Driving circuit of light emitting diode and ghost phenomenon elimination circuit thereof

1. Field of the Invention

The present invention relates to a driving circuit of a light emitting diode; in particular, to a driving circuit having the function of eliminating ghost phenomenon.

2. Description of Related Art

Light emitting diodes (LEDs) are small, power saving, and durable components. Along with the maturity of the manufacturing processes and cost reducing, more and more products are using light emitting diode as a light source. The light emitting diode is used in several kinds of equipments, such as headlight lamp of a vehicle, traffic light, word displayer, signboard, large screen video display, simple and building illumination, and LCD backlight.

Please refer to FIG. 1 . FIG. 1 shows a schematic diagram of a conventional driving device of a light emitting diode. The driving device of the light emitting diode mainly includes a driving line selector 110 and a driving circuit 120 . The driving line selector 110 may select the conducted driving lines L 1 and L 2 . Every driving line L 1 or L 2 is connected with several respective light emitting diodes D 1 ˜D 4 , as shown in FIG. 1 . The driving circuit 120 is for controlling driving currents of the light emitting diodes D 1 ˜D 4 , and has several current driving terminals OUT 1 and OUT 2 which correspond to different light emitting diodes D 1 ˜D 4 . In detail, the driving circuit 120 has a current source circuit for controlling the currents flowing into the current driving terminals OUT 1 and OUT 2 , in order to control the brightness of the light emitting diodes D 1 ˜D 4 .

In the multi scanning driving structure, such as two scanning or four scanning, the driving line selector 110 must scan several sets of light emitting diodes in one single frame period. When the scanning processes are switching, a ghost phenomenon may occur because of the parasitic capacitance of the light emitting diodes, and the phenomenon may get worse due to the increase of the switch actions.

The object of the present invention is to provide a driving circuit of a light emitting diode (LED) and a ghost phenomenon elimination circuit thereof. The driving circuit may pull up the voltage level of a current driving terminal to a high voltage level during a black insertion period, in order to reduce the problem of ghost phenomenon of the light emitting diode.

In order to achieve the aforementioned objects, according to an embodiment of the present invention, a driving circuit of the light emitting diode is disclosed. The driving circuit includes a current driving unit and a ghost phenomenon elimination circuit. The current driving unit has at least one current driving terminal, and the ghost phenomenon elimination circuit includes a ghost phenomenon elimination unit and a counter unit. The ghost phenomenon elimination unit is coupled to the current driving terminal, for adjusting the voltage level of the current driving terminal according to an enable signal. The counter unit is coupled to the ghost phenomenon elimination unit, for counting a gray scale clock signal, in order to determine a black insertion period and output the enable signal to the ghost phenomenon elimination unit during the black insertion period. After that, the ghost phenomenon elimination unit may pull up the voltage level of the current driving terminal to a high voltage level according to the enable signal.

Seeing from another aspect, the present invention provides a ghost phenomenon elimination circuit which is associated with a driving circuit of a light emitting diode. A current driving unit of the driving circuit has at least one current driving terminal, and an output timing of the current driving terminal corresponds to a gray scale clock signal. The ghost phenomenon elimination circuit includes a ghost phenomenon elimination unit and a counter unit. The ghost phenomenon elimination unit is coupled to the current driving terminal, for adjusting the voltage level of the current driving terminal according to an enable signal. The counter unit is coupled to the ghost phenomenon elimination unit for counting a gray scale clock signal, in order to determine a black insertion period and output the enable signal to the ghost phenomenon elimination unit during the black insertion period. After that, the ghost phenomenon elimination unit may pull up the voltage level of the current driving terminal to a high voltage level according to the enable signal.

On the basis of the above, the present invention determines the black insertion period by counting the gray scale clock signal, and pulls up the voltage of the current driving terminal during the black insertion period, for speeding up the time of turning off the light emitting diode, in order to eliminate the problem of ghost phenomenon.

For further understanding of the present disclosure, reference is made to the following detailed description illustrating the embodiments and examples of the present disclosure. The description is only for illustrating the present disclosure, not for limiting the scope of the claim.

The drawings included herein provide further understanding of the present disclosure. A brief introduction of the drawings is as follows:

FIG. 1 shows a schematic diagram of a conventional driving circuit of a light emitting diode;

FIG. 2 shows a schematic diagram of a driving circuit of a light emitting diode according to a first embodiment of the present invention;

FIG. 3 shows a schematic diagram of a partial circuit of a current driving unit 222 according to the first embodiment of the present invention;

FIG. 4 shows a schematic diagram of a driving circuit according to the first embodiment of the present invention;

FIG. 5 shows a schematic diagram of a driving circuit of a light emitting diode according to a second embodiment of the present invention; and

FIG. 6 shows a schematic diagram of a driving circuit of a light emitting diode according to a third embodiment of the present invention.

The aforementioned illustrations and following detailed descriptions are exemplary for the purpose of further explaining the scope of the present invention. Other objectives and advantages related to the present invention will be illustrated in the subsequent descriptions and appended drawings. Moreover, the same numerical expressions may represent similar components.

The First Embodiment

Please refer to FIG. 2 . FIG. 2 shows a schematic diagram of a driving device of a light emitting diode (LED) according to a first embodiment of the present invention. The driving device 200 includes a driving line selector 210 and a driving circuit 220 . The driving line selector 210 is used for scanning driving lines L 1 and L 2 which are connected with respective several light emitting diodes D 1 ˜D 4 . The driving line selector 210 may connect to the driving line L 1 through a PMOS transistor P 1 and an NMOS transistor N 1 . The PMOS transistor P 1 is connected between a driving voltage VDD and the driving line L 1 , and the NMOS transistor N 1 is connected between a ground terminal GND and the driving line L 1 . The driving line selector 210 may determine whether to provide the driving voltage VDD to the driving line L 1 or not by controlling the PMOS transistor P 1 and the NMOS transistor N 1 , for driving the corresponding light emitting diodes D 1 , D 2 .

The driving line selector 210 may connect to driving line L 2 through a PMOS transistor P 2 and an NMOS transistor N 2 , and the circuit structure thereof is similar to the aforementioned circuits, thus it is not described repeatedly. The circuit structure of the driving line selector 210 may be adjusted for matching different scanning manners, such as two scanning or four scanning. The two scanning or four scanning manners mean that a single current driving terminal OUT 1 drives two sets (or arrays) or four sets (or arrays) of light emitting diodes in one frame period.

The driving circuit 220 includes a current driving unit 222 and a ghost phenomenon elimination circuit 223 . The ghost phenomenon elimination circuit 223 includes a counter unit 226 and a ghost phenomenon elimination unit 224 . Moreover, the ghost phenomenon elimination unit 224 has several voltage output circuits 231 and 232 which are coupled to several current driving terminals OUT 1 and OUT 2 of the current driving unit 222 respectively, in order to adjust the voltage levels of the current driving terminals OUT 1 and OUT 2 . The counter unit 226 is coupled to the ghost phenomenon elimination unit 224 and the current driving unit 222 , for determining a black insertion period of the screen according to a gray scale clock signal GCK, and for outputting an enable signal EN to the ghost phenomenon elimination unit 224 during the black insertion period. The voltage output circuits 231 and 232 may pull up the voltage levels of the current driving terminals OUT 1 and OUT 2 to a high voltage level during the black insertion period according to the enable signal EN. The gray scale clock signal GCK is a clock signal which is frequently used in a driving circuit of light emitting diodes, and is mainly used for determining frame periods and calculating pulse density modulation signals. However, the present embodiment is not limited thereby.

Generally, the driving circuit 220 may determine output timings of the current driving terminals OUT 1 and OUT 2 according to the gray scale clock signal GCK. By selectively adjusting the output timings of the current driving terminals OUT 1 and OUT 2 , the driving circuit 220 may adjust the average brightness of the light emitting diodes D 1 ˜D 4 .

During the black insertion period, the driving circuit 220 may reduce the currents of the current driving terminals OUT 1 and OUT 2 to zero, that is, turns off the light emitting diodes D 1 ˜D 4 for inserting black screens. The insertion of black screens may reduce the ghost phenomenon of the screen. Therefore, during the black insertion period, the current driving unit 222 may be disabled for stopping driving the light emitting diodes D 1 ˜D 4 . The current driving unit 222 may be disabled according to a disable signal DN outputted by the counter unit 226 , or according to the controls of outer signals, only if the timings are synchronous to the black insertion period. The present embodiment does not restrict the control manners of the driving circuit 220 .

In addition, the current driving unit 222 may be disabled during the black insertion period according to the changes of the voltage level of the enable signal EN. It is worth noting that the disabling of the current driving unit 222 only represents turning off the current driving terminals OUT 1 and OUT 2 rather than restricting whether the circuit of the current driving unit 222 stops operating or not. The manners of turning off the current driving terminals OUT 1 and OUT 2 may be using a switch for cutting off its current paths, however, the present embodiment is not limited thereby.

In other words, when the counter unit 226 detects the signal of inserting black screens, it may simultaneously enable the ghost phenomenon elimination unit 224 and disable the current driving unit 222 , for allowing the ghost phenomenon elimination unit 224 to pull up the voltages of the current driving terminals OUT 1 and OUT 2 to a high voltage level, and to reduce the current values passing through the current driving terminals OUT 1 and OUT 2 . Therefore, the ghost phenomenon of the light emitting diodes D 1 ˜D 4 may be reduced. The mentioned high voltage level may be designed according to actual needs, thus the present embodiment is not restricted thereby. The counter unit 226 may use the number or waveform of the gray scale clock signal GCK for determining the black insertion period. For example, one black insertion period may be generated every 1024 or 2048 pulses, or a specific pulse waveform may be used for generating the black insertion period, wherein the detection manner thereof is not limited in the present embodiment. In addition, the black insertion period may also be generated according to actual design requirements, and it is not restricted in this embodiment.

The current driving unit 222 of the driving circuit 220 may include several channel circuits 322 or several circuits which are able to generate a plurality of current sources, which may be used for determining the current amounts flowing through the light emitting diodes D 1 ˜D 4 when they turn on. Please refer to FIGS. 2 and 3 at the same time. FIG. 3 shows a schematic diagram of a partial circuit of the current driving unit 222 according to the first embodiment of the present invention. The channel circuit 322 is applicable for controlling the driving current and the driving timing of the current driving terminal OUT 1 . The channel circuit 322 includes a shift resister unit 331 , a lock unit 332 , a data selection unit 333 , a pulse density modulation unit 334 , a constant current driving unit 335 , a scan switching controller 336 , and a scan counter 337 . The lock unit 332 is coupled to the shift register unit 331 and the data selection unit 333 , the pulse density modulation unit 334 is coupled to the data selection unit 333 and the constant current driving unit 335 , and the voltage output circuit 231 of the ghost phenomenon elimination unit 224 is coupled to the current driving terminal OUT 1 . The scan switching controller 336 is coupled to the data selection unit 333 , and the scan counter 337 is coupled to the pulse density modulation unit 334 for counting the gray scale clock signal, in order to output a counting signal to the pulse density modulation unit 334 .

The shift register unit 331 may store the pixel data according to a data clock signal DCK and a data signal DI, and may transmit the data signal DI to the next channel circuit. The data signals outputted by the shift register unit 331 are represented by the symbol DO. The lock unit 332 locks the data stored in the shift register unit 331 according to a lock signal LAT. The date selection unit 333 may choose the corresponding gray scale data to the pulse density modulation unit 334 according to the driving timing. For example, if the structure of the light emitting diode display is two scanning, that means two sets of light emitting diodes are scanned within one frame period, and the data selection unit 333 thereof may have two sets of gray data. The scan switching controller 336 may acquire the currently scanned pixels (light emitting diodes) according to the gray scale clock signal GCK, and then controls the data selection unit 333 for selecting corresponding gray scale data.

The scan counter 337 is used for counting the gray scale clock signal GCK, and outputting the result thereof to the pulse density modulation unit 334 . The pulse density modulation unit 334 may serve as a comparison unit, for comparing the counting result with the gray scale data, in order to generate a pulse density modulation signal to the current driving unit 335 . According to that, the constant current driving unit 335 may drive a constant current source circuit, which allows the constant current source circuit to conduct for a corresponding time length during one frame period. The driving manners may be a pulse width modulation signal, but the duty cycle thereof is separated into several small periods which are averagely distributed in the whole frame period. The driving manner may reduce the ghost phenomenon and increase screen qualities.

It is worth noting that the counter unit 226 , the scan counter 337 and the scan switching controller 336 generate outputs according to the gray scale clock signal GCK, thus they may be integrated in one signal counter circuit or be implemented by different counter circuit, which are not limited in the present embodiment. The counter unit 226 directly accords to the gray scale clock signal GCK for enabling the ghost phenomenon elimination unit 224 and disabling the constant current driving unit 335 . Generally, the driving chip of the light emitting diodes has the pin for receiving the gray scale clock signal GCK, thus the ghost phenomenon elimination circuit 223 according to the present invention may be directly integrated into conventional driving chip of the light emitting diodes without setting extra pins or control signals for controlling it. In addition, the current driving unit 222 may use different circuits for matching different design requirements, and the circuit structure thereof is not limited by FIG. 3 .

The voltage output circuits 231 and 232 may be implemented by switch components, and the disabling function of the current driving unit 222 may also be implemented by the switch components. Please refer to FIG. 4 . FIG. 4 shows a schematic diagram of the driving circuit according to the first embodiment of the present invention. The ghost phenomenon elimination unit 224 has switches SW 1 and SW 2 for implementing the functions of the voltage output circuits 231 and 232 in FIG. 2 . The switch SW 1 is coupled between a high voltage VP and the current driving terminal OUT 1 , and is controlled be the enable signal EN. The switch SW 2 is coupled between the high voltage VP and the current driving terminal OUT 2 , and is controlled by the enable signal EN. The current driving unit 222 includes switches SW 3 and SW 4 which are respectively coupled between a current source 410 and the current driving terminal OUT 1 , and between a current source 420 and the current driving terminal OUT 2 . The current driving functions of the current driving unit 222 are implemented by using the current sources 410 and 420 . However, the circuit structure is not limited thereby.

When the counter unit 226 detects a black insertion period, it may output an enable signal EN for turning on the switches SW 1 and SW 2 , and output a disable signal DN for turning off the switches SW 3 and SW 4 , in order to insert black screen and reducing ghost phenomenon. When the counter unit 226 does not detect the black insertion period, it may turning off the switched SW 1 and SW 2 , and turning on the switches SW 3 and SW 4 , for normally driving the light emitting diodes D 1 ˜D 4 . When the switches SW 1 and SW 2 are turning on, the high voltage VP may be outputted to the current driving terminals OUT 1 and OUT 2 , for raising the voltages of the cathodes of light emitting diodes D 1 ˜D 4 to high voltage levels, in order to turn off the light emitting diodes D 1 ˜D 4 and to reduce the generation of the ghost phenomenon.

The current driving unit 222 may have several current driving terminals, and the ghost phenomenon elimination unit 224 may have several voltage output circuits which correspond to the current driving terminals. The numbers of the current driving terminal and the voltage output circuit are not restricted by this embodiment. By the aforementioned descriptions of embodiments, the person skilled in the art may easily derive the implementation manner, and the present embodiment may not repeat them again.

In addition, the switches SW 1 and SW 2 may be implemented by transistors, such as a PMOS transistor and an NMOS transistor, and the present embodiment is not limited thereby. The NMOS transistor is an abbreviation of N channel metal oxide semiconductor field effect transistor, and the PMOS is an abbreviation of P channel metal oxide semiconductor field effect transistor.

The Second Embodiment

Please refer to FIG. 5 . FIG. 5 shows a schematic diagram of a driving circuit of a light emitting diode according to a second embodiment of the present invention. The differences between FIGS. 5 and 4 are PMOS transistors P 51 , P 52 of the ghost phenomenon elimination unit 524 , and NMOS transistors N 51 , N 52 of the current driving unit 522 . The PMOS transistor P 51 is coupled between the current driving terminal OUT 1 and the high voltage VP, and the PMOS transistor P 52 is coupled between the current driving terminal OUT 2 and the high voltage VP. The gates of the PMOS transistors P 51 and P 52 are coupled to the counter unit 226 . The NMOS transistor N 51 is coupled to the current path of the current driving terminal OUT 1 , and the NMOS transistor N 52 is coupled to the current path of the current driving terminal OUT 2 . The gates of the NMOS transistors N 51 and N 52 are coupled to the counter unit 226 . During the black insertion period, the enable signal EN outputted by the counter unit 226 is low voltage, which may turn on the PMOS transistors P 51 and P 52 and turn off the NMOS transistors N 51 and N 52 . On the other hand, during the normal operation processes, the enable signal EN outputted by the counter unit 226 is high voltage, which may turn off the PMOS transistors P 51 and P 52 and turn on the NMOS transistors N 51 and N 52 .

In another embodiment of the present invention, the voltage output circuits 231 and 232 may be implemented by NMOS transistors, and the current driving terminals OUT 1 and OUT 2 may be implemented by PMOS transistors. At this time, the counter unit 226 may output high voltage enable signal EN during the black insertion period, and output low voltage enable signal EN during normal operation processes. After aforementioned descriptions, the person skilled in the art may derive other implementation manners, thus they are not described redundantly.

The Third Embodiment

Please refer to FIG. 6 . FIG. 6 shows a schematic diagram of a driving circuit of a light emitting diode according to a third embodiment of the present invention. The differences between FIGS. 6 and 4 are that the ghost phenomenon elimination unit 624 includes the switches SW 1 and SW 2 and diodes 610 and 620 which may be used for implementing the functions of the voltage output circuits 231 and 232 in FIG. 2 . The switch SW 1 is coupled between the cathode of the diode 610 and the current driving terminal OUT 1 , and the anode of the diode 610 is coupled to the high voltage VP. The switch SW 2 is coupled between the cathode of the diode 620 and the current driving terminal OUT 2 , and the anode of the diode 620 is coupled to the high voltage VP. The switches SW 1 , SW 2 have control terminals coupled to the enable signal respectively. The switch SW 1 and the diode 610 are one of the implementations of the voltage output circuit 231 , and the switch SW 2 and the diode 620 are one of the implementation of the voltage output circuit 232 . However, the implementations of the voltage output circuits 231 and 232 are not limited by FIG. 6 .

The high voltage may be generated by outer circuits or inner voltage output circuits which its switches SW 1 and SW 2 are coupled to the power source lines for receiving the high voltage VP through the diodes 610 , 620 . The diodes 610 and 620 are respectively coupled between the switch SW 1 , SW 2 and the power source lines, which have the capability of preventing current from flowing back to the power source lines for receiving high voltage VP through the current driving terminals OUT 1 and OUT 2 . When the voltages of the current driving terminals OUT 1 and OUT 2 are larger than the high voltage VP, the diodes 610 and 620 may prevent the voltages of the current driving terminals from being transmitted to the circuits for generating the high voltage VP, which may avoid influencing or damaging the circuits for generating the high voltage VP.

The operation manners of the switches SW 1 and SW 2 are as shown in the descriptions of FIG. 4 . After the aforementioned descriptions, the one skilled in the art may derive their implementation manners, thus they are not described redundantly.

In addition, it is worth noting that the coupling relations between the components described above may include direct or indirect electrical connections, only if the connections are able to do the requisite signal transmissions, and they are not limited in the present invention. The technical means described in the aforementioned embodiments may be used independently or dependently, and the components thereof may be increased, eliminated, adjusted, or replaced according to the function and design requirements, which are not limited in this invention. After the aforementioned descriptions, the one skilled in the art may derive their implementation manners, thus they are not described redundantly.

On the basis of the above, the ghost phenomenon elimination circuit may generate high voltage to the current driving terminals during the black insertion period according to the gray scale clock signal, which may achieve the objection of eliminating the ghost phenomenon.

Some modifications of these examples, as well as other possibilities will, on reading or having read this description, or having comprehended these examples, will occur to those skilled in the art. Such modifications and variations are comprehended within this disclosure as described here and claimed below. The description above illustrates only a relative few specific embodiments and examples of the present disclosure. The present disclosure, indeed, does include various modifications and variations made to the structures and operations described herein, which still fall within the scope of the present disclosure as defined in the following claims