METHODE DE PROGRAMMATION PAR COURANTS ECHELONNES POUR AFFICHEURS AMOLED

METHOD FOR SCALED CURRENT PROGRAMMING OF AMOLED DISPLAYS

Abrégé anglais

Disclosed is a technique for fast programming of the current cell and/or
current mirror circuits, in
particular current-programmed active matrix organic light emitting diode
(AMOLED) displays.

Description

CA 02503283 2005-04-08
FIELD OF THE INVENTION
The present invention generally relates to a light emitting device displays,
and particularly, to a
driving technique for AMOLED. The present technique enhances the settling time
of the current-
programmed AMOLED displays.
BACKGROUND OF INVENTION
The small current required by the OLEDs coupled with a large parasitic
capacitance increases the
settling time of the programming of the current-programmed AMOLED displays.
More
importantly, design of external driver supplying that current is very
challenging and impractical.
For example, in the CMOS technology, the transistors must work in sub-
threshold regime to
provide the small current required by the OLEDs. Therefore, to use current-
programmed
AMOLED pixel circuits, suitable driving schemes are desirable.
Current scaling is one method that can be used to manage the challenges
associated with the
small current required by the OLEDs. In the current mirror pixel circuit, the
current scaling can
be done by resizing the mirror transistors. If the size of the drive
transistor is smaller than the size
of the other mirror transistor, the current passing through the OLED is scaled
down. However,
this method is not applicable for other current-programmed pixel circuits.
Also, by resizing the
two mirror transistors the effect of mismatch increases.
SUMMARY OF INVENTION
This invention provides a simple current programming technique, suitable for
the use in
AMOLED displays. Despite the other current scaling technique, resizing is not
required in the
new technique, resulting in a better matching between two mirror transistors
in the current-mirror
pixel circuits.

CA 02503283 2005-04-08
FIG. 1 current scaling in a current-mirror pixel circuit with virtual-ground
technique.

CA 02503283 2005-04-08
FIG. 1 shows a current-mirror pixel circuit with the new current scaling
technique as an example.
Here, T3, T4 are the switch network, 10. During the programming cycle T3 and
T4 are ON. A
current equal to (IB+IP) is passing through T3 and T4, and the gate-source
voltage of T1 is self
adjusted to ( Ip SIB + YT ) where VT is the threshold voltage of the drive
transistor, Tl. Here, IP
is the programming current, and IB the bias current. However, the voltage
stored in the storage
IP + IB - YB + YT . Since the source of T2 is connected to VGND, the
capacitance, 11, is VAS = ,
current flowing through the OLED during the programming time is
IP;Xer = IP + IB + ~.Yez - 2~ ~ VB ~ (IP + IB)
If IB » IP, we can rewrite the pixel current as
IP~er = Ir + (Ie +,(3.VB2 - 2~ ~ VB ~ IB )
By choosing VB properly (VB= ~ ), the pixel current becomes equal to the
programming
current (IP). Therefore, it avoids unwanted emission during the programming
cycle.
During the driving cycle, VGND is zero, so that the gate-source voltage of Tl
(T2) becomes
equal to Vas. Therefore, the current of the pixel is given by
Ipixel = Ip + IB + ~3.YB2 - 2~ ~ YB ~ (IP + IB) .
If IB » IP, we can rewrite the pixel current as
z
IP~er = Ir + (Ie +,Q.VB - 2~ ~ VB ~ Ia )
By choosing VB properly (VB= ~ ), the pixel current becomes equal to the
programming
current (IP).

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