The present invention relates to an apparatus for driving a light emitting device and, more particularly, to a driving apparatus for an EL device.
Attention is paid to an EL (electroluminescence) display as a display apparatus which can be substituted for a liquid crystal display and in which a low electric power consumption, a high display quality, and a thin size can be realized. The EL display has an organic compound in which excellent light emitting performance can be expected and is used as a light emitting layer of an EL device that is used in the EL display. The device has a high efficiency and a long service life which can endure a practical use.
A full-color image display can be accomplished by selecting an organic material which can perform a light emission of red (R), green (G), or blue (B) (i.e., a first, a second, or a third primary color) as an emitting material which is applied to the light emitting layer (RGB method). It can be also accomplished by a CCM (Color Changing Mediums) method using a color converting layer for each of the RGB colors as disclosed in xe2x80x9cNikkei Electronicsxe2x80x9d, Vol. 1.29 (No. 654), pp. 99-103, 1996, or the like.
The organic EL device (hereinafter, simply referred to as an EL device) can be expressed by an electrical equivalent circuit as shown in FIG. 1.
As will be understood from FIG. 1, the EL device can be expressed by a configuration comprising a capacitive component C and a component E having diode characteristics connected in parallel with the capacitive component. Generally, the EL device is a capacitive light emitting device.
When a light emission driving voltage is applied to the EL device, charges corresponding to a capacitance first flow to an electrode as a displacement current and are accumulated. When the voltage exceeds a certain voltage (light emission threshold voltage) that is peculiar to the device, a forward current starts to flow from an anode into an organic layer serving as a light emitting layer and light emission occurs at an intensity that is proportional to the driving current.
FIGS. 2 to 4 show light emitting characteristics (L-I, I-V, and L-V characteristics: where xe2x80x9cLxe2x80x9d, xe2x80x9cIxe2x80x9d, and xe2x80x9cVxe2x80x9d denotes a light emission luminance, a driving current, and a driving voltage, respectively) of the EL device. When the driving voltage exceeding the light emission threshold value is applied to the EL device, light emission occurs at a luminance that is proportional to the driving current in accordance with the driving voltage. When the applied driving voltage is equal to or lower than the light emission threshold value, no driving current flows and the light emission luminance is also almost equal to zero.
As a method of driving a color panel using the EL device, it is known that a simple matrix driving method can be applied. A driving method of performing a resetting operation to discharge accumulated charges in each EL device arranged in a matrix form just before scanning lines are switched (hereinafter, referred to as a reset driving method) has been disclosed in Japanese Laid-Open Patent Publication (Kokai) No.H09-199136 (1997) by the same applicant as that of the present invention. The reset driving method will now be described with reference to FIGS. 5 to 8.
EL devices E1,1 to En,m serving as pixels are arranged in a matrix form. One end (anode side of the diode component E of the equivalent circuit) of each EL device is connected to an anode line and the other end (cathode side of the diode component E) is connected to a cathode line at each intersecting position between anode lines A1 to An arranged along the vertical direction and cathode lines B1 to Bm arranged along the horizontal direction, respectively.
A cathode line scanning circuit 1 and an anode line driving circuit 2 are provided as light emission driving means for the EL device. The cathode line scanning circuit 1 has a function to individually decide an electric potential of each cathode line in order to select a cathode line to be scanned. In more detail, scan switches 51 to 5m corresponding to the cathode lines B1 to Bm connect either a reverse bias voltage VB (for example, 10V) or a ground potential (0V) to the corresponding cathode lines.
The anode line driving circuit 2 has a function to individually supply a driving current through each anode line. In more detail, current sources 21 to 2n are provided in correspondence to the anode lines A1 to An. Currents which are generated in the current sources flow individually to the anode lines A1 to An through drive switches 61 to 6n.
The anode lines A1 to An are also connected to an anode resetting circuit 3. The anode resetting circuit 3 has shunt switches 71 to 7n each provided every anode line. When the shunt switch is turned on, the corresponding anode line is connected to the ground potential.
Each of the cathode line scanning circuit 1, the anode line driving circuit 2 and the anode resetting circuit 3 is controlled by a light emission control circuit 4. The light emission control circuit 4 controls each circuit in order to display an image carried by image data in accordance with an image data signal supplied from an image data generating system (not shown).
That is, the light emission control circuit 4 generates a scanning line selection control signal to the cathode line scanning circuit 1, selects any of the cathode lines B1 to Bm corresponding to a horizontal scanning period of the image data, and connects it to the ground potential. The control circuit 4 switches the scan switches 51 to 5m so that the reverse bias voltage VB is applied to the other cathode lines. The scan switches 51 to 5m are, therefore, subjected to a switching control according to what in called a-line-at-a-time scanning such that they are sequentially switched to the ground potential every horizontal scanning period. The cathode line connected to the ground potential acts as a scanning line for enabling the EL devices connected to the cathode line to perform a light emission.
The anode line driving circuit 2 performs a light emission control to the scanning line that is being scanned. The light emission control circuit 4 generates a drive control signal (driving pulse) indicating a result of a discrimination with respect to which one of the EL devices connected to the scanning line is allowed to perform the light emission at which timing for which duration in accordance with image information of the image data. The light emission control circuit 4 supplies the generated control signal to the anode line driving circuit 2.
The anode line driving circuit 2 controls the on/off operations of the drive switches 61 to 6n in response to the control signal and supplies the driving current to the EL device in accordance with the pixel information through the anode lines A1 to An. The EL device to which the driving current is supplied, thus, performs a light emission according to the pixel information.
The anode resetting circuit 3 is provided to perform the resetting operation. The resetting operation is performed in response to the reset control signal from the light emission control circuit 4. The anode resetting circuit 3 turns on any of the shunt switches 71 to 7n corresponding to the reset target anode line indicated by the reset control signal and turns off the other switches. The operation of a reset driving method based on the above configuration will now be described.
An operation flow will be explained as an example hereinbelow where after the cathode line B1 is scanned and the EL devices E1,1 and E2,1 are allowed to emit the light, the scan is shifted to the cathode line B2 and the EL devices E2,2 and E3,2 are allowed to emit the light. For simplicity of explanation, the EL device which performs the light emission is shown by a diode symbol and the EL device which does not perform the light emission is shown by a capacitor symbol. The reverse bias voltage VB which is applied to the cathode lines B1 to Bm is set to the same voltage 10V as a power voltage of the apparatus.
First, in FIG. 5, the scan switch 51 is switched to the 0V position as a reference voltage and the cathode line B1 is scanned. The reverse bias voltage 10V as a predetermined voltage is applied to the other cathode lines B2 to Bm via the scan switches 52 to 5m.
The current sources 21 and 22 are connected to the anode lines A1 to A2 via the drive switches 61 and 62. The other anode lines A3 to An are connected to the ground potential 0V via the shunt switches 73 to 7n.
In case of FIG. 5, therefore, only the EL devices E1,1 and E2,1 are biased in the forward direction, the driving currents flow from the current sources 21 and 22 as shown by arrows, and only the EL devices E1,1 and E2,1 emit the light. In FIG. 5, each of the EL devices shown by hatched regions in the capacitors is charged to a polarity as shown in the diagram. The following reset control is performed just before the scan is shifted from the light emitting state shown in FIG. 5 to a state where the light emission of the EL devices E2,2 and E3,2 as shown in FIG. 8 is performed.
That is, before the scan target is shifted from the cathode line B1 in FIG. 5 to the cathode line B2 in FIG. 8, first, as shown in FIG. 6, all of the drive switches 61 to 6n are turned off, all of the scan switches 51 to 5m and all of the shunt switches 71 to 7n are switched to the 0V position, and all of the anode lines A1 to An and cathode lines B1 to Bm are once set to 0V (all-resetting operation by 0V). Since all of the anode lines and the cathode lines are set to the same electric potential of 0V in the all-resetting operation by the voltage of 0V, the electrical charges charged in each EL device pass through the routes as shown by the arrows in the diagram and are discharged. The electrical charges charged in all of the EL devices instantaneously become to 0.
After the charged charges in all of the EL devices are become to 0 this manner, by switching only the scan switch 52 corresponding to the cathode line B2 to the 0V position as shown in FIG. 7, the cathode line B2 is scanned. At the same time, the current sources 22 and 23 are connected to the corresponding anode lines by the drive switches 62 and 63, the shunt switches 71 and 74 to 7n are turned on, and the anode lines A1 and A4 to An are connected to 0V.
When the cathode line B2 is scanned through the switching operation of the switches and the charged charges in all of the EL devices are set to 0 as mentioned above, the charging currents flow to the EL devices E2,2 and E3,2 to be subsequently subjected to the light emission via a plurality of routes as shown by the arrows in FIG. 7. A capacitor C of each EL device is instantaneously charged.
That is, not only the charging current flows to the EL device E2,2 through the route of (the current source 22xe2x86x92drive switch 62xe2x86x92anode line A2xe2x86x92EL device E2,2xe2x86x92scan switch 52) but also the charging current simultaneously flows through the route of (the scan switch 51xe2x86x92cathode line B1xe2x86x92EL device E2,1xe2x86x92EL device E2,2xe2x86x92scan switch 52) the route of (the scan switch 53xe2x86x92cathode line B3xe2x86x92EL device E2,3xe2x86x92EL device E2,2xe2x86x92scan switch 52), . . . , and the route of (the scan switch 5mxe2x86x92cathode line Bmxe2x86x92EL device E2,mxe2x86x92EL device E2,2xe2x86x92scan switch 52). Since the EL device E2,2 is instantaneously charged up to the light emission threshold value with large charging current through those plural routes, it can be momentarily shifted to a stationary state of the light emission shown in FIG. 8.
Since the EL device E3,2 is also instantaneously charged up to the light emission threshold value with the charging currents by those plural routes as shown in FIG. 7, it can be momentarily shifted to a stationary state of the light emission shown in FIG. 8.
As mentioned above, according to the reset driving method, since all of the cathode lines and anode lines are once connected to 0V as a ground potential and are reset before the control is shifted to the light emission control mode for the next scanning line, when the scanning line is switched to the next scanning line, the EL devices to be subjected to the light emission on the switched scanning line are quickly charged up to the light emission threshold value. Thus, rapid increase of the light emission of the devices can be realized.
Although the EL devices other than the EL devices E2,2 and E3,2 to be subjected to the light emission are also charged through the routes as shown by the arrows in FIG. 7, since the charging direction in this instance is the reverse bias direction, the EL devices other than the EL devices E2,2 and E3,2 do not cause an erroneous light emission.
Although the case of using the current sources 21 to 2n as driving sources has been mentioned in the examples of FIGS. 5 to 8, the above driving method can be similarly realized by using voltage sources.
Further, a Japanese Laid-Open Patent Publication (Kokai) No.H09-232074 (1997) discloses that the reset driving method can be realized not only by the all-resetting operation of the EL devices by 0V as mentioned above but also by another predetermined reset voltage or by resetting the necessary EL devices.
In the state just after the switching of the scanning line shown in FIG. 7, a voltage of about VB [V] (in the example, 10V) which will be a value enough for the light emission threshold value is applied to the EL devices E2,2 and E3,2 to be subjected to the light emission and they are instantaneously charged by the flow of the current from the reverse bias voltage source, thereby preparing so that they can perform the light emission immediately after the drive switches 62 and 63 are turned on.
The light emission control including the above-mentioned preparation will now be described. FIG. 9 shows the light emission control mode by the reset driving method described above and the driving pulses which can be supplied individually as control signals to the drive switches in the anode line driving circuit 2 in correspondence to the mode.
As shown in FIG. 9, the light emission control mode includes a scanning mode as a period of time during which any of the cathode lines B1 to Bm is activated and a resetting mode as a period of time during which the operation as shown in FIG. 6 is performed subsequently to the activating period. The scanning mode and the resetting mode are executed every horizontal scanning period (1H) of the image data.
While the driving pulse shows the high level in the scanning mode, one of the drive switches 61 to 6n corresponding to the driving pulse is turned on and the light emission of the EL device is continued. At this time, the driving current which is supplied to the EL device is constant.
The longer the period of time during which the driving pulse is at the high level, the longer the light emitting time of the EL device and light emission luminance can be increased. A bright state can be formed by increasing the width of the driving pulse, therefore, and a dark state can be formed by decreasing the driving pulse width, so that a multi-stage gradation control can be accomplished. The gradation control is executed on the basis of a PWM (pulse width modulation).
The luminance of the actual output light of the EL device which is obtained in the gradation control is as shown in FIG. 10. FIG. 10 shows a state of a change in luminance L of the output light of the EL device at the maximum gradation (designated maximum luminance) at which the driving pulse is held at the high level for a period of time in the scanning mode.
Just after the resetting, the luminance of the EL device shows a relatively steep rising state and reaches the maximum luminance by the driving of the output voltage VB from the reverse bias power source and the output current of the constant current source. The luminance immediately drops and then becomes stable at the luminance corresponding to the designated gradation only by the driving current from the constant current source. The stable light emission is maintained until the next resetting mode.
The driving by the reverse bias power source and the constant current source just after the resetting corresponds to the operation of the xe2x80x9cpreparationxe2x80x9d mentioned above, namely, the operation of FIG. 7 and the subsequent driving only by the constant current source corresponds to the operation of FIG. 8.
According to the light emission control, the light emission of the EL device is rapidly activated by the preparing operation just after the resetting, thereby enabling the driving to be shifted smoothly to the driving only by the subsequent driving pulses from the constant current source. An area surrounded by the luminance curve in FIG. 10 and the time (t) axis corresponds to the light emission amount and the substantial luminance corresponds to the area.
It is, therefore, necessary to keep the relation between the area and one gradation (pulse width of the driving pulse) constant. Unless otherwise, it is considered that the linearity of the gradation is lost. Particularly, it is required to keep the relation constant even if the operating environment changes in terms of the display quality.
The present invention is made in consideration of the foregoing drawbacks and it is an object of the invention to provide an EL device driving apparatus which enables substantial light emission luminance characteristics of the device to be kept constant even if an environmental temperature fluctuates.
According to one aspect of the present invention, there is provided an EL device driving apparatus comprising: a driving unit for selectively supplying a light emission driving energy to the EL devices; a temperature sensing unit for sensing an operation temperature of the EL devices; and a temperature compensating unit for changing the light emission driving energy in accordance with the operation temperature.
According to another aspect of the present invention, the driving unit comprises: a plurality of first electrode lines; a plurality of second electrode lines which intersect the first electrode lines; and a light emission control unit for selecting any of the first electrode lines every horizontal scanning period of an image signal that is supplied, selecting any of the second electrode lines in correspondence to a pixel position in the horizontal scanning period, applying a reverse bias voltage to portions between non-selected lines among the first electrode lines and non-selected lines among the second electrode lines, and supplying a driving current to portions between the selected electrode line among the first electrode lines and the selected electrode line among the second electrode lines, wherein the EL devices are arranged in a matrix form in which one of each of electrodes and another electrode are connected to one of the first electrode lines and one of the second electrode lines, respectively, and the temperature compensating unit changes a magnitude of the reverse bias voltage in accordance with the operation temperature.
Further, the light emission control unit has a resetting unit for performing a resetting operation to extract charges accumulated in the EL devices every the horizontal scanning period.
Still further, the temperature compensating unit decreases a magnitude of the reverse bias voltage in accordance with an increase in the operation temperature and increases the magnitude of the reverse bias voltage in accordance with a decrease in the operation temperature.
According to further aspect of the present invention, the driving unit comprises: a plurality of first electrode lines; a plurality of second electrode lines which intersect the first electrode lines; and a light emission control unit for selecting any of the first electrode lines every horizontal scanning period of an image signal that is supplied, selecting any of the second electrode lines in correspondence to a pixel position in the horizontal scanning period, applying a reverse bias voltage to portions between non-selected lines among the first electrode lines and non-selected lines among the second electrode lines, and supplying a driving current to portions between the selected electrode line among the first electrode lines and the selected electrode line among the second electrode lines, wherein the EL devices are arranged in a matrix form in which one of each of electrodes and another electrode are connected to one of the first electrode lines and one of the second electrode lines, respectively, and the temperature compensating unit changes a magnitude of the driving current in accordance with the operation temperature.
Further, the light emission control unit has a resetting unit for performing a resetting operation to extract charges accumulated in the EL devices every the horizontal scanning period.
Still further, the temperature compensating unit decreases a magnitude of the driving current in accordance with an increase in the operation temperature and increases the magnitude of the driving current in accordance with a decrease in the operation temperature.
According to still further aspect of the present invention, the driving unit comprises: a plurality of first electrode lines; a plurality of second electrode lines which intersect the first electrode lines; and a light emission control unit for selecting any of the first electrode lines every horizontal scanning period of an image signal that is supplied, selecting any of the second electrode lines in correspondence to a pixel position in the horizontal scanning period, applying a reverse bias voltage to portions between non-selected lines among the first electrode lines and non-selected lines among the second electrode lines, and supplying a driving current to portions between the selected electrode line among the first electrode lines and the selected electrode line among the second electrode lines, wherein the EL devices are arranged in a matrix form in which one of each of electrodes and another electrode are connected to one of the first electrode lines and one of the second electrode lines, respectively, and the temperature compensating unit changes a magnitude of the supplying period of time of the driving current in accordance with the operation temperature.
Further, the light emission control unit has a resetting unit for performing a resetting operation to extract charges accumulated in the EL devices every the horizontal scanning period.
Still further, the temperature compensating unit decreases the driving period of time in accordance with an increase in the operation temperature and increases the driving period of time in accordance with a decrease in the operation temperature.
Further, the temperature sensing unit includes a thermistor.