This application claims the benefit of Taiwan application Serial No. 100128770, filed Aug. 11, 2011, the subject matter of which is incorporated herein by reference.
1. Field of the Invention
The invention relates in general to a display and a driving method thereof.
2. Description of the Related Art
Further, an anode of an OLED 12 of the AMOLED pixel 10 is a transparent indium tin oxide (ITO) having a high work function. Thus, during an element manufacturing process, a special procedure is needed to reduce the work function of the ITO in order to obtain a reliable OLED element having preferred characteristics, and so the overall manufacturing process is made more complicated.
The disclosure is directed to a display and a driving method thereof. Through a threshold voltage compensation mechanism, under circumstances of a same data input, each organic light-emitting diode (OLED) pixel of the display is able to provide a same output current instead of a current that degrades with time.
According to an aspect of the disclosure, a display including a panel is provided. The panel includes multiple OLED pixels, each including an OLED, a driving transistor, a switch transistor, a first compensation block and a second compensation block. The driving transistor has a first terminal coupled to an anode of the OLED, a second terminal for receiving an operating voltage, and a control terminal for receiving a data voltage. The switch transistor has a first terminal coupled to the control terminal of the driving transistor, a second terminal for receiving the data voltage, and a control terminal for receiving a first control signal. The first compensation block is coupled to the first terminal and the control terminal of the driving transistor. The second compensation block is coupled to the first terminal of the driving transistor, and receives the first control signal and the data voltage.
According to another aspect of the disclosure, a driving method of a display is provided. The display includes a panel. The panel includes multiple OLED pixels, each including an OLED, a driving transistor, a switch transistor, a first compensation block and a second compensation block. The driving transistor has a first terminal coupled to an anode of the OLED, a second terminal for receiving an operating voltage, and a control terminal for receiving a data voltage. The switch transistor has a first terminal coupled to the control terminal of the driving transistor, a second terminal for receiving the data voltage, and a control terminal for receiving a first control signal. The first compensation block is coupled to the first terminal and the control terminal of the driving transistor. The second compensation block is coupled to the first terminal of the driving transistor, and receives the first control signal and the data voltage. The driving method includes steps below. In a reset phase, the first compensation block is reset, so that the first compensation block has a reference voltage and the data voltage, and the first control signal cuts off the driving transistor via the switch transistor and the second compensation block. In a compensation phase, the second compensation block couples a potential at the first terminal of the driving transistor to a low-level voltage, so that the driving transistor becomes floating on and discharges until cutoff, and the first compensation block maintains a voltage difference between the voltage at the first terminal of the cutoff driving transistor and the reference voltage as well as the data voltage. In a light-emitting phase, the OLED is turned on, so that the first voltage at the terminal of the driving transistor is a driving voltage, and the first compensation block feeds the voltage difference between the reference voltage and the voltage at the first terminal of the driving transistor in the compensation phase as well as the driving voltage back to the control terminal of the driving transistor.
According to yet another aspect of the disclosure, a display including a panel is provided. The panel includes multiple OLED pixels, each including an OLED, a driving transistor, a switch transistor, a first compensation block and a second compensation block. The driving transistor has a first terminal coupled to an anode of the OLED, a second terminal for receiving an operating voltage, and a control terminal for receiving a data voltage. The switch transistor has a first terminal coupled to the control terminal of the driving transistor, a second terminal for receiving the data voltage, and a control terminal for receiving a first control signal. The first compensation block is coupled to the second terminal and the control terminal of the driving transistor. The second compensation block is coupled to the second terminal of the driving transistor, and receives the first control signal and the data voltage.
The above and other aspects of the disclosure will become better understood with regard to the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings.
The disclosure is directed to a display and a driving method thereof. Through a threshold voltage compensation mechanism, under circumstances of a same data input, each OLED pixel of the display is able to provide a same output current instead of a current that degrades with time.
The display according to one embodiment includes a panel, a gate driver and a source driver. The panel includes a plurality of OLED pixels. The gate driver is for enabling the OLED pixels. The source driver is for driving the OLED pixels. In the description below, an N-type MOS transistor is taken as an example for explaining the embodiment. It should be noted that the disclosure is not limited to an N-type MOS transistor, and a P-type MOS transistor or a BJT transistor may also be implemented based on actual design requirements.
The first compensation block 220 is coupled to the first terminal and the control terminal of the driving MOS transistor MOS_dri. The second compensation block 230 is coupled to the first terminal of the driving MOS transistor MOS_dri, and receives the first control signal Sn and the data voltage Data. In a reset phase, the first compensation block 220 is reset and thus has a reference voltage REF and the data voltage Data, and the first control signal Sn cuts off the driving MOS transistor MOS_dri via the switch MOS transistor MOS_sw and the second compensation block 230.
In a compensation phase, the second compensation block 230 couples a potential at the first terminal of the driving MOS transistor MOS_dri to a low-level voltage, such that the driving MOS transistor MOS_dri becomes floating on and discharges until cutoff. Meanwhile, the first compensation block 220 maintains a voltage difference between the voltage at the first terminal of the cutoff driving MOS transistor MOS_dri and the reference voltage REF as well as the data voltage Data. In a light-emitting phase, the first compensation block 220 turns on the driving MOS transistor MOS_dri to drive the OLED 210, and maintains the voltage difference (between the reference voltage REF and the voltage at the first terminal of the driving MOS transistor MOS_dri in the compensation phase), so as to feed the voltage at the first terminal of the turned on driving MOS transistor MOS_dri back to the control terminal of the turned on driving transistor MOS_dri.
In
The second capacitor C2 has a first terminal (a node A) coupled to a second terminal of the second MOS transistor T2, and a second terminal coupled to the first terminal of the driving MOS transistor MOS_dri. The third capacitor C3 has a first terminal coupled to the second terminal of the second MOS transistor T2, and a second terminal coupled to the control terminal of the driving MOS transistor MOS_dri. The third MOS transistor T3 has a first terminal coupled to the first terminal of the third capacitor C3, a second terminal coupled to the second terminal of the third capacitor C3, and a control terminal for receiving a second enable signal XEn or a second control signal Sn′.
In a compensation phase t2, the first control signal Sn cuts off the switch MOS transistor MOS_sw, the potential at the node G is maintained at the data voltage Data, and the potential at the node A is maintained at the reference voltage REF. The first control signal Sn also cuts off the first MOS transistor T1 and swings to the high-potential cathode voltage ELVSS to cut off the OELD 210. Further, the potential at the node S is coupled to a low-level voltage V(s) by a parasitic capacitance Cgs1 of the first MOS transistor T1. The low-level voltage can be calculated by an equation (1) below, where Cp is a bypass capacitance associated with the node S:
V(s)=Data+(Low−High)×(Cgs1/(Cgs1+C2+Cp)) (1)
The voltage difference between the gate voltage of the driving MOS transistor MOS_dry and the threshold voltage can be calculated by an equation (2) below:
Assuming Low is −10V, High is 10V, and Cgs1 and C2 are 0.2 pf, and the bypass capacitance Cp is neglected, the equation (2) may be simplified as Vgs−Vt=10−Vt. Therefore, when the threshold voltage Vt is smaller than 10V, the low-level voltage V(s) prompts the driving MOS transistor MOS_dri to become floating on and to discharge to a cutoff state. At this point, the potential at the node S is a cutoff potential Data−Vt. The voltage difference between the node A and the node S equal to (REF−Data+Vt) is maintained by the second capacitor C2.
The compensation phase t2 can substantially be defined by the second enable signal XEn or the second control signal Sn′. In the disclosure, the compensation phase t2 and a data write period (i.e., the reset phase t1) are independent from each other, so that the time of the compensation phase may be appropriately adjusted instead of being limited to one data write period (i.e., scan line active time). Thus, compensation accuracy is further increased to make the disclosure even more suitable for a large-size, high-resolution display device.
In a light-emitting phase t3, the first enable signal En cuts off the second MOS transistor T2, the second enable signal XEn or the second control signal Sn′ turns on the third MOS transistor T3, and charge sharing occurs between the node A and the node G. As a result, the driving MOS transistor MOS_dri is turned on, the cathode voltage ELVSS is restored to a low potential, and the potential at the node S is fed back to the node A by via the second capacitor C2 to maintain the voltage difference (REF−Data+Vt) in the compensation phase t2. At this point, the potential at the node S is Voled, the potential at the node A is (REF+Voled−Data+Vt), and the potential at the node G is the same as that at the node A. Thereof, the gate-source voltage difference of the driving MOS transistor MOS_dry is Vgs=(REF−Data+Vt). An output current I_dry of the driving MOS transistor MOS_dri is as shown in an equation (e), where Kp is ½(μ)(Cox)(W/L), μ is a carrier mobility, Cox is capacitance per unit area, and W/L is a width-length ratio.
I_dri=Kp×(Vgs−Vt)2=Kp×(REF−Data)2 (3)
It is observed from the equation (3) that, the output current I_dri of the driving MOS transistor MOS_dri is irrelevant to the threshold voltage Vt and the voltage of the OLED 210. That is to say, the OLED pixel 200 of the disclosure is capable of compensating the threshold voltage difference of the driving MOS transistor MOS_dri as well as outputting a same current instead of a current that degrades with time under circumstances of a same data input. Meanwhile, the OLED 200 of the disclosure is also capable of compensating the voltage change in the OLED 210, and has a constant output current that does not change as the voltage of the OLED 210 increases with time under circumstances of a same data input.
As previously stated, the disclosure may also implement a P-type MOS transistor.
The disclosure further provides a driving method for an OLED pixel. The OLED pixel includes an OLED, a driving transistor, a switch transistor, a first compensation block and a second compensation block. The driving transistor has a first terminal coupled to an anode of the OLED, a second terminal for receiving an operating voltage, and a control terminal for receiving a data voltage. The switch transistor has a first terminal coupled to the control terminal of the driving transistor, a second terminal for receiving the data voltage, and a control terminal for receiving a first control signal. The first compensation block is coupled to the first terminal and the control terminal of the driving transistor. The second compensation block is coupled to the first terminal of the driving transistor, and receives the first control signal and the data voltage.
The driving method for an OLED pixel includes steps below. In a reset phase, the first compensation block is reset, so that the first compensation block has a reference voltage and the data voltage, and the first control signal cuts off the driving transistor via the switch transistor and the second compensation block. In a compensation phase, the second compensation block couples a potential at the first terminal of the driving transistor to a low-level voltage such that the driving transistor becomes floating on and discharges until cutoff. Meanwhile, the first compensation block maintains a voltage difference between the voltage at the first terminal of the cutoff driving transistor and the reference voltage as well as the data voltage. In a light-emitting phase, the OLED is turned on, such that the voltage at the first terminal of the driving transistor is a driving voltage, and the first compensation block feeds the voltage difference between the voltage at the first terminal of the driving transistor and the reference voltage as well as the data voltage in the compensation phase back to the control terminal of the driving transistor.
Operation principles of the above driving method for an OLED pixel can be appreciated with reference to descriptions associated with
It is illustrated in the display and the driving method for the display according to the disclosed embodiments that, the OLED pixel of the display has a self-test capability on the threshold voltage through a threshold voltage compensation mechanism, and feeds back the driving voltage of the driving transistor so that each OLED pixel outputs a same current value instead of a current that degrades with time under circumstances of a same data input. Meanwhile, with the self-test capability on the threshold voltage provided by the threshold voltage compensation mechanism, the driving voltage of the driving transistor is fed back so that an output current of the OLED pixel does not change as the voltage of the OLED increases with time under circumstances of a same data input.
Further, as far as each OLED pixel in the disclosed display and the driving method thereof are concerned, in the reset phase, the OLED pixel is non-existent in a discharging path, inferring that not only unnecessary power consumption is prevented but also IR drop is not incurred when the OLED pixel is applied to a large-size display device. Moreover, in the disclosure, the compensation phase and the data write period are independent from each other, so that the time of the compensation phase may be appropriately adjusted instead of being limited to one data write period of a scan line active time. Thus, compensation accuracy is further increased to make the disclosure even more suitable for a large-size, high-resolution display device.
While the disclosure has been described by way of example and in terms of the preferred embodiments, it is to be understood that the disclosure is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.
Number | Date | Country | Kind |
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100128770 | Aug 2011 | TW | national |