Patent Publication Number: US-9852689-B2

Title: Circuit and method for driving an array of light emitting pixels

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a U.S. continuation application of U.S. application Ser. No. 14/568,517, filed Dec. 12, 2014, now allowed, which is a continuation of U.S. application Ser. No. 14/046,480, filed Oct. 4, 2013, now U.S. Pat. No. 8,941,697, which is a continuation of U.S. application Ser. No. 13/113,651, filed May 23, 2011, now U.S. Pat. No. 8,553,018, which is a continuation of U.S. application Ser. No. 10/554,754, filed Oct. 28, 2005, now U.S. Pat. No. 7,978,187, which is a U.S. national phase of International Application No. PCT/CA2004/001742, filed Sep. 23, 2004, which claims the benefit of priority of Canadian Patent Application No. 2,472,689, filed Jun. 29, 2004, and Canadian Patent Application No. 2,443,206, filed Sep. 23, 2003, each of which are incorporated by reference in their entirety. 
    
    
     COPYRIGHT 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. 
     FIELD OF INVENTION 
     The present invention generally relates to a light emitting device display techniques, and more particularly, to a technique for driving light emitting elements that uses a feedback architecture during programming to compensate for instability and non-uniformity of the pixels. 
     BACKGROUND OF THE INVENTION 
     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 HE 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. 
     SUMMARY OF THE INVENTION 
     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 MED 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 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention encompasses a technique for driving of columns of pixels where each pixel comprises a light emitting element, in particular, an organic light emitting diode (OLED). 
       FIG. 1  presents a display device having a feedback control-system architecture  10  and an array of addressable pixels  11 . The pixels  11  are controlled by a select line driver  12  and data driver  13 . As shown in  FIG. 1 , a separate feedback control unit  14  is provided on each column line of the array. The feedback control unit  14  of a given column is connected to each pixel in the column via a signal line  15  and a feedback line  16 . A block of reference elements  17 , located on the display substrate, may also be provided. The block of the reference elements  17  includes some elements of the pixel circuit for input signal corrections and may also include a photo-sensor that is optically coupled to a light emitting element to implement a luminance feedback. 
     The structure of a given pixel  11 , according to an embodiment of the invention is shown in  FIG. 2 . As shown in  FIG. 2 , the pixel has an OLED  21 , a current driving circuit  22 , controlled by the stored voltage level using a storage capacitor  23 , a feedback circuit  24 , and switches S 1  and S 2 . The switches S 1  and S 2  can be any suitable switching device, but are preferably an insulating gate type field effect transistor. The pixel  11  operates in writing and hold modes. In the writing mode, when select line (s) are activated, the switches S 1  and S 2  are turned on, and the current driving circuit  22  receives the signal voltage from control unit  14 , while the on-pixel feedback circuit  24  feeds the voltage feedback signal. The driving current through the OLED  21  can thereby be accurately controlled through the use of negative feedback. In the hold mode, the switches S 1  and S 2  are turned off and the driving circuit  22  provides the driving current having a current level in accordance with the voltage level to the storage capacitor  23 . 
       FIG. 3A  shows a pixel drive circuit according to an alternative embodiment and a circuit diagram of the control unit  14 . Controlling signals are shown in  FIG. 3B . 
     The pixel drive circuit comprises three transistors  34 , 36  and  38 , a resistor  32 , a storage capacitor Cs and an OLED  31 . The pixel drive circuit is connected to a select line, a feedback line, and a signal line. A power supply node having a positive potential Vdd and common ground are also shown. 
     Transistors  34 , 36  and  38  can be fabricated using amorphous silicon, poly silicon, appropriate organic semiconductors and NMOS or CMOS technologies. The on-pixel feedback circuit is consisted of a thin film resistor  32  that can be fabricated with any appropriate material and technology, which provides sufficient stability. For instance, in amorphous silicon technology the resistor  32  can be fabricated using N+ amorphous silicon or N+ microcrystalline silicon. 
     The drain terminal of driving transistor  36  is connected to the cathode of OLED  31 . The source terminal of transistor  36  is connected to resistor  32  and the gate terminal is connected to the signal line through transistor  34 . Resistor  32  is connected between the source terminal of transistor  36  and the common ground. 
     Transistors  34  and  38  are driving switch and feedback switch transistors, respectively. The gate terminals of transistors  34  and  38  are connected to the select line. The source terminal of transistor  34  is connected to the signal line and the drain terminal is connected to the gate terminal of transistor  36 . The source terminal of transistor  38  is connected to the feedback line and the drain terminal is connected to resistor  32 . All OLEDs of the different pixels have a common anode electrode, connected to the voltage supply node (Vdd). Storage capacitor Cs is connected between the gate terminal of transistor  36  and common ground. IL can be connected between gate and source terminals of transistor  36 . For the latter, capacitor Cs can be implemented by the gate-source capacitance of transistor  36 . 
     The external controlling unit  33  in its simplest form is a high-gain, low offset difference amplifier with a negative feedback connection. 
     During the writing mode, the select signal goes high, turning on transistors  34  and  38 , As a result, the driving transistor  36 , along with the external difference amplifier  33  and resistor  32  make a circuit with negative feedback. The difference in the voltage level between an input signal voltage and a voltage drop across the resistor  32  is amplified by the difference amplifier  33 , adjusting the potential on the gate of transistor  36 . After the initial transients the output current stabilizes and in the case of a high-gain feedback loop the current passing through the OLED  31  is: 
                     I   OLED     =       Vinp   Rf     .             (   1   )               
During the hold mode, the select line goes low, so transistors  34  and  38  are turned off and the pixel is disconnected. Since the gate voltage of driving transistor  36  is stored in capacitor Cs, the drive current does not change during the hold mode.
 
     In the configuration shown in  FIG. 3A , the current of the pixel  31  depends on the absolute resistance of resistor  32 , which is not desirable due to possible inherent inaccuracy and poor thermal stability of integrated resistors.  FIG. 4  presents an architecture, according to another embodiment of the invention that addresses this by implementing a reference resistor  42  and an external data current source  41 . The reference resistor  42  is made with the same material as integrated resistors and formed on the display substrate. This enhances the temperature stability of the circuit. The programmed level of the drive current for this circuit is: 
                       I   OLED     =     Iinp   ⁢     Rr   Rf         ,           (   2   )               
where Rr is the resistance of the reference resistor  42 , and Rf is the resistance of the feedback resistor  32 . The above equation indicates a considerable improvement in the accuracy of the programming current because of insensitivity of the resistance ratio to the temperature variations.
 
     A current pixel drive circuit according to another embodiment of the invention and a section of the column driver circuitry are shown in  FIG. 5 . The circuit is similar to the circuit of  FIG. 3A  however, in the circuit of  FIG. 5 , the cathode of OLED  51  is common and is connected to a negative power supply potential Vss. As a result, the cathode of the OLEDs is not patterned. 
     The anode of OLED  51  is connected to the source terminal of transistor  56 . The feedback resistor  32  is connected between the drain terminal of transistor  56  and ground node. The voltage level of the select line during the writing mode should be high enough to guarantee that transistor  54  is in“on”state for the entire output current range. The feedback line in this configuration is connected to the non-inverting input of the difference amplifier  33  to provide a negative feedback. 
       FIGS. 6A, 7 and 8  illustrate pixel drive circuit, according to other aspects of the invention wherein p-channel MOS transistors are used. 
       FIG. 6A  shows a pixel circuit, according to another embodiment of the invention. The feedback switch use transistor  68  is p-channel transistor. The circuit is similar to the circuit of  FIG. 5 , however the implementation of the PMOS transistor requires an additional select line.  FIG. 6B  shows corresponding waveforms for select line A and select line B. The advantage of this circuit over the circuit of  FIG. 5  is the lower voltage swing for the select lines that is required. 
       FIG. 7  shows a pixel circuit according to another embodiment of the invention. The transistors  76  and  74  are p-channel transistors and the transistor  78  is an n-channel transistor. As an embodiment of  FIG. 7  this circuit also has two select lines marked as A and B having reduced voltage swing. 
     In the pixel circuit shown in  FIG. 8 , all of the transistors are p-channel transistors. Here the anode of the OLED  51  is connected to the drain terminal of the transistor  76  and the common cathode electrode of the OLED  51  is connected to the negative power supply potential Vss. 
       FIGS. 9 and 10  show configurations of the pixel circuits according to alternative embodiments of the invention. In these pixel circuits, the current driving circuitry is based on a current mirror architecture, i. e. transistors  96  and  97  and  108  and  110 . The current level of the signal current and the current level of the drive current are proportional. In the circuit of  FIG. 9 , all transistors are n-channel transistors and in the circuit of  FIG. 10  all transistors are p-channel transistors. 
     In  FIG. 10 , the feedback resistor  32  is connected between the drain terminal of transistor  106  and common ground. The gate terminals of the transistors  106  and  110  are connected. In the circuit of  FIG. 9 , the cathode electrode of OLED  31  is connected to the drain terminal of transistor  97 , the anode is common and the transistor  97  is the driving transistor and is connected to OLED  31 . In the circuit of  FIG. 10 , the cathode of the OLED  51  is common and the anode is connected to the drain terminal of the transistor  110 . 
     During writing mode, the transistors  104  and  108  are in an“on”state, thus the transistor  106  alone with feedback resistor  32  and external control unit (the difference amplifier  33 ) form a feedback loop. The transistor  110  does not directly take part in the feedback loop, but since the transistors  110  and  106  have same gate-source voltage, the current, of the transistor  110  is proportional to the current of the transistor  106 . The ratio of current through transistors  110  to  106  is determined by the aspect ratios of these transistors. In these circuits, the feedback resistor  32  and the OLED  31  of  FIG. 9 and 51  of  FIG. 10 , are not in the same current path thus a higher lifetime is expected. 
     Several methods have been used to reduce the charge injection and clock feed-through effects in integrated circuits. As the simplest approach, a dummy transistor that is driven by the inverse signal of the select line connected to the gate of driving transistor can reduce both charge injection and clock feed-through errors caused by the driving switch. The drain and source terminals of the dummy transistor are connected to the gate of the driving transistor.  FIG. 11  shows an example of such modification for the embodiment of  FIG. 3 . The width of dummy transistor  118  is half of the width of driving transistor  116 . It will be apparent to one skilled in the art that the width of the dummy transistor  118  can be any appropriate size. 
       FIG. 12  is a schematic circuit diagram of another embodiment of a pixel circuit, column control unit and a reference cell according to the present invention. Here, the implemented luminance feedback improves the linearity of the video signal—light output characteristics, and also provides a compensation for brightness instability caused by instability of the organic material, ageing, temperature changes, or other environment stresses. The compensating circuit with luminance feedback includes a resistor R 1 , a difference amplifier  121 , and a NMOS transistor  122 , which are parts of the control unit, and the elements of the reference cell  123  including an OLED  124 , and photodiode  125 . The photodiode  125  is optically coupled to the reference OLED  124  to form a feedback current signal in response to emitted light. The circuit is balanced when the input current passing through the resistor R 1  is equal to a feedback current generated by the photodiode  125 . The current flowing through OLED  124  via transistor  122  and resistor  42  is an input signal for next stage of the device, which is the same as the embodiment of  FIG. 4 . 
       FIG. 13  is a schematic diagram of an alternative embodiment of the embodiment of  FIG. 4 . In this embodiment, diodes D 1  and D 2  are used in place of feedback resistor R 1  and reference resistor R 2  of  FIG. 3 , respectively. As circuit functionality with reasonably low error in the programmed current level requires a good match between the reference diode and pixel diodes, the fabrication technology must be efficient for fabrication of the diode array with reproducible forward current-voltage characteristics. 
     A schematic diagram of a circuit according to another embodiment of the invention is shown in  FIG. 14 . This circuit implements a common cathode OLED array configurations. in the writing mode, the input current from an external current data source  41  creates a voltage drop across a reference OLED  141 . A difference amplifier  33  in negative feedback connection is designed to hold the same voltage level on a pixel OLED  142 . During the hold mode, the current with a programmed current level flows through both the transistor  146  and the OLED  142  due to the voltage stored across the capacitor Cs. 
     Although the exemplary embodiments of the present invention are described in conjunctions with OLEDs, it is also contemplated other similar display elements such as a light emitting diode (LED) could be used in other embodiments. 
     The present invention has been described with regard to one or more embodiments. However, it will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.