Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a method for addressing
a matrix multiplexed thin film switched liquid crystal display
(LCD) and to a LCD arrangement, particularly adapted for that
me-thod.
Copending Canadian patent application serial Nos.
332,35~ filed July 23S 1979, 328,660 filed May 30, 1979 and
332,471 filed July 24, 1979, in the name of the present assignee,
all disclose matrix multiplexed LCDs. Briefly, these LCDs consist
of a pair of transparent confining plates with a layer of liquid
crystal sandwiched between them. Formed on the inside surfaces
of the plates are opposed row-column arrays of electrodes. To
locally alter the optical transmissivity of a picture element
or pel of the display, a selection voltage is applied between
the appropriate pair of opposed electrodes. This has the effect
of subjecting an intervening part of the LC layer to an electric
field which alters a field related optical characteristic of the
LC and thus changes the optical transmissivity of the LCD in
that locality.
In order to reduce the number of addressing leads
required, one scheme For matrix multiplexing the LCD is to
interconnect the pel electrodes on one plate by electrical leads
extending in the column direction and to interconnect the pel
electrodes on the other plate by electrical leads extending in
the row direction. In an addressing scheme for the display, a
series of scan pulses Vs, are, for example, applied sequentially
to each of the row leads, (scan lines) while reverse polarity
data pulses Vd are applied periodically to selected ones of the
column leads (data lines). To turn on a picture element at a
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selected row and column intersection, a selection voltage equal
to the difference between Vs and Vd is made great enough to
locally alter the field related optical characteristic of the
LC. Non-selected pulses in a scanned line are subjected to a
voltage equal to the sum of the opposite po1arity voltages Vs
and Vd.
Several factors combine to limit the number of
lines that can be multiplexed in a LCD.
Firstly, at the instant a pel is selected, other,
non-selected pels in the selected column also experience a
pulse Vd~ For one address period, the rms voltage experienced
by these pels is insufficient to turn them on, but if N pels
in a column are switched on and off in a single field scan, a
non-selected pel in that column will experience Vd for N
address periods. This may be enough to turn the non-selected
pel on. It can be shown that the ratio of rms voltage
experienced by a selected pel to that experienced by a non-selected
pel is:-
V selected
V non-selected ~ (V - Vd)2 + Vd2 (N-1)
As N increases, the ratio becomes smaller and, since field effect
materials such as LCs do not have a sharp threshold distlnguishing
on from off, the contrast ratio between selected and non-selected
pels becomes poorer. At a certain number of matrix rows, the
contrast ratio becomes unacceptable.
This problem is compounded for LC displays which
have a narrow viewing angle. Also, since the electro-optic
response of field effect materials is generally temperature
dependent, then if a pel is to be off at VnOn select (at high
p ), an on at VseleCt (at low temperature), the
djfference between VnOn select and Vselect mus 9
than for constant temperature operation. For the above reasons,
the known level of multiplexing displays is limitedO
This problem can be alleviated by placing a
controlling switch in series with each pel at the intersections
of scan and data lines. In use, pulses Vd or ¦Vsl - ¦Vd¦ do not
activate a switch pel combination whereas a selection pulse
IVsl + IVdl does activate the switch, whereupon the liquid
crystal experiences voltage.
The copending applications mentioned previously
disclose several -forms of LCD controlling switches fabricated
by thin film techniques, the most favoured being a switch based
on a MIM (metal-insulator-metal) device which functions by
tunnelling or trap depth modulation. In a typical addressing
scheme for switch controlled LCD pels, a waveform for a selected
pel consists of an alternating series of positive and negative
pulses, a scan pulse polarity reversal in one direction coinciding
with polarity reversal in the opposite direction of data pulses.
Polarity reversal is necessary in order to prevent any net DC
component through the LC which would result in irreversible
electrochemical degradation of the LC.
It has now been observed that the per-formance of a
LCD controlled by thin film switches can be improved by driYing
LC pels with alternating series of pulses, a series of one polarity
alternating with a corresponding series of reverse polarity.
Owing, it is suspected, to the establishment of
equilibrium between current carrier trapping and de-trapping
rates in the thin film switches being non-instantarleous, the
current to a selected switch - LC pel combination does not
reach an optimum, steady state value on application of a singie
pulse. Each time that the polarity is changed, drift is quenched
and current builds again towards the equilibrium value. If
instead of reversiny polarity after every pulse, a series of
successive unipolar pulses are applied, the current level
increases as each pulse is applied up to an asymptotically
approached limit. Subsequently, polarity is reversed. Since
a selected pel addressed by this scheme experiences a larger
average electric field than a selected pel addressed from an
identical power source by pulses of alternating polarity, the
visual contrast between selected and unselected pels is yreater.
For unselected pels which~ during a scan period, are subjected
only to a voltage Vs or ¦Vsl - ¦Vd¦, the increase in current
with time can be made negligible with appropriate choice of
voltage levels.
As an alternative to securing a greater contrast
ratio by this addressing system, a higher level of multiplexing
can be achieved if the initial contrast ratio is maintained.
An embodiment of the invention will now be
described, by way of example, with reference to the accompanying
drawings in which:-
Figure 1 is a circuit schematic drawing of a switch
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control`led LCD and associated drive circuitry,
Figure 2 is a perspective sectional view with part cut
away of a LCD picture element controlled by a thin film switch;
Figure 3 shows voltage pulse trains for application to
selected and non-selected pels of the Figure 2 LCD, and
Figure 4 shows the variation with time, for a particular
se1ected MIM switch - LCD pel combination3 of selection voltage (Figure
4(a)), of MIM switch current (Figure 4(b)); and of LC voltage IFigure
4(c)) using the same time axis.
Referring in detail to Figure 1, the elec~rical
components of a matrix multiplexed LCD include a serîes of row
conductors 12 and a series of column conductors 14. As shown in Figure
2, the conductors 12 and 14 are formed on a pair of glass plates 1~, 20
with a layer of twisted nematic LC 22 sealed between them and linear
polarizers 23 applied to their outside surfaces. The polarizers 23
have their polarizing axes perpendicular to one another. The inner
surface of the plates 18, 20 are treated in a manner known in the art
so that in ~he absence of an applied electric field, LC molecules
adjacent each plate line up with the ax;s of polarization. The
20 longitudinal axes of the LC rnolecules twist through a right angle
across the thickness of the LC layer 22. By applying a voltage across
selected regions of the LC layer, the LC can be caused to undergo
localized molecular reorientation. Light passing into the LCD through
one plate is polarized and then, at the other plate, is extinguished,
the applied voltage thus reducing the optical transmissivity of the
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LCD. The picture elements or pels are de-fined by a row-column array of
transparent electrodes 24 on the inside surfaces of plates 18 and 20.
The electrodes 24 on the plate 18 are connected in rows by row
conductors 12 and the electrodes 24 on the plate 20 are connected in
columns by column conductors.
The pels 16~ which in Figure 1 are represented by the
crossover locations of conductors 12 and 14, each have a dedicated MIM
switch 17. As shown in Figure 2 each MIM consists of a thin film 27 of
tantalum adjacent the glass substrate, an insulating thin film 2~ oF
10 anodized tantalum, and a top contact thin film 30 oP NiCr:Au. The MIM
switch operates by a combination of tunnelling and carrier trap depth
modulation.
Referring back to Figure 1, the scan and data lines 12
and 1~ are driven by scan and data bipolar drivers 32 and 34. The
scanning sequence is set by a ring counter 36 under the control of a
clock generator 38. Also driven by the clock generator is a frequency
divider 40 which determines when polarity reversal of the two bipolar
drivers takes place.
Referring now to Figure 3a, there are shown waveforms
20 for addressing a selected picture element at row N, column M, of the
matrix multiplexed display 10. The voltage experienced by the selected
element is a series of electrical pulses each of voltage ¦Vsl + ¦Vd¦
where Vs is the scan voltage applied to the row conductors 12 on one
plate and Vd is the data voltage applied ~o a particular column
conductor 1~.on the other plate 20. After four pulses, spaced from one
another by the matrix scan time3 t, of the display, the polarity of pulses
applied to the particular row and column conductors is reversed
and the selected elements experience pulses of voltage
- ¦Vsl - ¦Vd¦. This reversal prevents any long term DC component
which would cause irreversible electrochemical breakdown oF the
LC. As shown in Figure 3b a non-selected pel at row N + 1,
column M experiences at any tirne a maximum voltage of ¦Vd¦ or
¦Vs~ - ¦Vd¦, the net DC component again being zero.
Figure 4 shows the voltage waveform for a selected
pel, (Figure 4a)~ together with the corresponding variation of
current I through a switch, (Figure 4_), and the voltage applied
across the LC (Figure 4c), all as a function of time. A current
pulse directed through the MIM switch decays as the LCD pel
charges, so reducing the voltage across the switch. In addition,
because of transient effects in the MIM switch, the current does
not reach a stable output value until a number of consecutive
unipolar pulses of the waveform have been applied. Because of
the switch transient effects, if pulse polarity is reversed
after every pulse, the current through the switch never reaches
a stable value. In effect, using such an addressing scheme for
a switched matrix multiplexed LCD of this type, the display is
never fully turned on.
As illustrated, by using a waveform in which
polarity reversal only takes place after a series of unipolar
pulses, the pel voltage reaches a value Vth at which the display
is fully turned on. The current decay time increases with
increasing LC pel capacitance. The voltage across the LC pel
depends on difference between the charging and discharging
time constants of the LC. As mentioned previously, the transient
effect of MIM switches occurs, it is thought, owing to the time
taken for the establishment of equilibrium between current
carrier trapping and de-trapping rates in the thin film MIM
switch. Other mechanisms may also be responsible for this
efFect. Certainly, other types of thin film switch which may
find use in matrix multiplexed LCDs show similar transient
behaviour so enabling this addressing scheme to be beneficially
used. The rate of polarity reversal must be greater than 30 Hz
which is the flicker fusion rate. The number of pulses between
polarity reversa7s is limited by the number of lines being
multiplexed.
