1. Field of the Invention
The invention relates to a circuit for an element of a light-emitting display and to a circuit for a light-emitting display having a plurality of elements. The invention furthermore relates to a method for controlling the elements of a light-emitting display.
2. Description of Related Art
Light-emitting displays, which generate light using light-emitting elements through which an electric current flows, comprise a multiplicity of light-emitting elements in a suitable arrangement. In this case, the light-emitting elements emit a luminous flux that is dependent on the electrical current flowing through them. The term luminous flux describes the total radiation power of the light source. In the following the term current is used to represent the electrical current. In the case of a matrix arrangement comprising a plurality of light-emitting elements, monochromatic or polychromatic images having a plurality of pixels are displayed. In the case of monochromatic images, the images are resolved into individual gray-scale values for the pixels. In this case, the gray-scale values are various luminous flux values. The various luminous flux values are generated by corresponding currents through the light-emitting elements. In the case of a polychromatic light-emitting display, a plurality of light-emitting elements of different colors usually interact. Various colors can be produced from the original colors of the light-emitting elements using additive color mixing for each pixel. The light-emitting elements comprise, inter alia, light-emitting diodes. Light-emitting diodes can be produced on the basis of semiconductive materials (for example silicon, germanium) but light-emitting diodes based on organic materials (OLED: “Organic Light-Emitting Diode”) are also available. A common feature of all these light-emitting diodes is that the luminous flux that is output depends on the electrical current through the light-emitting element.
In the case of organic light-emitting diodes (OLEDs), in particular, the current/voltage characteristic curve is greatly dependent on ageing and on process parameters during production.
In organic light-emitting diodes, light is generated by passing a direct current through the organic diode material. In this case, the organic light-emitting diode is forward-biased. It has been found that the forward voltage of the OLED may vary from pixel to pixel and increases over time. It has likewise been found that the current for generating a particular luminous flux remains relatively stable over time.
In today's light-emitting displays comprising light-emitting elements which are arranged in a matrix arrangement and have individual current control means, the individual light-emitting elements are driven successively in lines or columns. FIG. 1 shows a light-emitting element for this type of driving. A current control means 4 is connected in series with a light-emitting element 8 between an operating voltage VDD and ground. A control signal is supplied to a control input of the current control means 4 via a switch 10. In this case, the control signal is a control voltage USet. The switch 10 is controlled in this case in such a manner that only a single light-emitting element in an arrangement of light-emitting elements is respectively driven. In the case of the driving scheme that is required for this circuit, the period of time during which the light-emitting diode radiates light is relatively short. The active period of time is reduced depending on the number of light-emitting elements present in the arrangement of the light-emitting display. Since the human eye is a natural system with a low-pass filter response, it is possible to compensate for the short active period of time by appropriately increasing the luminous flux during the active period of time.
Light-emitting displays in which each current control means is permanently driven by a control signal are also conceivable. The switch 10 can then be dispensed with. However, the multiplicity of requisite control lines reduces the area available for light to emerge on the screen.
In the case of the light-emitting element shown in FIG. 2, a signal holding means 6 has been added to the circuit described above between the control electrode of the current control means 4 and the operating voltage VDD. The control signal USet applied when the switch 10 is closed is kept constant by the signal holding means 6 when the switch is open until a new control signal USet is applied. This makes it possible to extend the active period of time during which the light-emitting element 8 radiates light. The active period of time now covers almost the entire period during which an image is composed. This reduces the requisite luminous flux that must be radiated during the active period of time. Since the observer's eye can now integrate a smaller luminous flux over a longer period of time, the same quantity of light is picked up and the same image impression as described with reference to FIG. 1 results. Since ageing and the change in the electrooptical properties of the OLED greatly depend on the current density of the electrical current through the OLED, this circuit offers the advantage of a slower change in the properties.
However, when a control voltage is used for driving, it is generally necessary to take account of the ageing-related change in the forward voltage of the OLED.
Another method for compensating for the time-dependent electrooptical properties involves the driving being carried out using control currents. To this end, a first current control means is connected upstream of each light-emitting element, that is to say each organic light-emitting diode, for example. The first current control means is connected to a second current control means in such a manner that a current mirror circuit results. In the case of the current mirror circuit, a reference current flows through the second current control means, a corresponding control signal becoming established on a control electrode of the second current control means. This control signal is supplied to the control electrode of the first current control means. If the first and second current control means essentially have the same properties, the current through the first current control means corresponds to the current through the second current control means. The same properties of the two current control means compensate for temperature related, production-related and ageing-related changes.
However, the driving method using currents is complex in terms of circuitry and requires a larger number of components than other known methods. The larger number of components in turn reduces the area available for generating light or forms passive regions which do not allow any light to pass through.
The currents used for driving must cover a wide range of values. In particular, the very small electrical currents for small luminous fluxes can be set only in a poorly reproducible manner. In addition, parasitic capacitances, the charge of which must be reversed by the currents, are formed by the connecting lines. When displaying moving images, for example television pictures, the charge is usually reversed 50 to 60 times a second, depending on the television standard used. Even higher image refresh rates are possible for computer monitors. Small control currents may consequently result in reductions in the image quality, whether as a result of delayed image composition, non-uniform image brightness distribution or the like. In addition, the very small currents which are, for example, in the nanoampere range (nA) can be set in a manner that can be reproduced only with great difficulty.
The use of an appropriate current mirror allows the currents required for control and the currents through the light-emitting elements to be selected independently of one another. In this way, it is possible, for example, to increase the currents required for control, while the currents through the light-emitting elements are in an advantageous range. Overall, however, this increases the control power required for driving.
FIG. 3 shows an element of a light-emitting display as was described in FIG. 2. The element is marked by a dashed frame 1. In this case, the control signal S is taken from the control electrode of a current control means 2. When the switch 10 is closed, the current control means 2 forms a current mirror circuit with the current control means 4 of the element 1. In a light-emitting display comprising a plurality of elements 1 in a grid arrangement, an individual control signal is supplied to each element 1 depending on the image content. To this end, a respective control current iprog is forced through the current control means 2. In this case, a control circuit (not shown in FIG. 3) successively actuates the switches 10 of the various elements 1 of the light-emitting display. The increased complexity of the circuit as compared with the circuits in FIGS. 1 and 2 can clearly be seen.
It has been found that, in the case of certain production methods for organic light-emitting diodes, the electrooptical properties of individual light-emitting elements are essentially the same in some regions. In this case, the term electrooptical properties relates to the current/voltage characteristic curve and the associated luminous fluxes. Suitable control of the production methods allows these regions of essentially the same electrooptical properties to be shaped in such a manner that these regions extend over light-emitting elements that are arranged in lines and/or columns. A correction value may thus be provided, during driving, for the respective regions of essentially the same electrooptical properties. However, it is also possible to provide correction values for individual elements. A control signal that has been corrected using the correction value is then used during driving in order to drive the element. This method is particularly suited to being combined with driving of the elements using a control voltage, thus making it possible to use the advantages of voltage driving, for example faster setting of the desired luminous fluxes.
It is now desirable to improve the driving of light-emitting displays having light-emitting elements of the type described above. To this end, it is desirable to obtain an improved element for light-emitting displays. In addition, it is desirable to obtain an improved method for calibrating light-emitting elements and a light-emitting display having light-emitting elements according to the invention.
The element specified in claim 1 achieves part of this object. The light-emitting display specified in claim 8 and the method specified in claim 11 achieve other parts of the object. Further developments of the invention are specified in the respective subclaims.
An element of a light-emitting display according to the invention has a current control means that is connected in series with a light-emitting means. A first switching means is arranged between a control line and a control electrode of the current control means. In a further embodiment, the current control means additionally has an associated signal holding means. When the first switching means is closed, a control signal is applied to the first current control means via the control line. In the case of elements which are arranged in a column and line raster, the first switching means, for example, selects the line in which the element is arranged, while the control line is provided for elements in a column. The current control means controls an electrical current that flows through the light-emitting means. The light-emitting means emits a luminous flux that is dependent on the electrical current. When the luminous flux has been set to a desired magnitude, the first switching means is opened and the next element that is connected to the same control line in a manner such that it can be switched is actuated. In this case, the magnitude of the control signal is corrected in accordance with a correction value that is stored for the respective element or for a group of elements. A memory is provided for individual elements or groups of elements in order to store the correction values. For carrying out the calibration or measurement method described further below, a second switching means is provided, which switchably connects the control line to a connection of the light-emitting means.
Correction is effected in such a manner that the values stored for a group of elements or for individual elements are used to calculate a characteristic curve that describes the electrical properties at various operating points. For the current control means, this may be, for example, a transistor characteristic curve. If the transistor characteristic curve is known, driving may be effected using a voltage that is used to set the desired electrical current. As described further above, the luminous flux output by the light-emitting means is essentially dependent only on the electrical current which flows through the light-emitting means. Driving the current control means using a suitable voltage thus makes it possible to set a desired luminous flux in a reproducible and accurate manner.
However, the circuit according to the invention for the element also makes it possible to measure the electrical properties of the light-emitting means. The electrical properties of those components of an element of a light-emitting display, which are essential for image rendition, can thus be advantageously determined and combined to form a set of correction values.
The circuit of the element allows for redetermining the correction values during a calibration mode or during operation. To this end, a second switching means is connected between the control line and the common circuit point of the first current control means and of the light-emitting means. The control line is connected to means for measuring currents and/or voltages. The electrical properties of the current control means or of the light-emitting means can be determined depending on the switching state of the first and second switching means and of the control line. The properties ascertained are stored in the memory and are used for correction during driving in the abovementioned manner.
In the case of light-emitting displays for rendering large-area images, for example in television sets, the images are produced in non-interlaced or in interlaced format. Non-interlaced or interlaced images are also referred to as “frames” and “fields”. In this case, the image area is split virtually and/or physically into lines and/or columns. When rendering images using interlaced images, a partial image that, for example, comprises only the even or only the odd lines of the entire image is then first of all rendered. The other interlaced image is then rendered. In the case of non-interlaced rendition, the entire image is composed. Interlaced rendition is also referred to as “interlaced scan” and non-interlaced rendition is referred to as “progressive scan”. When rendering moving images, the non-interlaced or interlaced images are also replaced at regular intervals with respective other images which have an altered image content in order to create the impression of fluid movements as a result. In this case, the image refresh rate is dependent on a respective television standard, for example.
The electrical properties of elements can be measured, for example, between the rendering of two successive interlaced or non-interlaced images. Appropriately switching the first and second switching means makes it possible to bridge the light-emitting means, with the result that no visible interfering effects occur during the measurements.
Driving the elements of the light-emitting display using a control voltage advantageously avoids the effects which result from driving using a control current together with the unavoidable parasitic capacitances. In comparison with current sources, voltage sources have a low impedance and may charge, or reverse the charge of, the parasitic capacitances in a more rapid manner. The setting time for a light-emitting display having elements according to the invention is reduced in comparison with a light-emitting display having conventional elements.
A light-emitting display according to the invention has elements which are arranged in columns and lines. Control lines for the current control means and the switching means are connected to one or more of the elements which are arranged in columns or lines, with the result that each element can be driven. During normal operation, that is to say during operation for the purpose of displaying images by means of the light-emitting display, the control lines are connected to a controllable DC voltage source. When the first switching means are closed, the controllable DC voltage source accordingly sets a control voltage at the control electrode of the current control means.
A light-emitting display having elements according to the invention can also be used, in a particularly advantageous manner, with a control signal that increases continuously from an initial value to a final value. Such a control signal is a sawtooth voltage, for example. In this case, the control signal can be applied to a plurality of elements in a parallel manner. When a voltage that is suitable for the desired luminous flux of an element has been reached, the first switching means associated with the respective element is opened. Using such a signal makes it possible to actuate a plurality of elements in columns and/or lines in a parallel manner. A signal of this type is described in DE-A-103 60 816.
In the case of the invention, the electrical properties of the current control means and of the light-emitting means are known at any point in time. The voltage supply for the light-emitting display can therefore be regulated in such a manner that the maximum voltage needed to generate the desired maximum luminous flux is provided. The requisite voltage increases over time in an ageing-related manner. It is thus possible to save a considerable amount of energy in comparison with a light-emitting display that is designed for a relatively high voltage from the outset and anticipates the ageing effects which are to be expected. In the case of a fixed supply voltage that has been preset at a high value, the excess voltage that is not required for actuation is converted, in the current control means, into heat losses which must be dissipated. The light-emitting display according to the invention thus enables economical operation throughout the entire service life.
The possibility of measuring and storing electrical properties of the components of elements of a light-emitting display during operation also yields advantages for the production of light-emitting displays. Nowadays, the switching and current control means of certain light-emitting displays are usually in the form of so-called thin-film transistors or TFTs and are produced in a first process step. The light-emitting means of certain light-emitting displays are applied in a further process step that is different than the first process step. The arrangement of the first and the second switching means allows the properties of components of the elements already to be measured in an early stage of the production of the light-emitting display. The measured values can then be written to the memory as starting values, with the result that a desired quality of the light-emitting display has already been achieved when the light-emitting display is put into operation for the first time. It is furthermore possible to also physically separate the production steps since the properties are either already stored or can easily be ascertained by measurement. Should the first measurements indicate faults as early as in individual process steps before completion of the light-emitting display, faulty parts can be identified in good time and further process steps can be stopped. The use of resources can thus be reduced.