Computer graphics processing and selective visual display system – Display driving control circuitry – Display power source
Reexamination Certificate
2001-12-28
2004-06-08
Chang, Kent (Department: 2673)
Computer graphics processing and selective visual display system
Display driving control circuitry
Display power source
C345S082000, C345S208000
Reexamination Certificate
active
06747639
ABSTRACT:
BACKGROUND OF THE INVENTION
Organic light emitting diode (OLED) devices are increasing becoming the display of choice for a wide range of applications. For example, OLED devices are increasingly being used as displays for computers, laptops, personal digital assistance and cellular phones, just to name a few of their ubiquitous applications. Following their example in liquid crystal display technology, there are two main system architectures for OLED displays—passive and active matrix displays. For high resolution passive matrix OLED displays, one row is addressed at a time. For example, in an OLED display with M rows and an average luminance of L, the pixels in the same row will be driven to a peak brightness of M*L. For a 1000 line display, the peak brightness could exceed 200,000 nits and the voltage required to drive the OLED pixels could exceed 20V. Thus, the passive matrix OLED device may become very inefficient and the display power consumption high.
In order to reduce the power consumption of an OLED display, an active matrix scheme may be highly desirable. In this case, every pixel typically has a switch, a memory cell and a power source. When a row of pixels is addressed, the pixel switch is turned on and data is transferred from the display drivers to the pixel memory capacitors. The charge is held in the capacitor until the row is addressed in the next frame cycle. Once the charge is stored in the capacitor, it turns on the power source to drive an OLED pixel and the pixel will remain on until the next address frame cycle.
As a device, an OLED is commonly characterized as a “current device”—as its light output is proportional to its current input. To achieve good control of the luminance uniformity and good control of gray scale across the entire display, a current source is typically used to drive the OLED device. Therefore, the power source used in an active matrix OLED is usually a current source.
One such current source architecture—as is known in the field of active matrix OLED display (AMOLED)—is shown in FIG.
1
. The basic scheme in the field of OLED displays is a two transistor circuit with one transistor being a switch for the data and the other one being a current source.
FIG. 1
depicts a typical thin film transistor
100
as is known in the art. The data line is connected to the drain (
104
) of transistor T1 (
102
) is connected and the select line is connected to the gate (
106
). The source of T1 is connected to a capacitor C
S
(
108
) and to the gate of transistor T2 (
110
). The drain of T2
112
is connected to Power and the source of T2 is connected to the pixel area
114
.
In operation, T1 is the switching transistor that allows data charges to be stored in the storage capacitor
108
. The stored charge in the storage capacitor
108
turns on the current source transistor T2
110
. The drain of the current source transistors T2 supplies the current to the pixel
114
whereby the brightness of the pixel is determined by the drain current in the transistor T2. The drain current (I
D
) of the transistor T2 is controlled by the charge stored at the storage capacitor
108
.
FIG. 2
shows the operating characteristics of transistor T2 as a plot of I
D
versus V
DS
. A family of curves are shown—with each curve depicting operation at a different V
GS
. As can be seen, dotted line
202
broadly defines two separate operating regions of transistor T2—the “linear region”
204
and the “saturation region”
206
, as is well known in the art. To operate transistor T2 as a current source, it is typical to select a V
GS1
in the saturation region of transistor T2. Once selected, the current is fairly constant and is independent of the value of V
DS1
. To control the luminosity of the pixel, it is again typical to select the V
GS
. As can be seen, with higher values of V
GS
, the greater the amount of I
D
flows through the pixel and, hence, increases its light output.
In constructing the circuit of
FIG. 1
, thin film transistors (TFTs) are typically used to fabricate the pixel power source because of their relatively low cost. TFTs are widely used in AMLCD today in most high resolution flat panel displays. Most of the TFT's used today for AMLCD are made with amorphous silicon (a-Si) because of the low manufacturing cost. However, a-Si TFT has inherently low carrier mobility (~1 cm
2
/V-s) and the transistor size is relatively large. This limits the resolution of the displays fabricated with a-Si as well as the capability of using it as a current source.
For displays with fine pitch, polycrystalline Si (p-Si) is used for TFT fabrication because the size of the TFTs can significantly reduced. Typically, the electron mobility in p-Si is close to 100 cm
2
/V-s while the hole mobility is about 50 cm
2
/V-s. Since current source is used to drive AMOLED displays (and, in particular, those employing OLED pixels), p-Si typically chosen for TFT fabrication because of the high current capability of p-Si. However, there are many issues associated with using p-Si for TFT fabrications—and particularly when used in OLED displays.
For example, since current sources are commonly used to drive the pixel, the current source TFTs need to have a high current capability. Even with p-Si, the transistor size has to be fairly large relative to the pixel size, resulting in low pixel fill factor. As a result, pixels have to be driven at a higher pixel brightness and this reduces the panel power efficiency and device lifetime. In addition to the cost disparity between a-Si and p-Si TFTs, it is desirable to use a-Si for the driver circuitry of an active matrix display.
Second, the pixel power consumption is then equal to I*(V
PIXEL
+V
DS
), where V
DS
is the source-drain terminal voltage across the TFT and V
PIXEL
is the voltage across the cathode and the anode of the pixel. As noted above, for current-source operation, a TFT is usually operated in its saturation region. Under this operation, V
DS
can be quite large, typically in the range of 5-7 V for p-Si. On the other hand, V
PIXEL
is only about 3 V (in particular, for OLED pixels). As a result, over 60% pixel power consumption is due to the TFT circuitry. Thus, it is highly desirable to reduce the power consumption of the TFT circuitry.
Additionally, there is a problem using TFTs for a current source. The current in the TFT current source is determined by the difference between V
GS
and the threshold voltage of the gate terminal, V
T
. The threshold voltages in p-Si TFT are typically non-uniform across the display. This non-uniformity has a big impact on the TFT drain current. Typically, I
D
~(V
GS
−V
T
)
2
; thus, a small variation in V
T
could have a big change in I
D
. Several alternative approaches have been proposed to use a more complex circuitry (3-5 TFTs) to compensate for the drift in the threshold voltage. This approach increases the process complexity and affects yield. Since more transistors per pixel are used in the display, it further decreases the pixel fill factor, resulting in a display with lower efficiency and poor lifetime.
SUMMARY OF THE INVENTION
One embodiment of the present invention recites a driver circuit for an active matrix display, said driver circuit comprising:
a first transistor, said first transistor comprising a source, a drain and a gate;
a storage capacitor, said storage capacitor comprising a terminal, said terminal connected to one line, said one line comprised of a group of said source and said drain of said first transistor;
a second transistor, said second transistor comprising a source, a drain and gate, wherein said gate is connected to said terminal of said storage transistor;
wherein said drain and said source of said second transistor are connected to one of group, said group comprising a power source and a pixel element respectively; and
further wherein storage capacitor is chargeable to sufficiently high voltage to operate said second transistor in its linear region of operation.
REFERENCES:
patent: 4458201 (1984-07-01), Koen
patent: 5471225 (1995-11-01), Pa
Chang Kent
Osram Opto Semiconductors GmbH
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