Canadian Patents Database / Patent 2567076 Summary

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(12) Patent: (11) CA 2567076
(54) English Title: VOLTAGE-PROGRAMMING SCHEME FOR CURRENT-DRIVEN AMOLED DISPLAYS
(54) French Title: CONFIGURATION D'UNE PROGRAMMATION PAR TENSION POUR ECRANS AMOLED ALIMENTES PAR COURANT
(51) International Patent Classification (IPC):
  • G09G 3/3208 (2016.01)
  • G09G 3/3225 (2016.01)
(72) Inventors :
  • AROKIA, NATHAN (Canada)
  • HUANG, RICK (Canada)
  • ALEXANDER, STEFAN (Canada)
(73) Owners :
  • IGNIS INNOVATION INC. (Canada)
(71) Applicants :
  • IGNIS INNOVATION INC. (Canada)
(74) Agent: GOWLING WLG (CANADA) LLP
(45) Issued: 2008-10-21
(86) PCT Filing Date: 2005-06-28
(87) PCT Publication Date: 2006-01-05
Examination requested: 2006-12-01
(30) Availability of licence: N/A
(30) Language of filing: English

(30) Application Priority Data:
Application No. Country/Territory Date
2,472,671 Canada 2004-06-29

English Abstract




A system and method for driving an AMOLED display is provided. The AMOLED
display includes a plurality of pixel circuits. A voltage-programming scheme,
a current-programming scheme or a combination thereof is applied to drive the
display. Threshold shift information, and/or voltage necessary to obtain
hybrid driving circuit may be acquired. A data sampling may be implemented to
acquire a current/voltage relationship. A feedback operation may be
implemented to correct the brightness of the pixel.


French Abstract

L'invention concerne un système et un procédé d'alimentation d'un écran AMOLED. L'écran AMOLED comprend une pluralité de circuits de pixels. Une configuration de programmation par tension, une configuration de programmation par courant, ou leur combinaison, est appliqué pour alimenter l'écran. Des données de décalage de seuil et/ou la tension nécessaire pour obtenir un circuit de commande hybride peuvent être acquises. Un échantillonnage de données peut être mis en oeuvre pour établir une relation courant-tension. Une opération de rétroaction peut être mise en oeuvre pour corriger la luminosité du pixel.


Note: Claims are shown in the official language in which they were submitted.


WHAT IS CLAIMED IS:


1. A system for driving a display which includes a plurality of pixel
circuits, each
having a plurality of thin film transistors (TFTs) and an organic light
emitting diode
(OLED), comprising:

a voltage driver for generating a voltage to program the pixel circuit through
a
data node;

a programmable current source for generating a current to program the pixel
circuit through the data node; and

a switching network for selectively connecting the data driver or the current
source to one or more pixel circuits through the data node.


2. A system according to claim 1, wherein the switching network includes:

a first switch for connecting the voltage driver to one or more pixel
circuits,
and

a second switch for connecting the current source to one or more pixel
circuits.

3. A system according to claim 2, wherein the switching network includes:

a shift register for controlling the operation of the first and second
switches.

4. A system according to claim 1, further comprising:

a analog/digital converter for sampling a voltage on the data node of the
pixel
circuit.


5. A system according to claim 1, further comprising:

a lookup table for storing a current/voltage information representing a
relationship between the program current on the data node and a program
voltage on
the data node associated with the program current on the data node.


6. A system according to claim 5, further comprising:

a sensing network for sensing a current consumed in the data node of the pixel

circuit, or the voltage at the data node of the pixel circuit, to correct the
lookup table.

21


7. A system according to claim 5, further comprising:

a module for correcting the voltage information during the voltage-based
programming.


8. A system according to claim 1, further comprising:

a programming circuit for acquiring the threshold voltage of the TFT from the
pixel circuit, having an analog to digital converter for converting an analog
threshold
voltage information to a digital threshold voltage information, and for
programming
the pixel circuit based on the digital threshold voltage information and the
voltage
associated with incoming video information.


9. A system for driving a pixel circuit having a plurality of thin film
transistors
(TFTs) and an organic light emitting diode (OLED), comprising:

a pre-charge controller for pre-charging and discharging a data node of the
pixel circuit to acquire threshold voltage information of the TFT from the
data node
using an external driving circuit outside the pixel circuit; and

a hybrid driving circuit for programming the pixel circuit based on the
acquired threshold voltage information and video data information displayed on
the
pixel circuit.


10. A system according to claim 9, wherein the hybrid driving circuit includes
a
capacitor coupled to the data node, and the capacitor is located outside the
pixel
circuit.


11. A system according to claim 9, wherein the hybrid driving circuit
includes:

a sampling circuit for sampling the threshold voltage information on the data
node,

a summer for summing the video data voltage and the sampled voltage
threshold information, and

a switch for selectively connecting the output of the summer to the data node.


12. A system according to claim 9, wherein the hybrid driving circuit
includes:


22


an analog to digital converter for converting the threshold voltage
information
to a digital threshold voltage information,

a microcomputer for storing the digital threshold voltage information and
summing the digital threshold voltage information and the voltage, and

a digital to analog converter for converting the summing result output from
the
microcomputer to an analog data and providing the analog data to the data
node.


13. A system according to claim 9, further comprising:

a programming circuit for providing a current on the data node to program the
pixel circuit.


14. A system according to claim 9, wherein the hybrid driving circuit includes
a
switching matrix for selecting one of a voltage programming mode and a current

programming mode to program the pixel by the selected programming mode.


15. A system for driving a pixel circuit having a plurality of thin film
transistors
(TFTs) and an organic light emitting diode (OLED), comprising:

a sampler for sampling, from a data node of the pixel circuit, a voltage
required to program the pixel circuit;

a programming circuit for programming the pixel circuit based on the sampled
voltage and video data information displayed on the pixel circuit.


16. A system according to claim 15, further comprising:

a current source for providing current to the pixel circuit during a
calibration
mode; and

a lookup table for storing a current/voltage information representing a
relationship between the programming current applied to the data node and the
sampled voltage associated with the current.


17. A system according to claim 16, wherein the lookup table is created to
each
pixel circuit.


18. A system according to claim 16, further comprising:


23


a correction calculation module for correcting data from a data source based
on the current/voltage information, obtained by programming the data node with
a
current,

during a writing mode, a voltage associated with the corrected data being
applied to the pixel circuit through the data node.


19. A system according to claim 16, further comprising:

a module for extracting a threshold voltage shift of the TFT based on the
sampled voltage obtained by current-programming through the data node.


20. A system according to claim 15, further comprising:

a lookup table for storing a current/voltage curve representing a relationship

between a current and a voltage necessary to program the current into the
pixel circuit
through the data node;

a module for correcting the current/voltage curve based on the sampled
voltage associated with information currently displayed on the pixel circuit,

during a writing mode, a voltage to be programmed being determined based
on the current/voltage curve.


21. A system according to claim 20, wherein the lookup table is created to
each
pixel circuit.


22. A system according to claim 20, further comprising:

a module for extracting a threshold voltage shift of the TFT based on the
corrected current/voltage curve.


23. A system according to any one of claims 1-22, wherein the system is
applicable to a current-programmed pixel circuit and a voltage-programmed
pixel
circuit.


24. A system according to any one of claims 1-22, wherein the TFT includes
amorphous silicon, polysilicon (n-type or p-type), crystalline silicon, or
organic based
TFT.


24


25. A system according to any one of claims 1-22, wherein the OLED includes
NIP or PIN OLED, and is locatable in the source or the drain of one or more
driving
TFTs.


26. A method of driving a pixel circuit having a plurality of thin film
transistors
(TFTs) and an organic light emitting diode (OLED), comprising:

selecting a pixel circuit and pre-charging a data node of the pixel circuit
using
an external circuit connected through the data node;

allowing the pre-charged data node to be discharged;

extracting a threshold voltage of the TFT through the discharging step; and
programming the pixel circuit, including compensating a programming data
based on the extracted threshold voltage using an external compensation
circuit.


27. A method according to claim 26, wherein the extracting step includes:
sampling the threshold voltage, and

recording the sampled threshold voltage, the compensating step utilizing the
recorded sampled threshold voltage.


28. A method according to claim 27, wherein the programming step includes:
subsequently programming the pixel circuit through the data node based on
the threshold voltage recorded using an external circuit.


29. A method according to claim 26, wherein the programming step includes:
programming information on the pixel circuit with a current-programming
scheme and a voltage-programming scheme.


30. A method of driving a pixel circuit having a plurality of thin film
transistors
(TFTs) and an organic light emitting diode (OLED), comprising:

sampling, from a data node of the pixel circuit, a voltage required to program

the pixel circuit;





programming the pixel circuit based on the sampled voltage and information
displayed on the pixel circuit.


31. A method according to claim 30, further comprising:

enabling a calibration mode, and implementing a current-programming
scheme to the pixel circuit,

the sampling step implementing sampling operation during the calibration
mode.


32. A method according to claim 31, further comprising:

creating, based on the sampling, a lookup table storing a current/voltage
correction information representing the current used to program the pixel via
the data
node and the sampled voltage associated with the current,

the programming step including the step of correcting data from a data source
based on the current/voltage correction information.


33. A method according to claim 30, further comprising:

storing a current/voltage correction information representing a current and a
voltage necessary to program the current into the pixel circuit, and

correcting the current/voltage correction information based on the sampled
voltage associated with information currently displayed on the pixel circuit.


34. A hybrid driving circuit for implementing the switching network according
to
claim 1, wherein the hybrid driving circuit is applicable to driving schemes,
including
drive schemes that use timing of the data, select or power inputs to the pixel
circuits
to achieve increased brightness uniformity, drive schemes that use current or
voltage
feedback, and drive schemes that use optical feedback.


35. A hybrid driving circuit for implementing the system according to claim 9
or
15 , wherein the hybrid driving circuit is applicable to any driving schemes,
including
drive schemes that use timing of the data, select or power inputs to the pixel
circuits
to achieve increased brightness uniformity, drive schemes that use current or
voltage
feedback, and drive schemes that use optical feedback.


26


36. A system according to any one of claims 1-22, wherein the OLED material
includes fluorescent, phosphorescent, polymer, or dendrimer.



27

Note: Descriptions are shown in the official language in which they were submitted.


CA 02567076 2006-12-01

WO 2006/000101 PCT/CA2005/001007

Voltage-Programming Scheme For Current-Driven AMOLED Displays
FIELD OF INVENTION

[0001] The present invention relates to a display technique, and more
specifically to
technology for driving pixel circuits.

BACKGROUND OF THE INVENTION

[0002] Active matrix organic light emitting diode (AMOLED) displays are well
known
in the art. The AMOLED displays have been increasingly used as a flat panel in
a wide
variety of tools.

[0003] The AMOLED displays are classified as either a voltage-programmed
display
or a current-programmed display. The voltage-programmed display is driven by a
voltage-programmed scheme where data is applied to the display as a voltage.
The
current-programmed display is driven by a current-programmed scheme where data
is
applied to the display as a current.

[0004] The advantage of the current-programming scheme is that it can
facilitate pixel
designs where the brightness of the pixel remains more constant over time than
with
voltage programming. However, the current-programming requires longer time of
charging capacitors associated with the column.

[0005] Therefore, there is a need to provide a new scheme for driving a
current-driven
AMOLED display, which ensures high speed and high quality.

SUMMARY OF THE INVENTION

[0006] The present invention relates to a system and method of driving a pixel
circuit in
an AMOLED display.

[0007] The system and method of the present invention uses Voltage-Programming
Scheme For Current-Driven AMOLED Displays.

[0008] In accordance with an aspect of the present invention there is provided
a system
for driving a display which includes a plurality of pixel circuits, each
having a plurality
of thin film transistors (TFTs) and an organic light emitting diode (OLED),
which


CA 02567076 2006-12-01

WO 2006/000101 PCT/CA2005/001007
includes: a voltage driver for generating a voltage to program the pixel
circuit; a
programmable current source for generating a current to program the pixel
circuit; and
a switching network for selectively connecting the data driver or the current
source to
one or more pixel circuits.

[0009] In accordance with a further aspect of the present invention there is
provided a
system for driving a pixel circuit having a plurality of thin film transistors
(TFTs) and
an organic light emitting diode (OLED), which includes: a pre-charge
controller for
pre-charging and discharging a data node of the pixel circuit to acquire
threshold
voltage information of the TFT from the data node; and a hybrid driving
circuit for
programming the pixel circuit based on the acquired threshold voltage
information and
video data information displayed on the pixel circuit.

[0010] In accordance with a further aspect of the present invention there is
provided a
system for driving a pixel circuit having a plurality of thin film transistors
(TFTs) and
an organic light emitting diode (OLED), which includes: a sampler for
sampling, from
a data node of the pixel circuit, a voltage required to program the pixel
circuit; and a
programming circuit for programming the pixel circuit based on the sampled
voltage
and video data information displayed on the pixel circuit.

[0011 ] In accordance with a further aspect of the present invention there is
provided a
method of driving a pixel circuit having a plurality of thin film transistors
(TFTs) and
an organic light emitting diode (OLED), which includes the steps of selecting
a pixel
circuit and pre-charging a data node of the pixel circuit; allowing the pre-
charged data
nodeto be discharged; extracting a threshold voltage of the TFT through the
discharging step; and programming the pixel circuit, including compensating a
programming data based on the extracted threshold voltage.

[00 12] This summary of the invention does not necessarily describe all
features of the
invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] These and other features of the invention will become more apparent
from the
following description in which reference is made to the appended drawings
wherein:
2


CA 02567076 2006-12-01

WO 2006/000101 PCT/CA2005/001007
[0014] Figure 1 is a block diagram showing a system for driving an AMOLED
display
in accordance with an embodiment of the present invention;

[0015] Figure 2 is a schematic diagram showing one example of a pixel circuit
of
Figure 1;

[0016] Figare 3 is a schematic diagram showing an example of a.hybrid driving
circuit,
which is applicable to Figure 1;

[0017] Figure 4 is an exemplary flow chart for showing the operation of the
hybrid
driving circuit of Figure 3;

[0018] Figure 5 is an exemplary timing chart for showing the operation of the
hybrid
driving circuit of Figure 3;

[0019] Figure 6 is a schematic diagram showing a further example of a hybrid
driving
circuit, which is applicable to Figure 1;

[0020] Figure 7 is an exemplary flow chart for showing the operation of the
hybrid
driving circuit of Figure 6;

[0021] Figure 8 is a schematic diagram showing a further example of a hybrid
driving
circuit, which is applicable to Figure 1;

[0022] Figure. 9 is an exemplary flow chart for showing the operation of the
hybrid
driving circuit of Figure 8;

[0023] Figure 10 is an exemplary timing chart for showing the operation of the
hybrid
driving circuit of Figure 8;

[0024] Figure 11 is a schematic diagram showing a further example of the pixel
circuit
of Figure 1;

[0025] Figure 12 is a block diagram showing a system for driving an AMOLED
display
in accordance with a further embodiment of the present invention;

[0026] Figure 13 is an exemplary flow chart for showing the operation of the
system of
Figure 12;

3


CA 02567076 2007-08-20

[0027] Figure 14 is an exemplary flow chart for showing the operation of the
system of
Figure 12;

[0028] Figure 15 is an exemplary timing chart for showing the operation of the
system
of Figure 12;

[0029] Figure 16 is an exemplary flow chart for a hidden refresh operation of
the
system of Figure 12;

[0030] Figure 17 is a diagram showing an example of a sample of the
current/voltage
correction curve;

[0031 ] Figure 18 is a diagram showing the current/voltage correction curve of
Figure
17 and an example of a newly measured data point:

[0032] Figure 19 is a diagram showing an example of a new current/voltage
correction
curve based on the measured point of Figure 18;

[0033] Figure 20 is a block diagram showing a further example of a programming
circuit for implementing a combined current and voltage-programming technique;

[0034] Figure 21 is a block diagram showing a system for driving an AMOLED
display
in accordance with a further embodiment of the invention; and

[0035] Figure 22 is a schematic diagram showing an example of a switch network
of
Figure 21; and

[0036] Figure 23 is a schematic diagram showing an example of a system for
correcting
the current voltage information of a pixel circuit.

DETAILED DESCRIPTION

[0037] Embodiments of the present invention are described using an AMOLED
display.
Drive scheme described below is applicable to a current programmed (driven)
pixel
circuit and a voltage programmed (driven) pixel circuit.

[0038] In addition, hybrid technique described below can be applied to any
existing
driving scheme, including a) any drive schemes that use sophisticated timing
of the
4


CA 02567076 2007-08-20

data, select, or power inputs to the pixels to achieve increased brightness
uniformity, b)
any drive schemes that use current or voltage feedback, c) any drive schemes
that use
optical feedback.

[0039] The light emitting material of the pixel circuit can be any technology,
specifically organic light emitting diode (OLED) technology, and in
particular, but not
limited to, fluorescent, phosphorescent, polymer, and dendrimer materials.

[0040] Referring to Figure 1, there is illustrated a system 2 for driving an
AMOLED
display 5 in accordance with an embodiment of the present invention. The
AMOLED
display 5 includes a plurality of pixel circuits. In Figure 1, four pixel
circuits 10 are
shown as an example.

[0041] The system 2 includes a hybrid driving circuit 12, a voltage source
driver 14, a
hybrid programming controller 16, a gate driver 18A and a power-supply 18B.
The
pixel circuit 10 is selected by the gate driver 18A (Vsel), and is programmed
by either
voltage mode using a node Vdata or current mode using a node Idata. The hybrid
driving circuit 12 selects the mode of programming, and connects it to the
pixel circuit
10 through a hybrid signal. A pre-charge signal (Vp) is applied to the pixel
circuit 10 to
acquire threshold Vt information (or Vt shift information) from the pixel
circuit 10.
The hybrid driving circuit 12 controls the pre-charging, if pre-charging
technique is
used. The pre-charge signal (Vp) may be generated within the hybrid driving
circuit 12,
which depends on the operation condition. The power-supply 18B (Vdd) supplies
the
current required to energize the display 5 and to monitor the power
consumption of the
display 5.

[0042] The hybrid controller 16 controls the individual components that make
up the
entire hybrid programming circuit. The hybrid controller 16 handles timing and
controls the order in which the required functions occur. The hybrid
controller 16 may
generate data Idata and supplied to the hybrid driving circuit 12. The system
2 may
have a reference current source, and the Idata may be supplied under the
control of the
hybrid controller 16.

[0043] The hybrid driver 12 may be implemented either as a switching matrix,
or as the
hybrid driving circuit(s) of Figure 3, 6, 8 or 20 or combination thereof.

5


CA 02567076 2007-08-20

[0044] In the description, Vdata refers to data, a data signal, a data line or
a node for
supplying the data or data signal Vdata, or a voltage on the data line or the
node.
Similarly, Idata refers to data, a data signal, a data line or a node for
supplying the data
or data signal Idata, or a current on the data line or the node. Vp refers to
a pre-charge
signal, a pre-charge pulse, a pre-charge voltage for pre-charging/discharging,
a line or
a node for supplying the pre-charge signal, pre-charge pulse or pre-charge
voltage Vp.
Vsel refers to a pulse or a signal for selecting a pixel circuit or a line or
a node for
supplying the pulse or signal Vs. The terms "hybrid signal", "hybrid signal
node", and
"hybrid signal line" may be used interchangeably.

[0045] The pixel circuit 10 includes a plurality of TFTs, and an organic light
emitting
diode (OLED). The TFT may be an n-type TFT or a p-type TFT. The TFT is, for
example, but not limited to, an amorphous silicon (a-Si:H) based TFT, a
polycrystalline
silicon based TFT, a crystalline silicon based TFT, or an organic
semiconductor based
TFT. The OLED may be regular (P-I-N) stack or inverted (N-I-P) stack. The OLED
can be located in the source or the drain of one or more driving TFTs.

[0046] Figure 2 illustrates an example of the pixel circuit 10 of Figure 1.
The pixel
circuit of Figure 2 includes four thin film transistors (TFTs) 20-26, a
capacitor Cs 28
and an organic light emitter diode (OLED) 30. The TFT (Tdrive) 26 is a drive
TFT that
is connected to the OLED 30 and the capacitor Cs 28. The pixel circuit of
Figure 2 is
selected by the select line Vsel, and is programmed by a data line DL. The
data line DL
is controlled by the hybrid signal output from the hybrid driving circuit 12
of Figure 1.
[0047] In Figure 2, four TFTs are illustrated. However, the pixel circuit 10
of Figure
1 may include less than four TFTs or more than four TFTs.

[0048] In the description, the terms "data line DL" and "data node DL" may be
used
interchangeably.

[0049] Referring to Figures 1-2, the data node DL is pre-charged and
discharged to
acquire the threshold Vt of a drive TFT (e.g., Tdrive 26 of Figure 2) or the
threshold Vt
shift. In the description, Vt shift, Vt shift information, Vt, and Vt
information may be
used interchangeably. The pixel circuit 10 is then consecutively programmed by
the
source driver 14 using voltage-programming. The acquired Vt shift information
is
6


CA 02567076 2007-08-20

utilized to compensate for degradation of the pixel circuit 10, thus
maintaining uniform
brightness of the display 5.

[0050] The process of acquiring Vt starts by applying Vsel to T120 and T2 22
to the
pixel circuit illustrated in Figure 2. Such action causes the drain and gate
of T3 24 to
be at the same voltage. This allows the Vt of T3 24 to be extracted by first
applying the
pre-charge voltage Vp to the data line DL, which is than allowed to be
discharged. The
rate of discharge is a function of Vt. Thus, by measure of the rate of
discharge, Vt can
be obtained.

[0051 ] Figure 3 illustrates an example of a hybrid driving circuit, which is
applicable to
the hybrid driving circuit 12 of Figure 1. The hybrid driving circuit 12A of
Figure 3
implements voltage programming technique. This is a driving scheme that is
located
outside of the pixel circuit, and can be used on any current-programmed pixel
circuit.
[0052] The hybrid driving circuit 12A of Figure 3 includes a charge
programming
capacitor Cc 32, which is part of the external driving circuit and not on the
pixel circuit.
It is located outside the pixel circuit. The charge programming capacitor Cc
32 is
provided between the data line Vdata and the data node DL. The pre-charge line
Vp is
also connected to the data node DL.

[0053] The hybrid driving circuit 12A is provided to a pixel circuit 10A
having four
TFTs (such as the pixel circuit of Figure 2). However, the pixel circuit I OA
may
include more than four TFTs or less than four TFTs.

[0054] The charge programming capacitor Cc 32 is provided to program the pixel
circuit 10A with a voltage that is equal to the sum of threshold Vt of the TFT
and Vdata,
scaled by a constant K. The constant is determined by the voltage division
network
formed by the charge storage capacitor (e.g. Cs 28 of Figure 2) and the charge
programming capacitor Cc 32.

[0055] Figure 4 illustrates an exemplary flow chart for showing the operation
of the
hybrid driving circuit 12A of Figure 3. At step S 10, pre-charge mode is
enabled. At
step S12, a pixel circuit is selected and pre-charging (Vp) is started. At
step S14, Vt
7


CA 02567076 2007-08-20

acquisition mode is enabled, and at step S 16, discharging (Vp) starts. The Vt
information is acquired through Cc 32. Then at step S18, writing mode is
enabled.
[0056] Figure 5 illustrates an exemplary timing chart for showing the
operation of the
hybrid driving circuit 12A of Figure 3. In the drawings, VdataO represents
voltage at
the data node (e.g. DL of Figure 2) of the pixel circuit; IdataO represents
current at the
data node (e.g. DL of Figure 2) of the pixel circuit.

[0057] The programming procedure starts by selecting the pixel to be
programmed with
the pulse Vsel. At the same time, the pre-charge pulse Vp is applied to the
pixel
circuit's data input (e.g. DL of Figure 2).

[0058] During the Vt acquisition phase, voltage on the data line (DL) is
allowed to be
discharged through the pixel circuit, which is in a current mirror connection
with the
Vsel line held high. The data line (DL) is discharged to a certain voltage,
and the Vt of
a drive TFT is extracted from that voltage. The voltage at Vdata is at ground.

[0059] During the programming (writing) phase, the calculated compensated
voltage is
applied to the data input line (DL) of the pixel circuit. The programming
routine
finishes with the lowering of the V sel signal.

[0060] The calculated compensated voltage is obtained through analog means of
a
charge programming capacitor Cc32. However, any other analog means for
obtaining
compensated voltage may be used. Further, any (external) digital circuit (e.g.
50 of
Figure 7) may be used to obtain the calculated compensated voltage.

[0061] The source driver (14 of Figure 1) supplies Vdata to the capacitor Cc
32. When
Vdata is increased from ground to the desired voltage level, the voltage at
Idata is equal
to (Vt+Vdata)*K.

[0062] The structure of Figure 3 is simple, and is easily implemented.

[0063] Figure 6 illustrates a further example of a hybrid driving circuit,
which is
applicable to the hybrid driving circuit 12 of Figure 1. The hybrid driving
circuit 12B
of Figure 6 is located outside of the pixel circuit, and implements voltage
programming
technique.

8


CA 02567076 2007-08-20

[0064] The hybrid driving circuit 12B includes a summer 40, a sample and hold
(S/H)
circuit 42 and a switching element 44. The S/H circuit 42 samples Idata and
holds it for
a certain period. The summer 40 receives Vdata and the output of the S/H
circuit 42.
The switching element 44 connects the output of the summer 40 to the data node
DL in
response to a programming control signal 46.

[0065] The hybrid driving circuit 12B utilizes the summer 40, instead of the
charge
coupling capacitor Cc 32, to produce programming voltage that is equal to the
sum of
Vt and Vdata. As the hybrid driving circuit 12B does not utilize a capacity,
programming voltage is not affected by the parasitic capacitance, and it has
less charge
feed-through effect. As the hybrid driving circuit 12B does not utilize a
charge storage
capacitor, programming voltage is not affected by the charge storage
capacitance. As
the hybrid driving circuit 12B does not utilize a charge programming
capacitor, it
achieves faster Vt acquisition time. Removal of the charge programming
capacitor
eliminates the charge dependency of the programming scheme. Thus the
programming
voltage is not affected by the charge being shared between the charge storage
capacitor
and the parasitic capacitance of the system. This results in a higher
effective
programming voltage.

[0066] Figure 7 illustrates an exemplary flow chart for showing the operation
of the
hybrid driving circuit 12B of Figure 6. During the Vt acquisition mode, the Vt
is
sampled at step S20, and new data is produced at step S22. When writing mode
is
enabled, the new data is supplied to the pixel circuit in response to the
programming
control signal (46) at S24. It is noted that the operation of the system
having the hybrid
driving circuit 12B is not limited to Figure 7. The new data may be produced
after step
S 18. The control signa146 may be enabled before step S 18.

[0067] During the Vt acquisition cycle, Vdata is at ground, and the voltage at
the data
node DL is equal to Vt of the TFT by the pre-charging/discharging operation
(Vp). The
voltage on the data node DL is sampled and holed by the S/H circuit 42. The Vt
is
provided to the summer 40 through the S/H circuit 42. When Vdata is increased
from
ground to the desired voltage level, the summer 40 outputs the sum of Vt and
Vdata.
The switch 44 turns on in response to the programming control signa146. The
voltage
9


CA 02567076 2007-08-20

at the data node DL goes to (Vt +Vdata). Timing chart for showing the
operation of the
system 2 having the hybrid driving circuit 12B is similar to that of Figure 5.

[0068] Figure 8 illustrates a further example of a hybrid driving circuit,
which is
applicable to the hybrid driving circuit 12 of Figure 1. The hybrid driving
circuit 12C
of Figure 8 implements voltage programming technique.

[0069] The hybrid driving circuit 13C is a direct digital hybrid driving
circuit. The
direct digital programming circuit 13C includes a microComputer uC 50 which
receives digital data (Vdada), a digital to analog (D/A) converter 52, a
voltage follower
54 for increasing current without affecting voltage, and an analog to digital
(A/D)
converter 56.

[0070] The threshold Vt of the drive TFT may increase slowly. Thus, it may not
be
necessary to acquire the threshold Vt of the drive TFT every programming
cycle. This
effectively hides the Vt acquisition for the majority of the programming
cycle. In the
direct digital hybrid driving circuit 13C, the threshold Vt acquired from the
pixel circuit
10A is digitalized at the A/D converter 56, and is stored in memory contained
in the uC
50. The digital data that defines the brightness of the pixel is added to the
Vt in the uC
50. The resulting voltage is then converted back to an analog value at the D/A
52,
which is programmed into the pixel circuit 10A. This programming method is
designed
to compensate for the slow process of the Vt acquisition.

[0071 ] Figure 9 illustrates an exemplary flow chart for showing the operation
of the
hybrid driving circuit 12C of Figure 8. At the Vt acquisition mode, the Vt is
sampled
and recorded at step S30. When writing mode is enabled, new data is provided
based
on the recorded data. It is noted that the operation of the system having the
hybrid
driving circuit 12C of Figure 8 is not limited to Figure 9. At the writing
mode, the data
which have been recorded may be used without implementing the Vt acquisition.
[0072] Figure 10 illustrates an exemplary timing chart for showing the
operation of the
hybrid driving circuit 12C of Figure 8. During the Vt acquisition, sampling by
the A/D
converter 56 is implemented. In a next cycle, the hybrid driving circuit 13C
may use the
Vt that has been previously acquired and has been recorded in the uC 50.



CA 02567076 2007-08-20

[0073] The conversion of the output on the data node DL by A/D can remove the
requirements of having to acquire the Vt every programming cycle. The Vt of
the pixel
circuit I OA may be acquired once every second or less. Thus, it may acquire
Vt for only
one row of the display per frame cycle. This effectively increases the amount
of time
for the pixel programming cycle. Less frequent need of Vt acquisition ensures
faster
programming time.

[0074] In the above description, Figure 2 is used to describe the pixel
circuit 10 of
Figure 1. However, the pixel circuit 10 is not limited to that of Figure 2.
The pixel
circuit 10 may be a pixel circuit illustrated in Figure 11 (J. Kanichi, J.-H.
Kim, J.Y.
Nahm, Y. He and R. Hattori "Amorphous Silicon Thin-Film Transistor Based
Active-Matrix Organic Light Emitting Display" Asia Display IDW 2001 pp. 315).
The
pixel circuit of Figure 11 includes four TFTs 64-70, a capacitor CST 72 and an
OLED
74. The TFT 78 is a drive TFT that is connected to the OLED 74 and the
capacitor CST
72. The pixel circuit of Figure 11 is selected by Vselectl and Vselect2, and
is
programmed by Idata. The voltage acquired is a combination of the voltage
across the
OLED 74 and T3 68. The technique compensates the voltage change of both the Vt
and
the OLED 74. Idata of Figure 11 corresponds to the data node DL of Figure 2.

[0075] Figure 12 illustrates a system for driving an AMOLED display in
accordance
with a further embodiment of the invention. The system 82 of Figure 12
includes a
hybrid programming circuit having a correction table 80, a source driver 14
for
implementing a voltage-programming scheme and a reference current source 94
for
implementing a current-programming scheme. The system 82 drives a display
having
a plurality of pixel circuits using the voltage-programming scheme and the
current-programming scheme.

[0076] A hybrid controller 98 is provided to control each component. In Figure
12, the
hybrid controller 98 is placed between the A/D converter 96 and the correction
table 80,
as an example. The hybrid controller 98 is similar to the hybrid controller 16
of Figure
1.

[0077] The pixel circuit driven by the system 82 may be the pixel circuit 10
of Figure
1, and may be a current programmed pixel circuit or a voltage programmed pixel
circuit.
I I


CA 02567076 2007-08-20

The pixel circuit driven by the system 82 may be implemented by Figure 2 or
Figure 11,
however, is not limited to those of Figures 2 and 11.

[0078] The hybrid programming circuit includes a correction calculation module
92 for
correcting data from the data source 90 based on the correction table 80 and
an A/D
converter 96. The data corrected by the correction calculation module 92 is
applied to
the source driver 14. The source driver 14 generates Vdata based on the
corrected data
output from the correction calculation module 92. Vdata from the source driver
14 and
Idata from the reference current source 94 are supplied to the hybrid driver
12.

[0079] The data source 90 is, for example, but not limited to, a DVD. The
hybrid driver
12 may be implemented either as a switching matrix, or as the digital
programming
circuit(s) of Figure 8, 20 or combination thereof. The A/D converter 96 may be
the A/D
converter 56 of Figure 8. The system 82 may implement the Vt acquisition
technique
described above using the A/D converter 96 (56).

[0080] The correction table 80 is a lookup table. The correction table 80
records the
relationship between current required to program the pixel circuit and voltage
necessary
to obtain that current. The correction table 80 is built for every pixel in
the entire
display.

[0081 ] In the description, the relationship between the current required to
program the
pixel circuit and the voltage necessary to obtain that programming current, is
referred to
as "current/voltage correction information", "current/voltage correction
curve", or

"current/voltage information", or "current voltage curve".

[0082] In Figure 12, the correction table 80 is illustrated separately from
the correction
calculation module 92. However, the correction table 80 may be included in the
correction calculation module 92.

[0083] The operation of the system of Figure 12 has two modes, namely display
mode
and calibration mode. In the display mode, the data from the data source 90 is
corrected
using the data in the correction table 80, and is applied to the source driver
14. The
hybrid driver 12 is not involved in the display mode. In the calibration mode,
the
current from the reference current source 94 is applied to the pixel circuit,
and the

12


CA 02567076 2007-08-20

voltage associated with the current is read from the pixel circuit. The
voltage is
converted to a digital data by the A/D converter 96. The correction table 80
is updated
with the correct value based on the digital data.

[0084] During the display mode, a voltage-programming scheme is implemented.
The
voltage on the data line (e.g. DL of Figure 2) of the pixel circuit determines
the
brightness of the pixels. The voltage required to program the pixel circuit is
calculated
from the pixel brightness to be displayed (from the incoming video
information)
combined with the current/voltage correction information stored in the
correction table
80. The information on the correction table 80 is combined with incoming video
information to ensure that each pixel will maintain a constant brightness over
long-term
use.

[0085] After the display has been used for a fixed period of time, the display
enters the
calibration mode. The current source 94 is connected to the data input node
(DL) of the
pixel circuit via the hybrid driver 12. Each pixel is programmed through a
current-programming scheme (where the level of current on the data line
determines the
brightness of the pixel), and the voltage required to achieve that current is
read by the
A/D converter 96.

[0086] The voltage required to program the pixel current is sampled at
multiple current
points by the A/D converter 96. The multiple points may be a subset of the
possible
current levels (e.g. 256 possible levels for 8-bit, or 64 levels for 6-bit).
This subset of
voltage measurements is used to construct the correction table 80 that is
interpolated
from the measurement points.

[0087] The calibration mode may be entered either through user's command or
may be
combined with the normal display mode so that the calibration takes place
during the
display refresh period.

[0088] In one example, the entire display may be calibrated at once. The
display may
stop showing incoming video information for a short period of time while each
pixel
was programmed with a current and the voltage recorded.

13


CA 02567076 2007-08-20

[0089] In a further example, a subset of the pixels may be calibrated, such as
one pixel
every fixed number of frames. This is virtually transparent to the user, and
the
correction information may still be acquired for each pixel.

[0090] When a conventional voltage-programming scheme is utilized, a pixel
circuit is
programmed in an open loop configuration, where there is no feedback from the
pixel
circuit regarding the threshold voltage shift of the TFTs. When a conventional
current-
programming scheme is utilized, the brightness of the pixel may remain
constant over
time. However, the current programming scheme is slow. Thus, the table lookup
technique combines the technique of the current-programming scheme with the
technique of the voltage-programming scheme. The pixel circuit is programmed
with
a current through a current-programming scheme. A voltage to maintain that
current is
read and is stored at a lookup table. The next time that particular level of
current is
applied to the pixel circuit, instead of programming with a current, the pixel
circuit is
programmed based on information on the lookup table. Accordingly, it attains
the
compensation inherent in the current programming scheme while attaining the
fast
programming time that is only possible with voltage-programming scheme.

[0091 ] In the above description, the correction table (lookup table) 80 is
used to correct
the current/voltage correction information. However, the system 82 of Figure
12 may
use the lookup table to correct the Vt shift and the current/voltage
correction
information at the same time in combination with the hybrid driving circuit of
Figure 3,
6,8or20.

[0092] For example, several voltage measurements are captured at many
different
current points by the A/D converter 96 (56). The hybrid controller 98 extracts
the Vt
shift information by extending the voltage versus current curve to zero
current point.
The Vt shift information is stored in an array of tables (correction table 80)
which is
applied to incoming display data.

[0093] The uC 50 of Figure 8 or 20 may utilize the lookup table to generate
appropriate
voltage and program the pixel circuit.

[0094] The hybrid circuits 12A of Figure 3 and 12B of Figure 6 may be
integrated into
the system of Figure 12.

14


CA 02567076 2007-08-20

[0095] Figures 13-14 illustrate exemplary flow charts for showing the
operation of the
system of Figure 12. Referring to Figure 13, at step S40, calibration mode is
enabled.
At step S42, a pixel circuit is selected and current programming is
implemented to the
selected pixel circuit. At step S44, a switch matrix enable signal is enabled.
Then the
connection to the pixel circuit is changed. The Vt is sampled at step s46, and
then the
correction table is created/corrected at step S48. Referring to Figure 14, at
step S50,
video data are corrected based on the correction table. Then at step S52, new
Vdata is
produced based on the corrected data.

[0096] It is noted that the writing mode may be implemented based on the
previously
created correction table without implementing the calibration mode. It is
noted that the
operation of the system of Figure 12 is not limited to Figures 13-14.

[0097] Figure 15 illustrates an exemplary timing chart for showing a
combination of
the Vt shift acquisition and the current/voltage correction. A switch matrix
enable
signal in Figure 15 represents a control signal for the hybrid driver 12 of
Figure 12.
[0098] Referring to Figures 12 and 15, the calibration mode (i.e. the
current-programming scheme) is enabled when the switch matrix enable signal is
high.
The programming mode (i.e. the voltage-programming scheme) is enabled when the
switch matrix enable signal is low. However, the calibration mode may be
enabled
when the switch matrix enable signal is low. The programming mode may be
enabled
when the switch matrix enable signal is high.

[0099] A/D sampling is implemented during the calibration mode. During the
calibration mode, the current from the reference current source 94 is applied
to the pixel
circuit. The voltage on the data input node is converted to a digital voltage
by the A/D
converter 56. Based on the digital voltage and current associated with the
digital
voltage, current/voltage correction information is recorded at the lookup
table. The Vt
shift information is generated based on the data in the correction table 80 or
the output
from the A/D converter 96.

[00100] The system 82 of Figure 12 may implement hidden refresh technique
for refreshing current/voltage correction information in addition to the table
lookup
technique described above.



CA 02567076 2007-08-20

[00101] Under the hidden refresh operation, new current/voltage correction
information is constructed while completely hidden from user's perception.
This
technique utilizes the information that is currently displayed on the screen
(i.e. the
incoming video data). By obtaining the pixel characteristics from the full
calibration
routine that has been performed during the manufacturing process of the
display, the
current/voltage correction information for each pixel in the display is known.
During
the display's usage, the current/voltage correction curve may shift due to the
change in
Vt. By measuring a single point along the current/voltage correction curve
(which is the
data currently displayed, that is part of the video image), a new
current/voltage
correction curve is extrapolated from the point so that it is fitted to the
measured point.
Based on the new current/voltage correction curve, the Vt shift information is
extracted
which is used to compensate for the shift in Vt.

[00102] Figure 16 illustrates an exemplary flow chart for the hidden refresh
operation of the system of Figure 12. First, a current/voltage correction
curve is
produced during the calibration process that is implemented during the
manufacturing
of the display (step S62). Figure 17 illustrates an example of a sample of the
current
voltage correction curve.

[00103] Referring to Figure 16, the next step is to measure a point along the
curve during the usage of the display. This point can be any point along the
curve, so
any data that the user currently has on the display can be used for
calibration (step S64).
Figure 18 illustrates the current voltage correction of Figure 17 and an
example of a
newly measured data point.

[00104] Referring to Figure 16, the last step is to shift the current/voltage
correction curve to fit the point of voltage verses current relationship that
is measured
(step S66). Figure 19 illustrates an example of a new current voltage
correction curve
based on the measured point of Figure 18.

[00105] The process associated with Figures 17-19 is implemented in the hybrid
controller 98 of Figure 12.

[00106] The system 82 of Figure 12 may implement a combined current and
voltage-programming technique. Figure 20 illustrates one example of a hybrid
driving
16


CA 02567076 2007-08-20

circuit for implementing the combined current and voltage-programming
technique.
The hybrid driving circuit of Figure 20 may be included in the hybrid driver
12 of
Figure 12.

[00107] In the hybrid driving circuit of Figure 20, the digital hybrid driving
circuit 12C and a current source 100 are provided to the data line DL of the
pixel circuit.
[00108] To enhance the circuit's ability to compensate for a change in the
current/voltage correction curve due to temperature, threshold voltage shift,
or other
factors, the pixel circuit programming is divided into two phases.

[00109] During the writing mode, the pixel circuit 10A is voltage-programmed
first to set the gate voltage of the driving TFT to an approximate value, then
followed
by a current programming phase. The current programming phase can then fine-
tune the
output current. The system of Figure 20 is faster than current programming and
has the
compensation capabilities of the current programming scheme.

[00110] In Figure 20, the digital hybrid driving circuit 12C is provided.
However, the combined current and voltage-programming technique may be
implemented by combining the hybrid driving circuit 12A of Figure 3 or 12B of
Figure
6 with the current source 100. The current source 100 may be the reference
current
source 94 of Figure 12.

[00111] The system 2 of Figure 1 may implement the hidden refresh technique
described above. The system 2 of Figure 1 may implement the combined current
and
voltage-programming technique. The system 2 of Figure 1 may include the hybrid
driving circuit of Figure 20 to implement the combined current and
voltage-programming technique.

[00112] Extension of the direct digital programming scheme is now described in
detail. The direct digital programming scheme (Figures 6, 8 and 20) can be
extended to
drive an OLED array (e.g. a 4T OLED array) using voltage programmed column
drivers, such as those used for driving Active Matrix Liquixd Crystal Display
(AMLCD), or voltage-programmed Active-Matrix Organic Light Emitting Diode
(AMOLED) displays, or any other voltage-output display driver.

17


CA 02567076 2007-08-20

[00113] Figure 21 illustrates a system for driving an AMOLED array having a
plurality of pixel circuits in accordance with a further embodiment of the
invention.
The system 105 of Figure 21 includes a voltage column driver 112, a
programmable
current source 114, a switching network 116, an A/D converter 118 and a row
driver
120.

[00114] The voltage column driver 112 is a voltage programmed column driver.
Each of the voltage column driver 112 and the row driver 120 may be any driver
that
has a voltage output, such as those designed for the AMLCD. The voltage column
driver 112 and the programmable current source 114 are connected to an OLED
array
110 through the switching network 116. The OLED array 110 forms an AMOLED
display, and contains a plurality of pixel circuits (such as 10 of Figure 1).
The pixel
circuit may be a current programmed pixel circuit or a voltage programmed
pixel
circuit.

[00115] The A/D converter 118 is an interface that allows an analog signal
(i.e.
current driving the display 110) to be read back as a digital signal. The
digital signal
associated with the current can than be processed and/or stored. The A/D
converter 118
may be the A/D converter 56 of Figures 8 and 20. The column driver 112 may be
the
source driver 14 of Figures 1 and 12.

[00116] The system 105 of Figure 21 implements the calibration mode and the
display mode as described above.

[00117] Figure 22 illustrates an example of the switch network 116 of Figure
21.
The switching network 116 of Figure 22 includes two MOSFET switches 122 and
124
that can switch the column of the display (110) from connecting to the column
driver
(112) to the combination of the current source (114) and the A/D converter
(118), and
vice versa. A shift register 126 is a source of the digital control signal
that controls the
operation of the MOS switches 122 and 124. An inverter 128 inverts an output
from the
shift register 126. Thus, when the switch 122 is on (off), the switch 124 is
off (on).
[00118] The switching network 116 may be located either off the glass in the
column driver (112) or directly on the glass using TFT switches.

18


CA 02567076 2007-08-20

[00119] Referring to Figures 21-22, the system 105 uses only one current
source
114. The voltage-programming drivers (such as, AMLCD drivers, or any other
voltage-output drivers) drive the rest of the display 110. The switching
matrix
(switching network 116) allows different pixels within the array of pixels to
be
connected to a single current source (114) through a time division method.
This allows
a single current source to be applied to the entire display. This lowers the
cost of the
driver circuit and speeds up the programming time for the pixel circuit.

[00120] The system 105 uses the A/D converter 118 to convert an analog output
of the data node (e.g. DL of Figure 2) of the pixel circuit to digital data.
The conversion
by the A/D converter 118 removes the requirements of having to acquire the Vt
every
programming cycle. The Vt of the pixel circuit may be acquired once every few
minutes. Thus it may acquire one column of the panel every refresh cycle.

[00121] Only one A/D 118 may be implemented for all the columns. The circuit
acquires only one pixel per frame refresh. For example, for a 320 by 240
panel, the
number of pixels is 76, 8000. For a frame rate of 30Hz, the time required to
acquire Vt
from all pixels for the entire frame is 43 minutes. This may be acceptable for
some
applications, providing that Vt does not shift substantially in an hour.

[00122] The parasitics only affect the amount of time to discharge the
capacitor
to acquire Vt. Since the circuit is voltage-programmed, it is not affected by
the
parasitics. Since Vt is only acquired one column per frame time, it can be
long. For
example, for a display with 320 columns that has a frame rate of 30Hz, each
frame time
is 33mS. For voltage programming, it is possible to program a pixel in 70uS.
For 320
columns, the time to update the display is 22mS, which still leave 11mS to
complete a
charge/discharge cycle.

[00123] The system 105 may implement the lookup table technique to
compensate for Vt shift and/or to correct the current/voltage information as
described
above

[00124] The system 105 may implement the hidden refresh technique to acquire
the Vt shift information and current/voltage correction information of each
pixel circuit
(10) in the display 110. This current/voltage correction information is used
to populate
19


CA 02567076 2007-08-20

a lookup table (e.g. a correction table 80 of Figure 12) that will then be
used to
compensate for the degradation in the pixel circuit, which is caused by aging.
To reduce
cost, the number of current-programmed circuits has been reduced so there is
only one
per display instead of one per column driver.

[00125] The system 105 may implement the combined current and
voltage-programming technique as described above.

[00126] Figure 23 illustrates a system for correcting the current/ voltage
information of the pixel circuit. In Figure 23, a display 130 is depicted as a
2T or 4T
OLED array. However, the display 130 may include a plurality of pixel
circuits, each
having three or more than four transistors. The display 130 may include
voltage-driven
pixel circuits or current-driven pixel circuits. The system of Figure 23 is
applicable to
the systems 2, 82, and 105 of Figures 1, 12 and 22.

[00127] As illustrated in Figure 23, a switch 132 is provided to disconnect
the
common electrode of the OLED. It is well known that two electrodes are
provided for
the OLED. One is connected to the pixel circuit, and the other is a common
electrode
connected to all OLEDs. It is noted that the common electrode may be Vdd or
GND
depending on the current sensing network 134 utilizing a high side common mode
sensor (such as, INA168 by TI). The current sensing network 134 measures the
current
through the common electrode.

[00128] According to the embodiments of the present invention, the major issue
with current-programmed pixel circuits, which is the slow programming time, is
solved.
The concept of using feedback to compensate the pixel circuit enhances the
uniformity
and stability of the display while retaining the fast programming capability
of the
voltage programmed drive scheme.

[00129] The present invention has been described with regard to one or more
embodiments. However, it will be apparent to persons skilled in the art that a
number
of variations and modifications can be made without departing from the scope
of the
invention as defined in the claims.


A single figure which represents the drawing illustrating the invention.

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Admin Status

Title Date
Forecasted Issue Date 2008-10-21
(86) PCT Filing Date 2005-06-28
(87) PCT Publication Date 2006-01-05
(85) National Entry 2006-12-01
Examination Requested 2006-12-01
(45) Issued 2008-10-21
Lapsed 2012-06-28

Payment History

Fee Type Anniversary Year Due Date Amount Paid Paid Date
Special Order $500.00 2006-12-01
Request for Examination $200.00 2006-12-01
Filing $400.00 2006-12-01
Maintenance Fee - Application - New Act 2 2007-06-28 $100.00 2007-06-26
Registration of Documents $100.00 2008-05-02
Maintenance Fee - Application - New Act 3 2008-06-30 $100.00 2008-06-20
Final $300.00 2008-08-08
Maintenance Fee - Patent - New Act 4 2009-06-29 $100.00 2009-06-19
Maintenance Fee - Patent - New Act 5 2010-06-28 $200.00 2010-06-16
Current owners on record shown in alphabetical order.
Current Owners on Record
IGNIS INNOVATION INC.
Past owners on record shown in alphabetical order.
Past Owners on Record
ALEXANDER, STEFAN
AROKIA, NATHAN
HUANG, RICK
Past Owners that do not appear in the "Owners on Record" listing will appear in other documentation within the application.

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