Method of driving an LCD employing an active matrix with short pulses for gray scale

• US 5,543,947 A
• Filed: 05/10/1994
• Issued: 08/06/1996
• Est. Priority Date: 05/21/1991
• Status: Expired due to Term

• Current Assignee: Semiconductor Energy Laboratory Co., Ltd.
• Original Assignee: Semiconductor Energy Laboratory Co., Ltd.
• Inventors: Hiroki, Masaaki, Mase, Akira, Takemura, Yasuhiko, Yamazaki, Shunpei
• Primary Examiner(s): NOT, DEFINED
• Assistant Examiner(s): Parker, Kenneth
• Application Number:
  US08/240,499
• Time in Patent Office: 819 Days
• Field of Search: 359/55, 359/57, 359/58, 359/59, 359/84, 359/56, 340/784, 345/97, 345/98, 345/148, 6/89
• US Class Current: 345/92
• CPC Class Codes: G02F 1/13624 having more than one switching element per pixel
• CPC Class Codes: G02F 1/136245 having complementary transistors
• CPC Class Codes: G09G 2300/0809 Several active elements per pixel in active matrix panels
• CPC Class Codes: G09G 2300/0823 used to establish symmetry in driving, e.g. with polarity inversion
• CPC Class Codes: G09G 2310/0251 Precharge or discharge of pixel before applying new pixel voltage
• CPC Class Codes: G09G 2330/08 Fault-tolerant or redundant circuits, or circuits in which repair of defects is prepared
• CPC Class Codes: G09G 3/2014 by modulation of the duration of a single pulse during which the logic level remains constant
• CPC Class Codes: G09G 3/3648 using an active matrix G09G3/367 - G09G3/3696 take precedence
• CPC Class Codes: G09G 3/3651 using multistable liquid crystals, e.g. ferroelectric liquid crystals

Abstract

Method of driving a liquid crystal electro-optical device in such a way that the device produces accurate gray tones with high reproducibility. In this device, a complementary TFT (thin film transistor) configuration consisting of at least one p-channel TFT and at least one n-channel TFT is formed per pixel electrode. One of the input/output terminals of this configuration is connected to the pixel electrode. The other is connected to a first signal line. All the gate electrodes of the TFTs of the configuration are connected to a second signal line. High-speed pulses are applied to one of the input/output terminals of the TFT configuration via the first signal line. A high-speed bipolar signal is applied to the gate electrodes in synchronism with the pulses, to electrically charge the pixel. After the completion of the charging, the bipolar signal is again applied to the gate electrodes to discharge the pixel. This series of operations is repeated to produce halftones.

Claims

1. A method of driving an active matrix type liquid crystal display device having a liquid crystal layer, a plurality of pixel electrodes arranged in the form of matrix, each of which electrodes is provided with a switching element for applying an electric field to said liquid crystal layer, said method comprising the steps of:

scanning said switching elements with scan signals a predetermined number of times within a unit time;

supplying data signals to predetermined ones of said switching elements during said scanning in order to apply an ON electric field thereto in the form of pulses to said liquid crystal layer through associated ones of said pixel electrodes,

wherein the duration of each of said pulses is short enough so that liquid crystal molecules in said liquid crystal layer can not completely follow the applied pulses, and the number of data signals supplied to the switching element in the unit time is a function of the desired tone of an image to be exhibited by said liquid crystal device.

2. The method of claim 1 wherein a gradational display can be exhibited by said device with said predetermined number of data signals.

3. The method of claim 1 wherein each of said switching element has a gate, an input, and an output where said gate is connected to said scan signals, said input is connected to a data signals and said output is connected to an associated one of said pixel electrodes.

4. A method of driving an active type liquid crystal device having a liquid crystal layer, a plurality of pixel electrodes arranged in the form of a matrix, each of which is provided with a switching element, a plurality of row lines connected to a gate of the corresponding switching element, and a plurality of column lines connected to an input terminal of the corresponding switching element, said method comprising the steps of:

(a) scanning said row lines in sequence with a row select signal;

(b) applying a date signal to said column lines in synchronization with the scanning of said row lines in order that an 0N voltage is applied to said pixels; and

(c) repeating steps (a) and (b) N times where N is an integer;

wherein the number of applications of said data signal during step (c) is varied from 0 to N with the applied ON voltage having a constant duration and a constant amplitude.

6. The method of claim 4 further comprising the step of removing said ON voltage applied to said pixel electrodes before each scanning the corresponding row lines, by scanning said corresponding row lines with another select signal during which said data signal is not applied to the corresponding column line.

5. A method of driving an active type liquid crystal device having a liquid crystal layer, a plurality of pixel electrodes arranged in the form of a matrix, each of which is provided with a switching element, a plurality of row lines connected to a gate of the corresponding switching element, and a plurality of column lines connected to an input terminal of the corresponding switching element, said method comprising the steps of:

(a) scanning said row lines in sequence with a row select signal;

(b) applying a data signal to said column lines in synchronization with the scanning of said row lines in order than an ON voltage is applied to said pixels; and

(c) repeating steps (a) and (b) N times where N is an integer and wherein the number of times said data signal is applied during step (c) is varied from O to N depending upon the desired tone of an image to be established by said liquid crystal device;

wherein a duration of the application of said ON voltage at one scanning is short enough so that liquid crystal molecules in said liquid crystal layer cannot completely follow the applied voltage.

7. A method of driving an active type liquid crystal device having a liquid crystal layer, a plurality of pixel electrodes arranged in the form of a matrix, each of which is provided with a switching element, a plurality of row lines connected to a gate of the corresponding switching element, and a plurality of column lines connected to an input terminal of the corresponding switching element, said method comprising the steps of:

(a) scanning said row lines in sequence with a row select signal;

(b) applying a data signal to said column lines in synchronization with the scanning of said row lines in order that an ON voltage is applied to said pixel electrode; and

(c) repeating steps (a) and (b) N times where N is an integer,

wherein a total duration of applications of said ON voltage during step (c) is monotonously increased with increasing the number of applications of said data signal during step (c).

8. A method of driving an active type liquid crystal device having a liquid crystal layer, a plurality of pixel electrodes arranged in the form of a matrix, each of which is provided with a switching element, a plurality of row lines connected to a gate of corresponding switching element, and a plurality of column lines connected to an input terminal of the corresponding switching element, said method comprising the steps of:

(a) scanning said row fines in sequence with a row select signal comprising a bipolar pulse; and

(b) applying a data signal to said column lines in synchronization with the scanning of said row lines in order that an ON voltage having a single polarity is applied to said pixel electrodes.