(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2002/0101172 A1
`(43) Pub. Date:
`Aug. 1, 2002
`Bu
`
`US 2002O101 172A1
`
`(54) OLED ACTIVE DRIVING SYSTEM WITH
`CURRENT FEEDBACK
`(76) Inventor: Lin-kai Bu, Tainan (TW)
`Correspondence Address:
`MARTINE & PENILLA, LLP
`710 LAKEWAY DRIVE
`SUTE 170
`SUNNYVALE, CA 94085 (US)
`(21) Appl. No.:
`09/823,180
`(22) Filed:
`Mar. 29, 2001
`(30)
`Foreign Application Priority Data
`
`Jan. 2, 2001 (TW)........................................ O901OOOT3
`
`Publication Classification
`
`(51) Int. Cl." ....................................................... G09G 3/10
`(52) U.S. Cl. .......................................................... 315/169.3
`
`ABSTRACT
`(57)
`The invention provides an organic light emitting diode
`active driving System with current feedback, thereby a
`driving current for organic light emitting diode is not
`affected by variation of characteristic parameters of thin film
`transistor under an active driving mode. The active driving
`System in accordance with the invention includes a transistor
`and a current comparator for driving an organic light emit
`ting diode. The transistor has two current carrying electrodes
`respectively connected to a cathode of the organic light
`emitting diode and ground, and a gate controlled by a data
`Signal. The current comparator has two input terminals
`respectively receive a reference current with predetermined
`value and a driving current flowing through the organic light
`emitting diode. The current comparator compares the refer
`ence current and the driving current, and then outputs a
`Voltage to the gate of the transistor in response to the
`comparison result So as to make the value of the driving
`current equal to that of the reference current. Therefore, the
`active driving System for organic light emitting diode array
`or flat panel display in accordance with the invention can
`achieve a desirable light emission uniformity.
`
`WFB
`
`CURRENT
`COMPARATOR
`
`3
`
`DATA SIGNAL
`
`4
`
`222
`
`22 221
`/
`
`2
`
`J-21
`212
`
`LG Display Co., Ltd.
`Exhibit 1011
`Page 001
`
`

`

`Patent Application Publication
`
`Aug. 1, 2002 Sheet 1 of 3
`
`US 2002/0101172 A1
`
`Vs
`
`3
`
`1
`223 - - -
`221
`
`- -
`22
`J/21
`212
`213
`22
`---------
`
`
`
`4
`
`2-
`
`Fig. 1
`(PRIOR ART)
`
`LG Display Co., Ltd.
`Exhibit 1011
`Page 002
`
`

`

`Patent Application Publication
`
`Aug. 1, 2002. Sheet 2 of 3
`
`US 2002/0101172 A1
`
`6
`
`-> If
`
`CURRENT
`COMPARATOR --IoLED
`
`()DRV
`
`()REF
`-
`
`Vs
`
`a.
`O
`2
`3-2 2.
`F - H -- 42
`INV
`542
`541
`535, -5
`
`a 4
`
`531
`
`DATA SIGNAL
`
`4
`
`L 532 53
`1-\/
`21
`J-21
`212
`
`23
`
`T223
`221
`- /
`2
`22
`
`23
`
`–
`
`Fig. 2
`
`LG Display Co., Ltd.
`Exhibit 1011
`Page 003
`
`

`

`Patent Application Publication
`
`Aug. 1, 2002. Sheet 3 of 3
`
`US 2002/0101172 A1
`
`
`
`Vs
`
`T542
`541
`535
`
`531
`
`s
`
`DATA SIGNAL
`
`4.
`
`2-
`
`T223
`221
`/
`22
`
`23
`
`L932 53 - I
`1-V
`211
`
`21
`3i.
`
`213
`
`LG Display Co., Ltd.
`Exhibit 1011
`Page 004
`
`

`

`US 2002/0101172 A1
`
`Aug. 1, 2002
`
`OLED ACTIVE DRIVING SYSTEM WITH
`CURRENT FEEDBACK
`
`BACKGROUND OF THE INVENTION
`0001) 1. Field of the Invention
`0002 The invention relates to an active driving circuit
`System for organic light emitting diode (OLED) and, more
`particularly, to an OLED active driving System for improv
`ing light emission uniformity of an array or flat panel display
`(FPD) made up of OLEDs by current feedback.
`0003 2. Description of the Related Art
`0004 Recently, since OLED arrays can generate rela
`tively high luminance of light and have relatively low
`production and operation costs, they are becoming more and
`more popular as FPDs. Besides, OLEDs can be fabricated in
`a variety of sizes from very Small (less than a tenth milli
`meter in diameter) to relatively large (greater than an inch)
`so that OLED arrays can be fabricated in a variety of sizes.
`Also, OLED arrays can generate most colors of light with
`relative ease and provide a very wide viewing angle.
`0005 All OLEDs work on the same general principles
`described as follows. Firstly, one or more layers of organic
`material are Sandwiched between two electrodes. A current
`is then applied to the OLEDS, causing negatively charged
`electrons to move into the organic material from the cathode.
`Positive charges typically referred to as holes move in from
`the anode. Then, the positive and negative charges meet,
`combine, and produce photons in the center layers (i.e., the
`organic material). The color of the photons depends on the
`electronic properties of the organic material in which the
`photons are generated.
`0006. As disclosed in U.S. Pat. No. 5,748,160, two
`dimensional OLED arrays typically contain rows and col
`umns of OLEDs. FIG. 1 shows one of the OLEDs, which is
`designated by reference numeral 1. Referring to FIG. 1, the
`OLED 1 is connected to a circuit block 2. The circuit block
`2 includes a first transistor 21 having a current carrying
`electrode 211 connected to a cathode of the OLED 1 and a
`current carrying electrode 212 connected to ground. The
`circuit block 2 further includes a Second transistor 22 having
`a current carrying electrode 221 connected to a gate elec
`trode 213 of the first transistor 21. Another current carrying
`electrode 222 of the Second transistor 22 Serves as a data
`Signal input terminal 4, and a gate electrode 223 of the
`Second transistor 22 Serves as a Scan Signal input terminal 3.
`Besides, a capacitor 23 is connected between the gate
`electrode 213 and ground as a Storage element So as to
`maintain the OLED 1 in an ON mode for a specific period
`of time, and control the flowing of Some fixed current,
`wherein the current value is determined by the gate-Source
`voltage Vgs of the first transistor 21.
`0007. The OLED 1 is addressed by Supplying a scan
`Signal to the gate electrode 223 of the Second transistor 22,
`and Supplying a data Signal to the current carrying electrode
`222. Specifically, the Scan Signal activates the Second tran
`Sistor 22 So that the data Signal is input to the gate electrode
`213 of the first transistor 21 through the current carrying
`electrodes 222 and 221. Thereby, the gate electrode 213 is
`activated. At this time, a current path is completed between
`the cathode of OLED 1 and ground. Since a Supply Voltage
`
`Vs is connected to the anode of OLED 1, the current flows
`through the OLED 1, which thus emits light.
`0008 OLEDs are typically current driven devices (i.e.,
`emit due to current flowing through them), as opposed to
`Voltage driven devices Such as liquid crystal displayS
`(LCDs). Therefore, in an array or FPD made up of OLEDs,
`it must be assured that each of the OLEDs is driven by the
`Same current under the same Supply Voltage in order to
`achieve Superior light emission uniformity. However, Since
`the first transistors 21 of the OLEDs do not have the same
`characteristic parameters, different driving currents can be
`generated under the same Supply Voltage. Therefore, the
`conventional array or FPD made up of OLEDs cannot
`achieve desirable light emission uniformity.
`SUMMARY OF THE INVENTION
`0009. In view of the above-mentioned requirement for
`light emission uniformity of OLED array or FPD, the
`invention provides an OLED active driving system with
`current feedback. With the OLED active driving system, a
`driving current for OLED is not affected by variation of
`characteristic parameters of thin film transistor under an
`active driving mode, so that the OLED array or FPD can
`achieve desirable light emission uniformity.
`0010. In one embodiment in accordance with the inven
`tion, a cathode of an OLED is connected to a current
`carrying electrode of a first transistor. A current carrying
`electrode of a Second transistor is connected to a gate
`electrode of the first transistor. Another current carrying
`electrode of the second transistor serves as a data signal
`input terminal, and the gate electrode Serves as a Scan Signal
`input terminal. A capacitor is connected between a gate
`electrode of the first transistor and ground as a Storage
`element. Two current carrying electrodes of a third transistor
`are respectively connected to an anode of the OLED and a
`comparison terminal of a current comparator. A gate elec
`trode of the third transistor is connected to the Scan Signal
`input terminal. Two current carrying electrodes of a fourth
`transistor are respectively connected to the anode of the
`OLED and a supply voltage. The gate electrode of the fourth
`transistor Serves to receive a reverse signal of the Scan
`Signal.
`0011. In order to make the driving current input from the
`third transistor into the OLED not affected by variation of
`characteristic parameters of a thin film transistor under
`active driving mode, another comparison terminal of the
`current comparator is connected to a reference current
`Source for receiving a reference current with predetermined
`value. The current comparator compares the driving current
`and the reference current, and then outputs a Voltage to the
`gate electrode of first transistor in response to the compari
`Son result. The gate electrode of the first transistor controls
`the value of driving current, and therefore the driving current
`is maintained at the value of reference current due to the
`feedback effect of the voltage.
`BRIEF DESCRIPTION OF DRAWINGS
`0012 FIG. 1 is a circuit diagram showing one unit of a
`conventional organic light emitting diode array;
`0013 FIG. 2 is a circuit diagram showing one unit of a
`organic light emitting diode array, with the use of an active
`driving System with current feedback, in accordance with the
`invention; and
`
`LG Display Co., Ltd.
`Exhibit 1011
`Page 005
`
`

`

`US 2002/0101172 A1
`
`Aug. 1, 2002
`
`FIG. 3 is a circuit diagram showing one example
`0.014
`of a current comparator in accordance with the invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`0.015 The above-mentioned objects, features, and advan
`tages of the invention will be more apparent from the
`following description, referring to the accompanying draw
`ings. Preferred embodiments in accordance with the inven
`tion will be described in detail with reference to accompa
`nying drawings.
`0016 Referring to FIG. 2, one unit of an OLED array or
`FPD in accordance with the invention includes an OLED 1,
`a first circuit block 2, a Second circuit block 5, and a current
`comparator 6. In FIG. 2, similar elements as in FIG. 1 are
`designated by Similar reference numerals. For the Sake of
`simplification, only the difference of the invention from the
`prior art will be described hereinafter.
`0.017. The second circuit block 5 includes a third tran
`sistor 53, which has a current carrying electrode 531 con
`nected to an anode of the OLED 1 and a current carrying
`electrode 532 connected to a comparison terminal of the
`current comparator 6. The third transistor 53 has a gate
`electrode 533 connected to a Scan Signal input terminal 3.
`The second circuit block 5 further includes a fourth transis
`tor 54, which has a current carrying electrode 541 connected
`to an anode of the OLED 1 and a current carrying electrode
`542 connected to Supply Voltage Vs. In the present inven
`tion, the supply voltage Vs is the supply voltage of the LCD
`panel. The fourth transistor 54 has a gate electrode 543 for
`receiving a reverse Signal of the Scan Signal through another
`Scan line.
`0.018. The current comparator 6 has two comparison
`terminals, which respectively receive a driving current I,
`and a reference current If Supplied from a reference current
`Source REF. The current comparator 6 has an output termi
`nal, which outputs a feedback Voltage V to a data Signal
`input terminal 4 in response to the obtained result of
`comparing of the driving current I
`and reference current
`If. The operation of the organic light emitting diode active
`driving System with current feedback in accordance with the
`invention will be described in detail.
`0.019
`First, as with the prior art, the scan signal and the
`data Signal are input to the first circuit block 2 through the
`Scan Signal input terminal 3 and the data Signal input
`terminal 4 respectively, in order to activate the Second
`transistor 22 and the first transistor 21. At this time, the Scan
`Signal and its reverse Signal are respectively input to the gate
`electrode 533 of the third transistor 53 and the gate electrode
`543 of the fourth transistor 54. Therefore, the third transistor
`53 is in an ON mode and the fourth transistor 54 is in an OFF
`mode So that a comparison terminal of the current compara
`tor 6 can receive the driving current I
`that flows through
`the OLED 1.
`0020. As described above, since the OLED is a current
`driven element, it is required that the driving currents Io
`flowing through respective OLEDs are the same under the
`Same display gray level in order to assure the light emission
`uniformity of the OLED array or FPD made up of OLEDs.
`To achieve the object, another comparison terminal of the
`current comparator 6 is connected to the reference current
`
`ref
`
`Source REF for receiving a reference current If with pre
`determined value. The current comparator 6 compares the
`driving current IoD and the reference current If, and then
`output a feedback voltage V to the gate electrode 213 of
`the first transistor 21 in response to the comparison result.
`The voltage of the gate electrode 213 controls the value of
`the driving current IoE, and therefore the driving current
`IoED is maintained at the Value of reference current If due
`to the feedback effect of the feedback Voltage V.
`0021 Specifically, in the case that the first to fourth
`transistors 21, 22, 53, 54 are all n-channel transistors, the
`current comparator 6 is designed that when the value of the
`driving current It is Smaller than that of the reference
`current It, a plus feedback Voltage V is output So that the
`Voltage of the gate electrode 213 increases, which then
`increases the driving current I. On the contrary, when
`the value of the driving current It is larger than that of
`the reference current I, a minus feedback voltage V is
`output So that the Voltage of the gate electrode 213
`decreases, which then decreases the driving current Io.
`Therefore, the current comparator 6 in accordance with the
`invention assures that the value of the driving current I
`is equal to that of the reference current Ir, which is not
`affected by different characteristic parameters of the first
`transistor 21.
`0022. When the aforementioned programming mode is
`finished, the Scan Signal is turned into low level So that the
`third transistor 53 is in an OFF mode and the fourth
`transistor 54 is in an ON mode. Therefore, the driving
`current I
`is input from the Supply Voltage Vs to the
`OLED 1. The gate voltage of the first transistor 21 main
`tained by the capacitor 23 is adjusted So that the driving
`current IoED is not affected by the characteristic parameters
`of the first transistor 21. Therefore, each OLED 1 driven by
`the same Voltage has the Same driving current IoED flowing
`through it. Therefore, the organic light emitting diode active
`driving System with current feedback in accordance with the
`invention can achieve the object of making a uniform light
`emission of the array or FPD made up of OLEDs.
`0023 FIG. 3 shows an example of the current compara
`tor 6 in accordance with the invention. Referring to FIG. 3,
`the current comparator 6 is made up of four p-type transis
`tors P1, P2, P3, and P4, and three n-type transistors N1, N2,
`and N3. Specifically, two p-type transistors P1 and P2 with
`the same threshold Voltage constitute a current mirror,
`wherein sources P1S and P2s of the transistors P1 and P2 are
`connected to a Supply Voltage Vpp, and gates P1g and P2g
`of the transistors P1 and P2 are connected to each other.
`Also, the gate P1g and drain P1d of the transistor P1 are
`connected to each other. The drain P1d of the transistor P1
`Serves as a comparison terminal of the current comparator 6,
`and is connected to a reference current Source REF that
`Supplies reference current If. Due to the current mirror, a
`current proportional to the reference current If is output
`from the drain P2d of the transistor P2. It is preferable that
`the proportional constant is 1.
`0024. Two p-type transistors P3 and P4 with the same
`threshold Voltage constitute a current mirror of driving
`current, wherein the Sources P3s and P4s of the transistors
`P3 and P4 are connected to the supply voltage Vpp, and
`gates P3g and P4g of the transistors P3 and P4 are connected
`to each other. Also, the gate P3g and drain P3d of the
`
`LG Display Co., Ltd.
`Exhibit 1011
`Page 006
`
`

`

`US 2002/0101172 A1
`
`Aug. 1, 2002
`
`transistor P3 are connected to each other. The drain P3d of
`the transistor P3 Serves as another comparison terminal of
`the current comparator 6, and is connected to the above
`mentioned current carrying electrode 532 of the third tran
`Sistor 53 through which flows the driving current I. Due
`to the current mirror, a current proportional to the driving
`current It
`is output from the drain P4d of the transistor
`P4. It is preferable that the proportional constant is 1.
`0025. The transistors N1 and N2 have the same threshold
`Voltage for providing a compare function for the current
`comparator 6. To go into details, the drain N1d of the
`transistor N1 receives a current corresponding to the refer
`ence current It, and the drain N2d of the transistor N2
`receives a current corresponding to the driving current
`I. Besides, the gates N1g and N2g of the transistors N1
`and N2 are connected to each other and the Sources N1S and
`N2S thereof are connected to ground. Also, the gate N1g and
`drain N1d of the transistor N1 are connected to each other.
`The transistor N3, as an output of the current comparator 6,
`has its gate electrode N3g connected to the drain N2d of the
`transistor N2, its drain electrode N3d connected to the
`Supply Voltage Vpp, and its Source electrode N3S connected
`to ground.
`0026. The transistors N1 and N2 are arranged into a
`current mirror Structure, and therefore there is a proportional
`relationship between the reference current If and the driv
`ing current IoED under a stable condition. It is preferable
`that the proportional constant is 1. In this case, when the
`value of the driving current I
`is Smaller than that of the
`reference current If a drain voltage V.Nea of the transistor
`N2 decreases So that a drain Voltage V output by the
`transistor N3 increases. Since the increase of the drain
`Voltage V.N. makes the gate Voltage of the first transistor 21
`in the first circuit block 2 to increase, the driving current
`I,
`increases. On the contrary, when the value of the driving
`current It is larger than that of the reference current Ir,
`the drain Voltage V.N. of the transistor N2 increases So that
`the drain voltage V output by the transistor N3 decreases.
`Since the decrease of the drain Voltage V.N. makes the gate
`voltage of the first transistor 21 in the first circuit block 2 to
`decrease, the driving current IoE decreases. Therefore, the
`organic light emitting diode active driving System with
`current feedback in accordance with the invention assures
`that the value of the driving current I
`equals to that of
`the reference current If. Thereby, the OLED array or FPD
`made up of OLEDS can achieve uniform light emission.
`0027. While the present invention has been particularly
`described, in conjunction with Specific examples, it is evi
`dent that many alternatives, modifications and variations
`will be apparent to those skilled in the art in light of the
`foregoing description. It is therefore contemplated that the
`appended claims will embrace any Such alternatives, modi
`fications and variations as decreaseing within the true Scope
`and Spirit of the present invention.
`What is claimed is:
`1. An organic light emitting diode active driving System
`with current feedback for driving an organic light emitting
`diode, comprising:
`a first transistor having a current carrying electrode con
`nected to a cathode of Said organic light emitting diode,
`a current carrying electrode connected to ground, and a
`gate electrode,
`
`a Second transistor having a current carrying electrode
`connected to a gate electrode of Said first transistor, a
`current carrying electrode as a data Signal input termi
`nal, and a gate electrode as a Scan Signal input terminal;
`a capacitor connected between Said gate electrode of Said
`first transistor and ground as a Storage element;
`a current comparator having two comparison terminals
`and an output terminal connected to Said data Signal
`input terminal;
`a third transistor having a current carrying electrode
`connected to an anode of Said organic light emitting
`diode, a current carrying electrode connected to a
`comparison terminal of Said current comparator, and a
`gate electrode connected to Said Scan signal input
`terminal; and
`a fourth transistor having a current carrying electrode
`connected to an anode of Said organic light emitting
`diode, a current carrying electrode connected to a first
`Supply Voltage, and a gate electrode for receiving a
`reverse signal of Said Scan Signal input terminal;
`wherein Said two comparison terminals of Said current
`comparator respectively receive a reference current
`with a predetermined value and a driving current flow
`ing through Said organic light emitting diode, compare
`Said reference current and Said driving current, and
`output a voltage to Said data input terminal in response
`to a comparison result, So that the value of Said driving
`current equals to that of Said reference current.
`2. The organic light emitting diode active driving System
`with current feedback as in claim 1, wherein Said current
`comparator comprises:
`a fifth transistor having a drain for receiving Said refer
`ence current, a Source connected to ground, and a gate
`connected to Said drain of Said fifth transistor;
`a sixth transistor having a drain for receiving Said driving
`current, a Source connected to ground, and a gate
`connected to Said gate of Said fifth transistor, and
`a Seventh transistor having a drain connected to Said data
`input terminal, a Source connected to ground, and a gate
`connected to Said drain of Said Sixth transistor.
`3. The organic light emitting diode active driving System
`with current feedback as in claim 2, wherein Said fifth, Sixth,
`and Seventh transistors are n-channel transistors, and a
`threshold voltage of said fifth transistor equals to that of said
`Sixth transistor.
`4. The organic light emitting diode active driving System
`with current feedback as in claim 2, wherein Said current
`comparator further comprises:
`an eighth transistor having a drain connected to a refer
`ence current Source and as a comparison terminal of
`Said current comparator, a Source connected to a Second
`Supply Voltage, and a gate connected to Said drain of
`Said eighth transistor;
`a ninth transistor having a drain connected to Said drain of
`Said fifth transistor, a Source connected to Said Second
`Supply Voltage, and a gate connected to Said gate of Said
`eighth transistor.
`5. The organic light emitting diode active driving System
`with current feedback as in claim 4, wherein Said eighth and
`
`LG Display Co., Ltd.
`Exhibit 1011
`Page 007
`
`

`

`US 2002/0101172 A1
`
`Aug. 1, 2002
`
`ninth transistors are p-channel transistors, which constitute
`a current mirror Structure, and a threshold Voltage of Said
`eighth transistor equals to that of Said ninth transistor.
`6. The organic light emitting diode active driving System
`with current feedback as in claim 2, wherein Said current
`comparator further comprises:
`a tenth transistor having a drain connected to Said current
`carrying electrode of Said third transistor as another
`comparison terminal of Said current comparator, a
`Source connected to Said Second Supply Voltage, and a
`gate connected to Said drain of Said tenth transistor,
`a eleventh transistor having a drain connected to Said
`drain of Said Sixth transistor, a Source connected to Said
`
`Second Supply Voltage, and a gate connected to Said
`gate of Said tenth transistor.
`7. The organic light emitting diode active driving System
`with current feedback as in claim 6, wherein Said tenth and
`eleventh transistors are p-channel transistors, which consti
`tute a current mirror Structure, and a threshold Voltage of
`Said tenth transistor equals to that of Said eleventh transistor.
`8. The organic light emitting diode active driving System
`with current feedback as in claim 1, wherein Said first,
`Second, third, and fourth transistors are n-channel transis
`torS.
`
`LG Display Co., Ltd.
`Exhibit 1011
`Page 008
`
`