Patent Application
20100020061
The present invention relates to a display device using capacitive elements, for example, organic EL (electro-luminescence) elements and the like as display pixels and a drive circuit thereof.
A display using a light-emitting display panel composed of light-emitting elements disposed in matrix is being widely developed. Attention is paid to an organic EL element, which uses an organic material for a light-emitting layer, as a light-emitting element used to the display panel. This is based on such a background that the efficiency of an EL element is enhanced to a level capable of withstanding a practical use and the life thereof is prolonged by using an organic compound, to which excellent light-emitting characteristics are expected, for a light-emitting layer of the EL element.
The above organic EL element is basically formed by laminating a transparent electrode constituting an anode, a light-emitting function layer containing an organic compound, and, for example, a metal electrode constituting a cathode on a transparent substrate. Accordingly, the organic EL element can be electrically replaced by a light-emitting element having diode characteristics and a parasitic capacitance component coupled in parallel with the light-emitting element, and thus it can be said that the organic EL element is a capacitive light-emitting element.
A passive drive type display panel, on which organic EL elements are disposed in matrix, has been partly in practical use as a display panel using the organic EL element.
A display panel 1 is composed by disposing M pieces of signal lines (hereinafter, also referred to as anode lines) A1 to AM in a longitudinal direction, disposing N pieces of scan lines (hereinafter, also referred to as cathode lines) K1 to KN in a lateral direction, and disposing organic EL elements (hereinafter, also referred to as light-emitting elements) E11 to EMN shown by parallel coupling members of diodes as light-emitting elements and capacitors as parasitic capacitances at the respective intersecting points (M×N positions in total) of the signal lines and the scan lines.
Then, one ends of the respective EL elements E11 to EMN which constitute pixels (anode terminals in equivalent diodes of the EL elements) are connected to the anode lines, and the other ends (cathode terminals in the equivalent diodes of the EL elements) are connected to the cathode lines in correspondence to the respective intersecting points of the anode lines A1 to AM along a vertical direction and the cathode lines K1 to KN along a horizontal direction. Further, the respective anode lines A1 to AM are connected to an anode line drive circuit 2 as a data driver, the respective cathode lines K1 to KN are connected to a cathode line scan circuit 3 as a scan driver, and they are driven by the drivers, respectively.
The anode line drive circuit 2 has constant current sources I1 to IM operating by making use of a drive voltage Vah and drive switches Sa1 to SaM and acts so that the currents supplied from the constant current sources I1 to IM are supplied to the respective EL elements E11 to EMN disposed in correspondence to the cathode lines by connecting the drive switches Sa1 to SaM to the constant current sources I1 to IM sides. Further, when the drive switches Sa1 to SaM do not supply the currents from the constant current sources I1 to IM to the respective EL elements, they are arranged to be connected to a ground side using the anode line as a reference potential point.
In contrast, the cathode line scan circuit 3 has scan switches Sk1 to SkN in correspondence to the respective cathode lines K1 to KN and acts to apply any one of a reverse bias voltage Vkh for preventing crosstalk emission or a ground potential as the reference potential point to a corresponding cathode line. With this operation, the cathode line scan circuit 3 acts to cause the respective EL elements to selectively emit light by connecting the constant current sources I1 to IM to desired ones of the anode lines A1 to AM while setting the cathode lines to the reference potential point (ground potential) at a predetermined cycle.
Note that, in the state shown in
Incidentally, the EL elements constituting the display panel have the parasitic capacitances described above. Thus, as to an example in which, for example, N pieces of the EL elements are connected to one signal line (anode line), synthesized capacitances N times as large as the respective parasitic capacitances are connected to the signal line as a load capacitance when viewed from the signal line.
Accordingly, a problem arises in that the current from the anode lines is consumed to charge the load capacitance at the beginning of a scan period, a time delay occurs to charge the load capacitance until the light emission threshold voltage (Vth) of the EL elements is sufficiently exceeded, and eventually the start-up of emission of the EL elements is delayed. In particular, when the constant current sources I1 to IM are used as a drive source as described above, since the constant current sources are a high impedance output circuit from the operating principle thereof, a current is restricted and thus the start-up of emission of the EL elements is prominently delayed.
Further, since the capacitances parasitizing to the EL elements of the non-selected scan lines are connected in parallel when a parasitic capacitance of one pixel is shown by Cpix, the capacitances can be shown by (N−1)·Cpix. Accordingly, the equivalent circuit shown in
In a circuit arrangement shown in
Further, the amount of flowing-out current is more increased when the non-selected scan lines have larger parasitic capacitances (N−1)·Cpix (when the number of the scan lines is larger). As a result, the emission of the EL elements connected to the selected scan line is delayed as shown by Is in
There are proposed a “peak boot method” and a “rush current method” as means for improving the emission start-up characteristics of the EL elements to be scanned, and these methods are disclosed in Patent Documents 1 and 2 and the like applied by the present applicants.
Patent Document 1: Japanese Patent Application Laid-Open Publication No. 9-232074
Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2002-229512
Note that the “rush current method” is arranged such that the parasitic capacitances of all the EL elements are reset to a constant charge amount each time a scan is performed, and there are proposed a “Vm-Vm reset method”, a “GND-GND reset method”, a “Vm-Vr reset method”, and the like depending on a type of a potential applied to both the ends of the EL elements.
Incidentally, according to a drive unit of the conventional display panel shown in
Further, when the constant current sources are arranged as an IC chip, since it is difficult to reduce the size thereof, an increase of cost is indispensable. Further, the constant current must have a certain degree of voltage drop to provide it with constant current characteristics, which acts as a factor for causing a power loss.
To solve the problems described above, it is also contemplated to drive the EL elements by a constant voltage. However, in this case, the degree of light emitting luminance of the EL elements is extremely changed by the environmental temperature and the change over time, from which a problem arises in that the EL elements cannot be practically used unless a countermeasure is employed.
Accordingly, an object of the present invention is to provide a display device and a method of driving the display device which simplify an arrangement of the data driver by providing a function which is replaced with the constant current sources by positively utilizing the operating principle of the “rush current method”.
Subsequently, as shown in
Accordingly, the anode terminal voltage of the selected EL element reaches a voltage near to Vkh as shown in
An amount of the charge Q injected to the parasitic capacitances of the EL elements at the time can be shown as follows.
Q=(N−1)·Cpix·(Vkh−Vth) (Expression 1)
Note that, for the purpose of simplification, N is set to a sufficiently large value and the parasitic capacitances Cpix of the selected EL elements are omitted.
Further, when it is assumed Vth≈Vf (Vf is a forward voltage of the EL elements) in Expression 1 in consideration of diode characteristics of the EL elements, Q can be shown as follows.
From Expression 2, the amount of charge injected to the selected EL element can be set to a constant value by setting Vkh to a value sufficiently larger than ΔVf which is an amount of change of Vf caused by an environment temperature and a change over time. More specifically, a drive near to a constant current drive can be realized. Further, Expression 2 suggests that the amount of charge injected to the selected EL element can be changed by the number N cathode lines for realizing a rush current and the cathode voltage Vkh, and it is considered that this can be applied to a dimmer control for controlling the overall brightness of the entire display surfaces, and the like.
As disclosed in first aspect of the invention, a display device according to the present invention, to which the basic principle achieved to solve the above problems is applied, has a plurality of signal lines and a plurality of scan lines which intersect with each other, a plurality of light-emitting elements connected between the plurality of signal lines and the plurality of scan lines, respectively at the respective intersecting points of the scan lines and the signal lines, a pulse power supply connected to at least one of the scan lines for outputting a pulse voltage, a data driver having a plurality of data selection means for selectively connecting the signal lines to non-light-emitting terminals connected to a non-light-emitting potential or to light-emitting terminals connected to rectifying means for interrupting a current flowing from the signal lines, and a scan driver having scan selection means for selectively connecting the scan lines to scan terminals connected to a scan selection potential.
Further, as described in eleventh aspect of the invention, a method of driving a display device made in order to solve the above problem is a method of driving a display device which has a plurality of signal lines and a plurality of scan lines which intersect with each other and a plurality of light-emitting elements connected between the plurality of signal lines and the plurality of scan lines, respectively at the respective intersecting points of the scan lines and the signal lines, and there are executed a scan line setting operation for setting a scan line to be scanned to a scan selection potential as well as applying a pulse voltage to at least one of the scan lines which are not scanned, and a light emission drive operation for supplying a rush current from the scan line to which the pulse voltage is applied to a light-emitting element to be emitted through a light-emitting element which is connected to a signal line to be selected and is not emitted a plurality of times in one scan period.
A display device according to the present invention will be explained below based on embodiments shown in the figures.
A data driver 2 of the embodiment shown in
In contrast, a scan driver 3 has switches Sk1 to SkN as scan selection means for selectively connecting respective scan lines K1 to KN disposed to the display panel 1 to scan terminals. More specifically, the scan terminals are connected to the ground potential as a scan selection potential, thereby the respective scan lines K1 to KN can be alternatively set to the ground potential.
Further, in the embodiment shown in
In the embodiment shown in
Note that
Subsequently, when a pulse voltage from the pulse power supply 4 falls as shown in
Here, when
More specifically, when the selected EL elements are emitted, (A) and (B) of
Accordingly, the gradation of the EL elements to be emitted can be controlled by controlling how many times the transitions shown in (A) and (B) of
Note that in the arrangement shown in
In contrast, the dimmer control can be realized by variably controlling the amplitude (peak value), the frequency, or the pulse width (DUTY) of the pulse voltage supplied from the pulse power supply 4. Further, since the current value of the rush current is changed depending on the number of scan lines to which the pulse voltage is applied, the dimmer control can be also performed by variably controlling the number of the scan lines to which the pulse voltage is applied.
In this case, since dimmer control means controls the switches Sk1 to SkN as the scan selection means in the scan driver 3 shown in
Incidentally, when the selected EL elements are caused not to emit light, sine the rush current directly flows to the non-light-emitting potential of the data driver 2, that is to say, to the ground potential as shown in
The power recovery circuit 5 is composed of a timing switch S0 and a capacitor C0 having a large capacitance. The timing switch S0 is switched in synchronization with the times at which a pulse voltage supplied from a pulse power supply 4 rises and falls, and the capacitor C0 accumulates the rush current which tends to directly flow to the ground potential through the timing switch S0.
Then, a terminal voltage of the capacitor C0 is supplied to a booster circuit 6 such as a DC-DC converter and the like. With this arrangement, the rush current discharged to a reference potential point (ground) can be utilized as a part of a power supply for driving a display device.
As can be understood from the above explanation, the timing switch S0 is connected to the capacitor C0 side when the pulse voltage of the pulse power supply 4 rises. At the time, as shown in
In contrast, the timing switch S0 is connected to the ground potential when the pulse voltage in the pulse power supply 4 falls. With this operation, a GND-GND reset shown
In an arrangement shown in
In contrast, a scan driver 3 has switches Sk1 to SkN as scan selection means for selectively connecting respective scan lines K1 to KN disposed to the display panel 1 to scan terminals. More specifically, the scan terminals are connected to the ground potential as a scan selection potential, thereby the respective scan lines K1 to KN can be alternatively set to the ground potential.
Note that
According to an arrangement shown in
According to the arrangement shown in
Further, when the capacitances of the respective capacitors C1 to CM are selected and adjusted in correspondence to the side where the switches Sk1 to SkN as the scan selection means are disposed, a phenomenon that the luminance of the EL elements is changed by the influence of a resistance existing in the scan lines, that is to say, the appearance of inclined luminance of the EL elements due to a cathode wiring resistance can be effectively suppressed.
Further, even in a mode in which pixels are not disposed in a dot matrix state in the display panel and which has different light emitting areas as in a segment and an icon, the balance of respective light emission luminances can be appropriately set by selecting the capacitances of the capacitors C1 to CM in correspondence to the mode.
Note that capacitive light-emitting elements such as EL elements and the like may be also used in place of the respective capacitors C1 to CM. When the light-emitting elements are caused to act as the respective capacitors C1 to CM, it is possible to perform a film forming process of EL elements acting as the respective capacitors C1 to CM simultaneously with a film forming process of respective EL elements when a display panel is molded. In this case, it is preferable to form masks on the upper surfaces of the EL elements and the like acting as the respective capacitors as necessary to prevent that they emit unnecessary light.
In the arrangement shown in
More specifically, the switches Sk1 to SkN as the scan selection means disposed to the scan driver 3 are provided with terminals for introducing the positive pole voltage of the voltage source E1. A negative terminal of the voltage source E1 is connected to the ground as a reference potential point of a circuit. When the switches Sk1 to SkN disposed to the scan driver 3 and switches Sa1 to SaM as data selection means disposed to the data driver 2 are selected such that they are placed in a state shown in
When the operation for applying the reverse bias voltage to the respective EL elements E11 to EMN is instantly performed to, for example, each one or each several frames, an effect of prolonging the life of the respective EL elements can be obtained without substantially adversely affecting an image displayed on the display panel 1.
Then, in the embodiment shown in
Note that since the display panel denoted by a reference numeral 1 and a data driver denoted by a reference numeral 2 in
In the arrangement shown in
When the selection switch S1, switches Sk1 to SkN as scan selection means disposed to a scan driver 3, and switches Sa1 to SaM as data selection means disposed to the data driver 2 are selected such that they are placed in a state shown in
The operation for applying the reverse bias voltage to the respective EL elements E11 to EMN is instantly performed to, for example, each one or each several frames as explained based on
More specifically, the display panel 1 shown in
In the EL elements disposed as described above, the amplitude (peak value) of a pulse voltage from a pulse power supply 4 is changed depending on the light emitting characteristics (light emitting efficiency) of the EL elements for emitting the respective colors. The amplitude of the pulse voltage from the pulse power supply 4 is changed in synchronization with a timing at which a scan of the scan lines is switched by the scan driver 3. As a result, an appropriate color balance can be realized because a pulse voltage having an optimum level to each emitted color can be applied.
Note that this is not limited to the case that the amplitude of the pulse voltage from the pulse power supply 4 is changed, and even if the frequency or the pulse width of the pulse voltage from the pulse power supply 4 is changed for each scan, the same effect can be obtained.
The precharge circuit 7 has a precharge switch S2 and a voltage source E2. The precharge switch S2 is switched in synchronization with the times at which a pulse voltage from a pulse power supply 4 rises and falls, and the voltage source E2 supplies a precharge voltage to a non-light-emitting line (ground side terminal) of a data driver 2 through the precharge switch S2. Note that the voltage value of the precharge voltage source E2 is set to a value approximately equal to or somewhat higher than the light emission threshold voltage (Vth) of EL elements.
Then, the precharge switch S2 is connected to the precharge voltage source E2 side when the pulse voltage of the pulse power supply 4 rises. With this operation, the voltage source E2 precharges a voltage in a forward direction to an EL element to be emitted in synchronization with the rush current supplied when the pulse voltage from the pulse power supply 4 rises.
Accordingly, since the EL element to be emitted is supplied with the rush current while being applied with the voltage in the forward direction from the voltage source E2, it can securely repeat a light emitting operation by the rush current.
In contrast, the precharge switch S2 is connected to a ground potential when the pulse voltage from the pulse power supply 4 falls. With this operation, a GND-GND reset shown
Note that, in the embodiments explained above, the change over time and the temperature dependency of the EL elements of the display panel can be compensated by appropriately controlling the amplitude (peak value), the frequency, and the pulse width of the pulse voltage of the pulse power supply 4 in accordance with the actually used time and the environmental temperature of the EL elements.
In the embodiments explained above, although a gradation control is realized by switching the switches Sa1 to SaM as the data selection means of the data driver, the gradation can be provided with gamma characteristics by appropriately controlling the number of the scan lines to which the pulse voltage is applied in addition the above operation.
Further, in the embodiments explained above, although the pulse voltage is applied to the plurality of scan lines from the one pulse power supply 4, each of the scan lines may be provided with the pulse power supply 4.
Further, the embodiments explained above show the example using the organic EL elements as the light-emitting elements disposed to the display panel. However, the display device according to the present invention can be also applied to a display device using other display panel provided with capacitive light-emitting elements having diode characteristics.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2006/321239 | 10/25/2006 | WO | 00 | 7/2/2009 |
