WO2013038151A1

WO2013038151A1 - Optical device

Info

Publication number: WO2013038151A1
Application number: PCT/GB2012/052187
Authority: WO
Inventors: John Richard Moore; Jonathan Paul Hannington; Terry Clapp
Original assignee: Cambridge Enterprise Limited; Dow Corning Corporation
Priority date: 2011-09-14
Filing date: 2012-09-06
Publication date: 2013-03-21
Also published as: CN204178683U; GB201115875D0; TW201312542A

Abstract

A liquid crystal device drive circuit has a positive power supply terminal, a negative power supply terminal, a ground terminal, a control input and an output terminal for connection to a liquid crystal device incorporating a composition having smectic-A properties. The circuit has a first controllable-device for coupling the positive power supply terminal to an output terminal, a second controllable device for coupling the negative power supply terminal to the output termmal and a third controllable device for coupling the ground terminal to the output terminal, the first second and third controllable devices being controllable in accordance with the control input.

Description

Optical device

The present invention is in the field of photonics. An embodiment relates to a driver circuit for an optical device using & materiai having smeetie A liquid crystal properties In one non-limiting embodiment the optical device is a display. In another it is a panel for aflecting the transmission of light, in yet another it is an amplitude spatial light modulator.

An embodiment relates to optica! devices in which a disordered state is produced by the process of SmA dynamic scatteriog and a clear, uniform state is induced by dielectric re-orientation. Such optica! devices can be used to provide variable amounts of light transmission- either locally, for example in "pixels" or across the whole device without the use of optical polarisers. Liquid crystals have molecules which tend to self order without freezing and thus gain crystalline attributes even though they still flow and may fill a container. The phases of liquid crystals are broadly a generalised sequenc of states that such a molecular fluid may pass through on the wa from being an. isotropic liquid until it freezes as a solid. In general such molecules will be typified by strong anisotropy. The form that this anisotropy takes can be considered where the molecule is typified by a high aspect ratio (much longer than wide, thus "rod^** or "lath" like), and may have dipole character, and anisotropic polarisahility. In these cases the average direction of molecular orientation is referred to as the "director".

Hematic liquid crystals typify the commonest liquid crystalline materials and are commonly used in liquid crystal flat screen devices and flat-panel displays. Extending the length of nematic mesogens. or other structural changes, very often causes them to show further phases upo cooling below the nematic phase, and before solidification, and at lower temperatures the typical character mm be of a "layered fluid''. Such layered liquid crystals are called "smectic" liquid crystals. Herein we will only consider the materials normally referred to as "smectic A", abbreviated to "SmA", liquid crystals. For example the prototypical "5CB" (4'»pentyi-4-bip!ieaylearhon.itrile), "50C.B" (is the ether linked pentyl, 4⁵-(pe.ntyiox.yV4-bip:hen>icarboriiiriie), is nematic. die "8CB* (4*«octyl- 4-bipheny.carbonifcrile) and "SQCB" (4'*{ocUyioxy)--4~biphenykarbonitri!e), each exhibit a SmA phase beneath the higher temperature nematic phase, where in the abbreviation "raCB" and "mOCB" m stands for an integer and refers to the number of carbon atoms in the alkyl or alkoxyi chain in 4-cyano-4'-n- alkylbipheny! and 4~eyano-4 n-alkoxybiphenyl, respectively; for example: 8GB = 4-cyano-4'-octyl^'biphenyl and

80CB*⁵4-cyano-4^,-octyloxybiphenyl

The molecules forming SmA phases have similar properties to those forming nematic phases. They are rod-like and usually have a positive dielectric amsotropy. The introduction of a strong transverse dtpole in order to induce a negative dielectric amsotropy lends to destabilise the SmA phase and may lead to increased chemical instability,

Smectic liquid crystals exhibit hysteresis in their switching so that dielectric re- orientation (or other disturbances of the smectic structure) does not rela when an applied electric field is removed. Unlike most nematic liquid crystal structures, dielectric-ally re-oriented SmA liquid crystals rest in the driven state until further forces are applied. A panel may be formed by taking planar sheets, for example of glass, and applying to these a transparent conducting layer, typically made of indium tin oxide, the conducting layers being connected to conductors so that a variable field may he applied. These two sheets may be formed into a panel for example separated by beads of uniform diameter (typically, say, 5-15 micrometers, dependent on desired cell thickness). This panel is then edge sealed with glue allowing one or more apertures for filling with the liquid crystal material.

Using such a cell a SrnA Liquid crystal layer may be formed by filling the panel (typically at an elevated temperature above the isotropic transition for the material). In the SmA devices discussed here, no alignment iayers are required unlike nematie devices where uriiforra alignment of the eel! is essential On filling and thermally cycling such a SmA pa el from room temperature to above t e isotropic transition and back again, the liquid crystal will exhibit textures that are typical for the phases. Whilst the nematie, with no surface alignment, may appear in the well-known Schlieren. texture where line defects or 'threads' scatter light, in the SmA a 'focal conic' texture is formed as a consequence of the layered structure of the SmA material, ^"There is a sharp spatial variation in the refractive index which results in light scattering. The appearance of these textures results from the a isotropy of the refractive index, which is highest when light is travelling orthogonal to the more polarisable axis of the average molecular direction. The variation in refractive index causes light scattering. When viewed (under a microscope) between crossed polarisers, contrast can also be observed between regions of different molecular orientations.

To electrically address a SmA liquid crystal panel an alternating (AC) field is normally applied. In non-doped materials, positive dielectric anisotropy of the LC will cause the re-arrangement of initially randomly aligned poly-domains, to align the raesogen with the field direction (normal to the glass surface). The panel w ll appear clear, as the average orientation of the anisotropic molecules is normal to the glass layer. For most non-doped SmA materials this situation is only reversible by heating the ceil to destroy the SmA alignment If a suitable ionic dopant is dissolved in the SmA liquid crystal host, then under the influence of DC or low frequency (e.g. 200 Hx) electric fields, two orthogonal forces attempt to orient the smectic A director:- i) Dielectric re-orientation as described above attempts to align the SmA director (indicating the average direction of the long molecular axis) in the field direction.

¾) Simultaneously, the movement of ions through the SmA electrolyte attempts to align the smectic A directo hi the direction in which ions find it easer to travel.

In SmA materials this is within the layers i.e. orthogonal to the field direction (i.e. the materials have positive dielectric anisotropy and negative conductivity anisotropy). The two competing forces give rise to an electro-hydrodynamic instability in the liquid crystal fluid that is referred to as 'dynamic scattering'. In smectic A materials the dynamic scattering state strongly scatters light and (in contrast to the similar state in nematic materials) the disruption of the SmA structure thai if produces remains after the electrical pulse causing it has terminated. The reversibility between the clear, uiiifomily oriented, state and the ion-transit induced, poly-domain, scattering state, depends upon the different frequency domains in which these processes operate. Dynamic scattering requires the field driven passage of ions through the liquid crystal fluid, it therefore occurs only with DC or low frequency AC drive.

Higher frequencies cause dielectric re-orientation (the ions cannot "move" at these frequencies) thus re-establishing a uniform orientation of the molecules.

Thus the combination of dielectric re-orientation (into a clear transparent state) and dynamic scattering (into a strongly light scattering state) in a suitably doped SmA phase (possessing positive dielectric anisotropy and negative conductivity anisotropy) can form the basis of an electrically addressed display. High frequencies (variable, typically >1000 Hz) drive the SmA. layer into an optically clear state and low frequencies (variable, typically < 200 Hz) drive it into the light scattering state. A key feature of such a display is that both these optica! states arc set up using short electrical addressing periods, and both persist indefinitely, or until they are re-addressed electrically. This is not true of the related phenomena in oematie liquid crystals. It is this property of electro-optic bistahility (or more accurately multi-stability) that allows SraA dynamic scattering displays to be matrix addressed without pixel circuitry and which results in their extremely low power consumption in page-oriented displays or in smart windows.

CN-101533162 and WO 2009/11 191 disclose an electrically controlled medium for controlling light includes two plastic thin film layers and a mixture layer is provided between the two thin film layers. The mixture layer consists of smectic liquid crystals, polymeric molecule materials and additives.

Conductive electrode layers 4 are provided on the sides of the two plastic thin film layers and the liquid crystal molecules exhibit different alignment states by controlling the size, frequency and acting time of the voltage applied to the conductive electrode layers . so that the electrically controlled medium for controlling light may be switched between a blurredly shielding state and a fully transparent state and even may be switched among a plurality of gradual stales of different gray levels. Optionally the aspects of the present invention specifically exclude the arrangement disclosed in this specification.

In one aspect there is provided a smectic- A liquid crystal device drive circuit having a positive power supply terminal, a negative power supply terminal, a ground terminal, a control input and an output terminal for connection to a smectic-A liquid crystal device, the circuit having a first controllable device for coupling the positive power supply terminal to an output terminal, a second controllable device for coupling the negative power supply terminal to the output terminal and a third controllable device for coupling the ground terminai to the output terminal, the first second and third controllable devices being controllable in accordance with the control input.

The drive circuit may further comprise control circuitry for coatroUing the first- third controllable devices, said control circuitry including logic circuitry

The drive circuit may have a control circuit power supply node for supplying drive to the control circuitry from a control circuit power supply referenced to -troratd.

The drive circuit may be dc coupled

In the drive circuit the first and second controllable devices may be configured to handle power supplies of more than 50 volts.

The first and second control lable devices may comprise respective MOSFETs and the third controllable device may comprise a bilateral pair of MOSFETs.

The logic circuitry may comprise open-collector inverter circuitry. in another aspect there is provided a liquid crystal device having a liquid crystal pane! and drive circuitry according to any preceding claim connected to provide drive voltages to the panel, the drive circuitry having an output node configured to provide dc- balanced positive voltage and negative voltage to the panel and to provide a ground potential to the panel, the liquid crystal composition comprising_., in weight %:

(a) 25 - 75% in. total of at least one siloxaae of the general formula I:

wherein

q ~ 1 to 12, e.g. 6 to JO,

t -~ 0 or L

k = 2 or 3,

A is a phenyl or c-yclohexyl ring which may be the same or different and are bonded together in par positions,

~ a Cx.3 alkyl group, e.g. methyl, which may he the same or different, X - a Cj.12 alk i group, and

Z - , CI, Br, I, CN, ¾ N0₂, Me¾, NCS, C¾, or OC¾, CF₃, OCF_3> 0¾F, CHF₂ especially CN;

(b) 0.001 - 1% in total of at least one quaternary ammonium sak of the general formula 11:

wherein:

T~ a methyl group or a silyl or sifoxane group and

v ~ 1 to 30, for example v~ 9 to 19, e.g. myristyl (v^™1 _> T^metliyl) or cetyl (v^l S nd ^eth l),

Rl, R2 and R3, which may be the same or different, are CM alky e.g. methyl or ethyl,

Q is an oxidatively stable ion, especially a CIO₄ ion,

(c) 20-65% in total of at least one polarisahje linear molecule having an alkyl chain, the molecule having the general formula ill:

wherein:

D stands for a C M straight chained alkyl or alkoxy group optionally containing one or more double bonds; k ^» 2 or 3,

A* is a phenyl, cyclohexyl, pyrimidine, L 3-dioxan.e, or 1, 4-bicyclo[2_s2,2]oetyl ring, wherein each A may be the same or different nd are bonded together in para positions, the terminal ring attached to Y optionally being a phenyl, and Y is Iocaied in the para position of the terminal ring of the group A and is selected from Z (as defined above in connection with Formula I), C{._½ straight chained alkyl, ._u straight chained alkoxy, OQ¾ NMe¾ CH_3s OCOC¾, and COC¾; and

(d) 2 ~ 20%, optionally 5 - 15, in total of at least one side chain liquid crystal polysiloxane of the general formula IV:

\

(O)t

(IV)

wherein:

a, b and e each independently have a value of 0 to 100 and are such that a+b+c has an average value in the range 3 to 200, e.g. 5 to 20; and a is such that the chain units of the formula Y~R₃Si.O~[SiR;rO]_a represents 0 to 25 mole percentage of the compound of the general formula IV, and c is such that the units of the formula chain -[SiiiR-Oj_t-RjSiOY represents 0 to 15 mole percentage of the compound of the general formula IV,

m - 3 to 20. e.g. 4 to 12:

t ^« 0 or 1,

k = 2 or 3

A is a phenyl or cyclohexyl ring which may be the same or different and the rings are bonded together in para positions. R ^~ a C-i.3 alkyl group, e.g. methyl, each of which may be the same or different, and

Y - a C. 2 alkyl group, a chromophore or a calar itie liquid crystal group and each of which may be the same or different, and

Z is as defined above in connection with Formula I and wherein the amounts and nature of the components are selected such thai the composition, possesses smeetk A layering, as detected by X-ray diffraction, The li quid crystal device may be pixeiiated.

The composition may he a composition as described in PCT/US 0/27328, claiming priority from US patent applicalioa 61/314039, incorporated herein by reference

In the drawings:

Figure 1 is a plan view of a first example of a liquid crystal panel.

Figure 2 is a cross-section along the line Π-Ι1Α of Figure 1;

Figure 3 is a cross-section similar to that of Figure 2, through a second example of a liquid crystal panel;

Figure 4 is a schematic diagram of an embodiment of a driver circuit for smectie A liquid crystal panels; and

Figure 5 is a schematic diagram of a powe supply for the driver circuit, of Figure 4, in the drawings, like reference signs refer to like parts.

Referring to Figs I and 2, a embodiment of a display panel 400 has first and second substrates 410,420. in this embodiment both of the substrates are transparent to visible light and are of glass, thus being generally rigid, in other embodiments, transparency and rigidity may not be required, and some

em odiments use substrates of relatively flexible polymer such as PET.

The panel 400 has a first set of electrodes 430 shown in Fig I as extending laterally across the device 400, and these are referred to for convenience herein as row electrodes. The panel 400 has a second set of electrodes 440 extendi ng perpendicular to the row electrodes 430, and these are referred to for convenience herein as row electrodes. It will be understood of course that the device 400 need not be oriented as shown. The electrodes 430» 440 in this embodiment are transparent to visible light. Examples of suitable materials are gold or T.TO.

The column electrodes 440 are disposed on the inner surface of the first substrate 4.10, and the row electrodes on the inner surface of the second substrate 420. The substrates are maintained in spaced relationship by spacers 450, shown here as spheres. The spacing between the substrates forms a chamber which contains a sniectie A composition 460 As previous discussed, the liquid crystal composition is a thermotropic liquid crystal composition exhibiting a smectic type A phase made up of multiple layers, wherein: under the influence of different electric fields applied between the electrodes, the alignment of die layers of the composition can become more ordered or more disordered, the composition has stable states in which the alignment of the layers of the composition are differently ordered including an ordered state, a disordered state and intermediate states, the composition being such that, once switched to a given state by an electric field, it remains substantially in that state when the field is removed.

In this embodiment, no alignment layer is provided. However alignment layers may be provided in other embodiments.

to In use, voltages applied between row electrodes and column electrodes influence the liquid crystal composition between trie relevant electrodes. For example, referring again to Fig 1. it wilt be seen that an exemplary row

electrode is marked 430a and an exemplary column electrode is marked 440a. if a low frequency, less than 200 Hz,- e.g. 50 Hz, 60 Hz,- voltage of suitable amplitude for the thickness of liquid crystal composition, the material of the composition directly associated with the electrode crossover will become scattered and will block transmission of visible light, if a relatively high frequency, over about 1 00 Hz- e.g. 2 KHz ~ voltage is applied, this will clear the composition at thai location and light will he transmitted through the composition. The time taken to scatter or respectively clea a region of the composition depends on the thickness, the voltage applied and the frequency.

Referring to Fig 3. a panel 500 is shown. This panel is generally similar to the one shown in Figs 1 and 2, except that the electrodes 530. 540 are generally continuous across the whole of the substrates 510, 520. In this embodiment the substrates 510,520 are transparent to visible light. Unlike the first embodiment the substrates are of polymer material,

Re erring to Fig 4, a driver circuit.200 for a liquid crystal device has a positive supply rail 210, a negative supply rail 220, a ground rail 230, and an output node 250, It consists of three parts, a logic part 160, a control part 165 and a power part 170. The driver circuit 200 is powered by a po er supply circuit- see Fig 5. Significantly, the logic part and control part are powered by a logic- level power supply (here 5 volts is chosen), and the circuit is configured so that this constant level supply ensures sufficient, drive to the components of the power part. The device is generally capacitive in nature; however there is also an ion current to supply during the scattering process and this is also quite substantial. The driver has to supply this during the ion tran fer in the excursions of the low frequency waveform. , Especially where it is large in stee it may draw a relatively heavy current when initially supplied with positive or negative-going power, and this needs to be taken into account. For example, without sufficient drive to the power part 170, under-driven power devices in the power part might cause the output levels to diverge from the voltages on the respective power rails.

In driving a panel containing a composition having smeeiie A liquid crystal properties it is important to provide DC balance. T e present embodiment provides square wave drive, and symmetrical waveform. DC balancing can be maintai ed by storting and stopping the waveform at a zero-crossing point and including an integral number of cycles.

It should of course be understood that, for example mark-space control of rectangular waves is envisaged, as opposed to square waves. In yet other arrangements, other waveshapes, for example sine wave drive may be employed,

It is important that, when addressing electrodes, the unselected electrodes be positively grounded.

However, grounding unselected rows means that all pixels in those rows will be subject to a column voltage of either the pixel select waveform or the pixel unselect waveform (in this case both having an amplitude excursion from +50 volts to ·- 50 volts), which is referred to in the art as the "error voltage".

Certain compositions discussed in this specification have a high error voltage tolerance- that is pixels will withstand a high error voltage without being affected by it. The ability to withstand a high error voltage means that the "one-third select" regime can be used successfully. However other drive arrangements may be used with those compositions, for example, those in which, a lower voltage or higher voltage is applied across unselected rows than a voltage of one third of the voltage used, to clear the selected pixels.

A reason for grounding unselected rows (or respectively columns) is that voltages eapaeiuVely-coupled, e.g. from adjacent row/column electrodes, cannot arise i the unselected electrodes. Such voltages, if they did arise, could exceed fee threshold voltage of the composition and thereby affect pixels that we e to be left unaltered. When no waveform is applied it is good practice to ground all rows and columns to improve lifetime. Any waveform generator, either for pixellated displays or unpixellated large panels, has to be able to drive an alternating positive and negative voltage across the liquid crystal cell, and also clamp both sides to the same voltage, in. this embodiment but not necessarily to ground (0 volts). Drive ends after an integral number of waveform cycles, and generators connect to ground in absence of drive requirements

The power part 170 has a first p-channel power MOSFET 310 with one end (source) of its main conduction path connected to the positive rail 210, and the other end (drain) of its main conduction path connected via a first resistor 311 to the output node 250. A first n-channel power MOSFET 320 has one end (source) of its main conduction path connected to the negative rail 220 and the other end (drain) of its main conduction path connected via a second resistor 3 1 to the output node 250. A bilateral n-channel power MOSFET pair 380, 381 has one end (one dram) of their main current path connected to the ground rail 230 and the other end (the other drain) connected via a third resistor 322 to the output node 250. The bilateral pair is provided as a pair of transistors, connected back to back to ensure dc isolation against both positive and negative output node voltages- the inherent diode in. the transistor structure would pass positive or negative output voltages to ground if only single transistor were used. A. fourth resistor 323 connects the common gates of the pair to the common source connection. In the control part 165, the gate of the first p-channel MOSPBT 310 is connected to the positive rail via a first pull-up resistor 325. and to the collector of a first common-base pull-down npn bipolar' transistor 341, whose base is connected to a logic power supply rail 260, here a 5 volt rail, and whose emitter is connected via a fifth resistor 324 to the drain of a first p-channei switching FET 382. The gate of the first p-channel switching FET 382 is connected to the logic power supply rail 260 via a second pull-u resistor 326. The gate of the first n-channel power MOSFET 320 is connected via a pull-down resistor 327 to the negative rail 220, and to the coikctor of a common-base pull-up pnp bipolar transistor 342. The gate of the common-base pull-up pnp bipolar transistor 342 is connected to the ground rail 230, and the emitter of the coramon-basc pull-up pnp bipolar transistor 342 is connected via a third pull-up resistor 330 to the logic power supply rail 260. The common gates of the bilateral pair 380,381 are connected to the drain of a p-channel switching FET 383, whose source is connected to the positive rail 210 via a source resistor 328. The gate of the p-channel switching FET 383 is connected via a second pull-up resistor 329 to the positive supply rail 210 and to the collector of a second common-base pn pull-down transistor 343. whose base is coupled to the logic power supply rail 260, and whose emitter leads to one end of a sixth resistor 330, in the logic part 160, three open-collector inverters are controlled by a first control, signal at a first node (G) and two by a second control signal at a second node (H). A first open-collector inverter 361 , connected to the first node (G), has its open-collector output node couple to the gate of the first p-ehannel switching FET 382 The source of the first p-channei switching FET 382 is connected to the open-collector of the second open-collector inverter 362, which has its input connected to the second node (H). The second node (11) also controls a third open-collector inverter 363, whose output is coupled to the emitter of the common-base pull-up npn bipolar transistor 342. The fourth ©pea-coliector inverter 364 is controlled by the first node (O), and its ouipuf is also coupled to the emitter of the common-base pull-up npn bipolar transistor 342. The fifth open-col lector inverter 365 is also controlled by the first node (O) and its output is connected to the other end of the sixth resistor 330.

The circuit is thus fully dc-coupled, and includes no capacitors. All the control part is referenced onl to the logic supply rail 260 and the ground rail 230,

In use, when first node (G) is low, the outputs of the first and fourth open- collector inverters 361, 364 are open-circuit. Hence the gate of the first p- channel switching FET 382 turns it on. Then second node (11) goes high, which causes the output of the second open-collector inverter 362 to go low, so thai the first p-channei switching FET 382 conducts. This pulls the emitter of the first common-base pull-down npn bipolar transistor 341 low, turning that transistor on, This in turn pulls down the gate of the first p-channel power MOSFET 310. turning it on, and connecting the output node 250 to the positive supply rail 210. Since the drive current is set by the control part without reference to the positive or negative rails, the circuit can be, and is dimensioned so that the pull-down current via fifth resistor 324 causes a specific voltage -in this embodiment approx lOv- across first pull-up resistor 325, so that the drive to the first p-channel power MOSFET 10 is independent of the voltage on rail 210, providing this voltage is above a low threshold (of approx 15 volts).

If first node (G) is low and second node (H) goes low, the first pull-up resistor 325 pulls up the gate of the first p-channel power MOSFET 311 hereby tunning it off. Both the third and fourth open-collector inverters 363 and 364 have open-circuit outputs and hence third pull-up resistor 330 turns on the common base pull-up npn transistor 342, which sinks current through pull-down resistor 327, Since the current is determined again only by the control part, this causes a drive voltage to the first n«chanael power MOSFET 320 that is independent of the supply rail voltage, and the first n-channd power MOSFET 320 is turned on to connect the negative rail 230 to the output node. Regardless of the state of the second node (IT), if the first node (G) goes high, the common base pull-u npn transistor 342 is cut off by the fourth open- collector inverter 364 output going to earth. This turns off first n-channel power MOSFET 321. The first p-channel switching FBI' 382 is turned off by the open-collector inverter 361. output going t earth, and this in turn ensures the first p-channel power MOSFE T 3 Ϊ 0 is turned off.

The fifth open-collector inverter 365 output goes low, and this causes second common-base pup pull-down transistor 343 to turn on. which in turn causes p- channel switching FET 383 to draw current via source resistor 328, to provide current to fourth resistor 322. thereby turning on the bilateral pair 380,381. This means the bilateral pair will sink charge from the output node to earth regardless of the polarity of the output node.

Fig 5 shows an exemplary power supply circuit 600 for the driver 200 of Fig 4.

The invention is not restricted to features of the embodiments as described but extends to the full scope of the claims.

Claims

1. A smectic-A liquid crystal device drive circuit having a positive power supply terminal a negative power supply temiinal, a ground terminal, control input and an. output terminal fo connection to a smectic-A liquid crystal device, the circuit having a first controllable device for coupling the positive power supply terminal to an output terminal, a second controllable device for coupling the negative power supply iermmal to the output terminal and a third controllable device for coupling the ground terminal to the output terminal, the first second and third controllable devices being controllable in accordance with the control input.

2. A drive circuit according to claim 1 -further comprising control circuitry !br controlling the first-third controllable devices, said control circuitry including logic circuitry

3. A drive circuit according to claim 2, having a control circuit power supply node for supplying drive to the control circuitry from a control circuit power supply referenced to ground,

4. A drive circuit according to any preceding claim, wherein the drive circuit is dc coupled

5. A drive circuit according to any preceding claim, wherein the first and second controllable devices are configured to handle power supplies of more than 50 volts.

6. A drive circuit according to any preceding claim, wherein the first and second controllable devices comprise respective MOSFETs and the third controllable device comprises a bi lateral pair of MOSFETs.

?. A drive circuit according to any preceding claim dependent on claim 2, wherein the logic circuitry comprises open-collector inverter circuitry.

8. A liquid crystal device having a liquid crystal panel and drive circuitry according to any preceding claim connected to provide drive voltages to the panel, the drive circuitry having an output node configured to provide de- balanced positive voltage and negative voltage to the panel and to provide a ground potentiai to the panel the liquid crystal composition comprising, in weight %:

(a) 25 - rmula I:

wherein

p - 1 to 10, e.g. 1 to 3,

q - 1 to 12, e.g. 6 to 10,

i - O or 1 _?

k ^ 2 or 3,

A is a phenyl or eyclohexyl ring which may be the same or different and are bonded together in para positions,

R^;;; a Cj.3 alley! group, e.g. methyl, which may be the same or different, X^™ a C¾,|2 alk l group, and

Z - F, CI, Br, L CN, N¾, N0_2> NMe₃, NCS, CH₃₍ or OC¾, CF₃, OCF₃, CH₂F\ CHF₂ especially CN:

(b) 0.001 - 1% in total of at least one quaternary ammonium salt of the general formula 11:

wherein:

T- a methyl group or a silyl or stioxane group and

v - 1 to 30, for example v= 9 to 19, e.g. myristyl (v=13, ί -methyl) or cctyl v^lS and T^raethyi),

Rl , R2 and R3, which may he the same or different, are CM alkyl. e.g. methyl or ethyl,

Q is an oxi datively stable iofi, especially a CI.O<j ion,

(c) 20-65% in total of at least one poiarisabls linear molecule having an alkyl chain, the molecule having the general formula Hi:

(HI)

wherein;

D stands for a Cj„i6 straight chained alkyl or aikoxy group optionally containing one or more double bonds;

lc^™ 2 or 3,

A^* is a phenyl, cyelohexyl pyrimiditie. 1 ,3-dioxane, or 1.,4 ncyclo[2,2,2]oetyl ring, wherein each A may be the same or different and are bonded together in para positions, the terminal ring attached to Y optionally being a phenyl and Y is located in the para position of the terminal ring of the group A and is selected from Z (as defined above in connection with Formula I), Cj.₁₆ straight chained alkyl, C_Mf) straight chained aikoxy, OCHF₂, NM¾, CI¾, OCOCH₅, and CQC¾; and

(d) 2 - 20%, optionally 5 ~ 15, in total of at least one side chain liquid crystal polysiloxane of the general formula IV:

(iV)

wherein:

a, b and c each independently have a value of O to 100 and are such that a+tH-c has an average value in the range 3 to 200. e.g. 5 to 20; and is such that the chain units of the formula Y-RjSiQ-tSiR -Oi represents 0 to 25 mole percentage of the compound of the general formula IV, and c is such thai the units of the formula chain -[SiHR-QJc-RjSiO-Y represents 0 to 15 mole percentage of the compound of the general formula TV,

m - 3 to 20, e.g. 4 to 12;

t = 0 or ! ,

k - 2 or 3

A is a phenyl or cyciohexyl ring which may he the same or different and the rings are bonded together in para positions,

R^∞ a Cj.j alkyl group, e.g. methyl, each of which may be the same or different, and

Y - a C_{t -}52 alkyl group, a cfaromophore or a calam c liquid crystal group and each of which may be the same or different, and

Z is as defined above in connection with Formula I .

and wherein the amounts and nature of the components are selected such that the composition possesses smectic A layering, as detected by X-ray diffraction.

9. A liquid crystal device accordmg to claim 8, wherein the panel is pixellated.