This application claims priority to and the benefit of Korean Patent Application No. 10-2010-0012464, filed in the Korean Intellectual Property Office on Feb. 10, 2010, the entire content of which is incorporated herein by reference.
1. Field
Aspects of embodiments according to the present invention relate to a pixel, a display device including the same, and a driving method thereof.
2. Description of the Related Art
Various kinds of flat display devices that are capable of reducing detriments of cathode ray tube (CRT) devices, such as their heavy weight and large size, have been developed in recent years. Such flat panel display devices include liquid crystal displays (LCDs), field emission displays (FEDs), plasma display panels (PDPs), and organic light emitting diode (OLED) displays. Among these flat panel display devices, the OLED display, which uses OLEDs to generate light by a recombination of electrons and holes for the display of images, has a fast response speed, low power consumption, excellent luminous efficiency, luminance, and viewing angle.
Generally, the OLED display is classified as a passive matrix OLED (PMOLED) and an active matrix OLED (AMOLED) according to a driving method of the OLED. Of these, the active matrix OLED, in which unit pixels are selectively lit, is used instead of the PMOLED for, its better resolution, contrast, and operation speed.
A typical pixel of the active matrix OLED includes the OLED, a driving transistor for controlling a current amount supplied to the OLED, and a switching transistor for transmitting a data signal controlling a light emitting amount of the OLED to the driving transistor. However, the driving transistor of the pixel of the active matrix OLED may generate a difference of current flowing to the OLED due to a variation of its threshold voltage or a variation of a power source voltage transmitted to its pixel. This, in turn, may cause luminance variation of the OLEDs from one pixel to another.
In particular, in order to realize high image quality of the display device, high frequency driving may be applied while applying driving timing to the driving circuit of each pixel. In this case, however, it may be difficult to ensure that the time that the threshold voltage of the driving transistor of each pixel is compensated is sufficient, such that the image quality may be deteriorated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.
Aspects of embodiments according to the present invention relate to a pixel, a display device using the same, and a driving method thereof that are capable of ensuring a sufficient threshold voltage compensation time under high resolution and high frequency driving when compensating for a threshold voltage of a driving transistor. More particularly, embodiments of the present invention provide for a driving circuit, a pixel, a display device including the same, and a driving method thereof that are capable of realizing high image quality by providing sufficient time to compensate a threshold voltage of a driving transistor when driving each pixel of the display device by the high resolution and high frequency driving method. The technical features of the present invention are not limited to the above, and other non-mentioned features will be clearly understood by a person of ordinary skill in the art by way of the following description.
According to an exemplary embodiment of the present invention, a display device is provided. The display device includes a display unit, a scan driver, a data driver, and a light emission control driver. The display unit includes a plurality of pixels. The pixels are coupled to a plurality of scan lines, a plurality of data lines, and a plurality of light emission control lines. The scan lines are for transmitting a plurality of scan signals. The data lines are for transmitting a plurality of data signals. The light emission control lines are for transmitting a plurality of light emission control signals. The scan driver is for transmitting the plurality of scan signals. The data driver is for transmitting the plurality of data signals. The light emission control driver is for transmitting the plurality of light emission control signals. Each of the plurality of pixels includes an organic light emitting diode (OLED), a driving transistor, a first transistor, and a first capacitor. The driving transistor is for transmitting a driving current to the OLED according to one of the data signals. The first transistor is for transmitting the one of the data signals to the driving transistor according to one of the scan signals. The first capacitor includes a first terminal and a second terminal. The first terminal is coupled to the first transistor. The second terminal is coupled to a gate electrode of the driving transistor. The first terminal is for receiving an assistance voltage and the second terminal is for receiving an initialization voltage during an initialization period. The initialization period is for initializing a gate voltage of the driving transistor. The driving transistor is further for diode-connecting and the first terminal is further for maintaining the assistance voltage during a threshold voltage compensation period. The threshold voltage compensation period is for compensating a threshold voltage of the driving transistor. The threshold voltage compensation period is longer than a scan period. The scan period is for turning on the first transistor according to a level of the one of the scan signals.
Each of the plurality of pixels may further include a first switch and a second switch. The first switch is for transmitting the initialization voltage to the second terminal. The second switch is for transmitting the assistance voltage to the first terminal.
The plurality of scan lines may include a plurality of second scan lines. The second scan lines are for transmitting an initialization signal to the plurality of pixels. The scan driver may further be for generating the initialization signal and transmitting the initialization signal to each of the pixels through a corresponding one of the plurality of second scan lines. The initialization signal is for controlling the switching operation of the first switch for transmitting the initialization voltage to the second terminal and of the second switch for transmitting the assistance voltage to the first terminal in the plurality of pixels.
For each of the pixels, the initialization signal may be an other one of the scan signals. The scan driver may be further for transmitting the other one of the scan signals earlier by a period corresponding to the threshold voltage compensation period than the one of the scan signals.
Each of the plurality of pixels may further include a first switch and a second switch. The first switch is for diode-connecting the driving transistor. The second switch is for transmitting the assistance voltage to the first terminal.
The plurality of scan lines may include a plurality of second scan lines. The second scan lines are for transmitting a threshold voltage compensation signal to the plurality of pixels. The scan driver may further be for generating the threshold voltage compensation signal and transmitting the threshold voltage compensation signal to each of the pixels through a corresponding one of the plurality of second scan lines. The threshold voltage compensation signal is for controlling the switching operation of the first switch for diode-connecting the driving transistor and of the second switch for transmitting the assistance voltage to the first terminal in the plurality of pixels.
Each of the plurality of pixels may further include a first switch. The first switch is for transmitting the driving current from the driving transistor to the OLED according to one of the light emission control signals during a light emitting period. During the light emitting period, the OLED is for receiving the driving current according to the one of the data signals, and emitting light in response to the received driving current.
Each of the plurality of pixels may further include a storage capacitor. The storage capacitor is coupled to a first power source and the gate electrode of the driving transistor. The storage capacitor is for charging a voltage corresponding to the threshold voltage of the driving transistor.
The threshold voltage compensation period may be at least twice the initialization period.
The threshold voltage compensation period may be at least 2 horizontal cycles.
According to another exemplary embodiment of the present invention, a pixel is provided. The pixel includes an organic light emitting diode (OLED), a driving transistor, a first transistor, and a first capacitor. The driving transistor is for transmitting a driving current to the OLED according to a transmitted data signal. The first transistor is for transmitting the data signal to the driving transistor according to a scan signal. The first capacitor includes a first terminal and a second terminal. The first terminal is coupled to the first transistor. The second terminal is coupled to a gate electrode of the driving transistor. The first terminal is for receiving an assistance voltage and the second terminal is for receiving an initialization voltage during an initialization period. The initialization period is for initializing a gate voltage of the driving transistor. The driving transistor is further for diode-connecting and the first terminal is further for maintaining the assistance voltage during a threshold voltage compensation period. The threshold voltage compensation period is for compensating a threshold voltage of the driving transistor. The threshold voltage compensation period is longer than a scan period for turning on the first transistor according to a level of the scan signal.
The pixel may further include a first switch and a second switch. The first switch is for transmitting the initialization voltage to the second terminal. The second switch is for transmitting an assistance voltage to the first terminal.
The first switch and the second switch may further be for receiving an initialization signal. The initialization signal is for controlling a switching operation of the first switch and the second switch from a scan driver. The scan driver is for generating and transmitting the scan signal and the initialization signal.
The initialization signal may be an other scan signal. The scan driver may further be for transmitting the other scan signal earlier by a period corresponding to the threshold voltage compensation period than the scan signal.
The pixel may further include a first switch and a second switch. The first switch is for diode-connecting the driving transistor. The second switch is for transmitting the assistance voltage to the first terminal.
The first switch and the second switch may further be for receiving a threshold voltage compensation signal. The threshold voltage compensation signal is for controlling a switching operation of the first switch and the second switch from a scan driver. The scan driver is for generating and transmitting the threshold voltage compensation signal.
The pixel may further include a first switch. The first switch is for transmitting the driving current from the driving transistor to the OLED according to a light emission control signal during a light emitting period. During the light emitting period, the OLED is for receiving the driving current according to the data signal, and emitting light in response to the received driving current.
The pixel may further include a storage capacitor. The storage capacitor is coupled to a first power source and the gate electrode of the driving transistor. The storage capacitor is for charging a voltage corresponding to the threshold voltage of the driving transistor.
The threshold voltage compensation period may be at least twice the initialization period.
The threshold voltage compensation period may be at least 2 horizontal cycles.
According to yet another exemplary embodiment of the present invention, a method for driving a pixel is provided. The pixel includes an organic light emitting diode
(OLED), a driving transistor, a first transistor, and a capacitor. The driving transistor is for controlling a current supplied to the OLED. The first transistor is for transmitting a data signal to the driving transistor. The capacitor is coupled between the driving transistor and the first transistor. The method includes initializing a gate voltage of the driving transistor, compensating a threshold voltage of the driving transistor, and transmitting a data signal to the driving transistor through the capacitor. A period for compensating the threshold voltage is longer than a period for transmitting the data signal to the driving transistor.
The initializing the gate voltage may include applying an assistance voltage to a first terminal of the capacitor coupled to the first transistor, and applying an initialization voltage to a second terminal of the capacitor coupled to a gate electrode of the driving transistor.
The compensating the threshold voltage may include applying an assistance voltage to the first terminal of the capacitor coupled to the first transistor, diode-connecting the driving transistor; and charging a voltage corresponding to the threshold voltage of the driving transistor to a storage capacitor while the driving transistor is diode-connected. The storage capacitor is coupled between a gate electrode of the driving transistor and a first power source.
The period for compensating the threshold voltage may be at least twice a period for initializing the gate voltage of the driving transistor.
The period for compensating the threshold voltage may be at least 2 horizontal cycles.
According to still another exemplary embodiment of the present invention, a method for driving a display device is provided. The display device includes a plurality of pixels. Each of the pixels includes an organic light emitting diode (OLED), a driving transistor, a first transistor, and a capacitor. The driving transistor is for controlling a current supplied to the OLED. The first transistor is for transmitting a data signal to the driving transistor. The capacitor is coupled between the driving transistor and the first transistor. The method includes initializing a gate voltage of the driving transistor, compensating a threshold voltage of the driving transistor, and transmitting a data signal to the driving transistor through the capacitor. A period for compensating the threshold voltage is longer than a period for transmitting the data signal to the driving transistor.
The initializing the gate voltage includes applying an assistance voltage to a first terminal of the capacitor coupled to the first transistor, applying an initialization voltage to a second terminal of the capacitor coupled to a gate electrode of the driving transistor.
The compensating the threshold voltage comprises applying an assistance voltage to the first terminal of the capacitor coupled to the first transistor, diode-connecting the driving transistor, and charging a voltage corresponding to the threshold voltage of the driving transistor to a storage capacitor coupled between a gate electrode of the driving transistor and a first power source while the driving transistor is diode-connected.
The method may further include applying and maintaining an assistance voltage to the first terminal of the capacitor coupled to the first transistor during a period for initializing the gate voltage and the period for initializing the threshold voltage.
A period for compensating the threshold voltage may be at least twice a period for initializing the gate voltage of the driving transistor.
The period for compensating the threshold voltage is at least 2 horizontal cycles.
According to exemplary embodiments of a pixel, a display device including the same, and a driving method thereof, sufficient time to compensate the threshold voltage of the driving transistor may be obtained under high resolution and high frequency driving to realize a display device of high image quality. Accordingly, in embodiments of the driving circuit of the pixel using the high resolution and high frequency driving method, the compensation period of the threshold voltage of the driving transistor is sufficient such that each of the plurality of pixels of an exemplary display device has a complete threshold voltage compensation capability. Thus, the display device may realize a high quality display.
The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of embodiments of the present invention.
The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention.
Constituent elements having the same structure throughout multiple embodiments are denoted by the same reference numeral and are described in a first embodiment. In later embodiments, descriptions of these same constituent elements may be omitted. In addition, to clarify description of embodiments of the present invention, parts not related to the description may be omitted. In addition, like reference numerals designate like elements and similar constituent elements throughout the specification. Further, power sources and their corresponding voltages may be referred to with the same reference name where the appropriate meaning is apparent from context.
Throughout this specification and the claims that follow, when it is described that an element is “coupled” to another element, the element may be “directly coupled” (e.g., connected) to the other element or “indirectly coupled” (e.g., electrically coupled or electrically connected) to the other element through one or more third elements. In addition, unless explicitly described to the contrary, the word “comprise” and variations such as “comprises” or “comprising” will be understood to imply the inclusion of stated elements but not the exclusion of any other elements.
Referring to
The plurality of pixels PXjk are located in crossing regions of the scan lines Gi1 to Gin, Gv1 to Gvn, and Gw1 to Gwn, the data lines D1 to Dm, and the light emission control lines EM1 to EMn, and are arranged substantially in a matrix. The pixels PXjk are supplied with a first power source voltage ELVDD, a second power source voltage ELVSS, a reset initialization voltage VINT, and an assistance voltage VSUS from a power supply unit 60 controlled through the signal controller 50. In an exemplary arrangement of the pixels PXjk, the plurality of scan lines Gi1 to Gin, Gv1 to Gvn, and Gw1 to Gwn for transmitting the scan signals extend substantially in a row direction and are substantially parallel to each other, while the plurality of data lines D1 to Dm extend substantially in a column direction and are substantially parallel to each other. However, the present invention is not limited thereto.
In the exemplary embodiment of
Referring to
The driving transistor Td includes a source electrode coupled to the first power source ELVDD, the drain electrode coupled to a third node N3, and the gate electrode coupled to a first node N1. The voltage at the gate electrode corresponds to the data signal. The driving transistor Td is for transmitting the driving current to the OLED according to the data signal transmitted to the pixel.
The first transistor T1 includes a source electrode coupled to a data line Dk for transmitting the data signal Vdata, a drain electrode coupled to a second node N2, and a gate electrode coupled to the scan line Gwj for transmitting the scan signal Gw (also denoted Gw[N] or Gw[j]). When the scan signal Gw is transmitted through the scan line Gwj such that the first transistor T1 is turned on, the data signal Vdata is transmitted to the first capacitor C1, and a voltage corresponding to the data signal is transmitted to the gate electrode of the driving transistor Td according to the voltage charged to the first capacitor C1.
In more detail, the first capacitor C1 includes the first electrode coupled to the first transistor T1 and the second electrode coupled to the gate electrode of the driving transistor Td. The storage capacitor Cst includes one terminal coupled to the gate electrode of the driving transistor Td, that is, the first node N1, and the other terminal coupled to the first power source ELVDD. The storage capacitor Cst maintains a difference of the gate electrode voltage and the source electrode voltage of the driving transistor Td.
If the data signal Vdata is transmitted to the first capacitor C1, a voltage divided according to the capacitance of the first capacitor C1 and that of the storage capacitor Cst is transmitted to the gate electrode of the driving transistor Td. This voltage is the voltage corresponding to the above-described data signal Vdata, and the storage capacitor Cst maintains the difference between this voltage and the first power source voltage ELVDD until the next data signal is written. That is, if the data signal Vdata is transmitted to the first capacitor C1, the voltage of the first node N1 is changed by a voltage corresponding to the difference between the data signal Vdata and the assistance voltage VSUS compared with a voltage at the first node N1 after a threshold voltage compensation period. This voltage is transmitted to the gate electrode of the driving transistor Td, and the voltage difference between the gate electrode and the source electrode of the driving transistor Td is uniformly maintained by the storage capacitor Cst.
The pixel PXjk according to an exemplary embodiment of the present invention includes a switch for transmitting an initialization voltage VINT and a switch for transmitting the assistance voltage VSUS during an initialization period for initializing the gate voltage of the driving transistor Td. In the exemplary embodiment of
In an exemplary embodiment of the present invention, the assistance voltage VSUS is applied during the period (for example, the initialization period) in which the initialization voltage VINT is applied, such that the voltage of the first electrode line of the first capacitor C1 may be prevented from being floated. In the exemplary embodiment of
In an exemplary embodiment of the present invention, the initialization signal Gi that is transmitted to the first switch M1 and the second switch M2 may be a signal that is generated and transmitted independently (for example, along a plurality of second scan lines Gi1 to Gin) from the scan signal Gw, which is generated in the scan driver 20 and transmitted by the plurality of scan lines Gw1 to Gwn. That is, the scan lines coupled to the pixel PXjk of
On the other hand, in another exemplary embodiment, the initialization signal may be a scan signal (not shown) that is transmitted at an earlier time (corresponding to a length of the threshold voltage compensation period) than the time when the corresponding scan signal Gw among the plurality of scan signals generated in the scan driver 20 of the display device 100 is transmitted to the scan line Gwj. For example, based on the pixel driving timing of
Here, the scan driver 20 is further for generating dummy scan signals to transmit from the first scan line Gi1 to the fifth scan line Gi5. In another exemplary embodiment of the present invention, it is determined that the length of the threshold voltage compensation period is 4 horizontal cycles, so there is a 5 horizontal cycle gap between the initialization signal and the corresponding scan signal. Accordingly, instead of the initialization signal Gi[N], Gw[N-5] is transmitted. An appropriate scan signal may be used instead of the initialization signal according to the length of the threshold voltage compensation period.
The third switch M3 is controlled by a threshold voltage compensation signal Gv. The third switch M3 is turned on during the threshold voltage compensation period, which is when the threshold voltage of the driving transistor Td is compensated. While the third switch M3 is turned on, the driving transistor Td is diode-connected. Concurrently (for example, simultaneously), since the fourth switch M4 is also controlled by the threshold voltage compensation signal Gv, during the threshold voltage compensation period, the fourth switch M4 is turned on, and the assistance voltage VSUS is transmitted from the assistance power source coupled to the fourth switch M4.
In more detail, the third switch M3 includes the third node N3, which is a source electrode coupled to the drain electrode of the driving transistor Td, the first node N1, which is a drain electrode coupled to the gate electrode of the driving transistor Td, and a gate electrode coupled to the scan line Gvj for transmitting the threshold voltage compensation signal Gv (also denoted Gv[N] or Gv[j]). The fourth switch M4 includes a source electrode coupled to the assistance power source for supplying the assistance voltage VSUS, a drain electrode coupled to the second node N2, and a gate electrode coupled to the scan line Gvj for transmitting the threshold voltage compensation signal Gv.
During the threshold voltage compensation period, the driving transistor Td is diode-connected by the turn-on of the third switch M3 such that the voltage corresponding to the threshold voltage of the driving transistor Td is charged at the first node N1. In this period, the fourth switch M4 concurrently (for example, simultaneously) receives the threshold voltage compensation signal Gv transmitted to the third switch M3 and is turned on. Accordingly, the fourth switch M4 transmits the assistance voltage VSUS to the second node N2.
As mentioned above, in order to solve the problem that a threshold voltage compensation period is reduced under high resolution and high frequency driving of the pixel, such that the image quality is deteriorated, the assistance voltage VSUS is concurrently (for example, simultaneously) input during the threshold voltage compensation period. Consequently, although the threshold voltage compensation period is lengthened to be more than a period (for example, a predetermined period, such as a horizontal cycle), the voltage floating at the second node N2 may be stable. Accordingly, in an exemplary embodiment of the present invention, although the assistance voltage VSUS is applied during the threshold voltage compensation period and the initialization period such that a relatively long threshold voltage compensation period is ensured, a stable driving circuit may be realized.
In
When the third switch M3 for diode-connecting the driving transistor Td is turned on, the voltage of the first node N1 where the storage capacitor Cst and the first capacitor C1 meet each other becomes the first power source voltage ELVDD offset by the threshold voltage of the driving transistor Td. That is, the voltage that is the threshold voltage of the driving transistor Td subtracted from the first power source voltage ELVDD, is transmitted to the first node N1 of the storage capacitor Cst and the first capacitor C1.
In the above-described circuit shown in
In an exemplary embodiment of the present invention, the threshold voltage compensation period for providing sufficient compensation of the threshold voltage of the driving transistor Td is not limited. However, it may be longer than the period in which the corresponding data signal is written, that is, when the scan signal Gw among the plurality of scan signals is transmitted to turn on the first transistor T1. In addition, according to another exemplary embodiment, the threshold voltage compensation period is more than at least twice the initialization period, or at least 2 horizontal cycles 2H.
For example, 1 line time is 14.8 us under FHD 60 Hz driving, however it may be 7.4 us under FHD 120 Hz high frequency driving.
In the driving timings of
Next, in a second period T2, the initialization signal Gi is increased (e.g., becomes the high level) after the initialization period such that the first switch M1 and the second switch M2 of
When the driving transistor Td is diode-connected by the turn-on of the third switch M3, the threshold voltage compensation period begins. At this point, the second electrode of the first capacitor C1, that is, the first node N1, is input with the voltage that is the threshold voltage of the driving transistor Td subtracted from the first power source voltage ELVDD. Concurrently (for example, simultaneously), the fourth switch M4 is also turned on such that the first electrode of the first capacitor C1 may be prevented from being floated. The threshold voltage compensation period is from the second period T2 to a fifth period T5.
In the embodiment of
In a sixth period T6, the threshold voltage compensation signal Gv is increased (e.g., becomes the high level), such that the third switch M3 and the fourth switch M4 of
After the scan period, that is, the period that the corresponding pixel among the plurality of pixels is written with the corresponding data signal in one frame such that light is emitted by the driving current, the corresponding scan signal Gw is increased (e.g., becomes the high-level) in the seventh period T7 after light emitting such that the first transistor T1 of
Referring to
The OLED emits light in light emitting period T14 after data input period T13 in which the voltage value corresponding to the data signal (for example, a predetermined data signal) of the current is applied after the threshold voltage compensation period T12.
As described above, the display device and the driving method according to an exemplary embodiment of the present invention may significantly reduce the change of the driving current caused by the variation of the threshold voltage of the driving transistor between the different pixels.
While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.
Number | Date | Country | Kind |
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10-2010-0012464 | Feb 2010 | KR | national |