(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2002/0033718A1
`Tam
`(43) Pub. Date:
`Mar. 21, 2002
`
`US 20020033718A1
`
`(54) CIRCUIT, DRIVER CIRCUIT, ORGANIC
`ELECTROLUMNESCENT DISPLAY DEVICE
`ELECTRO-OPTICAL DEVICE, ELECTRONIC
`APPARATUS, METHOD OF CONTROLLING
`THE CURRENT SUPPLY TO AN ORGANIC
`ELECTROLUMINESCENT PIXEL, AND
`METHOD FOR DRIVING ACIRCUIT
`
`(75) Inventor: Simon Tam, Cambridge (GB)
`Correspondence Address:
`OLIFF & BERRIDGE, PLC
`P.O. BOX 19928
`ALEXANDRIA, VA 22320 (US)
`(73) Assignee: SEIKO EPSON CORPORATION, 4-1
`Nishishinjuku 2-chome, Tokyo 1 63.0811
`(JP)
`
`(21) Appl. No.:
`
`09/899,915
`
`(22) Filed:
`(30)
`
`Jul. 9, 2001
`Foreign Application Priority Data
`
`Jul. 7, 2000 (GB)......................................... OO16816.1
`
`Publication Classification
`
`(51) Int. Cl." ....................................................... H03K 5/00
`(52) U.S. Cl. ................................................................ 327/94
`
`(57)
`
`ABSTRACT
`
`Adriver circuit operating in Stages that comprise a program
`ming Stage and a reproduction Stage, the circuit comprising:
`a plurality current paths each of which passes through the
`circuit a current driven element, a transistor connected So as
`operatively to control the current Supplied to the Said ele
`ment, a capacitor connected for Storing an operating Voltage
`of the transistor during the programming Stage, and Switch
`ing means which control the current paths, the arrangement
`being Such that one of the current paths does not include the
`Said element. No current is applied to the current driven
`element by the current controlling transistor during the
`programming Stage and thus the overall power consumption
`is reduced. Furthermore, the circuit can be operated from a
`normal Supply Voltage rather than requiring a high bias
`Voltage. During the programming Stage, the circuit uses a
`current Sink rather than a current Source. Preferably, the
`current driven element is an electroluminescent element.
`
`
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`Woe
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`Patent Application Publication Mar. 21, 2002. Sheet 1 of 8
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`US 2002/0033718A1
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`Wselect
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`Patent Application Publication Mar. 21, 2002 Sheet 2 of 8
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`US 2002/0033718A1
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`Patent Application Publication Mar. 21, 2002 Sheet 3 of 8
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`US 2002/0033718A1
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`Patent Application Publication Mar. 21, 2002. Sheet 4 of 8
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`Patent Application Publication Mar. 21, 2002. Sheet 5 of 8
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`Patent Application Publication Mar. 21, 2002. Sheet 6 of 8
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`Patent Application Publication Mar. 21, 2002. Sheet 7 of 8
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`US 2002/0033718A1
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`Probe
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`Readout
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`Patent Application Publication Mar. 21, 2002. Sheet 8 of 8
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`US 2002/0033718A1
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`Mar. 21, 2002
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`CIRCUIT, DRIVER CIRCUIT, ORGANIC
`ELECTROLUMNESCENT DISPLAY DEVICE
`ELECTRO-OPTICAL DEVICE, ELECTRONIC
`APPARATUS, METHOD OF CONTROLLING THE
`CURRENT SUPPLY TO AN ORGANIC
`ELECTROLUMINESCENT PIXEL, AND METHOD
`FOR DRIVING A CIRCUIT
`0001. The present invention relates, inter alia, to a driver
`circuit. One particular application of Such a driver circuit is
`for driving a pixel of an organic electroluminescent device.
`0002 An organic electro-luminescent (OEL) element
`comprises a light emitting material layer Sandwiched
`between an anode layer and a cathode layer. Electrically, this
`element operates like a diode. Optically, it emits light when
`forward biased and the intensity of the emission increases
`with the forward bias current. It is possible to construct a
`display panel with a matrix of OEL elements fabricated on
`a transparent Substrate and with at least one of the electrode
`layers being transparent. One can also integrate the driving
`circuit on the Same panel by using low temperature poly
`silicon thin film transistor (TFT) technology.
`0003. In a basic analog driving scheme for an active
`matrix OEL display, a minimum of two transistors are
`required per pixel (FIG.1): T is for addressing the pixel and
`T is for converting the data Voltage Signal into current
`which drives the OEL element at a designated brightness.
`The data signal is stored by the storage capacitor Co.
`when the pixel is not addressed Although p-channel TFTs
`are shown in the figures, the same principle can also be
`applied for a circuit with n-channel TFTs.
`0004. There are problems associated with TFT analog
`circuits and OEL elements do not act like perfect diodes. The
`light emitting material does, however, have relatively uni
`form characteristics. Due to the nature of the TFT fabrication
`technique, Spatial variation of the TFT characteristics exists
`over the entire panel. One of the most important consider
`ations in a TFT analog circuit is the variation of threshold
`Voltage, AV, from device to device. The effect of Such
`variation in an OEL display, exacerbated by the non perfect
`diode behaviour, is the non-uniform pixel brightness over
`the display panel, which Seriously effects the image quality.
`Therefore, a built-in circuit for compensating a dispersion of
`transistor characteristics is required.
`0005. A circuit shown in FIG. 2 is proposed as one of
`built-in for compensating a variation of transistor charac
`teristics. In this circuit T is for addressing the pixel. T.
`operates as an analog current control to provide the driving
`current T. connects between the drain and gate of T and
`toggles T. to be either a diode or in Saturation. T acts as a
`Switch. Either T or T can be ON at any one time. Initially,
`T, and T are OFF, and T is ON. When T is OFF, T, and
`T are ON, and a curt of known value is allowed to flow into
`the OEL element, through T. This is the programming Stage
`because the threshold Voltage of T is measured with T.
`operating as a diode (with T. turned ON) while the pro
`gramming current is allowed to flow through T, through T.
`and into the OEL element. T shorts the drain and gate of T
`and turns T. in to a diode. The detected threshold voltage of
`T is Stored by the capacitor C connected between the gate
`and Source terminals of T when T and T are Switched
`OFF. Then T is turned ON, the current is now provided by
`V. If the slope of the output characteristics were flat, the
`
`reproduced current would be the same as the programed
`current for any threshold Voltage of T detected. By turning
`ONT, the drain-Source Voltage of T is pulled up, So a flat
`output characteristic will keep the reproduced current the
`Same as the programmed current. Note that AV shown in
`FIG. 2 is imaginary, not real.
`0006 Aconstant current is provided, in theory, during the
`active programming Stag, which is t to ts in the timing
`diagram shown in FIG. 2. The reproduction Stage Starts atts.
`0007. The circuit of FIG. 2 is advantageous but there is
`an on-going desire to reduce power consumption. In par
`ticular, implementation of the current-Source in the circuit of
`FIG. 2 requires a bias Voltage (VAs) in addition to the
`Supply Voltage (VDE). Although the Supply Voltage (VDE)
`could be increased to cover the required bias Voltage
`(VAs)—which would have the advantage of reducing the
`component count, there is Sill an overall increase in System
`power consumption to program with any value of data
`current (IDA).
`0008 Attention is, by the present invention, drawn to the
`fact that all currents passing through the circuit of FIG. 2
`pass through the OEL element. The Significance of this to the
`present invention will be apparent from the description
`given here .
`0009. According to a first aspect of the present invention
`there is provided a driver circuit operating in Stages that
`comprise a programming Stage and a reproduction Stage, the
`circuit comprising:
`0010 a plurality of current paths each of which
`passes through the circuit,
`0011)
`a current driven element,
`0012 a transistor connected so as operatively to
`control the current Supplied to the Said element,
`0013 a capacitor connected for storing an operating
`Voltage of the transistor during the programming
`Stage, and
`0014 Switching means which control the current
`paths, the arrangement being Such that one of the
`current paths does not include the Said element.
`0015 According to a second aspect of the present inven
`tion there is provided a driver circuit for driving a pixel of
`an electroluminescent device, the pixel including an elec
`troluminescent element and the circuit comprising:
`0016 a transistor connected so as operatively to
`control the current Supplied to the electrolumines
`cent element,
`0017 a capacitor connected for storing an operating
`Voltage of the transistor during a programming Stage,
`0018 a first Switching means connected so as to
`establish when operative a current path through the
`transistor during the programming Stage, and
`0019 a second Switching means connected so as to
`establish when operative a current path through the
`transistor and the electroluminescent element during
`a reproduction Stage, wherein the first Switching
`means is connected Such that the current path during
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`Mar. 21, 2002
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`the programming Stage does not pass through the
`electroluminescent element.
`0020. According to a third aspect of the present invention
`there is provided a driver circuit for driving a pixel of an
`electroluminescent device, the pixel including an electrolu
`minescent element and the circuit comprising:
`0021 a transistor connected so as operatively to
`control the current Supplied to the electrolumines
`cent element,
`0022 a capacitor connected for storing an operating
`Voltage of the transistor dug a programming Stage,
`0023 a first Switching means connected so as to
`establish when operative a current path through the
`transistor during the programming Stage,
`0024 a second Switching means connected So as to
`establish when operative a current path through the
`transistor and the electroluminescent element during
`a reproduction Stage, and
`0025 a current sink, the first Switching means being
`connected Such that the current path during the
`programming Stage is through the transistor to the
`current Sink.
`0026. According to a fourth aspect of the present inven
`tion there is provided a method of controlling the current
`Supply to an electroluminescent element comprising the
`Steps of providing a current path during a programming
`Stage which path does not pass through the electrolumines
`cent element and of providing a current path during a
`reproduction Stage which path does pass through the elec
`troluminescent element.
`0.027 According to a fifth aspect of the present invention
`there is provided a method of controlling the current Supply
`to an electroluminescent element comprising the Steps of
`providing a current path during a programming Stage which
`path connects to a current Sink and of providing a current
`path during a reproduction Stage which path passes through
`the electroluminescent element
`0028. According to a sixth aspect of the present invention
`there is provided an electroluminescent display device com
`prising one or more driver circuits according to any of the
`fist to third aspects of the present invention.
`0029. According to a seventh aspect of the present inven
`tion there is provided an electronic apparatus incorporating
`an electroluminescent display device according to the Sixth
`aspect of the present invention.
`0.030. According to an eigth aspect of the present inven
`tion there is provided a circuit comprising a current driven
`element the circuit providing a first current path including
`the current driven element and a Second current path not
`including the current driven element
`0031. According to a nineth aspect of the present inven
`tion there is provided a circuit comprising a current driven
`element, the circuit providing a first current path flowing a
`current through the current driven element, and a Second
`current path not flowing current through the current driven
`element.
`0032. According to a tenth aspect of the present invention
`there is provided a method for driving a circuit comprising
`
`a current driven element and a transistor that controls a
`current Supplied to the current driven element, comprising a
`Step of determining a gate Voltage of the transistor based on
`a predetermined current.
`0033. It will be noted that according to the present
`invention no current is applied to the current driven element
`by the current controlling transistor during the programming
`Stage. In accordance with the invention in an electrolumi
`neScent device a pixel driver circuit can be implemented
`without degrading the perceived image presented by the
`electroluminescent device. It has the benefit of reducing the
`overall power consumption compared with the prior art in
`which the same current is supplied to the OEL element
`during both the programming and the reproduction Stage.
`Furthermore, the circuit can be operated from a normal
`Supply Voltage rather than requiring a high bias Voltage as in
`the prior art. In effect, the present invention provides for
`Separation of the programming and the reproduction current
`paths. This enables a number of advantages to be achieved.
`For example, if there is no current flow through the OEL
`element during the programming Stage then the program
`ming Stage operates more quickly-Since the arrangement
`avoids the slow down caused by the parasitic capacitance of
`the OEL element.
`0034 Embodiments of the present invention will now be
`described by way of further example only and with reference
`to the accompanying drawings, in which:
`0035 FIG. 1 shows a conventional OEL element pixel
`driver circuit using two transistors,
`0036 FIG. 2 shows a known current programmed OEL
`element driver with threshold Voltage compensation.
`0037 FIG. 3 shows a pixel driver circuit according to a
`first embodiment of the present invention,
`0038 FIG. 4 shows a pixel driver circuit according to a
`Second embodiment of the present invention,
`0039 FIG. 5 shows several pixels in a matrix display
`wherein each pixel uses the circuit of FIG. 4,
`0040 FIG. 6 is a schematic sectional view of a physical
`implementation of an OEL element and pixel driver accord
`ing to an embodiment of the present invention,
`0041 FIG. 7 is a simplified plan view of an OEL display
`panel incorporating the present invention,
`0042 FIG. 8 shows another embodiment of a pixel driver
`circuit according to the present invention,
`0043 FIG. 9 shows another embodiment of a pixel driver
`circuit according to the present invention,
`0044 FIG. 10 is a schematic view of a mobile personal
`computer incorporating a display device having a pixel
`driver according to the present invention,
`004.5 FIG. 11 is a schematic view of a mobile telephone
`incorporating a display device having a pixel driver accord
`ing to the present invention,
`0046 FIG. 12 is a schematic view of a digital camera
`incorporating a display device having a pixel driver accord
`ing to the present invention,
`0047 FIG. 13 illustrates the application of the driver
`circuit of the present invention to a magnetic RAM,
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`0048 FIG. 14 illustrates the application of the driver
`circuit of the present invention to a magnetoresistive ele
`ment,
`0049 FIG. 15 illustrates the application of the driver
`circuit of the present invention to a capacitance Sensor or a
`charge Sensor,
`0050 FIG. 16 illustrates the application of the driver
`circuit of the present invention to a night vision camera, and
`0051 FIG. 17 is a brief outline of an alternative imple
`mentation of the circuit of FIG. 4.
`0.052 A pixel driver circuit according to a first embodi
`ment of the present invention is shown in FIG. 3. Transistor
`T operates as an analog current control to provide the
`driving current to the OEL element. Also, the Storage
`capacitor C is connected between the gate and the Source of
`transistor T. In the circuit of FIG. 2, a current source is
`operatively connected to the Source of transistor T by
`transistor T, during the programming Stage, and current is
`thus applied to the OEL element. In the embodiment of the
`present invention, transistor T operatively connects transis
`tor T to a current Sink during the programming Stage. That
`is, according to the present invention, during the program
`ming Stage no current is Supplied through transistor T to the
`OEL element. In the circuit of FIG. 3, the drain of transistor
`T is connected to the Source of transistor T. Via the
`Source/drain path of transistor T. The Source of transistor T
`is connected to the gate of transistor T and the gates of
`transistors T and T are connected together. The program
`ming Voltage V is applied to the gates of T and T.
`Transistor T, which is Switched off during the programming
`Stage, connects the drain of T and the Source of T to the
`OEL element. During the programming stage, transistor T
`operatively connects transistor T to a current Sink which is
`tied to ground or a reference Voltage.
`0053. The circuit of FIG. 3 operates in the programming
`Stage with T. Switched off and T and T. Switched on. T
`being Switched on has the effect of making T act as a diode
`and T connects this diode to the data current Sink. AS a
`result, capacitor C charges (or discharges, depending on the
`voltage Stored during the previous frame). Capacitor C
`charges to the gate/Source Voltage of transistor T and thus
`Stores the Voltage (Vcs, corresponding to the data current
`I) which will control the current Supply to the OEL
`element during the reproduction Stage. At the end of the
`programming Stage, T and T are Switched off. The Voltage
`Vs is Stored on C for the remainder of the frame period.
`AS will be readily apparent from the circuit diagram and this
`description, in accordance with the present invention there is
`no requirement for a bias Voltage to provide a current Source.
`That is, the Supply voltage (V) in FIG.3 is determined by
`T and by the OEL element and there is no requirement for
`a high Voltage to power a current Source. The maximum
`Voltage required by the circuit is thus Significantly less than
`that required by the circuit of FIG. 2.
`0054) At the start of the programming stage, with T.
`Switched off, it is found that the OEL element exhibits a
`parasitic capacitance which discharges through the device.
`The rate of charging of C determines the time taken for the
`programming Stage. In accordance with circuits embodying
`the present invention, the capacitance of C can be relatively
`Small and thus the charging can be very rapid. As a conse
`
`quence, the period for which no current is applied to the
`OEL element by T is very short compared with the whole
`Same. These factors, together with the persistence of vision
`of the human eye means that there is no perceptible degra
`dation of a displayed image.
`0055 The off resistance of T can be important, because
`aft C has been charged and T is switched off, the off
`resistance of T can affect the Voltage acroSS C for the rest
`of the frame period. Thus, the gate/Source capacitance of T
`should preferably be small compared with C.
`0056 The reproduction voltage V is applied to the gate
`of transistor T. At the beginning of the reproduction Stage,
`in the circuit of FIG. 3, T is switched on and T and T.
`Switched off. As a result, T. acts as a current Source with
`Vs biased by C, thus Supplying current to the OEL
`element. At the end of the reproduction Stage T is Switched
`off, T and T. remain Switched off his completes one cycle.
`The driving waveform is indicated in FIG. 3.
`0057 FIG. 4 illustrates a second embodiment according
`to the present invention. The circuit of FIG. 4 differs from
`that of FIG. 3 in the connection of transistor T. In the
`circuit of FIG. 4. T is connected to C through the drain/
`Source path of T. The circuit of FIG. 4 is preferred to that
`of FIG. 3 because T is not in the current path during the
`programing Stage. Otherwise the operation and effects of the
`Second embodiment are similar to those of the first embodi
`ment.
`0.058 FIG. 5 is a circuit diagram showing a number of
`pixels in an active matrix display, with each pixel imple
`mented in accordance with the circuit of FIG. 4. To simplify
`the illustration, a monochrome display device is shown.
`Since the circuit is of an active matrix, pixels on the same
`row are addressed at the Same time. Transistor T is respon
`Sible for pixel addressing, So its Source terminal is connected
`to the current data line shared by a column of pixels.
`Because of this the leakage current of T should be kept to
`a minimum. This can be ensured by using a multi-gate
`Structure for T. In addition to a multi-gate Structure, a
`lightly doped drain (LDD) structure can also reduce the
`leakage current.
`0059 FIG. 6 is a schematic cross-sectional view of the
`physical implementation of the pixel driver circuit in an
`OEL element structure. In FIG. 6, numeral 132 indicates a
`hole injection layer, numeral 133 indicates an organic EL
`layer, and numeral 151 indicates a resist or Separating
`structure. The Switching thin-film transistor 121 and the
`n-channel type current-thin-film transistor 122 adopt the
`Structure and the proceSS ordinarily used for a low-tempera
`ture polysilicon thin-film transistor, Such as are used for
`example in known thin-film transistor liquid crystal display
`devices Such as a top-gate Structure and a fabrication proceSS
`wherein the maximum temperature is 600 C. or less.
`However, other Strut and processes are applicable.
`0060. The forward oriented organic EL display element
`131 is formed by: the pixel electrode 115 formed of A1, the
`opposite electrode 116 formed of ITO, the hole injection
`layer 132, ad the organic EL layer 133. In the forward
`oriented organic EL display element 131, the direction of
`current of the organic EL display device can be set from the
`opposite electrode 116 formed of ITO to the pixel electrode
`115 formed of A1.
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`0061 The hole injection layer 132 and the organic EL
`layer 133 may be formed using an ink-jet printing me
`employing the resist 151 as a separating Structure between
`the pixels. The opposite electrode 116 formed of ITO may be
`formed using a Sputtering method However, other methods
`may also be used for forming all of these components.
`0062) The typical layout of a full display panel employ
`ing the present invention is shown schematically in FIG. 7.
`The panel comprises an active matrix OEL element 200 with
`analogue current program pixels, an integrated TFT Scan
`ning driver 210 with level shifter, a flexible TAB tape 220,
`and an external analogue driver LSI 230 with an integrated
`RAM/controller. Of course, this is only one example of the
`possible panel arrangements in which the present invention
`can be used.
`0.063. The structure of the organic EL display device is
`not limited to the one described here. Other structures are
`also applicable.
`0064. With reference for example to the circuit of FIG.3,
`it will be appreciated that the invention provides a data
`current Source in this case forte OEL element The circuit is
`readily extended So as to provide an amplified and/or
`multiple level (current) output. The principle of Such a
`circuit cam be understood with reference to FIG. 8. the
`circuit of FIG. 8 comprises the circuit of FIG. 3 with an
`additional drive transistor Ts and an additional Switching
`transistor T added. The Source of T is connected to V
`and its gate receives the same drive Voltage Signal as is
`applied to the gate of transistor T. The drain of transistor Ts
`is Series connected to the drain of transistor T and the
`Source of T is connected to the common point of connection
`of transistors T, T- and T. The gate of transistor T is
`connected to the gate of transistor T. If it is assumed that the
`characteristic of transistor T is W/L and that the character
`istic of transistorTs is selected to be (N-1)W/L then a current
`amplification of
`outlinxN
`0065) is achieved. It is the current which flows through
`the current Sink, i.e. It in FIGS. 3 and 4. It is the current
`which flows through the OEL element. Thus the circuit of
`FIG. 8 can be used to reduce the value of I
`compared
`with the circuit of FIGS. 3 and 4, while maintaining the
`Same current trough the OEL element. Lowering the value of
`IDA has the advantage of enabling the operating Speed of the
`circuit to be increased. Lowering the value of Ir also has
`the advantage of lowering the transmission loSS experienced
`acroSS a matrix of pixels, which is particularly important
`with respect to large Scale display panels.
`0.066. Of course, additional stages-each adding their
`own circuit of additional transistors Ts and T-can be
`added. With the Switching transistors T. Series connected
`and each receiving its own gate drive Signal-as shown in
`FIG. 9 (AB etc)-different current values can be selected to
`pass through the OEL element, resulting in different inten
`Sities of light output.
`0067 Preferably the circuits shown in FIGS. 3 to 9 are
`implemented using thin film transistor (TFT) technology,
`most preferably in polysilicon.
`0068 The present invention is particularly advantageous
`for use in Small, mobile electronic products Such as mobile
`
`phones, computers, CD players, DVD playerS and the like
`although it is not limited hereto.
`0069. Several electronic apparatuses using the above
`organic electroluminescent display device will now be
`described.
`0070 <1: Mobile Computers
`0071 An example in which the display device according
`to one of the above embodiments is applied to a mobile
`personal computer will now be described.
`0072 FIG. 10 is an isometric view illustrating the con
`figuration of this personal computer, In the drawing, the
`personal computer 1100 is provided with a body 1104
`including a keyboard 1102 and a display unit 1106. The
`display unit 1106 is implemented using a display panel
`fabricated according to the present invention, as described
`above.
`0073) <2: Portable Phones
`0074 Next, an example in which the display device is
`applied to a display Section of a portable phone will be
`described& FIG. 11 is an isometric view illustrating the
`configuration of the portable phone. In the drawing, the
`portable phone 1200 is provided with a plurality of operation
`keys 1202, an earpiece 1204, a mouthpiece 1206, and a
`display panel 100. This display panel 100 is implemented
`using a display panel fabricated according to the present
`invention, as described above.
`0075) <3: Digital Still Camerad
`0076) Next, a digital still camera using an OEL display
`device as a finder will be describes. FIG. 12 is an isometric
`View illustrating the configuration of the digital Still camera
`and the connection to external devices in brief.
`0077. Typical cameras sensitize films based on optical
`images from objects, whereas the digital still camera 1300
`generates imaging Signals from the optical image of an
`object by photoelectric conversion using, for example, a
`charge coupled device (CCD). The digital still camera 1300
`is provided with an OEL element 100 at the back face of a
`case 1302 to perform display based on the imaging Signals
`from the CCD. Thus, the display panel 100 functions as a
`finder for displaying the object A photo acceptance unit 1304
`including optical lenses and the CCD is provided at the front
`side (behind in the drawing) of the case 1302.
`0078 When a cameraman determines the object image
`displayed in the OEL element panel 100 and releases the
`Shutter, the image Signal from the CCD are transmitted and
`stored to memories in a circuit board 1308, In the digital still
`camera 1300, video signal output terminals 1312 and input/
`output terminals 1314 for data communication are provided
`on a side of the case 1302. As shown in the drawing, a
`television monitor 1430 and a personal computer 1440 are
`connected to the Video signal terminals 1312 and the input/
`output terminals 1314, respectively, if necessary. The imag
`ing signals stored in the memories of the circuit board 1308
`are output to the television monitor 1430 and the personal
`computer 1440, by a given operation.
`0079 Examples of electronic apparatuses, other than the
`personal computer shown in FIG. 10, the portable phone
`shown in FIG. 11, and the digital still camera shown in FIG.
`12, include OEL element television sets, view-finder-type
`
`SAMSUNG EX. 1013 - 13/15
`
`

`

`US 2002/0033718A1
`
`Mar. 21, 2002
`
`and monitoring-type video tape recorders, car navigation
`Systems, pagers, electronic notebooks, portable calculators,
`word processors, WorkStations, TV telephones, point-of
`sales system (POS) terminals, and devices provided with
`touch panels. Of course, the above OEL device can be
`applied to display Sections of these electronic apparatuses,
`0080. The driver circuit of the present invention can be
`disposed not only in a pixel of a display unit but also in a
`driver disposed outside a display unit.
`0081. In the above, the driver circuit of the present
`invention has been described wit reference to various dis
`play devices. The applications of the driver circuit of the
`present invention are much broader than just display devices
`and include, for example, its use with a magnetoresistive
`RAM, a capacitance Sensor, a charge Sensor, a DNA Sensor,
`a night vision camera and many other devices.
`0082 FIG. 13 illustrates the application of the driver
`circuit of the present invention to a magnetic RAM. In FIG.
`13 a magnetic head is indicated by the reference MH.
`0083 FIG. 14 illustrates the application of the driver
`circuit of the present invention to a magnetoresistive ele
`ment. In FIG. 14 a magnetic head is indicated by the
`reference MH. and a magnetic resistor is indicated by the
`reference MR.
`0084 FIG. 15 illustrates the application of the driver
`circuit of the present invention to a capacitance Sensor or a
`charge sensor. In FIG. 15 a sense capacitor is indicated by
`the reference C. The circuit of FIG.15 is also applicable
`to other applications, Such as a fingerprint Sensor and a DNA
`SCSO.
`0085 FIG. 16 illustrates the application of the driver
`circuit of the present invention to a night vision camera. In
`FIG. 16 a photoconductor is indicated by refrenece R.
`0.086. In the embodiments illustrated with reference to the
`above Specific description the transistors have been shown
`as p-channel type transistors. This is not limiting of the
`invention. For example, FIG. 17 is a brief outline of an
`alternative implementation of the circuit of FIG. 4. In FIG.
`17 n-channel transistors are used throughout the circuit,
`except for the drive transistor which is retained as a p-chan
`nel transistor.
`0087. It will be apparent to persons skilled in the art that
`other variations and modifications can be made to the
`arrangements described with respect to FIGS. 3 to 16
`without departing from the Scope of the invention.
`
`1. A driver circuit operating in Stages that comprise a
`programming Stage and a reproduction Stage, the circuit
`comprising:
`a plurality of current paths each of which passes through
`the circuit,
`a current driven element,
`a transistor connected So as operatively to control the
`current Supplied to the Said element,
`a capacitor connected for Storing an operating Voltage of
`the transistor during the programming Stage, and
`
`Switching means which control the current paths, the
`arrangement being Such that one of the current paths
`does not include the Said clement.
`2. A driver circuit for driving a pixel of an electrolumi
`neScent device, the pixel including an electroluminescent
`element and the circuit comprising,
`a transistor connect So as operatively to control the current
`Supplied to the electroluminescent element,
`a capacitor connected for Storing an operating Voltage of
`the transistor during a programming Stage,
`a first Switching means connected So as to establish when
`operative a current path through the transistor during
`the programming Stage, and
`