Patent Publication Number: US-2009219270-A1

Title: Apparatus for driving an lcd display with reducted power consumption

Description:
The driving circuit for an LCD (e.g., an active matrix LCD) can be divided in two parts: a source and a gate driver. The gate driver controls the gates of the transistors to select and deselect the pixels of a specific row. The source drivers provide the required voltage level to all sub-pixels of the currently selected row corresponding to the desired intensity for each color. For this purpose, the source drivers typically comprise analog output buffers. 
     LCD driver circuits include more and more channels in a single chip, while the output voltage range, and, consequently, the analog supply voltage become larger in order to provide an increased dynamic range and color depth. Due to the high number of channels and the increased supply voltage, one of the most important parameters of a driver circuit, namely the overall power consumption, is mainly determined by the power consumption of the analog output buffers. 
     Conventional source drivers contain two different types of analog output buffers. In some implementations so-called polarity dependent drivers containing N and P output buffers (herein referred to as N-buffers and P-buffers) are employed. The full supply voltage range of the source driver is supplied to these output buffers, but they work only in the upper or the lower regime of the supply voltage range. 
     There are also display implementations where so-called rail-to-rail output buffers (herein referred to as P rail-to-rail buffers and N rail-to-rail buffers) are employed. These output buffers are typically positioned between two power supply rails of the supply voltage regime. 
     In  FIG. 1  an example of a conventional source driver  100  with polarity dependent output buffers  1 ,  2  is shown. In this example each pixel of the display (i.e., each output  103 ,  104  of the driver  100 ) may be driven either by a P output buffer  1  or N output buffer  2 , depending on the polarity at the respective inputs  101 ,  102  of the P output buffer  1  or the N output buffer  2 . As indicated in  FIG. 1 , the positive part of the gamma curve  3  is applied to the input  101  of the P output buffer  1 , whereas the negative part of the gamma curve  4  is applied to the input  102  of the N output buffer  2  so that both buffers  1  and  2  are always in use. A consequence of this design is that the supply voltages have to be defined and “hard-wired” during the design of the source driver chip  100  and cannot be altered afterwards. Since the supply of the two output buffers  1 ,  2  is provided by the two power rails VDDH, VSSH, these buffers  1 ,  2  must be composed by high voltage transistors. The power is so high in this case because both buffers  1 ,  2  use the entire supply voltage range between VDDH and VSSH. 
     Another disadvantage of this design is that due to the fact that high voltage transistors are required, quite some chip area is occupied. 
       FIG. 2  presents a conventional architecture with rail-to-rail buffers. In  FIG. 2  part of a driver chip  110  with one such rail-to-rail buffer  7  is shown. When such a rail-to-rail buffer  7  is used, this single buffer  7  has to drive both positive  8  and negative gamma  9  voltages. The buffer  7 , however, still operates in the whole supply voltage range, thus having the same disadvantages of the high voltage transistors, namely increased power consumption and large size. 
     For both cases presented, the DC power consumption of the output buffers can be calculated as: 
       TotalPowerPerChannel=VDDH•Iddh_average 
     where Iddh_average is the average current flowing through the two buffers  1  and  2  in  FIG. 2  or through the buffer  7  in  FIG. 2 . 
     For the whole driver chip  100  or  110 , this value must be multiplied by the number of channels N channels . 
       TotalPowerPerChip=TotalPowerPerChannel•N channels   
     The major drawback of both designs is, as mentioned, the high power consumption and the large chip area. 
     It is thus an object of the present invention to provide a driving scheme for use in a display that consumes less power than conventional display drivers, and that enables the design of smaller driver chips. 
     This and other objects are accomplished by an apparatus according to claim  1 . 
     According to the present invention an apparatus for driving an LCD display is provided where a new and inventive TFT LCD driving technique is employed. The apparatus comprises a source driver operating between a first and a second power supply rail. The source driver has at least one power buffer arranged between these power supply rails. The power buffer provides at an output a virtual voltage of about half the voltage being available between the two power supply rails. Furthermore, the source driver comprises a large number of P- and N-buffers (depending on number of output channels, typically several hundreds). As P-buffers and N-buffers either rail-to-rail buffers or polarity dependent buffers can be employed. The P-buffer is situated between the first power supply rail and the output where the virtual voltage is made available. The N-buffer is situated between the output where the virtual voltage is made available and the second power supply rail. According to the present invention, the P-buffer is driven by a positive gamma voltage curve and the N-buffer is employed such that it is driven by a negative gamma voltage curve. 
     According to the present invention, the reduction of the power consumption and area typically occupied by the high voltage transistors is achieved by using a new and inventive TFT LCD driving technique providing for a reduced power consumption. 
     According to another embodiment of the present invention, a set of switches is employed in order to be able to operate a buffer during a first load cycle between the first power supply rail and a rail where the virtual voltage is made available and during a subsequent load cycle between the virtual voltage rail and the second power supply rail. This embodiment has the advantage that a constant offset is ensured. 
     Further advantageous embodiments of the apparatus are provided in the dependent claims. 
     Instead of having DC consumption over the full power supply range, as in case of conventional drivers shown in  FIG. 1 and 2 , the present invention uses only about half of the supply voltage for each output buffer. A strong power buffer is employed in order to create a virtual voltage acting as a power supply for the N-buffers and as ground for the P-buffers. According to the present invention, this virtual voltage is created internally inside the driver circuit, and is in the most preferred embodiment shared by all channels (i.e., by all N- and P-buffers of an integrated circuit) of the driver circuit. 
     In another embodiment of the present invention the proposed power reduction technique is used in conjunction with rail-to-rail output buffers. 
     In yet another embodiment of the present invention the proposed power reduction technique is used in conjunction with polarity-dependent buffers. 
     The power consumed by the driver circuits in accordance with the present invention is about half of the power consumed by the conventional architecture. 
     Another advantage of the present invention is the area reduction, due to the fact that low voltage transistors can be used instead of high voltage transistors. This is possible because the highest voltage across the transistors is always about half of the potential difference between the two power supply rails. 
     Another embodiment of the invention is characterized in that the offset of each channel of the driver circuit is kept constant in the whole working range. Since the polarity of the output voltage changes with every load cycle, a set of switches is employed. As each of the two buffers (N-buffer and P-buffer) only works in their own supply regime, cross selection switches may be employed in this embodiment in order to change polarity. 
    
    
     
       For a more complete description of the present invention and for further objects and advantages thereof, reference is made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic representation of a conventional display driver using polarity dependent output buffers; 
         FIG. 2  is a schematic representation of a conventional display driver using a rail-to-rail buffer; 
         FIG. 3  is a schematic representation of a first embodiment of the present invention using a power buffer and two rail-to-rail buffers; 
         FIG. 4  is a schematic representation of a second embodiment of the present invention; 
         FIG. 5A  is another schematic representation of the second embodiment of the present invention during a frame N; 
         FIG. 5B  is another schematic representation of the second embodiment of the present invention during a frame N+1; 
         FIG. 6  is another embodiment of the present invention. 
     
    
    
     A first embodiment of the present invention is presented in  FIG. 3 , depicting part of a source driver  200  for LCD displays. The apparatus comprises of a power divider  33 , a power buffer  22 , a P rail-to-rail buffer  20  and an N rail-to-rail buffer  21 . 
     The power divider  33  is made of two resistors R being arranged in series between the power supply rails VDDH  30 . 1  and VSSH  30 . 2 , having a middle node  29  connected to an input of the power buffer  22 . 
     The power buffer  22  is arranged between the power supply rails VDDH  30 . 1  and VSSH  30 . 2 , having one of its inputs connected to the middle node  29  of the power divider  33  and connected to an output  32 . This kind of arrangement is herein referred to as voltage follower or unity gain configuration. This power buffer  22  provides at its output  32  a virtual voltage VV of about half the voltage that is available between the two power supply rails VDDH and VSSH. 
     The P rail-to-rail buffer  20  is situated between the first power supply rail VDDH and the virtual voltage VV. This P rail-to-rail buffer  20  is driving positive gamma voltages and shares the Iddh DC current with the N rail-to-rail buffer  21 . The respective input signal is herein referred to as V input P. That is, the signals V input P corresponding to the positive part of the gamma curve is applied to an input  27  of the P rail-to-rail buffer  20 . 
     The N rail-to-rail buffer  21  is being situated between the virtual voltage VV and the second power supply rail VSSH, and is driving negative gamma voltages V input N. That is, the signals V input N corresponding to the negative part of the gamma curve is applied to an input  28  of the N rail-to-rail buffer  21 . 
     The operating range is divided into two different phases (load cycles or frames), where Frame N is shown on  FIG. 3  as  23  and Frame N+1 shown on  FIG. 3  as  24 . During the first phase (Frame N), the output  25  of the P rail-to-rail buffer  20  drives a column of the display (not shown) and during the second, subsequent phase (Frame N+1) the output  26  of the N rail-to-rail buffer  21  drives the column of the display. That is, while one column is being served by one of the buffers ( 20  or  21 ), the respective other buffer ( 21  or  20 ) is connected to a neighboring column of the display. 
     Please note that in  FIG. 3  only part of a source driver  200  is shown. A real source driver  200  comprises at least one power buffer  22  and a plurality of pairs of P rail-to-rail buffers  20  and N rail-to-rail buffers  21 . The number of pairs of buffers corresponds to the number of channels N channels . 
     According to  FIG. 3  and assuming one can derive the following formula to calculate the total power consumed by the source driver  200 : 
     
       
         
           
             
               TotalPowerPerChip 
               = 
               
                 
                   
                     VDDH 
                     2 
                   
                   · 
                   
                     I 
                     ddh 
                   
                   · 
                   
                     N 
                     channels 
                   
                 
                 + 
                 
                   Ivb 
                   · 
                   
                     V 
                     DDH 
                   
                 
               
             
             ; 
           
         
       
     
     where Ivb is the current “consumed” by the power buffer  22  and Iddh the current “consumed” by the buffers  20 ,  21 . From this equation one can derive that the power consumption of the inventive source driver  200  is at about half of the power consumption of a conventional source driver (if one disregards the power consumed by the power buffer  22 ). 
     In  FIG. 4  another embodiment of an apparatus  300  of the present invention is depicted. As illustrated in this Figure, the P rail-to-rail buffer and the N rail-to-rail buffer each comprise two stages, where the first stage is referred to as input stage  28  and the second stage is referred to as output stage  27 . The input stage  28  of the P rail-to-rail buffer comprises an input buffer  31  and the input stage  28  of the N rail-to-rail buffer comprises an input buffer  32 . The output stage  27  of the P rail-to-rail buffer comprises two power transistors  25 . 1 ,  25 . 2  serving as P output buffer, and the output stage  27  of the N rail-to-rail buffer comprises two power transistors  26 . 1 ,  26 . 2  serving as N output buffer. Each of the input buffers  31  or  32  can be connected to either output stage  27  or  28 , thus arranging a voltage-follower (or unity gain configuration). 
     A set of switches SwPP- 1 , SwPN- 1 , SwGP- 1 , SwGN- 1 , SwFb- 1 , SwOut- 1  and SwPP- 2 , SwPN- 2 , SwGP- 2 , SwGN- 2 , SwFb- 2 , SwOut- 2  is provided in order to be able to change the polarity of the output signals at the output pads Pad 1  and Pad 2 . These switches are controlled so that during a first frame (Frame N) the input V input P, i.e., the positive part of the gamma curve (P gamma), is “connected” via the input buffer  31  and the output stage with transistors  25 . 1 ,  25 . 2  to the Pad 1  and a respective first display channel. During the subsequent second frame (Frame N+1), the input V input N, i.e., the negative part of the gamma curve (N gamma), is “connected” via the input buffer  31  and the output stage with transistors  25 . 1 ,  25 . 2  to the Pad 1 . During the first frame (Frame N), the input buffer  31  operates between the voltages VDDH and VV whereas during the second frame (Frame N+1) the input buffer  31  operates between the voltages VV and VSSH. During the first frame (Frame N) the input V input N, i.e., the negative part of the gamma curve (N gamma), is “connected” via the input buffer  32  and the output stage with transistors  26 . 1 ,  26 . 2  to the Pad 2  (and a respective second display channel) and during the subsequent second frame (Frame N+1), the input V input P, i.e., the positive part of the gamma curve (P gamma), is connected via the input buffer  32  and the output stage with transistors  26 . 1 ,  26 . 2  to the Pad 2 . During the first frame (Frame N), the input buffer  32  operates between the voltages VV and VSSH whereas during the second frame (Frame N+1) the input buffer  32  operates between the voltages VDDH and vv. 
     The embodiment depicted in  FIG. 4  has the advantage that the offset of each channel is kept constant in the whole working range, since the same input buffers  31 ,  32  are used to drive one and the same output pad with the positive and negative parts of the gamma curve. Since the polarity of the output voltages changes with each frame (load cycle) a set of switches, as illustrated in  FIG. 4 , must be used. Since each of the buffers  31 ,  32  can only work in its own supply regime, the output signals have to be changed using cross-selection switches, as shown. In order to keep the offset of each channel constant over the whole range of the gamma curve, additional switches SwPP- 1 , SwPN- 1 , and SwPP- 2 , SwPN- 2  are used to commutate the supply lines for both input buffers  31 ,  32 . 
     It is sufficient to just commutate the input buffers of the apparatus  300  since the offset is caused by the input buffers  31 ,  32  mainly. This means that it is not necessary to commutate the elements of the output stages  27 . The output stages  27  can be strongly connected to the supply lines VDDH, VV and VV, VSSH, respectively. This approach allows to saves chip area since for the commutation of the output stages  27  strong and large switches would be required. 
     In order to better illustrate this embodiment, additional details are described in connection with the  FIGS. 5A and 5B . In  FIG. 5A , part of a inventive apparatus  300  are shown during a first frame (Frame N).  FIG. 5B  shows the same apparatus  300  during a second frame (Frame N+1). The  FIGS. 5A and 5B  are drawn such that the commutation of the supply regimes becomes visible. 
     The apparatus  300  comprises two input buffer  31 ,  32 . The input buffers  31 ,  32  are two identical operational amplifiers (without output stage), which, when connected to an output stage, create a voltage follower (or unity-gain) configuration. Each of them is capable of handling the input and output voltages in the whole range between two supply rails. This feature is referred to as Rail-to-Rail operation. The amplifiers are implemented in such a way that they may be supplied between any two supply rails that exist inside the apparatus  300  (e.g. inside the source driver). This feature is referred to as floating amplifiers. 
     The apparatus  300  further comprises two high-voltage output stages (Outstage- 1  and Outstage- 2 ). These two high-voltage output stages are firmly connected between the corresponding supply rails. That is, the OutStage- 1  is connected between VDDH and VV whereas the OutStage- 2  is connected between VV and VSSH. 
     There is a set of paired switches that allows exchanging the signals from the wires marked with arrows. For example, the switches SwPP- 1  and SwPP- 2  together serve as a paired switch. In one position, as shown for a Frame N in  FIG. 5A , they provide a connection between the terminal vdd of the buffer  31  and the upper power supply VDDH, and, respectively, terminal vdd of the buffer  32  and the virtual power supply VV. In another position, depicted for the Frame N+1 in  FIG. 5B , these switches SwPP- 1  and SwPP- 2  provide a connection of the terminal vdd of the buffer  31  to the virtual power supply VV, and, respectively, between the terminal vdd of the buffer  32  and the upper power supply VDDH. 
     The paired switches SwPP- 1  and SwPP- 2  and the paired switches SwPN- 1  and SwPN- 2  are used to connect each buffer  31 ,  32  between either supply rails VDDH and VV or VV and VSSH. 
     The virtual voltage VV is provided by a power buffer, as in case of  FIG. 3 , for instance. 
     The paired switches SwIn- 1  and SwIn- 2  at the input side of the input buffers  31 ,  32  are used to connect the inputs of either buffer  31 ,  32  to either signal source V input P (positive part of the gamma curve) or V input N (negative part of the gamma curve). 
     The paired switches SwGP- 1  and SwGP- 2  and the paired switches SwGN- 1  and SwGN- 2  are used to connect the gates of the transistors  25 . 1 ,  25 . 2 ,  26 . 1 ,  26 . 2  of the output stages OutStage- 1  and OutStage- 2  to the controlling signals of either buffer  31 ,  32 . 
     The paired switches SwOut- 1  and SwOut- 2  are used to redirect the output signal of the OutStage- 1  and OutStage- 2  to either output pad Pad 1  or Pad 2 . 
     The paired switches SwFb- 1  and SwFb- 2  are used to provide feedback input for each buffer  31 ,  32  from the output of the appropriate output stage OutStage- 1  or OutStage- 2 . 
     Using a set of paired switches as illustrated in  FIGS. 4 and 5A ,  5 B, the offset of each channel is kept constant during the positive part of the gamma curve and the negative part of the gamma curve, since the same input buffer is used for both parts of the gamma curve. In general, the toggling all the paired switches is equivalent to exchanging the two buffers (placing buffer  31  instead of buffer  32  and vice-versa), and exchanging the two output pads Pad 1  and Pad 2 . 
     In  FIG. 6  an apparatus  400  is shown which comprises a gate driver  402  and a source driver  401  for driving the pixels of a display panel. The display panel is schematically shown by a grid comprising M rows and N columns. The invention is implemented inside the source driver  401 . In the present embodiment of the invention, the source driver  401  comprises a plurality of integrated circuits  200 / 300 . The source driver  401  is supplied be an upper voltage VDDH and a lower voltage VHHS. Each of the integrated circuits  200 / 300  comprises one power buffer. These power buffers provide a virtual voltage which is about half the voltage between the two power supply rails VDDH and VSSH. In  FIG. 6 , the power buffers and the virtual voltage VV are schematically depicted. A the output side of each integrated circuit  200 / 300  there is a number of P-buffers and N-buffers for driving the channels of the display. In  FIG. 6 , the P-buffers and N-buffers are schematically depicted as a row of triangles. 
     The P-buffers of the integrated circuit  200 / 300  are situated between the upper power supply rail VDDH and the virtual voltage VV and the N-buffers are situated between the virtual voltage VV and the lower power supply rail VSSH. If switches are provided, as in  FIGS. 4 ,  5 A and  5 B, the supply of the buffers can be commutated. 
     According to the present invention, embodiments are possible where several intermediate virtual voltages VV 1  through VVn are provided (with n=2, 3 . . . ) by a corresponding number of power buffers.