Abstract:
A driving circuit on a liquid crystal display (LCD) panel and associated control method is provided. The LCD panel connected to a display control circuit via a flexible print circuit (FPC) includes a master source driver, for inputting a digital image signal in compliance with a first electrical specification via an FPC board and converting the digital image signal to a gate driving signal and a slave source driving signal, which are in compliance with a second electrical specification; a gate driver, for receiving the gate driving signal in compliance with the second electrical specification; and a slave source driver, for receiving the slave source driving signal in compliance with the second electrical specification. The master source driver, the slave source driver and the gate driver drive a thin-film transistor (TFT) on the LCD panel.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
     This patent application is based on Taiwan, R.O.C. patent application No. 098114456 filed on Apr. 30, 2009. 
     FIELD OF THE INVENTION 
     The present invention relates to a driving circuit on a liquid crystal display (LCD) panel and associated control method, and more particularly, to a driving circuit with low swing voltage on an LCD panel. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  shows a schematic diagram of a conventional LCD panel and a driving circuit thereof. Generally, an LCD panel  10  is divided into a display area  12  and a non-display area  14 . The display area  12  comprises a thin-film transistor (TFT) array, and the non-display area  14  comprises a gate driver  20 , a master source driver  40  and a slave source driver  60  for controlling transistors of the TFT array. 
     Taking an LCD panel with a resolution of 800*480 as an example, the gate driver  20  needs 480 lines connected to the TFT array to control each of the 480 rows, and therefore the gate driver  20  is also referred as a row driver. The master source driver  40  and the slave source driver  60  are referred as column drivers. Take a TFT array controlled by red (R), green (G) and blue (B) colors as an example. The master source driver  40  and the slave source driver  60  need 2400 (800*3) lines connected to the TFT array to control 800 columns. Hence, the master source driver  40  needs 1200 lines and the slave source driver  60  needs 1200 lines. 
     Conventionally, a display controller  82  on a circuit board  80  outside the LCD panel  10  outputs a first digital image signal and a second digital image signal to the master source driver  40  and the slave source driver  60  on the LCD panel  10  via two flexible print circuits (FPC)  42  and  44 , respectively. The master source driver  40  generates a gate driver signal to the gate driver  20  according to the first digital image signal. 
     The first digital image signal and the second digital image signal outputted by the display controller  82  are of transistor-transistor logic (TTL) logic level or complementary metal-oxide semiconductor (CMOS) logic level. That is, the high level is 3.3 V and the low level is 0 V. Similarly, the gate driving signal outputted by the master source driver  40  is also of a TTL or CMOS logic level. 
     Cost and area of the FPCs  42  and  44  connected between the circuit board  80  and the LCD panel  10  are proportional to each other. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a driving circuit on an LCD panel, with a signal transmitted between drivers of the LCD panel having a low swing voltage and slow transition speed. 
     The present invention provides an LCD panel connected to a display controller. The LCD panel comprises a main source driver for receiving a digital image signal in compliance with a first electrical specification from the display controller and converting the digital image signal to a gate driving signal and a slave source driving signal in compliance with a second electrical specification, a gate driver for receiving the gate driving signal in compliance with the second electrical specification, and a slave source driver for receiving the slave source driving signal in compliance with the second electrical specification. The main source driver, the slave source driver and the gate driver drive a TFT array on the LCD panel according to the digital image signal, the gate driving signal and the slave source driving signal. 
     The present invention further provides a method for transmitting an LCD driving signal with low electromagnetic interference (EMI). The method comprises receiving a digital image signal in compliance with a first electrical specification; generating a gate driving signal and a slave source driving signal in compliance with a second electrical specification according to the digital image data; and driving a TFT array on the LCD panel according to the digital image signal, the gate driving signal and the slave source driving signal, respectively. 
     The advantages and spirit of the present invention can be further understood with the following detailed description and drawings. However, the present invention is not limited to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a LCD panel and a driving circuit thereof according to the prior art. 
         FIG. 2  is a schematic diagram of an LCD panel and a driving circuit thereof in accordance with an embodiment of the present invention. 
         FIG. 3  is a schematic diagram of an LCD panel and a driving circuit thereof in accordance with another embodiment of the present invention. 
         FIG. 4A  is a schematic diagram of a conversion output unit in accordance with an embodiment of the present invention. 
         FIG. 4B  is a schematic diagram of a conversion output unit in accordance with another embodiment of the present invention. 
         FIG. 5  is a schematic diagram of a conversion input unit in accordance with an embodiment of the present invention. 
         FIG. 6  is a flow chart of a method for transmitting an LCD driving signal with low EMI in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 2  shows a schematic diagram of an LCD panel and a driving circuit thereof in accordance with an embodiment of the present invention. An LCD panel  100  comprises a display area  112  and a non-display area  114 . The display area  112  comprises a TFT array; and the non-display area  114  comprises a gate driver  120 , a main source driver  140  and a slave source driver  160  for driving transistors of the TFT array. 
     A display controller  182  on a circuit board  180  utilizes an FPC  150  to transmit a digital image signal to the main source driver  140 , which generates a slave source driving signal and a gate driving signal to the slave source driver  160  and the gate driver  120 , respectively. The digital image signal, the gate driving signal and the slave source driving signal are of TTL or CMOS logic level. Since the cost and area of the FPC  150  are proportional to each other, the FPC  150  for transmitting the digital image signal reduces cost while improving signal quality. Detailed description on the FPC  150  is disclosed below. 
     Since the gate driving signal and the slave source driving signal are of TTL or CMOS logic level with frequencies about tens of MHz, a fast high-low transition may cause serious EMI. In order to solve the foregoing problem, a shielding metal covering traces between the slave source driving signal and the gate driving signal on the LCD panel is applied for reducing EMI. 
     Preferably, the TTL or CMOS logic level signals, i.e., the gate driving signal and the slave source driving signal, are converted to signals with a low swing voltage and slow transition speed, so as to reduce EMI without the shielding metal. 
       FIG. 3  shows a schematic diagram of an LCD panel and a driving circuit thereof in accordance with another embodiment of the present invention. A master source driver  190  comprises a main circuit  192  and a conversion output unit  194 . The main circuit  192  mainly operates in the identical way as the master source driver  140  illustrated in  FIG. 2 . The main circuit  192  receives the digital image signal outputted by the display controller  182  and outputs a slave source driving signal and a gate driving signal of TTL or CMOS logic level to the conversion output unit  194 . 
     The conversion output unit  194  receives and converts the slave source driving signal and the gate driving signal of TTL or CMOS logic level to signals with a low swing voltage and slow transition speed. The gate driver  120  and the slave source driver  160 , respectively comprising conversion input units  122  and  162 , convert the received signals with the low swing voltage and slow transition speed to a gate driving signal and a slave source driving signal of TTL or CMOS logic level. Therefore, signals transmitted on the LCD panel are the gate driving signal and the slave source driving signal with low swing voltages and slow transition speed. Therefore, according to this embodiment of the present invention, not only is shielding metal spared but EMI is also effectively reduced. 
       FIG. 4A  shows a conversion output unit in accordance with an embodiment of the present invention. The conversion output unit comprises a first PMOS transistor Mp 1 , a second PMOS transistor Mp 2 , a first NMOS transistor Mn 1  and a second NMOS transistor Mn 2 . For example, the first PMOS transistor Mp 1  has its source connected to a 3.3V voltage source, and its gate connected to its drain to form a diode connected transistor. The second NMOS transistor Mn 2  has its source connected to ground, and its gate connected to its drain to form a diode connected transistor. 
     The second PMOS transistor Mp 2  and the first NMOS transistor Mn 1  form a buffer  196 . The second PMOS transistor Mp 2  has its drain connected to the drain of the first PMOS transistor Mp 1 , and the first NMOS transistor Mn 1  has its gate connected to the gate of the second PMOS transistor Mp 2  to form an input end of a buffer  196 . The second PMOS transistor Mp 2  has its drain connected to the drain of the first NMOS transistor Mn 1  to form an output end of the buffer  196 , and the first NMOS transistor Mn 1  has its source connected to the drain of the second NMOS transistor Mn 2 . 
     Referring to  FIG. 4A , the first PMOS transistor Mp 1  and the second NMOS transistor Mn 2  are configured as diode connected transistors. Suppose that a drain voltage of the first PMOS transistor Mp 1  is V 1  and a drain voltage of the second NMOS transistor Mn 2  is V 2 , where Vcc&gt;V 1 &gt;V 2 &gt; 0 . Therefore, a conversion output unit illustrated in  FIG. 4A  reduces the voltage swing from 3.3V to a difference by subtracting V 2  from V 1 . 
     Referring to  FIG. 4B  showing a schematic diagram of a conversion output unit in accordance with another embodiment of the present invention. The conversion output unit comprises a first buffer  202 , a second buffer  204 , a third buffer  206 , a first delay unit  208  and a second delay unit  210 . The first buffer  202 , the second buffer  204  and the third buffer  206 , being identical to the buffer  196  illustrated in  FIG. 4A , are connected to voltage sources V 1  and V 2 . The first delay unit  208  and the second delay unit  210  have different delay time. 
     A signal input end is directly connected to an input end of the first buffer  202 . The signal input end is connected to an input end of the first delay unit  208 , of which an output end is connected to an input end of the second buffer  204 . The signal input end is also connected to an input end of the second delay unit  210 , of which an output end is connected to an input end of the third buffer  206 . 
     In this embodiment, in addition to reducing a swing voltage from 3.3V to V 1 -V 2 , the conversion output unit illustrated in  FIG. 4B  further reduces a first transition slope to a second transition slope. 
       FIG. 5  shows a schematic diagram of a conversion input unit in accordance with an embodiment of the present invention. The conversion input unit comprises a comparator  220 , a first resistor R 1  and a second resistor R 2 . The first resistor R 1  and the second resistor R 2  are serially connected between a 3.3V voltage source and the ground to form a voltage dividing circuit for providing a threshold voltage Vth to a positive input end of the comparator  220 . A signal input end is connected to a negative input end of the comparator  220 . In this embodiment, in addition to increasing a voltage swing from V 1 -V 2  to 3.3V, the conversion input unit also increases a second transition slope to a first transition slope. 
       FIG. 6  shows a flow chart of a method for transmitting an LCD driving signal with low EMI in accordance with an embodiment of the present invention. In Step  610 , a master source driver receives a digital image signal via an FPC and converts the digital image signal to a gate driving signal and a slave source driving signal, which are compliant with another specification such as a High-Speed Transceiver Logic (HSTL) specification or a Stub Series Terminated Logic (SSTL) specification. For example, the digital image signal is compliant with a low voltage differential signal (LVDS) specification. The swing voltage and the transition speed of the gate driving signal and the slave source driving signal, compliant with the another specification, is smaller than those of signals compliant with a TTL or CMOS logic specification. It should be noted that, the swing voltage and the transition speed of the gate driving signal and the slave source driving signal compliant with the another specification are adjustable. In Step  620 , a gate driver receives and converts the gate driving signal in compliance with the another specification to the gate driving signal in compliance with TTL or CMOS logic specification. The gate driving signal in compliance with TTL or CMOS logic specification swings between a first level and a second level, and the gate driving signal of the another specification swings between a third level and a fourth level. Preferably, the first level is greater than the third level, the third level is greater than the fourth level, and the fourth level is greater than the second level. In Step  630 , the slave source driver receives and converts the slave source driving signal in compliance with the another specification to the slave source driving signal in compliance with TTL or CMOS logic specification. In Step  640 , the master source driver, the gate driver and the slave source driver drive a TFT array on an LCD panel with the gate driving signal and the slave source driving signal in compliance with the another specification, respectively. 
     To sum up, the present invention provides a driving circuit on an LCD panel, with a signal transmitted on the LCD panel having a low swing voltage and adjustable slow high-low transition, such that EMI of the LCD panel is significantly reduced. 
     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not to be limited to the above embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.