Display unit having brightness control function

Abstract: A display unit comprising a thin film EL display panel, a scanning side switching circuit connected to a scanning side electrode, a data side switching circuit connected to a data side electrode, a scanning side drive circuit which outputs a high voltage pulse to the scanning side switching circuit and a data side drive circuit which outputs a signal voltage to the data side switching circuit, in which means for decreasing a pulse width of a high voltage pulse supplied from the scanning side drive circuit to the scanning side switching circuit in accordance with an increase of the level of a high voltage generated by a high voltage power supply in the scanning side drive circuit.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a display unit specifically a capacitive flat matrix display (referred to as a thin film EL display hereinafter). 2. Description of the Prior Art Fig. 6 is a block diagram showing a structure of a common thin film EL display unit. display panel 1 is formed of a thin film EL element. In the thin film EL element, belt-shaped transparent electrodes are arranged in parallel on a glass substrate, a three-layer structure is formed by laminating a dielectric material, an EL layer thereon and the dielectric material thereon, and then belt-shaped back electrodes are arranged in parallel in a direction crossing at a right angle to the transparent electrodes. The thin film El element is driven by a comparatively high voltage of approximately 200V as can be seen from the applied voltage-brightness characteristic shown in Fig. 7. In the display panel 1, the transparent electrodes of the thin film EL element are designated by data side electrodes D1 to Dm and the back electrodes of the thin film EL element are designated by scanning side electrodes S1 to Sn. A data side switching circuit 2 is a circuit for individually applying a modulation voltage VM to each of data side electrodes D1 to Dm, which circuit comprises a data side output port group 3 connected to each of the data side electrodes D1 to Dm and a logical circuit 4 which receives display data corresponding to each of the data side electrodes D1 to Dm and turns the data side output port group 3 on and off in accordance with the display data. A scanning side switching circuit 5 is a circuit for sequentially applying writing voltages VW1 and -VW2 (VW1 = VW2 + VM) to the scanning side electrodes S1 to Sn in order, which circuit comprises a scanning side output port group 6 connected to each of the scanning side electrodes S1 to Sn and a logical circuit 7 which turns the scanning side output port group 6 on and off in accordance with the order of the scanning side electrodes S1 to Sn. A drive circuit 8 is a circuit for generating a high voltage for driving the display panel 1 from a constant reference voltage VD, which circuit comprises a modulation drive circuit 9 for applying the modulation voltage VM to the data side output port group 3 and a writing drive circuit 10 for applying the writing voltages VW1 and -VW2 to the scanning side output port group 6. A driving logical circuit 11 is a circuit for generating various timing signals necessary for drive of the display panel 1 in accordance with an input signal such as display data D, a data transfer clock CK, a horizontal synchronizing signal H or a vertical synchronizing signal V. Fundamental drive of the display unit, in which a period over two first and second fields is one cycle, is performed by applying the modulation voltage VM corresponding to the display data which decides emission or non-emission, to the data side electrodes D1 to Dm, while applying the voltage VW1 in the first field and the voltage -VW2 in the second field as the writing voltage to the scanning side electrodes S1 to Sn in order. By this display drive, a superimposed effect or an offset effect of the writing voltages VW1, -VW2 and the modulation voltage VM is generated at each pixel where the data side electrodes D1 to Dm and the scanning side electrodes S1 to Sn cross. As the thin film EL element forming the display panel 1 shows the applied voltage-brightness caracteristic shown in Fig. 7, the voltage VW1 of an emission threshold voltage Vth or more or the voltage VW2 of the emission threshold voltage Vth or less is applied to the pixel as an effective voltage by the superimposed effect and the offset effect of the writing voltages VW1, -VW2 and the modulation voltage VM, so that each pixel acquires an emission or non-emission state and a predetermined display can be obtained. Therefore, the effective voltage whose polarity is inverted is alternately applied to one pixel in the first and second fields, whereby symmetrical AC drive which is ideal for the thin film EL element can be performed in the two fields of one cycle. Fig. 8 is a block diagram showing a structure of the writing drive circuit 10 and the driving logical circuit 11 in detail. The writing drive circuit 10 comprises a high voltage power supply 13 which generates a hign voltage HV and a switching element 12 for obtaining pulse-shaped writing voltages VW1 and VW2 which correspond to the timing when the scanning side output port group 6 specifies the row of each pixel in the display panel 1 by intermittently supplying the high voltage HV to the scanning side output port group 6. On and off of the switching element 12 is controlled by a control signal HVC from the driving logical circuit 11. In addition, the driving logical circuit 11 comprises a memory 14 such as a read only memory and the control signal HVC is output in accordance with the timing written in the memory 14. Fig. 9 comprises timing charts showing the timing of the drive of the display unit, in which Fig. 9(1) shows a vertical synchronizing signal V, Fig. 9(2) shows a pulse waveform of the writing voltage applied to the scanning side Electrodes S1 to Sn and Fig. 9(3) shows a waveform of the high voltage HV output from the high voltage power supply 13 in the writing drive circuit 10. According to the conventional display unit, there is fluctuation in the high voltage HV output from the high voltage power supply 13 in the writing drive circuit 10 as shown in Fig. 9. Therefore, the amplitude of the pulse voltage applied as the writing voltages VW1 and -VW2 varies according to the scanning side electrode. As a result, a brightness difference is generated between scanning lines on a screen, causing a display quality to be considerably deteriorated. More specifically, as shown in Fig. 9(1), after the writing voltage is applied to the last scanning side electrode Sn, there is a blank period in the vertical synchronizing signal V before it is applied to the first scanning side electrode S1 in the next field. For this period a load to the high voltage power supply 13 is decreased and then an output level of the high voltage power supply 13 is increased as shown in Fig. 9(3). Thus, even if the writing voltage starts to be applied to the scanning side electrode S1, the output level does not immediately return to a predetermined value and the output level is kept high for a while. As a result, the writing voltage applied to the first scanning side electrode S1 is higher than that applied to the last scanning side electrode Sn, so that a brightness difference between the scanning lines is generated. As means for solving the above problems, it is thought that load fluctuation of the high voltage power supply 13 itself should be held down. However, in this case, it is necessary to insert a large capacity capacitor into an output stage of the high voltage power supply 13 or increase control precision of the control circuit for the high voltage power supply 13, causing in increase in the number of parts and the cost. SUMMARY OF THE INVENTION It is an object of the present invention to provide an EL display unit in which a brightness difference is prevented from being generated between scanning lines on a screen without increasing its cost. According to the present invention, as defined by claim 1, there is provided an EL display unit in which an array of pixels is formed at the crossing points of a set of scanning electrodes and a set of data electrodes extending transverse to said scanning electrodes, and in which scanning drive circuitry includes a selection voltage generation means and is operable to apply the selection voltage as pulses to the scanning electrodes in sequence, characterised in that said scanning drive circuitry includes means for altering the pulse width of the selection voltage pulses to compensate for variation in the level of the selection voltage, as output by said selection voltage generation means, such that said pulse width is reduced as said selection voltage level is increased. The preamble of claim 1 reflects the state of the art according to EP-A-O 345 399, which discloses a capacitive display apparatus in which the widths of modulation pulses applied to data electrodes are varied according to the selected scanning electrode so as to compensate for the effect on the pixel display brightness down the display screen of the line resistance of the data electrodes. According to the present invention, when an amplitude of the selection voltage pulse (writing voltage) is increased in accordance with an increase of an output level of the selection voltage generation means (high voltage power supply) in the scanning drive circuitry, a pulse width of the selection voltage pulse is accordingly decreased. As a result, a pixel on any scanning electrode on a screen has a uniform brightness and the display can be implemented with uniform brightness of the pixels irrespective of the positions of their associated scanning electrodes. Since a pixel on any scanning electrode on a screen can have the same brightness without being influenced by output fluctuation of the power supply, the display can be implemented with uniform brightness without increasing the cost. DESCRIPTION OF THE DRAWINGS Three embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a timing chart showing the timing of drive of a display unit in accordance with a first embodiment of the present invention; Fig. 2 is a view showing a main part of a circuit structure of a display unit in accordance with a second embodiment of the present invention; Fig. 3 is a timing chart showing a timing of drive of the display unit; Fig. 4 is a view showing a main part of a circuit structure of a display unit in accordance with a third embodiment of the present invention; Fig. 5 is a timing chart showing a timing of drive of the display unit; Fig. 6 is a block diagram showing a schematic structure of a conventional thin film EL display unit; Fig. 7 is a view showing an applied voltage-brightness characteristic of the thin film EL element; Fig. 8 is a block diagram showing a main part of a circuit structure of the conventional thin film EL display unit; and Fig. 9 is a timing chart showing a timing of drive of the conventional thin film EL display unit. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 is a timing chart showing a timing of drive of a display unit in accordance with a first embodiment of the present invention, in which Fig 1(1) shows a waveform of a vertical synchronizing signal, Fig. 1(2) shows a pulse waveform of a writing voltage and Fig. 1(3) shows a high voltage output waveform of the high voltage power supply 13. A thin film EL display unit is shown in this embodiment of the present invention and its schematic structure is the same as the common thin film EL display unit shown in Figs. 6 and 8, so that its structure is not shown and its description is omitted here. According to the first embodiment of the present invention, timing data is previously written in the memory 14 so that a control signal HVC may be output, which signal is applied from the memory 14 of the driving logical circuit 11 shown in Fig. 8 to the switching element 12 of the writing drive circuit 10 and whose pulse width is narrower than in the common display unit while the first few lines of the scanning side electrodes, for example the scanning side electrode S1 to the scanning side electrode S4, are specified, and is gradually increased as the scanning side electrode is sequentially specified. Therefore, according to the display unit of the present embodiment, after the high voltage HV of the high voltage power supply 13 whose level is increased in the blank period starts to apply a writing voltage to the scanning side electrode, it is gradually decreased to a predetermined level, while the amplitude of the writing voltage is accordingly increased. The greater is the amplitude thereof, the narrower becomes the pulse width thereof, as shown in Fig. 1(2). The greater is the amplitude of the writing voltage, the greater is the effective voltage applied to the pixel of the display panel 1. Thus, brightness of the pixel is also increased as can be seen from the applied voltage-brightness characteristic shown in Fig. 7. Meanwhile, the shorter is the period of voltage application, more specifically, the narrower the pulse width of the writing voltage becomes, the shorter an emission time of the pixel becomes. As a result, the brightness of the pixels on each of the scanning side electrodes S1 to Sn is about the same. Fig. 2 is a view showing connection of the writing drive circuit 10 and the driving logical circuit 11 of the thin film EL display unit in accordance with a second embodiment of the present invention. The structure of the writing drive circuit 10 and the driving logical circuit 11 is the same as the conventional structure shown in Fig. 8 except that the control signal HVC output from the driving logical circuit 11 is converted to another control signal HVC 2 by a converting circuit 15 and applied to the switching element 12 of the writing drive circuit 10. More specifically, the converting circuit 15 comprises an inverter 16 which inverts the control signal HVC output from the driving circuit 11, an integrating circuit 20 comprising a diode 17, a resistor 18 and a capacitor 19 for integrating the signal inverted by the inverter 16, an integrating circuit 24 comprising a diode 21, a resistor 22 and the capacitor 23 for integrating the vertical synchronizing signal V, and a comparator 25 which compares an output HVC 1 of the integrating circuit 20 with an output V1 of the integrating circuit 24. Fig. 3 is a timing chart showing operation of the converting circuit 15, in which Fig. 3(1) shows a waveform of the vertical synchronizing signal V, Fig.3(2) shows a waveform of the control signal HVC output from the driving logical circuit 11, Fig. 3(3) shows waveforms of the signals HVC1 and V1 output from the integrating circuits 20 and 24, respectively and Fig. 3(4) shows a waveform of the control signal HVC2 output from the converting circuit 15. Next, the operation of the converting circuit 15 will be described in reference to the timing charts shown in Fig 3. The control signal HVC shown in Fig. 3(2) is inverted by the inverter 16 and then converted to the signal HVC1 having an integration waveform shown by a solid line in Fig 3(3) by the integrating circuit 20. Meanwhile, the vertical synchronizing signal V shown in Fig. 3(1) is converted to the signal V1 having an integration waveform shown by an alternate long and short dash line in Fig. 3(3). The signal HVC1 is input to an inversion input terminal of the comparator 25 and the signal V1 is input to a non-inversion input terminal of the comparator 25, so that the comparator 25 outputs the control signal HVC2 which becomes high level only while the signal V1 is at a high level as compared with the signal HVC1 as shown in Fig 3(4), which control signal HVC2 is applied to the switching element 12 of the writing drive circuit 10. The control signal HVC2 is a signal corresponding to the control signal HVC shown in Fig 3(2) and its pulse width is sufficiently narrow at the beginning of the field and then gradually increased to be like the pulse width of the original control signal HVC. As a result, the writing voltage from the writing drive circuit 10, whose pulse width is controlled by the control signal HVC2, has the same waveform as the pulse waveform shown in Fig 1(2). Therefore, in this embodiment of the present invention also, brightness of the pixels is uniform in a vertical direction on the screen of the display panel 1 without being influenced by the fluctuation of the high voltage output of the high voltage power supply 13. Fig. 4 is a view showing a connection structure of the writing drive circuit 10 and the driving logical circuit 11 in a thin film EL display unit in accordance with a third embodiment of the present invention. According to this embodiment of the present invention, the control signal HVC output from the driving logical circuit 11 is converted to another control signal HVC4 by the converting circuit 26 to be applied to the switching element 12 in the writing drive circuit 10 and other structure is the same as that of the second embodiment of the present invention. More specifically, the converting circuit 26 comprises a filter 31 comprising resistors 27 and 28, a capacitor 29 and a diode 30 which passes an AC component HV1 of the high voltage HV output from the high voltage power supply 13, an integrating circuit 35 comprising a diode 32, a resistor 33 and a capacitor 34 which integrates the control signal HVC output from the driving logical circuit 11, and a comparator 36 which compares the output signal HV1 from the filter 31 with the output signal HVC3 from the integrating circuit 35 . Fig. 5 is a timing chart showing operation of the converting circuit 26, in which Fig. 5(1) shows a waveform of the vertical synchronizing signal V, Fig. 5(2) shows a waveform of the control signal HVC output from the driving logical circuit 11, Fig. 5(3) shows waveforms of the signals HV1 and HVC3 output from the filter 31 and the integrating circuit 35, respectively and Fig. 5(4) shows a waveform of the control signal HVC4 output from the converting circuit 26. Next, the operation of the converting circuit 26 will be described in reference to the timing chart shown in Fig. 5. The control signal HVC shown in Fig. 5(2) is converted to the signal HVC3 having the integration waveform shown by a solid line in Fig. 5(3) by the integrating circuit 35. Meanwhile, the AC component HV1 having the waveform shown by an alternate long and short dash line in Fig. 5(3) is taken out from the high voltage HV by the filter 31. The AC component HV1 is input to the inversion input terminal of the comparator 36 and the signal HVC3 is input to the non-inversion input terminal of the comparator 36, so that the comparator 36 outputs the control signal HVC4 which becomes high level only while the signal HVC3 is at high level as compared with the AC component HV1 as shown in Fig.5(4). Then, the signal is input to the switching element 12 of the writing drive circuit 10. The control signal HVC4 is a signal corresponding to the control signal HVC and its pulse width is sufficiently narrow at the beginning of the field and then gradually increased to be like the pulse width of the original control signal HVC. As a result, the writing voltage from the writing drive circuit 10, whose pulse width is controlled by the control signal HVC4 has the same waveform as the pulse waveform shown in Fig. 1(2). Therefore, in this embodiment of the present invention also, the brightness of the pixels is uniform in a vertical direction on a screen of the display panel 1 without being influenced by the fluctuation of the high voltage output of the high voltage power supply. While only certain presently preferred embodiments have been described in detail, as will be apparent to those skilled in the art, certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims.

Claims: An EL display unit in which an array of pixels is formed at the crossing points of a set of scanning electrodes (S₁-S n ) and a set of data electrodes (D₁-D m ) extending transverse to said scanning electrodes, and in which scanning drive circuitry (5,10,11) includes a selection voltage generation means (13) and is operable to apply the selection voltage as pulses to the scanning electrodes in sequence, characterised in that said scanning drive circuitry (5,10,11) includes means (14;15;26) for altering the pulse width of the selection voltage pulses to compensate for variation in the level of the selection voltage, as output by said selection voltage generation means (13), such that said pulse width is reduced as said selection voltage level is increased. An EL display unit according to claim 1, wherein the means (14;15;26) for altering said pulse width operates to produce selection voltages of relatively narrower width for the scanning electrodes corresponding to the first few lines of each field of the displayed image. An EL display unit according to claim 2, wherein the scanning drive circuitry includes a switching element (12) which selectively outputs, according to a control signal, the selection voltage output by said selection voltage generation means (13), thereby generating said selection voltage pulses. An EL display unit according to claim 3, wherein the means for altering said pulse width comprises a memory (14) storing timing data for said control signal. An EL display unit according to claim 3, wherein the means for altering said pulse width comprises a converting circuit (15) comprising: first integrating means (20) for integrating a control signal comprising pulses having a predetermined width; second integrating means (24) for integrating a vertical synchronizing signal; and a comparator (25) for comparing an output of the first integrating means with an output of the second integrating means, and outputting, according to the result of the comparison, a converted control signal for controlling said switching element (12). An EL display unit according to claim 3, wherein the means for altering said pulse width comprises a converting circuit (26) comprising: means (31) for extracting an AC component of the selection voltage output from said selection voltage generation means (13); integrating means (35) for integrating a control signal comprising pulses having a predetermined width; and a comparator (36) for comparing an output signal from the means for extracting the AC component with an output from the integrating means, and outputting, according to the result of the comparison, a converted control signal for controlling said switching element (12).

Inventors: HARADA SHIGEYUKI, KISHISHITA HIROSHI, OHBA TOSHIHIRO, SAKAMOTO ATSUSHI, YAMAMOTO KYOICHI, HARADA, SHIGEYUKI, KISHISHITA, HIROSHI, OHBA, TOSHIHIRO, SAKAMOTO, ATSUSHI, YAMAMOTO, KYOICHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1991
Main classification: G09G3
IPC codes: G09F9, G09G3