MATRIX DISPLAY DEVICE

Abstract: Power consumption in driving ICs for electro-optical devices is reduced by driving all pixels in a frame to one extreme state and then introducing intermediate levels (grey-levels, colours) by multiplexing, using a reduced selection pulse width. In this way the number of level transitions for the extreme states and hence power dissipation is reduced.

Description: MATRIX DISPLAY DEVICE The invention relates to a display device compnsing an electro-optical display matenal between a first substrate provided with row electrodes, and a second substrate provided with column electrodes, in which overlapping parts of the row and column electrodes define picture elements, the device further compnsing dnving means for providing the row electrodes with selection pulses having a selection pulse-width and a selection pulse voltage and for providing the column electrodes with data pulses Such display devices are commonly known as passive displays and are used in e.g. mobile phones and portable computers A general way of dnving these types of displays is known as multiplexing: the RMS-voltage across a picture element, or pixel, determines the light transmission. In passive displays each column electrode as well as each row electrode is common to several pixels. Generally, time-multiplexing is used, m which (subsequent) rows of pixels are selected subsequently dunng a row selection penod, while data-voltages are simultaneously supplied to the column electrodes, dependent on the information to be wntten. After all rows have been selected (one frame time), this is repeated To obtain grey-values (or colours if a birefnngent liquid crystal effect is used, such as ECB or STN), pulses of different pulse-width are used for different grey-values (or colours), which implies switching of the data pulse dunng each selection penod at least once, if data is available. For each switching action, the pixel capacitance has to be loaded or reloaded, which is a major source of current ( power ) consumption in LCD dnving circuitry It is an object of the present invention to overcome the above mentioned problems at least partly A display device according to the invention is therefore charactenzed in that the device compnses means for diminishing dunng operation the selection pulse widths within a frame time based on an extreme pulse width of the data pulses to be applied to the column electrodes within a frame time and further dnving means for applying, in operation, correction voltages across the picture elements dunng said frame time The invention is based on the recognition that said diminishing of pulse widths renders the above-mentioned switching superfluous for the pulses related to the lightest and/or darkest colour or grey-value within a frame. To guarantee the nght colour (grey-value), all pixels are given an extra voltage simultaneously before or after a frame Because this correction depends on the RMS-value to be corrected, either a voltage correction dunng the full frame time can be applied, or a pulse width correction can be applied A first embodiment is charactenzed in that the means for diminishing the selection pulse widths compnse means to dimmish the selection pulse widths by the minimum pulse width of a data pulse withm the frame time. Preferably, the further dnving means compnse means for providing row electrodes simultaneously with a pulse having the minimum data pulse width withm the frame time dunng the remainder of the frame time and means for providing the column electrodes simultaneously with a data-pulse. This guarantees optimum contrast A further embodiment is charactenzed in that the means for diminishing the selection pulse widths compnse means to diminish the selection pulse widths by the minimum difference between the selection pulse width and the width of a data pulse within the frame time Preferably, the further dnving means then compnse, for example, means for providing row electrodes simultaneously with a pulse having a pulse width equal to the minimum difference between the maximum pulse width of a selection pulse and the width of a data pulse withm the frame time dunng the remainder of the frame time and means for providing the column electrodes simultaneously with a non-data pulse This guarantees optimum contrast again These and other aspects of the invention will be elucidated with reference to the embodiments descnbed hereinafter In the drawings Fig. 1 is a diagrammatic cross-section of a part of the display device, together with a diagrammatic representation of the dnve section, Fig 2 is a diagrammatic representation of the display device, while Figs 3 to 8 show diagrammatically a plurality of dnve pulses The Figures are diagrammatic and not to scale Corresponding elements are generally denoted by the same reference numerals. Fig. 1 is a diagrammatic cross-section of a part of a liquid crystal display device compnsing a liquid crystal cell 1 with a twisted nematic liquid crystal matenal 2 which is present between two supporting plates or substrates 3 and 4 of, for example, glass or quartz, provided with selection electrodes 5 and data electrodes 6, respectively, in this embodiment. In this case, the liquid crystal matenal has a positive optical amsotropy and a positive dielectnc anisotropy and a low threshold voltage. If necessary, the device compnses polanzers (not shown) whose polanzation directions are, for example, mutually crossed perpendicularly The device further compnses onentation layers, 7, and 8, which onent the liquid crystal matenal on the inner walls of the substrates in such a way that the twist angle is, for example, 90°. The picture display device is of the passive type. Incoming information 11 is processed if necessary, in the dnve section 10 and stored in a data register 12 and presented to the data electrodes 6 via data signal lines 16 Pixels, here arranged in rows and columns, are selected by successively selecting row electrodes 5 which are connected to a multiplex circuit 14 via row signal lines 15. The lines 17 ensure the mutual synchronization between the multiplex circuit 14 and the data register 12. After all row electrodes have been selected, this selection is repeated, this is effected at the frame frequency Fig 3 shows data signals (Fig 3 a ) for one column and row selection signals (Figs 3 b c d e f )for a passive display device using 1 n multiplexing The rows 1, 2, 3, 4, .., n are successively selected by means of row selection pulses having a pulse width t w and a voltage V s Dunng non-selection, a non-selection voltage ( 0V in this example) is applied. The frame time t F is thus nt w , in a subsequent frame time, the data and row signals are inverted. In this particular example, the display has only five rows so that the frame time is 5t w The bnghtness of a selected pixel is determined by the voltage on the data electrodes 6 which, in this example, switches between two values, a data voltage V d and a non-date voltage V n _, in this example V d and 0 V The pulse width of a data pulse dunng each selection pulse (width t w ) determines the grey-value or colour of the picture element, dependent on the display effect used (pulse width modulation) As can be seen in Figure 3 a , the data voltage switches from 0 V to V d and back dunng each selection time t w , which is at the expense of much energy in the line dnving circuitry As can be seen in Figure 3 a too, the minimum pulse width t d mm of the data voltage pulses occurs in the time period t 2 -t 3 , related (in this example) to the darkest grey-value. According to the invention all selection pulses t w and all data pulses are diminished in width by said amount t d, mm -This is shown in Figure 4, where the selection pulses now have a pulse width t w ' (to'- ti', ti'- 1 2 ', etc.) equal to u - t d . mm • To obtain the right RMS-voltage across a pixel all pixels of the column concerned have to be driven to the ON state ( voltage V d during selection) during the prescribed time within a frame. To this end, all row electrodes get an extra selection pulse having a pulse width t d . m after t 5 ' . In this particular example all rows 1,2, ..5 get this extra pulse simultaneously, but this is not absolutely necessary, provided the pulses are applied within the original frame time I F The resulting frame time can now, however, be chosen as 5t w '+ t dιπun . If necessary, this smaller frame time can be used to drive the display at a higher frequency, thereby reducing flicker. However, the main advantage can be seen in the form of the data voltage (Fig. 4 a ), in which one pulse (the minimum data pulse) has completely disappeared. This will lead to a considerable decrease in switching dissipation in the driving circuitry, especially since these minimum data pulses are generally related to the darkest (or lightest) pixels, which pixels form a background colour or grey-value in most images. In most applications the original frame time, in this example 5t w , is maintained (t F = 5t w ). Instead of reducing the selection pulse width by a value t dιmιn , the reduction may also be based on the maximum data pulse width t d , max . In this case, all row electrodes receive a selection pulse having a pulse width t w " (to"- tj", ti"- 1 2 ", etc.) equal to t d , m ax= t w - t rem -, see Figure 5 , which represents the same pulse pattern as Figure 3. To obtain the right RMS- voltage across a pixel, all pixels of the column concerned have to be driven to the ON state (voltage V d during selection), but also to the OFF-state, during the right time again. To this end all row electrodes get an extra selection pulse having a pulse width t rem after t 5 ", while all columns are driven to the OFF-state by applying a non-data voltage (0 V). The resulting frame time may be reduced to t F = st w "+ t rem (Figures 5,6), although in most applications the original frame time tF = 5t w is maintained again. There are similar advantages as mentioned with respect to the example of Figure 4. Since the RMS-voltage during a frame determines the light transmission of a pixel, a lower voltage may be applied to the columns, during a longer selection time after t 5 ", as is shown in Figure 6 by means of broken lines. The greatest advantage is obtained if both principles are combined. This is shown in Figure 7, in which all row electrodes 7,15 receive a selection pulse having a pulse width t w ' "(to' "- tι " \ ti ' "- 1 2 "\ etc.) equal to t d , m ax - t d .m_n= t w - t rem - d.mm- To obtain the right RMS-voltage across a pixel all pixels of the columns concerned have to be driven to the ON and OFF state again, in this case by means of an extra selection pulse having a pulse width t rem + t . m i n after t 5 '". The resulting frame time may now be reduced to tF"= 5tw'"- t rem - t d . m i n - The actual frame frequency is, however, determined by the actual application again. If the original frame t F = 5t w is maintained the advantages of less dissipation remain. A further reduction of dissipation of the driving circuitry can be obtained by "mirroring". This is shown in Figure 8 for the for the first two data pulses of Figure 7. Shifting of the end of the data pulse of the first selection period (t 0 '"- ti'") towards ti '" cause the data-pulses of two subsequent selections to be combined in one pulse, leading to a reduction of dissipation again, now in the column driver or the column driver part of a display driver. The shifts td, min , trem , by which the selection pulse width during a frame is reduced is determined, for example, by means of a microprocessor, in which all data voltages for a frame are stored, for example, by storing their width as a number of time-slots. Each original pulse width t w is divided into a number of time-slots, for example, 64. The duration of a data pulse is measured, for example by comparing with a running counter and the resulting value is stored in the memory of said microprocessor. After storing the values of each frame, the pulse widths to be applied for said frame are determined and submitted to the driving electronics. On the other hand, the shifts td, m i n > t.em can be derived directly from the minimum and maximum data pulse width by relating the end of the data pulses to values in a counter and using the counter value related to said minimum and maximum pulse width as a means to adopt the pulse widths, for example, by means of multiplexers , shift registers and other logic circuitry. In summary, the invention provides a way of reducing power consumption in driving ICs for electro-optical devices by driving all pixels in a frame to one extreme state and then introducing intermediate levels (grey-levels, colours) by multiplexing using a reduced selection pulse width. In this way, the number of level transitions for the extreme states and hence power dissipation is reduced.

Claims: CLAIMS: 1. A display device comprising an electro-optical display material between a first substrate provided with row electrodes and a second substrate provided with column — electrodes, in which overlapping parts of the row and column electrodes define picture elements, the device further comprising driving means for providing the row electrodes with selection pulses having a selection pulse width and a selection pulse voltage and for providing the column electrodes with data pulses, characterized in that the device comprises means for diminishing in operation the selection pulse widths within a frame time based on an extreme pulse width of the data pulses to be applied to the column electrodes within the frame time and further driving means for applying, in operation, correction voltages across the picture elements during said frame time. 2. A display device as claimed in claim 1 characterized in that the means for diminishing the selection pulse widths comprise means to diminish the selection pulse width by the minimum pulse width of a data pulse within the frame time. 3. A display device as claimed in claim 2 characterized in that the device comprises further driving means comprise means for providing the row electrodes simultaneously with a pulse having the minimum data pulse width within the frame time during the remainder of the frame time and means for simultaneously providing the column electrodes with a data pulse. 4. A display device as claimed in claim 1 or 2 characterized in that the means for diminishing the selection pulse widths comprise means to diminish the selection pulse widths by the minimum difference between the selection pulse width and the width of a data pulse within the frame time. 5. A display device as claimed in claim 4 characterized in that the further driving means comprise means for providing row electrodes simultaneously with a pulse having a pulse width equal to the minimum difference between the maximum pulse width of a selection pulse and the width of a data pulse within the frame time during the remainder of the frame time and means for simultaneously providing the column electrodes with a non-data pulse. 6. A display device as claimed in claim 2 or claim 4 characterized in that the device comprises means for causing data-pulses of two subsequent selections to be combined in one pulse.

Inventors: PLANGGER GUIDO, PLANGGER, GUIDO
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1999
Main classification: G09G3
IPC codes: G02F1, G09G3