The present application is related to U.S. Ser. No. 11/427,139, filed concurrently herewith, of John W. Hamer and Gary Parrett, entitled “Active Matrix Display Compensation”.
The present invention relates to an active matrix-type display apparatus for driving display elements.
In recent years, it has become necessary that image display devices have high-resolution and high picture quality, and it is desirable for such image display devices to have low power consumption and be thin, lightweight, and visible from wide angles. With such requirements, display devices (displays) have been developed where thin-film active elements (thin-film transistors, also referred to as TFTs) are formed on a glass substrate, with display elements then being formed on top.
In general, a substrate forming active elements is such that patterning and interconnects formed using metal are provided after forming a semiconductor film of amorphous silicon or polysilicon etc. Due to differences in the electrical characteristics of the active elements, the former requires ICs (Integrated Circuits) for drive use, and the latter is capable of forming circuits for drive use on the substrate. In liquid crystal displays (LCDs) currently widely used, the amorphous silicon type is widespread for large-type screens, while the polysilicon type is more common in medium and small screens.
Typically, organic EL elements are used in combination with TFTs and utilize a voltage/current control operation so that current is controlled. The current/voltage control operation refers to the operation of applying a signal voltage to a TFT gate terminal so as to control current between the source and drain. As a result, it is possible to adjust the intensity of light emitted from the organic EL element and to control the display to the desired gradation.
However, in this configuration, the intensity of light emitted by the organic EL element is extremely sensitive to the TFT characteristics. In particular, for amorphous silicon TFTs (referred to as a-Si), it is known that comparatively large differences in electrical characteristics occur with time between neighboring pixels, due to changes in transistor threshold voltage. This is a major cause of deterioration of the display quality of organic EL displays, in particular, screen uniformity. Uncompensated, this effect can lead to “burned-in” images on the screen.
Goh et al. (IEEE Electron Device Letters, Vol. 24, No. 9, pp. 583-585) have proposed a pixel circuit with a precharge cycle before data loading, to compensate for this effect. Compared to the standard OLED pixel circuit with a capacitor, a select transistor, a power transistor, and power, data, and select lines, Goh's circuit uses an additional control line and two additional switching transistors. Jung et al. (IMID '05 Digest, pp. 793-796) have proposed a similar circuit with an additional control line, an additional capacitor, and three additional transistors. While such circuits can be used to compensate for changes in the threshold voltage of the driving transistor, they add to the complexity of the display, thereby increasing the cost and the likelihood of defects in the manufactured product. Further, such circuitry generally comprises thin-film transistors (TFTs) and necessarily uses up a portion of the substrate area of the display. For bottom-emitting devices, the aperture ratio is important, and such additional circuitry reduces the aperture ratio, and can even make such bottom-emitting displays unusable. Thus, there exists a need to compensate for changes in the electrical characteristics of the pixel circuitry in an OLED display without reducing the aperture ratio of such a display.
It is therefore an object of the present invention to provide an apparatus and method of compensating for changes in the electrical characteristics of the pixel circuitry in an OLED display.
This object is achieved by an apparatus for determining an adjustment to a signal voltage for compensating for changes in the threshold voltage (Vth) for a drive transistor in a pixel drive circuit in an active matrix OLED display having at least one OLED light-emitting pixel, comprising:
a) the pixel drive circuit having a data line, a power supply line, a drive transistor having source, drain, and gate electrodes, and a switch transistor having source, drain, and gate electrodes;
b) the source or drain electrode of the drive transistor being electrically connected to the power supply line, and the other of the source or drain electrode being electrically connected to the OLED light-emitting pixel;
c) the source or the drain electrode of the switch transistor being electrically connected to the gate electrode of the drive transistor, and the other of the source or drain electrode being electrically connected to the data line;
d) first means for applying a first voltage to the power supply line which is either positive or negative for causing current to flow in a first direction through the drive transistor which causes the OLED light-emitting pixel to produce light in response to the signal voltage;
e) second means for applying a second voltage to the power supply line opposite in polarity to the first voltage so that current will flow through the drive transistor in a second direction opposite to the first direction until the potential on the gate electrode of the drive transistor causes the drive transistor to turn off;
f) third means for producing a threshold-voltage-related signal on the data line which is a function of the potential on the gate electrode of the drive transistor; and
g) fourth means responsive to the threshold-voltage-related signal for calculating the adjustment to the signal voltage.
It is an advantage of the present invention that it can compensate for changes in the electrical characteristics of the thin-film transistors of an OLED display. It is a further advantage of this invention that it can so compensate without reducing the aperture ratio of a bottom-emitting OLED display and without increasing the complexity of the within-pixel circuits.
Turning now to
Turning now to
The above embodiments are constructed wherein the drive transistors and switch transistors are n-channel transistors. It will be understood by those skilled in the art that embodiments wherein the drive transistors and switch transistors are p-channel transistors, with appropriate well-known modifications to the circuits, can also be useful in this invention.
In practice in active-matrix displays, the capacitance is often not provided as a separate entity, but in a portion of the thin-film transistor sections that form the drive transistor.
Turning now to
Current will then flow through drive transistor 210 in a second direction, that is, electrons will flow from power line 110 to ground 150, and charge the COLED capacitor. As the charge on COLED is increased, the potential between the source and drain electrodes of drive transistor 210 is reduced. Simultaneously, the potential on the gate electrode of drive transistor 210 (which is isolated by switch transistor 180) will shift to maintain the ratio of the potential difference from the gate to source and drain in proportion to the inverse of the ratio of respective capacitances:
Vgp/Vgo=Cgo/Cgp (Eq. 1)
The current flow will continue until the potential Vgp between gate electrode 215 of drive transistor 210 and power supply line 110 falls to the value of the drive transistor threshold voltage, which causes the drive transistor to turn off. By turn off, it is meant that the current flow through drive transistor 210 is substantially zero. However, it is known in the art that transistors can leak small amounts of current under threshold voltage or lower conditions; such transistors can be successfully used in this invention. For illustration purposes, we are assuming in this example that the threshold voltage Vth of drive transistor 210 is 3.0V.
Vgate=PVDD2+Vth (Eq. 2)
After the voltages have equilibrated as shown in
Vout=f(Vgate) (Eq. 3)
The transfer function f(x) can be inverted, represented by f−1(x). The threshold voltage is calculated from the measured voltage by:
Vth=f−1(Vout)−PVDD2 (Eq. 4)
Alternatively, before activating switch transistor 180 and measuring the potentials, an additional step can be done wherein the potential of power supply line 110 can then be changed to a third voltage. This will redistribute the potentials based upon the capacitances, as shown in
wherein PVDD3 represents the third voltage (e.g. zero in this example) applied to power supply line 110. In this case the threshold voltage can be calculated from the measured voltage by:
This last step of reducing the reverse driving potential (
As the threshold voltage of a transistor can change with usage, it can be necessary to calculate an adjustment for the threshold voltage. This is the difference between the currently-calculated threshold voltage and the initial threshold voltage:
Adjustment=Vth−Vthi (Eq. 7)
where Vthi represents the initial threshold voltage of the transistor.
Turning now to
In order to determine an adjustment to a signal voltage for compensating for changes in the threshold voltages (Vth) for the drive transistors of OLED display 250, it is necessary to apply a second voltage opposite in polarity to the first voltage to the power supply line and the pixel drive circuit and thus place the OLED in an inoperative condition, as described above. Voltage supply 270, which is a negative power supply in this embodiment, applies a second voltage (PVDD2) opposite in polarity to the first voltage to power supply line 110 via switch 265. As described above, this causes current to flow through the drive transistor in a second direction opposite to the first direction of normal operation, until the potential on the gate electrode of the drive transistor causes the drive transistor to turn off. Switch 265 can also optionally switch the circuit to a third voltage state (PVDD3), e.g. ground 150. During the second and third voltage operations, data line 120 can become an output line providing a threshold-voltage-related signal that is a function of the potential on gate electrode 215 of drive transistor 210. Switch 285 connects data line 120 during such data output to a correlated double sampling circuit 290 which is responsive to the threshold-voltage-related signal. In the case of multiple data lines 120, each data line can have its own correlated double sampling circuit 290, or there can be fewer correlated double sampling circuits, with multiplexing to allow sequential data sampling of all data lines. Correlated double sampling circuit 290 comprises integrator 310, low pass filter 320, correlated double sampling unit 330, sample-and-hold element 340, and analog-to-digital converter 350. Correlated double sampling circuit 290 is known and is a commercially available integrated circuit for amplification and readout of small charge over long data wires. An example is ISC9717 from Indigo. The data from correlated double sampling circuit 290 goes to a processor 315, which can store it in memory 325 as raw data, or can include computation circuitry for calculating the current threshold voltage for the drive transistor via Eq. 4 or Eq. 6, or via a lookup table. Processor 315 can calculate an adjustment to the signal voltage, via Eq. 7, from the difference between the current threshold voltage and the initial threshold voltage, which is the threshold voltage of drive transistor 210 before any aging takes place. The initial threshold voltage can be measured when pixel drive circuit 200 is new, and subsequently stored in memory. During operation of OLED display 250, processor 315 can apply the adjustment to the signal voltage through digital-to-analog converter 280, which can adjust the signal voltage, and thus apply the adjustment to data line 120, through switch transistor 180 of pixel drive circuit 200, to gate electrode 215 of drive transistor 210. Processor 315 and memory 325 can be made of individual integrated circuits or encapsulated in a single package as an SiP (System in Package). Memory 325 can also be built into processor 315 as an SoC (System on Chip).
In practice, circuits such as correlated double sampling circuit 290 make two measurements for each pixel. The first measurement is made on data line 120 without a signal, e.g. with switch transistor 180 turned off, wherein correlated double sampling circuit 290 obtains the noise level of the data line. The second measurement is made after the potentials have equilibrated, as in
Turning now to
Those skilled in the art will understand that other embodiments are possible. For example, after Step 440, the drive voltage can be set to another voltage, such as zero, by connecting ground 150 to power supply line 110 via switch 265, after which current flows again to reach the state shown in
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
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