Patent Application
20110134021
The present invention relates to color sequential display schemes, including but not limited to color sequential displays employing liquid crystal displays (LCD's) and thin film transistor (TFT) LCD's. More particularly, the present invention relates to driving circuits for color sequential displays.
The popularity of flat panel displays and their application in computer monitors and High Definition Televisions has grown to unprecedented heights. In particular, there has been a growing interest in flat panel displays comprising liquid crystals, and such LCD's have now surpassed the cathode ray tube (CRT) in sales as the preferred choice of consumers worldwide.
One of the reasons that LCDs have become the most popular of flat panel displays is a combination of their brightness, durability and reduced cost of construction, which has made such displays more affordable than ever.
Recent innovations in LCD technology include color-sequential scanning. More particularly, color-sequential technology is a new technology that is gaining favor because it uses less power than traditional LCD technology and does not require color filters (an example of such filters 105 is shown in
As shown in
However, as color-sequential technology takes advantage the temporal integration of images as perceived by the human brain, the LEDs are turned on and off for a short period of time, and the combination of LEDs turned on and off are repeated at a frequency sufficient so that viewers preferably can perceive the full color image comprised of the different color frames (such as shown in
The present invention provides a way to reduce the costs of construction and size of the color-sequential scan displays by simplifying the scan display circuitry by reducing the current sources in a three color string (such as RGB) by two thirds, so that one current source rather than three current sources are used. In cases where there are more than three strings of colors used, the present invention's use of a single current source in place of multiple current sources also provides a savings that can be even greater than the reduction using the RGB strings of LEDs and the common current source.
In an exemplary aspect of the present invention, a light emitting diode (LED) driver for an LED backlight of a color-sequential liquid crystal display (LCD) includes a plurality of LED strings of respectively different predetermined colors, wherein each LED string includes one or more LEDs emitting light at a wavelength substantially corresponding to a particular predetermined color; a plurality of switches respectively coupled to the plurality of LED strings so that each LED string is coupled to a respective switch, wherein the plural switches are opened and closed according to control output according to a respective control signal; a current source is switchably coupled respectively and sequentially to each of the plurality switches strings to provide current to control operation of each of the plurality of LED strings to output light emitted at the particular predetermined color in a sequence color. The respective control signal opens and closes a respective switch from the plurality of switches sequentially to permit a respective LED string to provide an output of the particular predetermined color for a specific period of time. Preferably, this can be timed such that a total output of the plurality of LED strings provides an output having a desired overall color and intensity which can be perceived by a viewer during a temporal integration of an output of each respective LED string.
The respective control signals preferably comprise pulse width modulated (PWM) control signals, although other controls schemes also could be used.
In addition, according to an exemplary aspect of the present invention, when a number of the plurality of LED strings is equal to “n” and a frequency of switching time of the respective control signal occurs at a frequency n times higher than a control signal frequency for operating LED strings in which each LED string has its own respective current source, the current source operates at a value of current about n times that of a value of current used for operating LED strings in which each LED string has its own respective current source.
For example, a three-string set of LEDs having respective colors such as red, green and blue to blend into a plurality of other colors would typically have, for purposes of illustration and explanation, supplied current of 60 ma from each operating current source (a), (b) and (c) shown in
Another way to describe this arrangement is that each control signal is switched with a frequency such that, during a period of time “T”, the LED string which operates in response to that control signal is activated for fraction of the time “T” (approximately T/n, where “n” is an integer of value 2 or greater). To compensate for the light which is not produced by the LED string during the balance of the time T (that being time approximately (T−T/n), the current source provides current to each active LED string at a level such that the intensity of light emitted by the associated activated LED string is correspondingly increased.
The above and other aspects, features and advantages of certain exemplary embodiments of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawing, in which:
A person or ordinary skill in the art should understand and appreciate that the drawings and their accompanying description have been provided for purposes of illustration and not for limiting the claimed invention to the examples shown and described herein. In addition, well-known configurations and detailed descriptions of well-known functions and structures incorporated herein may be omitted to avoid obscuring appreciation of the subject matter of the present invention by a person of ordinary skill in the art.
Now referring to
Still referring to
It is also within the spirit and scope of the invention that next to the colored LEDs, (such as the RGB), there can be an extra string of white LEDs, as white can be used more than other colors in certain applications, and it may be more desirable to include a string of white LEDs rather than mix multiple wavelengths of different LEDs to create white light. Additionally, one or more of the RGB strings could be omitted. The white LEDs could include, but are in no way limited to a color LED (such as for example blue Indium-Gallium-Nitride InGaN) coated with, for example, suitable phosphor, so as to permit tuning of the light. The invention even can extend to a system using only white LEDs, and a single color of LEDs.
Referring now to
In addition, while the examples shown and described herein involve PWM control signals, the present invention is not limited to a particular type of control signal, and only requires that the control signal can open and close the switches in the timewise manner described herein.
The controller 611 is operatively coupled to both driving unit 617 for driving the liquid crystal array 619, and to the first driving unit 613 having a current source according to the present invention for driving the LED array 615 of a backlight. The controller 611 may receive from a formatting unit the RGB information, or may include the formatting information. Preferably, the LED array 615 typically including LED strings of color sub-pixel elements is arranged in alignment with the LC array 619 (typically behind the LC array 619). The LEDs are activated to illuminate in a pattern so as to transmit colored light to the LC cells to permit the LC to output color frames having sequences of, in this case, red, green and blue LED strings having an on and off pattern such that the frames are perceived as blended colors according to a color selection provided by the controller. To drive the LED strings in LED array 615 the current source of driving unit 613 operates at a current typically three times greater than devices having a 1:1 ratio of current sources per LED string. The driving unit 613 opens and closes switches to activate selected LED strings via PWM signals. The PWM signals typically open and close at three times the frequency of devices having three different color strings of LEDs and a 1:1 ratio of current sources to LED strings.
Finally,
At step 703, a current source is switchably coupled to a selected one of the plurality of switches, respectively and sequentially via action of respective control signals so as to provide connection of the associated one of the respective LED strings to the current source to output light at the times it is desired to display a particular predetermined color (corresponding to that LED string) in a sequence.
At step 705, the respective switch is opened and closed to provide an output of the particular color for a specified period of time such that a total output of the plurality of LEDs over a predetermined time period provides an overall color that can be perceived by a viewer during a temporal integration of an output of each respective LED string. As previously discussed, the frequency and brightness of the LED string outputs are required to be sufficient to be seen by the human eye such that the brain perceives a blended color formed by the output of light from the plurality of strings.
Accordingly, the present invention provides a cost reduction in the construction of an LCD device by using less silicon, yet still provides a color-sequential display in which the colors are perceived as in conventional color-sequential displays.
In addition, the above-described methods according to the present invention can be realized in hardware or as software or computer code that can be stored as machine readable code in a medium such as a ROM, an RAM, a floppy disk, a hard disk, a flash memory, or a magneto-optical disk, or downloaded over a network, so that the methods described herein can be rendered in such software using a general purpose microprocessor, general purpose computer, or a special processor or in programmable or dedicated hardware, such as an ASIC or FPGA.
As would be understood in the art, the computer, the processor or the programmable hardware include memory components, e.g., RAM, ROM, Flash, etc. that may store or receive software or computer code that when accessed and executed by the computer, processor or hardware implement the processing methods described herein.
The present invention has been described with respect to particular embodiments and with reference to certain drawings, but the invention is not limited thereto, but rather, is set forth only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, for illustrative purposes, the size of some of the elements may be exaggerated and not drawn to a particular scale. Where the term “comprising” is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun, e.g. “a” “an” or “the”, this includes a plural of that noun unless something otherwise is specifically stated. Hence, the term “comprising” should not be interpreted as being restricted to the items listed thereafter; it does not exclude other elements or steps, and so the scope of the expression “a device comprising items A and B” should not be limited to devices consisting only of components A and B. This expression signifies that, with respect to the present invention, the only relevant components of the device are A and B.
Furthermore, the terms “first”, “second”, “third” and the like, if used in the description and in the claims, are provided for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances (unless clearly disclosed otherwise) and that the embodiments of the invention described herein are capable of operation in other sequences and/or arrangements than are described or illustrated herein.
