WO2004034367A1

WO2004034367A1 - Driving arragement for dap liquid crystal devices

Info

Publication number: WO2004034367A1
Application number: PCT/GB2003/004302
Authority: WO
Inventors: Paul William Herbert Surguy
Original assignee: Central Research Laboratories Limited
Priority date: 2002-10-10
Filing date: 2003-10-06
Publication date: 2004-04-22
Also published as: GB0223556D0

Abstract

A driving arrangement for a liquid crystal device including a dyed DAP (Deformation of Aligned Phase) liquid crystal material (2); the driving arrangement including driving means (7, 8) for applying driving voltages (9a, 9b, 10a, 10b) to said material (2) to effect optical switching thereof; the driving means including generating means (7) for generating said voltages in accordance with a cyclically repetitive waveform of predetermined frequency; the waveform comprising a combination of pulsed voltage portions including, in each cycle, at least one portion of relatively high voltage (9a) and at least one portion of relatively low voltage (10a); the relative voltages of the said respective portions and their relative durations and/or numbers being arranged such that the power consumption of the device is reduced as compared with the power consumption associated with conventional driving square waveforms of around 1kHz frequency and +/-5 volts amplitude.

Description

DRIVING SIGNALS FOR DAP LIQUID CRYSTAL DEVICES

This invention relates to driving arrangements for DAP (Deformation of Aligned Phase) liquid crystal devices, and particularly to such arrangements for dyed DAP devices.

Dyed DAP devices, which have been known for some 30 years, typically utilise a dye dissolved in a nematic liquid crystal contained in a cell about 5micrometers thick, and the liquid crystal is typically doped to achieve a pitch length of the order of 10 micrometers. The performance of early DAP devices however was poor and interest has returned to the technology only in recent years with the advent of improved dyes and liquid crystal materials.

Even when advantage is taken of such improvements, however, there remain difficulties associated with the use of DAP devices; a principal difficulty being the requirement for relatively high driving voltages (typically +/- 5 volts or more) to achieve switching at speeds compatible with video rates, and to achieve reasonable contrast. Since the principal application for DAP technology lies in low-power portable devices, such as mobile telephones, the power consumption associated with the use of such relatively high driving voltages presents a problem.

DAP devices are typically driven with square waveforms at about 1kHz and with voltage amplitudes, as aforesaid, of around +/- 5 or more; the power consumption

2 depending upon V . If the voltage is merely reduced, however, to reduce the power consumption, the contrast of the display is reduced and the switching speed is compromised, in particular because the turn-on time of the device is increased.

The invention aims to address the aforementioned problem by permitting the power consumption of dyed DAP devices to be reduced without substantial compromise in either contrast or switching speed. According to the invention, there is provided a driving arrangement for a liquid crystal device including a dyed DAP liquid crystal material as specified in the claims.

According to a first preferred embodiment of the invention, the waveform comprises alternating positive and negative half-cycles; wherein each half cycle comprises a first portion of said relatively high voltage and of relatively short duration and a second portion of said relatively low voltage and of relatively long duration, said portions not overlapping.

Preferably, in said first preferred embodiment, the first and second portions in each half cycle of the waveform are substantially contiguous but, if desired, at least one further portion may be included therebetween and/or before and/or after said first and second portions; said further portion or portions being either of zero or finite voltage.

Further preferably, in the first preferred embodiment, the said relatively short duration is l/20^th of the half cycle time, or less, and the said relatively long duration is 19/20^th of the half cycle time or more.

It is still further preferred, in said first preferred embodiment, that said relatively high voltage is between 7 volts and 15 volts; preferably 12 volts, and said relatively low voltage is between 2.5 volts and 8 volts; preferably 3.3 volts, but always less than the relatively high voltage.

In further preferred embodiments of the invention, said relatively low voltage is equal to or less than 50% of the relatively high voltage.

In general, the voltages and durations of said first and second portions are selected such that the power consumption of the device is reduced as compared with the power consumption associated with conventional driving square waveforms of around 1kHz frequency and +/-5volts amplitude, without substantially compromising the optical performance of the device, in terms of contrast and switching speed. In a particularly preferred form of the first preferred embodiment, wherein the said relatively short duration is 1/20 .th of the half cycle time; the said relatively long duration is 19/20^th of the half cycle time; the said relatively high voltage is 12 volts; and said relatively low voltage is 3.3 volts, the power consumption of the device is reduced by 30% as compared with that associated with the use of conventional driving square waveforms of around 1kHz frequency and +/-5volts amplitude, and moreover the device exhibits improved contrast and switching performance.

According to a second preferred embodiment of the invention, each cycle of the waveform comprises a first portion comprising a single pair of positive and negative- going pulses of said relatively high voltage and a second portion comprising a plurality of pairs of positive and negative-going pulses of said relatively low voltage.

In all embodiments of the invention, each cycle of the waveform is preferably dc balanced, thereby avoiding the possible build up of unbalanced charges which can lead to erroneous and uncontrolled switching at one or more locations of the device.

In order that the invention may be clearly understood and readily carried into effect, one embodiment thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 shows, in block diagrammatic form, a liquid crystal device and a driving arrangement therefor in accordance with one example of the invention;

Figure 2 shows a driving waveform utilised in a first embodiment of the invention; and

Figure 3 shows a driving waveform utilised in a second embodiment of the invention.

Referring now to Figure 1, a liquid crystal device comprises a cell 1 in which dyed DAP liquid crystal material 2 is sandwiched between plates of glass 3 and 4. This construction is conventional, and the glass plates support, on their respective inner surfaces, sets 5 and 6 of transparent electrical conductors; set 5 being configured as row conductors and set 6 as column conductors. The transparent electrical conductors are conveniently formed of indium tin oxide (ITO) as is conventional, and the selective energising of the row and column conductors enables individual small volumes of the liquid crystal material 2 (such volumes being conveniently referred to as pixels) to be rendered transparent or opaque (or reflective or non-reflective) depending on the information that is to be displayed by the device 1.

In alternative electrode configurations, suitable for micro-displays or active matrix displays, a common electrode is applied to one surface of the liquid crystal material 2 and an array of square electrodes, each defining an individual picture element, or pixel, is applied to the other surface of the material 2. In all cases, the electrodes need not be applied directly to the liquid crystal material and may, if preferred, be applied to a thin film of interposed material and/or to the glass plates 3 or 4.

In any event, and as mentioned above, such devices are conventionally driven with square drive waveforms having a frequency of around 1kHz and amplitude of +/- 5volts; the view having been developed by those skilled in the art that drive waveforms exhibiting those characteristics are necessary in order for such devices to exhibit acceptable optical characteristics, such as contrast and switching speed.

In accordance with this embodiment of the invention, a cell driver circuit 7, suitably synchronised and powered by a control circuit 8, generates driving waveforms of the form shown in Figure 2, whereby, in each half cycle of the waveform, a relatively high voltage (+/- 12 volts) is applied for a relatively short duration (say l/20^th of the half cycle time) and a relatively low voltage (+/-3.3 volts) is applied for a relatively long duration (say the remaining 19/20^th of the half cycle time). By this means, the power consumption of the device 1 is reduced by 30%, as compared with that associated with the use of conventional driving square waveforms of around 1kHz frequency and +/- 5volts amplitude, and moreover the device exhibits improved contrast and switching performance.

It will be appreciated that the precise voltage amplitudes used, and the relative durations for which they are applied, may be varied to suit different operational circumstances and different materials. In this general connection, the relatively high voltage may be selected to lie between 7 volts and 15 volts; and the relatively low voltage may be selected to lie between 2.5 volts and 8 volts, but always less than the relatively high voltage, and preferably at most 50% of the said relatively high voltage.

The rationale behind the selection of the above operating ranges is that, regarding the relatively low voltage, this is ineffective below 2.5 volts, whereas the liquid crystal effect saturates at around 8 volts. With regard to the relatively high voltage, this provides no additional benefit if raised above 15 volts, whereas it has no effect below 7 volts. Underscoring the choice of parameters, of course, is the overall objective of reducing power consumption whilst not unduly compromising the optical performance of the device, especially in terms of contrast and switching speed.

Furthermore, the periods for which the relatively high and relatively low voltages are applied need not be contiguous, and in some embodiments they are indeed separated by one or more periods when zero voltage, or finite voltages, are applied. Moreover, in other embodiments, different voltage amplitudes (including zero) may be applied before and/or after the relatively high and relatively low voltages, as well as, or instead of, separations between the relatively high and relatively low voltages.

The reduction in power consumption attributable to the invention can be quantified by comparing the power consumption for equivalent half-cycles in the drive waveform, say the positive half cycles. In general, the ratio of the power consumption associated with a device according to the invention to the power consumption associated with a prior art waveform is given by the equation relationship:

(tιV_{2 +} t₂V₃ )/ TVι

where T represents the half-cycle period, and Vi the conventional driving voltage of 5 volts; ti the relatively long period (in this example 19T/20) for which the relatively low voltage (V₂ ), e.g. 3.3 volts, is applied and t₂ represents the relatively short period (in this example T/20) for which the relatively high voltage (N₃), e.g. 12 volts is applied.

Applying the working values from the preferred embodiment into the above equation indicates the power consumption to be reduced to 70% of that associated with the conventional (+/-5 volts) driving waveform.

In addition to significantly reducing power consumption, as illustrated above, the invention can also improve the optical characteristics of the device, particularly contrast and switching speed. This, in turn, means that more grey levels can be displayed. Moreover, for any period while the device is required to display static images only, the relatively high voltage parts of the waveform can be omitted, allowing the driving voltage to be maintained at the level of the relatively low voltage throughout each half cycle, in order to minimise power consumption.

In accordance with a second preferred embodiment of the invention, the waveform illustrated schematically in Figure 3 is employed to drive the liquid crystal device shown in figure 1.

In Figure 3, it will be observed that each cycle of the waveform comprises a first portion comprising a single pair (9a, 9b) of positive and negative-going pulses of said relatively high voltage and a second portion comprising a plurality of pairs (such as 10a, 10b) of positive and negative-going pulses of said relatively low voltage. In this example, eight pairs of the low voltage pulses such as 10a and 10b are used, giving an overall ratio of 8:1 as between the low voltage and high voltage pulses.

In the particular waveform configuration shown in Figure 3, the low voltage pulses such as 10a and 10b are of the same duration as the high voltage pulses such as 9a and 9b. This need not necessarily be the case, however, and the low voltage pulses such as 10a and 10b may consist of or include pulses of different durations. In one alternative example, the waveform of Figure 3 was varied by utilising four pairs of low voltage, each of twice the duration shown in Figure 3. In another alternative arrangement, the low voltage pulses consisted of two pairs of pulses having the same duration as those shown as such as 10a and 10b sandwiching a single pair of pulses of twice the duration of the pulses 10a and 10b.

In general, according to the invention, the relative voltages of the high and low voltage portions of the waveform and their relative durations and/or the relative numbers of pulses included therein, are arranged such that the power consumption of the device is reduced as compared with the power consumption associated with conventional driving square waveforms of around 1kHz frequency and +/-5volts amplitude.

Claims

1. A driving arrangement for a liquid crystal device including a dyed DAP (Deformation of Aligned Phase) liquid crystal material (2); the driving arrangement including driving means (7, 8) for applying driving voltages (9a,

9b, 10a, 10b) to said material (2) to effect optical switching thereof; the driving means including generating means (7) for generating said voltages in accordance with a cyclically repetitive waveform of predetermined frequency; the waveform comprising a combination of pulsed voltage portions including, in each cycle, at least one portion of relatively high voltage (9a) and at least one portion of relatively low voltage (10a); the relative voltages of the said respective portions and their relative durations and/or numbers being arranged such that the power consumption of the device is reduced as compared with the power consumption associated with conventional driving square waveforms of around 1kHz frequency and +/-5 volts amplitude.

2. An arrangement according to claim 1 wherein the waveform comprises alternating positive and negative half-cycles; wherein each half cycle comprises a first portion (9a) of said relatively high voltage and of relatively short duration and a second portion (10a) of said relatively low voltage and of relatively long duration, said portions not overlapping.

3. An arrangement according to claim 2 wherein the first and second portions (9a, 10a) are substantially contiguous.

4. An arrangement according to claim 2 wherein at least one further portion is included between said first and second portions, and/or before and/or after said first and second portions.

5. An arrangement according to claim 4 wherein said further portion or portions is of zero voltage.

6. An arrangement according to any of claims 2 to 5 wherein the said relatively short duration is l/20^th of the half cycle time, or less, and the said relatively long duration is 19/20^lh of the half cycle time or more.

7. An arrangement according to any of claims 2 to 6 wherein said relatively high voltage is between 7 volts and 15 volts and said relatively low voltage is between 2.5 volts and 8 volts, but always less than the relatively high voltage.

8. An arrangement according to claim 7 wherein said relatively low voltage is at most 50% of the said relatively high voltage.

9. An arrangement according to claim 7 or claim 8 wherein said relatively high voltage is 12 volts and said relatively low voltage is 3.3 volts.

10. An arrangement according to any of claims 2 to 9 wherein said relatively high and relatively low voltages and the relatively short and relatively long durations are so related that the power consumption of the device is reduced by substantially 30% as compared with that associated with the use of conventional driving square waveforms of around 1kHz frequency and +/- 5 volts amplitude.

11. An arrangement according to claim 1 wherein each cycle of the waveform comprises a first portion comprising a single pair of positive- (9a) and negative- (9b) going pulses of said relatively high voltage and a second portion comprising a plurality of pairs of positive and negative-going pulses (10a, 10b) of said relatively low voltage.

12. An arrangement according to claim 11 wherein said single pair of pulses and the pulses of said plurality of pairs are of the same duration and frequency.

13. An arrangement according to claim 11 wherein said single pair of pulses and the pulses of at least one of said plurality of pairs differ as to their durations and frequencies.