Display, pixel circuit, and method

Active Matrix Organic Light Emitting Diode (AMOLED) displays, novel pixel circuits therefor, and methods of programming the pixel circuit and measuring the current of the pixel circuit and OLED thereof are disclosed. One pixel circuit includes four TFT transistors, a storage capacitor and an OLED device and is programmed with use of voltage supplied through a data line. One method measures currents of the OLED and the pixel circuit through the data line by a readout circuit.

FIELD OF THE PRESENT DISCLOSURE

The present disclosure relates to active matrix organic light emitting diode (AMOLED) displays and particularly to pixel circuits thereof and methods of driving and measuring pixel and organic light emitting diode (OLED) currents in order to extract pixel and OLED parameters.

BRIEF SUMMARY

According to a first aspect there is provided a display system comprising: an array of pixel circuits arranged in rows and columns, a pixel circuit of the array of pixel circuits including: a drive transistor including a source terminal coupleable to a data line of the display system; a storage capacitor coupled across a gate terminal and the source terminal of the drive transistor; and a light emitting device coupleable to a drain terminal of the drive transistor different from the source terminal, and a controller for driving the pixel circuit in a plurality of operation states for the pixel circuit including a programming state for programming the storage capacitor of the pixel circuit with use of a data voltage provided over the data line, and a measurement state for measuring a current from the pixel circuit over the data line.

In some embodiments, the display system further comprises a readout circuit coupleable to the data line for measuring the current from the pixel circuit over the data line.

In some embodiments, the readout circuit comprises an integrator for integrating said current from the pixel during said measuring and generating an output voltage corresponding to said integrated current, and an analog to digital converter for converting said output voltage into a digital code output.

In some embodiments, the readout circuit is not coupleable to the pixel circuit via a signal line different from the data line for measuring the current from the pixel circuit.

In some embodiments, the measurement state for measuring a current from the pixel circuit comprises an organic light emitting diode (OLED) measurement state for measuring an OLED current from the pixel circuit passing through said light emitting device.

In some embodiments, the pixel circuit further comprises a reference line coupleable to a gate terminal of the drive transistor, and in which the controller, during the OLED measurement state, couples the gate terminal of the drive transistor to the reference line and provides a reference voltage over the reference line sufficient to turn on the drive transistor such that it acts as a closed switch, couples the source terminal of the drive transistor to the data line and provides a data voltage over the data line sufficient to turn on the light emitting device.

In some embodiments, the display system further comprises a readout circuit coupleable to the data line for measuring the current from the pixel circuit over the data line, the readout circuit comprising an integrator for integrating said OLED current from the pixel during said measuring and generating a corresponding output voltage, and an analog to digital converter for converting said output voltage into a digital code output, in which the controller couples the gate terminal of the drive transistor to the reference line with use of a first transistor in the pixel circuit, and couples the source terminal of the drive transistor to the data line with use of a second transistor coupled between the source terminal and the data line.

In some embodiments, the measurement state for measuring a current from the pixel circuit comprises a pixel circuit measurement state for measuring a pixel circuit current from the pixel circuit passing through said drive transistor according to the voltage difference across the storage capacitor, said pixel circuit measurement state subsequent to the programming state.

In some embodiments, the pixel circuit further comprises a reference line coupleable to a gate terminal of the drive transistor, in which the controller, during the pixel circuit measurement state, decouples the reference line from the gate terminal of the drive transistor to maintain the voltage difference across the storage capacitor, and couples the source terminal of the drive transistor to the data line.

In some embodiments, the display system further comprises a readout circuit coupleable to the data line for measuring the current from the pixel circuit over the data line, the readout circuit comprising an integrator for integrating said pixel circuit current from the pixel circuit during said measuring and generating a corresponding output voltage and an analog to digital converter for converting said output voltage into a digital code output, and in which the controller during the pixel circuit measurement state, decouples the reference line from the gate terminal with use of a first transistor coupled between the gate terminal of the drive transistor and the reference line, and couples the source terminal of the drive transistor to the data line with use of a second transistor coupled between the source terminal and the data line.

In some embodiments, the pixel circuit comprises transistors which are only p-type thin film transistors (TFTs), and in which said light emitting device is an OLED.

According to a second aspect there is provided a method of driving a display system, the display system including an array of pixel circuits arranged in rows and columns, a pixel circuit of the array of pixel circuits including: a drive transistor including a source terminal coupleable to a data line of the display system; a storage capacitor coupled across a gate terminal and the source terminal of the drive transistor; and a light emitting device coupleable to a drain terminal of the drive transistor different from the source terminal, the method comprising: driving the pixel circuit in a plurality of operation states for the pixel circuit including: programming the storage capacitor of the pixel circuit with use of a data voltage provided over the data line during a programming state, and measuring a current from the pixel circuit over the data line during a measurement state.

In some embodiments, measuring the current from the pixel circuit comprises coupling a readout circuit to the data line and measuring said current from the pixel circuit with use of said readout circuit.

In some embodiments, measuring said current from the pixel circuit with use of said readout circuit comprises integrating said current from the pixel circuit, generating a corresponding output voltage, and converting said output voltage into a digital code output.

In some embodiments, measuring the current from the pixel circuit comprises measuring an OLED current from the pixel circuit passing through said light emitting device during an OLED measurement state.

In some embodiments, the pixel circuit further comprises a reference line coupleable to a gate terminal of the drive transistor, and in which measuring the OLED current during the OLED measurement state comprises, coupling the gate terminal of the drive transistor to the reference line, providing a reference voltage over the reference line sufficient to turn on the drive transistor such that it acts as a closed switch, coupling the source terminal of the drive transistor to the data line, and providing a data voltage over the data line sufficient to turn on the light emitting device.

In some embodiments, measuring the OLED current during the OLED measurement state comprises: coupling the gate terminal of the drive transistor to the reference line with use of a first transistor in the pixel circuit; coupling the source terminal of the drive transistor to the data line with use of a second transistor coupled between the source terminal and the data line; and coupling a readout circuit to the data line and measuring said current from the pixel circuit with use of said readout circuit, including, integrating said OLED current from the pixel circuit, generating an output voltage corresponding to the integrated current, and converting said output voltage into a digital code output.

In some embodiments, measuring said current from the pixel circuit comprises measuring a pixel circuit current from the pixel circuit passing through said drive transistor according to the voltage difference across the storage capacitor, during a pixel circuit measurement state subsequent to the programming state.

In some embodiments, measuring the pixel current during the pixel circuit measurement state comprises decoupling the reference line from the gate terminal of the drive transistor to maintain the voltage difference across the storage capacitor and coupling the source terminal of the drive transistor to the data line.

In some embodiments, measuring the pixel circuit current during the pixel circuit measurement state comprises: decoupling a reference line from the gate terminal of the drive transistor with use of a first transistor coupled between the gate terminal of the drive transistor and the reference line; coupling the source terminal of the drive transistor to the data line with use of a second transistor coupled between the source terminal and the data line; and coupling a readout circuit to the data line and measuring said current from the pixel circuit with use of said readout circuit, including, integrating said pixel circuit current from the pixel circuit, generating an output voltage corresponding to the integrated current, and converting said output voltage into a digital code output.

The foregoing and additional aspects and embodiments of the present disclosure will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments and/or aspects, which is made with reference to the drawings, a brief description of which is provided next.

DETAILED DESCRIPTION

An OLED device is a Light Emitting Diode (LED) in which the emissive electroluminescent layer is a film of organic compound that emits light in response to an electric current. This layer of organic material is situated between two electrodes; typically, at least one of these electrodes is transparent. Compared to conventional Liquid Crystal Displays (LCDs), Active Matrix Organic Light Emitting Device (AMOLED) displays offer lower power consumption, manufacturing flexibility, faster response time, larger viewing angles, higher contrast, lighter weight, and amenability to flexible substrates. An AMOLED display works without a backlight because the organic material of the OLED within each pixel itself emits visible light and each pixel consists of different colored OLEDs emitting light independently. The OLED panel can display deep black level and can be thinner than an LCD display. The OLEDs emit light according to currents passing through them supplied through drive transistors controlled by programming voltages. The power consumed in each pixel has a relation with the magnitude of the generated light in that pixel.

The quality of output in an OLED-based pixel depends on the properties of the drive transistor, which is typically fabricated from materials including but not limited to amorphous silicon, polysilicon, or metal oxide, as well as properties of the OLED itself. In particular, the critical drawbacks of OLED displays include luminance non-uniformity due to the electrical characteristic variations of the drive transistor such as threshold voltage and mobility as the pixel ages and image sticking due to the differential aging of OLED devices. In order to maintain high image quality, variation of these parameters must be compensated for by adjusting the programming voltage. In order to do so, those parameters are extracted from the driver circuit. The measured information can then be used to inform subsequent programming of the pixel circuits so that adjustments may be made to the programming taking into account the measured degradation.

Aspects of the present disclosure include a novel pixel circuit in display panels and methods to drive and measure the pixel and OLED current in order to extract parameters of the pixel. The pixel circuit includes a Light-Emitting Device (LED), such as an Organic Light Emitting Diode (OLED), a storage capacitor and Thin Film Transistors (TFTs). Some methods include supplying voltage or current to the pixel circuit from the source via the data line and measuring an electric current in the data line. Some methods further include converting the measured current to voltage for further processing. For example, a source driver having a ReadOut Circuit (ROC) may be utilized for measuring a current from the pixel circuit. In some embodiments, the current from the pixel circuit can be either the current of the driving TFT or the current of the OLED. The current is converted into a corresponding voltage and then an Analog-to-Digital Convertor (ADC) is used to convert the voltage to a digital code, i.e. a 10 to 16 bit digital code. The digital code is provided to a digital processor for further processing.

FIG. 1is a block diagram of an exemplary OLED display system100according to an embodiment. The display system100includes a display panel108, a source driver110which includes a Readout Circuit (ROC)112, a gate driver104, a controller114, a memory storage116, a reference generator106, and a supply voltage block102. The display panel108includes a plurality of pixels200arranged in “n” rows and “m” columns. Each pixel200has a pixel circuit including four Thin Film Transistors (TFTs), a storage capacitor and an OLED as shown inFIG. 2. Each pixel200is individually programmed to emit light with specific luminance values. The digital controller114receives digital video data indicative of information to be displayed on the display panel108. The controller114sends signals136comprising digital video data to the source driver110and signals134to the gate driver104to drive the pixels200in the display panel108in row by row basis to display the information indicated. The plurality of pixels200associated with the display panel108thus comprise a display array (“display screen”) adapted to dynamically display information according to the input digital data received by the controller114. The display screen108can display, for example, video information from a stream of video data (not shown) received by the controller114. The supply voltage block102provides a constant or an adjustable supply for the display panel108which is controlled by the signals132from the controller114. The reference generator block106provides constant or adjustable reference voltages for the display panel108which is controlled by the signals140from the controller114.

FIG. 1is illustrated with only two pixels200aand200bin the display panel108for sake of simplicity and illustrative purposes. The display system100can be implemented with a plurality of similar pixels, such as the pixel200and the display panel size is not restricted to a particular number of rows and columns of pixels. For example, the display system100can be implemented with a display panel with a number of rows and columns of pixels commonly available in displays for mobile devices, monitor-based devices, TVs, and projection devices.

According to an embodiment, an exemplary pixel circuit200of a display system ofFIG. 1, is shown inFIG. 2, the pixel circuit comprising four p-type TFTs (221,222,223and224), a storage capacitor (Cs)212, an OLED device230, and input with three control signals. A drive transistor221is coupled in series with the OLED230, and the storage capacitor212is coupled across a source and a gate of the drive transistor221. Transistor222, controlled by EM[i], is coupled between the source of the drive transistor221and VDD, transistor223controlled by WR[i] is coupled between the source of the drive transistor221and the data line130, while transistor224controlled by RST[i] is coupled between the gate of the drive transistor221and the reference line126. Control signals EM[i]206, WR[i]208and RST[i]210are control signals of the ith row, and are the emission, write, and reset signal respectively for the pixel circuit200. All the control signals are provided by the gate driver block104, as controlled by controller114, as shown inFIG. 1. The reference voltage VREFis common for all pixels located in each row. These reference voltages VREF[i] and VREF[n] are provided over reference lines126iand126nby the reference voltage generator106. The pixel circuit200includes a storage capacitor Cs212, for storing the data voltage VDATAprovided by the source driver110over the data line130and for allowing the pixel circuit200to drive the OLED device230after being addressed. As such, the display panel108including a pixel circuit200, is an active matrix display array. The transistors that have been utilized in the pixel circuit200are p-type Thin Film Transistors (TFTs), but implementations of the present disclosure are not limited to pixel circuits having a particular polarity of transistor or only to pixel circuits having thin-film transistors.

FIG. 1is illustrated with only two pixels200aand200bin the display panel108. As shown inFIG. 1, the pixel200aillustrated as the top-left pixel in the display panel108represents a “ith” row and “jth” column, is coupled to an emission signal line120ifor an emission signal EM[i], a write signal line122ifor a write signal WR[i], a reset signal line124ifor a reset signal RST[i], a supply line128jfor a supply voltage VDD[j], a data line130jfor a data voltage VDATA[j], and a reference line126ifor a reference voltage VREF[i].

As shown inFIG. 1, the pixel200billustrated as the bottom-right pixel200in the display panel108represents a “nth” row and “mth” column, is coupled to an emission signal line120nfor an emission signal EM[n], a write signal line122nfor a write signal WR[n], a reset signal line124nfor a reset signal RST[n], a supply line128mfor a supply voltage VDD[m], a data line130mfor a data voltage VDATA[m], and a reference line126nfor a reference voltage VREF[n].

As shown inFIG. 1, the gate driver104provides the EM, WR, and RST signals for the emission signal lines120i,120n, the write signal lines122i,122n, and the reset signal lines124i,124n. These signals are utilized to control the pixels200in the display panel108in order to program the pixels200or to measure the pixel or OLED currents through the use of the data lines (130j,130m). The data line130conveys programming information such as a programming voltage or a programming current to the pixel200from the source driver110to the pixel200in order to program the pixel200to emit a desired amount of luminance according to the digital data received by the controller114. The programming voltage or current can be applied to the pixel200during a programming operation of the pixel200so as to charge a storage device within the pixel200, such as a storage capacitor, thereby enabling the pixel200to emit light with the desired amount of luminance during an emission operation following the programming operation. For example, the storage device in the pixel200can be charged during a programming operation to keep the data voltage and then apply it to one or more of a gate or a source terminal of the driving transistor during the emission operation, thereby causing the driving transistor to convey the driving current through the OLED according to the voltage stored on the storage device.

Generally, in the pixel200, the driving current that is conveyed through the light emitting device by the driving transistor during the emission operation of the pixel200is a current that is supplied by the supply line (e.g. the supply line128jand128m). The supply line128can provide a positive supply voltage202(e.g., the voltage commonly referred to in circuit design as “VDD”). In some implementations, a negative or zero (0V) supply voltage VSS204can be provided over a second supply line to the pixel200. For example, each pixel can be coupled to a first supply line128and a second supply line (not shown) coupled with VSS, and the pixel circuits200can be situated between the first and second supply lines to facilitate driving current between the two supply lines during emission or other states of the pixel circuit.

In some embodiments, the display system100also includes a Readout Circuit (ROC)112which is integrated with the source driver110. The data line (130j,130m) connects the pixel200to the readout circuit112. The data line (130j,130m) allows the readout circuit112to measure a current associated with the pixel200and thereby extract information indicative of a degradation of the pixel200. The Readout circuit112converts the associated current into a corresponding voltage. In some embodiments, this voltage is converted into a 10 to 16 bit digital code and is sent to the digital control114for further processing or compensation.

In some embodiments, there are three modes of operations for the display system including a drive mode, a pixel measurement mode, and an OLED measurement mode.

Drive Mode

A timing diagram for the control signals of the pixel circuit200in the drive mode is shown inFIG. 3. The timing diagram shown inFIG. 3comprises three states which include, programming the pixel during a programming state301, an In-Pixel Compensation state (IPC) state302, and an emission state303during which the pixel emits light. During the programming state301, the storage capacitor Cs212is first charged to VDATA−VREF, which is the difference between the voltage of the data line130and the voltage of the reference line126. During the In-Pixel Compensation (IPC) state302the voltage stored on the capacitor212changes by ΔVIPC. During the emission state303, the drive transistor221drives the OLED device230with a current corresponding to the stored data voltage causing it to emit light.

During the programming state301as shown inFIG. 6, the emission signal EM[i]206is set to VDD, i.e. EM[i]=VDD. This turns off the transistor222. The write signal WR[i]208and the reset signal RST[i]210are set to zero, i.e. WR[i]=0 and RST[i]=0. These signals turn on the transistors223and224and connect the node221g(common with the gate of the drive transistor221) to VREFand the node221s(common with the source of the drive transistor221) to VDATA. The storage capacitor Cs212is charged to VDATA−VREFwhich is the difference between the voltage on the data line130and the voltage on the reference line126. At the end of the programming state301, the voltage stored in the storage capacitor Cs212is equal to:
VCs=VDATA−VREF(1)

During the In-Pixel Compensation (IPC) state302as shown inFIG. 7, the emission signal EM[i]206and the write signal WR[i]208are set to VDD, i.e. EM[i]=VDD and WR[i]=VDD. These signals turn off the transistors222and223. The node221sis disconnected from the data line130. The reset signal RST[i]210is set to zero, i.e. RST[i]=0. This turns on the transistor224. The drive transistor221is turned on and IPC is performed in this state. At the end of this state, the voltage stored in the storage capacitor Cs212is equal to:
VCs=VDATA−VREF−ΔVIPC(2)
where ΔVIPCis the voltage drop during this state.

During the emission state303as shown inFIG. 8, the emission signal EM[i]206is set to zero, i.e. EM[i]=0 and the write signal WR[i]208and the reset signal RST[i]210are set to VDD, i.e. WR[i]=VDD and RST[i]=VDD. These signals turn on the transistor222and turn off the transistors223and224. The drive transistor221drives the OLED device230with the pixel current Ipixelcorresponding to the voltage stored in the capacitor212and the characteristics of the drive transistor221. Therefore the luminance of the OLED device230, determined by Ipixel, is dependent upon a programming of the capacitor212and the characteristics of the drive transistor T1.

Pixel Measurement Mode

The pixel current is measured in the pixel measurement mode. A timing diagram for the control signals of the pixel circuit200in the pixel measurement mode is shown inFIG. 4. The timing diagram shown inFIG. 4comprises four states which include, a programming state401, an IPC state402, an off state403during which the TFTs and OLED are turned off, and a pixel current measurement state404.

During the programming state401as shown inFIG. 9, the emission signal EM[i]206is set to VDD, i.e. EM[i]=VDD, turning off transistor222. The write signal WR[i]208and the reset signal RST[i]210are set to zero, i.e. WR[i]=0 and RST[i]=0. These signals turn on the transistors223and224and connect the node221gto VREFand the node221sto VDATA. The storage capacitor Cs212is charged to VDATA−VREFwhich is the difference between the voltage on the data line130and the voltage on the reference line126. At the end of this state, the voltage stored in the storage capacitor Cs212is equal to:
VCs=VDATA−VREF(3)

During the In-Pixel Compensation (IPC) state402as shown inFIG. 10, the emission signal EM[i]206and the write signal WR[i]208are set to VDD, i.e. EM[i]=VDD and WR[i]=VDD. These signals turn off the transistors222and223. The node221sis disconnected from the data line130. The reset signal RST[i] signal210is set to zero, i.e. RST[i]=0. This turns on the transistor224. The drive transistor221is turned on and IPC is performed in this state. At the end of this state, the voltage stored in the storage capacitor Cs212is equal to:
VCs=VDATA−VREF−ΔVIPC(4)
where ΔVIPCis the voltage drop during this state.

During the off state403as shown inFIG. 11, the emission signal EM[i]206, the write signal WR[i]208, and the reset signal RST[i]210are set to VDD, i.e. EM[i]=VDD, WR[i]=VDD and RST[i]=VDD. These signals turn off the transistors222,223and224and disconnect the node221sfrom the data line130and the node221gfrom the reference line126. During the off state403, no current is passing through the OLED230and it is off during this state.

During the pixel current measurement state404as shown inFIG. 12, the emission signal EM[i]206and the reset signal RST[i]210are set to VDD, i.e. EM[i]=VDD and RST[i]=VDD. The write signal WR[i]208is set to zero, i.e. WR[i]=0. The write signal WR[i]208turns on the transistor223and the node221sis connected to the data line130. In this state, the data line130is connected to the ROC112to measure the pixel current IPixel232. The drive transistor221drives the OLED device230with the pixel current Ipixelcorresponding to the voltage stored in the capacitor212and the characteristics of the drive transistor221. The pixel current Ipixel232is measured in this state and this current is converted to a corresponding voltage252which is quantized to 10 to 16 bit digital code256by the ADC254.

In some embodiments, in order to characterize the drive transistor221, pixel measurement is performed more than once, utilizing different voltages to program the capacitor212. In some embodiments, two points of an I-V curve for the drive transistor221are extracted using two different programming voltages for the capacitor and measuring the resulting two different pixel currents Ipixel, and the rest of the I-V curve is extrapolated with use of those two points.

OLED Measurement Mode

In this mode, in order to determine the I-V characteristic of the OLED device which is utilized to compensate aging of the OLED, the OLED current is measured. A timing diagram for the control signals of the pixel circuit200in the OLED measurement mode is shown inFIG. 5. The timing diagram shown inFIG. 5comprises only one state which is the OLED measurement state501.

During the OLED measurement state501as shown inFIG. 13, the emission signal EM[i]206is set to VDD, i.e. EM[i]=VDD and the write signal WR[i]208and the reset signal RST[i]210are set to zero, i.e. WR[i]=0 and RST[i]=0. The write signal WR[i]208turns on the transistor223and the node221sis connected to the data line130. In this state, the reference voltage VREFof the reference line126is switched to the lowest voltage, i.e. VREF=0. The reset signal RST[i]210turns on the transistor224therefore the node221gis connected to the reference line126which has a reference voltage VREFset to zero. The data voltage VDATAis set to a voltage greater than zero such that the drive transistor221is turned on in this state and behaves like a closed switch. Since the drive transistor221behaves as a switch, the data voltage VDATAis provided to the node221d, and is also set to a voltage great enough (VDATA>VOLED) such that the OLED230turns on. In this state501, the data line130is connected to the Readout Circuit (ROC)112to measure the OLED current IOled234. The OLED current IOled234is measured in this mode and is converted to a corresponding voltage252which is quantized to 10 to 16 bit digital code256by an Analog-To-Digital Converter (ADC)254.

In some embodiments, in order to characterize the I-V characteristic of the OLED230, the OLED measurement is conducted more than once, utilizing different data voltages VDATAeach sufficient to turn on the drive transistor221as a switch and great enough (VDATA>VOLED) to turn on the OLED230, with whatever voltage spacing is desired to create an I-V characteristic curve of a desired resolution.

The ROC112as shown inFIG. 12andFIG. 13includes an integrator248, an analog to digital converter (ADC)254, and one switch240coupling the coupling the ROC112to the data line130at the integrator248. The integrator248includes a reset switch246and an integrating capacitor CI258in parallel and connected between a first input242and an output of the integrator248and a bias voltage VBcoupled to a second input244of the integrator248. During measurement, the switch130is closed and the integrator246integrates the current coming from pixel200(Ipixel232or Ioled234) and converts it to a corresponding voltage252. The output voltage of the integrator252is applied to the ADC254and this voltage is converted to 10 to 16 bit digital code256by the ADC254.

Although the embodiments have been described with functionality of the transistors resulting from the application of particular example voltage values such as “VDD” or “0” or “VSS”, it is to be understood that in different contexts, the application of “high” and “low” voltages of appropriate different voltage values may be used to effect the same functionality from transistors and do not represent a departure from the embodiments disclosed above.