Stable driving scheme for amoled displays using feedback elements

CA 02551237 2006-06-27 Applicant: Ignis Innovation Inc.

Inventor:

Dr.

Arokia Nathan 55 Culpepper Dr.

Waterloo, ON, N2L3G1 CANADA Citizenship: Canadian G.

Reza Chaji 507-196 Westmount Road North Waterloo, ON, N2L3G5 CANADA Canadian resident Filing: Canada or USA I CA 02551237 2006-06-27 ABSTRACT Disclose 7s a stable AMOLED dis la to deaL-mdth.. instability- of backplane and w ic prov thermal, or other means of transduction.

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 luminance by adjusting the gate voltage of the drive transistor.

Although the new method can be used for each individual pixel circuits, it can be applied to some reference pixel circuits to provide aging knowledge for an offpanel algorithm.

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.

CA 02551237 2006-06-27 FIG. 1: shows two pixel circuits for the new driving technique.

FIG. 2: shows charge-pump amplifier configuration to extract the pixel aging.

FIG. 3: shows trans-resistance amplifier configuration to extract the pixel aging.

I CA 02551237 2006-06-27 FIG. 1 shows two pixel circuits that can provide constant luminance over the lifetime of the display. FIG. 1(a) is a pixel circuit with OLED 10 at the source of the drive transistor T1 and FIG. 1(b) is a pixel circuit with OLED 10 at the drain of the drive transistor Tl.

The pixel circuits comprise two switches T2 and T3, a drive transistor T1, OLED 10, a storage capacitor CS, and a generic sensor (e.g. optical, thermal, and etc) S1.

Here, Vbl can be connected to ground, Vdd, or the source of the drive TFT.

All the transistors can be fabricated using amorphous silicon, nano/macro crystalline silicon, poly silicon, cmos, and organic.

Also, the transistors can be replaced by PMOS.

During the programming cycle, node A is charged to a programming voltage through T2 while SEL[i] is high.

The aging of the drive TFT Tl and OLED 10 can be compensated in two different ways: in-pixel compensation, and of-panel calibration.

In-pixel compensation During the programming cycle of next adjacent row, SEL[i+l]/SEL[i-1] is high.

Therefore, the voltage at node A is discharged through the sensor S1.

However, the amount of discharged voltage depends on the resistance of the sensor which is controlled by the OLED luminance or temperature.

Therefore, as the pixel ages the amount of discharged voltage will reduce.

Of-panel calibration The idea is to extract the aging of the pixel by reading back the sensor, and calibrate the programming voltage in order to compensate for the pixel aging including VT shift and OLED degradation.

FIG. 2 shows using a charge-pump configuration to read back the discharged voltage.

At the beginning of the read-back cycle, Swl is ON, and so the line is charged to Vb2.

Also, Cl is charged to a voltage, Vpie as a result of leakage contributed from all the pixels connected to the line.

Then SEL[i] goes high and so the discharged voltage is developed across C1.

The difference between the two extracted voltages can be used to calculate the pixel aging.

Since the sensor S1 is OFF most of the time in this configuration, the sensor Si ages very slightly.

Also, the sensor Si can be biased correctly to suppress its degradation significantly.

More importantly, this method can be used for aging extraction of the sensor Sl itself.

At this mode, the pixel is OFF, and so the difference between the extracted voltage and the voltage extracted for the OFF state of a fresh pixel results in the extraction of sensor S1 degradation.

FIG. 3 shows a trans-resistance amplifier structure to extract the pixel aging.

At the beginning of the read-back cycle, Swl is ON while SEL[i] is low.

Therefore, the leakage current is extracted as the output voltage of trans-resistance amplifier.

Next, SEL[i] goes high and so the sensor current related to the luminance or temperature of the pixel is read back as the output voltage of trans-resistance amplifier.

Using the two extracted voltages, one can calculate the pixel aging.

Since the sensor S1 is OFF most of the time in this configuration, the sensor S1 ages very slightly.

Also, the sensor S1 can be biased correctly to suppress its degradation significantly.

More I CA 02551237 2006-06-27 importantly, this method can be used for aging extraction of the sensor Sl itself.

At this mode, the pixel is OFF, and so the difference between the extracted voltage and the voltage extracted for the OFF state of a fresh pixel results in the extraction of sensor S1 degradation.

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