Note: Descriptions are shown in the official language in which they were submitted.
~~'O 92102925 ~ ~ S S ~ ~ ~ PCf/GB91/01263
1
Multiplex addressing of ferro-electric liquid crystal displays
This invention relates to multiplex addressing of ferro-electric liquid
crystal displays. Such displays use a tilted chiral smectic C, T, or ~
liquid crystal material.
Liquid crystal devices commonly comprise a thin layer of a liquid
crystal material contained between two glass slides. Optically
transparent electrodes are formed on the inner surface of both slides.
When an electric voltage is applied to these electrodes the resulting
electric field changes the molecular alignment of the liquid crystal
molecules. The changes in molecular alignment are readily observable
and form the basis for many types of liquid crystal display devices.
In ferro electric liquid crystal devices the molecules switch between
two different alignment directions depending on the polarity of an
applied electric field. These devices have a degree of bistability and
tend to remain in one of the two switched states until switched to the
other switched state. This allows the multiplex addressing of guite
large displays.
One common multiplex display has display elements, ie pixels, arranged
in an x, y matrix format for the display of e.g., alpha numeric
characters. The matrix format is provided by forming the electrodes on
one slide as a series of column electrodes, and the electrodes on the
other slide as a series of row electrodes. The intersections between
each column and row form addressable elements or pixels. Other matrix
layout are known, e.g, polar co-ordinate ~r - ~), and seven bar numeric
displays.
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2
There are many different multiplex addressing schemes. A common feature
is application of a voltage, called a strobe voltage to each row or line
in sequence. Coincidentially with the strobe applied at each row,
appropriate voltages, called data voltages, are applied to all column
electrodes. The differences between the different schemes lies in the
shape of the strobe and data voltage waveforms.
European Patent Application 0,306,203 describes one multiplex addressing
scheme for ferro electric liquid crystal displays. In this application
the strobe is a unipolar pulse of alternating polarity, and the two data
waveforms are rectangular waves of opposite sign. The strobe pulse
width is one half the data waveform period. The combination of the
strobe and the appropriate one of the data voltages provides a switching
of the liquid crystal material.
Other addressing schemes are described in GB 2,146,473-A;
~-2,173,336A~ ~-2.173,337-A; ~-2.173.629-A; wo 89/05025 ; ~arada
et aI 1985 S.I.D Digest Paper 8.4 pp 131-I34; and Lagerwall et al 1985
TEEE. IDRC pp 213-221; Proc 19$$ IEEE, IDRC p 98-101 Fast Addz~essing
for Ferro Electric 3.C Display Panels, P Maltese et al.
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3
The material may be switched between its two states by two strobe pulses
of opposite sign, in conjunction with a data waveform. Alternatively, a
blanking pulse may be used to switch the material into one state, and a
single strobe pulse used with an appropriate data pulse to selectively
switch back pixels to the other state. Periodically the Sign of the
blanking and the. strobe pulses are alternated to maintain a net zero
d.c. value.
These blanking pulses are normally greater in amplitude and length of
application than the strobe pulses so that the material switches
irrespective of which of the two data waveforms is applied to any one
intersection. Hlanking pulses may be applied on a line by line basis
ahead of the strobe, or the whole display may be blanked at one time, or
a group of lines may be simultaneously blanked.
One known blanking scheme uses blanking pulse of equal voltage (V) time
(t) product Vt, but opposite polarity, to the strobe pulse i!t product.
The blanking pulse has an amplitude of half and a time of application of
twice that of the strobe pulse. These values ensure the blanking and
strobe have a net zero d.c. value without periodic reversal of
polarity. Experimental use has shown the scheme to have a poor
performance.
Another known scheme with a blanking pulse is described in EP 0,38,293.
This uses a conventional d.c. balanced strobe pulse (of equal periods
of opposite polarity) with a similar d.c. balanced blanking pulse (of
equal periods of opposite polarity) in which the width of, the blanking
pulse may be several times that of the strobe pulse. Such a scheme has
a net zero d.c. value without periodic reversal of polarity of blanking
and strobe waveforms.
The feature of d.c. balance is particularly important in projection
displays since if it is desired to switch the gap between pixels to one
optical state then periodic reversal of polarities is not permissible.
WO 92/02925 PCT/GB91/01263
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One problem with existing displays is the time taken to address complex
displays. In order to drive complex displays at video frame rates it is
necessary to address the display quickly. Contrast ratio can also be
improved by addressing quickly so that the column waveform is at a
correspondingly high frequency. However, merely increasing the speed of
addressing will not always result in correct switching, An object of
the present invention is to reduce the time taken to address a matrix
display and to improve display contrast.
According to this invention a method of multiplex addressing a ferro
electric liquid crystal matrix display formed by the intersections of a
first set of electrodes and a second set of electrodes comprises the
steps of:-
addressing each electrode individually in the first set of electrodes,
such addressing being either by application of a strobe waveform of
pulses of positive and negative values, or by application of a blanking
pulse followed by a strobe pulse with periodic polarity reversal to
maintain a net zero d.c. value,
applying one of two data waveforms to each electrode in the second set
of electrodes synchronised with the strobe waveform, both data waveforms
being of alternating positive and negative values with one data waveform
the inverse of the other data waveform. the period of the data waveforms
(2ts) being twice that of a single strobe pulse (ts),
Characterised by:- extending in time the end of each strobe pulse,
whereby each intersection is addressed with a pulse of appropriate sign
and magnitude to turn that intersection to a desired display state once
per complete display address period and an overall net zero d.c. value.
' WO 92/02925
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The strobe waveform may be first a zero in the first period, ts,
followed by a non zero voltage (main) pulse for a period greater than
ts, eg (1.5. 2Ø 2.5. 3.0 or more) x ts. The strobe waveform ~y
have a non zero voltage in the first is period of the same or different
polarity to the remainder of the strobe; this first voltage pulse being
of variable amplitude to provide a temperature compensation. The strobe
waveform may be followed by a non zero valtage for a time period of
opposite polarity to the main voltage pulse, eg greater than ts, ts,
or less than ts.
The liquid crystal material may be switched between its two states by
coincidence of a strobe pulse and an appropriate data waveform.
Alternatively the material may be switched into one of its state by a
blanking pulse and subsequently selected pixels switched back to the
other state by coincidence of a strobe pulse and an appropriate data
waveform;
The blanking pulse may be in two parts; a first part of opposite
polarity to the second. Both parts of the blanking pulse are arranged
to have a voltage time product Vt that combines with the Vt product of
the single strobe to give a net zero d.c. value.
Extending the time length of the strobe pulse means an overlapping of
addressing in sucessive electrodes in the first set of electrodes. Such
overlapping effectively increases the width of the switching pulse
whilst not affecting the other waveforms and thus reduces the total time
taken to address a complete display whilst maintaining a good contrast
ratio between elements in the two different switched states.
W~ 92/02925 PCTlGB91101263
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Each strobe pulse may be immediately preceded by a smaller prepulse of
the same or opposite sign to that of the associated strobe pulse. This
prepulse may be used to change the switching characteristics of the
liquid crystal material. It may be used as part of a temperature
compensation. In this case the temperature of the material is sensed
and the amplitude of the prepulse adfusted as appropriate.
Each strobe pulse may be immediately followed by a pulse of opposite
sign.
WO 92/02925 ~ ~ ~ ~ ~ ~ ~ PCTlGB91/01263
7
According to this invention a multiplex addressed liquid crystal display
comprises:
a liquid crystal cell formed by a layer of liquid crystal material
contained between two cell walls, the liquid crystal material being a
tilted chiral smectic material having a negative dielectric anisotropy,
the cell walls carrying electrodes Formed as a first series of
electrodes on' one wall and a second series of electrodes on the other
cell walls, the electrodes being arranged to form collectively a matrix
of addressable intersections, at least one of the cell walls being
surface treated to provide surface alignment to liquid crystal
molecules along a single direction;
driver circuits For applying a strobe waveform in sequence to each
electrode in the first set of electrodes;
driver circuits for applying data waveforms to the second set of
electrodes;
waveform generators for generating a strobe waveform, and two data
waveforms for applying to the driver circuits;
and means for controlling the order of data waveforms so that a desired
display pattern is obtained;
characterised by:- a data waveform generator that generates two sets of
waveforms of equal amplitude and frequency but opposite sign, each data
waveform~comprising d.c. pulses of alternate sign, and a strobe
generator that generates a strobe pulse of greater duration than one
half a data waveform period, each strobe pulse extending into an address
period of the next electrode.
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A simple analysis of the liquid crystal switching
behaviour (electro-optic pulse response of ferroelectric
liquid crystals, by F C Saunders, J R Hughes, H A Pedlingham
and M J Towler, Royal Signals, and Radar Establishment
Malvern Worc England, Liquid Crystals 1989, vol 6 No. 3,341-
347) yields the following expression for the field at which
the response time - voltage switching characteristic of the
liquid crystal material exhibits a minimum response time.
Ps
Emin = . . . equation (1)
. Eo I OE ( sin26
where Emin is the field at which the response
time-voltage switching characteristic of the liquid crystal
material exhibits a minimum response time.
Eo is the permittivity of free space DE is the
(negative) dielectric anisotropy of the liquid crystal
material
0 is the cone angle of the liquid crystal material
Ps is the spontaneous polarisation.
This simple analysis holds true for only some
materials and their values of Ps and OE can be adjusted to
achieve desired operating voltages. Recent work (ref. E P
Raynes, The Physics of Displays for the 1990's, in Fine
Chemicals for the Electronics Industry II, Chemical
Applications for the 1990's, pp 130-146; Jones, Raynes, and
Towler, The Importance of Dielectric Biaxiality for Ferro
electric Liquid Crystal Devices, 3rd International Conference
on Ferro electric Liquid Crystals, Univ of Boulder Colerado
i i i.
CA 02088770 2002-08-06
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USA 24-28 June 1991) has shown that the dielectric
biaxiality is important for the existence of a minimum in
the response time - voltage characteristic. The data for
Figures 16-20 described below were obtained experimentally.
One broad aspect of the invention provides a
method of multiplex addressing a ferroelectric liquid
crystal matrix display comprising a layer of liquid crystal
material contained between two cell walls carrying
electrodes formed as a first set of electrodes on one cell
l0 wall and a second set of electrodes on the other cell wall,
the electrodes comprising a matrix of addressable
intersections, at least one of the cell walls being surface
treated to provide surface alignment to liquid crystal
molecules along a single direction, said method comprising
the steps of: generating for each electrode in the first set
of electrodes a waveform comprising a pulse of one polarity
and a pulse of opposite polarity, at least one of said
pulses comprising a strobe waveform, said strobe waveform
comprising a first time period having a duration of is
immediately followed by a time period greater than is when
the voltage level is a greater amplitude than the voltage
level during the first time period; generating two data
waveforms of alternating positive and negative value pulses
with one data waveform pulse the inverse of the other data
waveform pulse, each data waveform pulse having a duration
ts; applying said strobe waveform separately to each
electrode in the first set of electrodes in synchronism with
said data waveforms with a time delay of 2ts between the
start of strobe waveforms being applied to any two
electrodes in the first set of electrodes; applying one of
said two data waveforms for a time period of 2ts to each
electrode in the second set of electrodes synchronized with
the strobe waveform applied to each electrode in the first
I I I I ~ 4,
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set of electrodes, whereby each intersection is addressed
with a pulse of appropriate sign and magnitude to turn that
intersection to a desired display state once per complete
display address period with an overall net zero d.c. value,
wherein two strobe pulses of opposite polarity are used in
addressing each intersection wherein an additional waveform
is applied to both sets of electrodes to produce a reduction
in the peak voltage applied to the electrodes.
Another broad aspect of the invention provides a
to method of multiplex addressing a ferroelectric liquid
crystal matrix display comprising a layer of liquid crystal
material contained between two cell walls carrying
electrodes formed as a first set of electrodes on one cell
wall and a second set of electrodes on the other cell wall,
the electrodes comprising a matrix of addressable
intersections, at least one of the cell walls being surface
treated to provide surface alignment to liquid crystal
molecules along a single direction, said method comprising
the steps of: generating for each electrode in the first set
of electrodes a first waveform comprising a pulse of one
polarity and a pulse of opposite polarity, at least one of
said pulses comprising a non-blanking strobe waveform
wherein a blanking waveform is one in which all pixels along
an electrode in the first set of electrodes are switched to
a first display state irrespective of a data waveform
applied to the electrodes in the second set of electrodes,
said strobe waveform comprising a first pulse having a
duration of is and a voltage level, said first pulse
immediately followed by a second pulse having a duration
greater than is and a voltage level of greater amplitude
than the voltage level of the first pulse; generating two
data waveforms of alternating positive and negative value
pulses with one data waveform pulse the inverse of the other
i,i i
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data waveform pulse, each data waveform pulse having a
duration ts; applying said strobe waveform separately to
each electrode in the first set of electrodes in synchronism
with said data waveforms with a time delay of 2ts between
the start of strobe waveforms being applied to any two
electrodes in the first set of electrodes; applying one of
said two data waveforms for a time period of 2ts to each
electrode in the second set of electrodes synchronized with
the start of the strobe waveform applied to each electrode
in the first set of electrodes, whereby each intersection is
addressed with the resultant of said strobe waveform and
appropriate data waveform to turn selected intersections to
a desired display state once per complete display address
period.
Another broad aspect of the invention provides a
method of multiplex addressing a ferroelectric liquid
crystal matrix display comprising a layer of liquid crystal
material contained between two cell walls carrying
electrodes formed as a first set of electrodes on one cell
wall and a second set of electrodes on the other cell wall,
the electrodes comprising a matrix of addressable
intersections, at least one of the cell walls being surface
treated to provide surface alignment to liquid crystal
molecules along a single direction, said method comprising
the steps of: generating for each electrode in the first set
of electrodes two non-blanking strobe waveforms of opposite
polarity, wherein a blanking waveform is one in which all
pixels along an electrode in the first set of electrodes are
switched to a first display state irrespective of a data
waveform applied to the electrodes in the second set of
electrodes, each strobe waveform comprising a first pulse in
a first time period having a duration of is and a voltage
level, said first pulse immediately followed by a second
~ i. i a . r:
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pulse in a second time period having a duration greater than
is and a voltage level of a greater amplitude than the
voltage level during the first time period; generating two
data waveforms of alternating positive and negative value
pulses with one data waveform pulse the inverse of the other
data waveform pulse, each data waveform pulse having a
duration ts; applying one of said two non-blanking strobe
waveforms separately to each electrode in the first set of
electrodes in synchronism with said data waveforms with a
l0 time delay of 2ts between the start of strobe waveforms
being applied to any two electrodes in the first set of
electrodes until all electrodes in the first set of
electrodes have received said one of said strobe waveforms,
then applying the other of said two non-blanking strobe
waveforms separately to each electrode in the first set of
electrodes in synchronism with said data waveforms with a
time delay of 2ts between the start of strobe waveforms
being applied to any two electrodes in the first set of
electrodes; applying one of said two data waveforms for a
time period of 2ts to each electrode in the second set of
electrodes synchronized with the start of the strobe
waveform applied to each electrode in the first set of
electrodes, whereby each intersection is addressed with the
resultant of said strobe waveform and appropriate data
waveform to turn selected intersections to a desired display
state once per complete display address period.
Another broad aspect of the invention provides a
method of multiplex addressing a ferroelectric liquid
crystal matrix display comprising a layer of liquid crystal
material contained between two cell walls carrying
electrodes formed as a first set of electrodes on one cell
wall and a second set of electrodes on the other cell wall,
the electrodes comprising a matrix of addressable
i i; i. ~
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se
intersections, at least one of the cell walls being surface
treated to provide surface alignment to liquid crystal
molecules along a single direction, said method comprising
the steps of: generating two data waveforms of alternating
positive and negative value pulses with one data waveform
pulse the inverse of the other data waveform pulse, each
data waveform pulse having a duration ts; generating for
each electrode in the first set of electrodes a blanking
waveform of duration greater than is and an amplitude such
that all pixels along an electrode in the first set of
electrodes are switched to a first display state
irrespective of which of said two data waveforms are applied
to the electrodes in the second set of electrodes;
generating for each electrode in the first set of electrodes
a non-blanking strobe waveform comprising a first pulse in a
first time period having a duration of is and a voltage
level, said first pulse immediately followed by a second
pulse in a second time period having a duration greater than
is and a voltage level of a greater amplitude than the
voltage level during the first time period; applying said
blanking waveform separately to each electrode in the first
set of electrodes with a time delay of 2ts between the start
of blanking waveforms being applied to any two electrodes in
the first set of electrodes; applying said strobe waveform
separately to each electrode in the first set of electrodes
in synchronism with said data waveforms with a time delay of
2ts between the start of strobe waveforms being applied to
any two electrodes in the first set of electrodes; and
applying one of said two data waveforms for a time period of
2ts to each electrode in the second set of electrodes
synchronized with the start of the strobe waveform applied
to each electrode in the first set of electrodes, whereby
each intersection is addressed with the resultant of said
strobe waveform and appropriate data waveform to turn
I : I: . ~i i
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selected intersections to a desired display state once per
complete display address period.
Another broad aspect of the invention provides a
multiplex addressed ferroelectric liquid crystal matrix
display comprising: a liquid crystal cell comprising a layer
of liquid crystal material contained between two cell walls
carrying electrodes formed as a first set of electrodes on
one cell wall and a second set of electrodes on the other
cell wall, the electrodes comprising a matrix of addressable
intersections, at least one of the cell walls being surface
treated to provide surface alignment to liquid crystal
molecules along a single direction; a strobe waveform
generator for generating for each electrode in the first set
of electrodes two non-blanking strobe waveforms of opposite
polarity, wherein a blanking waveform is one in which all
pixels along an electrode in the first set of electrodes are
switched to a first display state irrespective of a data
waveform applied to the electrodes in the second set of
electrodes, each strobe waveform comprising a first pulse in
a first time period having a duration of is and a voltage
level, said first pulse immediately followed by a second
pulse in a second time period having a duration greater than
is and a voltage level of a greater amplitude than the
voltage level during the first time period; a data waveform
generator for generating two data waveforms of alternating
positive and negative value pulses with one data waveform
pulse the inverse of the other data waveform pulse, each
data waveform pulse having a duration ts; at least one
driver circuit for applying one of said two non-blanking
strobe waveforms separately to each electrode in the first
set of electrodes in synchronism with said data waveforms
with a time delay of 2ts between the start of strobe
waveforms being applied to any two electrodes in the first
i i i ~ i.
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set of electrodes until all electrodes in the first set of
electrodes have received said one of said strobe waveforms,
then applying the other of said two non-blanking strobe
waveforms separately to each electrode in the first set of
electrodes in synchronism with said data waveforms with a
time delay of 2ts between the start of strobe waveforms
being applied to any two electrodes in the first set of
electrodes; at least one driver circuit for agplying one of
said two data waveforms for a time period of 2ts to each
electrode in the second set of electrodes synchronized with
the start of the strobe waveform applied to each electrode
in the first set of electrodes, whereby each intersection is
addressed with the resultant of said strobe waveform and
appropriate data waveform to turn selected intersections to
a desired display state once per complete display address
period.
Another broad aspect of the invention provides a
multiplex addressed ferroelectric liquid crystal matrix
display comprising: a liquid crystal cell comprised of a
layer of liquid crystal material contained between two cell
walls carrying electrodes formed as a first set of
electrodes on one cell wall and a second set of electrodes
on the other cell wall, the electrodes comprising a matrix
of addressable intersections, at least one of the cell walls
being surface treated to provide surface alignment to liquid
crystal molecules along a single direction; a data waveform
generator for generating two data waveforms of alternating
positive and negative value pulses with one data waveform
pulse the inverse of the other data waveform pulse, each
data waveform pulse having a duration ts; a blanking
waveform generator for generating for each electrode in the
first set of electrodes a blanking waveform of duration
greater than its and an amplitude such that all pixels along
o
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an electrode in the first set of electrodes are switched to
a first display state irrespective of which of said two data
waveforms are applied to the electrodes in the second set of
electrodes; a strobe waveform generator for generating for
each electrode in the first set of electrodes a non-blanking
strobe waveform comprising a first pulse in a first time
period having a duration of is and a voltage level, said
first pulse immediately followed by a second pulse in a
second time period having a duration greater than is and a
voltage level of a greater amplitude than the voltage level
during the first time period; at least one driver circuit
for applying said blanking waveform separately to each
electrode in the first set of electrodes with a time delay
of 2ts between the start of blanking waveforms being applied
to any two electrodes in the first set of electrodes; at
least one driver circuit for applying said strobe waveform
separately to each electrode in the first set of electrodes
in synchronism with said data waveforms with a time delay of
2ts between the start of strobe waveforms being applied to
any two electrodes in the first set of electrodes; and at
least one driver circuit for applying one of said two data
waveforms for a time period of 2ts to each electrode in the
second set of electrodes synchronized with the start of the
strobe waveform applied to each electrode in the first set
of electrodes, whereby each intersection is addressed with
the resultant of said strobe waveform and appropriate data
waveform to turn selected intersections to a desired display
state once per complete display address period.
w~ 9z~oz9zs 2 ~ $ ~ ~ ~.l ~ PCT/GB91/01z63
t;,;.; ,
9
The invention will now be described by way of example only with
reference to the accompanying drawings of which:-
Figure 1 is a diagrammatic view of a time multiplex addressed x, y
matrix;
Figure 2 is a cross section of part of the display of Figure 1 to
an enlarged scale;
Figure 3 is a graph of log time against log voltage showing
switching characteristics of a smectic material for two
differently shaped addressing waveforms;
Figures 4-8 show different strobe and data waveform diagrams that.
can be used;
Figure 9 show waveform diagrams having a strobe modified from that
of Figure 4;
Figure IO show blanking, strobe and data waveforms diagrams,
Figure 1I show strobe. data, and addressing waveforms used in a
prior art display.
Figures 12 a, b show waveform diagrams for addressing the 4 x 4
element display shown in Figure 13;
Figure 13 is a 4 x 4 element array showing some intersections
switched to an ON state with the remainder in an OFF state;
Figures 14, I5 show plots of contrast ratio against applied
voltage pulse width for two different materials,
Figures 16-20 are log tine against log applied voltage graphs
showing the switching characteristics of one material with
different applied waveforms;
Figures 21, 22 show different blanking, strobe and data
waveforms;
Figures 23, 24 show row and column waveforms for a prior art
display; .
Figures 25, 26 show row and column waveforms for a modification of
Figure 6.
~Y
W~O 92!02925 PCT/GB91/01263
The display 1 shown in Figures l, 2 comprises twa glass malls 2, 3
spaced about 1-6 lxn apart by a spacer ring 4 and/or distributed spacers.
Electrode structures 5. 6 of transparent tin~oxide are formed an the
inner face of both walls. These electrodes are shown as raw and column
forming an X, Y matrix but may be of other forms. For example, radial
and curved shape for an r, 8 display, or of segments form far a digital
seven bar display.
A layer '~ of liquid crystal material is contained between the walls 2. 3
and spacer rang 4.
Polarisers 8. 9 are arranged in front of and behind the cell 1. Row 10
and column 11 drivers apply voltage signals to the cell. Two sets of
waveforms are generated for supplying the row and column drivers 10, 11.
A strobe wave form generator 12 supplies row waveforms, and a data
waveform generator 13 supplies ON and OFF waveforms to the column
drivers 11. Overall control of timing and display format is controlled
by a control logic unit 14. Temperature of the liquid crystal layer 7
is measured by a thermocouple 15 whose output is fed to the strobe
generator 12. The thermocouple 15 output may be direct to the generator
or via a proportioning element 16 e.g. a programmed ROA! chip to vary
one part of the strobe pulse and or data waveform.
WO 92/02925 PCT/GB91/01263
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Prior to assembly the cell walls are surface treated in a known manner,
e.g. by applying a thin layer of polyimide or polyamide, drying and,
where appropriate, curing and buffing with a cloth (e.g. rayon) in a
single direction, Rl, R2. Alternatively a thin layer of e.g. silicon
monoxide may be evaporated at an oblique angle. These treatments
provide a surface alignment for the liquid crystal molecules. The
alignment/rubbing directions R1, R2 may be parallel as anti parallel.
When suitable unidirectional voltages are applied the molecules director
align along one of two directions D1, D2 depending an polarity of the
voltage. Ideally the angle between D1, D2 is about 45°. In the absence
of an applied electric field the molecules adopt an intermediate
alignment direction between R1, R2 and the directions D1, D2.
The device may operate in a transmissive or reflective mode, In the
former light passing through the device e.g. from a tungsten bulb is
selectively transmitted or blocked to form the desired display. In the
reflective mode a mirror is placed behind the second polarises 9. to
reflect ambient light back through the cell 1 and two palarisers. By
making the mirror partly reflecting the device may be operated both in a
transmissive and reflective mode.
Pleochroic dyes may be added to the material 7. In this case only one
polarises is needed and the layer thickness may be 4-101am.
Wa 92/0Z925 PCr/GB91/Og2G3~
Z2
Suitable liquid crystal materials are:-
Merck catalogue reference number SCE 8 (available from Merck Ltd Poole,
England) which has a Ps of about 5nC/square cm at 30°C, a
dielectric
anisotropy of about -2.0, and a phase sequenco of:- Sc 5g°C Sa
79°C N
98°C.
Mixture A which contains 5x racemic dopant and 3% chiral dopant in the
host;
Mixture B which contains 9.5x racemic dopant and 3.5% chiral dopant in
the host.
Host F F
C6H13O-~0 - 0 0 CSH11 37$ by weight
F F
C$H1~0 0 0 0 C5H11 41~
F ~
C6H130-~- 0 0 CSHli 14~
F ~
C$H1~0 0 0 0~-CSHil $ °/
Dopant (bath racemic and chiral)
s
C9H190-~0 -~-C00-CH.CH3
CH
The t denotes chirality, without it the material is racemic.
WO 92/02925
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13
Both mixtures A, B have a Ps of about ~nC/square cm at 30°C and a
dielectric anisotropy of about -2.3.
Mixture A has the phase sequence Sc 100°C Sa 111°C N
136°C.
Mixture B has the phase sequence Sc 8~°C 118°C N
132°C.
Liquid crystal material at an intersection of a row and column electrode
is switched by application of an addressing voltage. This addressing
voltage is obtained by the combination of applying a strobe waveform Vs
to the row electrode, and a data waveform Vd to the column electrode.
ie:- Vr = Vs - Vd
where Vr = instantaneous value of addressing waveform
Vs = instantaneous value of strobe waveform, and
Vd = instantaneous value of data waveform
Chiral tilted smectic materials switch on the product of voltage and
time. This characteristic is shown in Figure 3. Voltage time products
above the curve will switch a material; below the curve is a
non-switching regime. Note, the switching characteristic is independent
of the sign of the voltage; i.e. the material switches for either a
positive or a negative voltage of a given amplitude. The direction to
which the materal switches is dependent on the polarity of voltage.
WO 92/02925 PCT/G~9g/~~2~~_
14
Two curves are shown in Figure 3 because the switching characteristic
depends upon the shape of the addressing voltage pulse combination.
The upper curve is obtained when the addressing. voltage is immediately
preceded by a small prepulse of opposite sign; e.g. a small negative
pulse followed by a larger positive pulse. The material behaves the
same on application of a small positive pulse followed by a large
negative pulse. This upper curve usually exhibits a turn round or a
minimum response time at one voltage. This is not as given by equation
1, since the switching behaviour is modified by the prepulse. The
small prepulse may be termed a leading pulse (Lp) and the larger
addressing pulse a trailing pulse (Tp). The upper curve applies for a
negative value of the ratio Lp/Tp.
The lower curve is obtained when the addressing voltage is immediately
preceded by a small pre-pulse of the same sign; i.e. a small positive
pulse followed by a larger positive pulse. The same applies for a small
negative pulse followed by a large negative pulse. The lower curve has
a positive Lp/Tp ratio. This lower curve has a different shape to that
of the upper curve; for some materials it may not have a minimum value
of a voltage.time curve.
The difference in shape between the two.curves allows a device to be
operated without ambiguity over quite a wide range of time values. This
is obtained by operating a device in a regime between the two curves
e.g. as shown in hatched lines.. Intersections required to be switched
are addressed by an addressing voltage having a shape where the lower
curve applies and where the voltage and pulse width lie above the Gurve.
Intersections not requiring to be switched either receive an addressing
voltage having the shape where the upper curve applies, and where the
voltage and pulse width lie below the curve, or only receive a data
waveform voltage. This is described in more detail below.
WO 92ro2925 ~ ~ ~ ~ ,~ ,~ ~ rcrr~~g~ro~~6~
r ~~,
1~
Figure 4 shows strobe, data, and addressing waveforms of one embodiment
of the present invention. The strobe waveform is first a zero for a
time period is followed by *3 for twice ts. This is applied to each row
in sequence, i.e. ~ one time frame period. The second part of the strobe
is a zero for one is period followed by +3 for twice ts. Again this is
applied to each row in sequence for ane time frame period. Complete
addressing of a display takes two time frame periods. The values of ~;~,
-3 are units of voltage given for the purpose of illustration, actual.
values are given later for specific materials.
Data waveforms are arbitrarily defined as data ON and data OFF, or D1,
and D2. Data ON has first a value of +1 for a first time period of is
followed by a -1 for a time period ts. This is repeated; 3.e. data ON
is an alternating signal of amplitude 1 and period 2ts. Data OFF is
similar but has an inital value of -1 followed by +l; i.e. the invea°se
of data ON. The first part of the data waveform, e.g. For data ON the
value of +1 for a time period ts, is coincident with the first part of
the strobe waveform, i.e. zero for time period ts.
The addressing waveform is the sum of strobe and data. The combination
of a positive strobe pulse and data ON is : -I, 4, 2, 1, -1, 1 etc.
The value 4 immediately preceded by -1 ensures the material switch
characteristics are governed by the upper curve of Figure 3. The
combination of a negative strobe pulse and data ON is:- -1, -2, -4, 1,
-1, 1 etc. The combination of smaller pulses of the same sign as the
large (-4) pulse ensures the material switch characteristics are
governed by the lower curve in Figure 3. Similarly a positive strobe
pulse and data OFF combine to give:- 1, 2, 4, -1, 1 etc; and a negative
strobe pulse and data OFF combine to give:- l, -4, -2, -1, 1, -1 etc.
WO 92/02925 .. . PGT/GR9g/~263_
f .,.:;'~.
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1~
When not receiving a strobe pulse each row is earthed, i.e. receives a
zero voltage. Each column receives either data 0~1 or data OFF
throughout. The effect. is that all intersections receive an alternating
signal, caused by the data waveforms, when not being addressed. This
provides an a.c, bias to each intersection and helps maintain material
in its switched state. Larger amounts of a.c. bias lead to improved
Contrast by the known a.c. stabilisation described in Proc ~lxh IDRC
194, pp 217-220.
Further a.c, bias may be provided, e.g. From a 50 KHz source, direct
onto those rows not receiving a strobe pulse. The effect on contrast
ratio of a.c. bias, both magnitude and pulse width is shown in Figures
14 and 15 For the materials SCES and mixture A. These show inherent
contrast ratio (CR) measured as a function of a.c. frequency as a cell
is switched between its two bistable states and measured at various
levels of a.c. bias.
Alternative strobe waveforms are shown in Figures 5 to 8. In Figure 5
the strobe is first a zero for 1 x ts, and 3 for 3 x ts, followed by its
inverse. In Figure 6 the strobe waveforms is first a zero f'or 1 x is
and 3 for 4 x ts, followed by its inverse. In Figure 7 the strobe
waveform is first a zero for 1 x ts, a 3 for 2 x ts, and -1 for 1 x ts;
this is followed by its inverse.
Figure 8 is a modification of Figure 4 and uses a non zero prepulse in
the strobe waveform. As shown the first part of the strobe is between
-1 and 1, not the zero value of Figure~4. The remainder of the strobe
is the same as in Figure 4, i.e. amplitude 3 for twice tr. The
resulting addressing waveform is then a first pulse of between -2 and -1
for both first and second fields. The effect of this prepulse is to
change the position of the switching curves, Figure 3 etc. varying the
value of the prepulse varies the shape and vertical position of the
curves as explained wth reference to Figures 16, and 17 below. Table 8
below shows how the switching time varies with temperature. Such a
variation can be reduced by varying the prepulse amplitude.
WO 92/02925 2 ~ g ~ "~'~ ~ PCT/~B9~oD~26:~
1~
Figure 9,shows a modification bf Figure 4. Is this modification the
strobe waveform is zero for the first is and 3 for the next l.5ts. this
l.5ts is merely one example since any value greater than is can be used
up to about Sts.
Figure ZO shows a single blanking pulse of amplitude 4 applied For 4ts.
This switches all the intersections to one switched state. A strobe is
then used to switch selected intersections to the other switched state.
Periodically the sign of the blanking and strobe are reversed to
maintain overall net zero d.c. voltages. The use of a blanking pulse
and single strobe can be applied to all the schemes of Figures 4-8. An
advantage of blanking and strobe systems is that the whole display can
be addressed in a single field time period.
By way of comparison Figure 11 shows strobe, data, and addressing
waveforms for a prior art display scheme, a mono. pulse addressing
scheme.
Figures 21, 22 show addressing schemes of this invention using a
blanking pulse and a single strobe pulse that provide a net zero d.c.
value.
In Figure 22 the blanking pulse is in two parts, a prepulse of opposite
sign to the main and blanking pulse. The function of the prepulse is to
give zero d.c. balance. The prepulse has a value of 3 for 4ts
immediately followed by -3 for 6ts. The strobe pulse is first a zero
for lts immediately.followed by 3 far 2ts; this strobe is the same as
the strobe in Figure 4. Data waveforms D1, D2 are also the same as in
Figure 4. The combination of blanking and D1 or D2 shows a large
negative Vt product which switches all pixels in the addressed row to
OFF. The strobe pulse in combination with D2 switches required pixels
to OId as described above with reference to Figure 4.
WO 92!02925 P~/~~~~/~~~~;~~
2Q8~'~'~ 0
Is
Figure 22 is similar to Figure 21 but has a different shape of blanking
pulse. This blanking pulse has a prepulse of amplitude 3 for 4ts
immediately followed by -4.5 for 4ts. The strobe pulse has amplitude 3
for 2ts as in Figure 4. The combination of blanking pulse and D1 and D2
is shown to provide a large negative Vt product that switches all
addressed rows to an OFF state. Again, selected pixels are switched to
ON by the strobe and D2.
The blanking pulses of Figures 21, 22 can be applied with the other
forms of strobe pulses shown in Figures 5-9 with amplitude and or Vt
product arranged to give net zero d.c. Far the example of Figure ~
where the first time slot of the strobe is varied eg with temperata~re,
the amplitude of the pre and/or main blanking pulse is also adjusted to
maintain a net zero d.c. value.
The blanking pulse may precede the strobe pulse by a variable amo~.ant
but there is an optimum position for response time, contrast and
visible flicker in the display. This is typicaly with blanking pulse
starting six lines ahead of the strobe pulse but is dependent upon
material parameters and the detail of the multiplex scheme.
Figures I2 a, b show the waveforms involved in addressing a 4 x 4 matrix
array showing information as shown in Figure 13. Solid circles are
arbitrarily shown as ON electrode intersections, i.e. display elements,
unmarked intersections are OFF. The addressing scheme is that used in
Figure 4.
W~O 92/02925
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19
The positive, or leading, strobe pulse is applied to each row 1 to ~ in
turn; this comprises the first field. After the last row is addressed
by the leading strobe pulse the negative, or trailing, strobe pulse is
applied to each row 1 to 4 in turn and comprises the second field. Note
there is an overlap between rows. For example the third is period for
row I occurs at the same as the first is period of row 2. This ove~:lap
is more noticable for displays using the strobe waveforms shown in
Figures 5, 6.
The data waveform data ON applied to column 1 remains constant because
each intersection in column is always ON. Similarly for column 2 the
data waveform is data OFF and remains constant because all intersections
in column 2 are OFF. For column 3 the data waveform is data OFF whilst
'rows 1 and 2 are addressed, changing to data ON whilst row 3 is
addressed, then changing back to data OFF whilst row 4 is addressed.
This means that column 3 receives data OFF for 4 x ts, data ON for 2 x
ts, data OFF for 2 x ts, a period of one field time, the time taken for
the positive strobe pulse to address every row. Similarly for column 4
the data waveform is data OFF for 2ts, data ON for 2ts, data OFF for
2ts, and date ON for 2ts. This is repeated for a further field pex°i~l
whilst the negative strnbe pulse is applied. Two field nerinc~~ p,.p
required to provide one frame period and completely address the display.
The above is repeated until a new display pattern is needed.
WO 92/02925 PC~'/f~~9'i/6~~:~~~..
i :: .,;.
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Resulting addressing waveforms are shown in Figure 12b. For
intersection row 1 column 1 (R1,C1) the material does not switch durixag
the first field period because the material switching follows the upper
curve of Figure 3, and time and applied voltage level are made to lie
below the switching curve. Instead the material switches during the
second field period where the material switches because of the lower
voltage/time requirements shown by the lower curve of F3~gure 3. t!
similar reasoning applies to intersection R1,C2 where the material
switches during the first field period. '
For intersection R3.C3 the material switches during the second field
period because the time/voltage applied during the first field period
does not reach the higher value required by the upper curve of Figure ~.
Intersection R4,C4 switches at the end of the second field period whilst
a negative strobe pulse is being applied.
The shape of waveforms appled to column 4 imposes difficulties. Due to
the ON-OFF-ON-OFF pattern of display the data waveform has a period
twice that of e.g. column 1. This can mean a Lower contrast ratio as
shown in Figures 14, 15 where longer pulse widths (low frequency) gives
markedly Lower contrast ratios. Additionally the amplitude of the non
switching but large addressing pulse in the first field contrasts with a
lower amplitude switching pulse in the second field. For this to switch
reliably a large difference is needed between the two switching curves
shown e.g. in Figure 3.
1~'O 92/02925 ~ ~ ~ ~ ~ ~ ~ PCT/GI39~/0926~
2I
The contrast ratio (CR) curves Figure 14 (mixture A) and Figure 15
(mixture SCE $) indicate the inherent contrast of a device when switched
between its two bistable positions in the presence of an ac bfas.
Clearly operatiow along the plateau of short pulse widths is desirable
for both good contrast and uniform contrast. Since the multiplexing
a.c. bias from the column waveform will carry variable Frequency
components dependent upon the pixel pattern, the contrast in the display
can vary. This is most noticeable in the asses of all pixels in one
state (highest frequency components) and alternate pixels of opposite
states (lowest frequency components) where there is a factor of two
difference in the column waveform frequency. Such two cases are
illustrated by Figures 12 and 13 for columns 1 and 4.
Figure 16 shows a log time/voltage graph showing switching
characteristics for the material SCE8 in a parallel rubbed cell with a
layer thickness of l.SYm at a temperature of 25°C. .The axes of the
graph are log is and log pulse amplitude voltage.
The curves are obtained in a calibration cell simulating the addressing
vaveforma shown in Figure 4. Two different addressing waveforms are
used. The first one, waveform I, is a small negative pulse (of -1)
applied for a time ts, followed by a larger positive pulse (of 5)
applied for a time 2ts, ie the Lp/Tp ratio is -0.166. A period of zero
volts is then followed by the inverse, i.e. a small positive pulse (of
1) and a negative larger pulse (of -5). Additionally a 50 KFiz square
wave signal is imposed on the addressing to give an a.c. bias and
simulate a data waveform. The small pulse is 0.166 the value of the
Large pulse at all the voltage levels used to provide the curve. This
first addressing waveform provides the upper curve. Values of
time/voltage above this curve provide switching of the cell whilst
values below the curve do not provide a switching.
~'VO 92!02925 ~ PCJf /Gi~9~~al~~:~
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~ 8 8'~'~ 0
22
The second addressing waveform, II, is first a positive small pulse of Z
applied for is immediately followed by a larger positive pulse of 4
applied for 2ts. After a period of zero volts this is inverted. The
small pulse is 0.25 the value of the larger pulse, ie Lp/Tp = 0.25.
Again a 50 KHz signal is imposed to provide an a.c. bia.s. This second
addressing waveform provides the lower curve. Values of time/voltage
above this curve provide switching of the cell; whilst values below the
curve do not provide a switching. With a strobe voltage of Vs = 50
volts, data voltage Vd = 10 volts, the operating range is Vs-Vd = 40
switching at 52 ~secs, Vs + Vd = 60 switching at about 480~.~secs.
Figure 1~ shows time voltage characteristics for the same addressing
scheme used in Figure 16 namely that of Figure 4, but modified by use of
a small pre pulse in the strobe waveform as in Figure 8. Figure 17
shows that the effect of the pre pulse is to move the vertical position
of the curves. This useful for temperature compensation where movement
of the curves due to temperature changes is counteracted by changing the
value of the pre pulse.
For the upper cuxwe the simulation addressing waveform is first a zee
voltage for is followed by a Iarger positive pulse of 6 for 2ts, ie
Lp/Tp = 0. After a number of time intervals is at zero volts the
inverse is applied to maintain a net zero do voltage. A 50 KHz waveform
is superimposed to provide a.c. bias.
For the lower curve the addressing waveform is first a small positive
pulse of 1 for is followed by a larger positive pulse of 2 far 2 ts, ie
Lp/Tp = 0/5. This is later reversed in polarity. A 50 KHz waveform is
superimposed to provide a.c. bias.
WO 92/02925 ~ ~ ~ ~ PCT/GB99/ti1%6~
23
The operating range for Vs = 50, Vd = i0 is:- lower curves, Vs-Vd = 40
switching at 42~aecs, and the upper curve. Vs + Vd =- 60 switching at
about 500~secs.
Figure 18 is similar to Figure 16 with an identical cell but using
simulations of the addressing waveforms of Figure 5. Thus the
addressing waveforms are -1, 6, 4, 6, (Lp/Tp _ -0.166) for the upper
curve, and 1, 4, 6, 4 (Lp/Tp = 0.25) for the lower curve. For Vs = 50,
Vd = 10, the lower curve switches at 38~s, and the upper curve switches
at about 210.
Figure 19 is similar to Figure 16 with an identical cell but using
simulations of the addressing waveforms of Figure °7. The addressing
waveforms are as shown, namely for the curve with points marked "+" the
values -1, 6, 6. -6 (Lp/Tp = -0.166} and for the curve with points
marked "o" 1, 4, 4, -4. The switching is complicated since the upper
curve has a re-entrant area where the material switches on the trailing
pulse instead of the main pulse. For Vs = 50, Vd = 10, the lower curve,
Vs - Vd = 40 switches at 58 to 240 and again at greater than 300Ws whexa
switching.is:.to_the.trailing pulse, The upper curve, Vs + Vd = 60 does
not show any switching at 60 volts. Thus multiplex operation on the
main pulse occurs between 58 and 240~.ts and on the trailing pulse at
greater than 300.
By way of comparison the log time/voltage characteristics are given in
Figure 20 for a conventional mono pulse addressing scheme using a
simulation of the strobe and data waveforms of Figure 11 in the same
cell as for Figure 11. For the upper curve the simulation addressing
waveform is a negative pulse of amplitude 1 unit for is followed by a
positive 6 units for tr. For the lower curve the addressing waveform is
a positive pulse of 1 unit for is followed by a positive pulse of 4
units for -- The pulse amplitudes are described as units to indicate
relative -.- ?s; the curves are obtained at the illustrated voltages.
For Vs = Vd = 10, the lower curve. Vs - Vd = 40 switches at 80~ts,
and the upper curve, Vs + Vd = 60 switches at about 950~xs.
W~ 92/02925 PCT/GB91/~R2~~~
24
Details follow of device characteristics for different liquid crystal
materials and different addressing waveforms. A single pixel teat cell
was constructed and addressed with a simulation of a 50 row display.
Different values of strobe, Vs, and data. Vd, voltage amplitude were
selected to give addressing voltage values such that switching voltages
lay above the lower curve of Figure 3 and non switching voltages lay
below the upper curve of Figure 3, and the value of ts, in ~ceconds.
adjusted to give a clear switching display. This ensured the cell was
operating in the area indicated by hatched lines of Figure 3. The value
of contrast ratio, CR, is the ratio of light transmitted in one switched
state relative to that transmitted in the other switched state; it as a
measure of the clarity of the display. CR is measured at the extremes
of the pulse width ts, or at specified values of ts. CR has been
optimised by adjusting one of the switched positions of the director in
the liquid crystal to correspond to a minimum transmission.
In the following tables the operating range of time is does not quite
match the information given by the volts/time plats of Figures 16-2C.
The reason for this is.thseefold. Firstly the simulations used in
Figures 16°20 are, not completely accurate.for all situations of
displs~
patterns. Secondly at longer pulse widths and correspondingly long
frame times an operator can discern flicker due to transient switching;
this can be interpreted as not-multiplexing. Thirdly at longer pulse
widths the contrast ratio becomes low, see Figures 14, 15. For example
a CR of 2 at 2~ and so it is difficult to determine whether a
material is switching or not.
Thus for practical displays the upper time limit should be taken as when
a display no longer usefully switches. This may be much less than the
actual switching time.
!V0 92/02925 2 U 8 8'~'~ D PGT/~~39~/U~1263
r....
z5
Material SCE$ in a 1.8~ thick layer at 25°C.
Table 1, addressing scheme of Figure 4
Vs Vd is CR
50 5 36-53 8-7
50 7.5 46-115 45-15
4o 1o 46-88 77-21.5
50 10 57-140 71-9.5
Table 2, addressing scheme of Figure 5
50 7.5 40-73 26-11
40 10 34-57 64-23
50 10 47-10o 67-17
Table 3, addressing scheme of Figure 7
50 5 44-280 17.5-5.4
50 7.5 62-225 62-5
40 10 56-186 87-5.8
50 l0 69-z13 70-4.8
Table 4, addressing scheme of Figure 11 (mono pulse)
50 5- 65-450 23-3
50 7.5 75-480 65-2.2
40 10 95-345 49-2.7
50 l0 83-370 63-2.3
CVO 9/02925 I'~T/~~39~/~i:~6;~._
E . ., .
2088'~'~0
26
Mixture B in a layer 1.7~mm thick at 30°C
Table 5, addressing scheme of Figure 4
Vs Vd is Cft (at lowest ts)
50 l0 22-78 51
50 7.5 17-82 33
40 l0 16-47 56
Table 6, addressing scheme of Figure 5
50 10 20-68 51
50 7.5 14-62 24
4o i0 13-36 53
40 7.5 10-37 7.2
45 7.5 10-42 1o
Table 7, addressing scheme of Figure 7
50 10 24-80 52
50 7~5 19-98 35
40 1o 18-66 68
wo ~zro2~2s pcrrc~9~ro~~s:~
~~~~7~~
27
Table $, addressing scheme of Figure 4, at different temperatures
50 10 39-123 48 25c
50 1o 21-73 59 3oc
5o 1o 1z-43 58 35c
50 10 7-25 26 40C
50 10 5-10 5 45~
Table 9, addressing scheme of Figure 5, at different temperature$
50 10 18-64 52 3oc
5o 1o 8-zo 13 4oc
5o 1o 8-37 44 35c
50 10 35-120 48 25c
Table 10, addressing scheme of Figure 11, at 30°C (mono pulse)
5o 1o 28-93 47
50 7.5 2#-i48 33
40 10 32-120 44
WrD 9~/02~ ~ ~ ~~ r~ O . ~ PCf/G~~udO~~ ~~~'
28
Material A in a layer 1.'7~.~m thick at 3o°C
Table 11, addressing scheme of Figure 4
40 l0 39-loo 46
50 l0 59-120 26
Table 12, addressing scheme of Figure 5
40 l0 33-85 48
50 l0 52-110 30
Table 13, addressing scheme of Figure ?
40 l0 40-150 46
50 10 64-220 23
Table 14. addressing scheme of Figure 1l (Mono Pulse)
40 10 56-150 32
5o so 66-300 22
W~ 92!02925 PL.'f/C~B9~1/01263
2~
Material Merck catalogue number SI7
Temperature 30G; 60V; 15V
Vs = Vd =
Table Z5
Addressing schemeFig 11 Fig 4 Fig Fig 'j
5
fastest slot time2~ 15 12 1'7
~.ts
longest slot time116 37 28 70
has
operating range 4.3X 2.5X 2.3X 4.1X
(time)
contrast ratio 41 . 84 80 76
(CR)
Brightness (x) 63 63 60 63
Operating Range time /
is:- longest fastest
slot slot time
Brightness (;G) cell betweenparallelpolarisers.
is compared with
no
Material RSRE A206: temperature 30°C, Vs = 30V, Vd = !0V.
!%~O 92/02925 ~'~T/G1B91/~R26~
f ~.,:....~;
Table 16
Addressing schemeFig 11 Fig Fig 5
4
fastest slot time60 27 20
us
operating range >2X 2.6X 2.2X 1X
(time)
contrast ratio 14 48 55
(CR)
Brightness (~) 77 67 60
Material RSRE AS500: A151 + 5~ dopant
A206 is:- 1 : 1
AS 500 : A 151 1:1 + 5,~ dopant
AS 500 is:-
F F
~'~H15 0 0 °~-f5H11 ~5i~
F F
CSHli-~0 0 0 CSHm 25~
F F
C7Ha5~ 0 0 C5H11
~'O 92/02925 ~ ~ ~ ~ PCI°/~1391/012~3
1
31
A151 is:-
F F
CsHii'~~ OCaHm 10%
F F
CaHl'0-~0 -~-~0~.-C'Hi5 20%
F F
C6H130-~0 0 0 CSH11 35%
F F
C$H1~0-~~-~-CSHl 35%
dopant
C9Hy90 0 0 -C00.CH.CH3
CN
2% chiral
3% racemic
The " denotes chirality, without it the material is racemic.
WO 92/02925 PC'lf/~B9~~~1Z~
E',.
3~
In ferroelectric liquid crystal devices it is known to seduce peals row
and column voltages by applying additional waveforms to both row and
column electrodes.
For examples Figures 23-24 show two different schemes for reducing the
peak voltage of prior art monopulse drive systems of Figure 9.
In Figure 23 a strobe (row) waveform is alternately a zero for I.ts and
a positive pulse of Vs for I.ts in the first field followed by a zero
far I.ts and a negative pulse of -Vs for I is in the second
field. The additional waveform is a positive Vs/2 during the first
field followed by a -Vs/2 in the second field. The resultant strobe
waveform varies between Vs/2 and -Vs/Z as shown. The data (column)
waveforms are alternate Vd and -Vd pulses each lasting for l.ts. The
additional waveform applied to each column is Vs/2 for the first field
followed by -Vs/2 for a second field time. The resultant data
waveform.is as shown to vary between Vd + Vs/2 and -(Vs/2 + Vd). The
effect of the additional waveform is to reduce the peak voltage of e.g.
50 volts to 35 volts.
An alternative to Figure 23 is shown in Figure 24. As before normal
strobe pulses are a zero for l.ts and a positive Vs for l.ts in the
first field time and a zero for I.ts then -Vs for l.ts in the second
field time. The additional waveform is a rectangular waveform of
period 2.ts applied for the first field time followed by its inverse
for the second field time. each varying between Vs/2 and -Vs/2
volts. The resultant strobe (row) wavefosms are as shown. Similarly
the data (column) waveforms are i°ectangular varying between +Vd and
-Vd. The additional waveform is the same as applied to the row
electrodes. The data (column) resultant wavefosm is as shown and
varies between Vs/2 + Vd and -(Vs/2 + Vd). Again this reduces the
peak voltage needed by display drivers from e.g. 50 volts to 35 volts.
evo 9xio2~zs ~ ~ ~ ~'~'~ ~ ~C;ro~~~~~°o~;t~s~
33
The same principles of Figures 23. 24 may be applied to the addressing
scheme of Figures 4-$ above. This is shown in Figure 25 which is a
modification of the scheme of Figure 5. Strobe pulses of a zero for
l.ts are followed by Vs for 3.ts in the first field time. Strobe
pulses of zero for l.ts and then -Vs for 3.ts follow in the second
field time. The strobe waveform is shown for rows 1, 2, 3 and 4 of a 'I
row display; two different strobes are shown for row 4 for reasons
explained later. The additional waveform applied to row (and also
column) electrodes is shown as Vs/2 for the first field time then -Vs/2
for the second field time. The resultant a~ow waveform for row 1 is
shown to be -Vs/2 far l.ts, Vs/2 for 3.ts, -Vs/2 for 4.ts, Vs/2 for
l.ts, -Vs/2 for 3.ts, and Vs/2 For 4.ts in the first and second field
times. The resultant of strobe and additional waveform and the row
indicated as row 4a is shown to have a large peak value of + and _
3Vs/2. The reason for this is the extended strobe pulse length whie;h
overlaps into the adjacent field. To overcome this row 4 is eithe~°
kept hidden from view or addressed with a zero strobe voltage as
indicated at row 4b. In a more practical example of e.g. a 12$ row
display, the.waveform generated would be programmed as for a 128 row .
display but only 127 rows used in the scheme of Figure 25. Should eve~a
longer strobe pulse be used e.g. as in Figure 6 line even more lines
will be left unused. Waveform applied to column electrodes are shown
in Figure 26. Date 1 and its enverse data 2 are as in Figure
5. The additional waveform is Vs/2 for one field time, and -Vs/2 for
the second field time. The resultant column waveform is shown to
vary between +/- (Vd + Vs/2). Thus for the scheme of Figure 5 with
Vs = 50 volts and Vd = l~ volts the scheme of Figures 25, 26 reduces
peak voltage to 35 volts.
