# Patent

## Title
Method of driving display panel and drive for carrying out same

## Abstract

A drive of a display panel, according to the invention, comprises first switching means SEGm for changing over between connection of the respective data lines SWsm to the side of respective variable current sources and connection thereof to a grounding side, second switching means SWc2 for changing over a potential of the respective scanning lines between a power supply potential VC and a grounding potential, a drive control circuit for controlling the first switching means and second switching means correspondingly to input data, a comparison circuit provided in each of the data lines, for outputting a control signal by comparing a potential from a voltage regulator with a potential of the respective data lines, and a current control circuit for controlling a current of the variable current source provided in each of the data lines based on results of comparison executed by the respective comparison circuit.

## Background
FIELD OF THE INVENTION
The present invention relates to a method of driving a panel display and a drive for carrying out the method, and in particular, to a method of driving an organic EL panel, and a drive for carrying out the method.
BACKGROUND OF THE INVENTION
As shown in FIG. 1A, a driving circuit of an organic EL panel generally has a constant current source 11 and switching means SWs1-SWsm, respectively, for every data line, and a cathodic power supply potential VC and switching means SWc1-SWcn, respectively, for every scanning line, against the organic EL panel having an organic EL element PEm,n, disposed at respective crossover points of a plurality of the data lines (anodic lines SEG1-SEGm) and a plurality of the scanning lines (cathodic lines COM1-COMn). These switching means are controlled by a drive control circuit 10 and can be turned into select state or unselect state, respectively.
In common operation to cause the organic EL panel to emit light for displaying, the switching means SWcn of the respective scanning lines COMn is turned ON (connected to a grounding potential VG) and OFF (connected to the cathodic power supply potential VC) in such a manner as to have operation waveforms shown in FIG. 2 at a predetermined time interval, thereby sequentially selecting panel rows to be lighted. At this time, the switching means SWsm of the data line SEGm connected to the organic EL element PEm,n to be lighted, in the panel row selected, is turned ON, and current is supplied thereto, whereupon the organic EL element PEm,n is caused to emit light.
Since emitted light luminance of the organic EL element PEm,n is dependent on a current value, values of current supplied to the respective data lines SEGm are required to be constant values equal to each other in order to avoid display unevenness.
In order to obtain a constant current, it is desirable that the driving circuit is under small effects of its dependency on an output voltage of the constant current source, a power supply voltage, manufacturing variations in constituent elements thereof, or so on.
A common structure of the organic EL element is as shown in FIG. 1B. Because a transparent, electrically conductive film (ITO film) as a constituent member thereof has resistance as large as about 10 to 20 Ω/•, the same is used on the side of the anodic data lines SEGm where a large current does not flow (on the order of several hundred μA to 1 mA) while a resistance material such as Al is used on the side of the cathodic scanning lines COMn.
However, when causing all the elements in panel rows to emit light, a large current of several tens of mA flows in the direction of the grounding potential VG in the scanning lines COMn via the switching means SWc1-SWcn.
Even in the case of the scanning lines COMn using a resistance material such as an Al cathodic wiring, there flows a large current corresponding to the panel element connected thereto and a current value necessary for light emission, so that a voltage applied to the panel element PEm,n positioned at a more distal end in relation to the grounding potential VG becomes very high.
Assuming that resistance of the scanning lines COMn is Rm,n, a current flowing through the resistance is Icm,n, ON resistance of the switching means SWcn is SWrn, and a voltage applied to the organic EL element PEm,n when all the panel elements emit light is Vm,n as shown in FIG. 3, the following equation results:

Vm,n=VC+SWrn*Ic1,n+R1,n*Ic1,n+R2,n*Ic2,n+ . . . +Rm,n*Icm,n

Herein, assuming that light-emitting display panel rows are 128 rows, resistance between the panel elements is Rm,n=r (Ω), and a current supplied to respective data lines SEGm is Im=i (A), the following equation results:




  
    
      
        
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That is, there occurs a potential as high as 8256 ri (V) owing to the resistance component of the scanning lines COMn.
Thus, since the farther from the grounding potential VG the EL element PEm,n is positioned at a distal end, the smaller a potential difference ΔV11 applied to the respective constant current sources 11 becomes, there have been cases where it becomes impossible to supply a constant current, depending on conditions such as dependency of the respective constant current sources 11 on output voltage, a constant current value, and a drive power supply voltage Vs.
Further, there is a tendency of an increase in the number of bits of a driver IC following an increase in the size of a panel screen, and such an increase in the number of the bits poses a problem in that not only deterioration in display unevenness, due to manufacturing variations, is brought about but also constant current characteristic dependent on resistance on the panel described above becomes susceptible to occurrence of faults.
SUMMARY OF THE INVENTION
The invention has been developed to resolve the problems encountered in the past, and it is an object of the invention to provide a method of driving a display panel, capable of preventing light emission faults from occurring to a panel by implementing stable supply of a constant current, and a drive for carrying out the method.
The invention provides in its first aspect a method of driving a display panel made up of (n×m) pieces of display elements each disposed at respective crossover points of a matrix, formed of n rows of scanning lines and m columns of data lines, wherein a current value of respective variable current sources for driving the respective data lines is controlled by comparing a potential of the respective data lines with a reference potential and based on results of such comparison.
Further, in accordance with a second aspect of the invention, there is provided a drive of a display panel comprising means for assuming during a display period of present display data a current correction value for each of the data lines in a succeeding display period on the basis a position of the date line, the number of the display elements, and a fixed value determined by the position of the date line, and current correction means for correcting a current value of the respective variable current sources on the basis of results of such assumption.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a circuit diagram for illustrating a conventional technology;
FIG. 1B is a schematic representation showing the construction of an organic EL element by way of example;
FIG. 2 is a waveform chart showing driving operation of a panel;
FIG. 3 is a circuit diagram for illustrating problems encountered by conventional technology;.
FIG. 4 is a circuit diagram of a drive of a display panel, according to a first embodiment of the invention;
FIG. 5 is a circuit diagram of a drive of a display panel, according to a second embodiment of the invention;
FIG. 6 is a detailed circuit diagram showing a variable current source 12, current control circuit 15m, and the periphery thereof, according to the first embodiment of the invention; and
FIG. 7 is a detailed circuit diagram showing a variable current source 12, current correction circuit 18m, and the periphery thereof, according to the second embodiment of the invention.



PREFERRED EMBODIMENTS OF THE INVENTION
Embodiments of the invention are described in detail hereinafter with reference to the accompanying drawings.
First Embodiment
FIG. 4 is a circuit diagram of a drive of a display panel, according to a first embodiment of the invention. As shown in the figure, a comparator 14m capable of comparing voltage levels with each other is connected to respective data lines SEGm. The respective comparators 14m are connected to a voltage regulator 13 for generating a reference voltage. An output of the respective comparators 14m is connected to respective current control circuits 15m for controlling respective variable current sources 12. Assuming that current variation occurs when a voltage applied to the respective variable current sources 12 is ΔV12, the reference voltage of the voltage regulator 13 is set to a power supply voltage Vs-ΔV12. The respective comparators 14m are made up of a differential amplifier.
Operation of the circuit of a configuration as described above is described hereinafter. Normal operation to cause the panel to emit light for displaying is executed by turning switching means SWcn of respective scanning lines COMn ON (connected to a grounding potential VG) and OFF (connected to a cathodic power supply potential VC) in such a manner as to have operation waveforms shown in FIG. 2 at a predetermined time interval, thereby sequentially selecting panel rows to be lighted. At this time, switching means SWsm of the data line SEGm connected to a panel element PEm,n to be lighted, in the panel row selected, is turned ON, and current is supplied thereto, whereupon the panel element PEm,n is caused to emit light.
At this time, a current at as large as several ten mA flows through the respective scanning lines COMn in the direction of the grounding potential VG via the switching means SWcn1-SWcn, respectively. Accordingly, a voltage applied to the panel element PEm,n disposed at a distal end from the grounding potential VG becomes very high.
Assuming that resistance of the scanning line COMn is Rm,n, a current flowing through the resistance is Icm,n, ON resistance of the switching means SWcn is SWrn, ON resistance of the switching means SWsm is SWrm and a voltage applied to the organic EL element PEm,n when the panel in whole emits light is Vm,n, the following equation results:

Vm,n=VC+SWrn*Ic1,n+R1,n*Ic1,n+R2,n*Ic2,n+ . . . +Rm,n*Icm,n+SWrm*Icm,n

so that when the applied voltage Vm,n of the respective data lines SEGm becomes higher than an output voltage of the voltage regulator 13, a decrease in current is detected by the respective comparators 14m made up of the differential amplifier, thereby increasing current of the respective variable current sources by the agency of the respective current control circuits 15m. 

Further, when an excessive increase in current causes the voltage to drop, and Vm,n becomes lower than the output voltage of the voltage regulator 13, an increase in current is detected by the respective comparators 14m, thereby decreasing the current of the respective variable current sources by the agency of the respective current control circuits 15m. 
FIG. 6 is a detailed circuit diagram showing the variable current source 12, the current control circuit 15m, and the periphery thereof. The variable current source 12 comprises a PMOS transistor 12Mm for supplying a constant current at the time of normal constant current operation and a transistor 12Sm for adjustment of the constant current. The PMOS transistor 12Mm generates the constant current by applying a constant voltage to the gate thereof.
The current control circuit 15m comprises an NMOS transistor switch 15ms, an NMOS resistance 15mn with the gate thereof connected to the data line voltage Vm,n in common with the gate of a PMOS resistance 15mp, and other resistances, and an output 15mout of the current control circuit 15m is set such that when the switch 15ms is ON, the transistor 12Sm can supply necessary current corresponding to the data line voltage Vm,n (A resistance ratio of the current control circuit 15m is set such that the transistor 12Sm for current adjustment operates in a liner region when it is within a range of the voltage Vm,n, requiring current adjustment. The output 15mout is changed by the NMOS resistance 15mn and PMOS resistance 15mp changing respective resistance values correspondingly to the voltage Vm,n, thereby adjusting a current value of the PMOS transistor 12Sm).
When the voltage Vm,n of the data line SEGm becomes higher than an output voltage 13 out of the voltage regulator 13 (that is, when a voltage between the source and drain of the PMOS transistor 12Mm becomes lower, resulting in a decrease of current), the decrease of current is detected by the comparator 14m made up of the differential amplifier. The comparator 14m turns ON the NMOS transistor switch 15ms of the current control circuit 15m, whereupon a current 115m flows in the current control circuit 15m, and the output voltage 15mout of the current control circuit 15m becomes lower, so that the PMOS transistor 12Sm of the variable current source 12 is turned into ON state, thereby increasing the current of the variable current source 12.
Thus, since the current can be increased or decreased by detecting variation in current, due to insufficiency in potential applied to the current source, the present embodiment is effective for reducing light emission faults of the panel.
Second Embodiment
FIG. 5 is a circuit diagram of a drive of a display panel, according to a second embodiment of the invention. As shown in FIG. 5, there is provided a light-emitting bit number detection circuit 16 (which can be made up of, for example, an adder) for detecting the number of light-emitting bits for a succeeding light-emitting period on the basis of data determining light-emission and non light-emission of respective panel elements. Further, there is provided a VO detection circuit 17m (which can be made up of, for example, a subtracter and an adder) for assuming and detecting a level of a voltage applied to the panel element for each of data lines SEGm and the respective VO detection circuits 17m are connected with the light-emitting bit number detection circuit 16. The respective VO detection circuits 17m are connected to respective current correction circuits 18m so as to be able to control current of respective variable current sources 12. The respective current correction circuits 18m are preset so as to be able to execute current correction by stages (for example, for every 10 μA) taking into account a voltage ΔV12 applied to the respective variable current sources 12, a panel resistance value, and dependency thereof on a constant current value.
Operation of the circuit of the drive in FIG. 5 is described hereinafter. Normal operation to cause a panel to emit light is executed by turning switching means SWcn of respective scanning lines COMnON (connected to a grounding potential VG) and OFF (connected to a cathode power supply potential VC) in such a manner as to have operation waveforms shown in FIG. 2 at a predetermined time interval, thereby sequentially selecting panel rows to be lighted. Switching means SWsm of the data line SEGm connected to a panel element PEm,n to be lighted, in the panel row selected, is turned ON, and current is supplied thereto, whereupon the panel element PEm,n is caused to emit light.
Display data in a display period between time t4 and t5 are normally transferred in a period between time t2 and t3 and are latched before stored in a register, and the light-emitting bit number detection circuit 16 detects the number d of display elements in a subsequent display period from the display data. The respective VO detection circuits 17m assume and detect a voltage generated depending on panel resistance for each of the data lines on the basis of the display data.
Assuming that, for example, in case all m bits emit light (d=m) as shown in FIG. 5, resistance of the scanning line COM1 formed of a conductor film, up to the data line SEG1, is R1,1, a constant current flowing through the respective data lines is I, a voltage applied to the panel element PE1,1, across the resistance, is V1,1, a current proportional to the number d of the display elements flows through R1,1. That is, V1,1=R1,1*m*I.
Similarly, the following equations result:

V2,1=V1,1+R2,1*(m−1)*I.

V3,1=V2,1+R3,1*(m−2)*I.

V4,1=V3,1+R4,1*(m−3)*I

V5,1=V4,1+R5,1*(m−4)*I

Assuming that resistance Rm,n between the respective data lines is all identical, the following equation results:

V1,1=α*m(α is a constant).

Similarly, the following equations result:

V2,1=α*(2m−1)

V3,1=α*(3m−3)

V4,1=α*(4m−6)

V5,1=α*(5m−10)

Accordingly, only a value A found from Vm,n=α*A is sufficient for detection by the respective VO detection circuits 17m. 
If the value A of any of the data line SEGm becomes higher than a level value B (the value B is a value pre-calculated from the voltage ΔV12 applied to the respective variable current sources 12, a panel resistance value, and the dependency on the constant current value) set in the current correction circuit 18m, the current is increased by +10 μA by the agency of the current correction circuit 18m. Further, if the value A of the data line SEGm becomes higher than a level value C set in the current correction circuit 18m, the current is further increased by +10 μA (20 μA in total) by the agency of the current correction circuit 18m. 
Thus, current correction to be made for a succeeding display period is determined during a preceding display period, thereby enabling a current as desired to be applied immediately upon start of a display period.
FIG. 7 is a detailed circuit diagram showing the variable current source 12, the current correction circuit 18m, and the periphery thereof. The variable current source 12 comprises a current source PMOS transistor 12Mm for supplying the constant current at the time of normal constant current operation and PMOS transistors 12Sm1, 12Sm2, for adjustment of the constant current. The current source PMOS transistor 12Mm generates the constant current by applying a constant voltage to the gate thereof.
The current correction circuit 18m comprises a plurality of digital comparators 18mdc1, 18mdc2 . . . , thereby presetting correction levels B, C, . . . , respectively. Respective outputs of the digital comparators control switching circuits 18SW1, 18SW2, . . . , respectively, thereby changing over respective voltages of the PMOS transistors 12Sm1, 12Sm2 of the variable current source 12 between a power supply voltage Vs and an output voltage of a constant voltage regulator 18mvr. The constant voltage regulator 18mvr outputs the voltage for controlling the PMOS transistors 12Sm1, 12Sm2, respectively. This control voltage is set so as to enable, for example, the PMOS transistor 12Sm1 to allow a current of 10 μA to flow therethrough.
The VO detection circuits 17m each are provided with an adder-subtractor, executing binary calculation. If a display position corresponds to an m-th bit from the side of the switching means SWcn1-SWcn, the following calculation is made based on the number d (binary number) of bits, as detected by the light-emitting bit number detection circuit 16:

A=m*d−β(β is a fixed value determined by m)

If, for example, a value A of any of the data lines SEGm becomes larger than the level value B as set in the current correction circuit 18m (that is, it is determined that the voltage Vm,n of the data line SEGm as calculated from the number of the light-emitting elements causes the constant current to decrease), the switching circuit 18SW1 is changed over by the comparator 18mdc1, and the constant voltage regulator 18mVR operates such that the output voltage thereof controls the gate of the PMOS transistors 12Sm1, thereby outputting the constant current.
If the value A of the data line SEGm becomes larger than the level value C as set in the current correction circuit 18m (that is, it is determined that the voltage Vm,n of the data line SEGm calculated from the number of the light-emitting elements causes the constant current to further decrease), the switching circuit 18SW2 is changed over by the comparator 18mdc2, and the constant voltage regulator 18mvr operates such that the output voltage thereof controls the gate of the PMOS transistors 12Sm2, and a current is further added to the current described above, thereby outputting the constant current.
Thus, since current can be increased by pre-assuming a decrease in current, due to the panel resistance, and detecting the same, it is possible to implement not only stable supply of current during the display period, but also fine adjustment of the current, so that the present embodiment is more effective for reducing light emission faults of the panel.

## Claims

1. A method of driving a display panel made up of (n×m) display elements respectively disposed at different crossover points of a matrix formed of n rows of scanning lines and m columns of data lines, the method comprising variably controlling respective constant current values for driving the respective data lines,
wherein said variably controlling the constant current values is implemented by individually comparing a reference voltage with a voltage of each of the respective data lines as driven by the constant current values, using respective comparators each having a first input connected to the reference voltage and a second input connected to a respective one of the data lines, and
wherein said variably controlling the constant current values is further implemented by supplying a constant current to the data lines by respective first transistors, and by supplying an adjustment current to the data lines by respective second transistors responsive to control signals output from the comparators.

2. The method of driving a display panel according to claim 1, wherein the display elements are organic EL elements.

3. A drive of a display panel for driving (n×m) display elements respectively disposed at different crossover points of a matrix formed of n rows of scanning lines and m columns of data lines, the display elements each having an anode connected to a respective one of the data lines and a cathode connected to a respective one of the scanning lines, the drive comprising:
first switching means for changing over between connection of the respective data lines to respective variable current sources and connection thereof to ground;
second switching means for changing over a potential of the respective scanning lines between a power supply potential and ground;
driving means for controlling the first switching means and second switching means responsive to input data;
comparison means respectively provided for each of the data lines, said comparison means each having a first input coupled to a respective one of the data lines and for outputting a control signal by comparing a reference voltage from a reference voltage generator with a potential of the respective one of the data lines; and
current control means for individually controlling respective current values flowing from the variable current sources to the respective data lines, based on respective results of comparison executed by the comparison means,
each of the variable current sources having a first transistor and a second transistor connected to the respective data lines, the first transistors supplying constant currents to the respective data lines, and the second transistors supplying adjustment currents to the respective data lines responsive to the control signals.

4. The drive of a display panel according to claim 3, wherein the comparison means detect a decrease in current of the respective variable current sources based on an increase in potential of the respective data lines to thereby control an increase of the current of the respective variable current sources, and detect an increase in the current of the respective variable current sources based on a drop in the potential of the respective data lines to thereby control a decrease of the current of the respective variable current sources.

5. The drive of a display panel according to claim 3, wherein the display elements are organic EL elements.

6. A drive of a display panel for driving (n×m) display elements respectively disposed at respective crossover points of a matrix formed of n rows of scanning lines and m columns of data lines, the display elements each having an anode connected to a respective one of the data lines and a cathode connected to a respective one of the scanning lines, the drive comprising:
a first switching unit that changes over between connection of the respective data lines to respective variable current sources and connection thereof to ground;
a second switching unit that changes over a potential of the respective scanning lines between a power supply potential and ground;
a drive control circuit that controls the first switching unit and the second switching unit responsive to input data;
comparators respectively provided for each of the data lines, the comparators each having a first input coupled to a respective one of the data lines, the comparators each output control signals by comparing a reference voltage from a voltage regulator with a potential of the respective one of the data lines; and
current control circuits respectively provided for each of the data lines, the current control circuits individually control current values flowing from the respective variable current sources to the respective one of the data lines, based on respective results of comparison by the comparators,
each of the variable current sources having a first transistor and a second transistor connected to the respective data lines, the first transistors supplying constant currents to the respective data lines and the second transistors supplying adjustment currents to the respective data lines responsive to the control signals.

7. The drive of a display panel according to claim 6, wherein the comparators detect a decrease in the current of the respective variable current sources based on an increase in the potential of the respective data lines to thereby control an increase of the current of the respective variable current sources, and detect an increase in the current of the respective variable current sources based on a drop in the potential of the respective data lines to thereby control a decrease of the current of the respective variable current sources.