Semiconductor Insights recently looked at the Aquos QVGA screen in the Sharp 905SH video phone. The screen puts all the drive circuitry in the display glass and therefore uses no driver chips. By integrating the driver ICs onto the LCD glass substrate, Sharp has reduced costs and probably improved reliability. Reviews of the Aquos screen in Sharp phones have been highly favorable. The on-glass row and column drive transistors are situated around two of the edges of the display. Like the Renesas and NEC QVGA display driver ICs, this display supports 262,144 colors.
Typically, 0.25-micron transistors exhibit about 400 microamps on current per micron of gate width for NMOS transistors and about half that for PMOS. At 0.18 µm, this is improved to about 650 µA/µm for NMOS and 300 µA/µm for PMOS. By comparison, a-Si TFTs have historically exhibited extremely poor ID characteristics.
The driver transistors on Sharp's LTPS Aquos LCD screen show a very respectable normalized drive current, of about 160 µA/µm for NMOS and 240 µA/µm for PMOS. This is still not on par with standard CMOS but is certainly good enough to integrate the drive transistors.
It appears that LTPS technology is real, and that some adjustments in the market for monolithic display drivers cannot be too far in the future. So who will manufacture tomorrow's display drivers? Certainly, leaders like Samsung, NEC and Sharp are well-positioned.
Before they became commoditized, display driver ICs competed on pixel count and color depth. Now chip companies that make display driver ICs will once again need to differentiate themselves in ways that compel equipment makers not to purchase displays with integrated drive circuits. One area to watch for is the integration of LED backlighting control functionality into the driver IC. The road ahead will certainly be highly competitive.
With display data rates in QVGA mobile phones approaching 100 Mbits/ second, there are several initiatives to reduce the amount of interconnect between display controllers and driver ICs. A narrower interface has the benefit of enabling thinner screen bezels. Also, clamshell phone design is simplified because fewer connections must be routed through the hinge between the phone body and display. The main objective however, is cost reduction. All of the new intrapanel interconnects are designed to send more bits (pixels and colors) over a narrower interface, thus reducing the number of expensive interconnections in the final product.
Display architectures
The leading display format for high-end mobile devices is 262k-color QVGA (240 RGB x 320 pixels). In April, NEC launched its µPD160290 240RGB source driver, with a whopping 68 billion colors (3 x 12 bits). The driver uses an interface called point-to-point mini-LVDS (PPmL), which has also been embraced by Texas Instruments and Thine Electronics for their timing controller chips. PPmL targets large, high-definition panels, while NEC's other interconnect initiative, current-mode average differential signaling (CMADS), targets handsets.
The NEC announcement follows February's announcement by Qualcomm that many of its CDMA chip sets will incorporate the Mobile Display Digital Interface (MDDI). Seiko-Epson, Toshiba and Renesas have hitched themselves to that wagon.
Both NEC and Qualcomm are responding, of course, to National Semiconductor, which appears to have achieved good traction with the point-to-point differential signaling (PPDS) scheme it launched last October. The scheme has since been licensed by ST Microelectronics, Magnachip, Sharp and HiMax.
And don't forget Solomon Systech's August announcement that it had launched its first products with the mini-RGB interface.
The winning display driver company will likely be the one that comes up with a multilingual interface, since the offerings all appear to be based on some variant of differential low-voltage differential signaling (LVDS). Time will tell.
By Rob Hilkes (robh@semiconductor.com), lead technology analyst, Semiconductor Insights (Kanata, Ontario)
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