U.S. patent application number 10/356989 was filed with the patent office on 2003-08-07 for pixel driving device for a liquid crystal display.
Invention is credited to Chung, Chao-Chun.
Application Number | 20030146895 10/356989 |
Document ID | / |
Family ID | 27657749 |
Filed Date | 2003-08-07 |
United States Patent
Application |
20030146895 |
Kind Code |
A1 |
Chung, Chao-Chun |
August 7, 2003 |
Pixel driving device for a liquid crystal display
Abstract
A pixel driving device for a liquid crystal display. The pixel
driving device includes a thin film transistor (TFT), a pixel
capacitor, a storage capacitor, a first common line, and a second
common line. The TFT includes a gate electrode coupled to a scan
line and a source coupled to the data line. The pixel capacitor is
coupled to the TFT drain electrode and the first common line, while
the storage capacitor is coupled to the TFT drain electrode and the
second common line. The first common line and the second common
line possess different common voltages. When a pixel voltage is
applied to the pixel, the pixel capacitor and the storage capacitor
possess different capacitor voltage values by being coupled to
different common lines, respectively.
Inventors: |
Chung, Chao-Chun; (Tainan,
TW) |
Correspondence
Address: |
THOMAS, KAYDEN, HORSTEMEYER & RISLEY, LLP
100 GALLERIA PARKWAY, NW
STE 1750
ATLANTA
GA
30339-5948
US
|
Family ID: |
27657749 |
Appl. No.: |
10/356989 |
Filed: |
February 3, 2003 |
Current U.S.
Class: |
345/97 |
Current CPC
Class: |
G09G 3/2011 20130101;
G09G 3/3648 20130101; G09G 2300/0876 20130101; G09G 2330/10
20130101 |
Class at
Publication: |
345/97 |
International
Class: |
G09G 003/36 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 7, 2002 |
TW |
091102288 |
Claims
What is claimed is:
1. A driving device for a pixel of a liquid crystal display, the
driving device comprising: a scan line; a data line; a thin film
transistor (TFT) including a gate electrode coupled to the scan
line, a source electrode coupled to the data line, and a drain
electrode; a pixel capacitor coupled to the drain electrode; and a
storage capacitor coupled to the drain electrode and the pixel
capacitor; wherein the pixel capacitor and the storage capacitor
have different capacitor voltage values when a pixel voltage is
applied to the pixel.
2. The driving device according to claim 1, wherein the liquid
crystal display is a normally white (NW) mode liquid crystal
panel.
3. The driving device according to claim 1, wherein the liquid
crystal display is a twisted nematic (TN) mode liquid crystal
panel.
4. The driving device according to claim 1, wherein when an upper
electrode of the storage capacitor and a lower electrode of the
storage capacitor are short-circuited, a capacitor voltage of the
pixel capacitor is maintained by disconnecting the pixel capacitor
from the data line.
5. The driving device according to claim 4, wherein a difference
between the capacitor voltages of the pixel capacitor and the
storage capacitor enables the pixel to display its minimum
brightness.
6. A pixel driving device for a pixel in a liquid crystal display,
the pixel being coupled to a scan line and a data line, and the
pixel driving device comprising: a thin film transistor (TFT)
including a gate electrode coupled to the scan line, a source
electrode coupled to the data line, and a drain electrode; a pixel
capacitor coupled to the drain electrode; a storage capacitor
coupled to the drain electrode; a first common line having a first
common voltage level, wherein the pixel capacitor is coupled
between the drain electrode and the first common line; and a second
common line having a second common voltage level, wherein the
storage capacitor is coupled between the drain electrode and the
second common line, and when a pixel voltage is applied to the
pixel, the pixel capacitor and the storage capacitor have different
capacitor voltage values by being coupled to the first and second
common lines respectively.
7. The pixel driving device according to claim 6, wherein the
liquid crystal display is a normally white (NW) mode liquid crystal
panel.
8. The pixel driving device according to claim 6, wherein the
liquid crystal display is a twisted nematic (TN) mode liquid
crystal panel.
9. The pixel driving device according to claim 6, wherein when an
upper electrode of the storage capacitor and a lower electrode of
the storage capacitor are short-circuited, a capacitor voltage of
the pixel capacitor is maintained by disconnecting the pixel
capacitor from the data line.
10. The pixel driving device according to claim 9, wherein a
difference between the first common voltage and the second common
voltage level enables the pixel to display its minimum
brightness.
11. A pixel driving device for a liquid crystal display, the pixel
driving device comprising: a scan line; a data line; a thin film
transistor (TFT) including a gate electrode coupled to the scan
line, a source electrode coupled to the data line, and a drain
electrode; a first power source; a second power source; a pixel
capacitor coupled between the drain electrode and the first power
source; and a storage capacitor coupled between the drain electrode
and the second power source.
12. The pixel driving device according to claim 11, wherein the
first power source provides a first voltage level and the second
power source provides a second voltage level, wherein the second
voltage level is different from the first voltage level.
13. The pixel driving device according to claim 12, wherein when an
upper electrode of the storage capacitor and a lower electrode of
the storage capacitor are short-circuited, a capacitor voltage of
the pixel capacitor is maintained by disconnecting the pixel
capacitor from the data line.
14. The pixel driving device according to claim 13, wherein a
difference between the first voltage level and the second voltage
level enables a pixel associated with the thin film transistor to
display its minimum brightness.
Description
[0001] This application claims the benefit of Taiwan application
Serial No. 091102288, filed on Feb. 7, 2002.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a pixel driving device, and more
particularly to a pixel driving device for a liquid crystal
display.
[0004] 2. Description of the Related Art
[0005] Display technology has seen great advances. Conventional
cathode ray tubes (CRTs) have been gradually superseded by liquid
crystal display (LCD) in the high-end display market. CRTs have
some major drawbacks such as large size and high radiation
emissions while LCD monitors have advantages of no radiation
emissions, low power consumption, and lightweight.
[0006] FIG. 1 is a schematic diagram showing a pixel driving device
for a pixel in a conventional thin film transistor liquid crystal
display (TFT-LCD) panel. The LCD panel includes a plurality of
pixels arranged as a matrix. Each pixel has a pixel driving device
for driving liquid crystal molecules of the pixel. The pixel
driving device includes a thin film transistor (TFT) having a gate
electrode coupled to a scan line S.sub.N and a source electrode
coupled to a data line D.sub.M. The pixel driving device further
includes a pixel capacitor C.sub.LC and a storage capacitor
C.sub.ST wherein the storage capacitor C.sub.ST stores charges to
hold a voltage across the pixel capacitor C.sub.LC, thus keeping
the gray scale of the pixel stable. A drain electrode of the TFT is
coupled to the pixel capacitor C.sub.LC and the storage capacitor
C.sub.ST. The storage capacitor C.sub.ST and the pixel capacitor
C.sub.LC are connected in parallel to a common line L.sub.COM. The
connection for the storage capacitor C.sub.ST is called a
conventional "C.sub.ST on common" mode.
[0007] When the LCD displays frames, a drive circuit sequentially
enables each scan line and turns on the TFTs of each row of pixels
on the panel. Meanwhile, the drive circuit sequentially applies
pixel voltages Vp from the data line corresponding to each of the
pixels. The pixel voltage Vp is applied to the pixel capacitor
C.sub.LC and the storage capacitor C.sub.ST. Meanwhile, the common
line also provides a common voltage. The capacitor voltages of the
pixel capacitor C.sub.LC and the storage capacitor C.sub.ST are
determined according to the voltage difference of the common
voltage and the pixel voltage Vp. The pixel capacitor voltage
difference is utilized to drive the liquid crystal molecules of the
pixel giving the pixel a desired gray scale value while the storage
capacitor voltage difference is utilized to hold the desired gray
scale stable. Since the storage capacitor C.sub.ST and the pixel
capacitor C.sub.LC are connected in parallel to the common line
L.sub.COM, the values of the capacitor voltages of the pixel
capacitor C.sub.LC and the storage capacitor C.sub.ST are the
same.
[0008] FIGS. 2A to 2B illustrate the arrangement of the liquid
crystal molecules in a twisted nematic (TN) mode liquid crystal
panel with and without the pixel voltage Vp applied, respectively.
In FIGS. 2A and 2B, the arrows show the indicating directions of a
front-plate alignment film 204 and a rear-plate alignment film 202
in the TN mode liquid crystal panel. In particular, the indicating
directions of the front-plate alignment film 204 and the rear-plate
alignment film 202 are perpendicular to each other. The directions
of long axes of the liquid crystal molecules 200 close to the
alignment films 202 and 204 are substantially parallel to the
indicating directions of the alignment films 202 and 204,
respectively. When no pixel voltage Vp is applied, the liquid
crystal molecules 200 gradually twist until the uppermost layer
close to the front-plate alignment film 204 is at a 90-degree angle
to the rear-plate alignment film 202, as shown in FIG. 2A. Under
these conditions, the liquid crystal molecules 200 possess high
light transmission rates, and the pixel's brightness reaches a
maximum. FIG. 2B shows that when the proper pixel voltage Vp is
applied, the liquid crystal molecules 200 are rotated to be in
parallel with the direction of the electric field. In this case,
the liquid crystal molecules 200 possess low light transmission
rate, and the brightness of the pixel is reduced.
[0009] During the manufacture of the panel, the gate electrode of
the TFT and the lower electrode of the storage capacitor C.sub.ST
for a pixel are formed in one manufacturing step. In addition, the
drain and source electrodes of the TFT, and the upper electrode of
the storage capacitor C.sub.ST for the pixel are all formed in
another manufacturing step. For the sake of description, the gate
electrode of the TFT and the lower electrode of the storage
capacitor C.sub.ST are referred to as a first metal layer M1, while
the drain and source electrodes of the TFT and the upper electrode
of the storage capacitor C.sub.ST are referred to as a second metal
layer M2. A silicon nitride (SiN.sub.x) layer is provided between
the lower electrode and the upper electrode of the storage
capacitor C.sub.ST to serve as a dielectric material between the
two plates of the storage capacitor C.sub.ST.
[0010] Due to the possibility for error when manufacturing the
panels, the silicon nitride layer between the lower electrode and
the upper electrode of the storage capacitor C.sub.ST may be doped
with impurities or other substances, or voids may be formed in the
silicon nitride layer. If this occurs, the first metal layer and
the second metal layer are short-circuited. If the two metal layers
short-circuit, the electrical potentials of the lower and upper
electrodes of the storage capacitor C.sub.ST for the pixel are
equal regardless of the magnitude of pixel voltage Vp applied to
the pixel. The voltage difference between the lower and upper
electrodes of the pixel of the liquid crystal panel would be zero.
The pixel in this case is faulty. In a TN mode liquid crystal
panel, when the above-mentioned problem occurs in the storage
capacitor of a pixel, the faulty pixel always displays its
brightness regardless of the applied pixel voltage Vp, and causes a
bright spot, especially, for a normally white TN mode liquid
crystal panel. When the liquid crystal panel has a bright spot, the
display quality of the liquid crystal panel is seriously degraded
and customers are not willing to buy these products.
SUMMARY OF THE INVENTION
[0011] In view of the above-mentioned problems, it is therefore an
object of the invention to provide a pixel driving device for a
liquid crystal display (LCD) with no bright spot. When a pixel of
the LCD becomes faulty due to a short-circuit between a first metal
layer and a second metal layer of the pixel storage capacitor, a
bright spot is prevented from appearing on the liquid crystal
panel. The influence of panel manufacturing errors upon the display
quality of the liquid crystal panel can thus be reduced.
[0012] The invention discloses a pixel driving device for a pixel
of a liquid crystal display. Each pixel is coupled to a scan line
and a data line. The pixel driving device includes a thin film
transistor (TFT), a pixel capacitor, a storage capacitor, a first
common line, and a second common line. The TFT includes a gate
electrode coupled to the scan line, a source electrode coupled to
the data line, and a drain electrode. The pixel capacitor is
coupled between the drain electrode of the TFT and the first common
line, while the storage capacitor is coupled between the drain
electrode of the TFT and the second common line. The first common
line has a first common voltage while the second common line has a
second common voltage. When a pixel voltage is applied to the
pixel, the pixel capacitor and the storage capacitor possess
different capacitor voltage values by being coupled to respective
common lines.
[0013] The invention further discloses a driving device for a
liquid crystal display. The driving device includes a scan line, a
data line, and a TFT. The TFT includes a gate electrode coupled to
the scan line, a source electrode coupled to the data line, and a
drain electrode. The driving device may further include a first
power source, a second power source, a pixel capacitor coupled
between the drain electrode and the first power source, and a
storage capacitor coupled between the drain electrode and the
second power source. The first power source provides a first
voltage, and the second power source provides a second voltage
level which is different from the first voltage level.
[0014] Other objects, features, and advantages of the invention
will become apparent from the following detailed description of the
preferred but non-limiting embodiment. The following description is
made with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic diagram showing a pixel driving device
for a pixel in a conventional TFT-LCD.
[0016] FIGS. 2A to 2B are schematic diagrams showing the
arrangement of the liquid crystal molecules in a twisted nematic
(TN) mode liquid crystal panel with and without the pixel voltage
Vp applied, respectively.
[0017] FIG. 3 is a schematic diagram showing a pixel driving device
for a pixel in a TFT-LCD of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] The storage capacitor and pixel capacitor are connected to
two different common lines with two different common voltages
giving them different capacitor voltage values. When the first
metal layer and the second metal layer of a pixel are
short-circuited unintentionally, a capacitor voltage can be
maintained across the pixel capacitor by disconnecting the
source/drain electrodes from the storage capacitor, for example,
with a laser. The capacitor voltage of the pixel capacitor is large
enough to arrange the liquid crystal molecules of the pixel in a
direction parallel to the electric field, which are arranged
between the front and rear plates of the liquid crystal panel, such
as a normally white (NW) mode TN LCD. Consequently, the brightness
of such faulty pixel can be decreased, preventing a bright spot on
the liquid crystal panel.
[0019] Referring to FIGS. 3, a pixel driving device for a pixel in
a TFT-LCD is illustrated according to a preferred embodiment of the
invention. In this embodiment, a pixel capacitor C.sub.LC and a
storage capacitor C.sub.ST are respectively coupled to a first
common line Lcom1 and a second common line Lcom2. The first common
line Lcom1 and the second common line Lcom2 are coupled to a first
power source and a second power source which can be positioned
outside the liquid crystal panel, respectively. The first power
source provides a first common voltage Vcom1, and the second power
source provides a second common voltage Vcom2, wherein the first
common voltage Vcom1 and the second common voltage Vcom2 are of
different voltage levels. In addition to the second common line
Lcom2, the second power source may also be connected to the gate
electrode of the TFT for the pixel. In this configuration, the
first common line Lcom1 provides the first common voltage Vcom1
while the second common line Lcom2 provides the second common
voltage Vcom2. When the pixel voltage Vp is applied to the pixel,
the capacitor voltage value of the pixel capacitor C.sub.LC is
determined by the pixel voltage Vp and the first common voltage
Vcom1. The capacitor voltage value of storage capacitor C.sub.ST is
determined by the pixel voltage Vp and the second common voltage
Vcom2. By being coupled to the different common lines with
different common voltages, the pixel capacitor C.sub.LC and the
storage capacitor C.sub.ST may have different capacitor voltage
values.
[0020] If the first metal layer and the second metal layer of the
storage capacitor C.sub.ST corresponding to a pixel are
short-circuited, the pixel capacitor is electrically disconnected
from the data line such that no voltage can be applied to the upper
electrode of the storage capacitor through the source or drain
electrodes. Therefore, the capacitor voltage of the pixel capacitor
C.sub.LC is not equal to 0 but to a difference between the first
common voltage Vcom1 and the second common voltage Vcom2, because
the common voltages coupled to the pixel capacitor C.sub.LC and the
storage capacitor C.sub.ST are different. By properly designing the
voltage levels of the second common voltage Vcom2, it can be
ensured that the difference between the first common voltage Vcom1
and the second common voltage Vcom2 is large enough to change the
orientation of the liquid crystal molecules arranged between the
front and rear plates of the pixel of the NW mode LCD panel.
Accordingly, such faulty pixel will not always display its maximum
brightness, and a bright spot is prevented from appearing on the
liquid crystal panel.
[0021] In this embodiment, the first common voltage may be, for
example, 4V and the second common voltage Vcom2 may be, for
example, -5 volts. In this case, when the two electrodes of the
storage capacitor C.sub.ST are short-circuited and then the TFT
(source electrode or drain electrode) is disconnected from the data
line, the difference between the first common voltage Vcom1 and the
second common voltage Vcom2 can cause the liquid crystal molecules
to arrange in a direction parallel to that of the electric field.
At this time, the liquid crystal molecules possess low light
transmission rates, and the pixel does not form a bright spot on
the liquid crystal panel. Instead, the pixel is totally dark, and
the influence of manufacturing inaccuracy upon the display quality
of the liquid crystal panel can be reduced.
[0022] In the pixel driving device for the LCD panel disclosed in
the above-mentioned embodiment of the invention, the storage
capacitor and the pixel capacitor may possess different capacitor
voltage values, because they are coupled to common lines with
different common voltage levels. Accordingly, when the first and
second metal layers of the storage capacitor in a pixel are
short-circuited and then the source/drain electrode is disconnected
from the pixel capacitor, a capacitor voltage is maintained across
the pixel capacitor. The level of the capacitor voltage is
substantially large enough to arrange the liquid crystal molecules
of the pixel in a direction parallel to the electric field, which
are inserted between the front and rear plates of the NW mode TN
LCD panel. Consequently, the brightness of such faulty pixel may be
decreased, thus preventing a bright spot on the liquid crystal
panel.
[0023] While the invention has been described by way of example and
in terms of a preferred embodiment, it is to be understood that the
invention is not limited thereto. On the contrary, it is intended
to cover various modifications and similar arrangements and
procedures, and the scope of the appended claims therefore should
be accorded the broadest interpretation so as to encompass all such
modifications and similar arrangements and procedures.
* * * * *