This application is based on and incorporates herein by reference Japanese Patent Applications No. 2000-281657 filed Sep. 18, 2000 and No. 2001-157630 filed May 25, 2001.
The present invention relates generally to a driving method for driving luminous elements to emit light, and more particularly to a driving method for driving current injection type luminous elements having a capacitance component, such as an organic electroluminescent device (EL).
A conventional organic EL driving method is disclosed in U.S. Pat. No. 5,844,368 (JP-A-9-232074). This driving system, which is shown in
Scanning switches 21 through 2n for selecting either the supply voltage (VCC) or ground potential (O V) are connected to the cathode lines B1 through Bn for sequentially scanning the cathode lines. The ground potential (0V) is sequentially applied to the cathode lines B1 through Bn by scanning while switching switches 21 through 2n sequentially to the ground terminal side at fixed time intervals. Drive switches 31 through 3m for selecting the current source 11 through 1m, that is, the drive source, or the ground potential (0V), are connected to anode lines A1 through Am. Drive current is supplied to a luminous element at a desired anode-cathode intersection by connecting current source 11 through 1m to anode line A1 through Am by switching the drive switches 31 through 3m on and off in synchronism with the scanning switches.
Driving the luminous elements E1.2 and E1.3 to emit light is described by way of example below. When scanning switch 21 is switched to the ground side and a ground potential is applied to a first cathode line B1 as shown in
In addition, the reverse bias voltage VCC, which is equal to the source voltage potential, is applied to each of the cathode lines B2 through Bn. The reverse bias voltage VCC is not applied to the cathode line B1 being scanned in order to prevent erroneous emission.
Each of the luminous elements E1.1 through En.m connected at each intersection may be represented by a luminous element E having a diode characteristic and a parasitic capacitor C connected in parallel, as shown by the equivalent circuit in FIG. 11. However, this driving method has the following problems due to the parasitic capacitor C within the equivalent circuit.
When the cathode line B1 is scanned and the anode line A1 is not driven, parasitic capacitor C1.1 of the luminous element E1.1 connected to the cathode line B1 currently being scanned is not charged. However, other parasitic capacitors C2.1 through Cn.1 of luminous elements E2.1 through En.1 are charged in a direction shown in FIG. 12A.
It is assumed that the scanning position is shifted from the cathode line B1 to the next cathode line B2 and the anode lines A1 and A2 are driven in order to cause the luminous elements E2.1 and E2.2 to emit light. The state of the circuit when anode line A1 is driven to drive luminous element E2.1 to emit light is shown in
When luminous element E2.1 is driven to emit light, not only is the parasitic capacitor C2.1 of the luminous element E2.1 charged, but the parasitic capacitors C3.1 through Cn.1 of the luminous elements E3.1 through En.1 connected to the other cathode lines B3 through Bn also are charged because currents flow into the capacitors in the direction as indicated by arrows. On the other hand, when luminous element E2.2 is driven to emit light, only parasitic capacitor C2.2 of luminous element E2.2 is charged as shown in FIG. 12D. It will be noted because the charge causing luminous elements E2.1 and E2.2 to emit light differs greatly, the time needed for the end-to-end voltage of luminous elements E2.1 and E2.2 to reach the level required for the luminous elements to emit light also differs greatly. Accordingly, the brightness of luminous elements E2.1 and E2.2 differs, resulting in uneven luminance.
Another matrix driving method is disclosed in JPA-9-232073. This method, drives organic EL elements to emit light by connecting organic EL elements at the anode line and cathode line intersections of the grid. This method first resets all scanning lines to the same voltage potential when shifting to the next scanning line. This increases the build up speed from applying a voltage to emission.
This method is described next with reference to FIG. 13 through FIG. 15.
In
Accordingly,
That is, before shifting the scan from the cathode line B1 in
After discharging the electric charge stored in all of the luminous elements to zero, only the scanning switch 22, which corresponds to the cathode line B2, is switched to the side of 0V to scan the cathode line B2 as shown in FIG. 15. At the same time, drive switches 31 and 33 shunt the anode lines A1 and A3 to the current sources 11 and 13, and drive switches 32 and 34 through 3m are switched to the O V side to apply 0V to the other anode lines A2 and A4 through Am. As a result, only luminous elements E2.1 and E2.3 are biased in the forward direction in the case shown in FIG. 15. Thus, drive current flows from current sources 11 and 13 as shown by arrows so that only luminous elements E2.1 and E2.3 emit light.
Differences in the charge state (FIG. 12A and
It should be noted here that it is desirable to apply the reverse bias voltage to the luminous elements when driving luminous elements such as organic EL elements in order to increase the service life of these elements. That is, it is desirable in the above method to apply VCC to the cathode line, apply 0V to the anode line, and apply the reverse bias (−VCC) to each luminous element at least once each frame period.
The voltages applied to each luminous element in the above system is shown in FIG. 16. In the state shown in
It is understood from the voltage applied to luminous element E1.2 that the reverse bias voltage is applied in the period when luminous element E2.2, for example, is not emitting light.
However, with respect to the voltage applied to luminous element E1.1, there is no period in which the reverse bias is applied because all luminous elements on anode line A1 are emitting light. This is not desirable with respect to luminous element service life.
Furthermore, as shown by the equivalent circuit in
With the above driving method, however, the capacitance on the cathode line of the luminous elements to emit light next is also discharged, thus requiring more time to charge the luminous element driven to emit light next, and the build-up speed to emission is thus slow. Fast scanning is therefore not possible.
It is an object of the present invention to provide an improved driving method for driving luminous elements, which solves uneven luminance problems resulting from differences in the charge states of the luminous elements and at the same time applies the reverse bias to increase service luminous element life.
It is another object of the present invention to provide an improved luminous element driving method, which enables a fast build-up to emission and fast scanning.
According to the present invention, a luminous device has a luminous element provided at each intersection of anode lines and cathode lines arranged in a matrix. The anode lines are one of scan lines and drive lines and the cathode lines are the other of scan lines and drive lines. The luminous element provided at an intersection of a desired drive line is driven to emit light in synchronism with scanning while scanning the scan lines at a specific frequency. When switching the scanning line, at least one of the scanning lines is first connected to a first voltage, and the remaining scanning lines are connected at the same time to a second voltage that is different from the first voltage.
The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:
The present invention will be described below with reference to preferred embodiments.
(First Embodiment)
FIG. 1 through
In FIG. 1 through
Scanning switches 21 through 264 for selecting either the source voltage (VCC) or the ground potential (0V) are connected to cathode lines B1 through B64 in order to carry out sequential scanning. Drive switches 31 to 3256 for selecting current sources 11 through 1256, that is, the drive source, or ground potential (0V) are connected to the anode lines A1 through A256.
The operation for emitting light is described next with reference to FIG. 1 through FIG. 5. The operation described below will be explained by referring to an example in which luminous elements E1.1 and E1.2 are driven to emit light by scanning cathode line B1, then elements E2.1 and E2.3 are driven to emit light by shifting scanning to cathode line B2, and then elements E3.1 and E3.2 are driven to emit light by shifting scanning to cathode line B3. Further, in order to facilitate this description, the luminous elements emitting light are indicated by the diode symbol and the other luminous elements that are not emitting light are indicated by the capacitor symbol.
In
In the case shown in
That is, before shifting the scan from the cathode line B1 in
When the scanning line is switched by the prior art driving method, the charge stored in all luminous elements becomes zero and the reverse bias is not applied to the luminous elements because all cathode lines and anode lines are reset to 0V. In this first embodiment, however, the reverse bias is always applied to the luminous elements on cathode line B1. Furthermore, when the scanning line shifts as shown in FIG. 2, the luminous elements on cathode line B2 driven to emit light build up to light emission at substantially the same time because the luminous elements on all anode lines are charged to the same state. As a result, luminance is not uneven.
As described above, after applying the reverse bias potential to the luminous elements on cathode line B1, scanning switches 23 through 264 corresponding to cathode lines B3 through B64 switch to the source voltage VCC side and cathode line B2 is scanned as shown in FIG. 3. Drive switches 31 and 33 switch to current sources 11 and 13 at the same time, driving luminous elements E2.1 and E2.3 to emit light. In the state shown in
That is, before shifting the scan from the cathode line B2 in
In this case, as shown in
As described above, after applying the reverse bias potential to the luminous elements on cathode line B2, scanning switches 21, 22 and 24 through 264 corresponding to cathode lines B1, B2 and B4 through B64 switch to the source voltage VCC side and cathode line B3 is scanned as shown in FIG. 5. Drive switches 31 and 32 switch to current sources 11 and 12 at the same time, thus driving luminous elements E3.1 and E3.2 to emit light.
The above operation continues until cathode line B64 is scanned.
The above driving method thus applies the reverse bias voltage to luminous elements on cathode line B1 when switching from cathode line B1 to B2, and applies the reverse bias voltage to luminous elements on cathode line B2 when switching from cathode line B2 to B3. Because this same operation repeats until cathode line B64 is scanned, the reverse bias voltage is always applied at least once to the luminous elements on all cathode lines. Furthermore, when luminous elements on the next cathode line are driven to emit light, build-up to emission is substantially simultaneous because the luminous elements on all anode lines are all charged to the same state when shifting scanning. Luminance is therefore not uneven.
In the state shown in
It will be understood that the driving method of the first embodiment connects at least one cathode line to the source voltage and applies 0V to the other cathode lines before switching from a selected scanning line to the next scanning line. As a result, uneven luminance resulting from charge state differences does not occur, and applies the reverse bias when switching the scanning line to improve luminous element service life.
It will also be understood that while this embodiment is described using an example in which only one cathode line is connected to the source voltage and 0V is applied to the remaining cathode lines when shifting the scanning line, the same effect can be achieved if two, three or more cathode lines are connected to the source voltage.
(Second Embodiment)
FIG. 7 through
To apply negative potential −Vdd, scanning switches 21 through 264 can be switched to source voltage VCC, the ground potential, or negative potential −Vdd where voltage Vdd is below the emission threshold voltage of the luminous elements.
In
In the case shown in
Before shifting the scan from the cathode line B1 in
Thus, after applying the forward bias potential to the luminous elements on cathode line B2, scanning switches 21 and 23 through 264 corresponding to cathode lines B1 and B3 through B64 switch to the source voltage VCC side, switch 22 corresponding to cathode line B2 switches to 0V, and cathode line B2 is scanned as shown in FIG. 9. Drive switches 31 and 32 switch to current sources 11 and 12 at the same time, causing luminous elements E3.1 and E3.2 to emit light.
The speed of build-up to emission is thus increased and fast scanning is possible because luminous elements E2.1, E2.2 . . . E2.256 on cathode line B2 are charged to forward bias Vdd at this time.
It will also be understood that, while this present embodiment is described using an example in which only one cathode line is connected to negative potential Vdd and 0V is applied to the remaining cathode lines when shifting the scanning line, the same effect can be achieved if two, three, or more cathode lines are connected to the source voltage.
It will also be noted that while these embodiments of this invention are described using luminous elements as an organic EL by way of example, the invention shall not be so limited. For instance, the present invention can be applied to a light-emitting diode or other current injection type luminous elements having a diode characteristic and a capacitor component.
Number | Date | Country | Kind |
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2000-281657 | Sep 2000 | JP | national |
2001-157630 | May 2001 | JP | national |
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Number | Date | Country |
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A-9-232073 | Sep 1997 | JP |
Number | Date | Country | |
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20020033782 A1 | Mar 2002 | US |