives to display complex patterns having numerous segments, because of the need to raise the lead pin total. Because it allows installation of several dense drivers, the BD series permits use of static drives even for complex display patterns. For example, in models installing a 96-bit driver in a two-chip series, each driver output in the static drive connects to every segment (anode) of the display patterns. The power supply normally transmits the grid voltage without passing it through the driver. Unlike dynamic drives, static drives face the disadvantage of having to increase the driver-chip number in step with the rising segment count. However, because pattern division by grid is not necessary, the static drive offers an advantage by placing few restrictions on the pattern shape or position. This advantage becomes apparent in systems that emphasize design, such as displaying patterns having custom designs.

Additionally, static drives operate at low voltages, down to 10 V. Static drives will yield lower power consumption, longer product life, and less noise than dynamic drives, when displaying the same kinds of patterns. Unlike dynamic drives, static drives allow operation without exhibiting flickering by grid scans. Engineers can easily raise luminance by increasing voltage. Engineers prefer dynamic drives in systems that display text in 5 x 7 dot matrices. Static drives are difficult to use when these image displays require a large segment total.

In dynamic drives, the driver output connects to the grid as well as to the anode. There are two kinds of systems, depending on the uses and display patterns. One system combines the anode (segment) and the grid-scan data in a single driver. The other system uses separate drivers for the grid and anode data. When using a single-chip driver for the grid and the anode, the grid scan and the anode data must combine in the software and undergo input as data. Such arrangements. however, contribute heavily to shrinking the size and cost of the entire system, because they help diminish the number of components and mounting costs of the VFD driver circuit.

The logic device in the driver requires several components. These include a filament power supply (just as in regular VFDs), a +5-V power supply for the logic circuit, and a display power supply. Noritake's BD series of VFDs features a lead-out construction. The interface features CMOS-level synchronous serial data input. This enables direct control from general purpose, low end micro processors .

The combination of BD series technology with thin film, fine wiring techniques makes Noritake BD series CIGs compatible with graphic display applications. Graphic VFDs using CIG techniques can substantially reduce the number of lead pins, making VFD mounting significantly easier. Additionally, BD series technology assures low voltage driving, low energy consumption and improved luminance.

Currently Noritake R&D is studying the use of the BD series for several kinds of scrolling message displays. Shipment of samples will take place following test production and testing is completed. The main feature of CIG design is the elimination of special, external VFD drivers. The BD series offers additional benefits in its compact chip design, and from the use of a static drive. In recent years, the use of portable electronics has increased dramatically. Noritake's BD series CIG technology will continue to offer solutions to the challenges of ever smaller product design. This includes efforts to shrink the drive circuit, a primary goal for engineers designing for compact products that consume less energy.

Rib Grid. Replacing metal grids with printed rib grids results in a VFD of greatly expanded design possibilities. Borrowing technology developed for printed plasma panels now used in color plasma display tubes, Noritake developed a rib grid VFD especially suited for consumer products requiring more "exciting", eye catching features, such as message displays and video games. Rib grid VFDs are unique in that the grid electrode is formed from layers of thick film printing. Because the grid electrode is printed directly onto the anode plate and fired, it offers a number of significant advantages over other technologies. Noritake's precise printing technology has made this new construction possible. A 50 micron rib is built up in thirteen printed silicon layers with a tolerance of ±5 micron.

The rib grid VFDs are constructed as a triode, similar to conventional VFDs, with a cathode (filament), an anode and a grid. The structural uniqueness relates to the grid electrode. In the rib grid VFD three dimensional partitioning ribs are formed by thick film printing and serve as the grid electrode in contrast to a conventional VFD which uses a metal grid mesh electrode. The rib grid consists of two layers, a base insulated rib layer and a conductive rib layer on top. The rib grid surrounds the phosphorous pattern. It is driven by supplying a positive voltage to the conductive layer of the rib grid.

There are several distinct advantages of rib grid construction. Design flexibility is enhanced. Since the grid is formed by printing, any pattern shape can be formed. With a traditional metal mesh grid, certain shapes are too difficult. More information can be designed with in the same size glass package because the space between characters is reduces by more than one half. The minimum space between characters for the rib grid design is O.9mm. conventional VFD structure requires 2.1 mm of space between characters. Rib grids are more reliable as well because they are fired and fixed to the glass plate and are therefore free from grid distortion caused by heat and vibrations.

U-Version Modules. Noritake's U-Version series of VFDs is designed to offer drop-in replacements for many standard LCD modules. By developing custom chips and by shrinking the envelope size using a combination of unique Noritake VFD features, the U-Version offers the benefits of VFD technology to engineers looking for improvements over limited LCD visibility and brightness.

Until recently, Noritake U-Version VFD modules were limited to 2 line x 20 or 24 characters, both of which use a 5 x 7 dot font format. Recently, however, 2 x 16, 2 x 40 and 4 x 20 formats have been added to the series. The 2 x 16 in particular represents a breakthrough in U-Version development in that it utilizes rib grid technology used in plasma display panels. Larger U-Version displays use mesh grids. In addition, the 2 x 16 uses a smaller substrate, made possible by reducing the number and size of components and by employing the J-lead system by which lead pins are bent toward the inside of the VFD, instead of the outside which is the conventional technology.

The module's substrate exterior, connector hole position and mounting position are the same as 2 x 16 LCD modules. In addition, the 4 x 20 and 2 x 40 U-Version modules were designed with BD chip-in-glass technology in order to reduce the pin outs and make the glass package smaller and compatible with LCD modules. Other features include an interface specification equivalent to that of LCD modules, a choice of either 4-bit or 8-bit parallel interface, and low power consumption. Future Noritake plans include 1 x 16 and 1 x 40 character U Version modules.

Conclusion

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