Electric lamp and discharge devices: systems – Plural power supplies – Plural cathode and/or anode load device
Reexamination Certificate
2002-03-21
2004-02-17
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Plural power supplies
Plural cathode and/or anode load device
C315S169300, C345S076000, C345S205000
Reexamination Certificate
active
06693385
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of driving a display device in which each pixel has a thin film transistor (hereinafter referred to as TFT). Specifically, the present invention relates to a method of driving a display device having an electro luminescence element, which is attracting attention as a light emitting element. The invention also relates to information equipment that use this driving method for a display device.
2. Description of the Related Art
A conventional method of driving a display device that has a light emitting element is described.
Shown here as an example of the light emitting element is an element in which an anode and a cathode sandwich an organic compound layer that emits light by electro luminescence effect upon application of the electric field (EL element).
The term EL element here refers to both an element that utilizes light emission (fluorescence) by shift from singlet exciton to the base state and an element that utilizes light emission (phosphorescence) by shift from triplet exciton to the base state.
An organic compound layer includes a hole injection layer, a hole transporting layer, a light emitting layer, an electron transporting layer, an electron injection layer, and the like. The basic structure of a light emitting element is a laminate of an anode, a light emitting layer, and a cathode layered in this order. The basic structure can be modified into a laminate of an anode, a hole injection layer, a light emitting layer, an electron injection layer, and a cathode layered in this order, or a laminate of an anode, a hole injection layer, a hole transporting layer, a light emitting layer, an electron transporting layer, an electron injection layer, and a cathode layered in this order.
A display device having a conventional light emitting element is described with reference to circuit diagrams in
FIGS. 13 and 14
which illustrate examples of structures of pixel and pixel portion.
FIG. 14
shows the structure of a pixel portion.
A pixel portion
1401
has x columns of pixels and y rows of pixels which form a matrix. Each of the pixels is denoted by
1400
. The symbols x and y denote arbitrary natural numbers.
The pixel portion
1401
is composed of source signal lines S
1
to Sx, gate signal lines G
1
to Gy, and power supply lines V
1
to Vx, and each pixel in the pixel portion has a switching TFT
141
, a driving TFT
142
, a storage capacitor
143
, and a light emitting element
144
.
The storage capacitor
143
is not indispensable if the device makes a positive use of a parasitic capacitance of a gate of the driving TFT
142
or the like.
Signals from a source signal line driving circuit (not shown in the drawing) are inputted to the source signal lines S
1
to Sx. Signals from a gate signal line driving circuit (not shown in the drawing) are inputted to the gate signal lines G
1
to Gy. A constant electric potential is given to the power supply lines V
1
to Vx.
The structure of each pixel
1400
in
FIG. 14
is described next with reference to FIG.
13
.
In each pixel, a gate signal line G that is one of the gate signal lines G
1
to Gy is connected to a gate electrode of the switching TFT
141
. The switching TFT
141
has a source region and a drain region one of which is connected to a source signal line S, namely, one of the source signal lines S
1
to Sx, and the other of which is connected to a gate electrode of the driving TFT
142
. The driving TFT
142
has a source region and a drain region one of which is connected to a power supply line V, namely, one of the power supply lines V
1
to Vx, and to one of electrodes of the storage capacitor
143
, and the other of which is connected to one of electrodes of the light emitting element
144
. The other electrode of the storage capacitor
143
is connected to power supply line V, namely, one of the power supply lines V
1
to Vx. Here, being connected means being in electrically conductive state.
Of an anode and a cathode of the light emitting element
144
in the pixel
1400
, the one that is connected to the driving TFT
142
is called a pixel electrode and the other is called an opposite electrode.
The operation of each pixel
1400
is described in detail below. The description employs reference symbols in
FIGS. 13 and 14
.
In a certain period, one of the gate signal lines G
1
to Gy is selected first. Every switching TFT
141
whose gate electrode is connected to the selected gate signal line is turned ON. Here, a TFT being turned ON means that the gate/source voltage (hereinafter referred to as gate voltage) of the TFT turns the drain/source thereof conductive. A selected signal line refers to a signal line receiving a signal electric potential to turn ON a TFT whose gate electrode is connected to the signal line.
Signals inputted from the source signal line driving circuit to the source signal line are inputted to the gate electrode of the driving TFT
142
through the drain/source of the switching TFT
141
that has been turned ON. The electric potential given to the gate electrode of the driving TFT
142
is held in the storage capacitor
143
. The signals inputted to the gate electrode of the driving TFT
142
turn the driving TFT
142
ON to cause a current to flow into the light emitting element
144
through the drain/source of the driving TFT
142
from the power supply line. Then the light emitting element
144
emits light at a luminance according to the amount of current it receives.
Driving methods for a display device are roughly divided into analog methods and digital methods. The analog methods as defined in this specification are methods of displaying an image by inputting analog signals to source signal lines. The digital methods as defined herein are methods of displaying an image by inputting digital signals to source signal lines.
An analog driving method is described first.
FIG. 18
shows a block diagram of an analog display device.
In
FIG. 18
, the display device is composed of a driving circuit portion and a pixel portion
1800
. The driving circuit portion consists of a source signal line driving circuit
1801
and a gate signal line driving circuit
1807
. The source signal line driving circuit
1801
and the gate signal line driving circuit
1807
are respectively placed only on one side of the pixel portion
1800
in FIG.
18
. However, the pixel portion
1800
may have the source signal line driving circuit
1801
on each side thereof and may have the gate signal line driving circuit
1807
on each side thereof. This arrangement is preferred in terms of drive efficiency and reliability of the display device.
Next, a detailed description is given on the structure of the source signal line driving circuit
1801
.
In an analog method, video signals inputted from the external to the source signal line driving circuit
1801
may be analog signals or digital signals. When digital signals are inputted from the external to the source signal line driving circuit
1801
and analog signals are to be outputted to the source signal lines, the digital signals have to be converted into analog signals by a digital/analog converter (hereinafter referred to as DAC) in the source signal line driving circuit or before output of the source signal line driving circuit is inputted to the source signal lines.
The driving circuit shown in the block diagram of
FIG. 18
receives digital video signals from the external to input analog signal voltage to the source signal lines.
The source signal line driving circuit
1801
is composed of a shift register
1802
, a latch circuit (hereinafter referred to as LAT)
1
(denoted by
1803
), an LAT
2
1804
, and a DAC
1805
.
The amount of information digital video signals have are n (n is an arbitrary natural number) bits.
Digital video signals of the respective bits out of n bits are inputted to the LAT
1
1803
. Digital video signals inputted from the external are subjected to serial/parallel conversion in advance and groups of n-bit signals to be sent to their respe
Fish & Richardson P.C.
Semiconductor Energy Laboratory Co,. Ltd.
Vu David
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