FIELD OF THE INVENTION
The present invention generally relates to a light emitting display devices, and particularly, to a driving technique for AMOLEDs, to reduce the effects of differential aging of the pixel circuits significantly.
SUMMARY OF INVENTION
The disclosed technique stabilizes the pixel current by adjusting the gate voltage of the drive transistor.
ADVANTAGES
The new technique does not require any more driving cycle or driving circuitry than the ones used in AMLCD displays, resulting in a low cost application for portable devices including mobiles and PDAs. Also, it is insensitive to the temperature change and mechanical stress.
FIG. I (a, b): shows two circuit diagrams for the new driving technique.
FIG. 2 (a, b): shows two circuit diagrams with modified discharging elements.
FIG. 3: shows the simulation results of FIG.I (a) with and without discharging effect.
FIG. 4 (a-c): shows two circuit diagrams with modified discharging method and corresponding signal diagram.
FIG. 5: shows the simulation results of FIG 4(a).
FIG. 5: shows the simulation results of FIG 4(b).
FIG. 1 shows two pixel circuits that can provide constant averaged current over the frame time.
The pixel circuits comprise a switch T2, a drive transistor T1, a discharging transistor Td, OLED
10, and a storage capacitor 11.
During the programming cycle, node A is charged to a programming voltage through T2 while SEL is high. During the drive cycle, node A is discharged through Td. Since Td and Tl has the same bias condition, they experience the same threshold voltage shift.
Considering that the discharge time is a function of transconductance of Td, the discharge time increases as the threshold voltage of T1/Td increases. Therefore, the average current of the pixel over the frame time remains constant. Td should be a very weak transistor with short width and long channel length.
In FIG. 1 (b), an increase in the OLED 10 voltage will result in longer discharge time. Thus, the averaged pixel current will remain constant even after the OLED degradation.
FIG. 2 shows two pixel circuits with modified discharging element. The pixel circuits comprise two switches T2 and T3, a drive transistor T1, a discharging transistor Td, OLED 20, and a storage capacitor 21.
During the programming cycle, node A is charged to a programming voltage through T2 while SEL is high. During the drive cycle, node A is discharged through Td. Since Td and T1 has the same bias condition, they experience the same threshold voltage shift.
Considering that the discharge time is a function of transconductance of Td, the discharge time increases as the threshold voltage of T1/Td increases. Therefore, the average current of the pixel over the frame time remains constant. Here, T3 forces Td in the linear regime of operation, and so reduce feedback gain. Therefore, Td can be a unity transistor with the minimum channel length and width. VB can be shared between the pixels of the entire panel or it can be connected to node A.
Also, T3 can be replaced by a resistor.
In FIG. 2 (b), an increase in the OLED 20 voltage will result in longer discharge time. Thus, the averaged pixel current will remain constant even after the OLED degradation.
FIG. 3 shows the simulation results for the pixel circuit proposed in FIG.
1(a). It is obvious that the averaged pixel current is stable for the new driving scheme whereas it drops dramatically if Td is removed from the circuit (conventional 2-TFT pixel circuit).
FIG. 4 shows two pixel circuits with modified discharging method. The pixel circuits comprise two switches T2 and T3, a drive transistor T1, a discharging transistor Td, OLED 40, and a storage capacitor 41.
During the programming cycle, node A is charged to a programming voltage through T2 while SEL[n] is high. During the second operating cycle, node A is discharged through Td. Since Td and T1 has the same bias condition, they experience the same threshold voltage shift.
Considering that the discharge time is a function of transconductance of Td, the discharged voltage decreases as the threshold voltage of T1/Td increases. Therefore, gate voltage of the drive transistor T1 is adjusted accordingly. Here, SEL [n] is the address line of the nth row, and SEL[n+1] is the address line of (n+1)th row.
In FIG. 4 (b), an increase in the OLED 40 voltage will result in higher gate voltage. Thus, the pixel current remains constant.
FIG. 5 shows the simulation results for the pixel circuit depicted in FIG. 4 (a). It is seen that the pixel current is highly stable even after a 2-V shift in the threshold voltage of the drive transistor T1.
FIG. 6 shows the simulation results for the pixel circuit depicted in FIG. 4 (b). It is seen that the pixel current is highly stable even after a 2-V shift in the voltage of the OLED 11.