A New 6-Bit Column Driver Architecture For Active-Matrix LCDs

Cost-Effective, High-Performance Design Offers Superior Display Quality at Low Power

Portable computer makers, who in the past have differentiated their products based on either CPU performance or graphics controller performance, are now turning their attention to the video subsystem as a way of distinguishing their products. Although new graphics controllers and active-matrix liquid-crystal displays offer the potential for a host of improved video and multimedia functions, a key component of the display subsystem is also necessary to support these capabilities: the display signal driver. A family of low-power 6-bit column drivers developed by Cirrus Logic provide this key, employing a new architecture that cost effectively delivers the performance required for superior display quality.

Displays in the Spotlight

What makes one portable computer different from another? In the past, some portables had faster, more powerful CPUs. This was due in part to Intel's strategy for allocating its supply of high-performance microprocessors. Intel favored certain computer makers, enabling them to receive these new processors well before others and create portable computers that stood out from the crowd based on their CPU performance. Today, however, all computer makers are receiving Intel's latest CPUs, including the new 3.3-volt Pentium and 486DX-4 processors, at the same time, and as a result, no vendor has a clear advantage here.

Graphics performance offered another avenue for differentiation. The emergence of the VESA local bus and accelerated graphics controllers for a time enabled portable computer makers to leapfrog over each other in this area. The evolution from no local bus to a 16-bit and then a 32-bit local bus, combined with the initial scarcity of accelerated graphics controllers, set the landscape for this competition in graphics performance. But now this technology has matured, and portable computers with 32-bit local bus and accelerated graphics controllers have become commonplace. Today the distinctions between portable computers in terms of graphics performance are rapidly diminishing.

Now portable computer makers are turning their attention to the display subsystem to provide the next-generation portable PCs with their most significant differentiating features. New graphics controllers are touting a variety of multimedia features, including 6- and 8-bit true-color capabilities and full-motion video. But computer makers first need a display that can support the capabilities of these new graphics controllers. Although today's active matrix color LCDs are able to handle these features, the signal drivers surrounding the display cannot.

Next-Generation Portable Color Displays

Portable PC makers are demanding a new generation of active-matrix LCD-based displays, combining new levels of functionality, features and form. With portable computers increasingly being used as desktop replacements, portable PC makers are looking to achieve the same CRT-like display quality desktop users now achieve with their monitors.

Most of all, CRT-like display quality means achieving the same color depth available on CRT-based displays on the active-matrix LCD. But while the standard VGA palette comprises 256K colors, of which 256 can be displayed simultaneously, the current active-matrix LCD palette is limited to only 512 or 4096 colors.

CRT-like display quality also means going beyond the current 640-by-480 pixel VGA display resolution to 800-by-600 and even 1024-by-768 resolutions. But portable PC makers want these larger displays without having to increase the overall physical size of the display housing. And many applications will require the simultaneous use of and external CRT monitor, requiring the active-matrix LCD to be driven at CRT refresh rates.

Power is also a concern. Today's active-matrix displays require from 3 to 5 watts, of which 1 to 1.5 watts are required by the drive electronics alone. Portable PC makers want to reduce the total display panel power dissipation to about 3 watts.

Of course, portable PC makers want to achieve all these goals -- greater color depth, higher resolution, physical size limits and reduced power -- within a cost structure that is suitable for their market.

The Signal-Driver Bottleneck

While even today's active-matrix LCDs are capable of delivering all the benefits of the new generation of graphics controllers, a bottleneck exists between the two. This bottleneck is the result of the limitations of today's active-matrix LCD signal drivers. These drivers also place severe limitations on a portable computer display's physical size and power consumption.

LCD signal drivers perform roughly the same function for an LCD as a RAM/DAC does for a CRT display. Under the direction of a display controller, the signal driver holds the digital pixel data from the graphics controller, a row at a time, and converts it to the appropriate analog voltage to achieve the corresponding color (or gray level) for each pixel on the LCD. The number of bits the signal driver can store for each pixel determines the number of colors the LCD can display, while the rate at which the driver can change the voltage to each pixel, and how accurately, determines display quality and how well it can support simultaneous CRT output for a given display size.

First-generation 3- and 4-bit signal drivers were "high-voltage" designs, operating at 15 volts, that delivered reasonable performance, but were just too power hungry and too expensive for mainstream portable PCs. Today's "low-voltage" 3- and 4-bit signal drivers are the predominant products on the market, operating at 3 to 5 volts. While offering a reasonable display quality with reduced power and cost, these signal drivers have serious limitations in color depth, performance and package size.

A 3- or 4-bit signal driver cannot provide CRT-quality output with a true VGA color palette, which requires at least 6 bits per pixel to achieve 256K colors (8 bits for true color). Today's 3- and 4-bit signal drivers also have a maximum pixel rate of about 30 megahertz, placing severe limitations on their ability to operate at CRT refresh rates for larger display sizes, a requirement for supporting simultaneous CRT output. And the maximum data rate drops even further when operating at 3 volts. Finally, the number of outputs and the physical die and package size of current signal drivers limit their ability to support the signal-bank designs necessary to achieve the maximum display-to-housing ratio.

Some 6-bit signal drivers are beginning to emerge, but even these do not meet most of the requirements for next-generation active-matrix displays. While providing a 256K color palette, these signal drivers are still physically large, and do not offer low-cost, low-power, high-performance operation.

The Key to Better Active Matrix Displays

A new signal driver architecture developed by Cirrus Logic, combining its extensive graphics controller experience and SmartAnalog technology, addresses the needs of active-matrix display and portable PC vendors. Utilizing a unique R-string digital-to-analog converter (DAC) and a single, external reference voltage source that significantly reduces the die size, Cirrus Logic's 6-bit signal drivers are able to achieve cost-effective CRT-like display quality for today's active-matrix LCDs and tomorrow's higher-resolution displays.

Ideally, makers of active-matrix LCDs and portable computers would like a signal driver providing:

Of course, they would like to have all these features at the smallest possible premium over current 3- and 4-bit offerings, and from a vendor capable of providing high-volume manufacturing support.

Cirrus Logic's new family of 6-bit drivers approach this ideal. They generate 64 precise analog voltages for each channel with a maximum output error of (12 millivolts, providing superior display quality. The drivers operate at pixel rates of up to 55 MHz (40 MHz at 3.3 volts) and the low-power CMOS process minimizes power consumption for a given pixel load. These high pixel rates combined with small die size enable the use of a single bank of column drivers, maximizing the active display area. The architecture is inherently robust and capable of extension to more channels and more bits per pixel, and the standard 0.8-micron CMOS process allows production to be easily ported to multiple foundries for very high volume production capability.

The small die size achieved through Cirrus Logic's R-string DAC architecture enables 256K-color active-matrix LCDs to be cost-effective enough to become the standard display for portable computers. Unlike traditional signal drivers that employ space-hungry individual high-current operational amplifier buffers on the output to each channel, Cirrus Logic's R-string DAC design utilizes a single, external buffer to provide the drive current for all the signal driver outputs, regardless of the number of drivers used. The external buffer's power consumption is minimized because it needs to supply only nine reference voltages.

Besides saving die space and reducing power consumption, the R-string DAC design provides significantly better channel-to-channel uniformity and higher yields. Nine external reference voltages are applied to each driver, and the single R-string DAC on each driver die converts them into 64 internal reference voltages. All drivers, and hence all channels, share the same nine reference voltages. And because only eight voltages are derived internally (3 bits of resolution) between each pair of external reference voltages, which is much less than the 7 to 8 bits typical IC processes can achieve, manufacturability is significantly enhanced. This in turn increases yields while providing accuracies far greater than the 3-bit resolution of the DAC.

The result is a die size as small as 16.5 by 1.66 micrometers. When provided in a tape carrier package (TCP), an input bonding pad pitch of 425 micrometers and an output bonding pad pitch of just 80 micrometers becomes possible, enabling the TCP height -- and consequently the LCD bezel size -- to be minimized, particularly for single-bank driver configurations.

Cirrus Logic is poised to provide active-matrix LCD vendors and portable PC makers with a family of signal drivers to meet their demanding requirements for cost-effective CRT-like display quality today and in the future. It's unique R-string DAC architecture achieves the highest performance at the lowest cost, while offering the ability to grow as display sizes and color palettes increase. Once again, Cirrus Logic has combined its extensive graphics controller experience and SmartAnalog technology to provide the superior display solution.

# # #