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Researchers Submit Patent Application, "Circuit and Method for Driving an Array of Light Emitting Pixels", for Approval

February 19, 2014



By a News Reporter-Staff News Editor at Electronics Newsweekly -- From Washington, D.C., VerticalNews journalists report that a patent application by the inventors Nathan, Arokia (Cambridge, GB); Vygranenko, Yuriy (Waterloo, CA); Jafarabadiashtiani, Shahin (Waterloo, CA); Servati, Peyman (Vancouver, CA), filed on October 4, 2013, was made available online on February 6, 2014.

The patent's assignee is Ignis Innovation Inc.

News editors obtained the following quote from the background information supplied by the inventors: "Recently active-matrix organic light-emitting diode (OLED) displays have become more attractive due to advantages over conventional liquid crystal flat displays. These advantages include the ability to fabricate OLED displays at a relatively low cost and high efficiency. Further the displays do not require backlighting and provide a wide viewing angle.

"An active-matrix organic light-emitting diode (AMOLED) display compromises an array of rows and columns of pixels, each having an OLED and some active devices such as thin film transistors. Since OLEDs are current driven devices the pixel circuit of an AMOLED should be capable of providing an accurate and constant drive current to achieve a consistent and uniform luminance.

"As disclosed in U.S. Pat. No. 5,748,160, a simple pixel circuit comprises two thin film transistors (TFTs) and an OLED. In this circuit, the OLED is connected to the drain terminal of a driving TFT and a gate terminal of the driving TFT is connected to a column line through a switching TFT. A storage capacitor connected between the gate terminal of the driving TFT and ground is used to maintain the voltage at the gate terminal of the driving TFT when the pixel circuit is disconnected from column line. For this circuit the current through OLED strongly depends on the characteristic parameters of the driving TFT. Since the characteristic parameters of a TFT, particularly, the threshold voltage under bias stress, vary by time, and such changes may differ from pixel to pixel, the induced image distortion may be unacceptably high.

"One of the methods that has been employed to make the current driving circuit less sensitive to the shift in the threshold voltage is programming the pixel with current instead of voltage. In this method, the OLED current is less dependent on the voltage-current characteristics of driving transistors. Implementations of current programmed pixel circuits for OLEDs have been disclosed e.g. Yi H E et al. 'Current Source a-Si: H Thin-Film Transistor Circuit for Active Matrix Organic Light-Emitting Displays', IEEE Electro Device Letters, Vol. 21, No. 12, p 590-592, December 2000). A drawback of the current programming method is that it is slow, particularly for low programming current levels, due to the large line capacitance. As a result, voltage programming methods are desirable considering their speed. This is particularly true for large area TVs and displays.

"Another method to make the drive current less sensitive to transistor parameters is to use current feedback. United States patent application 20020101172A1 provides a driving system with current feedback. An external current comparator compares the pixel current with a reference current and generates an appropriate signal to control the pixel current. One drawback of the disclosed method is that the control signal is a current, which can limit the programming speed. Another drawback of the method is that the anode and cathode electrode of each OLED have to be patterned, which creates reliability concerns in the currently used OLED fabrication process.

"Luminance feedback is another method that has been used to stabilize OLED luminance. As described in U.S. patent application 20030151569 feedback readout circuits responsive to the feedback signal representing the light output of the OLED can be used to provide brightness control. A drawback of the disclosed method is that every pixel requires a photo-sensor that is optically coupled to the OLED. This results in integration issues. Another drawback is that the low level of the feedback signal generated by a photo-sensor may lead to the poor signal-noise ratio, thereby narrowing the dynamic range of the system."

As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "The present invention provides several driving circuits having a feedback control-system architecture that can be used for driving a column of the light emitting devices and are suitable for use in SMOLED displays. In the present invention, a feedback voltage is generated by an on-pixel feedback circuit or element. This voltage is used to adjust the programming voltage of the pixel.

"According to an aspect of the invention each pixel in the column is connected to the feedback-type control unit via signal line and feedback lines, and receives a scanning clock signal via select line connection terminal. The programming voltage applied to the pixel through the signal line sets the driving current through the light emitting element. The programming voltage can be accurately adjusted by an external control unit through the use of feedback voltage generated by the on-pixel feedback circuit. The feedback voltage is proportional to the driving current of the light emitting element and is used to set the programming voltage so as to achieve the desired driving current despite presence of any instability (shift in characteristics of transistors and light emitting element) and non-uniformity across pixels

"The column control unit may be connected to the block of reference elements formed on the display substrate in order to correct an error in the output current level caused by inaccuracy of the pixel components or temperature drift. The block of reference elements may also include a photo-sensor optically coupled to the light emitting element in order to provide a luminance feedback compensation for brightness variations induced by instability of organic material or temperature changes.

"According to another aspect of the invention a pixel circuit for use in a display is provided. The display comprising a plurality of pixels with each pixel having a select line, a signal line, a feedback line. The pixel circuit comprising a light emitting element, a drive part for providing a drive current to the light emitting element, the drive part having a storage capacitor and a switch use transistor having a gate terminal connected to the select line, and a first terminal connected to the signal line, and a second terminal, and an on-pixel feedback element for generating a feedback voltage representing a drive current provided to the light emitting element, the feedback signal being provided to the feedback line.

"According to another aspect of the invention a pixel circuit for use in a display is provided. The display comprising a plurality of pixels with each pixel having a first select line, a second select line, a signal line, a feedback line. The pixel circuit comprising a light emitting element, a drive part for providing a drive current to the light emitting element, the drive part comprising a storage capacitor, a switch use transistor having a gate terminal connected to the first select line, a first terminal connected to the signal line and a second terminal, a drive use transistor having a gate terminal connected to the second terminal of the switch use transistor, a first terminal and a second terminal connected to the light emitting element, and an on-pixel feedback circuit for generating a feedback voltage representing a drive current provided to the light emitting element. The feedback circuit comprising a resistor connected between the second terminal of said drive use transistor and a potential, and a feedback transistor having a gate connected to the second select line, a first terminal connected to the first terminal of the drive use transistor and a second terminal connected to the feedback line.

"According to another aspect of the invention a pixel circuit for use in a display is provided. The display comprising a plurality of pixels with each pixel having a select line, a signal line, a feedback line. The pixel circuit comprising a light emitting element, a drive part for providing a drive current to the light emitting element, the drive part comprising, a storage capacitor, a switch use transistor having a gate terminal connected to the select line, a first terminal connected to the signal line and a second terminal, a drive use transistor having a gate terminal connected to the second terminal of the switch use transistor, a first terminal and a second terminal connected to the light emitting element, and an on-pixel feedback circuit for generating a feedback voltage representing a drive current provided to the light emitting element. The feedback circuit comprising a resistor connected between the second terminal of said drive use transistor and a potential, and a feedback transistor having a gate connected to the select line, a first terminal connected to the first terminal of the drive use transistor and a second terminal connected to the feedback line.

"According to another aspect of the invention a display device is provided. The display device comprising a select line, signal line to which a voltage signal in accordance with both brightness and feedback information is supplied, a feedback line to which a feedback voltage signal in accordance with current level of drive current is supplied, a plurality of pixels forming an array of pixels, each pixel of the plurality formed on a substrate at an intersecting portion of said scanning line and said signal and feedback lines, each pixel comprising a light emitting element, a current driving circuit having a storage capacitor and a switch use transistor, and a feedback circuit to provide feedback signals representing a current output of said current driving circuit, a display column control circuit for receiving input signals, adjusting the input signals using a reference circuit formed on the substrate at each column, and modifying the input signals in response to the feedback signals from pixels in the column to produce a desired brightness level of said light emitting element in a selected pixel, and a selecting line drive circuit for successively activating selecting lines.

"According to another aspect of the invention a method of driving a plurality of light emitting elements arranged in a column at a desired brightness is provided. The method comprising the steps of selecting one pixel of a plurality of pixels in the column, establishing the desired brightness of a reference light emitting element by adjusting a reference current flowing through the light emitting element in response to a photocurrent from a photo-sensor that is optically coupled with the reference light emitting element, converting the reference current into a corresponding voltage level, transmitting the voltage level to the selected pixel, converting the voltage level into a drive current and generating a feedback signal representing a drive current level, adjusting the voltage level in response to the feedback signal from the selected pixel to establish a drive current substantially equal to the reference current, storing the adjusted voltage level, and driving the light emitting element with the drive current in accordance with the adjusted voltage level to produce the desired brightness level in the pixel.

"Advantages of the present invention include the ability to provide a stable current to the light emitting diode over time, thereby maintaining image quality. Moreover, the combination of the external current feedback for pixel programming and luminance feedback for data signal preprocessing provides brightness control and compensation despite instability and non-uniformity in pixels. The circuits occupy a small area and are voltage programmed with voltage feedback. The use of voltage for programming and feedback improves the programming speed, which is necessary for large area displays and TVs.

"This summary of the invention does not necessarily describe all features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

"These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings wherein:

"FIG. 1 is a block diagram of an example of the configuration of a display device with feedback control-system architecture according to an embodiment of the invention;

"FIG. 2 is a block diagram of pixel architecture according to an embodiment of the invention;

"FIG. 3A is a circuit diagram of a pixel circuit and column control unit according to an embodiment of the invention;

"FIG. 3B depicts the corresponding waveforms for the circuit of FIG. 3A according to an embodiment of the invention;

"FIG. 4 is a circuit diagram of a modification. of the embodiment of FIG. 3A;

"FIG. 5 is a schematic of a pixel circuit for a common cathode OLED configuration according to an embodiment of the invention;

"FIG. 6A is a circuit diagram of a column control unit and a pixel circuit having p-channel type transistors according to an embodiment of the invention;

"FIG. 6B depicts the corresponding waveforms for the circuit of FIG. 6A according to an embodiment of the invention;

"FIG. 7 is a circuit diagram of a column control unit and a pixel circuit with a p-channel type transistor switch according to an embodiment of the invention;

"FIG. 8 is a circuit diagram of a column control unit and a pixel circuit having p-channel and n-channel type transistors according to an embodiment of the invention;

"FIG. 9 is a circuit diagram of a column control unit and a pixel circuit with a current mirror as current driving circuit according to an embodiment of the invention;

"FIG. 10 is a circuit diagram of a modification of the embodiment of FIG. 9;

"FIG. 11 is a circuit diagram of a modification of the embodiment of FIG. 3;

"FIG. 12 is a circuit diagram of a pixel circuit, column control unit and a reference cell with implemented luminance feedback according to an embodiment of the invention;

"FIG. 13 is a circuit diagram of a pixel circuit and column control unit with a reference diode according to an embodiment of the invention;

"FIG. 14 is a circuit diagram of a pixel circuit, column control unit with a reference OLED according to an embodiment of the invention;

"FIG. 15 is a circuit diagram of a modification of the embodiment of FIG. 14."

For additional information on this patent application, see: Nathan, Arokia; Vygranenko, Yuriy; Jafarabadiashtiani, Shahin; Servati, Peyman. Circuit and Method for Driving an Array of Light Emitting Pixels. Filed October 4, 2013 and posted February 6, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=4650&p=93&f=G&l=50&d=PG01&S1=20140130.PD.&OS=PD/20140130&RS=PD/20140130

Keywords for this news article include: Electronics, Light-emitting Diode, Ignis Innovation Inc..

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Source: Electronics Newsweekly


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