Active matrix display device

The invention relates to an active matrix display device (6) comprising a display panel (2) with a matrix of display pixels (3), and row and column electrodes (11,12) coupled to the display pixels (3). Each of the display pixels (3) has a current mirror circuit adapted to receive a programming current (Iprog) via the column electrodes (11) and to reproduce the programming current (Iprog) for driving an emissive element (14). The display device (6) is further arranged to execute a calibration phase wherein a calibration voltage (Vcal) is applied at each column electrode (11) before the programming current (Iprog) is applied.

The invention relates to an active matrix display device comprising a display panel with a matrix of display pixels and row and column electrodes coupled to said display pixels, each of said display pixels having a current mirror circuit adapted to receive a programming current via said column electrodes and to reproduce said programming current for driving an emissive element.

US 2001/0052606 discloses a display device comprising a matrix of pixels at the area of crossings of row and column electrodes. The pixels each comprise a current mirror circuit to cope with transistor uniformity issues as a result of differences between drive transistors with respect to the charge carrier mobility and threshold voltage.

The current signals in these types of display devices are very low and the voltages involved show large spreads resulting in the disadvantage of long programming times for the display pixels.

It is an object of the invention to provide a display device, wherein the voltage is well-defined thereby allowing a reduction of the programming time for the display pixels.

This object is achieved by providing an active matrix display device that is further arranged to execute a calibration phase wherein a calibration voltage is applied at each column electrode before said programming current is applied and said calibration voltage is substantially maintained at said column electrode for each of said display pixels until said programming current is applied.

In this way the display device can be controlled such that the column lines are at a well-defined voltage at the moment that the programming current is applied to the display pixels. In other words, the display device is enabled to both apply a calibration voltage to the respective column electrodes and stabilize this calibration voltage for each display pixel along the column electrode. As a result, current programming of the display pixels may be performed faster. This advantage is particularly important for high resolution displays. An additional advantage is that the programming voltage is no longer dependent on the power supply voltage for the display pixel. It is noted that for a color display each of the column electrodes for the red, green and blue display subpixels may be fed with a common calibration voltage that is maintained at the display subpixels until the programming current for that subpixel is applied. It is further noted that the invention does not require that the calibration phase is executed each time that a programming current is applied to a display pixel, although this may be preferable to achieve an optimal effect.

In an embodiment of the invention the display device is arranged for simultaneous execution of said calibration phase for more than one row of said display pixels. In this way loss in addressing time as a result of the calibration phase is reduced or even negligible. If leakage is sufficiently low, the calibration stage can be performed at once for all rows of the display panel. The calibration phase may e.g. be executed each frame time.

In an embodiment of the invention each of said column electrodes or lines is coupled to at least one switch to apply said calibration voltage. This switch may be provided as a separate switch on the display panel, e.g. near the edge, or be implemented in the column driver. In an embodiment of the invention the switch connects said column electrodes to ground to obtain a calibration voltage of zero Volts such that the column line is at this well-defined voltage before application of the programming current. Alternatively a non-zero calibration voltage is applied, which may be advantageous in that a negative power supply voltage for the column driver that contains the programming current sources may be omitted.

In an embodiment of the invention each of said display pixels comprises calibration circuitry having a capacitor and a transistor whose current carrying electrodes are connected between said column electrode and a first plate of said capacitor, and is arranged to charge said capacitor prior to said calibration phase and to discharge during said calibration phase via said transistor such that the gate of said transistor carries a voltage substantially equal to the sum of said calibration voltage and a threshold voltage of said transistor. Such a display device is suited to execute the calibration phase.

In an embodiment of the invention the calibration circuitry comprises one or more switches to control said charging and discharging of said capacitor and the display device comprises a display controller to control said switches, e.g. via the row selection circuit.

In an embodiment of the invention a second plate of the capacitor is connected either to ground or to a substantially constant voltage supply. Preferably the second plate of the capacitor is connected to ground. However, the manufacturing technology employed for the display device may complicate or prevent a connection to ground of this plate, in which case connection to a constant voltage supply is preferred.

In an embodiment of the invention the display device comprises common calibration circuitry to execute said calibration phase for several display pixels along said column electrode. Such an arrangement may save space on the display panel as the calibration circuitry may be shared by some display pixels.

According to an aspect of the invention the product comprises the display device according to the invention and signal processing circuitry. The product may be a handheld device such as a mobile phone, a Personal Digital Assistant (PDA) or a portable computer as well as a device such as a monitor for a Personal Computer, a television set or a display on e.g. a dashboard of a car.

Preferably the display panel is a high resolution display panel as especially for such display panels the invention reduces or eliminates the effects of the voltage drop over the power lines for the display pixels. Further, the column line capacity is larger for such displays.

The invention also relates to a method for calibrating an active matrix display device comprising a display panel with a matrix of display pixels, and row and column electrodes coupled to said display pixels, each of said display pixels comprising a current mirror circuit adapted to receive a programming current via said column electrodes and to reproduce said programming current for driving an emissive element, comprising the steps of:

applying a calibration voltage to each column electrode before said programming current is applied;

substantially maintaining said calibration voltage at said column electrode until said programming current is applied.

The method results in a faster current programming for the display pixels as the column electrode is at a well-defined voltage at the moment of applying the programming current.

In an embodiment of the invention the calibration voltage is applied for more than one row of said display panel at once. Preferably the calibration stage is performed for the entire display at once, such that loss of addressing time is minimal.

The invention will be further illustrated with reference to the attached drawings, which show a preferred embodiment according to the invention. It will be understood that the invention is not in any way restricted to these specific and preferred embodiments.

FIG. 1shows a product1comprising an active matrix display device6and signal processing circuitry SP. The display device6comprises an active matrix display panel2having a plurality of display pixels3arranged in a matrix of rows4and columns5. The display panel2is an active matrix display comprising display pixels3containing polymer light emitting diodes (PLEDs) or small molecule light emitting diodes (SMOLEDs). The display panel2may be a high resolution display panel as the available programming times in such display panels are very small.

The product1may be a television receiver, in which case the signal processing circuitry SP may include circuitry for receiving a television signal and converting the television signal into a format for driving a data input10of the display device6. Alternatively, the product1may be a handheld device such as a mobile phone or a PDA, a portable computer or a monitor for a personal computer or any other product with a display device. In these cases the signal processing circuitry SP may include data processing circuitry.

FIG. 2shows a schematical illustration of an active matrix display device6, comprising a PLED display panel2of the product1as shown inFIG. 1having current emissive elements. The display device6comprises a display controller7, including amongst others a row selection circuit8and a column driver9. A data signal, comprising information or data such as for (video) images to be presented on the display panel2, is received via data input10by the display controller7. The data are written as programming currents to the appropriate display pixels3via the column driver9and data lines11. The selection of the rows4of display pixels3is performed by the row selection circuit8via selection lines12, controlled by the display controller7. Synchronization between selection of the rows4of display pixels3and writing of the data to the display pixels3is performed by the display controller7. Moreover the display controller7may control the power supply of the display pixels3via power line13.

FIG. 3shows a current programmable display pixel3in a current mirror configuration for a display panel2shown inFIG. 2. A driving transistor T2is used in both programming the display pixel3and in driving an emissive element14, such as a PLED element. The application of the programming current over the data line11is indicated by the current source Iprog. During the programming period a transistor T4connects a capacitor C with a current carrying electrode of the driving transistor T2while the emissive element14is isolated from the driving transistor T2by a transistor T3. During this programming phase the data input programming current is forced through T2while the capacitor C is charged or discharged depending on the previously programmed value to reach the associated gate-source voltage VGSfor T2. Now, by opening T1and T4and by closing T3, the drain current of the driving transistor T2is fed to the emissive element14. The memory function of the capacitor C assures that the current is a perfect copy of the programming current signal received over line11.

The current I through the driving transistor T2is:
I=Iprog=μ(V−Vt)2
wherein μ is the mobility of the charge carriers, Vt the threshold voltage of the driving transistor T2and V the gate-source voltage of the driving transistor T2. It is assumed here that the current I from the driving transistor T2is indeed identical to the programming current Iprog, which is a reasonable assumption for a display pixel3with a current mirror circuit. The programming voltage Vprogrepresenting the voltage that results from the application of the programming current Iprogtherefore yields:
Vprog=Vcc−Vt−√(Iprog/μ)
wherein Vccis the voltage supplied over the power line13. The current mirror circuit of the display pixel3shown inFIG. 3has the advantageous feature that at low frequencies, despite differences in mobility μ and threshold voltages Vt of the driving transistors between the various display pixels3, the current through the emissive element is an almost exact copy of the received programming current.

The programming currents Iprogare typically low, i.e. in the order of nanoamperes in the dark region to microamperes at full brightness of the emissive element14. The line capacitance of the column electrode11may be in the order of 100 pF. Thus for a difference in the programming voltage Vprogof 1 Volt between the upper and lower display pixel3, a programming current of 10 nanoamperes results in a period of 10 milliseconds to bring the column electrode11to the required voltage Vprog. Such long stabilization times limit operation of the display panel2at high frequencies. For high resolution displays2the capacitance of the column electrode11increases, thereby yielding worse performance. Further, the trend to use higher resolutions results in a decrease of the programming currents for each display pixel3.

FIG. 5shows a part of an active matrix display device6incorporating a display pixel3according to an embodiment of the invention. The display pixel3comprises circuitry identical to that shown inFIG. 4. Identical reference numerals indicate similar components of the circuitry in the display pixels3. The display pixel3further comprises calibration circuitry including switches S5and S6, a capacitor Ccaland a transistor Tcal. The capacitor Ccalhas one plate connected to ground and the other plate connected to the gate of the transistor Tcal. This plate and the gate of the transistor Tcalare connected via the switch S5to the voltage Vccof the power line13. Further this plate and the gate of Tcalare connected to a current carrying electrode of the transistor Tcalvia the switch S6. This current carrying electrode is further connected to the current mirror circuit of the display pixel3shown inFIG. 3. The other current carrying electrode of the transistor Tcalis connected to the column electrode11. The switches S5and S6may be controlled by the display controller7via the row selection circuit via selection lines12(not shown inFIG. 5) as are the other switches. It should be appreciated that switches S5and S6can be implemented as transistors in the display pixel3according to the invention.

It is further noted that the capacitor Ccalis not necessarily connected to ground, although this is a preferred arrangement. Instead the capacitor plate may be connected to a substantially stable voltage, such as Vcc.

Further the column electrode11is connected to a voltage Vcalvia a switch Scal.

An example of the operation of the active matrix display device6shown inFIG. 5is provided inFIGS. 6A-6C.

InFIG. 6Athe display pixel3is not programmed and the voltage over the capacitor C may cause T2to drive the current emissive element14. It should be appreciated that the invention does not require that light is emitted from the emissive element14. The switch S5is closed such that Ccalis charged to a level equal to Vccsaturating the calibration transistor Tcalprior to the calibration phase. However, as S1and S6are open, no current flows through Tcal.

FIG. 6Bshows an example for the implementation of the calibration phase. Still switch S1is open such that the display pixel3is not programmed by charging the capacitor C. In this calibration phase the switch Scalis closed applying a calibration voltage Vcalof e.g. 0 Volts to the column electrode11. Further switch S6is closed leading to a discharge of the calibration capacitor Ccalresulting in a current through the switch S6and the transistor Tcal. The gate voltage of Tcalwill decrease until Tcalstops conducting, the gate voltage then yielding the threshold voltage Vt of the transistor Tcal. At this moment the voltage of the column electrode11is well-defined at 0 Volts. This calibration voltage is substantially maintained at the column electrode11for each display pixel3until the current signal Iprogis applied in the programming phase as illustrated inFIG. 6C.

It should be appreciated that if Vcalis set at a non-zero voltage V1, Tcalwill stop conducting if the gate voltage equals Vt+V1. If Vcalis chosen to have a non-zero value V1, the column driver9can be implemented without a negative voltage supply. Such a supply may be required if the column driver(s)9is to absorb currents at zero volts on the column electrode11.

It should further be appreciated that during the calibration phase the emissive element14may still emit light as programmed in a prior programming phase.

FIG. 6Cillustrates the programming phase wherein the display pixel3is programmed by charging the capacitor C to the adequate voltage. Accordingly, switches S1and S4are closed and switch S3is opened. Further the switch Scalis opened to allow the programming current to flow into the display pixel3of the column electrode11. The capacitor Ccalensures maintenance of the voltage on the column electrode11after opening of the switch Scal. As S5and S6are opened the gate voltage of the calibration transistor Tcalwill not change and is fixed at the threshold voltage Vt. The programming current will flow through Tcal, S1and S4such that the voltage over the capacitor C increases or decreases to a value where the current through the driving transistor T2is equal to the programming current Iprog.

It is noted that the switches S1and S6are open for the non-programmed display pixels3along the column electrode11as displayed e.g. inFIG. 6A. The states of the other switches S3, S4and S5are not essential for the invention. If e.g. a non-addressed display pixel3is to emit light, switch S3is closed and switch S4is open. If the display pixel3should not emit light for a particular percentage of the frame time when it is not addressed, i.e. a reduced duty cycle applies, the switch S3should be open for this percentage of the frame time.

The calibration phase described above is executed row-wise for each column5. However, it is advantageous to execute the calibration phase for more than one row4of display pixels3at a time or even for the whole display panel2at once. The latter option requires the charge on Ccalto be sufficiently stable, i.e. no or negligible leakage, over the relevant period of time, i.e. the time during which the calibration voltage Vcalshould be maintained for the display pixel3. The initiation of the calibration phase for one or more rows4can be controlled from the display controller7.

The result of the calibration phase is that the display pixels3can be quickly current programmed as a result of the reduced voltage swing. Only in extreme cases the voltage swing on the column electrode11may be a few volts. Typically if the programming current increases from 1 nanoampere to 1 microampere, the voltage swing is a few millivolts which is considerably less than in the prior art display devices. As a consequence display panels2with higher resolutions can be applied. Further, the programming voltage Vprogis no longer dependent on the voltage Vccof the power line13. The gist of the invention is that the modified display pixel circuit features a well-defined input voltage that is independent of the spread in the characteristics of the driving transistors T2between the various display pixels3on the display panel2. The considerable reduction of the voltage swing on the column electrodes11enhances the current programming speed such that displays with higher resolutions can be operated. A disadvantage of the active matrix display device6according to the invention is the increase in the area accommodated by circuitry for each display pixel3which is detrimental for the aperture of the display pixel. However, for top emission display panels2, wherein the light of the emissive element14is emitted away from the display pixel circuitry, this is not an issue.

The purpose of the calibration circuitry in the display pixel3is to deal with the variation in the threshold voltages of the driving transistor T2in the display pixel3itself such that the long column electrode11does not experience such a variation. The variation however is still present between Tcaland T2in the display pixel. In this part such a variation is less or not harmful because of the low line capacity. As the line capacitance is relatively low, the use of a single calibration circuit for more than one display pixel3at the same column electrode11is possible, as shown inFIG. 7. In this embodiment, the line capacity is slightly higher compared to the arrangement wherein each display pixel or display subpixel has a dedicated calibration circuitry, since this capacity is increased by the line distance between Tcaland S1of the different display pixels3. However this line capacity is still significantly lower than that of the column electrode11.