JPH06230745A - El light emitting device and its driving method - Google Patents

Abstract

PURPOSE:To reduce electric power consumption and to prevent the erroneous light emission occurring due to field-through in the EL light emitting device for regulating the light emission quantity of an EL element by changing the frequency of a driving power source. CONSTITUTION:The high level voltage (on-voltage) of a writing data voltage is so changed as to be proportional to the frequency of the driving power source Va to prevent the impression of the voltage above the satd. voltage and to reduce the electric power consumption. The low level voltage (off-voltage) of the writing data voltage is so changed as to be proportional to the frequency of the alternative power source Va, there by, the difference between the light emission threshold voltage dropped by the change in the frequency and the voltage occurring due to field-through generated in the gate of a first thin-film transistor QD at the time of impressing the low level voltage (off-voltage) is assured and the erroneous light emission of the EL element CEL is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an EL light emitting device used in a drive system of an active matrix type EL display device or an electronic printing device and a driving method thereof, and more particularly, to a total emission amount of all EL elements. An apparatus and a driving method for performing adjustment.

[0002]

2. Description of the Related Art An EL light emitting device is composed of a plurality of ELs arranged in an array.
(Electroluminescence) A light emitting device that electrically stimulates elements in a fixed order to generate a desired optical signal. Since the EL elements can be integrated on one glass substrate with high accuracy, a two-dimensional array of a plurality of elements is a display panel, and a one-dimensional array is an image bar of an electronic printer. Available as These light emitting devices have advantages such as high space utilization efficiency because they can be made thin and lightweight, easy incorporation into a portable device, and an excellent display without bleeding. Much research has been conducted in recent years.

Among EL light emitting devices, an active EL light emitting device has a basic circuit element such as a switching element required for control for each EL element, and is configured to be driven for each EL element. It is formed by being integrated on a glass substrate by using thin film technology. The active EL light emitting device will be described with reference to FIGS. 7 and 8. One bit of this EL light emitting device is, as shown in FIG. 7, a first switching element (thin film transistor).
It has a QD, an EL element CEL that is on / off controlled by the switching element QD, and an AC power supply Va that drives the EL element CEL. That is, when the gate voltage VGD of the switching element QD exceeds the threshold voltage of the switching element QD, the switching element QD is turned on and an AC voltage is applied across the EL element CEL. When the gate voltage VGD is equal to or lower than the threshold voltage of the switching element QD, the switching element QD is turned off.
A data holding capacitor Cs is connected between the gate and drain of the switching element QD so that the charging voltage of the data holding capacitor Cs becomes equal to the gate voltage VGD.

The gate side of the switching element QD is connected to the source side of the switching element QW so that a data signal is applied to the drain side of the switching element QW and a strobe signal is applied to the gate side of the switching element QW. It has become. Therefore, the charging voltage of the data holding capacitor Cs is set to the switching element QW.
If it is controlled by, the light emission of the EL element CEL can be controlled. That is, when the data signal is at a high potential (high level), the switching element Q is driven by the strobe signal.
When W is turned on, the switching element QD is turned on and the EL element CEL is turned on. If the data signal has a low potential (low level), the switching element QD is turned off.

Therefore, as shown in the timing chart of FIG. 9, when the strobe signal 1 is at a high potential (high level), the data signals 1 to n are stored in the data holding capacitors Cs of the EL light emitting devices, respectively. Strobe signal 1
If this operation is repeated for ~ m, the EL shown in FIG.
In each bit of the light emitting device, the EL element CEL can emit light in accordance with each data signal of one cycle Ts.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention In the EL light emitting device as described above, when the light emission amount of all the EL elements CEL is collectively adjusted according to the use environment or the use condition (adjustment of the overall brightness), The method like was used. The EL element CEL, as shown in FIG.
Since the on / off drive is performed by the voltage control of the drive voltage Vpk that obtains on and the voltage equal to or lower than the light emission threshold voltage VTEL at the non-light emission luminance Loff, the drive voltage Vpk and the threshold VTE
The brightness can be adjusted by changing the drive voltage between L and L. The brightness is adjusted by changing the data voltage applied to the switching element QW and by changing the effective voltage applied to the EL element CEL. Since the brightness of the EL element CEL changes depending on the driving frequency (see FIG. 5), it is adjusted by changing the driving frequency fa of the AC power supply Va.

When the above-mentioned power supply voltage is changed, the EL elements arranged in the substrate have variations in the luminance-voltage characteristics. Therefore, even if the power supply voltage is changed collectively, uniform adjustment cannot be made in each EL element. There was a problem. When the data voltage is changed to change the voltage applied to the gate of the switching element QD and the effective voltage applied to the EL element is changed, the power consumption in the switching element QD is increased. .

Further, when the above drive frequency is changed, there are the following problems. That is, as shown in FIG. 4, the brightness of the EL element changes depending on the drive frequency, but in the brightness-data voltage characteristic with the drive frequency as a parameter, as shown in FIG. The high level voltage (ON voltage) (VDA1) changes depending on the drive frequency fa. Therefore, the drive frequency fa
The maximum display brightness (Lon
When the data voltage corresponding to 1) is VDA1, the high level voltage of the data voltage that saturates the brightness (Lon2) becomes VDA2 when the display frequency is lowered to the desired display brightness (Lon2) by setting the drive frequency fa2. Therefore, when the data voltage VDA1 is applied as it is, the voltage of VDA2 or more becomes an excessive applied voltage. As a result, the power consumption of the data write circuit and the peripheral drive circuit is wasted due to the voltage difference (VDA1-VDA2).

On the other hand, when the frequency is lowered to reduce the luminance, the light emission threshold voltage is lowered accordingly. When a low level voltage (off voltage) based on a data signal is applied, a gate voltage VGD due to field through is generated at the gate of the switching element QD. As a result, there is a problem that the EL element CEL emits light erroneously when the gate voltage VGD becomes higher than the light emission threshold voltage.

The present invention has been made in view of the above circumstances, and in the case of collectively adjusting the light emission amount (luminance) of all EL elements constituting the EL light emitting device by changing the driving frequency, the peripheral circuit It is an object of the present invention to provide a structure and a driving method for reducing the power consumption of the EL element and preventing the erroneous light emission of the EL element.

[0011]

According to another aspect of the present invention, there is provided an EL light emitting device having an EL element which is turned on / off by a first thin film transistor, an AC power source for driving the EL element, and a gate of the first thin film transistor. A plurality of EL light emitting circuits each including a second thin film transistor for controlling a voltage are provided, and a write data voltage is applied to a drain side of the second thin film transistor by a data driver E
The L light emitting device is characterized by including an adjusting unit that collectively changes the write data voltage for all the EL light emitting circuits so as to be proportional to the frequency of the AC power supply.

According to a second aspect of the present invention, there is provided a method for driving an EL light emitting device, wherein the EL is ON / OFF controlled by the first thin film transistor.
Write data applied to the drain side of the second thin film transistor, which includes an element, an AC power supply for driving the EL element, and a second thin film transistor whose source side is connected to the gate side of the first thin film transistor In a driving method of an EL light emitting device, which controls the gate voltage of the first thin film transistor by voltage to control the on / off of the second thin film transistor to drive the EL element, the voltage is proportional to the frequency of the AC power supply. The write data voltage is changed to adjust the emission brightness of the EL element.

[0013]

According to the present invention, when the frequency of the AC power source for driving the EL element is changed to change the light emission amount of the EL light emitting element, the high level of the write data voltage is proportional to the frequency of the AC power source. By changing the voltage (ON voltage), it is possible to prevent application of a voltage equal to or higher than the saturation voltage and reduce power consumption.

Further, by changing the low level voltage (OFF voltage) of the write data voltage so as to be proportional to the frequency of the AC power supply, the light emission threshold voltage which decreases due to the change in frequency and the low level voltage (OFF voltage) are applied. At this time, a difference from the voltage due to field through generated in the gate of the first thin film transistor can be secured, and erroneous light emission of the EL element can be prevented.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A 1-bit equivalent circuit of an active EL light emitting device according to an embodiment of the present invention will be described with reference to FIG. This EL light emitting device has a first switching element (thin film transistor) QD, an EL element CEL that is on / off controlled by the switching element QD, and an AC power supply Va that drives the EL element CEL.
A data holding capacitor Cs is connected between the gate and drain of the switching element QD, and the source side of the switching element QW is connected to the gate side of the switching element QD. A data signal is applied from the data driver 1 to the drain side of the switching element QW, and a strobe signal is applied from the strobe driver 2 to the gate side of the switching element QW. Further, the AC power supply Va is configured so that the drive frequency fa can be changed in order to adjust the brightness of the entire light emitting device.

The data signal and strobe signal are adapted to change in accordance with the driving frequency fa of the AC power supply Va. That is, the drive frequency fa of the AC power supply Va
Is detected, and high-level and low-level DC voltages corresponding to the drive frequency fa are output (frequency f
The frequency / voltage converter 3 is connected. Each DC voltage output from the frequency / voltage converter 3 is amplified by the amplifiers 4 and 4 so as to have a desired voltage, and a power supply line that determines the "high level" (voltage VDD) of the output circuit of the data driver 1 5. "Low level" of output circuit of data driver 1 and strobe driver 2
It is connected to a power supply line 6 that determines (voltage VSS).

A data signal corresponding to each bit of the data signals 1 to n (serial data signal) shown in FIG. 9 converted into parallel signals by the shift register 7 is input to the data driver 1. The data driver 1 is composed of a circuit as shown in FIG. 2, and when the voltage VDD (high level signal) supplied from the power supply line 5 when the data signal is “high” is “low”, the power supply line 6 is supplied. The supplied voltage VSS (low level signal) is output to the data signal supply line 8.

In the strobe driver 2, when the strobe signal 1 from the shift register (not shown) is "high", the voltage V'DD (high level signal) supplied from the power line 9 to the strobe driver 2 becomes "high". The voltage VSS (low level signal) supplied from the power supply line 6 when “low” is output to the strobe signal supply line 10. Therefore, the voltage VDD (high level signal) and the voltage VSS (low level signal) output to the data signal supply line 8 and the voltage VSS output to the strobe signal supply line 10
(Low level signal) is the drive frequency fa of the AC power supply Va.
It changes according to.

Next, driving of the operation of the EL light emitting device will be described with reference to the equivalent circuit diagram of FIG. 1 and the timing chart of FIG. The ON (high level) voltage as the data signal 1 is the strobe signal 1 and the capacitor C
When written to s, the voltage is held by the capacitor Cs and the gate voltage VGD of the switching element QD becomes "Vg.
h ”, the switching element QD becomes conductive and E
The L element CEL emits light (time t1 to t2). When an off (low level) voltage as the data signal 1 is written in the capacitor Cs as a strobe signal, the voltage is held in the capacitor Cs, the gate voltage VGD of the switching element QD becomes “Vgl”, and the switching element QD is non-conductive. The state is reached (time t2 to t3). At this time, a field through voltage divided by the gate-drain capacitances Cgd and Cs is generated as the drain voltage VDV of the switching element QD, and reaches the maximum peak value Vglpk.

As shown in FIG. 6, when the driving frequency fa is used as a parameter for the luminance-data voltage characteristic, the data voltage VDAn at which the saturated luminance LONn reaches the driving frequency fa (n) is the driving frequency. Changes in proportion to. This means that when the drive voltage Va is a sine wave, E
This is because when the drive current required to drive the L element CEL is IEL, the drive current IEL is proportional to the frequency fa as shown in the following equation.

[0021]

[Formula 1] IEL = (2πfa) VpkCEL · sin (2πfa
・ T + π / 2)

If the on-current flowing through the switching element QD is higher than the drive current, it is sufficient for the EL element to emit light, but if it is lower than that, the luminance is lowered. This corresponds to the luminance characteristics below each VDAn in FIG. Therefore, when the brightness of all EL elements CEL is collectively adjusted by changing the driving frequency fa, the minimum necessary voltage corresponding to the saturation brightness for each frequency may be set as the high level voltage of the data signal. . The data driver 1 of the embodiment shown in FIG. 1 is adjusted so that this voltage is output as the voltage VDD. Therefore, by setting the minimum necessary voltage to the high level voltage VDD of the data signal, it is possible to reduce the power consumption in the peripheral drive circuit when writing the ON voltage of the data signal into the capacitor Cs.

Further, when the data signal is an off (low level) voltage, it becomes the gate voltage Vglpk of the switching element QD due to the field through described above. When the driving frequency is lowered, the light emission threshold voltage VTEL at the luminance LOFF is correspondingly lowered, and the margin with the gate voltage Vglpk is reduced with respect to this voltage, and erroneous light emission is likely to occur. . Therefore, according to the above-described embodiment, the off (low level) voltage VSS of the data signal is adjusted to be low according to the drive frequency fa. Therefore, it is possible to secure a margin with the gate voltage Vglpk of the switching element QD with respect to the light emission threshold voltage VTEL that provides the luminance LOFF, and prevent erroneous light emission of the EL element CEL due to conduction of the switching element QD.

When changing the off (low level) voltage of the data signal, the strobe driver 2 is set so that the voltage VSS supplied to the data signal supply line 8 becomes equal to the off (low level) voltage VSS of the strobe signal. Is turned off (low level) by the power supply line 6, it is possible to prevent light emission due to malfunction.

That is, when the off (low level) voltage of the data signal is lowered according to the frequency of the AC power supply and the off (low level) voltage of the strobe signal is not changed at all, in the switching element QW, the source part (data The gate voltage VGS between the signal supply line 8 side) and the gate portion (strobe signal supply line 10 side) becomes large, and the switching element QW may be turned on even though the strobe signal is off (low level). As a result, the EL element CEL is caused to emit light by mistake. Therefore, when the off (low level) voltage of the data signal is lowered, the off (low level) voltage of the strobe signal is similarly lowered to prevent the rise of the gate voltage VGS in the switching element QW, thereby causing the erroneous light emission. Prevent.

[0026]

According to the present invention, the light emission amount (luminance) of the EL element is changed by changing the frequency of the AC power source for driving the EL element.
When changing the, the high level voltage (ON voltage) of the write data voltage is proportional to the frequency of the AC power supply.
It is possible to prevent the application of a voltage equal to or higher than the saturation voltage and reduce the power consumption by changing.

Further, by changing the low level voltage (off voltage) of the write data voltage so as to be proportional to the frequency of the AC power supply, the light emission threshold voltage which decreases due to the change in frequency and the low level voltage (off voltage) are applied. At this time, a difference from the voltage due to field through generated in the gate of the first thin film transistor can be secured, and erroneous light emission of the EL element can be prevented.

[Brief description of drawings]

FIG. 1 is an equivalent circuit explanatory diagram for 1 bit of an EL light emitting device according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a driver unit according to the embodiment.

FIG. 3 is a drive timing chart for causing one bit of the EL light emitting device to emit light.

FIG. 4 is a luminance-driving voltage characteristic diagram of an EL light emitting element.

FIG. 5 is a luminance-driving frequency characteristic diagram of an EL light emitting element.

FIG. 6 is a luminance-data voltage characteristic diagram of an EL light emitting element.

FIG. 7 is an equivalent circuit explanatory diagram of 1 bit of a conventional EL light emitting device.

FIG. 8 is an explanatory diagram of an equivalent circuit of the entire EL light emitting device.

FIG. 9 is a drive timing chart of the EL light emitting device.

[Explanation of symbols]

1 ... Data driver, 2 ... Strobe driver,
3 ... Frequency / voltage converter, 4 ... Amplifier,
5, 6 ... Power supply line, 7 ... Shift register, 8 ... Data signal supply line, 9 ... Power supply line, 10 ... Strobe signal supply line, QD ... Switching element (first thin film transistor), QW ... Switching element (second) Thin film transistor), CEL ... EL element, Va ... AC power supply

Claims (2)

[Claims] 1. An EL device comprising an EL element controlled to be turned on / off by a first thin film transistor, an AC power supply for driving the EL element, and a second thin film transistor for controlling a gate voltage of the first thin film transistor. In an EL light emitting device having a plurality of light emitting circuits, and a write data voltage is applied to the drain side of the second thin film transistor by a data driver, all the write data voltages are set to be proportional to the frequency of the AC power supply. EL
E having adjusting means for collectively changing the light emitting circuit
L light emitting device. 2. An EL element controlled to be turned on / off by a first thin film transistor, an AC power supply for driving the EL element, and a second thin film transistor whose source side is connected to a gate side of the first thin film transistor. An EL device that controls the gate voltage of the first thin film transistor by a write data voltage applied to the drain side of the second thin film transistor and turns on and off the second thin film transistor to drive the EL element. A method for driving an EL light emitting device, wherein the write data voltage is changed so as to be proportional to the frequency of the AC power supply, and the emission brightness of the EL element is adjusted.