Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2002-08-05
2004-10-26
Shalwala, Bipin (Department: 2673)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S078000, C345S082000, C345S092000, C345S098000, C345S100000, C315S169100, C315S169300
Reexamination Certificate
active
06809706
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a drive circuit for a luminous element in a display device, and specifically relates to a drive circuit for a display device appropriate for driving a current-controlled luminous element such as organic and inorganic EL (Electro Luminescence) elements and an LED (Light Emission Diode) whose luminance is controlled by a current flowing through it.
2. Description of the Related Art
A display device where scan lines and signal lines form a matrix, and luminous elements such as organic and inorganic EL elements and LEDs are provided individual intersections of the scan lines and the signal lines to display a character as a dot matrix is widely used for a television set, a portable terminal, and an advertising board. Especially, since the elements constituting the pixels are luminous elements, this type of display devices do not require a back light for illumination while a liquid crystal display device requires it, have characteristics such as a wide view angle, and thus are attracting attention. Especially, an active drive display device, which includes switching elements integrated into the individual pixels on the matrix, and holds an image represented by the pixels for a certain period, has characteristics such as higher luminance, higher resolution, and lower power consumption compared with a passive drive display device which includes only luminous elements, and thus is especially attracting attention recently.
For this type of display device, conventionally a drive circuit shown in
FIG. 1
has been used generally. In this conventional drive circuit, a scan line
201
turns on a switching transistor Tr
201
, a voltage on the data line
202
is written to a hold capacitor C
202
, and then the drive transistor Tr
202
is turned on. A current corresponding to conductivity determined by the gate-source voltage of the drive transistor Tr
202
flows through an EL element
200
. Namely, the voltage of the data line
202
conducts analog control of gradation display. However, since the channel in a polysilicon thin film transistor used for the active drive display device is polycrystal silicon, variation of the characteristics is remarkably large compared with single crystal silicon. Thus, when the same gate voltage is written, the current varies depending on the pixels due to the variation of the characteristics of the drive transistor Tr
202
, a luminance becomes uneven, and consequently high gradation display becomes difficult. To overcome this defect, a drive circuit which is not affected by variation in a threshold voltage is disclosed on pages 438 to 441 by Sarnoff Corp. in “SID 99 DIGEST” in 1998 published by Society for Information Display.
The following will describe the operation thereof while referring to FIG.
2
and FIG.
3
.
All of thin film transistors (Tr
101
to Tr
104
) are constituted by P-channel transistors. In a period {circle around (1)}, all of the transistors Tr
101
to Tr
104
are turned on, and a current flows through an EL element
100
. In a period {circle around (2)}, the transistor Tr
104
turns off, a current flows on a path indicated by an arrow until the gate-source voltage Vgs of the transistor Tr
102
reaches a threshold voltage Vth, and the transistor Tr
102
turns off when Vgs=Vth. In a period {circle around (3)}, the transistor Tr
103
turns off, and the voltage on a data line
102
changes VDD to Vdata. Then, the voltage generated between the both ends of the capacitor C
102
, namely the gate-source voltage Vgs of the transistor Tr
102
, becomes −VDD+Vth+C
101
·(VDD−Vdata)/(C
101
+C
102
). In a period {circle around (4)}, when the transistor Tr
104
turns on, current I flowing through the EL element
100
is (W·u·Cox/2·L)·((−C
102
·VDD−C
101
·Vdata)/(C
101
+C
102
))
2
if the transistor Tr
102
is used in the saturation region. Since this expression does not include the threshold voltage Vth, even if there is a variation in Vth, the current is not affected. Here, “L” and “W” respectively indicate channel length and channel width of the transistor Tr
102
, “u” is mobility, and “Cox” is gate dielectric film capacitance.
However, in this drive circuit, as the equation for calculating the current I described above clearly shows, though the variation of the threshold of the transistor can be compensated, the mobility of the transistor cannot be compensated. Thus, when there is a variation in the mobility, the luminance of the individual pixels fluctuates, and unevenness in the luminance occurs. Also, since this drive circuit requires two control liens in addition to the four transistors, the two capacitors, the scan line, and the data line, a pixel circuit becomes complicated, and the following two problems also occur.
The first problem is that probability of defects in production increases due to the complicated pixel circuit, and thus the yield decreases.
The second problem is that it is necessary to increase the current to provide intended luminance due to decrease of aperture ratio, and thus the power consumption increases.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a drive circuit for a display device which does not present a luminance unevenness even when there is a variation in characteristics of a transistor, and to provide a drive circuit for a display device enabling a high gradation display.
In addition, another object of the present invention is to provide a drive circuit for a display device which prevents decrease in the yield and the aperture ratio, and decreases the price and the power consumption by simplifying the constitution of a pixel circuit.
A drive circuit for a display device according to the present invention is a drive circuit for use in a display device with a plurality of pixels arranged as a matrix and luminous elements being provided for the individual pixels. The drive circuit comprises:
drive transistors provided for the individual luminous elements and driving said luminous elements, said luminous element and said drive transistor in each of the pixels being serially provided between a first power supply and a second power supply;
a first switching transistor provided in each of the pixels for supplying a gate of said drive transistor with a control signal for controlling said drive transistor; and
a differential amplifier for comparing a voltage of a connection point between said luminous element and said drive transistor in each of said pixels, and a control voltage input in said differential amplifier and indicating luminance of the pixel, and, thereby generating said control signal, wherein
said control signal is supplied for the gate of said drive transistor through said first switching transistor.
In this drive circuit for a display device, as another aspect of the present invention, a second switching transistor may supplies said differential amplifier with said voltage of said connection point between said luminous element and said drive transistor in each of said pixels.
Also, both of said first switching transistor and said second switching transistor may be controlled by the same second control signal.
Said drive circuit for driving a display device may comprise a hold capacitor holding a voltage between the gate and the source of said drive transistor.
As another aspect of the present invention, a circuit for canceling an input offset may be provided for the differential amplifier.
As another aspect of the present invention, the differential amplifier may be formed on the same substrate as the pixel.
In addition to these constitutions, it is possible to further constitute such that the control voltage which is supplied for the display device, and indicates the luminance of the pixel is applied to the inverted input terminal (−) of the differential amplifier, and simultaneously, the voltage between the luminous element and the drive transistor is applied to the non-inverted input terminal (+) of the differential amplifier.
Si
Lewis David L
McGinn & Gibb PLLC
Shalwala Bipin
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