JP2000235370A - Drive assembly for organic electroluminescent element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a drive assembly for an organic EL element which does not require such a TFT element capable of exactly making V-I conversion of the variable quantity of a signal level to a variable quantity of current. SOLUTION: Gradation control is applied during one frame period by using a pulse width fluctuation signal according to an input signal by each one pixel with the drive assembly 10 for the organic electroluminescent element of active matrix constitution. The gradation control is executed by converting the amplitude of the input signal to a pulse width modulation or to the pulse width modulation and amplitude modulation. The gradation control is executed by previously making input with the pulse width modulation signal and expanding the pulse width.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an organic electroluminescence (electroluminescence) device.
nce: EL) element driving device, and in particular, organic thin film E
The present invention relates to an organic EL element driving device used for a flat emission type organic thin film EL display as a device to which an L element is applied.

[0002]

2. Description of the Related Art In recent years, as a device using an organic thin film EL element utilizing the EL phenomenon of an organic thin film, an organic thin film EL device has been developed.
2. Description of the Related Art A flat light-emitting organic thin-film EL display has been proposed in which an element structure is a unit pixel, and the unit pixel is two-dimensionally arranged on a single support substrate and driven in a matrix.

[0003] As a first step in this proposal,
An organic EL display using a simple matrix has been researched and developed. In the case of a simple matrix, for example, assuming that a matrix of m rows × n columns is configured, a data signal is supplied to the n column side and a scanning signal is supplied to the m row side, and the m row side is sequentially supplied at predetermined intervals. It is driven to compose a screen by scanning.

In this simple matrix drive,
As the screen size increases, the scanning time for one row decreases, so that the average luminance of the screen decreases or power consumption increases to increase the luminance.

Therefore, as a display at the next stage,
Active matrix displays are being researched and developed. For example, Japanese Patent Application Laid-Open No. 9-305139 discloses a display device in which a light emitting element such as an organic EL element is driven in an active matrix.

FIG. 8 is a partial detailed view of a display section of a conventional organic EL display device. As shown in FIG. 8, the display unit includes m × n pixels P11 to Pm arranged in a matrix.
n (only P11 to P22 are shown), and each of the pixels P11 to Pmn emits light according to the supplied constant current.

An analog video signal Sv is amplified by a video amplifier, and the characteristics of the video signal are corrected by a V / I correction circuit and supplied to these pixels P11 to Pmn. In this case, the individual pixels P11 to P11
The video signal Sv is intermittently supplied to Pmn after being sequentially time-divided by the scanning control circuit. Note that the scanning control circuit performs scanning control at the timing of the supplied synchronization signal Sync.

Each of the pixels P11 to Pmn is provided with a driving means, which is a so-called active matrix drive. The video signal supplied intermittently to each of the pixels P11 to Pmn is converted into the next video by the next frame cycle. Holding means for holding until a signal is supplied, and a field effect transistor driven by a constant current according to the level of the video signal held by the holding means
(sistor: FET) element.

[0009] Then, each pixel P
A constant current for driving 11 to Pmn is supplied, and each of the pixels P11 to Pmn emits light according to the supplied constant current. Thus, gradation control at a stage corresponding to the video signal is performed.

For example, the pixel P11 is connected to the field effect transistor TR-11 in the organic EL element O-EL1.
Operates as an analog switch, and the pixel P1
1 is opened when a video signal is given, and the input video signal is applied to the capacitor C1 and the gate of the field effect transistor TR-1.

The field effect transistor TR-11 is controlled so as to be turned on only during a period when a video signal is supplied to the pixel P11. The cycle of turning on is set, for example, every frame.

In this manner, the video signal captured by the pixel P11 is held by the capacitor C1 until the next video signal is supplied in the next frame. Further, since the holding voltage of the capacitor C1 is applied to the gate of the field effect transistor TR-1, a constant current according to the gate voltage flows through the drain of the field effect transistor TR-1.

The drain current is controlled by the organic EL element OE.
L1 is supplied as a cathode current and the organic EL element
Light emission is performed with a current corresponding to the gradation during the frame period.

FIG. 9 is a circuit diagram of one pixel of the display section of FIG. As shown in FIG. 9, the signal line 1 is a video signal (see FIG. 8), the control line 2 is a line synchronization signal (see FIG. 8), and the switching element 3 is a field effect transistor T.
R-11 (see FIG. 8).

The storage capacitor 4 is a capacitor C1 (FIG. 8).
), A driving TFT (thin-film tran)
Sistor) 5 is connected to the field effect transistor TR-1.
The organic EL element 6 is an organic EL element O-EL1 (see FIG. 8).
Respectively. The organic EL element 6 is directly connected to the power supply line 7 and connected to the ground line 8 via the driving TFT 5.

When the control line 2 is in the active state and the switching element 3 is in the conductive state, the input signal from the signal line 1 is held by the holding capacitor 4 for one frame period, the gate of the driving TFT 5 is turned on, and the organic EL element 6 is turned on. An electric current is applied to emit light.

When a gray scale is provided in this display device, a method of varying the level of an input signal applied to the signal line 1 is generally used. In this case, for the organic EL element 6 that requires current driving, an element that can accurately perform the VI conversion from the variable amount of the signal level to the variable amount of the current is necessary.

[0018]

However, it is difficult and difficult to manufacture such a TFT element 5 without variation over the entire screen.

It is an object of the present invention to provide an organic EL element driving device which does not require a TFT element capable of converting a variable amount of a signal level into a variable amount of current accurately.

[0020]

In order to achieve the above object, an organic electroluminescence element driving device according to the present invention comprises: an organic electroluminescence element driving device having an active matrix structure;
It is characterized in that gradation is applied during one frame period using a pulse width modulation signal corresponding to an input signal.

With the above configuration, in the organic electroluminescence element driving device having the active matrix configuration, gradation is applied to each pixel in one frame period using a pulse width modulation signal corresponding to an input signal. As a result, it is possible to provide an organic EL element driving device that does not require a TFT element capable of accurately converting a variable amount of a signal level into a variable amount of a current.

[0022]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 is a circuit diagram of one pixel of an active matrix panel of an organic EL device driving device according to a first embodiment of the present invention.

As shown in FIG. 1, the organic EL element driving device 10 includes a power supply line 11, a signal line 12, a control line 13, a first switching element 14, a storage capacitor 15, and a driving TFT 1.
The organic EL element 18 is driven by an active matrix circuit having a switching TFT 6 and a switch TFT 17.

A predetermined drive voltage is applied to the power supply line 11, and the ground line 19 is grounded. Further, a predetermined voltage for operating the driving TFT 16 is applied to the setting voltage line 20.

An N-channel (Nch) TFT element, which is a switch TFT 17, is disposed in parallel with the storage capacitor 15 of the active matrix circuit, and pulse width modulation (pu) is performed.
The driving TFT 1 held by the holding capacitor 15 by adding the PWM signal formed by the first width modulation (PWM) signal conversion circuit 21.
6 is discharged to the ground line 19.

The organic EL element 18 is directly connected to the power supply line 11, and is connected to the ground line 19 via the N-channel drive TFT 16. The drive TFT 16 converts a drive voltage applied from the power supply line 11 to the ground line 19 into a drive current corresponding to a control voltage applied to the gate electrode, and supplies the drive current to the organic EL element 18.

One end of a storage capacitor 15 is connected to the gate electrode of the driving TFT 16 as voltage holding means, and the other end of the storage capacitor 15 is connected to a ground line 19.
The first switching element 1 serving as switching means is connected to the storage capacitor 15 and the gate electrode of the driving TFT 16.
4 is connected to one end.

The control terminal (gate electrode) of the first switching element 14 is connected to the control line 13, and the remaining terminals are connected to the set voltage line 20. A predetermined voltage is applied to the set voltage line 20 (may be connected to the power supply line 11), and the first switching element 1
4 and is supplied to the gate electrode of the driving TFT 16.

The signal line 12 has a PWM signal conversion circuit 21
Is connected, and the output of the PWM signal conversion circuit 21 is connected to the gate electrode of the switch TFT 17. The control line 13 is also connected to a PWM signal conversion circuit 21. The PWM signal conversion circuit 21 converts an input signal from the signal line 12 into a PWM signal in synchronization with a signal on the control line 13, and Output to TFT17.

That is, the circuit for one pixel of the organic EL element driving device 10 (see FIG. 1) is different from the circuit for one pixel of the conventional organic EL display device (see FIG. 9) by the switch TFT 17 and the PWM signal conversion circuit. 21 is added.

FIG. 2 is a configuration diagram of an image display device using the organic EL element driving device of FIG. As shown in FIG. 2, the organic EL device driving device 10 is used as a part of an image display device 22. In this image display device 22, (m × n) organic EL devices are mounted on one circuit board. (Not shown) are arranged in m rows and n columns.

The m power lines 11 are connected to each other to make one, and one DC power source 23 is connected. The m ground wires 19 are also connected to each other to be one, and are grounded by being connected to a large-capacity component such as a main body housing (not shown).

Each of the m signal lines 12 is individually connected to m signal drivers 24 for generating a control signal, and each of the n control lines 13 is for generating a control signal. N control signal drivers 25 are individually connected.

In the case of the image display device 22, the signal driver 24 supplies data signals such as video signals for m rows as voltage signals, and the control signal driver 25 sequentially outputs drive signals for each horizontal scanning period. In addition, setting voltage generator 2
6 outputs the set voltage to the set voltage line 27.

All of these drivers 24 and 25 are connected to one integrated control circuit (not shown). This integrated control circuit includes m signal drivers 24 and n control signal drivers 25. Integrated control of the matrix drive.

It is also possible to provide n setting voltage generators 26 for each column (see FIG. 3), change the voltage for each column, and cancel the influence of wiring resistance and the like. Alternatively, it is possible to offset the influence of the wiring resistance and the like for each row by providing m rows. If one pixel is prepared for all pixels, the luminance of the entire screen can be changed. If the pixel is connected to the power supply line 11 in one pixel, the setting voltage generator 26 can be omitted.

FIG. 3 is a signal waveform diagram in the organic EL element driving device of FIG. As shown in FIG. 3, in the organic EL element driving device 10, a control signal (see (a)) is input to the control line 13 and the first switching element 1
4 is turned on, and in this state, the signal line 12 is turned on.
Then, a signal for one pixel for causing the organic EL element 18 to emit light (see (b)) is input.

The PWM signal conversion circuit 21 operates in synchronization with the signal on the control line 13 and converts the signal from the signal line 12 into a PWM signal.
After being converted into a signal (see (c)), it is inverted (see (d)) and output to the gate of the switch TFT 17.

On the other hand, when a set voltage is applied to the set voltage line 20, this voltage is held in the storage capacitor 15 via the first switching element 14. Since the holding voltage of the holding capacitor 15 is applied to the gate electrode of the driving TFT 16, the driving voltage constantly applied to the power supply line 11 is converted into a driving current by the driving TFT 16 and
8 is supplied.

The amount of the current is determined from the holding capacitance 15 to the driving TF.
Since it corresponds to the voltage applied to the gate electrode of T16,
The organic EL element 18 emits light at a luminance corresponding to the signal supplied to the signal line 12, and the operation state is determined by the holding voltage of the holding capacitor 15 even when the first switching element 14 is turned off. Will be maintained.

Thereafter, the switch TFT 17 is operated by the gate signal (see (d)) of the switch TFT 17, and the drive TFT 16 is operated by the holding voltage (see (e)). Then, the corresponding current Ie1 flows through the organic EL element 18, and the organic EL element 18 is turned off in the middle of one frame period.

Since the length of the light-off period is determined by the PWM signal (see (c)), it means that gradation is applied by the input signal. If this is applied to the image display device 22 using the organic EL element driving device 10, an image can be displayed on the organic EL panel.

With the organic EL element driving device 10, even in a TFT element in which the variable amount of the signal level cannot be accurately converted to the variable amount of the current, the active matrix driving is performed by the organic EL, and the gradation is changed. Can be turned on. This converts the input video signal to a PWM signal,
This is because, even if accurate VI conversion is not performed, by applying blanking for varying the emission time of the organic EL during one frame period, gradation can be provided only by the on / off operation. (Second Embodiment) FIG. 4 is a circuit diagram of one pixel of an active matrix panel of an organic EL device driving device according to a second embodiment of the present invention.

As shown in FIG. 4, the organic EL device driving device 30 is different from the organic EL device driving device 10 of FIG. 1 in that a first signal line 31 is used instead of the set voltage line 20 and the signal line 12.
And a second signal line 32, the first switching element 14 is connected to the first signal line 31, and the second signal line 32 is connected to the PWM signal conversion circuit 33.

That is, the organic EL element driving device 30
Is provided with a first signal line 31 in place of the set voltage line 20 of the organic EL element driving device 10, except for the operation related to the first signal line 31. This is the same as the EL element driving device 10.

FIG. 5 is a signal waveform diagram in the organic EL element driving device of FIG. As shown in FIG. 5, in the organic EL element driving device 30, a control signal (see FIG.
Is turned on, and in this state, the second signal line 3 is turned on.
2, a signal for one pixel (see (b)) for causing the organic EL element 18 to emit light is input.

The PWM signal conversion circuit 33 operates in synchronization with the signal on the control line 13, converts the signal from the second signal line 32 into a PWM signal (see (c)), and then inverts the signal ((d) )) And output to the gate of the switch TFT 17.

On the other hand, when a certain voltage is applied to the first signal line 31, this voltage is stored in the storage capacitor 15 via the first switching element 14. Since the holding voltage of the holding capacitor 15 is applied to the gate electrode of the driving TFT 16, the driving voltage constantly applied to the power supply line 11 is converted into a driving current by the driving TFT 16 and the organic
It is supplied to the L element 18.

The amount of the current is determined by the driving TF
Since it corresponds to the voltage applied to the gate electrode of T16,
The organic EL element 18 emits light at a luminance corresponding to the signal supplied to the first signal line 31, and this operation state is maintained even when the first switching element 14 is turned off. Maintained by holding voltage.

The voltage applied to the first signal line 31 is complementary to the voltage applied to the second signal line 32. For example, the voltage level of the first signal line 31 is roughly varied, and the voltage of the second signal line 32 is
An operation of finely varying the PWM signal is possible (see (c)).

Thereafter, the switch TFT 17 is operated by the gate signal (see (d)) of the switch TFT 17, and the drive TFT 16 is operated by the holding voltage (see (e)). Then, the corresponding similar current Ie1 flows through the organic EL element 18, and as in the first embodiment, 1
The organic EL element 18 is turned off in the middle of the frame period, and as a result, gradation is applied by the input signal.

In the organic EL device driving device 30, a wider variable range of luminance can be obtained, and a further effect of increasing the contrast can be expected. (Third Embodiment) FIG. 6 is a circuit diagram of one pixel of an active matrix panel of an organic EL device driving device according to a third embodiment of the present invention.

As shown in FIG. 6, the organic EL device driving device 35 is different from the organic EL device driving device 10 of FIG.
The WM signal conversion circuit 21 is replaced with a pulse width expansion circuit 36, and a P line is used instead of the signal line 12 for inputting an analog signal.
A signal line 37 for inputting a WM signal is provided, and the signal line 37 is connected to the pulse width expansion circuit 36.

In the organic EL element driving device 35, the operation between the signal line 12 of the organic EL element driving device 10 and the PWM signal conversion circuit 21 is the same as the operation between the signal line 37 and the pulse width expansion circuit 36. Other operations are the same as those of the organic EL element driving device 10 of the first embodiment.

FIG. 7 is a signal waveform diagram in the organic EL element driving device of FIG. As shown in FIG. 7, in the organic EL element driving device 35, a PWM signal ((b) as a gradation signal created on the signal line 37 based on a digital signal obtained from a preceding device (not shown). See). This PWM signal is supplied to a pulse width expansion circuit 36.
And is enlarged at a fixed magnification (see (c)).

The switch TFT 17 is operated by the gate signal of the switch TFT 17 (see (d)), and the drive TFT 16 is operated by the holding voltage (see (e)), so that the current Ie1 flows through the organic EL element 18. The organic EL element 18 is turned off in the middle of one frame period, and gradation is applied by an input signal.

Therefore, in the organic EL element driving device 35, even if the video signal subjected to the digital signal processing is used,
An effect that the circuit scale does not increase and the power consumption does not increase can be expected. This is because the digitized signal can be input to the organic EL panel, which is the final stage of image display, and applied with gradation without returning to an analog signal again.

As a result, it is possible to provide an organic EL element driving device which does not cause an increase in circuit scale and an increase in power consumption even when a digitized signal is used.

That is, in recent years, the digitization of the signal processing circuit has been progressing even for the video signal. However, when inputting to the organic EL panel which is the final stage of image display, the digitized signal is converted into an analog signal again. Will be returned to. In order to perform such digital-to-analog conversion, an increase in circuit scale is inevitable, which leads to an increase in power consumption. We can respond to this request.

As described above, in the organic EL element driving devices 10, 30, and 35 described in the above embodiments, in the active matrix type organic EL panel, the gradation is controlled by performing the pulse width modulation, and the PWM signal is controlled. Is the storage capacity 15
Accordingly, the gate voltage of the driving TFT 16 held for one frame period is inserted only for a predetermined period in one frame period, and during this period, the emission control of the organic EL element is turned off to perform gradation control.

That is, the amplitude of the input signal is converted into pulse width modulation, or converted into pulse width modulation and amplitude modulation, or the pulse width of the input signal previously input with the pulse width modulation signal is expanded. Outputs from the PWM signal conversion circuits 21 and 33 and the pulse width expansion circuit 36 are connected to the switch TFT1.
7 to turn on the switch TFT 17 and forcibly discharge the voltage held by the holding capacitor in the active matrix driving to the ground line, thereby performing gradation control. , Using a pulse width modulation signal corresponding to the input signal, to apply gradation during one frame period.

As a result, even in a TFT element in which the variable amount of the signal level cannot be accurately converted into the variable amount of the current, the active matrix driving can be performed by the organic EL to give a gradation.

That is, the organic EL element driving device according to the present invention applies gradation by PWM in an organic EL driving circuit using active matrix driving TFTs. Normally, in the case of the active matrix driving, the gray scale is applied by voltage modulation (AM modulation), because it is easy to realize on the principle of the active matrix.

By the way, if the organic EL element driving device according to the present invention is configured as described above, the gradation can be controlled by PWM.
When an attempt is made to construct a circuit, it is expected that the aperture ratio of a liquid crystal pixel that must transmit light will be extremely low. Therefore, it seems that PWM is not practical for a liquid crystal panel driven by active matrix.

However, the organic EL element is self-luminous, and it is not necessary to provide a light-transmitting portion unlike liquid crystal. Therefore, in the case of an organic EL element display device, it is necessary to make a structure in which the organic EL element is overlaid on the TFT element. Can be.

[0066]

As described above, according to the present invention, in an organic electroluminescence element driving device having an active matrix structure, a pulse width modulation signal corresponding to an input signal is used for each pixel during one frame period. Therefore, it is possible to provide an organic EL element driving device that does not require a TFT element capable of accurately converting a variable amount of a signal level into a variable amount of a current into a variable amount of a current.

Also, using a digitized signal,
It is possible to provide an organic EL element driving device which does not cause an increase in circuit scale and an increase in power consumption.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of one pixel of an active matrix panel of an organic EL element driving device according to a first embodiment of the present invention.

FIG. 2 is a configuration diagram of an image display device using the organic EL element driving device of FIG. 1;

FIG. 3 is a signal waveform diagram in the organic EL element driving device of FIG.

FIG. 4 is a circuit diagram of one pixel of an active matrix panel of an organic EL element driving device according to a second embodiment of the present invention.

FIG. 5 is a signal waveform diagram in the organic EL element driving device of FIG.

FIG. 6 is a circuit diagram of one pixel of an active matrix panel of an organic EL element driving device according to a third embodiment of the present invention.

FIG. 7 is a signal waveform diagram in the organic EL element driving device of FIG.

FIG. 8 is a partial detailed view of a display unit of a conventional organic EL display device.

9 is a circuit diagram of one pixel of the display unit of FIG. 8;

[Explanation of symbols]

 10, 30, 35 Organic EL element driving device 11 Power supply line 12, 37 Signal line 13 Control line 14 First switching element 15 Storage capacitor 16 Driving TFT 17 Switch TFT 18 Organic EL element 19 Ground line 20 Setting voltage line 21, 33 PWM signal conversion circuit 22 Image display device 23 DC power supply 24 Signal driver 25 Control signal driver 26 Setting voltage generator 27 Setting voltage line 31 First signal line 32 Second signal line 36 Pulse width expansion circuit Ie1 Current

Claims (8)

[Claims] 1. An organic electroluminescence element driving device having an active matrix structure, wherein gradation is applied to each pixel during one frame period using a pulse width modulation signal corresponding to an input signal. Luminescent element driving device. 2. The organic electroluminescence device driving device according to claim 1, wherein the gradation control is performed by converting the amplitude of the input signal into pulse width modulation. 3. The organic electroluminescence element driving device according to claim 1, wherein the gradation control is performed by converting the amplitude of the input signal into pulse width modulation and amplitude modulation. 4. A driving thin film transistor for converting a driving voltage of an organic electroluminescence element into a driving current corresponding to a control voltage applied to a gate electrode and supplying the driving current, and a pulse width modulation signal for discharging a gate voltage of the driving thin film transistor 4. The organic electroluminescence element driving device according to claim 2, further comprising: a pulse width modulation signal conversion circuit that forms 5. The organic electroluminescence element driving device according to claim 1, wherein gradation control is performed by expanding a pulse width of an input signal previously input by a pulse width modulation signal. 6. A driving thin film transistor which converts a driving voltage of an organic electroluminescence element into a driving current corresponding to a control voltage applied to a gate electrode and supplies the driving current, and a pulse which expands the pulse width modulation signal at a constant magnification. The organic electroluminescence element driving device according to claim 5, further comprising a width expansion circuit. 7. The organic electroluminescence element driving device according to claim 4, further comprising a switch thin film transistor that is turned on by a gate signal input and discharges a gate voltage of the driving thin film transistor to a ground line. 8. The organic electroluminescent element is turned off only for a predetermined period in one frame period by forcibly discharging a voltage held by a storage capacitor in the active matrix driving to a ground line. The organic electroluminescence element driving device according to claim 1, wherein a tone is applied.