This Non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 104125303 filed in Taiwan, Republic of China on Aug. 4, 2015, the entire contents of which are hereby incorporated by reference.
Field of Invention
The present invention relates to a pixel circuit and, in particular, to a pixel circuit of a LED display panel.
Related Art
When the scan line 114 is enabled, the transistor 102 is turned on. The data signal voltage Vdata is transmitted from the transistor 102 to the gate of the transistor 104. Then, the transistor 104 is enabled according to the data signal voltage Vdata, and generates a driving current I1 to drive the OLED 106 to emit light according to the power source VDD. Accordingly, the level of the driving current I1 is determined based on the level of the data signal voltage Vdata instead of a constant current. This variable driving current I1 may cause the damage of the transistor 104 and the OLED 106, thereby decreasing the lifetime of the component.
In the digital driving OLED pixel, the brightness of the OLED 146 is related to the duty cycle of the switching signal SW. As shown in
Regarding to the flicker issue, the conventional solution is to increase the frequency of the scan signals. In the above example, the frequency of the scan signals must be increased to four time of the original to eliminate the flicker issue. Unfortunately, this solution will cause the increase of power consumption and need a more complex circuit design, and thus is not a suitable solution. Since the loads on different scan lines are varied, the pixels connected to the scan line may not be properly renewed if the frequency of the scan signals increases. Besides, the general LED display panel also has the above-mentioned flicker issue.
Therefore, it is desired to properly solve the flicker issue of the LED (OLED) display panel.
An objective of the present invention is to provide a display panel and a pixel circuit that can solve the flicker issue caused by the short duty cycle of pixels.
To achieve the above objective, the present invention discloses a pixel circuit, which includes a lighting element, a driving signal generating unit and a switching unit. The driving signal generating unit compares a data signal with a reference signal so as to generate a PWM driving signal. The status of the PWM driving signal is determined according to a comparing result of the data signal and the reference signal. The switching unit is electrically connected to a power source and the lighting element, and generates a driving current to drive the lighting element to emit light according to the PWM driving signal.
In one embodiment, the switching unit is a first transistor having a first source/drain optionally electrically connected to the power source or coupled to the power source through the lighting element, a second source/drain optionally grounded through the lighting element or directly grounded, and a gate electrically connected to the driving signal generating unit for receiving the PWM driving signal.
In one embodiment, the lighting element is a light-emitting diode (LED) or an organic light-emitting diode (OLED).
In one embodiment, the driving signal generating unit includes a comparator, a second transistor and a capacitor. The comparator has a first input end and a second input end for receiving the reference signal. The second transistor receives the data signal outputted from a data line and a scan signal outputted from a scan line. The second transistor is electrically connected to the first input end of the comparator for transmitting the data signal to the first input end according to a status of the scan signal. The capacitor has one end electrically connected to the first input end and another end grounded for storing a level of the data signal so as to generate an input voltage. The comparator compares the level of the data signal with the reference signal so as to generate the PWM driving signal.
In one embodiment, when a level of the reference signal is larger than a level of the input voltage, the comparator outputs the PWM driving signal of a first status. Otherwise, when the level of the reference signal is smaller than the level of the input voltage, the comparator outputs the PWM driving signal of a second status.
In one embodiment, the reference signal is a triangular wave signal, a square wave signal, or a sine wave signal.
To achieve the above objective, the present invention also discloses a display panel, which includes a substrate and a plurality of pixel circuits arranged on the substrate. At least one of the pixel circuits includes a lighting element, a driving signal generating unit and a switching unit. The driving signal generating unit includes a data signal with a reference signal so as to generate a PWM driving signal. The status of the PWM driving signal is determined according to a comparing result of the data signal and the reference signal. The switching unit is electrically connected to a power source and the lighting element, and generates a driving current to drive the lighting element to emit light according to the PWM driving signal.
In one embodiment, the switching unit is a first transistor having a first source/drain optionally electrically connected to the power source or coupled to the power source through the lighting element, a second source/drain optionally grounded through the lighting element or directly grounded, and a gate electrically connected to the driving signal generating unit for receiving the PWM driving signal.
In one embodiment, the lighting element is a light-emitting diode (LED) or an organic light-emitting diode (OLED).
In one embodiment, the driving signal generating unit includes a comparator, a second transistor and a capacitor. The comparator has a first input end and a second input end for receiving the reference signal. The second transistor receives the data signal outputted from a data line and a scan signal outputted from a scan line. The second transistor is electrically connected to the first input end of the comparator for transmitting the data signal to the first input end according to a status of the scan signal. The capacitor has one end electrically connected to the first input end and another end grounded for storing a level of the data signal so as to generate an input voltage. The comparator compares the level of the data signal with the reference signal so as to generate the PWM driving signal.
In one embodiment, when a level of the reference signal is larger than a level of the input voltage, the comparator outputs the PWM driving signal of a first status, and when the level of the reference signal is smaller than the level of the input voltage, the comparator outputs the PWM driving signal of a second status.
In one embodiment, the display panel further includes a plurality of scan lines, a plurality of data lines and a plurality of reference signal lines. The scan lines are electrically connected to the pixel circuits for transmitting the scan signal. Herein, the scan lines extend along a first direction and arranged in parallel along a second direction, which is substantially perpendicular to the first direction. The data lines are electrically connected to the pixel circuits for transmitting the data signal. Herein, the scan lines extend along the second direction and arranged in parallel along the first direction. The data lines are intercrossed with the scan lines so as to define a plurality of pixel areas, and the pixel circuits are arranged in the pixel areas, respectively. The reference signal lines are electrically connected to the pixel circuits for transmitting corresponding reference signals.
In one embodiment, the reference signal lines are arranged in parallel along the first direction or the second direction, and a waveform of the reference signal transmitted through at least one of the reference signal lines is different from a waveform of the reference signal transmitted through another one of the reference signal lines.
In one embodiment, the reference signals transmitted through of the reference signal lines are all the same.
In one embodiment, the reference signal is a triangular wave signal, a square wave signal, or a sine wave signal.
As mentioned above, the present invention can compare the data signal with a reference signal so as to generate a PWM driving signal for driving the lighting element to emit light. Accordingly, the flicker issue of the displayed image can be prevented. In addition, when the image has flicker, it is simply to increase the frequency of the reference signal Vref so as to eliminate the flicker phenomenon. Thus, this invention can easily solve the flicker issue without complex circuit and large power consumption.
The invention will become more fully understood from the detailed description and accompanying drawings, which are given for illustration only, and thus are not limitative of the present invention, and wherein:
The present invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings, wherein the same references relate to the same elements.
The driving signal generating unit 26 receives data signal Data from a data line and a scan signal Scan from a scan line, and then compares the data signal Data with a reference signal Vref so as to output a pulse width modulation (PWM) driving signal PWM to the switching unit 24. Then, the switching unit 24 generates a driving current Idrive to drive the lighting element 22 to emit light according to the PWM driving signal PWM. Preferably, the driving current Idrive is constant, but this invention is not limited.
The switching unit 24 can be carried out by a first transistor 242. In this embodiment, the first transistor 242 is a PMOS transistor, but this invention is not limited. The first source/drain (source) of the first transistor 242 is electrically connected to the power source VDD, and the second source/drain (drain) of the first transistor 242 is electrically connected to the lighting element 22 and then grounded through the lighting element 22. In another embodiment, the source of the first transistor 242 is coupled to the power source VDD through the lighting element 22, and the drain of the first transistor 242 is directly grounded. In this current embodiment, the lighting element 22 is an OLED having one end connected to the first transistor 242 and the other end grounded. In another embodiment, the lighting element 22 can also be an inorganic LED, and this invention is not limited.
Referring to
The comparator 262 has a first input end IN1, a second input end IN2, and an output end OUT. The first input end IN1 is electrically connected to the source of the second transistor 264, and the second input end IN2 receives the reference signal Vref. The output end OUT of the comparator 262 is electrically connected to the gate of the first transistor 242 for transmitting the PWM driving signal PWM to the first transistor 242.
To be noted, the above-mentioned comparators and the circuits thereof are not to limit the present invention, and those skilled persons should know that using different comparator circuits is still within the scope and spirit of the invention.
In this embodiment, the reference signal Vref is a triangular wave signal, but this invention is not limited thereto. In other embodiments, the reference signal Vref can be a sine wave signal, a square wave signal, a pulse signal, or the likes. When the level of the reference signal Vref is lower than that of the data signal Data (the level of the data signal voltage Vdata as the dotted line shown in the figure), the comparator 262 will output a low level PWM driving signal PWM. Alternatively, when the level of the reference signal Vref is higher than that of the data signal Data, the comparator 262 will output a high level PWM driving signal PWM. When the PWM driving signal PWM is in a low level, the first transistor 242 is enabled to generate the driving current Idrive to drive the lighting element 22 (e.g. an LED) to emit light. In another case, if the first transistor 242 is an NMOS transistor, it will be enabled to drive the lighting element 22 to emit light as the PWM driving signal PWM is in a high level.
In this invention, the lighting element 22 is driven by the PWM driving signal PWM, and the PWM driving signal PWM is generated according to the comparison result of the data signal Data and the reference signal Vref. Herein, the frequency of the reference signal Vref is larger than the renewal frequency of the frame. Preferably, the frequency of the reference signal Vref is multiple times (e.g. four, five or six times) of the renewal frequency of the frame. Accordingly, if the lighting element 22 is driven with a non-fully brightness (e.g. 50% brightness), this non-fully brightness period is shorter. Taking the 25% brightness as an example, the light emitted by the OLED 146 of
In general, human eyes can view the flicker images with the lighting frequency lower than 180 Hz, and some sensitive eyes can view the flicker images with the lighting frequency lower than 240 Hz. In this invention, if the image still has the flicker phenomenon, the user can simply increase the frequency of the reference signal Vref to eliminate the flicker phenomenon. For example, the reference signal Vref can be adjusted to increase the lighting frequency (over 180 Hz, and preferably over 240 Hz) of the lighting element 22 in the frame time F1, and the complex procedure in the conventional art is not needed.
In addition, the display device 5 further includes a scan driver 52 and a data driver 54. The scan driver 52 is electrically connected to the scan lines SL for outputting the scan signals Scan through the scan lines SL. The data driver 54 is electrically connected to the data lines DL for outputting the data signals Data through the data lines DL.
In particular, the display device 5 further includes a plurality of reference signal lines VL, which are arranged on the display panel 50 and electrically connected to the pixel circuits of the display panel 50, respectively. The reference signal lines VL transmit the reference signals Vref to the electrically connected pixel circuits. In some embodiments, the waveform of one reference signal Vref transmitted from one of the reference signal lines VL is different from the waveform of another reference signal Vref transmitted from another one of the reference signal lines VL. In this embodiment, the waveforms of all reference signals Vref transmitted from the reference signal lines VL are the same, and this invention is not limited.
As shown in
In summary, the present invention can compare the data signal with a reference signal so as to generate a PWM driving signal for driving the lighting element to emit light. Accordingly, the flicker issue of the displayed image can be prevented. In addition, when the image has flicker, it is simply to increase the frequency of the reference signal Vref so as to eliminate the flicker phenomenon. Thus, this invention can easily solve the flicker issue without complex circuit and large power consumption.
Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments, will be apparent to persons skilled in the art. It is, therefore, contemplated that the appended claims will cover all modifications that fall within the true scope of the invention.
Number | Date | Country | Kind |
---|---|---|---|
104125303 | Aug 2015 | TW | national |