Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-12-21
2002-06-25
Wu, Xiao (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S205000
Reexamination Certificate
active
06411273
ABSTRACT:
TECHNICAL FIELD
This invention relates to a driver circuit for an active matrix liquid crystal display comprising a resistive dividing type digital to analog converter circuit.
BACKGROUND ART
A driver circuit for an active matrix liquid crystal display capable of displaying multi-scale gray images or full-color images generally comprises a digital-to-analog converter circuit (DAC) so that analog video signals are outputted. A capacitor based DAC is well known in the art as one type of such a DAC. However, such a capacitor based DAC has a drawback in that a linear output voltage characteristic cannot be obtained easily when such a capacitor based DAC is employed in constructing a driver circuit for a liquid crystal display (LCD). In view of this drawback, a resistive dividing type DAC using resistance elements has also been employed for an LCD driver circuit.
Among such resistive dividing type DACs, some types are constructed utilizing resistance elements and switching elements, both having an individual component form, but many types are constructed within a single crystalline silicon integrated circuit (IC) and formed in a chip form. Such a driver IC has been used for conventional LCD driver circuits. Specifically, in those driver circuits having resistive dividing type DACs, driver ICs are attached onto an array substrate by a tape automated bonding or by directly mounting onto the array substrate.
However, such an LCD driver circuit has drawbacks as described in the following.
(1) A driver IC is essential as a component of an LCD, and therefore the component cost is high.
(2) A step of mounting the driver IC onto an array substrate is inevitably required.
(3) The thickness of an LCD is increased corresponding to the thickness of the driver IC, and in addition, the driver IC requires a large area in the array substrate. These have been the major obstacles in the attempts to reduce the physical sizes and thickness of LCDs.
(4) Furthermore, in conventional driver circuits utilizing a crystalline silicon, a silicon in which an n-type or p-type impurity of approximately 10
16
/cm
3
is doped is typically employed for resistance elements that constitute DACs in the driver circuits. A resistance value of these resistance elements must be controlled with extremely high precision by controlling a concentration of these impurities so as to suppress an output variation among these DAC chips. As a result, in order to produce a chip with an extremely precise resistance value, an ion implantation method should be employed in doping an n-type or p-type impurity in a crystalline silicon. However, by this method, it is extremely difficult to suppress a variation of the resistance value within a predetermined range when a large chip size or a large number of the chip is required, and moreover a throughput until completing the driver circuit is low.
In addition to the above drawbacks, conventional LCD driver circuits have such drawbacks as described below, in view of reducing their power consumption.
A resistive dividing type DAC is a well-known circuit, and while some of the DACs are constructed utilizing the resistance elements and switching elements having an individual component form, many of the DACs commercially available have a chip form incorporated within a single crystalline silicon (c-Si) IC. Recently, there have been developed techniques intended to incorporate an LCD driver circuit including such DACs on a glass substrate by forming thin-film transistors (herein after referred as ‘TFTs’) utilizing poly-silicon (p-Si). However, because p-Si TFTs are inferior to c-Si transistors' in their performance and thereby in the power efficiency in the circuits, the effective reduction in power consumption has been difficult for such an LCD driver circuit incorporated on the grass substrate, although such a circuit has certain advantages such as low cost and small sizes owing to the fact that they are capable of eliminating driver ICs. In conventional LCD driver circuits, a waste of power consumption is noted particularly during a writing period for source lines and pixel electrodes. The discussion will now focus on this account. Generally speaking, a large amount of capacitive load is connected to a source line because a certain capacitance is generated in each intersection of a source line and gate line, or in each gap between a source line and counter electrode. Therefore, when a driving voltage is outputted from a driver circuit to a source line, a potential of the source line does not reach a required voltage for driving liquid crystals immediately after the driving voltage is outputted, and a certain amount of time is required until a desired voltage is obtained. After this amount of time, i.e., a writing period to a source line elapses, a gate scanning pulse is outputted to a pixel transistor, and thereby a potential of a pixel electrode reaches a desired voltage. Or, a gate scanning pulse is outputted almost simultaneously with an output of a driving voltage, and a potential of a pixel electrode reaches a desired voltage corresponding to the change of a potential of a source line. A writing to a pixel electrode is thus completed according to either of the above described manners. Therefore, there is essentially no need to continue applying a predetermined driving voltage to a source line. Nevertheless, in conventional driving methods, such a writing period to a source line or a pixel electrode has been made equivalent to one horizontal synchronizing period. This is because the writing to a source line or a pixel electrode has been controlled by a horizontal synchronizing signal. As a result, in prior arts, the driver circuit continues a normal operation for applying a driving voltage even during the period in which there is no need to keep applying a predetermined driving voltage to a source line. This has been a major drawback from the viewpoint of reduction in power consumption.
In view of the above-described disadvantages in prior arts, it is therefore an object of the present invention to provide a driver circuit for an active matrix liquid crystal display in which, by eliminating a driver IC from a component of an LCD, the component cost is reduced, the manufacturing steps are simplified, and moreover the reduction of sizes and thickness is achieved.
It is another object of the present invention to provide a driver circuit for an active matrix liquid crystal display in which the reduction of power consumption is achieved by reducing a current consumption of the digital-analog converter circuits therein during a period excluding a normal operation period.
DISCLOSURE OF THE INVENTION
In accordance with the first aspect of the invention, there is provided a driver circuit for an active matrix liquid crystal display formed on an array substrate of the liquid crystal display comprising:
a resistive dividing type digital-to-analog converter circuit (DAC), comprising a plurality of resistance elements and a plurality of switches related to the resistance elements;
the driver circuit constructed so that an output signal from the DAC is outputted as a driving voltage for a liquid crystal display portion of the liquid crystal display via a current amplifier element having an impedance conversion function wherein a voltage amplification ratio of the current amplifier element is 1;
the driver circuit characterized in that the resistance elements are composed of an impurity-containing semiconductor layer formed on the array substrate.
According to the above-mentioned construction, it is made possible to reduce the component cost of an LCD since the driver circuit is formed on the array substrate without using driver ICs as a component for the LCD. Moreover, it is also made possible to reduce the manufacturing cost since the step of mounting the driver ICs onto the substrate is made unnecessary. In addition, the reduction in the thickness and sizes of an LCD can be achieved easily. In particular, the above-mentioned semiconductor layer is formed simultaneously with a step of forming pix
Adachi Katsumi
Izuchi Masumi
Kaneko Naomi
Nakamura Mika
Nanno Yutaka
Parkhurst & Wendel L.L.P.
Wu Xiao
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