A DRIVING AND AGING EXTRACTION SCHEME FOR CURRENT PROGRAMMED AMOLED PIXEL CIRCUIT WITH NO UNWANTED EMISSION

METHODE DE COMMANDE ET DE DETERMINATION DU VIEILLISSEMENT POUR CIRCUIT DE PIXELS AMOLED A COURANT PROGRAMME SANS EMISSION NON DESIREE

English Abstract

Disclosed is a technique to reduce the unwanted emission in active matrix
light emitting displays,
in particular, active matrix organic light emitting diode (AMOLED) displays.
Also, the technique
can be adopted in hybrid driving scheme to extract the actual threshold
voltage shift/mismatch of
the drive transistor.

Description

CA 02521986 2005-10-17
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 unwanted emission and extract the
precise aging
of the pixel circuits.
SUMMARY OF INVENTION
The disclosed technique turns off the light emitting device during the
programming/calibration
cycle, and so it prevents the unwanted emission and effect of light emitting
device on the pixel
aging.
ADVANTAGES
The unwanted emission is reduced significantly resulting in a higher
resolution. Also, individual
extraction of circuit aging and light emitting device aging become possible,
leading in a more
accurate calibration.

CA 02521986 2005-10-17
FIG. 1: An example of current mirror pixel circuit.
FIG. 2: Pixel configuration during the calibration/programming cycle.
FIG. 3: Simulation results for the voltage on IDATA line.
Z

CA 02521986 2005-10-17
The disclosed driving technique can be applied to any current mirror pixel
circuit fabricated in
any technology including poly silicon, amorphous silicon, crystalline silicon,
and organic
materials.
FIG.1 shows an example of a current mirror AMOLED pixel circuit. The pixel
comprises two
switches T3 and T2, a drive transistor T4, a programming transistor T1, OLED
10, and a storage
capacitor 11.
In the conventional current programming, SEL goes high, and a programming
current (IP) is
applied to IDATA. Considering that the width of T1 is 'n' times larger than
the width of T4, the
current flowing through the OLED 10 during the programming cycle is (n+1)IP.
Since'n' should
be large to gain significant speed improvement, the unwanted emission becomes
considerable.
In the disclosed driving scheme, VDD goes to a reference voltage during the
programming cycle.
The reference voltage should be chosen carefully so that the voltage at node B
becomes smaller
than the ON voltage of the OLED 10. As a result, the OLED 10 turns off and the
unwanted
emission is zero. FIG. 2 shows the pixel configuration at this cycle. Here,
the voltage of the
IDATA line consists of VP+VT+ 4VT where VP includes the drain-source voltage
of T4 and the
gate-source voltage of T1, VT is the threshold voltage of Tl (T4), and ~VT is
the VT
shift/mismatch. At the end of programming cycle, VDD goes to its original
value, and so voltage
at node B goes to VpLED~ However, the gate voltage of T4/T1 is fixed and
stored in the storage
capacitor 11 since the switches are OFF. Therefore, the pixel current during
the driving cycle
becomes independent of the VT.
More importantly, this method can be adopted in hybrid driving scheme to
extract the VT-shift or
VT-mismatch. It is obvious that the only variant parameter in the voltage of
the IDATA line is the
VT shift/mismatch. Therefore, OVT can be extracted and the programming data
can be calibrated
with 4VT.
The simulation result for the voltage on IDATA line for different VT is
depicted in FIG. 3. As is
can be seen, the voltage of IDATA line includes the shift in the threshold
voltage of T1 and T4.
Here, the programming current is 1 ~A.
3

Representative Drawing