Patent Abstract:
An output circuit whose outputting signals having large voltage swing by using switches with low voltage tolerance is provided. The output circuit includes: operation amplifiers, receiving positive input voltage and negative input voltages, respectively; transmission gates, passing output signals from the operation amplifiers, respectively; switch transistors, passing output signals from the transmission gates as an output signal of the output circuit, pulling up the output signal from one of the transmission gates, and pulling down the output signal from the other of the transmission gates.

Full Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates to an output circuit in a driving circuit and a driving method of a display device, more particularly to an output circuit in a driving circuit and a driving method of a display device using low voltage switches. 
   2. Description of Related Art 
   A liquid crystal display (LCD) has many advantages such as light weight, small size, low power consumption and little radiation, and has been widely used in recent years. 
   In general, a LCD includes a panel, a gate driver that orderly actuates a gate line of the panel, and a source driver that transmits image data to each source line of the panel. The source driver at least includes a shift register, a data latch, a D/A converter and an output circuit. In polarity inversion, output voltages from the source driver may drive, for example, from +5V to −5V. That is, for positive polarity, the output voltages are from +5V to 0V, and for negative polarity, the output voltages are from −5V to 0V. In this case, to achieve a voltage swing of 10V for the source driver, switches in the output circuit must have a voltage tolerance of at least 10V, which may lead to a large chip area of the source driver. 
   Therefore, there needs a new output circuit with a small voltage tolerance using low voltage switches, thus reducing the chip area of the source driver. 
   SUMMARY OF THE INVENTION 
   One of the objects of the invention is to provide an output circuit with a large voltage swing by using low voltage switches, thus reducing the chip area of the source driver. 
   To at least achieve the above and other objects, the invention provides an output circuit suitable in a driving circuit for a display device. The output circuit includes: a first operation amplifier, receiving a first input voltage; a second operation amplifier, receiving a second input voltage; a first transmission gate, passing an output signal from the first operation amplifier under control of a first enable signal; a second transmission gate, passing an output signal from the second operation amplifier under control of a second enable signal; a first switch, passing an output signal from the first transmission gate under control of a first switch control signal for generating an output signal of the output circuit; a second switch, passing an output signal from the second transmission gate under control of a second switch control signal for generating the output signal of the output circuit; a third switch, pulling up the output signal from the second transmission gate under control of a third switch control signal; a fourth switch, pulling down the output signal from the first transmission gate under control of a fourth switch control signal; a first inverter, receiving and inverting the first enable signal for generating an inverted signal thereof, the first transmission gate being conducted or non-conducted based on the first enable signal and the inverted signal thereof; and a second inverter, receiving and inverting the second enable signal for generating an inverted signal thereof, the second transmission gate being conducted or non-conducted based on the second enable signal and the inverted signal thereof. 
   Further, the invention provides a method for driving a display device via low voltage tolerance switches. The method comprises the steps of: amplifying a first input voltage or a second input voltage; passing the amplified first input voltage under control of a first enable signal; passing the amplified second input voltage under control of a second enable signal; switching the passed and amplified first input voltage as a driving voltage for the display device under control of a first switch control signal; and switching the passed and amplified second input voltage as the driving voltage for the display device under control of a second switch control signal. 
   Further, the amplified first input voltage is passed under control of the first enable signal and an inverted signal thereof. The amplified second input voltage is passed under control of the second enable signal and an inverted signal thereof. The passed and amplified second input voltage is pulled up under control of a third switch control signal. The passed and amplified first input voltage is pulled down under control of a fourth switch control signal. The first input voltage or the second input voltage is amplified in unity gain. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  shows a circuit diagram of an output circuit in a driving circuit for a display device. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawing. 
   In the embodiment, an output circuit can drive a pair of input voltages to an output voltage between +5V to 0V and −5V to 0V only using switches with a voltage tolerance of 5V. 
     FIG. 1  shows a circuit diagram of an output circuit in a driving circuit for a display device. As shown in  FIG. 1 , the output circuit includes operation amplifiers OP 21 ˜OP 22 , inverters INV 21 ˜INV 22 , transmission gates TM 21 ˜TM 22  and switches TP 21 ˜TP 22  and TN 21 ˜TN 22 . In this embodiment, three reference voltages VDDA (+5V), VSSA (0V) and VDDAN (−5V) are used. In general, only one of the input voltages INP and INN is asserted in the operation of the output circuit. In other words, if one of the input voltages INP and INN has non-zero voltage, the other one will be 0V. 
   The operation amplifier OP 21  is operated under VDDA and VSSA. The operation amplifier OP 21  has an inverting input terminal, a non-inverting input terminal and an output terminal. The operation amplifier OP 21  receives a positive input voltage INP via the non-inverting input terminal. The positive input voltage INP has a voltage swing between +5V and 0V. The output signal from the output terminal of the operation amplifier OP 21  is feedback to the inverting input terminal of the operation amplifier OP 21 . In other words, the operation amplifier OP 21  has a unity gain. 
   The operation amplifier OP 22  is operated under VDDAN and VSSA. The operation amplifier OP 22  has an inverting input terminal, a non-inverting input terminal and an output terminal. The operation amplifier OP 22  receives a negative input voltage INN via the non-inverting input terminal. The negative input voltage INN has a voltage swing between −5V and 0V. The output signal from the output terminal of the operation amplifier OP 22  is feedback to the inverting input terminal of the operation amplifier OP 22 . In other words, the operation amplifier OP 22  has a unity gain. 
   The inverter INV 21  receives and inverts an enable signal ENP into an inverted signal thereof. The inverter INV 21  is operated under VDDA and VSSA. The enable signal ENP is further coupled to the transmission gate TM 21 . The inverted signal of the enable signal ENP output from the inverter INV 21  is also coupled to the transmission gate TM 21 . The enable signal ENP has at least two logic states, positive logic high state (+5V) and logic low state (0V). 
   The inverter INV 22  receives and inverts another enable signal ENN into an inverted signal thereof. The inverter INV 22  is operated under VDDAN and VSSA. The enable signal ENN is further coupled to the transmission gate TM 22 . The inverted signal of the enable signal ENN output from the inverter INV 22  is also coupled to the transmission gate TM 22 . The enable signal ENN has at least two logic states, negative logic high state (−5V) and logic low state (0V). 
   The transmission gate TM 21  receives the output signal from the operation amplifier OP 21 . The transmission gate TM 21  is operated under VDDA and VSSA. The transmission gate TM 21  is conducted or non-conducted under control of the enable signal ENP and the inverted signal of the enable signal ENP. When the enable signal ENP is in the positive logic high state, the transmission gate TM 21  is conducted. When the enable signal ENP is in the logic low state, the transmission gate TM 21  is non-conducted. The transmission gate TM 21  generates an output signal PNET to the switches TP 21  and TN 22 . In general, when the transmission gate TM 21  is conducted, the output signal PNET from the transmission gate TM 21  has the same voltage value as the positive input voltage INP. 
   The transmission gate TM 22  receives the output signal from the operation amplifier OP 22 . The transmission gate TM 22  is operated under VDDAN and VSSA. The transmission gate TM 22  is conducted or non-conducted under control of the enable signal ENN and the inverted signal of the enable signal ENN. When the enable signal ENN is in the logic low state, the transmission gate TM 22  is conducted. When the enable signal ENN is in the negative logic high state, the transmission gate TM 22  is non-conducted. The transmission gate TM 21  generates an output signal PNET to the switches TP 21  and TN 22 . The transmission gate TM 22  generates an output signal NNET to the switches TN 21  and TP 22 . In general, when the transmission gate TM 22  is conducted, the output signal NNET from the transmission gate TM 22  has the same voltage value as the negative input voltage INN. 
   In the embodiment, the switches TP 21 ˜TP 22  and TN 21 ˜TN 22  are implemented by P-type MOSFETs and N-type MOSFETs, respectively. However, the invention is not limited thereby. 
   The switch TP 21  has a source terminal coupled to the output signal PNET from the transmission gate TM 21 , a gate terminal receiving a switch control signal SWN and a drain terminal coupled to an output signal SOUT of the output circuit. Further, the bulk terminal of the switch TP 21  is coupled to the source terminal of the switch TP 21 . The switch control signal SWP has at least two logic states, negative logic high state (−1.8V) and logic low state (0V). 
   The switch TP 22  has a source terminal coupled to VSSA, a gate terminal receiving a switch control signal SWNB and a drain terminal coupled to the output signal NNET from the transmission gate TM 22 . Further, the bulk terminal of the switch TP 22  is coupled to the source terminal of the switch TP 22 . The switch control signal SWNB has at least two logic states, negative logic high state (−5V) and logic low state (0V). 
   The switch TN 21  has a source terminal coupled to the output signal NNET from the transmission gate TM 22 , a gate terminal receiving a switch control signal SWP and a drain terminal coupled to the output signal SOUT of the output circuit. Further, the bulk terminal of the switch TN 21  is coupled to the source terminal of the switch TN 21 . The switch control signal SWN has at least two logic states, positive logic high state (+1.8V) and logic low state (0V). 
   The switch TN 22  has a source terminal coupled to VSSA, a gate terminal receiving a switch control signal SWPB and a drain terminal coupled to the output signal PNET from the transmission gate TM 21 . Further, the bulk terminal of the switch TN 22  is coupled to the source terminal of the switch TN 22 . The switch control signal SWPB has at least two logic states, positive logic high state (+5V) and logic low state (0V). 
   In this embodiment, the positive input voltage INP has a voltage swing between VDDA (+5V) and VSSA (0V) and the negative input voltage INN has a voltage swing between VDDAN (−5V) and VSSA (0V). Further, four scenarios are described below. In scenario A, the positive input voltage INP is between VDDA and 0.5*VDDA, i.e. +5V˜+2.5V. In scenario B, the positive input voltage INP is between 0V and 0.5*VDDA, i.e. 0V˜+2.5V. In scenario C, the negative input voltage INN is between VDDAN and 0.5*VDDAN, i.e. −5V˜−2.5V. In scenario D, the negative input voltage INN is between 0 and 0.5*VDDAN, i.e. 0V˜−2.5V. 
   Scenario A: INP Between VDDA˜0.5*VDDA 
   In scenario A, the signals ENP, SWPB, SWP, ENN, SWNB and SWN are positive logic high (+5V), logic low (0V), logic low (0V), negative logic high (−5V), negative logic high (−5V) and logic low (0V), respectively. Therefore, the transmission gate TM 21 , the switches TP 21  and TP 22  are turned on; and the transmission gate TM 22 , the switches TN 21  and TN 22  are turned off. Because the transmission gate TM 21  is turned on, the output signal from the operation amplifier OP 21 , having the same voltage value as the positive input voltage INP, is passed by the transmission gate TM 21  and the output signal PNET from the transmission gate TM 21  has the same voltage value as the positive input voltage INP. Because the switch TP 21  is turned on, the output signal SOUT has the same voltage value as the output signal PNET. In other words, SOUT=PNET=INP. In scenario A, the reason why the switch TP 22  is turned on relies on that, in worst case, if in initial state, the signal NNET has a non-zero negative voltage value, the ON switch TP 22  pulls high the signal NNET to 0V. Under scenario A, V SG  and V DG  of the switches TP 21  and TP 22  and V GS  and V GD  of the switches TN 21  and TN 22  are listed as Table 1. 
   
     
       
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               TP21 
               TP22 
                 
               TN21 
               TN22 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               V SG   
               +2.5 V~+5 V 
               +5 V 
               V GS   
               0 V 
               0 V 
             
             
               V DG   
               +2.5 V~+5 V 
               +5 V 
               V GD   
               −5 V~−2.5 V 
               −5 V~−2.5 V 
             
             
                 
             
           
        
       
     
   
   From Table 1, it is known that, V SG  (or V GS ) and V DG  (or V GD ) of anyone of the switches in scenario A is not higher than +5V (or −5V). 
   Scenario B: INP Between VSSA˜0.5*VDDA 
   In scenario B, the signals ENP, SWPB, SWP, ENN, SWNB and SWN are positive logic high (+5V), logic low (0V), negative logic high (−1.8V), negative logic high (−5V), negative logic high (−5V) and logic low (0V), respectively. Therefore, the transmission gate TM 21 , the switches TP 21  and TP 22  are turned on; and the transmission gate TM 22 , the switches TN 21  and TN 22  are turned off. Because the transmission gate TM 21  is turned on, the output signal from the operation amplifier OP 21 , having the same voltage value as the positive input voltage INP, is passed by the transmission gate TM 21  and the output signal PNET from the transmission gate TM 21  has the same voltage value as the positive input voltage INP. Because the switch TP 21  is turned on, the output signal SOUT has the same voltage value as the output signal PNET. In other words, SOUT=PNET=INP. In scenario B, the reason why the switch TP 22  is turned on is similar to that in scenario A. In other words, in worst case, if in initial state, the signal NNET has a non-zero negative voltage value, the ON switch TP 22  pulls high the signal NNET to 0V. Under scenario B, V SG  and V DG  of the switches TP 21  and TP 22  and V GS  and V GD  of the switches TN 21  and TN 22  are listed as Table 2. 
   
     
       
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
                 
               TABLE 2 
             
             
                 
                 
             
             
                 
               TP21 
               TP22 
                 
               TN21 
               TN22 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               V SG   
               +1.8 V~+4.3 V 
               +5 V 
               V GS   
               0 V 
               0 V 
             
             
               V DG   
               +1.8 V~+4.3 V 
               +5 V 
               V GD   
               −2.5 V~0 V 
               −2.5 V~0 V 
             
             
                 
             
           
        
       
     
   
   From Table 2, it is known that, V SG  (or V GS ) and V DG  (or V GD ) of anyone of the switches in scenario B is not higher than +5V (or −5V). 
   Scenario C: INN Between 0.5*VDDAN˜VDDAN 
   In scenario C, the signals ENP, SWPB, SWP, ENN, SWNB and SWN are logic low (0V), positive logic high (+5V), logic low (0V), logic low (0V), logic low (0V) and logic low (0V), respectively. Therefore, the transmission gate TM 21 , the switches TP 21  and TP 22  are turned off; and the transmission gate TM 22 , the switches TN 21  and TN 22  are turned on. Because the transmission gate TM 22  is turned on, the output signal from the operation amplifier OP 22 , having the same voltage value as the positive input voltage INN, is passed by the transmission gate TM 22  and the output signal NNET from the transmission gate TM 22  has the same voltage value as the positive input voltage INN. Because the switch TN 21  is turned on, the output signal SOUT has the same voltage value as the output signal NNET. In other words, SOUT=NNET=INN. In scenario C, the reason why the switch TN 22  is turned on is similar to that in scenario A. In other words, in worst case, if in initial state, the signal PNET has a non-zero positive voltage value, the ON switch TN 22  pulls low the signal PNET to 0V. Under scenario C, V SG  and V DG  of the switches TP 21  and TP 22  and V GS  and V GD  of the switches TN 21  and TN 22  are listed as Table 3. 
   
     
       
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
                 
               TABLE 3 
             
             
                 
                 
             
             
                 
               TP21 
               TP22 
                 
               TN21 
               TN22 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               V SG   
               0 V 
               0 V 
               V GS   
               +2.5 V~+5 V 
               +5 V 
             
             
               V DG   
               −2.5 V~−5 V 
               −2.5 V~−5 V 
               V GD   
               +2.5 V~+5 V 
               +5 V 
             
             
                 
             
           
        
       
     
   
   From Table 3, it is known that, V SG  (or V GS ) and V DG  (or V GD ) of anyone of the switches in scenario C is not higher than +5V (or −5V). 
   Scenario D: INN Between 0.5*VDDAN˜VSSA 
   In scenario D, the signals ENP, SWPB, SWP, ENN, SWNB and SWN are logic low (0V), positive logic high (+5V), logic low (0V), logic low (0V), logic low (0V) and positive logic high (+1.8V), respectively. Therefore, the transmission gate TM 21 , the switches TP 21  and TP 22  are turned off; and the transmission gate TM 22 , the switches TN 21  and TN 22  are turned on. Because the transmission gate TM 22  is turned on, the output signal from the operation amplifier OP 22 , having the same voltage value as the positive input voltage INN, is passed by the transmission gate TM 22  and the output signal NNET from the transmission gate TM 22  has the same voltage value as the positive input voltage INN. Because the switch TN 21  is turned on, the output signal SOUT has the same voltage value as the output signal NNET. In other words, SOUT=NNET=INN. In scenario D, the reason why the switch TN 22  is turned on is similar to that in scenario A. In other words, in worst case, if in initial state, the signal PNET has a non-zero positive voltage value, the ON switch TN 22  pulls low the signal PNET to 0V. Under scenario D, V SG  and V DG  of the switches TP 21  and TP 22  and V GS  and V GD  of the switches TN 21  and TN 22  are listed as Table 4. 
   
     
       
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
                 
               TABLE 4 
             
             
                 
                 
             
             
                 
               TP21 
               TP22 
                 
               TN21 
               TN22 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               V SG   
               0 V 
               0 V 
               V GS   
               +1.8 V~+4.3 V 
               +5 V 
             
             
               V DG   
               −2.5 V~0 
               −2.5 V~0 
               V GD   
               +1.8 V~+4.3 V 
               +5 V 
             
             
                 
             
           
        
       
     
   
   From Table 4, it is known that, V SG  (or V GS ) and V DG  (or V GD ) of anyone of the switches in scenario D is not higher than +5V (or −5V). 
   From the above description, in any scenario, voltage drop between any two terminals of any of the switches TP 21 ˜TP 21  and TN 21 ˜TN 22  is not higher than +5V (VDDA) or −5V (VDDAN). Therefore, in the embodiment, the output signal SOUT from the output circuit has a voltage swing of +5V˜−5V by using switches having low voltage tolerance, for example, only 5V tolerance. A switch having low voltage tolerance has a reduced circuit layout. Thus, the output circuit in the embodiment has a reduced circuit area. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing descriptions, it is intended that the present invention covers modifications and variations of this invention if they fall within the scope of the following claims and their equivalents.

Technology Classification (CPC): 6