Using LTPS technology, TMD has integrated not only the row and column driver components, but also switching elements that help demux the outputs from the display driver. The active circuits to switch in/out the gate-drive signals--including the drivers themselves--around the periphery of the pixel array consume little more than 2 mm on the long edge of the glass. This amount of glass fits comfortably beneath the plinth of the handset's slider display assembly. Other peripheral circuitry consumes less than a millimeter on the other edges of the glass.
While better than traditional amorphous-silicon transistors, LTPS transistors are still relatively poor compared with standard CMOS. Historically, the transistors integrated into the display have been limited to drivers. These are big, beefy transistors that don't need to be particularly fast. Now, switching and timing control circuits are being integrated, as demonstrated by TMD, Samsung, Sharp and others.
The next logical transistor component to integrate is the display memory. It is becoming increasingly difficult to cram an entire picture's worth of SRAM onto the driver chip. Handset makers have tried to incorporate sufficient memory in display driver ICs to enable a single full-resolution, full-color display. Storing a complete image in a driver IC mounted on the display glass conserves battery power because the refresh information does not need to be transmitted through the hinge or slider assembly from the display controller, which normally resides in the main body of the handset. This is practical up to about QVGA resolution. At WVGA resolution, 480 x 800 x 18 bits, or about 6.6 Mbits of SRAM, are required.
Like the increased number of driver outputs, this requirement pushes up the driver's die area. We have not yet seen display driver ICs break into the 90-nanometer process node, partly because the output driver voltages and currents are still esoteric in that geometry. Assuming it's just around the corner, integrating this amount of display memory into a WVGA driver at 90-nm technology will require about 15 mm2, if traditional 6T SRAM cells are used.
One trade-off we expect is lower color depth (or perhaps resolution) for wallpaper and standby images. Semiconductor Insights counted 992,340 SRAM bit cells on the driver IC in the W52T mobile handset. The cell size was about 3.8 microns2, indicating a lithography node in the 0.15- to 0.18-meter region. Far short of 6.6 Mbits, this memory is perhaps sufficient for low-power operational modes.
At FPD International 2006, Sanyo Epson, now Epson Imaging Devices, launched LTPS displays with in-pixel SRAM bit cells. The advantage of integrating SRAM into the display is that there is a lot of room. There are limitations to the technology, however. Epson Imaging Device's display uses only one memory bit cell for each subpixel, with the memory cells being situated in the pixels. The result is a simple on/off state stored for each subpixel, resulting in eight colors.
One of the challenges with integrating display memory into the display glass is that the memory stores digital color information, while the pixel transistor requires an analog voltage. Putting memory cells on the glass will require binary weighted reference voltages to be summed on the glass prior to delivering that summed voltage to the pixel gate.
(Click on image to enlarge)
Off-the-shelf display drivers, with three source drivers for each column, a gate driver for each row and a complete screen's worth of memory with 18 bits of color information per pixel, will become less common. In the future, careful design partitioning between the driver and the glass will optimize multiplexed outputs, display memory and drive circuitry according to the unique requirements of the display subassembly.
Rob Hilkes is technology manager for analog/mixed signal at analyst firm Semiconductor Insights (Kanata, Ontario). Hilkes holds a bachelor's of applied science degree in electrical engineering from the University of Waterloo.
Related Articles:
Boosting LCD brightness for mobile video without increasing power
New realities for display drivers
Toshiba unveils advanced displays
