Abstract:
The present invention provides a driving circuit and a driving method for light-emitting diodes (LEDs), and a display apparatus using the same. The driving circuit comprises a power switch and a dimmer circuit. The method comprising: providing a pulse width modulation (PWM) dimming signal and a high frequency dimming signal; and multiplying the PWM dimming signal and the high frequency dimming signal for providing a driving signal to the power switch. The present invention can improve a dimming effect of the LEDs.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a driving circuit and a driving method for light-emitting diodes (LEDs), and more particularly to a driving circuit and a driving method for LEDs applicable to a backlight module and a display apparatus. 
     BACKGROUND OF THE INVENTION 
     Liquid crystal displays (LCDs) have been widely applied in electrical products. Currently, most of LCDs are backlight type LCDs which comprise a liquid crystal panel and a backlight module. According to the position of the backlight source, the backlight module can be a side-light type or a direct-light type in order to provide LCDs with backlight. 
     Light emitting diodes (LEDs) have several beneficial characteristics, including low electrical power consumption, low heat generation, long operational life, small volume, good impact resistance, fast response and excellent stability for emitting color light with stable wavelengths. These characteristics have made the LEDs suitable for light sources of the backlight module. 
     Currently, in an LED driving circuit, a pulse width modulation (PWM) dimming signal is utilized to control a current switch of the LEDs for dimming the light of the LEDs. 
     However, the dimming signal is a low frequency PWM signal, and thus the light of the LEDs can not be dimmed frequently during a specific dimming period, deteriorating the dimming effect of the LEDs 
     As a result, it is necessary to provide a driving circuit and a driving method for the LEDs, and a display apparatus using the same to solve the problems existing in the conventional technologies, as described above. 
     SUMMARY OF THE INVENTION 
     The present invention provides a driving circuit and a driving method for LEDs, and a display apparatus using the same, so as to solve the dimming problem using a low frequency PWM signal. 
     A primary object of the present invention is to provide a driving circuit for driving a plurality of light-emitting diodes, and the driving circuit comprises: a power switch connected to the light-emitting diodes; a resist connected to the power switch and electrically connected to ground; and a dimmer circuit connected to the power switch, wherein the dimmer circuit comprises a multiplier configured to multiply a PWM dimming signal and a high frequency dimming signal, so as to provide a driving signal to the power switch, and the high frequency dimming signal is obtained by transforming an image signal of a display panel. 
     In one embodiment of the present invention, the power switch is an n-channel metal-oxide-semiconductor (NMOS) transistor. 
     In one embodiment of the present invention, a drain electrode of the power switch is connected to the LEDs, and a source electrode of the power switch is electrically connected to the resist, and a gate electrode of the power switch is connected to the dimmer circuit. 
     In one embodiment of the present invention, the high frequency dimming signal is provided by a timing controller. 
     In one embodiment of the present invention, the high frequency dimming signal is a PWM signal with high frequency. 
     A further object of the present invention is to provide a driving method for driving a plurality of light-emitting diodes, wherein the light-emitting diodes are electrically connected to a power switch, and the driving method comprises: providing a PWM dimming signal; providing a high frequency dimming signal, wherein the high frequency dimming signal is obtained by transforming an image signal of a display panel; and multiplying the PWM dimming signal and the high frequency dimming signal for providing a driving signal to the power switch. 
     In one embodiment of the present invention, the high frequency dimming signal is provided by a timing controller. 
     In one embodiment of the present invention, the high frequency dimming signal is a PWM signal with high frequency. 
     In one embodiment of the present invention, the PWM dimming signal and the high frequency dimming signal are multiplied by a multiplier of a dimmer circuit. 
     Another object of the present invention is to provide a display apparatus, and the display apparatus comprises: a display panel; a timing controller; and a backlight module comprising: a back bezel; a plurality of light-emitting diodes disposed on the back bezel; and a driving circuit electrically connected to the light-emitting diodes for driving the light-emitting diodes, wherein the driving circuit comprises: a power switch connected to the light-emitting diodes; a resist connected to the power switch and electrically connected to ground; and a dimmer circuit connected to the power switch, wherein the dimmer circuit comprises a multiplier configured to multiply a PWM dimming signal and a high frequency dimming signal provided by the timing controller, so as to provide a driving signal to the power switch, and the high frequency dimming signal is obtained by transforming an image signal of a display panel. 
     The driving circuit and the driving method of the present invention can dim the light of the LEDs for many times in a short dimming period, so as to improve the dimming effect of the LEDs and the contrast of the display panel, thus improving the display quality of the display apparatus. 
     The structure and the technical means adopted by the present invention to achieve the above and other objects can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a backlight module and a display panel according to a first embodiment of the present invention; 
         FIG. 2  is a circuit diagram showing the driving circuit according to the first embodiment of the present invention; and 
         FIG. 3  is a diagram showing waveforms of a PWM dimming signal and a high frequency dimming signal according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following embodiments are referring to the accompanying drawings for exemplifying specific implementable embodiments of the present invention. Furthermore, directional terms described by the present invention, such as upper, lower, front, back, left, right, inner, outer, side and etc., are only directions by referring to the accompanying drawings, and thus the used directional terms are used to describe and understand the present invention, but the present invention is not limited thereto. 
     In the drawings, structure-like elements are labeled with like reference numerals. 
     Referring to  FIG. 1 , a cross-sectional view showing a backlight module and a display panel according to a first embodiment of the present invention is illustrated. The driving circuit  150  of the present invention is used for driving a plurality of LEDs  120 . The LEDs  120  can be connected in series as at least one LED strip, so as to act as a light source of the backlight module  100 . The backlight module  100  may be realized as a side-light type backlight module or a direct-light type backlight module disposed opposite to a display panel  101  (such as an LCD panel), thereby forming a display apparatus (such an LCD apparatus). The display apparatus of the this embodiment further comprises timing controller (Tcon)  102  for providing an image signal to the display panel  101 , and providing a corresponding backlight driving signal to the backlight module  100 . 
     Referring to  FIG. 1  again, in this embodiment, the backlight module  100  may be the direct-light type backlight module which comprises a back bezel  110 , the plurality of LEDs  120 , a circuit board  130 , a reflective layer  140 , the driving circuit  150  and at least one optical film  160 . The back bezel  110  is configured to carry the LEDs  120  and the circuit board  130 . The LEDs  120  can be disposed on the circuit board  130  and electrically connected to the driving circuit  150  through the circuit board  130  for emitting light to the display panel  101 . The circuit board  130  may be a printed circuit board (PCB) or a flexible printed circuit (FPC). The reflective layer  140  is formed around the LEDs  120  (such as formed on the circuit board  130  or the back bezel  110 ) for reflecting the light of the LEDs  120 . The driving circuit  150  is electrically connected to LEDs  120  by the circuit board  130 . The optical film  160  is disposed above the LEDs  120  for improving the light uniformity and light efficiency thereof. 
     Referring to  FIG. 2 , a circuit diagram showing the driving circuit according to the first embodiment of the present invention is illustrated. The driving circuit  150  of the present embodiment comprises a power switch Q 1 , a resist R 1  and a dimmer circuit  151 . The power switch Q 1  is connected between the LEDs  120  and the resist R 1 . The dimmer circuit  151  is connected to the power switch Q 1  for transmitting a driving signal to the power switch Q 1 , so as to dim the light of the LEDs  120 . In this case, the dimmer circuit  151  can be integrated into an IC chip. 
     In this embodiment, the power switch Q 1  may be an enhancement mode n-channel metal-oxide-semiconductor (NMOS) transistor, and a drain electrode thereof is connected to the LEDs  120 , and a source electrode thereof is electrically connected to the resist R 1 , and a gate electrode thereof is connected to the dimmer circuit  151 . 
     Referring to  FIG. 2  again, the dimmer circuit  151  of this embodiment is connected to the timing controller  102  and the power switch Q 1 . The dimmer circuit  151  comprises a multiplier  152 , and the multiplier  152  is connected to the power switch Q 1  for receiving a PWM dimming signal and a high frequency dimming signal. The PWM dimming signal is provided by an external system (not shown), and is transmitted to the dimmer circuit  151  by the timing controller  102 . In general, the PWM dimming signal is a low frequency PWM signal. The high frequency dimming signal is provided by the timing controller  102 , and the high frequency dimming signal is obtained by transforming an image signal of a display panel  101 , wherein a frequency of the high frequency dimming signal is higher than a frequency of the PWM dimming signal which is transmitted from the external system. For example, the frequency of the high frequency dimming signal may be two times or more than three times the PWM dimming signal. The multiplier  152  of the dimmer circuit  151  can multiply the PWM dimming signal and the high frequency dimming signal which are transmitted from the timing controller  102 , so as to form the multiplied driving signal. Therefore, the dimmer circuit  151  can provide the multiplied driving signal to act as a driving waveform of the power switch Q 1 . 
     When dimming the light of the LEDs  120 , the system (not shown) can transmit the PWM dimming signal to the timing controller  102 , and the PWM dimming signal can be provided to the driving circuit  150  by the timing controller  102 . At the same time, corresponding to different frames (such as a dark frame or a bright frame) of the display panel  101 , the timing controller  102  can transform the image signal of the display panel  101  into the high frequency dimming signal (the PWM signal with high frequency) and provide the high frequency dimming signal to the dimmer circuit  151  of the driving circuit  150 , so as to allow the multiplier  152  of the dimmer circuit  151  to multiply the PWM dimming signal and the high frequency dimming signal which are transmitted from the timing controller  102 . Therefore, a high-frequency PWM dimming signal can be superimposed on the PWM dimming signal with low frequency which is provided by the external system, and the driving signal with the high-frequency PWM dimming signal is provided to the power switch Q 1  of the driving circuit  150 , so as to control the power switch Q 1  for dimming the light of the LEDs  120 . 
     Referring to  FIG. 3 , a diagram showing the waveforms of the PWM dimming signal and the high frequency dimming signal according to the present invention is illustrated. In comparison with the PWM dimming signal and the high frequency dimming signal corresponding to the bright frame, when the display panel  101  displays a dark frame or a frame with low-brightness, the PWM dimming signal with low frequency merely turn on the power switch Q 1  once during a short dimming period. At this time, since the multiplier  152  of the dimmer circuit  151  can superimpose the high frequency dimming signal on the PWM dimming signal for providing the driving signal. Therefore, the dimmer circuit  151  of the driving circuit  150  can dim the light of the LEDs  120  for many times during the short dimming period. 
     Therefore, with the use of the LED driving circuit and method of the this embodiment, in the short dimming period, the LEDs  120  can be dimmed for many times, so as to improve the dimming effect of the LEDs  120  and facilitate a contrast of the display panel  101  for improving a display quality of the display apparatus. 
     In this embodiment, the driving circuit can be also applicable to a side-light type backlight module. At this time, the backlight module may comprise a back bezel, a plurality of LEDs, a circuit board, a reflective layer, the driving circuit, at least one optical film and a light guide plate. The light guide plate is disposed on the back bezel. The LEDs are disposed on the circuit board to form a light bar which can be disposed at one side of the light guide plate to emit light thereto, and the light is guided to be outputted by the light guide plate. The driving circuit is electrically connected to LEDs by the circuit board for driving the LEDs. 
     As described above, in comparison the conventional dimming method using the low frequency PWM signal, the circuit and method for driving the LEDs of the present invention can multiply the PWM dimming signal and the high frequency dimming signal. Therefore, with the use of the LED driving circuit and method of the present invention, the LEDs  120  can be dimmed for many times, so as to improve the dimming effect of the LEDs and the contrast of the display panel, thus improving the display quality of the display apparatus. 
     The present invention has been described with a preferred embodiment thereof and it is understood that many changes and modifications to the described embodiment can be carried out without departing from the scope and the spirit of the invention that is intended to be limited only by the appended claims.