Computer graphics processing and selective visual display system – Display driving control circuitry
Reexamination Certificate
1998-12-21
2002-06-18
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Display driving control circuitry
C345S087000, C345S090000, C345S098000, C345S100000, C345S211000, C345S212000, C345S213000, C345S099000
Reexamination Certificate
active
06407732
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to drive circuits and specifically to low power drivers for liquid crystal display technologies.
BACKGROUND OF THE INVENTION
The demand for Liquid Crystal Displays (LCD) continues to exceed supply. LCD's are implemented as screens on almost all types of digital devices, including watches, personal computers, video monitors, portable computers (e.g., laptops, notebooks, handheld, palm) and projection displays. The size of the display area has steadily grown while general performance of LCD's has steadily improved in the last years. But an important issue is the power dissipation of the growing LCD's.
Users are steadily looking for increased display size and higher resolution. Enhancing both of these features, however, consumes more and more energy. New designs for portable digital devices, in particular, are aiming at lowering the power dissipation of every component and therefore increasing battery life.
Among the different factors contributing to the power dissipation of an LCD are the background illumination and the signal or image information transfer. The background illumination can be completely eliminated in applications where the natural incident light can be used so that the LCD operates in a reflection mode. In one aspect, this invention relates to the power reduction related to the signal or image information. This signal information transfer is related to the charging and discharging of a matrix of capacitive LC-pixels.
The most popular and most widely used LCD's are based on Twisted Nematic, Super Twisted Nematics and Cholesterics. Displays fabricated with these kinds of LCD-materials operate with polarizers and analyzers, hence restricting the use of back light free operation. This induces optical losses such that more power is needed for the back light illumination or higher levels of natural incident light are required.
More recently much effort has been spent in the development of Polymer Dispersed LCD's as described by Ikeno et al., “A 23-cm Diagonal Bright Reflective Guest-Host TFT-LCD”, SID 1995 Digest, pp. 333-336. These Polymer Dispersed LCD's do not use polarizers, thereby saving back light power or allowing a lower level of natural light illumination. Unfortunately, the driving voltages for the Polymer Dispersed LCD pixels are higher and therefore any energy saved from lower power back light illumination is lost. The present invention can drastically lower the power dissipated when driving the pixels even at extended voltage levels, such that eventually the LCD consumes less energy.
Several methods, or addressing schemes, have been developed for sending signals or image information to LCD's. The three most important are : direct addressing, and passive and active matrix addressing. Direct addressing, usually used in watches and calculators, is great for simple alphanumeric characters, since one signal controls one segment of pixels. However, direct addressing is unrealistic for larger systems because of the large number of wires that need to be interfaced.
In a matrix system, the number of wires can be greatly reduced by splitting up the display into a grid of wires called rows and columns, with a pixel at the intersection of each row and column. Matrix displays can be grouped into two categories, passive matrix liquid crystal displays (PMLCD) and active matrix liquid crystal displays (AMLCD).
A PMLCD is the simplest display for achieving low power, low cost and small size. In a PMLCD, only a LC-pixel is located at the intersection of each column and row. PMLCD's have, in general, less performance than the AMLCD's but are much simpler to fabricate and therefore preferred for smaller, less accurate displays. In an AMLCD, an extra nonlinear element is introduced at each pixel location to enhance the nonlinear behavior (i.e., contrast) of each pixel. This extra nonlinear element can be a two-terminal device or a three-terminal device. The number of terminals at the pixel location influences the driving scheme.
The trend toward larger, higher definition displays in notebook computers is forcing display manufacturers to seek new electrical drive methods for the integrated circuit that drives the LCD. Current methods for driving the electrical signals onto these displays have been proposed to address significant issues with power dissipation and image quality.
For example, Erhart et al. (“Charge-Conservation Implementation in an Ultra-Low-Power AMLCD Column Driver Utilizing Pixel Inversion”, SID 1997 Digest, pp. 23-26) implemented a capacitively based energy recovery method for AMLCD displays. At the beginning of each row time, the column busses are shorted together to a supplemental capacitor, which naturally maintains a potential halfway between average upper and average lower voltage. The maximum power saving of this method is limited to 50%.
Okumura et al. (“Multifield driving method for reducing LCD Power dissipation”, SID 1995 Digest, pp. 249-252) proposed a multi-field driving method for reducing LCD power dissipation. In this method, the image refresh rate is lowered without flicker occurrence by dividing the field image into an odd number of interlaced sub-field images. One sub-field flicker is compensated by the other sub-field flickered images. The power reduction is here limited to 30%.
In another proposal formulated by Sakamoto et al. (“Half-Column-Line driver method for Low-Power and Low-Cost TFT-LCDs”, SID 1997 Digest, pp. 387-390), the number of column drivers is halved and the number of row drivers doubled. This technique can lead again to a power reduction of 50%.
The driving power of the LCD's schemes for two terminal devices has been improved by increasing the number of voltage levels applied to the select line as outlined by R. A. Hartman (“Two-Terminal Devices Technologies for AMLCDs”, SID 1995 Digest, pp. 7-9). The excellent image quality demands higher power dissipation. The system of the present invention is compatible with these improved schemes but further reduces the power dissipation.
In some cases, panel manufacturers are returning to direct drive displays. Direct drive refers to the ability of the column driver chips to “directly” provide the alternating voltage and the variable magnitude. See, for example, Erhart et al. (“Charge-Conservation Implementation in an Ultra-Low-Power AMLCD Column Driver Utilizing Pixel Inversion”, SID 1997 Digest, pp. 23-26). This early drive technique had been abandoned by many of the major LCD manufacturers due to cost concerns and replaced by common backplane node driving. :Although direct drive requires higher voltage driver circuits, substantial power dissipation and image quality improvement could be reached compared to traditional drive methods. The complementary driving schemes, direct drive and common backplane node, can both benefit from the driving circuit and method described herein. But even the prior art methods proposed to date have not provided satisfactory reduction of power dissipation.
The cost of the LCD is partially influenced by the glass quality and the integration possibility of the peripheral driver circuits on the LCD substrate. This is discussed by Stewart et al., “Circuit Design for a-silicon AMLCDs with Integrated Drivers”, SID 1995 Digest, pp. 89-92 and Aoyama et al., “Inverse Staggered poly-Si and Amorphous Si Double Structure Thin Film Transistors and LCD Panels with Peripheral Driver Circuits Integration”, IEEE Trans. Elect. Devices 43(5), pp. 701-705 (1996). Drivers and nonlinear elements integrated on poly-Si substrates feature low resistances but also require expensive high-quality glass resistant to high temperature processing. The technological tendency has been toward laser annealed hydrogenated amorphous silicon (a-Si:H), which features low resistance values and process temperatures and therefore cheaper glass. The invention proposed here can strongly benefit from these technological improvements as explained below.
SUMMARY OF THE INVENTION
In one aspect, t
Kuijk Maarten
Stiens Johan
Hjerpe Richard
Rose Research L.L.C.
Slater & Matsil LLP
Zamani Ali
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