Saturating column driver for grey scale LCD

A saturating column driver for an LCD substrate. Digital data is entered into a register in a serial fashion to produce a series of single-column addresses. The single-column addresses are latched and apparatus is provided for translating video binary words into grey scale codes. Also provided are switches responsive to the translated video binary words which switch a voltage input corresponding to the video binary word through to the LCD substrate to generate the desired grey scale.

BACKGROUND OF THE INVENTION 
I. Field of the Invention 
The invention applies to video signal interface apparatus, and more 
particularly to apparatus which provides a means of driving the column of 
an LCD active matrix with a variable voltage to produce grey scale but 
with only drivers which operate in a saturated mode. 
II. Discussion of the Prior Art 
State-of-the-art analog column drivers used in active matrix LCD grey scale 
applications dissipate power at a high rate. This high power dissipation 
limits the temperature operating range of the active matrix, reduces 
reliability and limits maximum panel size and pixel density. 
FIG. 1 shows a conventional grey scale liquid crystal display (LCD) column 
drive circuit in block diagram form. In such conventional circuits, a 
logical "1" is entered into a shift register 10 and propagated therein to 
produce one single-column address at a time. This is done sequentially 
until all columns have been addressed. Each time a column is addressed, 
the appropriate switch S is activated and a sample-and-hold capacitor C1A, 
for example, is selected to store the video voltage. During this setup, 
capacitor C1B which was accessed one line earlier by input switch B1 is 
providing the video voltage for that column for the particular row 
currently being output through output switch A2 and an analog line driver 
20 to the display. There are several variations on this theme, such as 
using a digital-to-analog converter (DAC) to store voltages instead of a 
capacitor in a keyed sample-and-hold circuit as described above. Such 
circuits require high power consumption and are very complex in comparison 
to the invention. Such complexity and high power consumption might be 
warranted if a larger number of grey levels were available. However, a 
typical LCD display is limited by construction and viewing angle variation 
to a small number of grey shades. For the example shown, 16 grey shades 
are used. In such cases, the extra power and increased complexity of more 
capable column drivers is not warranted. 
SUMMARY OF THE INVENTION 
A grey scale column driver which uses devices operated only in a saturated 
mode for an LCD substrate is disclosed. The saturating column driver 
comprises register means for entering digital data to produce a column 
voltage address; means for latching the column voltage address connected 
to the register means; means for voltage level translating the column 
voltage address connected to the latching means; and means for different 
column voltages switching responsive to the column voltage address 
connected to the translating means. A plurality of voltage generator means 
for generating a plurality of electronic signals is connected to the 
switching means so that when the switching means is activated, at least 
one of the voltage generator means supplies an electronic signal through 
the switching means to drive the column of the LCD substrate. 
It is one object of the invention to decrease power in active matrix LCD 
column drivers by taking advantage of the finite capability of an LCD to 
present grey scale without viewing angle difficulties. 
Other features objects and advantages of the invention will become apparent 
to one skilled in the art through the drawings herein wherein like 
reference numerals refer to like elements, and the detailed description of 
the preferred embodiment and claims herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention will be discussed herein with reference to an illustrative 
embodiment. Those skilled in the art will recognize that many alternative 
embodiments can be accomplished using the teachings of this invention and 
that the invention is not limited to the embodiment illustrated for 
explanatory purposes. 
FIG. 2 shows a functional block diagram of one embodiment of the LCD column 
driver of the invention. LCD column driver 30 comprises a shift register 
32 connected to a latch, or flip-flop 34 which is in turn connected to a 
series of demultiplexers 36. The plurality of demultiplexers 36 are in 
turn connected to level shifter 40 which controls a plurality of output 
drivers 101 through 164 in this example. As shown in FIG. 2, and as is 
well understood in the art, the level shifter 40 shifts the voltage levels 
received from the demultiplexers from a logical level to a switching level 
sufficient to control the output drivers. The output drivers may 
advantageously be comprised of transistors, for example, FETS 
appropriately sized to switch the voltages through to the LCD substrate. 
Lines V1 to V16 are connected to linear amplifiers 101 through 164 wherein 
each of the lines V1 through V16 represent a different video level. These 
16 drives are located external to the LCD glass, thus their power 
dissipation does not have a significant effect on the reliability and 
environmental range of the LCD display as is the case in prior art 
systems. The video lines V1 to V16 are small in number compared to the 
number of columns in a typical LCD display. 
An external voltage generator 54, FIG. 4 generates voltages V1 through V16. 
This may be, for example, a simple voltage divider string connected to a 
reference supply as shown in FIG. 4. The buffer inverters 50 as shown in 
FIG. 4 provide a low impedance, sign reversible, drive to the LCD 
substrate 200 as shown in FIG. 2. Referring again to FIG. 4, a switch 52 
may be included in the system so that an external signal can be injected 
directly to any column. Note that the external generator 54 and switch 52 
may be located on any of the plurality of video signals V1 through V16. 
Those skilled in the art will also recognize that any number of video 
signals may be thus supplied to produce varying levels of grey scale. 
Notice that in FIG. 4 the system proposed uses analog video transmission 
which is digitized to provide addresses for switching FET transistors. 
Various other methods could be used to directly transmit digitized video 
to produce those same addresses. Further, notice that the video signals V1 
through V16 may be distributed equally or unequally in various increments 
depending on the application. DC levels can be designed dynamically to 
match the signal from a TV camera, which has significant .gamma. for 
example. For test purposes a linear voltage distribution may be desirable. 
Referring again to the external generator setup of FIG. 4, those skilled in 
the art will recognize that this configuration will allow injection of 
test signals into the LCD. These signals could be located anywhere on the 
line and would allow the user to inject clean signals which could be used, 
for example, for test purposes to detect bad pixels. Each voltage supply 
line V1, V2-V16, is optionally configured to receive voltage from either 
the resistor network or a set of external generators 501-516 switched 
through set of switches 551-566. The objective of the external generators 
is to provide an optional reference signal to each voltage level. In an 
alternative embodiment of the invention, the voltage sent to the LCD can 
be either a combination of the driver voltages or the external generator 
voltages. 
Referring now to FIGS. 3, 3a and 3b a video interface block diagram is 
shown incorporating one embodiment of the invention. The grey shade 
voltage generator 56 is connected to buffer inverters 58 and 60 which 
output the grey levels V1 through V16 Note that in the application shown 
in FIG. 3, an odd/even "ping pong" scheme is used. Except for the addition 
of the apparatus of the invention, such systems are well known in the art. 
Having explained the physical embodiment of the invention, the operation of 
the invention will now be explained in detail with reference to the 
illustrative embodiment of FIG. 2. In operation, data is clocked into 
shift register 32 in serial fashion. Shift register 32 may advantageously 
be a 4.times.64 BIT device. The shift register 32 then produces one 
singlecolumn address at a time until all 64 columns have been addressed. 
This data is then latched as appropriate and passed through latch 34 to a 
plurality of demultiplexers 36. Those skilled in the art will appreciate 
that the latch could be a flip-flop. The demultiplexers 36 operate on the 
data in 4 bit video binary words and decode the video binary words into 
grey scale codes. In the example shown, the demultiplexers used are 4:1 
demultiplexers. The demultiplexers translate the 4 bit video binary words 
into a single FET switch closure for each column. There are preferably 16 
possible FET switch connections. Then, as discussed above, lines V1 to V16 
are connected to linear amplifiers, each preferably representing a 
different voltage level. The voltage line V1 through V16 is switched 
through the selected FET, as determined by the grey scale code. 
Referring now to FIG. 5, an alternative method of the invention is shown. 
In operation, data is clocked into shift register 32 in serial fashion as 
in the operation of FIG. 2. Shift register 32 may advantageously be a 
64.times.4 bit device. The shift register produces a single column address 
for each of the 64 columns. The data is latched and sent through to a 
plurality of translation devices 136 that receive the data and translate 
it to a grey code scale. The output of the translators 136 are then sent 
to a series of switches (T1-T16) that control the LCD substrate 200. Those 
skilled in the art will appreciate that the latching mechanism can also be 
implemented as flip-flops. In FIG. 2, the translation devices 136 are 
shown as demultiplexers 36. As discussed above, lines V1-V16 are connected 
to each switch (T1-T16), each line preferably representing a different 
voltage level. The voltage lines are then gated through the switches 
controlled by the output of the translation devices 136. This accomplishes 
the grey scale coding by uniquely switching one voltage to the liquid 
crystal display substrate 200. 
As shown in FIG. 4, the voltage along one of the voltage supply lines 
V1-V16 could be switched by switch 52 to receive an external voltage 
generated by an external voltage generator 54. 
This invention has been described herein in considerable detail in order to 
comply with the Patent Statutes and to provide those skilled in the art 
with the information needed to apply the novel principles and to construct 
and use such specialized components as are required. However, it is to be 
understood that the invention can be carried out by specifically different 
equipment and devices, and that various modifications, both as to 
equipment details and operating procedures can be accomplished without 
departing from the scope of the invention itself.