The present invention relates to display devices and in particular to improving the display quality thereof. The invention also relates to a method and an electronic apparatus.
One example of a display device to which the present invention relates is an organic electroluminescent display device. Organic electroluminescent devices (OELDs) comprise a layer (active layer) of organic light emitting material, often a light emitting polymer, sandwiched between two electrodes which are used to pass a current through the active material. The device essentially behaves like a diode and the intensity of light emission is a function of the forward bias current which is applied. The devices are good candidates for the fabrication of display panels.
A basic requirement for a display panel is an ability to display good quality graphical images. This is dependent upon the ability of the individual pixels to generate a range of brightness intensity. The image quality improves as the number of gray scales increases. The conventionally used standard is 3×8 bit colour, equivalent to 256 gray scales per colour. This standard is used in many current day applications.
Various methods of generating gray scales with an analog driving circuit have been proposed for OELD displays. The conventional technique is to drive the OELD with a voltage dependent current and this has allowed the implementation of active matrix OELD displays. A typical arrangement is illustrated in
As shown in
The variation of threshold voltages of the transistors is, however, a very significant problem in the practical implementation of the above described display panels. Another significant problem is the high power consumption of these circuits.
An alternative method of providing gray scaling is to use an area dithering technique in which each pixel is divided in to a number of sub-pixels, preferably with binary weighted areas. Each sub-pixel is driven either fully on or fully off. Thus a digital driver can be used and power consumption reduced. However, this technique has the disadvantage that the panel size is increased (because each pixel is replaced by a number of sub-pixels and, in the limit, each sub-pixel is the same size as a conventional pixel) and also there is a large increase in the number of signal lines required (because of the need to address each sub-pixel).
Against this background, it is an object of the present invention to provide a display device with good gray scale capabilities which mitigates the above mentioned disadvantages.
According to the present invention there is provided a display device comprising a driver circuit which modulates the duty cycle of the on-state of a pixel during a frame period.
Thus, the present invention provides pulse width modulation of the on-period of a pixel and the integrating function of the human eye perceives this as modulation of the intensity of the emitted light. Modulation of the on-period is in stark contrast to the conventional control of brightness, ie control of the instantaneous amplitude of the current supplied.
Embodiments of the present invention will now be described in more detail by way of further example only and with reference to the accompanying drawings, in which:
A description will first be given of the pixel level configuration according to one embodiment of the present invention. Thus,
In a line-at-a-time driving scheme, Vsel sets the state of the pass-gate SW1 of the pixel elements on the same row. When pass-gate SW1 is closed, the data voltage Vdat is transferred to the inverting input of the comparator 12 and to the capacitor C1. Then, when pass-gate SW1 is opened the data voltage is memorised by capacitor C1. The waveform Vsaw is then initiated. When the voltage, V+, at the inverting input of the comparator 12 is less than the voltage, V−, at the non-inverting input thereof, the comparator outputs a LO signal which puts the light emitting element 14 in to the on-state. When the voltage, V+, at the inverting input of the comparator 12 is greater than the voltage, V−, at the non-inverting input thereof, the comparator outputs a HI signal which puts the light emitting element 14 in to the off-state. As a result the data voltage stored by the capacitor C1 modulates the duration for which the light emitting element 14 remains in the on-state during a frame period.
The frame period might typically be 20 mS and with the response time of the light emitting element 14 being of the order of nano-seconds, the speed of the polysilicon TFTs and any stray capacitance become the limiting factors in operation of the driving scheme. That is, exceptionally effective switching can be obtained.
In the circuit illustrated in
A description will now be given of the detailed considerations which apply to the practical implementation of the comparator 12 used in the circuit of
Since a separate comparator is provided for each pixel, the circuit area and power consumption of the comparator should be kept as low as possible. Further, in order to achieve a high number of gray scales, the comparator must be able to distinguish a small difference in input voltages. For example, if it is desired to implement 256 gray scales with a voltage swing of 0V to 5V then clearly something of the order of ΔV=19.5 mV is appropriate. Thus switching must be very fast but, from the previous discussion, it is well within the ability of the described circuit.
A detailed circuit diagram of one implementation of the comparator 12 of
The differential stage 16 comprises the drain-source series connection circuit of transistors 20, 21 and 23 connected between the VDD rail and ground, together with the similarly connected circuit of transistors 20, 22 and 24, wherein transistors 22 and 24 are connected in parallel with transistors 21 and 23. The respective gates of transistors 21 and 22 provide the two input terminals (+), (−) of the comparator 12, whereas the gate of transistor 20 receives a bias voltage Vbias. The output stage 18 comprises two transistors, 25 and 26, which are source-drain series connected between the VDD rail and ground. The output Vout of the comparator is taken from the connection between the transistors 25 and 26 and the gates thereof receive there input from the junction between transistors 21 and 23.
The circuit illustrated in
A description will now be given of various aspects of implementing a display panel incorporating the above described embodiment of pixel level circuitry.
Basically all pixels in the same row of the matrix share the same driving waveform, denoted by Vsaw/m where m indicates that it is the mth-row of the matrix which is being considered. When rows are addressed sequentially, the driving waveforms for the next row, denoted by Vsaw/m+1, should incorporate a delay or phase shift of Tframe/M, where Tframe is the frame period and M is the total number of rows in the matrix. Thus if the display is driven externally a total of M interconnections are required. This can be a problem for high resolution displays. Thus, according to one embodiment of the present invention there is provided an integrated waveform generator, by which the number of interconnections required can be reduced.
Ideally, however, the function of the generators is to provide the same waveform with a unique phase shift for each row of pixel elements. The precise timing and data voltage relationship becomes a major challenge when the spatial variation of TFT characteristics over a display panel is taken into account. However, this problem can be solved by providing the master clock Vmaster and the reference voltage source Vref to ensure that outputs from all waveform generators are the same but different in phase shift.
The waveform generator should be synchronised to Vscan/m, and thus the signal Vscan/m can be used as a trigger.
From the foregoing description, a generalised synchronous driving scheme is illustrated in
The display can also be driven asynchronously. An asynchronous driving scheme is shown in
It is also possible to incorporate gamma compensation into the driving waveform. This is illustrated in
As noted above, the circuit has four inputs (Vgray, φ1, φ2 and Vscan) and one output (Vsaw). The input waveforms are shown in
Waveform Vgray operates between 0V and a maximum level, say h. Waveforms φ1 and φ2 are non-overlapping clock pulses and Vscan is the same signal as in the scan line. When Vscan goes HI, data is transferred to the pixel storage capacitor as described above. At the same time, Vscan signals SW30 to close so that the input of the unity gain buffer is at 0V and C10 is discharged. Effectively, this acts as a reset and zeros the output. When Vscan goes LO, SW30 is opened. Waveform Vgray=0V when SW20 is closed and SW10 is opened. The transition of Vgray from 0V to h raises the input voltage at the unity gain buffer. If C10=C20, this increment equals h/2. When Vgray=h, SW20 is opened and SW10 is closed. The unity gain buffer 32 input voltage is stored by C10. This voltage is reflected by the output of the unity gain buffer and is connected to C20 while Vgray returns to 0V. Next SW10 is opened and then SW20 is closed, and then Vgray will transit from 0V to h. This will increase further the voltage at the input of the unity gain buffer 32. If C10=C20, this increment equals h/2 and the resulting voltage becomes h. This continues and the output of the unity gain buffer 36 takes on a step shape. If the output is passed through the low pass filter L.P. the output signal becomes a smooth ramp.
It may be appreciated that the described arrangements according to the present invention can utilise existing analog video signals as input signals.
An example was implemented using the circuits described above, with polysilicon TFTs. Using a data voltage range of 0V to 5V, 256 gray scales were implemented.
After the data transfer, which typically occurs in the first 20 μs, the frame period was divided into 256 sections. For a frame rate of 50 cycles/s, the time difference for each additional gray scale is given by Δt=1/50÷256=78.125 μs, and the corresponding data voltage difference is given by ΔV=5÷256=19.53 mV. It is noted that for gray scale=0 the OELD must not be turned on at all.
The current required by the driver is small compared to the current flowing in to the electroluminescent element.
Generally, the image quality which can be achieved with the present invention has been found to be superior to conventional Liquid Crystal Displays and at least equal to that of conventional CRT displays. In addition, the low power consumption required by the display device of the present invention makes it ideal for mobile and portable apparatus.
Modifications
As will already be appreciated, although much of the detail given above in relation to specific embodiments has been in terms of organic electroluminescent display devices; the present invention is also applicable to other types of display devices. Further, althought the above described embodiments have mentioned specific implementation using TFT technology, usually in polysilicon,; the present invention is not limited to the use of TFT technology. The invention is applicable not only to thin film transistor technology but also to silicon based transistors. Silicon based transistors can be arranged on a display substrate using several different methods. For example, silicon based transistors can be arranged in a liquid.
The present invention is advantageous for use in small, mobile electronic products such as mobile phones, computers, CD players, DVD players and the like—although it is not limited thereto.
Several electronic apparatuses using a display device according to the present invention will now be described.
<1: Mobile Computer>
An example in which the display device according to one of the above embodiments is applied to a mobile personal computer will now be described.
<2: Portable Phone>
Next, an example in which the display device is applied to a display section of a portable phone will be described.
<3: Digital Still Camera>
Next, a digital still camera using an OEL display device as a finder will be described.
Typical cameras sensitize films based on optical images from objects, whereas the digital still camera 1300 generates imaging signals from the optical image of an object by photoelectric conversion using, for example, a charge coupled device (CCD). The digital still camera 1300 is provided with an OEL element 100 at the back face of a case 1302 to perform display based on the imaging signals from the CCD. Thus, the display panel 100 functions as a finder for displaying the object. A photo acceptance unit 1304 including optical lenses and the CCD is provided at the front side (behind in the drawing) of the case 1302.
When a cameraman determines the object image displayed in the OEL element panel 100 and releases the shutter, the image signals from the CCD are transmitted and stored to memories in a circuit board 1308. In the digital still camera 1300, video signal output terminals 1312 and input/output terminals 1314 for data communication are provided on a side of the case 1302. As shown in the drawing, a television monitor 1430 and a personal computer 1440 are connected to the video signal terminals 1312 and the input/output terminals 1314, respectively, if necessary. The imaging signals stored in the memories of the circuit board 1308 are output to the television monitor 1430 and the personal computer 1440, by a given operation.
Examples of electronic apparatuses, other than the personal computer shown in
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