P-40 / Y.-S. Son
`
`P-40: A Novel Data Driving Method and Circuits for AMOLED Displays
`
`Young-Suk Son, Sang-Kyung Kim, Yong-Joon Jeon, Young-Jin Woo, Jin-Yong Jeon,
`Geon-Ho Lee, and Gyu-Hyeong Cho
`Div. of Electrical Engineering, Dept. of EECS, KAIST, Yuseong-gu, Daejeon, Korea
`
`
`
`transient charging current generated from an adjacent data line are
`described.
`2.
`Operation Principle
`Long data line charging/discharging time prevents the application
`of current-mode driving to display panels. Figure 2 shows the
`proposed data driver architecture. In order to enhance the data
`driving speed, the transient cancellation feedback (TCF) output
`driver has been composed. The TCF output driver adaptively
`provides the required transient charging current for a data line
`parasitic capacitance and transfers data current to the pixel circuit
`simultaneously. The adaptive generation of the transient charging
`current for a data line capacitance shortens the data driving time
`because the transient current adaptively charges the data line
`capacitance instead of the data current.
`GRAM
`
`T-Con.
`
`n
`
`m
`
`C-DAC
`
`TCFD
`
`Shift Register
`
`Sampling Latch
`
`Holding Latch
`
`nnn
`
`R BG
`
`Reference
`Generator
`
`n
`
`m
`
`C-DAC
`
`TCFD
`
`Abstract
`A novel current-mode driving method and driving circuits are
`proposed for AMOLED displays. The data driving speed in
`conventional current-mode driving methods
`is dramatically
`enhanced by the proposed method and circuits. The proposed
`method shortens data driving time with feedback of the transient
`charging current from the parasitic capacitance of an adjacent
`data line. The current consumption and chip size of the data
`driver IC are reduced by a half, because one data driving circuit
`can drive two data lines of a display panel. 10 nA data current
`can be transferred to the pixel circuit of a display panel with sub-
`nA accuracy in 15 µsec. The data line parasitic resistance and
`capacitance are 4 kohm and 20 pF, respectively.
`1.
`Introduction
`low power
`response,
`fast
`Offering wide viewing angle,
`consumption, and thinness, AMOLED displays hold promise as
`next generation flat panel displays. Compared to LCDs, PDPs,
`and CRTs, however, the quality of AMOLED displays from a
`display uniformity point of view is not satisfactory.
`Various driving schemes and pixel circuits have been introduced
`in efforts to improve the display uniformity of AMOLED displays
`due to variations of pixel-to-pixel characteristics [1-9]. These
`driving schemes, including the proposed driving method, can be
`technologically categorized as shown in Figure 1.
`Driving Schemes
`
`Hybrid
`
`Analog
`
`Digital
`
`Feedback
`
`TCF
`
`PWM
`
`Current
`
`Voltage
`
`TRG
`
`ARG
`
`Current
`
`Voltage
`
`
`
`Figure 1. Tree of data driving schemes
`Voltage-mode driving methods and pixel circuits cannot
`compensate mobility variation effects on display uniformity,
`although they provide fast driving speed [5].
`In terms of display uniformity, the driving method and the pixel
`circuit that offer the best performance are current-mode driving
`and current-copy pixel, respectively [5]. However, high speed
`data driving is beyond attainment with the conventional current-
`mode data driver for large pre-charge error voltages, because data
`currents must drive data lines and pixels in the end of the scan
`time [6].
`In this paper, a novel data driving method and circuits that
`remarkably reduce data driving time by utilizing feedback of the
`
`AMOLED Display Panel
`
`SCAN
`Driver
`
`Figure 2. Current-mode data driver architecture with the output
`drivers
`Figure 3 shows the conceptual operation of the TCF output driver.
`For fast transfer of a data current (IDATA) to a pixel current (Ipixel),
`the parasitic capacitance of a data line (CPP) should be charged
`swiftly with a large current. The variable current source (IBT),
`which is controlled by sensing the voltage variation at node A
`(VA), adaptively supplies the required transient charging current
`for the data line. VA senses the change of IDATA and generates the
`control voltage (VB) according to this change. IBT has two current
`components, one is static (IB) and the other is transient. This
`transient component is the charging current of CPP. The data
`driving speed and accuracy depend on the accuracy and
`promptness of the IBT.
`The implemented circuits of the TCF concept given in Figure 3
`are shown Figure 4. The capacitance and the initial voltage of the
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`ISSN0006-0966X/06/3701-0343-$1.00+.00 © 2006 SID
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`from the current flowing in this adjacent data line, as shown in
`Figures 4 and 5.
`
`n-bit wide
`digital data
`n
`
`Current DAC
`
`IDATA
`
`VDD
`
`VREF
`
`Path Exchanger
`
`EQEN
`
`IB
`
`IB
`
`ESCAN
`OSCAN
`PCEN
`VPC
`
`SCAN
`Driver
`
`ESCAN(1)
`OSCAN(1)
`
`ESCAN(2)
`OSCAN(2)
`
`ESCAN(M)
`
`OSCAN(M)
`
`DL(1)
`
`DL(2)
`
`Pixel
`
`Pixel
`
`Pixel
`
`Pixel
`
`Pixel
`
`Pixel
`
`
`
`P-40 / Y.-S. Son
`
`data line should be known first so as to generate the required
`transient charging current.
`VDD
`
`n-bit wide
`digital data
`n
`
`Current DAC
`
`IBT(t)
`
`IDATA
`VA
`
`C
`
`A
`
`M
`
`Control
`Circuit
`
`VREF
`
`VDD
`
`VB
`
`B
`
`DL
`
`SCAN
`
`RPP
`
`Ipixel
`
`D
`
`IB
`
`CPP
`
`CST
`
`DTFT
`
`
`
`Data line parasitic
`capacitance and
`resistance
`Figure 3. Conceptual operation of the transient cancellation
`feedback (TCF) driving method
`The data line capacitance can be extracted from an adjacent data
`line, because the capacitance of a data line is almost the same as
`that of an adjacent data line. The adjacent data line provides the
`data line capacitance by disabling the SCAN signal, as shown in
`Figure 4.
`
`n-bit wide
`digital data
`n
`
`Current DAC
`
`Source-type
`
`VDD
`
`M4
`
`M3
`
`IDATA
`
`A
`
`②②②②
`
`VREF
`
`①①①①
`
`M1
`
`M2
`
`RPP
`
`CPP
`
`RPP
`
`VDD
`
`VDD
`
`ESCAN
`
`OSCAN
`
`Figure 5. Application of the data driver to a display panel
`The required quantity of charge for a data line is given in equation
`(1) and the quantity of generated charge from an adjacent data line
`is given in equation (2). The error charge, i.e., the difference
`between these two charge quantities, is given in equation (3). CPP
`is the equivalent data line capacitance, CST is the storage
`capacitance of a pixel, and ∆VDL is the voltage swing of a data
`line. The parasitic capacitance of a data line can be charged
`quickly as the error charge converges to zero by the operation of
`two feedback-loops. The loop ① monitors states of transferring
`IDATA to Ipixel and generates a control voltage for the loop ②.
`The loop ② generates the transient charging current according to
`the control voltage. When the error charge converges to zero, the
`variation of voltage at node A no longer exists and IDATA is
`transferred to Ipixel at the same time.
`(
`)
`∆
`=
`+
`∆⋅
`Q
`C
`C
`V
`
`DL
`DL
`ST
`PP
`
`(∆⋅
`=
`∆
`+
`V
`V
`C
`Q
`GS
`DDL
`PP
`DL
`(
`∆
`−
`⋅
`=
`V
`V
`C
`Q
`GS
`GS
`error
`
`
`V
`GS
`C
`
`ST
`
`−
`+
`
`1
`
`)
`
`1
`
`PP
`
`2
`
`IB
`
`CPP
`
`IB
`
`CST
`
`DTFT
`
`CST
`
`DTFT
`
`Ipixel
`
`Disabled pixel of an
`adjacent data line
`(a) Source-type TCF circuit
`
`VDD
`DTFT
`
`CST
`
`VDD
`DTFT
`
`CST
`
`Ipixel
`
`ESCAN
`
`OSCAN
`
`RPP
`
`CPP
`
`RPP
`
`CPP
`
`VDD
`
`IB
`
`IB
`
`n-bit wide
`digital data
`n
`
`Current DAC
`
`Sink-type
`
`M1
`
`①①①①
`
`M2
`
`VREF
`
`IDATA
`
`②②②②
`
`A2
`
`M4
`
`M3
`
`
`
`
`
`(b) Sink-type TCF circuit
`Figure 4. Implemented circuits for transient cancellation feedback
`driving
`Equalization and pre-charge function define the initial voltage of
`the data line and its adjacent data line, as shown in Figure 5.
`Hence, the required transient charging current can be predicted
`
`
`
`Panel Applications and Operation
`3.
`Timing Diagram
`Figures 5 and 6 show the application of the proposed driving
`circuits and the operation timing diagram, respectively.
`Trow
`
`Trow/2
`
`Trow/2
`
`IDATA_EN
`
`T EQEN
`
`EQEN
`
`PCEN
`
`ESCAN
`or OSCAN
`
`SCAN_STOP
`
`TPCEN
`
`TOSCAN
`
`TESCAN
`
`TSS
`Figure 6. Operation timing diagram
`
`
`
`
`
`
`
`)2
`
`∆⋅
`V
`DL
`
`(1)
`
`(2)
`
`(3)
`
`
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`P-40 / Y.-S. Son
`
`The possibilities of 10 nA and several-nA data driving are verified
`from the results with 10 µsec and 15 µsec driving times,
`respectively. Figure 9 shows that the data driving time fluctuation
`is not significant even with ±300 mV threshold voltage (VT)
`variations of DTFT in a pixel circuit.
`
`offset=0.3V
`offset=0.2V
`offset=0.1V
`offset=0.0V
`offset=-0.1V
`offset=-0.2V
`offset=-0.3V
`
`10.0
`
`20.0
`Time (μμμμsec)
`(a) IDATA = 100 nA
`
`
`30.0
`
`
`
`offset=0.3V
`offset=0.2V
`offset=0.1V
`offset=0.0V
`offset=-0.1V
`offset=-0.2V
`offset=-0.3V
`
`10.0
`
`20.0
`
`30.0
`
`120
`
`100
`
`80
`
`60
`
`40
`
`20
`
`Pixel Current (nA)
`
`0
`0.0
`
`1.5
`
`1
`
`0.5
`
`Pixel Current (μμμμA)
`
`0
`0.0
`
`Time (μμμμsec)
`(b) IDATA = 1 µA
`Figure 9. Data driving performance with ±300 mV VT variation
`(SCAN enabled at 10 µsec)
`The performance of the proposed driving method and circuits are
`summarized in Table 1.
`
`
`
`The path exchanger in Figure 5 enables one data driving circuit to
`drive two data lines by selecting the connection between a driving
`circuit and two data lines of a panel from ESCAN and OSCAN
`signals. The function of the path exchanger is advantageous in
`terms of chip size and current consumption. One complete scan
`time (Trow) is composed of ESCAN (TESCAN), OSCAN (TOSCAN),
`two equalizations (TEQEN), and two scan stops (TSS), as shown in
`Figure 6. EQEN makes the average data line voltage of a data line
`and its adjacent data line while PCEN adjusts this average voltage
`if necessary. The functioning of EQEN and PCEN significantly
`shortens the data driving time.
`4.
`Simulation Results
`A distributed R-C model of 4 kohm resistance and 20 pF
`capacitance is applied as a data line load in order to evaluate the
`driving performance of the proposed driving method and circuit.
`The settling characteristics of the pixel currents in response to
`data currents from 2 nA to 10 µA are shown in Figure 7.
`
`IDATA=20 nA
`IDATA=40 nA
`IDATA=60 nA
`IDATA=80 nA
`IDATA=100 nA
`
`100
`
`80
`
`60
`
`40
`
`20
`
`0
`
`Pixel Current (nA)
`
`IDATA=2 nA
`IDATA=4 nA
`IDATA=6 nA
`IDATA=8 nA
`IDATA=10 nA
`
`10
`
`20
`
`30
`
`0
`
`
`Time (μμμμsec)
`Time (μμμμsec)
`(a) 2 nA-step driving (b) 20 nA-step driving
`
`10
`
`20
`
`30
`
`10
`
`02468
`
`Pixel Current (nA)
`
`0
`
`IDATA=2 µA
`IDATA=4 µA
`IDATA=6 µA
`IDATA=8 µA
`IDATA=10 µA
`
`12
`
`10
`
`02468
`
`Pixel Current (µA)
`
`IDATA=200 nA
`IDATA=400 nA
`IDATA=600 nA
`IDATA=800 nA
`IDATA=1000 nA
`
`10
`
`20
`
`30
`
`0
`
`10
`
`20
`
`30
`
`1200
`
`1000
`
`800
`
`600
`
`400
`
`200
`
`Pixel Current (nA)
`
`0
`
`0
`
`Time (μμμμsec)
`Time (μμμμsec)
`(c) 200 nA-step driving (d) 2 µA-step driving
`Figure 7. Step responses (SCAN enabled at 10 µsec)
`The feasibility of nA-order data driving is verified from the
`simulation results. Figure 8 shows the error currents according to
`the data currents with the given data driving times.
`
`
`
`Time=10 µsec
`Time=15 µsec
`Time=20 µsec
`
`500
`
`1000
`Data Current (nA)
`
`1500
`
`2000
`
`
`
`10
`
`5
`
`0
`
`-5
`
`Error Current (nA)
`
`-10
`
`0
`
`Figure 8. Current driving speed and accuracy according to data
`currents
`
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`organic light emitting diodes on the design of active matrix
`OLED displays,” IEEE IEDM, 1998, pp. 875-878
`[2] S. W-B. Tam, Y. Matsueda, M. Kimura, H. Maeda, T.
`Shimoda, and P. Migliorato, “Poly-Si driving circuits for
`organic EL displays,” Electronic Imaging 2001, paper 4925-
`20
`[3] A. Yumoto, M. Asano, H. Hasegawa, and M. Sekiya, “Pixel-
`driving methods for large-sized poly-Si AM-OLED
`displays,” IDW, 2001, pp. 1395-1398
`[4] K. Inukai, H. Kimura, M. Mizukami, J. Maruyama, S.
`Murakami, J. Koyama, T. Konuma, and S. Yamazaki, “4.0-in
`TFT-OLED displays and a novel digital driving method,”
`Proc. of SID Sym., 2000, pp. 924-927
`[5] M. Ohta, H. Tsutsu, H. Takahara, I. Kobayashi, T. Uemura,
`and Y. Takubo, “A novel current programmed pixel circuit
`for active matrix OLED displays,” Proc. of SID Sym., 2003,
`pp. 108-111
`[6] M. Shimoda, K. Abe, H. Haga, H. Asada, H. Hayama, K.
`Iguchi, D. Iga, H. Imura, and S. Miyano, “An integrated
`poly-Si TFT current data driver with a data-line pre-charge
`function,” Journal of the SID, 2003, pp. 461-466
`[7] H. Kageyama, H. Akimoto, Y. Shimizu, T. Ouchi, N. Kasai,
`H. Awakura, N. Tokuda, K. Kajiyama, and T. Sato, “A 2.5-
`inch OLED display with a three-TFT pixel circuit for
`clamped inverter driving,” Proc. of SID Sym., 2004, pp.
`1394-1397
`[8] H. In, I. Jeong, J. Kang, and O. Kwon, and H. Chung, “A
`novel feedback-type AMOLEDs driving method for large-
`size panel applications,” Proc. of SID Sym., 2005, pp. 252-
`255
`[9] S. Jafarabadiashtiani, G. Chaji, S. Sambandan, D.
`Striakhilev, A. Nathan, and P. Servati, “A new driving
`method for a-Si AMOLED displays based on voltage
`feedback,” Proc. of SID Sym., 2005, pp. 316-319.
`
`P-40 / Y.-S. Son
`
`Table 1. Achieved specifications
`Load conditions
`DL resistance :
`
`DL capacitance :
`
`2.2 kohm for QGVA
`3.7 kohm for VGA
`6.6 pF for QVGA
`13.5 pF for VGA
`
`Simulation load condition
`DL resistance :
`DL capacitance :
`
`4.0 kohm
`20 pF
`
`Driving speed and accuracy
`10 μμμμsec for ± 3 nA error current
`15 μμμμsec for ± 0.1nA error current
`Current consumption
`8 μμμμA/channel effectively
`
`
`
`Conclusions
`5.
`The retard of data driving speed in conventional current-mode
`driving methods due to the parasitic capacitance of a data line is
`overcome by TCF driving. The operation of this circuit is verified
`with a Cadence SPECTRE simulation. The proposed method
`maintains data driving speed and accuracy even with variation of
`TFTs and massive parasitic capacitance. One data driving circuit
`drives two data lines through the use of a path exchanger. The
`proposed approach can be applied for driving poly-silicon panels
`as well as a-Si panels. It is shown that the TCF driving method is
`a viable solution to the speed limitation of conventional current-
`mode driving methods.
`
`References
`7.
`[1] R. Dawson, Z. Shen, D.A. Furst, S. Connor, J. Hsu, M.G.
`Kane, R.G. Stewart, A. Ipri, C.N. King, P.J. Green, R.T.
`Flegal, S. Pearson, W.A. Barrow, E. Dickey, K. Ping, S.
`Robinson, C.W. Tang, S. Van Slyke, F. Chen, J. Shi, M.H.
`Lu, and J.C. Sturm, “The impact of the transient response of
`
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`346 • SID 06 DIGEST
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`SAMSUNG, EXH. 1027, P. 4
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