BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to electronic displays and in particular to LED control circuitry for back lighting LCD displays and the like.
2. Description of Related Art
Liquid crystal displays (LCD) are common type of electronic display. FIG. 1 shows a conventional LCD display which includes an LCD panel 20 which produces various pixels to define an image. A back light unit 24 operates to provide light to the LCD panel, with a light conductor plate 22 being disposed between the LCD panel and the back light unit 24. The back light unit 24 uses LEDs that provides improved power consumption, brightness and weight as compared to other back lighting devices such as cold cathode fluorescent lamps. In order to provide a color display, the LED backlighting unit may include a combination of red, green and blue (RGB) LEDs that are driven using what is termed the field sequential color (FSC) driving method. This method displays a color by relying upon the after image effect in human vision. As can be seen in the timing diagram of FIG. 2, one frame of image (or picture frame) divided into three sub-pictures (sub-frames). At the beginning of a typical picture frame, there is a Red sub-picture, followed by a Green sub-picture which is then followed by a Blue sub-picture. At the beginning of the Red sub-picture, the LCD panel 20 is refreshed and the Red LEDs in backlight unit 24 are driven. Thus, during the Red sub-picture, the Red components of the image is displayed. The same sequence is carried out during the subsequent Green and Blue sub-pictures. The separate color sub-pictures are not detected by the human eye, with the result being a full color image generally free of flicker.
Note that the LED backlight panel 24 of FIG. 1 is suited for relatively large LCD displays. For smaller displays, the backlighting LEDs are disposed on the edges of the LCD panel 20 so that the overall thickness of the display is substantially reduced. A light guide (not depicted) is than used diffused the light over the panel equally.
The circuitry for driving the LED backlight unit 24 using the FSC driving method is typically located in a circuit module separate from the unit itself. A typical circuit module may be in the form of an integrated circuit disposed in a integrated circuit package, with that package having a limited number of pins (electrical terminals) for interfacing with the LED unit 24 and other external circuit components.
There is a need for a circuit module for driving a LED backlight panel using the FSC driving method and other methods that requires only a limited number of pins but yet is capable of providing an optimized drive to the individual LEDs of the panel. As will become apparent to those skilled in the art after a reading of the following Detailed Description of the Invention together with the drawings, the present invention addresses these and other shortcomings of prior art LED driver circuits.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts a prior art LCD color display which includes and LED back light unit utilizing Red, Green and Blue LEDs.
FIG. 2 is a timing diagram illustrating the field sequential color (FSC) driving method in accordance with the prior art.
FIG. 3 shows an LED control module in accordance with one embodiment of the present invention connected to an LED matrix.
FIG. 4 shows an LED control module in accordance with another embodiment of the present invention connected to an LED matrix.
FIGS. 5A and 5B show a modification of the LED arrays of FIGS. 3 and 4 where multiple LEDs, as opposed to a single LED, are disposed between selected row and conductor lines.
FIG. 6 is a timing diagram of an alternative FSC driving method.
DETAILED DESCRIPTION OF THE INVENTION
Referring again to the drawings, FIG. 3 shows one embodiment of the present invention. A control module 28 is disclosed for driving an LED matrix array. The control module is typically implemented in the form of an integrated circuit enclose by a circuit package that contains the electrical terminals such as pins for interfacing with the external components. The circuit package may also be a surface mounted device (SMD) which utilizes contact bumps which function as the electrical terminals of the device. The LEDs of the array are arranged in rows and columns, with the array being preferably configured for Field Sequential Color driving. The array includes a first row 44 of eight Red LEDs, a second row 46 of Green LEDs and third row 48 of Blue LEDs. The cathodes of the LEDs in row 48 are all connected to a first row conductor line 42A which is to be connected to a pin (electrical terminal) 32A of the control module 28. The cathodes of the LEDs in row 46 are all connected to a second row conductor line 42B which is to be connected to a pin 32B of the control module, with the cathodes of the LEDs in row 44 are all connected to a third row conductor line 42C which is to be connected to pin 32C of the control module 28.
Each of the eight columns of LEDs has an associated column conductor line 40A-40H, with each of the column conductor lines to be connected to respective control module pins 30A-30H. The anodes of the LEDs in a particular column are connected to the column conductor line associated with that column. By way of example, the anodes of the Red, Green and Blue LEDs on one column are connected to the column conductor line 40A associated with that column.
The control module 28 includes three transistor switches 36A, 36B and 36C having respective switch terminals connected to separate ones of the control module pins 32A, 32B and 32C (the row driver pins). The opposite switch terminals of the transistor switches are connected in common to the circuit ground of the control module. As will be described, the states of switches 36A, 36B and 36C are independently controlled by an FSC Drive Control 50 of module 28. Each of the column driver pins 30A-30H has an associated LED driver 34A-34H disposed within control module 28, with the LED drivers each being independently controlled by the FSC Drive Control 50 (the control lines are not depicted). Note that the details for implementing Control 50 are conventional and would be readily apparent to those skilled in the art upon reading the present disclosure. Thus, in order to avoid obscuring the true nature of the present invention in unnecessary detail, such details are not presented here.
The FSC Driver Control 50 operates in synchronization with the controller (not depicted) for controlling the LCD. Referring to both the timing diagram of FIG. 2 and the FIG. 3 LED array and control module, a typical picture frame begins by refreshing the LCD for the Red sub-picture image. After the refresh, the eight Red LCDs of the array are driven (activated). In order to carry out this action, the FSC Drive Control 50 will switch transistor 36C to an ON state, with the other switches 36B and 36C remaining OFF. Thus, row conductor line 42C is grounded, with lines 42B and 42C remaining open circuited. The FSC Drive Control 50 will further actuate each of the LED drivers 34A-34H, with the drive current being selected for driving the Red LEDS, since the cathodes of the Green and Blue LEDs are unconnected to ground. At the end of the Red sub-picture, Control 50 turns the eight LED drivers off so that the Red LEDs are deactivated. In addition, Control 50 turns transistor 36C OFF. Following the end of the Red sub-picture, the LCD is refreshed for the Green sub-picture. Next, the Green LEDs 46 are driven when Control 50 turns transistor switch 36B ON and enables LED drivers 34A-34H. Once again, the drive current can be optimized by Control 50 for driving Green LEDs. At the end of the Green sub-picture, the Green LEDs are deactivated, with the sequence being repeated for the Blue sub-picture when Blue LEDs 48 are activated.
The LED drivers 34A-34H are preferably implemented to provide drive currents with controlled precision, with 12 bit current resolution being preferred. Thus, the FSC Drive Control 50 can independently control each of the drivers thereby providing the capability of matching the drive characteristics to the individual LEDs. As previously noted, the optimum drive currents for differing color LEDs are different, with Control 50 being able to make the appropriate adjustments depending upon the color of the sub-picture. In addition, the drivers can be configured to provide a feedback signal to Control 50 regarding the state of each LED so that the drive to the individual LEDs in a given row can also be optimized by Control 50.
FIG. 4 shows another embodiment 52 of the subject control module. In this case, the LEDs are arranged in the matrix so that the cathodes are connected to the associated column conductor lines 58A-58H and the anodes are connected to the associated row conductor lines 60A-60C. Eight LED drivers are included in the module 52 which are connected to the respective module pins 30A-30H. The LED drivers are configured to sink, rather than source, current present on the column conductor lines. Once again the states of the LED drivers are independently controlled by FSC Drive Control 50. Switching transistors 56A, 56B and 56C are provided having one set of terminals separately connected to the respective module pins 32A, 32B and 32C. The other set of the switch terminals connected are common to a positive voltage source Vdd. Switches 32A, 32B and 32C are separately controlled by FSC Control 50 so that a selected one of the three row conductor lines 60A, 60B and 60C can be connected to supply Vdd.
During a typical FSC drive sequence, the Red LED row 60 is activated during the Red sub-picture when Control 50 turns switch 56A on and further activates the eight LED drivers 54A-54H. Thus, drive current will flow from voltage source Vdd through ON switch 56A and through the eight Red LEDs into the LED drivers. Once again, it is preferred that the LED drivers 54A-54H have the same control features previously described in connection with drivers 34A-34H of the first embodiment of FIG. 3.
The previously described LED matrices described in connection with FIGS. 3 and 4 utilize a single LED for each N×Y column/row location. It would be possible to replace the single LEDs of these matrices with two or more LEDs, usually of the same color. By way of example, FIG. 5A show a pair of LEDs connected between column conductor line 40A and row conductor line 42C of FIG. 3. As a further example, FIG. 5B shows a pair of LEDs connected between row conductor line 60A and column conductor line 58A. In order to accommodate the driving of multiple LEDs at the same time by a single driver, it will be necessary to adjust the drive voltage level including voltage Vdd of FIG. 4 but also the LED driver 54A-54H and 34A-34H characteristics.
FIG. 6 is a timing diagram of an alternative FSC driving method. In this case, a picture frame is divided into four sub-frames rather than three. During first, second and third sub-frames, the respective Red, Green and Blue LEDs predominate, with Blue LEDs period being repeated in the fourth sub-frame. Since only a single color LED is driven at any one time, each of the sub-frame includes a a sequence of Red, Green and Blue drive period, with the drive periods being interleaved. Thus, as can be seen from the FIG. 6 diagram, the three sets of color LEDs are time multiplexed during a given sub-frame. During the sub-frame, the Red LEDs of row 44 of FIG. 3 are turned ON for a first duration following by the Green LEDs of row 46 followed by the Blue LEDs of row 48, with this interleaving of colors taking place throughout the remainder of the sub-frame. The contribution of the three LED colors during a given sub-frame is determined by the relative total times that a given color LED is turned ON and by the brightness of each color. During the first sub-frame, the color Red predominates. During second sub-frame, the color Green predominates, with the color Blue predominating in the sub-frame. During the fourth sub-frame the color Blue again predominates. When a color predominates in a sub-frame, the total duration that the predominating color LED is driven exceeds the total duration that any one of the other colors LEDs is driven during that sub-frame.
Thus, a novel control module for driving an LED array has been disclosed. Although two embodiments have been described in some detail, it is to be understood that certain changes can be made by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.
Patent Grant
8456093
