An integrated poly-Si TFT current data driver with a data-line pre-charge function
`
`M. Shimoda
`K. Abe
`H. Haga
`H. Asada
`H. Hayama
`K. Iguchi
`D. Iga
`H. Imura
`S. Miyano
`
`Abstract — We have developed an integrated poly-Si TFT current data driver with a data-line pre-
`charge function for active-matrix organic light-emitting diode (AMOLED) displays. The current data
`driver is capable of outputting highly accurate (±0.8%) current determined by 6-bit digital input data.
`A novel current-programming approach employing a data-line pre-charge function helps achieve
`accurate current programming at low brightness. A 1.9-in. 120 × 136-pixel AMOLED display using
`these circuits was demonstrated.
`
`Keywords—OLED,currentdatadriver,currentprogram,pre-charge.
`
`Introduction
`1
`Poly-Si thin-film-transistor (TFT) OLED displays hold
`great promise for use in mobile phones, PDAs, PCs, and
`TVs, offering wide viewing angles, fast response, low power
`consumption, and panel thinness.
`To obtain uniform images on poly-Si TFT AMOLED
`displays, it is necessary to ensure uniformity in the bright-
`ness of each pixel. This brightness is dependent on the cur-
`rent in the OLED that is driven by each pixel circuit, and an
`important issue for poly-Si TFT OLED displays has been
`the pixel-driving technology needed to achieve uniformity
`in OLED currents. The main difficulty is non-uniformity in
`the current–voltage characteristics of the poly-Si TFTs that
`form part of the pixel circuit. A number of pixel-driving
`schemes have been proposed to overcome this disadvan-
`tage.1–7 Among them, those that employ a current-program-
`ming approach,4,5 which works to compensate for variations
`in both Vt and mobility in poly-Si TFTs, are effective in
`achieving good uniformity. Unfortunately, however, this
`approach suffers from a serious drawback. Specifically, the
`lowness of its current for a dark gray scale means that (1)
`programming time will be relatively long and (2) a highly
`accurate current data driver will be required. While such
`driving accuracy might be externally supplied, as with, for
`example, drivers composed of crystal-Si MOSFETs that
`offer high uniformity in current–voltage characteristics, the
`use of such drivers would increase programming time fur-
`ther because of the increase in high parasitic capacitance
`induced by the connection between the data drivers and the
`display panel. What is needed, then, is to integrate high-
`accuracy current data drivers into the AMOLED display
`substrate itself.
`Further, there is also high parasitic capacitance in the
`data-line capacitors, and the need at low current levels to
`charge these data-line capacitors would also increase pro-
`
`gramming time. To deal with this we need a data-line pre-
`charge function.
`In this paper, we propose for AMOLED displays an
`integrated poly-Si TFT 6-bit current data driver and a novel
`current-programming approach that employs a data-line
`pre-charge function. The proposed design reduces current-
`programming times at low brightness and achieves accurate
`current programming. We also demonstrate a 1.9-in. poly-Si
`TFT OLED display which employs these circuits.
`
`Current data driver
`2
`2.1 Driver configuration
`Figure 1 is a block diagram of the poly-Si TFT 6-bit current
`data driver, which contains 60-bit shift-register 1, 6-bit data
`registers, 6-bit data latches, 60-bit shift register 2, 60 6-bit
`digital-to-current converters (DCCs), a standard current
`source circuit, pre-charge circuits, and 1-to-2 selectors.
`These last five components form the design’s “current signal
`block.”
`Each 6-bit data register receives 6-bit digital gray-
`scale data when scanning signals are output from 60-bit shift
`register 1, and it outputs this data to each 6-bit data latch.
`The 6-bit data latches send 6-bit digital data to the 6-bit
`DCCs. The 6-bit DCCs carry out two different operations.
`One is to memorize values of the six standard currents (Is,
`Is × 2, Is × 4, Is × 8, Is × 16, Is × 32) generated by the inte-
`grated standard current source circuit in the driver. The
`other is to output the gray-scale currents (Is, Is × 2,..., Is ×
`62, or Is × 63) determined by the digital data from the 6-bit
`data latch. Each 6-bit DCC is composed of 6 1-bit DCCs
`connected in parallel. Each 6-bit DCC sends an analog cur-
`rent signal to the pre-charge circuit. A 1-to-2 selector out-
`puts the signals to each of two outputs of the driver, one
`after the other. This is accomplished over a single horizontal
`period. 60-bit shift register 2 outputs scanning signals to
`
`Revised version of a paper presented at the 9th International Display Workshops (IDW ‘02) held December 4–6, 2002, in Hiroshima, Japan.
`The authors are with SOG Research Laboratories, NEC Corp., 1120 Shimokuzawa, Sagamihara, Kanagawa 229-1198 Japan;
`telephone +81-42-771-0689, fax -0780, e-mail: m-shimoda@ct.jp.nec.com.
`K. Abe is with the 5th System LSI Division, 2nd Business Development 0perations Unit, NEC Electronics Corp., Japan
`© Copyright 2003 Society for Information Display 1071-0922/03/1103-0461$1.00
`
`Journal of the SID 11/3, 2003
`
`461
`
`SAMSUNG, EXH. 1004, P. 1
`
`

`

`FIGURE 1 — Block diagram of current data driver, which contains shift
`register 1, 6-bit data registers, 6-bit data latches, shift-register 2, 6-bit
`digital-to-current converters (DCCs), a standard current source circuit,
`pre-charge circuits, and 1-to-2 selectors.
`
`6-bit DCCs. The output timing of the scanning signals is
`synchronous with the 6-bit DCC’s operation of memorizing
`the values of current supplied by the standard current
`source circuit.
`
`1-bit digital-to-current converter
`2.2
`The current-copier-type 1-bit DCCs of which the 6-bit
`DCC is composed are able to output a highly accurate cur-
`
`FIGURE 2 — Basic 1-bit current-copier-type DCC, composed of a
`current-copier circuit and a switching TFT (SW3) controlled by 1-bit
`digital data signals sent from a 6-bit data latch.
`
`462
`
`Shimoda et al./ An integrated poly-Si TFT current data driver
`
`FIGURE 3 — Operation states of
`the current-copier-type 1-bit
`digital-to-current converter: the memorizing state, the outputting state
`for 1-bit data “H,” and the outputting state for 1-bit data “L.”
`
`rent. In this section, in order to clarify what makes this
`accuracy possible, we give a detailed explanation of the 1-bit
`DCC’s design and operations.
`Figure 2 shows the basic circuit of a 1-bit DCC. It is
`composed of a current-copier circuit and a switching TFT
`(SW3) controlled by 1-bit digital data signals sent from a
`6-bit data latch. The current-copier circuit is composed of a
`driving TFT, two memorizing-switches TFT (SW1, SW2)
`controlled by a signal [MS (1-to-60)] sent from 60-bit shift
`register 2, and a hold capacitor.
`Figure 3 shows the three operation states of the 1-bit
`DCC: the memorizing state, the outputting state for 1-bit
`data “H,” and the outputting state for 1-bit data “L.” In the
`memorizing state, SW1 and SW2 are ON, SW3 is OFF, and
`the hold capacitor and the gate capacitor of the driving TFT
`have been charged in order to send a standard current (Is,
`Is × 2, Is × 4, Is × 8, Is × 16, or Is × 32) into the driving TFT
`(in Fig. 3, this current is “Is”). In the outputting states, SW1
`and SW2 are OFF, SW3 is ON or OFF, and the driving TFT
`outputs “Is” or zero so that the hold capacitor and the driv-
`ing TFT’s gate capacitor can retain their charge so as to send
`“Is.” Note that of the four TFTs in this 1-bit DCC, three are
`switches; the fourth is the “driving TFT” used for memoriz-
`ing and outputting.
`
`Experiments
`2.3
`We have experimentally fabricated the 6-bit DCC described
`above. Its measured output characteristics are shown in
`Fig. 4. Here, the output current value of the driver
`increases as the value of the gray scale increases, which
`indicates 6-bit monotony for the current output. Figure 5
`shows deviations among the MSB (63 gray-scale) output
`currents for four outputs in each frame, as measured for five
`
`SAMSUNG, EXH. 1004, P. 2
`
`

`

`3 Novel current-programming approach
`with data-line pre-charge function
`As previously noted, the 6-bit DCCs are able to output cur-
`rent accurately, with 6-bit resolution. The problem of long
`current-programming time remains, however, and in this
`section we explain our novel current-programming approach,
`which employs a data-line pre-charge function in order to
`reduce current-programming times and to achieve accurate
`current-programming at low gray-scale levels, This
`approach has become possible because the current data
`driver has been integrated into the display substrate itself.
`
`Circuit design
`3.1
`Figure 6 is a diagram of both the pixel circuit and the inte-
`grated current data driver containing a pre-charge circuit.
`The pre-charge circuit is composed of one TFT (T1) and a
`voltage-follower amplifier. TFTs (T2 ~ T4) are switching
`TFTs. The current source indicated in the diagram repre-
`sents a 6-bit DCC which generates an analog current on the
`basis of 6-bit digital data that has been input to it, as was
`previously noted in Section 2. The output of the data driver
`is connected to a data line via a selector, which is used to
`select an appropriate data line for connection.
`The pixel circuit contains five TFTs: one driving TFT
`(T5) and four switching TFTs (T6 ~ T9). The gate length
`and width of the driving TFT (T5) are designed to be the
`same as those of the driving TFT in the pre-charge circuit.
`The switching TFTs are controlled by signals that are output
`by an integrated gate driver to three pixel circuit inputs
`[Write scan1 (WS1), Write scan 2A/B (WS2A/B), and Erase
`scan (ES)]. In order to control the emission period, the
`switching TFT (T8) is used to cut off the current to the
`OLED, and the switching TFT (T9) is used to initialize the
`OLED.
`
`FIGURE 4 — Current driver output characteristics.
`
`FIGURE 5 — Deviation among four output currents for four outputs in
`each frame, as measured for five samples.
`
`samples. As may be seen, at MSB, where the output current
`is the highest, the current driver satisfies 6-bit accuracy
`(±0.8%) requirements.
`
`FIGURE 6 — The schematic diagram of both the pixel circuit and the current data driver with the pre-charge circuit.
`
`Journal of the SID 11/3, 2003
`
`463
`
`SAMSUNG, EXH. 1004, P. 3
`
`

`

`FIGURE 7 — Timing chart illustrating current data driver and pixel
`circuit operations. The signals Write scan1 (WS1), Write scan 2A/B
`(WS2A/B), and Erase scan (ES) are output by an integrated gate driver.
`
`Circuit operations
`3.2
`Figure 7 is a timing chart which illustrates how the current
`data driver and pixel circuit work.
`A single horizontal period contains both the pre-
`charge and current-programming periods. In the pre-
`charge period, T2 and T3 are turned on, T4 is turned off,
`and current from the current source flows into T1. A volt-
`age-follower amplifier outputs the gate voltage of T1 into
`the gate electrode of T5, with T6 and T7 turned on. This
`pre-charge function rapidly pre-charges the voltage in the
`gate electrode of T5 so that a current roughly equivalent to
`the value of the current source flows through T5.
`In the subsequent current-programming period, T4 is
`turned on, T2 and T3 are turned off, the pre-charge circuit
`is disconnected, and the current from the current source is
`directly supplied to the pixel circuit. While a pre-charge
`error may be generated if there exist variations in charac-
`teristics between T1 and T5, or if there is an offset voltage
`in the voltage-follower amplifier, this error can be compen-
`sated for during current-programming periods. Setting the
`current from the current source high makes it possible to
`compensate for the greater pre-charge error that occurs
`during current-programming periods. This improves the
`accuracy of current programming significantly.
`After the horizontal period, T6 and T7 are turned off,
`T8 is turned on, and a current programmed by the data
`driver is supplied to the OLED. The emission period is con-
`trolled by erase-scan signals which turn T8 and the reset
`TFT T9 on and off. Setting the emission period short makes
`it possible to set the current of the current source high
`enough to obtain the same average brightness over succes-
`sive frame periods.
`
`Simulation results
`3.3
`Figures 8(a) and 8(b) show, respectively, simulation results
`for transient response in cases both with and without pre-
`
`464
`
`Shimoda et al./ An integrated poly-Si TFT current data driver
`
`FIGURE 8 — Simulation results for transient response in cases both with
`(a) and without (b) pre-charge. Both (a) and (b) show response for levels
`from the current source of from 0 to 80 nA.
`
`charge. Both show response for levels from the current
`source of from 0 to 80 nA. As may be seen in Fig. 8(b),
`without pre-charge, the value of the current from the cur-
`rent source cannot be programmed into the pixel circuit
`because that current, 80 nA, is too small to discharge, within
`the horizontal period, T5 gate capacitance and data-line
`capacitance sufficiently for 80 nA to flow through T5. By
`way of contrast, as may be seen in Fig. 8(a), with pre-charge,
`programming can be performed because both the T5 gate
`capacitance and data-line capacitance can be discharged
`sufficiently within the pre-charge period by the voltage-fol-
`lower amplifier in the pre-charge circuit.
`Figure 9 shows simulation results for OLED current
`accuracy vs. emission ratio (emission period/maximum
`emission period, expressed in %) for three cases of pre-
`charge error with respect to green (G), which has the small-
`est OLED current value among R, G, and B. We have
`confirmed that setting smaller emission periods makes
`higher accuracy possible. We have also found that a 64-level
`gray scale can be achieved by setting the emission ratio to
`
`SAMSUNG, EXH. 1004, P. 4
`
`

`

`FIGURE 9 — Simulation results for OLED current accuracy vs.emission
`ratio (emission period/maximum emission period, expressed in %) for
`three cases of pre-charge error.
`
`roughly 15% at 0.5 V of pre-charge error, about 40% at 0.2
`V, and about 70% at 0.1 V.
`
`4 Displays
`We have fabricated an AMOLED display based on the
`design presented here. Specifications for the developed dis-
`play are summarized in Table 1. The display contains gate
`drivers, a current data driver, and level shifters. Both RGB
`digital data signals (6 bits each) and digital control signals
`are supplied at 3 V from an external controller. Figure 10
`shows a sample display image. Here, the emission ratio is
`about 40%. Good pixel-to-pixel luminance uniformity and
`gray-scale images have been achieved with this display.
`
`Conclusion
`5
`We have developed an integrated poly-Si TFT current data
`driver based on a current-copier technique and a novel cur-
`rent-programming approach with a data-line pre-charge
`function. The integrated current data driver eliminates of
`the need for external current sources, reduces cost, and
`
`TABLE 1 — Display specifications.
`Display size
`1.9 in.
`120 × RGB × 136
`Pixel count
`Pixel pitch
`96 ppi
`>150 cd/m2
`Peak luminance
`Digital 6 bit × RGB
`Input data
`Input level
`3 V I/F
`Current data driver
`Gate driver
`Level shifter
`
`Integrated circuits
`
`FIGURE 10 — Image on the fabricated AMOLED display.
`
`increases reliability in AMOLED display systems, and the
`novel current-programming approach improves the accu-
`racy of current programming at low brightness. These cir-
`cuits make it possible to produce AMOLED displays with
`good gray-scale image quality and good pixel-to-pixel lumi-
`nance uniformity.
`
`Acknowledgments
`We are very grateful to Mr. S. Kaneko, head of the SOG
`Research Labs for his important guidance and encourage-
`ment. We also thank the members of the SOG Research
`Labs for their help in fabricating the poly-Si TFT sub-
`strates, and we further wish to express our appreciation to
`the members of the R&D Technical Support Center in NEC
`for thier valuable support during the course of this work.
`
`References
`1 R Dawson et al, IEEE IEDM ‘98, 875 (1998).
`2 K Inukai et al, SID Intl Symp Digest Tech Papers, 924 (2000).
`3 M Kimura et al, Proc IDW ‘99, 171 (1999).
`4 I M Hunter et al, AMLCD2000 Digest, 249 (2000).
`5 R Hattori et al, AMLCD2001 Digest, 223 (2001).
`6 T Sasaoka et al, SID Intl Symp Digest Tech Papers, 384 (2001).
`7 A Yumoto et al, Proc IDW ‘01, 1395 (2001).
`8 K Abe et al, Proc EuroDisplay, 279 (2002).
`
`Masamichi Shimoda received his B.E. degree in
`electrical engineering from Meiji University in
`1988. He joined NEC Corporation in 1988 and
`has recently been engaged in the research and
`development of polycrystalline-silicon TFT cir-
`cuits and their application devices.
`
`Journal of the SID 11/3, 2003
`
`465
`
`SAMSUNG, EXH. 1004, P. 5
`
`

`

`Koichi Iguchi received his B.S degree in elec-
`tronic engineering from the University of Elec-
`tro-Communications in 1995. He joined NEC
`Corporation in 1995. He has recently been
`engaged in the research and development of
`polycrystalline-silicon TFT circuits and OLED
`displays.
`
`Daisuke Iga received his B.S. and M.S. degrees
`in electrical engineering and electronics from
`Aoyama Gakuin University, Tokyo, Japan in
`1994 and 1996, respectively. He joined NEC
`Corporation in 1996 and has recently been
`engaged in the development of polycrystalline-
`silicon TFTs, with particular respect to their LCD
`applications.
`
`Hironori Imura received his B.E. degree from
`Osaka University in 1984. He joined NEC Corpo-
`ration in 1984 and has recently been engaged in
`the research and development of OLED dis-
`plays, with a particular focus on OLED process
`technology and polycrystalline-silicon TFT
`process technology.
`
`Soichiro Miyano received his B.E. degree from
`Kyoto University, Kyoto, Japan, in 1977. He
`joined NEC Corporation in that year and has
`worked on silicon micro-machining technolo-
`gies, as well as on such devices as silicon micro-
`accelerometers and vacuum microelectronics
`devices. Currently, he is responsible for poly-
`crystalline-silicon TFT process, device, circuit,
`and panel technologies for AMOLEDs. Mr.
`Miyano is a member of IEEE.
`
`Katsumi Abe received his B.S. and M.S. degrees
`in nuclear engineering from Kyoto University in
`1993 and 1995, respectively. He joined NEC
`Corporation in 1995, and in 2002, he moved to
`NEC Electronics Corporation. In recent years,
`he has been engaged in the research and devel-
`opment of polycrystalline-silicon TFT circuits
`and their application devices. Mr. Abe is a
`member of the Society for Information Display.
`
`Hiroshi Haga received his B.E. degree in image
`science and technology from Chiba University
`in 1994. Since joining the Display Device Research
`Laboratory, Functional Devices Research Labo-
`ratories of NEC Corporation in 1994, he has
`been engaged in the research and development
`of polycrystalline-silicon TFT circuits and their
`application devices. Mr. Haga is a member of
`the Society for Information Display, the Institute
`of Electronics, Information and Communication
`Engineers, and the Institute of Image Information and Television Engi-
`neers.
`
`Hideki Asada received his B.E. and M.E. degrees
`in electrical engineering from Keio University in
`1985 and 1987, respectively. In 1987, he joined
`the Central Research Laboratories, NEC Corpo-
`ration. Since then, he has been engaged in the
`research and development of polycrystalline-
`silicon TFT circuits and their application
`devices. Mr. Asada is a member of the Society
`of Information Display.
`
`Hiroshi Hayama received his B.S., M.S., and
`Ph.D. degrees from the Tokyo Institute of Tech-
`nology, Tokyo, Japan in 1978, 1980, and 1995,
`respectively. He joined the Central Research
`Laboratories, NEC Corporation, Kawasaki,
`Japan in 1980, where he has been engaged in
`research on highspeed CMOS/SOS, three-dimen-
`sional integration of ICs, display-driver LSIs,
`thin-film transistors on amorphous substrates,
`and TFT-LCDs. Dr. Hayama is a member of the
`Society of Information Display, the Institute of Electronics, Information
`and Communication Engineers of Japan, and the Japan Society of
`Applied Physics.
`
`466
`
`Shimoda et al./ An integrated poly-Si TFT current data driver
`
`SAMSUNG, EXH. 1004, P. 6
`
`