Display device including a correction circuit for the input signals

Abstract: The response rate of a liquid crystal display device is increased in that capacitive variations in the liquid crystal mixture caused by a different drive voltage or varying capacitances in drive transistors are taken into account in advance. If necessary, the required corrections are performed with a microprocessor, but they are preferably stored in advance in a look-up table.

Description: The invention relates to a display device according to the introductory part of claim 1. A display device of the type described in the opening paragraph is generally known as an active driven display device. In liquid crystal display devices the capacitance associated with a pixel may vary with the drive voltage; this may detrimentally influence the response time. Another source which leads to an erroneous first adjustment and hence a delayed response in such a picture display device using active drive is the so-called DC offset voltage which occurs when using the drive mode using thin-film transistors. One of the objects of the present invention is to eliminate the above-mentioned drawbacks as much as possible. It is based, inter alia, on the recognition that variations of the voltage across a pixel caused by (voltage-dependent) capacitances in the switch can be taken into account in advance. To this end a display device according to the invention is characterized according to the characterizing part of claim 1. In this way the pixels are submitted to a pre-adapted drive voltage which at least substantially prevents the above-described delay. The use of a look-up table for controlling drive signals is known per se from GB 2.207.272A. In this application temperature variations are taken into account. EP 0 196 889 describes the correction of drive voltages in order to compensate for varying pixel capacitances. During selection of a pixel an offset voltage which is also defined by the capacitance of the pixel can be generated across the pixel with a value of: V off = V R C X C X +C LC (V R : amplitude of selection pulse during falling edge, C X : capacitance of drive element, i.e. the gate-drain capacitance of a thin-film transistor, C LC : voltage-dependent capacitance of the liquid crystal). As a result, the voltage across the pixels acquires a value which differs from the externally applied signal voltage. Since both C X and C LC may be voltage-dependent, a correction can be defined for the drive voltage of a pixel. This correction can be performed for one of the capacitances C X , C LC separately, or combined for both. The correction stated above can be performed, for example, directly by means of a microprocessor, but this is usually rather cumbersome. Therefore, a look-up table is used. A RAM, ROM or microprocessor may be present as a separate unit, but it may alternatively form part of a larger memory or drive system which is already present for, for example, signal processing. The invention will now be described in greater detail with reference to some embodiments and the drawings in which Fig. 1 shows diagrammatically a display device according to the invention; Fig. 2 shows the drive of a pixel via a thin-film transistor and Fig. 3 shows the associated drive signals and voltages across the pixel. The Figures are diagrammatic; corresponding components are generally denoted by the same reference numerals. The display device of Fig. 1 comprises a plurality of pixels 4, for example, liquid crystal pixels arranged in rows and columns. These pixels are driven via switching elements 5, in this example, TFT's and are arranged in a matrix configuration. Information present at the column electrodes 3 is presented to the pixels 4 by successively selecting (energizing) row electrodes 2. Row electrodes 2 are successively selected by means of, for example, a shift register 6, while the information to be presented for a selected row of pixels is stored in a register 7. An incoming video signal 10 may be directly connected to the register 7 for this purpose. The voltages at the column electrodes 3 are then equal to the presented video voltages for each pixel. Dependent on the drive mode, the switching elements 5 used in the matrix (TFTs), the column voltages and the selection voltages at the row electrodes 2, which voltages originate from the shift register 6, a pixel 4 is subjected to a voltage V I during selection. The liquid crystalline material which is used for the pixels has a given voltage-dependent dielectric constant. The capacitance of a pixel is therefore voltage-dependent and a given capacitance C I is associated with the voltage V I . In Fig. 1 the incoming video signal 10 is converted by means of an A/D converter 11 into digital signals of, for example, 8 bits. Fig. 2 shows diagrammatically a pixel 4 which is driven by a thin-film transistor 25 and which forms part of a display device arranged in a matrix configuration comparable with that of Fig. 1. A row electrode 2 is connected to the gate electrode 26 of the transistor 25, while the column electrode 3 is connected to the source contact 27. The drain contact 28 is connected to the pixel 4 which has a voltage-dependent capacitance (C LC ). The capacitance 29 represents a capacitance C X associated with the transistor 25 (channel capacitance, gate-drain capacitance). Due to capacitive coupling this capacitance produces an offset voltage across the pixel with a value of: V off = V R .C X C X +C LC at the falling edge of a selection pulse 30 (Fig. 3a) on the row electrode 2. (V R : amplitude selection pulse, falling edge). Since C LC is voltage-dependent (and is thus a function of the voltage across the pixel), V off is also voltage-dependent. A high capacitance C LC leads, for example, to a response on the pixel as is illustrated by means of curve a in Fig. 4b, whereas a lower value gives rise to curve b. The voltage drop V off across the pixel can be compensated again by employing a correction compensating for this voltage drop, dependent on the applied drive voltages. To this end the external signal 10 is applied to an A/D converter 11 (Fig. 1). It addresses a look-up table 20 whose output supplies a (digitized) corrected voltage value and which, if desired, also is corrected for voltage dependency of C X . A correction voltage 31 is obtained via a D/A converter 21. The normally processed signal 32 from the processor 34 is added to the correction voltage by means of the circuit 33 which applies the correct voltage via shift register 7 to the column electrodes 3. The look-up table 20, which comprises, for example, a ROM or RAM, is programmed in such a way that a corrected drive value defined by the above-mentioned formula is passed on (in a digital form) to the D/A converter 21. The corrected column voltages converted to analog values are then also loaded into the register 7. The drive circuit 22, which receives a synchronizing signal 23, ensures the mutual synchronization of the different switches, registers, memories, etc. via drive lines 24.

Claims: A display device (1) comprising an electro-optical medium between two supporting plates, provided with a system of pixels (4) arranged in rows and columns, means for providing selection voltages to row electrodes (2) to select thin film transistors (25) and means or providing column voltages to column electrodes (3), each thin-film transistor (25) having a gate electrode (26) connected to a row electrode, a source contact (27) connected to a column electrode, and a drain contact (28) connected to the pixel, wherein the display device is provided with correction means (11,20,21) for correcting external signals for off-set voltages due to capacitive effects, which correction means comprise an A/D converter (11) for converting the incoming signal into digital signals addressing a look-up table (20), the look-up table being arranged for supplying at its output a corrected digital signal for addressing a D/A converter (21) for providing a correction voltage for a pixel, the correction voltage being determined in the look-up table by (V R .C x )/(C x + C LC ), in which V R is the amplitude of a selection pulse during falling edge, C x is the capacitance of the drive element of the pixel and C LC is the capacitance of the electro-optical medium. A display device according to Claim 1, wherein the correction means determine the correction voltage, dependent on the voltage dependent capacitance of the electro-optical medium. A display device according to Claim 1 or 2, wherein the correction means determine the correction voltage, dependent on the voltage dependent capacitance of the drive element of the pixel.

Inventors: HARTMAN ROBERT ARNOLD, KNAPP ALAN GEORGE, HARTMAN, ROBERT ARNOLD, KNAPP, ALAN GEORGE, Hartman, Robert Arnold, Int. Octrooibureau B.V., Knapp, Alan George, Int. Octrooibureau B.V.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1991
Main classification: G09G3
IPC codes: G09G3