Patent Publication Number: US-7224338-B2

Title: Signal processing circuit and liquid crystal display device using the same

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a signal processing circuit, and particularly to a circuit that drives a liquid crystal device in a liquid crystal display device. 
   2. Description of the Related Art 
   As shown in  FIG. 4 , a liquid crystal driving circuit is formed comprising a digital circuit and an analog circuit. 
   The digital circuit includes a shift register  101  to which data is input column-by-column, data registers  102  that receive data to be displayed from the shift register  101  and that temporarily store the received data, and data latches  103  that hold the data which is being displayed. 
   The analog circuit includes D/A (digital-to-analog) converters  104  that convert digital data to analog data based on a predetermined relationship, and buffers  105  for power amplification of the converted analog data to drive liquid crystal devices. 
   A liquid crystal display device has the structure shown in  FIG. 5 , including a source driver  200  that outputs and supplies display data to a liquid crystal device  203  connected with each column line, and a gate driver  201  that outputs a control signal to a gate of a transistor connected with a desired row line and that supplies the display data to the liquid crystal devices corresponding to the row line to be displayed and thereby writing the display data to the predetermined liquid crystal devices. Due to their low-power-consumption, liquid crystal display devices have recently seen increased usage in portable devices such as cellular phones. In order to support smaller portable devices, the demands for liquid crystal display devices having even lower power consumption have increased. 
   In the conventional liquid crystal driving circuit such as shown in  FIG. 4 , the power consumption of the analog circuit is higher than the digital circuit. In particular, the power consumption required for digital-to-analog conversion of the D/A converters  104  and power amplification of the buffers  105  to drive the liquid crystal devices occupies 70% to 80% of the overall power consumption of the liquid crystal display device. 
   It is therefore necessary to reduce the power consumption of the D/A converters  104  and the buffers  105  in order to efficiently achieve low power consumption of the overall liquid crystal driving circuit. 
   In one known mechanism, the operations of the buffers  105  and the D/A converters  104 , which require high power consumption; are turned on or off by a source driver according to an external signal (see Japanese Unexamined Patent Application Publication No. 2001-188499). The structure of such a source driver will be described with reference to  FIG. 6 . 
   A control signal to provide low power consumption is input to a buffer  106 , an inverter circuit  108 , and a switch  107  in a standby mode. 
   In the standby mode, the power supply to the buffer  106  is turned off and the power supply to the inverter circuit  108  is turned on, so that a signal from the inverter circuit  108  is output from the switch  107 . 
   In a non-standby mode, the power supply to the buffer  106  is turned on and the power supply to the inverter circuit  108  is turned off, so that a signal from the buffer  106  (analog grayscale data generated by the D/A converter  109 , corresponding to a certain number of gray levels) is output from the switch  107 . 
   Therefore, the operation of the buffer  106  is controlled, thus realizing low power consumption. 
   However, this structure allows for only chip-based batch control of the output modes of the source driver because of standby-mode control. 
   In this structure, the most significant bit (MSB) of the input digital grayscale data is output to the inverter circuit  108  in the standby mode so as to output binary data. 
   Thus, only eight colors are represented by using one-bit (the most significant bit) grayscale data for each of R (red), G (green), and B (blue); whereas, 260,000 colors are represented by using 6-bit grayscale data for each of R, G, and B. The above-described structure provides fewer displayable colors, resulting in degradation in image quality. 
   It is therefore difficult to provide low power consumption for a normal display without reducing the number of displayable colors and the image quality. Accordingly, what is needed is an improved liquid crystal driving circuit providing low power consumption while maintaining the number of gray levels for the image. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a liquid crystal driving circuit which achieves low power consumption without reducing the number of gray levels and degrading the image quality. 
   In one aspect, the present invention provides a signal processing circuit including a D/A conversion circuit for converting input digital data into an analog voltage and outputting the converted analog voltage, a first switch for selecting one of a plurality of supplied voltages and outputting the selected voltage as a supply analog voltage, a second switch for selecting one of the converted analog voltage and the supply analog voltage and outputting the selected voltage as an analog voltage, and a detection circuit for determining whether or not the input digital data matches internal set data, and, when it is determined that the input digital data matches the set data, switching the first switch so that the supply analog voltage corresponding to the input digital data is output and switching the second switch so that the supply analog voltage is output. 
   Thus, when the detection circuit determines that the input digital grayscale data matches certain set data, the detection circuit turns off the power supply to the D/A conversion circuit having a D/A converter and a buffer, and changes the output modes of the first and second switches so that one of the input different voltages is selected, that is, so that the voltage corresponding to the input digital grayscale data is selected from the plurality of voltages input to the first switch and the selected voltage is output as an analog-grayscale-voltage driving signal. Therefore, when the signal processing circuit of the present invention is used for D/A conversion in a liquid crystal display device, the power consumption can be greatly reduced in a source driver, thus realizing a low-consumption-power device. 
   In a signal processing circuit in accordance with embodiments of the present invention, the detection circuit may have digital data corresponding to the supply analog voltage as the set data, and the correspondence between the set digital data and the supply analog voltage may be similar to the correspondence between the input digital and the converted analog voltage in the D/A conversion circuit. 
   Thus, a voltage having a similar value to the converted voltage output from the D/A conversion circuit can be supplied via the first and second switches. This allows for conversion of the digital grayscale data into the corresponding analog-grayscale-voltage driving signal without using the D/A conversion circuit. Since the D/A conversion circuit is not used to convert digital grayscale data into a specific voltage, the power consumption is reduced. 
   Moreover, in the signal processing circuit of the present invention, input digital grayscale data is checked for each column line to determine, for each column line, which of the converted signal (converted analog voltage) from the D/A conversion circuit and the supply signal (supply analog voltage) from the first switch is to be output as an analog-grayscale-voltage driving signal. When it is determined that the digital grayscale data indicates an intermediate gray level, the output signal from the D/A conversion circuit is output as analog grayscale data. Thus, the number of gray levels is not reduced. Therefore, the power consumption can be reduced without degradation in image quality. 
   In a signal processing circuit in accordance with embodiments of the present invention, when the detection circuit determines that the input digital data matches the internal set data, a power supply to the D/A conversion circuit having a D/A converter and a buffer can be turned off. 
   Thus, a power supply to the D/A conversion circuit having a D/A converter and a buffer can be controlled, if necessary, thus reducing unnecessary power and reducing the power consumption. 
   In a signal processing circuit in accordance with embodiments of the present invention, the first switch can select one of a power supply voltage supplied from a power supply circuit and 0 V (ground voltage) by switching, and can output the selected voltage as the supply analog voltage. 
   Thus, the power supply to the D/A conversion circuit is turned off when a power supply voltage and a ground voltage, which require high power consumption of the D/A conversion circuit, are output, and the output modes of the first and second switches are changed so that an analog grayscale voltage of a driving signal can be obtained from a power supply unit. Since the D/A conversion circuit is not used during this operation, low power consumption can be realized. 
   In a signal processing circuit in accordance with embodiments of the present invention, the first switch can select a predetermined voltage in a range between a power supply voltage supplied from a power supply circuit and 0 V by switching, and can output the selected voltage as the supply analog voltage. 
   An analog grayscale voltage corresponding to the most frequently used gray level or analog grayscale voltages corresponding to a plurality of frequently used gray levels can be supplied from the first switch, thus allowing more accurate control of the operations of the D/A converter and the buffer. Therefore, low power consumption can be realized also in this situation. 
   A signal processing circuit in accordance with embodiments of the present invention may further include a counter for counting each piece of input digital data in units of predetermined ranges, and a power supply generation circuit for generating a voltage corresponding to the set data and supplying the generated voltage to the first switch, wherein the detection circuit may select a piece of or a plurality of pieces of digital data having a high input count from the digital data as a result of counting and may set the selected piece or pieces of digital data as the set data in units of the predetermined ranges. 
   Thus, the most frequently used gray level is detected during actual usage, and is set as the set data in addition to the gray levels corresponding to the power supply voltage and the ground voltage so as to supply the corresponding analog grayscale voltage from the first switch. This allows more real-time control of the operations of the D/A converter and the buffer. Low power consumption can thereby be realized. 
   In another aspect, the present invention provides a liquid crystal driving circuit using the above-described signal processing circuit as a driving voltage generation circuit for supplying an analog voltage corresponding to input digital grayscale data to each of a plurality of signal lines. 
   Because of the aforementioned advantages of the driving voltage generation circuit, the liquid crystal driving circuit of the embodiments of the present invention can achieve significantly reduced power consumption levels. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a driving voltage generation circuit according to a first embodiment of the present invention; 
       FIG. 2  is a block diagram of a driving voltage generation circuit according to a second embodiment of the present invention; 
       FIG. 3  is a block diagram of a driving voltage generation circuit according to a third embodiment of the present invention; 
       FIG. 4  is a block diagram of a source driver in a liquid crystal display device; 
       FIG. 5  is a concept diagram of the liquid crystal display device; and 
       FIG. 6  is a block diagram of a driving voltage generation circuit of the related art. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The structure of a source driver according to an embodiment of the present invention will be described with reference to the drawings. 
   First Embodiment 
     FIG. 1  is a concept diagram of a driving voltage generation circuit according to a first embodiment of the present invention. 
   A source driver according to embodiments of the present invention is similar to the source driver  200  used in the liquid crystal display device shown in  FIG. 5  having the structure shown in  FIG. 4 , whereas a driving voltage generation circuit  150  has the structure shown in  FIG. 1 . 
   The driving voltage generation circuit shown in  FIG. 1  includes at least a detection circuit  1 , a D/A converter  2 , a buffer  3 , and switches  4  and  5 . 
   The detection circuit  1  determines whether or not digital grayscale data input from a latch (not shown), e.g., the latch  103  shown in  FIG. 4 , matches internal set data that is set in advance. 
   Assuming that digital grayscale data to be converted into an analog grayscale voltage is 6-bit data, the set data is, for example, digital grayscale data indicating gray levels 63 and 0, which require higher power consumption than the remaining gray levels. 
   The D/A converter  2  converts the input digital grayscale data into an analog voltage signal based on a predetermined correspondence between digital grayscale data and analog voltages, and outputs the converted analog voltage signal. 
   This correspondence between the input digital grayscale data and analog voltage based on the D/A converter  2  is similar to a correspondence between the set data of the detection circuit  1  and the corresponding supply analog voltage to be transmitted from the output terminal of switch  4 . 
   The detection circuit  1  controls a power supply to operate the D/A converter  2 . 
   The buffer  3  amplifies the converted signal output from the D/A converter  2 , and supplies the sufficiently-power-amplified driving signal to each column line, to which a source of a transistor is connected, via the transistor to drive a liquid crystal device. 
   The detection circuit  1  also controls a power supply to the buffer  3  for power amplification. 
   When the buffer  3  amplifies converted analog voltage signals for gray levels 63 and 0, the buffer  3  must drive a plurality of internal transistors. Thus, higher power is required for gray levels 63 and 0 than the remaining gray levels. 
   The switch  4  has input terminals to which predetermined voltages, that is, a power supply voltage VD and a ground voltage GND, are input. The switching control of the switch  4  is performed by the detection circuit  1  so that either voltage is to be supplied to the switch  5  as a supply analog voltage. 
   The switch  5  receives the supply signal (supply analog voltage) output from the switch  4  and the converted signal (converted analog voltage) output from the buffer  3 . The switching control of the switch  5  is also performed by the detection circuit  1  so that either signal is to be output as an analog-grayscale-voltage driving signal to drive a liquid crystal device. 
   The operation of the liquid crystal display device in accordance with the first embodiment will be described with reference to  FIGS. 1 ,  4 , and  5 . 
   For simplicity of description, 6-bit digital grayscale data (for the grayscale range between gray level 0 and gray level 63) is employed. 
   When the digital grayscale data indicates gray level “63” (white in grayscale), represented by “1 (MSB) 11111 (LSB); 3F (hexadecimal)”, the D/A converter  2  outputs the converted signal of the power supply voltage VD as the converted analog voltage. When the digital grayscale data indicates gray level “0” (black in grayscale), represented by “000000;00”, the D/A converter  2  transmits as an output signal (i.e., outputs) the converted signal of the ground voltage GND as the converted analog voltage. 
   When digital grayscale data for any of the grayscale range of gray levels 62 through 1 is input, the D/A converter  2  transmits as an output signal, as an intermediate-gray-level converted analog voltage, the converted signal of the converted analog voltage, between the power supply voltage VD and the ground voltage GND, corresponding to the digital grayscale voltage based on a predetermined correspondence. 
   In this case, the set data in the detection circuit  1  is data for gray levels “3F” and “00”, which require higher power consumption of the D/A converter  2  and the driver  3  than the remaining gray levels. 
   When digital grayscale data is transmitted from the data register  102  to the input of latch  103 , the latch  103  holds the digital grayscale data, and supplies the digital grayscale data to the detection circuit  1  and the D/A converter  2 . 
   The detection circuit  1  determines whether or not the digital grayscale data received at the input terminal matches any of the internal set data “3F” and “00” by comparing these data. 
   When the detection circuit  1  determines that the digital grayscale data received at the input terminal does not match “3F” or “00”, the detection circuit  1  controls the switch  5  so that the converted signal from the buffer  3  is transmitted to each column line as a driving signal. 
   When the detection circuit  1  determines that the digital grayscale data received at the input terminal matches the set data “3F” or “00”, the detection circuit  1  turns off the power supply to the D/A converter  2  and the buffer  3  (or the power supply to the buffer  3 ). 
   The detection circuit  1  also controls the switch  4  so that the supply analog voltage corresponding to the digital grayscale data received at the input terminal of the detection circuit is transmitted as an output signal. 
   For example, when it is determined that the input digital grayscale data (i.e., received at the input terminal) is “3F”, the supply signal of the supply analog voltage (power supply voltage) VD is supplied from the switch  4 . 
   Accordingly, when the input digital grayscale data is “3F”, the detection circuit  1  controls the switch  4  so as to transmit a supply signal of a supply analog voltage of the power supply voltage VD; when the input digital grayscale data is “00”, the detection circuit  1  controls the switch  4  so as to transmit as output a supply signal of a supply analog voltage of the ground voltage GND. 
   The detection circuit  1  further controls the switch  5  so that any of the supply signals is supplied. 
   This switching control allows the supply signal from the switch  4  to be supplied from the switch  5  to each column line as a driving signal. 
   As described above, in the liquid crystal driving circuit according to the first embodiment, the detection circuit  1  turns off the power supply to the D/A converter  2  and the buffer  3  when the detection circuit  1  determines that the input digital grayscale data matches the set data “3F” or “00”, and further changes the output modes of the switches  4  and  5  so that the supply signal of the analog grayscale voltage corresponding to the power supply voltage VD or the ground voltage GND is transmitted as the output signal. Therefore, low power consumption can be realized. 
   The power consumption of the D/A converter  2  and the buffer  3  occupies 70% to 80% of the overall power consumption of the source driver  200 . Turning off the power supply to the D/A converter  2  and the buffer  3  contributes to significantly reduce power consumption. 
   The liquid crystal driving circuit according to the first embodiment is often used to drive liquid crystal devices of a portable device for providing character data display, etc., by switching the switches  4  and  5 . The advantage of low power consumption is useful particularly for such applications. 
   In the liquid crystal driving circuit according to the first embodiment, it is determined, for each of the column lines shown in  FIG. 4 , whether or not input digital grayscale data matches “3F” or “00”, and it is determined, for each column line, which of the converted signal (converted analog voltage) from the buffer  3  and the supply signal (supply analog voltage) is to be output as an analog-grayscale-voltage driving signal. When it is determined that the digital grayscale data indicates an intermediate gray level, the output signal from the D/A converter  2  and the buffer  3  is output as analog grayscale data. Thus, the number of gray levels is not reduced. Therefore, low power consumption can be realized without degradation in image quality. 
   Second Embodiment 
     FIG. 2  is a concept diagram of a driving voltage generation circuit according to a second embodiment of the present invention. 
   In the driving voltage generation circuit shown in  FIG. 2 , similar elements to those shown in  FIG. 1  are given the same reference numerals, and a description thereof is omitted. 
   The difference from the driving voltage generation circuit of the first embodiment is that a switch  11  further receives an intermediate voltage Vn similar to an analog grayscale voltage corresponding to, for example, a frequently used gray level, in addition to the predetermined voltages input to the switch  4  shown in  FIG. 1 , i.e., the power supply voltage VD and the ground voltage GND. 
   In accordance with the structure of the switch  11 , the digital grayscale data corresponding to the power supply voltage VD, the ground voltage GND, and the intermediate voltage Vn, represented by “3F”, “00”, and “NN (any gray level value)”, respectively, are set as the set data of a detection circuit  10 . 
   The detection circuit  10  determines whether or not the input digital grayscale data, i.e., that received at the input terminal, matches any of the set data “3F”, “00”, and “NN”. 
   When it is determined that the input digital grayscale data matches any of the set data, the detection circuit  10  controls the switch  11  so that a supply signal of a supply analog voltage having a similar voltage value to the analog grayscale voltage corresponding to the matched set data is supplied, and also controls the switch  5  so that the supply signal from the switch  11  is transmitted from the output terminal as a grayscale signal. 
   The structure and operation of the remaining components are similar to those of the driving voltage generation circuit of the first embodiment. 
   According to the second embodiment, therefore, in addition to the advantages of the first embodiment, an analog grayscale voltage corresponding to the most frequently used gray level or analog grayscale voltages corresponding to a plurality of frequently used gray levels can be supplied from the switch  11 , thus allowing more accurate control of the operations of the D/A converter  2  and the buffer  3 . Therefore, low power consumption can be realized. 
   Third Embodiment 
     FIG. 3  is a concept diagram of a driving voltage generation circuit according to a third embodiment of the present invention. 
   In the driving voltage generation circuit shown in  FIG. 3 , similar elements to those shown in  FIG. 2  are given the same reference numerals, and a description thereof is omitted. 
   The difference from the driving voltage generation circuit of the second embodiment is that the driving voltage generation circuit of the third embodiment shown in  FIG. 3  further includes a power supply generation circuit  14  for generating an intermediate voltage equivalent to an analog grayscale voltage corresponding to a frequently used gray level to be supplied to the switch  11 . 
   The driving voltage generation circuit of the third embodiment further includes a counter  13 . Each time a pixel is input, the counter  13  counts each of the gray levels to be used in the input pixel, for example, 64 gray levels that ranges from “63” to “00”, and outputs the gray level having the highest input count to a detection circuit  12  and the power supply generation circuit  14 . 
   Alternatively, the counter  13  may select a plurality of gray levels having a high input count, and may output the selected gray levels to the detection circuit  12  and the power supply generation circuit  14 . 
   The detection circuit  12  outputs control signals indicating count start and count end to the counter  13  so that the counter  13  can count input pixels per screen. 
   In response to the count-start control signal, the counter  13  starts to count a pixel. In response to the count-end control signal, the counter  13  outputs the selected gray level to the detection circuit  12  and the power supply generation circuit  14 . 
   The power supply generation circuit  14  generates a voltage Vn corresponding to the gray level (digital grayscale data) input from the counter  13 , and outputs the voltage Vn to the switch  11 . 
   The detection circuit  12  captures, as digital grayscale data for a frequently used intermediate grayscale, the gray level output from the counter  13  in response to the count-end control signal output from the detection circuit  12 , so that the captured gray level is further set as the set data in addition to the gray levels “63” and “00”. 
   When it is determined that the input digital grayscale data matches any of the set data, the detection circuit  12  controls the switch  11  so that a supply signal of a supply analog voltage having a similar voltage value to the analog grayscale voltage corresponding to the matched set data is supplied, and also controls the switch  5  so that the supply signal from the switch  11  is output as a grayscale signal. 
   The structure and operation of the remaining components are similar to those of the driving voltage generation circuit of the second embodiment. 
   According to the third embodiment, therefore, in addition to the advantages of the first and second embodiments, the most frequently used gray level is detected during actual usage, and is set as the set data in addition to the gray levels “63” and “00” so as to supply the corresponding analog grayscale voltage from the switch  11 . This allows more real-time control of the operations of the D/A converter  2  and the buffer  3 . Low power consumption can also be realized. 
   Some embodiments of the present invention have been described in detail with reference to the drawings; however, these embodiments are merely examples. The specific structure is not limited to the structure shown in such embodiments, and a variety of modifications may be made to the present invention without departing from the scope and spirit of the invention. 
   Currently, general-purpose liquid crystal driver ICs (integrated circuits) designed for portable usage allow switch-output-mode driving (for example, eight-color) or amp-output-mode driving (for example, 260,000-color) to be selected by a mode specification signal. 
   According to one of the applications of the present invention, therefore, a detection circuit outputs a mode specification signal for selecting the switch output mode if digital grayscale data input in units of scanning lines or frames has the same bit level (either all 1&#39;s or all 0&#39;s), and, otherwise, outputs a mode specification signal for selecting the amp output mode. 
   This enables power consumption control of liquid crystal devices in each driver output area, and the power consumption can therefore be reduced in a driving circuit.