Liquid-crystal displays are quickly becoming the dominant display technology for a wide variety of applications-from mobile appliances such as handsets, PDAs and MP3 players to large-format systems like wide-aspect-ratio high-definition televisions. In particular, mobile phones are requiring more-advanced LCDs to facilitate video transmission, gaming and camera capabilities. These newer displays require LCD drivers that deliver better resolution and higher levels of integration. At the heart of these displays are the row and column drivers. The Isron IS2341, a cutting-edge LCD driver that exemplifies some of the design issues chip makers are grappling with in this market, is a fully integrated LCD driver targeted for use in 128 RGB (384-column outputs) x 160-row thin-film-transistor (TFT) mobile-phone displays. The device-equipped with the necessary LCD drivers, display memory, power supply circuitry and control logic-came to MagnaChip Semiconductor in February, when that Seoul, South Korea, company acquired International System and Electronics Corp. (Isron), a display driver vendor based in Osaka, Japan.
This part has the smallest, most effective die size of all the LCD drivers that Semiconductor Insights has analyzed, with a large memory, large number of output drivers and very narrow die aspect ratio. Compared with typical LCD drivers, often built at 0.5 micron, it is manufactured with an advanced process, at 0.25 micron.
Future is LCDs
Flat-panel display technology has been progressing quickly over the past few years, with drastic increases in visual quality and constantly decreasing price points. There are a number of different technologies behind the evolution that have enabled these improvements. The displays are using a variety of materials, including TFTs and contrast-enhancement films, to provide vivid images with high refresh rates to eliminate "ghosting." To make the image show up on the screen, display driver components are required.
To reduce size and manufacturing complexity, the Isron IS2341 is mounted directly onto the glass of the display.
Generally, manufacturers seek the greatest number of outputs per component, thereby reducing the number of devices needed in a system to display the image. Plasma drivers can have upward of 192 outputs, while LCD drivers usually have more than double that amount. The driver will depend on the application it is being used for; the app will determine whether the part will drive segmented, character-cell or dot-matrix displays. It will also determine whether the driver supports monochrome, gray-scale or color display modes.
Active-matrix LCDs, which offer a brighter and sharper display than passive varieties, use a TFT arranged on a glass substrate at the intersection of each row and column line, to access each dot on the display. The voltage stored at the dot adjusts the "twist" of the liquid crystal and determines the amount of light that is transmitted through the dot. Three adjacent dots are combined to make one pixel using a color filter array of red, green and blue that is placed over the display with a resolution of 256 shades. In combination, these shades can produce more than 16.8 million colors. Most mobile applications keep the number of colors at 65,000 or less, however, to reduce the size and power consumption by limiting the number of transistors.
Row drivers activate the rows of the display, while column drivers set the required voltage on all of the dots in the activated row. A controller is used to coordinate the timing of the row and column drivers and provides a constant stream of display data to the column drivers. The controller may also modify the display data to enhance the image quality. In larger displays there are several row driver ICs, several column driver ICs and a controller IC. But for a small display, the functions of all three ICs can be integrated into a single chip, such as the Isron device.
Color filter pixel sites on LCD panel are shown; panel operates in row- or frame-polarity inversion modes.
Different packaging technologies are employed for different application spaces, including flat packs; ball grid arrays; and bare dice for chip-on-glass, chip-on-board or chip-on-flex implementations. The two latter solutions take significantly more space, and in a mobile application this increases the overall size of the system. It is also more expensive, since a cable or board is required.
Recently, ICs such as the IS2341 have begun to be mounted directly onto the glass of the display. This approach reduces the size of the display module and the complexity of manufacturing. Because there is no surrounding package, costs are further reduced.
While chip-on-glass is one of the more recent developments in LCD technology, some system makers are starting to use organic light-emitting diode displays. OLED is an ultrathin, bright and colorful display that does not require backlighting, allowing for reduced power and smaller solutions. More work is still needed for OLED technology to reach the mainstream, however, since display sizes are limited and the price is not competitive with LCDs. Currently, there is no way to leverage the low production cost of OLEDs, because of the extreme care required to seal the devices hermetically inside the glass. This results in an overall construction that is quite similar to that of LCDs.
Three DMOS transistor types are used, differing in symmetry and drain diffusion.
The IS2341 LCD driver supports 15-bit color resolution (32k colors), although its display RAM supports 18-bit color resolution (368,928-bit capacity). The chip-on-glass die is manufactured with a low-cost CMOS four-metal-layer process. The output driver bond pads are staggered in two rows to minimize the effective pad pitch at 29 microns.
Many LCD drivers are produced in older fabs using 0.5-micron or older technology. The IS2341 is designed in a 0.25-micron process node, resulting in a die that measures 17.3 mm2 with only a 1-mm die width. This die size is the smallest seen for an LCD driver combined with row and column controllers.
The row driver pads are located at the two ends of the front side of the die. The column driver outputs take up the rest of that side. The pads on the backside of the die are used for input to the device.
On both ends of the die are high-voltage analog circuits with row drivers and row-driver circuits. The digital control circuitry is concentrated in one block near one end of the die. The column drivers and pads take up almost 70 percent of one side of the die. The reference voltage generator for the column driver digital-to-analog converters is located at the center of the die. There are four blocks of SRAM.
Three types of DMOS transistors are used on this device for row drivers and some of the voltage generation. The first DMOS transistor has a symmetric structure, with the source and drain diffusions separated from the channel region by shallow-trench isolation (STI). There are likely N-P-type wells under the STI which connect the highly doped N-P-type source and drain diffusion to the channel region under the gate oxide. The poly gate covers about two-thirds of the STI width.
The second DMOS type uses an asymmetric structure with the source diffusion separated from the channel region by an STI, while the third DMOS type has an asymmetric structure with the drain diffusion separated from the channel region by an STI. Here again, the IS2341 takes a unique route, using an STI silicon structure instead of the more-typical local oxidation of silicon to deliver significant space and power savings while offering higher performance and lower-noise operation.
The column drivers have 5-bit resolution, making 32 voltage levels available for output. The 32 reference voltages for the column drivers are divided into five segments, where the endpoint of each segment can be adjusted and the reference voltages are linearly interpolated between the endpoints of the segments. All of the column drivers use the same 32 reference voltages, so the panel can only be operated in a row-polarity or a frame-polarity inversion mode (as opposed to dot-inversion mode).
While Isron uses a chip-on-glass technique to implement its LCD driver, joint venture Sony and Toyoda Automatic Loom Works has chosen different materials in its design. The ACX309, an LCD panel used in the Sony DSC-P9 digital still camera, was the first commercial product to employ low-temperature polycrystalline silicon (LTPS) as an active material for TFT displays.
Polycrystalline silicon is known to have better electrical properties than amorphous silicon and thus reduces the power consumption of the display. However, it is more difficult to incorporate in the TFT LCD technology. Nevertheless, the use of LTPS instead of amorphous silicon has made it possible to integrate the pixel array and the driver circuitry onto a single glass plate.
LTPS is expected to replace the more-common LCD amorphous-silicon substrates as existing fabs are retooled and new fabs are built. In fact, LTPS is already the basis for premium LCDs, since LTPS allows higher resolution, lower power consumption and fewer external chips, and it greatly simplifies electrical connections between the display module and the rest of the device.
Price reductions and increased image quality have propelled LCD technology to widespread use on both a large scale, as in wide-screen televisions, and a small scale, such as in mobile devices. MagnaChip's recent purchase of Isron catapulted the company to the top of the LCD driver market in one step. Combining row and column drivers in a single component, at a small process node, positions its IS2341 driver device well ahead of most competitors. With the small die and ease of integration that come with a chip-on-glass implementation, the part makes it possible for mobile devices to provide vivid color images without sacrificing form factor.
Gregory A. Quirk (firstname.lastname@example.org), systems analyst at Semiconductor Insights (Kanata, Ontario)