PMRME: A parallel multi-resolution motion estimation algorithm and architecture for HDTV sized H.264 video coding

A

Novel

Top

Emission Pixel Circuit Compensating

for

TFT

Threshold

Voltage Variation and Luminance

Degradation

of OLED

Chih-Lung Lin*,

Kuo-Chao Liao

Abstract-This work demonstrates a novel pixel circuit bottom emission [3]. However, the aperture ratio of the pixel composed of top emission architecture with a voltage circuit with top emission architecture can be significantly programming method. Besides compensating for the increased [6].

threshold voltage variation of TFTs, the proposed circuit During recent years, the degradation and lifetime of OLED can improve the luminance degradation of active-matrix are the main concerns. With long term operation, luminance organic light-emitting diode (AMOLED) by using OLED and efficiency of OLED often decay gradually. Thus,

voltage as a feedback of OLED luminance degradation. In numerouscompensation schemesareproposedtopostponethe addition, the reversed-bias voltage applied to OLED can degradation of OLED[4], [5], [7], [8]; theseapproachescanbe suppress the degradation of OLED. The simulation results categorized as optical-feedback compensation [7], and circuit

showthat the proposed circuit can effectively compensate compensation [4], [5], [8]. Although optical-feedback scheme for TFT threshold voltage variation and improve can compensate for OLED luminance degradation, the luminancedegradation of OLED. compensation efficiency is sensitivetoambientlight [4]. Shieh

etal. proposedan acdriving method to prolong the lifetime of

OLEDs by using the reversed-bias voltage [8]. However, AVTH Index Terms-Active-matrix organic light-emitting is not compensated and the display doesn't perform well diode (AMOLED), reversed-bias voltage, top mission, against non-uniformity. Lee etal. proposed a novel pixel circuit

degradation with the stable current independent of AVTH of TFTs and

OLED,but luminance still drops due to OLED degradation [5]. Forthe purpose of holding the same luminance, our previous

I.

INTRODUCTION

research proposed the OLED current can be compensated by a

Organic light-emitting diode(OLED) displays have attractive feedback of OLED voltage [4]. OLED degradation can be

attributes,suchaswide viewangle, highcontrastratio, and fast effectively improved, but overmuch TFTs affect the aperture

response time [1]. However, process variation in electrical ratio and luminance efficiency.

characteristics of low-temperature polycrystalline-silicon The proposedpixel circuit with topemission architecture can

(LTPS) or hydrogenated amorphous silicon (a-Si:H), such as

significantly

improve the aperture ratio. Meanwhile, the

threshold voltage (VTH), causes differences in the OLED reversed bias voltage can suppress the degradation ofOLED

currentamongpixelsand the display is non-uniformity. Thus, and the luminance can be improved by the feedback various compensation circuits have been published and compensation of OLED voltage [8]. Therefore,thedisplay has

generally classified bythecurrent

[1]

andvoltage programming better uniform luminance and a high degree of immunity to

methods [1]-[5]. Althoughthe current method can compensate

AVTH

for AMOLEDdisplay. The simulation results show that for the thresholdvoltagevariation(AVTH),the constant current the proposed circuit can compensate for

AVTH

and improvethe

source is difficult to design for controlling the small current luminance by OLED voltage compensation. level and the charging time is prolonged for low data current

[2].The voltage programming method can effectively II. PROPOSED PIXEL CIRCUIT OPERATION

compensate for TFTthreshold voltage variation. Besides, the Fig. I shows the equivalent circuit of the proposed circuit and aperture ratio is a critical issue for AMOLED applications. driving signals. The proposed pixel is composed of a driving FewerTFTscanimprove aperture ratio in a pixel circuit with TFT

(TI),

four switching TFTs (T2-T5), two storage capacitors (C1-C2) and an OLED, three scan lines(VSCAN1-VSCAN3), a data line (VDATA) and PVDD is a power supply line. VREF is a The authors are with the Department of Electrical Engineering, constant voltage source line and set to 3V. The scan lines

National Cheng Kung University, Tainan, 701-01, Taiwan, Republic (VSCAN1_VSCAN3) range is between -20-20V. PVDD, Vp and of China (e-mail: c1iami~cueut). VSS are set to -2 3V, -6V and -9V, respectively. A\VDATA range is about between-6.5--4V. C1 and C2are 0.3pf. VOLED

is the applied voltage when thecurrent passesthroughOLED.

PVDD

TheOLED current canbe analyzed by thegate-sourcevoltage ofTI.T2is used for the diode connection of TI. VSCAN1-VSCAN3

are used to control TFTs (T2-T5) individually. VREF is a

VSCAN2 VSCAN3

reversed-biasvoltage usedtoextend the lifetime of OLED. The

frame ofa

pixel

is as showed in

Fig.

1(b),

and the

operation

REF

scheme and compensation principle of the proposed pixel

V

T3

T4

circuitaredescribedasfollows.

1) ResetPeriod: During this period, T2, T3,T4andT5 are

TlICTl

allonand TI is in diode connection. Thevoltages of node l

NI,N2,and N4 are

allVREF,

thevoltageof node N3 is Vp

-andPVDDis low. At thistime,VNIislarger thanPVDD

and the reversed-biasvoltage canaccelerate therecoveryof _

OLEDdegradation. Thevoltagesofnodes, VNI,VN2, VN3 T

and VN4atthisperiodare

VSCA

_

T5

VNI =VN2=VN4 VREF

VN3 = (i)

VPATA

2) Programming Threshold Voltage Period: During the (a)

second

period,

T4 and OLEDareoff. BecauseofTI in ) ) (3 f)()

diode

connection,

Ci starts to discharge throughTi until (1) ( (3 ( (

thegatevoltageofTI (VN4)iS VSS+VTH TFTI (thethreshold

voltage

of

TI).

The

voltages

of

nodes,

VN1,

VN2, VN3 and _..._... 3

VN4atthisperiodare

PVDD

V

4 o 4#i 4'.4.4. 20V

NI N2 N4 TH_TFT1

VN3

VP

(2) VCAI.2

... .

X..-.*20V

3)

Data

Input

Period: T2is off. At the

beginning,

because .-j20V

VN3 is

changed

from

Vp

to

VP+AVDATA,

VN4is

changed

_... ...

from

VSS+VTH_TFTI

to

VSS+VTH_TFTi+AVDATA.

Meanwhile,

V.At

VP

PVDDisturned

high

and thecurrent

begins

topass

through

A

OLEDand TI. Therefore, VNI and VN2areboth

PVDD-VOLED. Thevoltageof each nodeatthisperiodis (b)

VNI

=VN2 =PVDD-

VOLED

Fig.

i.

(a) Equivalent circuit of the proposed pixel circuit. (b)

VN3=VP+

AVDATA

Driving signals.

(i)

Reset

Period,

(2)

Programming

Threshold

VN4

=VSS+

VTH

TFTI+

AVDATA

(3)

VoltagePeriod,(3) DataInputPeriod(4)HoldingVOLED Period,

and

(5)

Emission Period.

4) HoldingVOLEDPeriod: Thevoltagesof CI and C2are

Therefore,

the OLED current

(ILED)

can be determined

by

N4-N3 and

N3-N2,

respectively,

thegate-sourcevoltage of

TI.

IOLEDiS

VCl

_{V ~~V}

VN4 -VN3 VSS

_{-v ~v +AV}+

VTH

TFTI_-PVDD+VoLED

VP

_~~OLEDI

_2kT

k1(VGS V

2

VC2

VN3 VN2 VP

+

AVDATA

-PVDD+

VOLED

(4) )

=-kl

(VN4

-VSS

VTH

TFTI

where

Vc,

andVc2arethe differentvoltagesstored inCI 2

andC2,individually. -

k,

(VSS +

VTH

T_T

+AVDATA

5) Emission Period: During emission period, T4 is turned - PVDD + VOLED + VREF VSS

2VTH_TFT1

on and VN2 is changed from PVDD-VOLED toVREF. i

Therefore, VN4 iS boosted from VSS±VTH_TFT1±AVDATA to =-k(AVDATA -PVDD +VO +VRF(5

VSS±VTH_TFT1±AVDATA± VJHJFPVDD±VOLED

due to the 2OE E 5

charge conservation of

Cil

and C2.

where k1 is COXW1/L1. 386

Besides, PVDD and VREF are both 3V. Therefore, the 1100

equation (5) can be simplified as follows. 1000 = PfoposedPixelCiraLtwithV aHOV

---+e ProposedPixel CircLtwith LViia +O03V 1;

900 ConventionalPixelCirctcwithAV -3V / ConventionalPixelCircutwithAV H=OV

IOLLD

=-kl(VOLLD + 0\ VI)ATA ) (6) 800 + Co nveConventionalPixeCircitwithAxVH=+0.3V

~~~~~~~2

X 00J

IOLED

2_+AVDATAT (6) 700

600 0

From equation (6), IOLED is independent of AVTH and only J affectedbyVOLED+AVDATA.Consequently,AVTHdoesn't affect O 400

the OLED current. Moreover, VOLED canprovide another part 300

-of the OLED current andimprove the display luminance due to 200 .

OLED degradation. In the first period of one frame, Reset 100_

Period, the reversed-bias voltagecanaccelerate the recovery of 0 I

OLEDdegradationand extend the OLED lifetime. -6.5 -6 _Inp1ut-5.5_DataVoltage-5 _(V) -4.5 -4

Fig. 3. Simulation of OLED current with threshold voltage

III. SIMULATION

RESULTS variation betweeninputdata range. Toconfirm thefunctionality of theproposed pixel circuit,the - X/5hA

simulation results are achieved by AIM-SPICE. Simulation

l4

nA

model parameters were extracted from the measurements of 9rn fabricated OLED

panels.

The

VTH

of

n-type

TFTis 1.88V and

the width andlength of TI are 12pm and 25ptm,respectively. -The width andlength of T2, T3, T4 and T5 are all 5,um. The

600X

thresholdvoltagevariation ofTFTisset to±0.3V.

Fig. 2 shows the gate voltage of TI with AVTH during one frame operation. The simulation result represents that the

voltage stored in Cl includes AVTH information and the

t0

uniformity of OLEDcurrent canbe achieved. 20

Fig.

3 shows the simulation of OLED current with AVTH I 46

. . ~~~~~~~~~~~~~~~00.t 0.2 Us 0A 0.5

between

input

data range.

Compared

with that of the V Si

(s

conventional 2T1Cpixelcircuit,OLEDcurrentoftheproposed Fig. 4. OLED current as a function of shift in OLED voltage.

pixelcircuit areobviouslyindependentof AVTH.Therefore, the

result illustrates that the proposed pixel circuit successfully iv. CONCLUSION

compensate for AVTH and has superior immunity against Inthis

study,

anoveltop emission architecture

pixel

circuit

non-uniformity.

with five TFTs and two capacitors is presented. Inaddition to

Fig.4shows the result of OLED current with VOLED shift. As

compensating

for the threshold

voltage variation,

the

proposed

VOLEDincreases,OLED currentcanbecompensatedtoimprove pixel circuit can increase the aperture ratio and suppress the the

degradation

of OLED.

degradation

of OLED by

reversed-bias

voltage

applications.

5.0 OLED luminance canbe

partially

compensated by

afeedback

voltage of VOLED in OLED current. Therefore, this proposed

| . . .. [

vl~1SS3v

1 | pixelcircuit can improve uniformity and the lifetime

of

OLED

-0.0 Gate\ I[tageofT1 |TH=LSSV displays.

// \~~~~~~%TH=1lS-0O3V

-X.0 v ACKONWLEDGMENT

The authors would liketothank AU

Optronics

Corporation,

Hsinchu, Taiwan, R.O.C., for its technical support and the

-o.ol,

X | National Science Council oftheRepublicofChina,

Taiwan,

for

______________________________________ _ , lfinancially supporting this research under Contract No. NSC

(2)1 (3)Eli} (5) 96-2221-E-006-131-MY3.

0O*u $5OOu 100Ou0 15OOiu 200Ou0

Tiiiw (seL REFERENCES

Fig. 2. The simulation of the gate voltage of the driving TFT

(Ti) with threshold voltage variation. (1) Reset Period, (2) [1] R. Dawson, Z. Shen, D. A. Furest, S. Connor, J. Hsu, M. G. Programming ThresholdVoltage Period, (3) Data Input Period Kane, R.G. Stewart, A. Ipri, C. N. King, P. J. Green, R. T. (4) Holding VOLED Period, and (5) Emission Period. Flegal, S. Pearson, W. A. Tang, S. Van Slyke, F. Chen, J.

Shi,M. H. Lu,andJ.C. Sturm,"Theimpact of the transient

response oforganic light emitting diodes onthe design of active matrix OLED displays," in IEEE IEDMMeeting,

1998, pp. 875-878.

[2] S. H. Jung, W. J. Nam, and M. K. Han, "A new

voltage-modulated AMOLED pixel design compensating for threshold voltage variation in poly-Si TFTs," IEEE

Electron Device Lett., vol. 25, no. 10, pp. 690-692, Oct.

2004.

[3] C. L.LinandT. T.Tsai, "Anovelvoltage drivingmethod using 3-TFT pixel circuit for AMOLED," IEEEElectron

Device Lett., vol. 28,no.6,pp.489-491,Jun.2007. [4] C.L.LinandY. C.Chen,"Anovel LTPS-TFTpixelcircuit

compensating for TFT threshold-voltage shift and OLED

degradation for AMOLED," IEEE Electron Device Lett., vol. 28,no.2,pp. 129-131,Feb. 2007.

[5] J. H. Lee, J.H. Kim, andM. K. Han, "A new a-Si:HTFT

pixel circuit compensating the threshold voltage shift of a-Si:H TFT and OLED for active matrix OLED," IEEE

Electron Device Lett., vol. 26, no. 12, pp. 897-899, Dec.

2005.

[6] M.-H. Lu, M. S. Weaver, T. X. Zhou, M. Rothman, R. C.

Kwong, M. Hack, and J. J. Brown, "High-efficiency

top-emitting organic light-emitting devices," Appl. Phys. Lett.,vol. 81,pp.3921-3923, 2002.

[7] D. Fish, N. Young, S. Deane, A. Steer, D. George, A.

Giraldo, H. Lifka, 0. Gielkens, and W. Oepts, "Optical

feedback for AMOLEDdisplay compensation using LTPS and a-Si:Htechnologies,"inProc. SIDTech. Dig., 2005,pp.

1340-1343.

[8] Y. C. LinandH. P. D. Shieh, "Improvement ofbrightness

uniformity byAC drivingscheme for AMOLED display,"

IEEEElectron DeviceLett., vol. 25, no. 11, pp. 728-730,

Nov.2004.