Proceedings of 6th Thin Film Materials & Devices Meeting
November 2-3, 2009, Kyoto, Japan
http://www.tfmd.jp/Pulse-Width Modulation with Current Uniformization
using Reduced Pixel Circuit for TFT-OLEDs
Daisuke Suzuki1, Masamichi Koike1, Shigeki Sawamura2,
Masakazu Kato3 and Mutsumi Kimura1,4
1

Ryukoku University, Seta, Otsu, 520-2194, Japan, mutsu@rins.ryukoku.ac.jp
Nara Institute of Science and Technology, Takayama, Ikoma 630-0192, Japan
3
Osaka University, Yamadaoka, Suita 565-0871, Japan
4
Innovative Materials and Processing Research Center, Seta, Otsu 520-2194, Japan2

Keywords: Pulse-Width Modulation, Current Uniformization, Pixel Circuit, TFT-OLEDAbstract. We have proposed a novel driving concept for TFT-OLEDs, pulse-width modulation
with current uniformization. This driving method can simultaneously achieve precise grayscaleand exceedingly improve luminance uniformity. Lately, we succeeded to reduce the number ofTFTs in a pixel circuit from nine to seven. Especially in this paper, its working and tolerance
against characteristic deviations of TFTs and OLEDs are confirmed using circuit simulation.
Introduction
TFT-OLEDs have been highlighted for a next generation of flat-panel displays [1-2]. One of
the most critical issues for TFT-OLEDs is luminance uniformity between neighboring pixels. Inorder to improve the luminance uniformity, many kinds of driving method were proposed[3-11].
Formerly, we and another group presented pulse-width modulations with self-biased inverter[6-7]. These driving methods can compensate characteristic deviation of TFTs. However, theycannot compensate characteristic deviation of OLEDs. On the other hand, recently, wepresented a time-ratio grayscale with current uniformization [11]. This driving method cancompensate characteristic deviations of both TFTs and OLEDs. However, it needs high-speed
scanning for high-resolution and many-grayscale displays.
We have proposed a novel driving concept for TFT-OLEDs, pulse-width modulation withcurrent uniformization [12]. This driving method can simultaneously achieve precise grayscaleand exceedingly improve luminance uniformity. Lately, we succeeded to reduce the number ofTFTs in a pixel circuit from nine to seven [13]. Especially in this paper, its working andtolerance against characteristic deviations of TFTs and OLEDs are confirmed using circuit
simulation [14].
Pulse-width modulation with current uniformization
This driving method can simultaneously achieve precise grayscale using the pulse-widthmodulation and exceedingly improve luminance uniformity using the current uniformization.
The pixel circuit consists of the Pwm-part, pulse-width modulation part, and Cu-part, current
uniformization part, as shown in Fig. 1. Here, the reduced TFTs and buslines that exist in the100228075-1

conventional pixel circuit of the pulse-width modulation with current uniformization are shownusing the dotted lines.
The working of the Pwm-part is explained as follows. First, during an addressing period,data-addressing operation is executed. Since Vin and Vout are connected, the voltage difference
between Vdata and Vsb, Vth of the inverter, can be stored in the Csig. Next, during alight-emitting period, pulse-width-modulation operation is executed. Since a sweep wave isapplied and coupled to Vin through the Csig, and the voltage difference between Vdata and Vsb
is already stored in the Csig, the Pwm-TFT can be correctly turned on, and Woled, light-emitting
time, can be exactly controlled.
On the other hand, the working of the Cu-part is explained as follows. First, during a
programming period, current-programming operation is executed. Since the Prg-TFT andSup-TFT are turned on, the proper voltage for Itft=Isup can be stored in the Cst. Since Isup isenough to charge the Cst and whole pixel circuit, Vpx is settled, and Itft is asymptotical to Isup.
Next, during a light-emitting period, current-reproduce operation is executed. Since Vgs of the
Dr-TFT is already stored in the Cst, Itft is unchangedly kept, and Ioled can be roughly equal toVdd

Vemit

DrTFT
Cu-partPwm-part

Vdd

ItftCstVscan

PrgTFT

Vcat

Inverter
EmitTFT

Addressing and programming period

Vscan

Vpx

Data signal

Vsig

Vin

Vsig

Vout

PwmTFT

Csig

Vin

Pixel
InvTFT

Ioled

Vinv

OLED

Isup
DriverSbTFT
Vscan

Vdata-Vsb

Vsb

Vout

Itft

Woled

Ioled

Light emitting

Vcat(a) Pixel circuit
Pwm-partCu-part

Sweep wave

Vdata
Vdata-Vsb

SupTFT

Light-emitting period(b) Timing chart

Pulse-width modulation part
Current uniformization part

TFT to connect input and output of self-biased inverter
Sb-TFT
Inv-TFTTFT to power inverter
Csig
Capacitor to store Vdata and Vsb
Dr-TFT
TFT to drive OLED with constant current
Prg-TFTTFT to program proper voltage in Cst
Sup-TFTTFT to supply constant current from Isup
Pwm-TFT TFT to control light emitting time by pulse-width modulation
Emit-TFT
TFT to define light-emitting period
Cst
Capacitor to store Vgs of Dr-TFT

VscanVsig
VdataVinVoutVsbVinv
VcatVpxVemit
Isup
Itft
Ioled
Woled

Scanning voltage
Signal voltage
Data voltage related to data signal
Input voltage inputted to inverter
Output voltage outputted from inverter
Vth of inverter detected by self-biased inverter
Voltage applied to Inv-TFT
Cathode voltage
Pixel voltage between Dr-TFT and OLED
Voltage applied to Emit-TFT
Current from constant current supply
Current through Dr-TFT
Current through OLEDLight-emitting time

Fig. 1
Pulse-width modulation with current uniformization.

100228075-2

Isup. Since only one operating point is used in the current-programming operation, and it cancoincide with the operating point in the current-reproduce operation, the current variation due tothe shift of the operating points does not occur.
Recently, we succeeded to reduce the number of TFTs in a pixel circuit from nine to seven.
First, the Inv-TFT to power off the inverter can be reduced because it does not matter whether
the inverter is powered on or off when the pixel circuit is not selected during the addressing andprogramming period. Next, the Emit-TFT to define the light-emitting period can be alsoreduced because the same function is obtained when the cathode voltage is varied. As a result,
we also reduce the number of buslines in an array panel from six to four.
Simulation results
We confirm the working of this driving method using circuit simulation as shown in Fig. 2.
Here, typical characteristics and optimized designs are used, the addressing and programming
period is 16 Ps per one scan line, the light-emitting period is 8 ms, and Vdata is 2.5 V byassuming that the pixel pitch is 200 Pm, the maximum luminance is 100 Cdm-2, the scan-line
number is 500, and the halftone grayscale is displayed. An original TFT model is implemented
in the circuit simulation [15]It is found in the simulated voltage and current waveforms shown in Fig. 2(b) that Woled canbe exactly controlled, whose value of 3.94 ms is almost similar to the required value of 4 ms,
and Ioled can be roughly equal to Isup, whose value of 0.998 PA is almost similar to therequired value of 1 PA.
We also confirm the tolerance against 'Vth of TFTs and OLEDs using circuit simulation asshown in Fig. 3. Here, the variations of Woled, Ioled and Loled, light-emitting brightness, whose

Pwm-part

p22S/An220.75n220.75C (pF)
0.1

Csig

TypeCst10pnnn

W
L
LDD(Pm) (Pm) (Pm)
52
S/A
220.751020.75
1020.75

Light-emitting period

104
Vscan

5

Vin=Vout

53

4
Vemit

3Vsig

0Vin02Isup-5Cu-partDr-TFT
Prg-TFT
Sup-TFT
Pwm-TFT

Addressing and programming period

Vsig

-51

Ioled=0.998uA

Isup

1

Woled=3.94ms-10-100

Itft-1508t (us)-15-1
16 00Ioled-1
2

4t (ms)

6C (pF)2(a) Design parameters

2Vout(uA)

Inverter-TFT
Inverter-TFT
Sb-TFT

W
L
LDD(Pm) (Pm) (Pm)(V)

Type(b) Voltage and current waveforms

Fig. 2Working confirmation of the driving method using circuit simulation.

100228075-3

8

value is estimated by multiplying Woled by Ioled, are evaluated.
It is found in Fig. 3(a) that the average value of Woled is almost similar to the required value
and the variation of Woled is sufficiently small against 'Vth of TFTs. Moreover, the averagevalue of Ioled is also almost similar to the required value, and the variations of Ioled is alsosufficiently small except for 'Vth of TFTs is -0.5 V. Consequently, so is Loled. It is also foundin Fig. 3(b) that the average value of Woled is almost similar to the required value and thevariation of Woled is sufficiently small against 'Vth of OLEDs. Moreover, the average value ofIoled is also almost similar to the required value, and the variations of Ioled is also sufficiently
small except for 'Vth of OLEDs is -1 V. Consequently, so is Loled. Since 'Vth of TFTs of-0.5 V and 'Vth of OLEDs of -1 V are considerably large values and those in mass-fabrication
technologies is probably small, it is verified that the pulse-width modulation with current
uniformization using reduced pixel circuit can simultaneously achieve precise grayscale andexceedingly improve luminance uniformity.

1.1

110

1100

4Loled (%)

4.1
Ioled (PA)

Woled (ms)

4.2

0.9

90

3.9
3.8

0.8
-0.5

00.580-0.50'Vth (V)0.5-0.50'Vth (V)Woled

0.5'Vth (V)

Loled

Ioled(a) 'Vth of TFTs
1.1

11011004Loled (%)

4.1
Ioled (PA)

Woled (ms)

4.2

0.9

90

3.9
3.8

0.8
-1-0.50'Vth (V)

Woled

0.5

180-1-0.5

00.5'Vth (V)Ioled

1-1-0.50'Vth (V)

Loled(b) 'Vth of OLEDs
Fig. 3Tolerance confirmation against characteristic deviations of TFTs and OLEDs.

100228075-4

0.51Conclusion
We have proposed a novel driving concept for TFT-OLEDs, pulse-width modulation withcurrent uniformization. This driving method can simultaneously achieve precise grayscale andexceedingly improve luminance uniformity. Lately, we succeeded to reduce the number ofTFTs in a pixel circuit from nine to seven. Especially in this paper, its working and tolerance
against characteristic deviations of TFTs and OLEDs were confirmed using circuit simulation.
We will confirm its working and tolerance against characteristic deviations of TFTs andOLEDs using actual pixel equivalent circuits in the near future.
Acknowledgments
The authors wish to thank Drs. Hiroyuki Hara, Satoshi Inoue, Hitoshi Fukushima, Tomoyuki
Kamakura, Simon W. B. Tam, some members of Seiko Epson, Prof. Tatsuya Shimoda of Japan
Advanced Institute of Science and Technology, Silvaco International, Silvaco Japan, Cadence
Design Systems, Cadence Design Systems Japan, Dr. Yoshiki Katsuyama of Cybernet Systems,and Dr. Tsuneo Munakata of Jedat.
This research is partially supported by a collaborative research with Seiko Epson Corporation,a research project of the Joint Research Center for Science and Technology of Ryukoku
University, a grant for research facility equipment for private universities from the Ministry ofEducation, Culture, Sports, Science and Technology, and a grant for special research facilities
from the Faculty of Science and Technology of Ryukoku University.
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