Patent Abstract:
There is provided a power circuit for a display-device-driving circuit for supplying a plurality of voltages to a driving circuit for time-division-driving a display device, wherein each of the plurality of voltages are output via a constant-voltage circuit constituted by a regulator for dividing a voltage supplied from a power source and keeping divided voltages at a certain voltage level, whereby a stable power source consuming a small power to a driving circuit as a power source for a display device to be time-division-driven can be obtained.

Full Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a power circuit for a display driver of a display device, particularly to a driving circuit of a liquid-crystal device. More particularly, the present invention relates to a display device for performing to be multiplex driving, still more particularly relates to a power supplying circuit for driving a liquid-crystal device used for a liquid-crystal-driving circuit.  
         [0003]     2. Related Background Art  
         [0004]      FIGS. 8 and 9  show conventional liquid-crystal-driving-power supplying methods for multiplex driving a liquid-crystal device.  
         [0005]      FIG. 8  is an illustration showing the resistance-dividing type for driving a liquid crystal at a ⅓ bias. Symbol  101  denotes a battery serving as a power source,  102  denotes a regulator connected to the battery  101  for keeping a voltage supplied from the battery  101  constant, and  103 ,  104 , and  105  denote resistances connected in series and having equal resistance values. One end of the resistance  103  is connected to the output of the regulator  102  and one end of the resistance  105  is connected to the negative side of the battery  101  and the ground side of the regulator  102  so as to divide an output voltage of the regulator  102 . In this case, when assuming the output voltage of the regulator  102  as 4.5 V, a voltage of 3.0 V can be obtained at the connection point between the resistances  103  and  104  and a voltage of 1.5 V can be obtained at the connection point between the resistances  104  and  105 .  
         [0006]     Symbol  106  denotes an LCD driver (LCDDR) for driving an LCD, which receives a voltage from the battery  101  and an output of the regulator  102  and voltages divided by the resistances  103 ,  104 , and  105  as LCD driving voltages.  
         [0007]     Symbol  107  denotes a liquid-crystal panel connected to the LCD driver, which is turned on/off by the LCD driver  106 .  
         [0008]      FIG. 9  is an illustration showing a charge-pump type for driving a liquid crystal at a ⅓ bias, in which symbol  108  denotes a battery serving as a power source,  109  denotes a regulator connected to the battery  108  to keep a voltage supplied from the battery  108  constant, and  110  denotes a charge-pump circuit for raising a voltage kept constant by the regulator  109 . The charge-pump circuit outputs an input voltage, and the twofold-raised voltage and threefold-raised voltage of the input voltage. In this case, when assuming an output voltage of the regulator  109  as 1.5 V, 3.0 V is obtained as a twofold-raised voltage and 4.5 V is obtained as a threefold-raised voltage.  
         [0009]     Symbol  111  denotes an LCD driver (LCDDR) for driving an LCD, which receives power from the battery  101  and a liquid-crystal-driving voltage from the charge pump  110 . Symbol  112  denotes a liquid crystal panel connected to the LCD driver, which is turned on/off by the LCD driver  106 .  
         [0010]     However, since in the case of the voltage divider by using resistance shown in  FIG. 8  in the above conventional examples, divided resistances are used to generate three types of voltages serving as LCD driver voltage sources, it is necessary to supply tens of microamperes or more to the current for the resistances of the voltage divider. Therefore, a mobile unit for always displaying data by using a battery as a power source has a problem that the service life of the battery is early completed because it consumes a large power.  
         [0011]     Moreover, the charge pump type shown in  FIG. 9  requires an oscillation circuit serving as a signal source for performing pumping-up and a capacitor for accumulating electric charges. Therefore, when the number of divided voltages for driving a liquid crystal increases, it is necessary to increase the number of capacitors by the increased number of divided voltages. Therefore, when the number of divided voltages to be raised increases, a problem occurs that the voltage ratio between the lowest voltage and the highest voltage increases and higher-side voltages do not become accurate integral multiples due to a switching loss and the like. Moreover, to supply stable power by using a charge-pump system, a circuit configuration becomes complex because it is necessary to stabilize power by using voltage stabilizer such as a series regulator and then boosting the power by a charge pump.  
         [0012]     Furthermore, though some of liquid-crystal drivers respectively having a built-in a charge pump circuit are marketed, most liquid-crystal drivers do not have a charge pump in general. Therefore, because charge-pump ICs for supplying LCD driver voltages are hardly marketed, it is actually difficult to constitute an LCD driver of a small current consumption by using a consumer IC.  
       SUMMARY OF THE INVENTION  
       [0013]     To solve the above problems, the present invention makes it possible to provide a display driver of display devices consuming a small current without requiring a complex circuit such as a charge pump circuit by using a series regulator of a very small current consumption constituted by a plurality of CMOS circuits or the like and thereby obtaining the voltages for multiplex driving mode from a voltage supplied from a power source such as a battery.  
         [0014]     Moreover, when a voltage is equal to or lower than a power-source voltage, an output voltage of a series regulator is stable and it is possible to accurately raise a low voltage and a high voltage by integral multiples like in the case of a charge pump system even if the number of divided driving voltages increases.  
         [0015]     The present invention relates to a power circuit for supplying a plurality of voltages to a driving circuit for multiplex driving mode, that is, a power circuit for a display driver in which the voltages are output via a constant-voltage circuit constituted by a regulator for dividing a voltage supplied from a power source and keeping the plurality of voltages at certain voltage levels.  
         [0016]     Moreover, the present invention is characterized by a power circuit in which the above constant-voltage circuit is a step-down regulator.  
         [0017]     Furthermore, the present invention is characterized by a power circuit in which the above constant-voltage circuit has means for correcting an output voltage in accordance with a temperature.  
         [0018]     Furthermore, the present invention is characterized by a power circuit using one regulator IC in which a plurality of the above constant-voltage circuits are integrated.  
         [0019]     Furthermore, the present invention is characterized by a power circuit in which each of the above constant-voltage circuits has at least two input terminals, one input terminal of each constant-voltage circuit is connected to the positive electrode of a power source, and the other input terminal of it is connected to the output of another constant-voltage circuit when connecting the constant-voltage circuits each other.  
         [0020]     Furthermore, the present invention is characterized by a power circuit in which each of the constant-voltage circuits has at least two input terminals and includes a first constant-voltage circuit in which the above input terminals are connected to the positive and negative electrodes of the above power source and a second constant-voltage circuit in which the above input terminals are connected to the positive electrode of a power source and the output of the first constant-voltage circuit when connecting the constant-voltage circuits each other.  
         [0021]     Furthermore, the present invention is characterized by a camera having a display device provided with a driving circuit using the above power circuit, and a data-copying function for copying the display data to the liquid-crystal device to a film. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  is a block diagram of a liquid-crystal-driving circuit of the present invention.  
         [0023]      FIG. 2  is an internal circuit diagram of a regulator circuit of the present invention.  
         [0024]      FIG. 3  is an illustration for explaining liquid-crystal-driving waveforms of the present invention.  
         [0025]      FIG. 4  is a block diagram of a camera using a liquid-crystal-driving circuit of the present invention.  
         [0026]      FIG. 5  is a block diagram of a liquid-crystal-driving circuit of another embodiment of the present invention.  
         [0027]      FIG. 6  is a block diagram of a liquid-crystal-driving circuit of further embodiment of the present invention.  
         [0028]      FIG. 7  is a block diagram of a liquid-crystal-driving circuit of still further embodiment of the present invention.  
         [0029]      FIG. 8  is a block diagram of a conventional liquid-crystal-driving circuit.  
         [0030]      FIG. 9  is a block diagram of a conventional liquid-crystal-driving circuit.  
         [0031]      FIG. 10  is an illustration showing a temperature characteristic of a driving voltage.  
         [0032]      FIG. 11  is a circuit diagram of a regulator having a temperature sensor. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]     The present invention is described below in detail in accordance with the illustrated embodiments.  
         [0034]      FIG. 1  is a block diagram showing a configuration of a display driver of an embodiment of the present invention. In this case, a liquid-crystal device is used as a display device to show an example using a regulator constituted by a CMOS circuit consuming a small current. In  FIG. 1 , symbol  1  denotes a battery serving as a power source and  2  denotes a regulator constituted by a CMOS circuit whose input is connected to the positive electrode of the battery  1  and whose VSS is connected to the negative electrode of the battery  1  to keep a voltage supplied from the battery  1  constant. Symbol  3  is a regulator constituted by a CMOS circuit whose input is connected to the positive electrode of the battery  1  and whose VSS is connected to the output of the regulator  2  to keep a voltage supplied from the battery  1  constant and  4  denotes a regulator constituted by a CMOS circuit whose input is connected to the positive electrode of the battery  1  and whose VSS is connected to the output of the regulator  3  to keep a voltage supplied from the battery  1  constant.  
         [0035]     In this case, the regulators  2 ,  3 , and  4  are regulators having an equal output voltage. Because the VSS of the regulator  3  is connected to the output of the regulator  2 , the output voltage of the regulator  3  becomes two times higher than the output voltage of the regulator  2  on the basis of the VSS of the regulator  2 . Furthermore, because the VSS of the regulator  4  is connected to the output of the regulator  3 , the output voltage of the regulator  4  is set to be three times higher than the output voltage of the regulator  2  on the basis of the VSS of the regulator  2 .  
         [0036]     Concrete voltage example is as follows. When assuming the voltage of the battery  1  as 5 V and the output voltage of each regulator as 1.5 V, the output voltage of the regulator  2  shows 1.5 V, that of the regulator  3  shows 3.0 V, and that of the regulator  4  shows 4.5 V.  
         [0037]     Then, symbol  5  denotes a liquid-crystal display driver (LCDDR) using the battery  1  as the power source of a circuit and the output of the regulator  2 ,  3 , or  4  as the LCD driver voltages, in which the output of the regulator  2  is connected to VL 1 , that of the regulator  3  is connected to VL 2  and that of the regulator  4  is connected to VL 3 . Symbol  6  denotes a liquid-crystal display device (liquid-crystal display panel) connected to the liquid-crystal display driver  5 , which outputs multiplex signals to drive the liquid-crystal display panel  6  by applying common signals COM 0 , COM 1 , and COM 2  and a segment signal of an SEG 21  from an SEG 0  through a common electrode and a segment electrode.  
         [0000]     (Circuit Diagram of Regulator)  
         [0038]      FIG. 2  is an illustration for explaining details of internal circuits of the regulators  2 ,  3 , and  4 , in which symbol  101  denotes a current source connected to the power input terminal of a regulator,  102  denotes a reference-voltage circuit for generating a reference voltage in accordance with the current supplied from the current source  101 ,  103  denotes a resistance whose one end is connected to the output terminal of a regulator,  104  denotes a resistance whose one end is connected to the resistance  103  and whose other end is connected to GND, and  105  denotes an operational amplifier whose negative input is connected to the output of the reference-voltage circuit  102  and whose positive input is connected to the resistances  103  and  104 . Symbol  106  denotes an output-voltage control device serving as a P-channel MOSFET whose source is connected to the power input of a comparator, whose drain is connected to the output of a regulator, and whose gate is connected to the output of the operational amplifier  105 . An input voltage is controlled to a specified output voltage by comparing a voltage obtained by dividing the output voltage of a regulator by the resistances  103  and  104  with the output voltage of the reference-voltage circuit by the operational amplifier  105  and controlling the gate voltage of the P-channel MOSFET in accordance with the output of the operational amplifier.  
         [0039]     In this case, the regulator is constituted by a CMOS to control an output voltage by controlling the gate voltage of the P-channel MOSFET for controlling a voltage. Thereby, it is unnecessary to use the base current of a voltage control device like in the case of a bipolar transistor and it is possible to decrease the current of a power source. Therefore, it is possible to greatly reduce current consumption.  
         [0040]      FIG. 3  is an illustration showing driving voltages to be applied to common and segment electrodes when driving a liquid-crystal panel in accordance with the multiplex driving mode, showing signal waveforms applied from the liquid-crystal driver  5  to the liquid-crystal display device  6 . A ⅓ bias system and a ⅓ duty system are described below as specific examples.  
         [0041]     For each segment signal (SEG) and common signal (COM), voltages VL 1 , VL 2 , VL 3 , and 0 V generated by the regulators  2 ,  3 , and  4  are output in accordance with a display state of a liquid crystal. In this case, the signal shown by “COM 0 -SEG 1  in  FIG. 3  as an ON voltage is applied to the liquid-crystal display portion connected with COM 0  and SEG 1  and the liquid-crystal display is turned on because the effective voltage of the ON voltage exceeds the turning-on voltage of the liquid crystal. Moreover, the signal shown by “COM 1 -SEG 2 ” in  FIG. 3  is applied to the liquid-crystal display portion connected with SEG 2  and the liquid-crystal display portion is turned off because the effective voltage is lower than the turning-on voltage of the liquid crystal display.  
         [0000]     (Explanation of Camera Using Liquid-Crystal Display Device)  
         [0042]      FIG. 4  is a block diagram showing an electrical configuration of a camera to which the above LCD driver and liquid-crystal display device are applied. In  FIG. 4 , symbol  11  denotes a microcomputer for controlling the whole of the camera,  12  denotes a RAM serving as memory means set to the outside of (or built in) the microcomputer  11 , and  13  denotes an EEPROM serving as nonvolatile memory means set to the outside of (or built in) the microcomputer  11 . Symbol  14  denotes a focus detecting sensor connected to the microcomputer  11  to perform autofocusing,  15  denotes a photometric circuit connected to the microcomputer  11  to measure the brightness of an object, and  16  denotes a lens control circuit connected to the microcomputer  11  to control an electronic circuit in an interchangeable lens removable from a camera body. Symbol  17  denotes an interchangeable lens which is removable from a camera body and is connected to the lens control circuit  16  and includes an electronic circuit for controlling a lens in accordance with a control signal supplied from the lens control circuit  16 .  
         [0043]     Symbol  18  (SW 1 ) denotes a switch connected to the microcomputer  11  to start photometry and focus detection and  19  (SW 2 ) denotes a switch connected to the microcomputer  11  to start exposure. Switches SW 1  and SW 2  are release switches respectively having a two-stage configuration. The switch SW 1  is turned on in accordance with the first stroke of a release switch and switches SW 1  and SW 2  are both turned on in accordance with the second stroke of the release switch.  
         [0044]     Symbol  20  denotes an AF (auto-focus)-region selecting switch connected to the microcomputer  11  to optionally select any one of a plurality of AF regions provided for the focus detecting sensor  14 ,  21  denotes a dial detecting circuit connected to the microcomputer  11  to detect operations of dials for various settings provided for a camera, and  22  denotes a setting dial connected to the dial detecting circuit  21  to perform various settings of a camera. It is possible to select an optional automatic AF region or automatic AF-region selection in which a camera automatically selects an AF region by the AF-region selecting switch  20  and the dial.  
         [0045]     Symbol  23  denotes a film loaded in a camera body (not illustrated),  24  denotes a film detecting circuit controlled by the microcomputer  11  to detect the position of the film  23 ,  25  denotes a photosensor driven by the film detecting circuit  24  to detect the position of the film  23 , and  26  denotes a film supplying circuit controlled by the microcomputer  11  to wind or rewind the film  23  by driving a film supplying motor  27 . Symbol  28  denotes a shutter control circuit controlled by the microcomputer  11  to control a shutter for exposure, and  29  denotes a shutter controlled by the shutter control circuit  28  to perform exposure.  
         [0046]     Then, symbol  30  denotes a liquid-crystal display driver connected to the microcomputer  11  to display various information on an LCD, which corresponds to symbol  5  in  FIG. 1 . Symbol  31  denotes an LCD power circuit for supplying LCD driver voltages to the liquid-crystal display driver  30 , which corresponds to the regulator  2 ,  3 , or  4  in  FIG. 1 . Symbol  32  denotes an LCD panel connected to the liquid-crystal display driver  30  to display various setting states, operation states, or exposure information of a camera, which corresponds to the LCD  6  in  FIG. 1 . Symbol  33  denotes a copying LCD connected to the liquid-crystal display driver  30  to copy photographing date or user setting information to the film  23 , which corresponds to the LCD  6  in  FIG. 1 . Symbol  34  denotes a lamp for copying the various information displayed on the LCD  33  to a film and  35  denotes a lamp control circuit for turning on the lamp  34  and copying data.  
         [0047]     Thus, a plurality of constant voltages are generated from a voltage supplied from a battery serving as a power source by using a plurality of regulators to turn on an LCD via a liquid-crystal display driver by using the certain voltages as LCD driver voltage sources.  
         [0048]     Moreover, it is possible to obtain different liquid-crystal-driving voltages by using one type of a regulator and thereby connecting the output of one regulator with VSS of another regulator.  
         [0049]     Though the LCD by multiplex driving mode has been described by a ⅓ bias and a ⅓ duty, it is also possible to use the above mode for another bias levels by changing the number of regulators.  
         [0050]     Furthermore, it is possible to decrease the current consumption and always display data easily in the case of a unit using a battery as a power source such as a camera by using this LCD power circuit.  
       Second Embodiment  
       [0051]      FIG. 5  is a block diagram showing a second configuration of the display driver of second embodiment of the present invention. In  FIG. 5 , symbol  36  denotes a battery serving as a power source and  37  denotes a regulator whose input is connected to the positive electrode of the battery  36  and whose VSS is connected to the negative electrode of the battery  36  to keep a voltage supplied from the battery  36  constant. In this case, the regulator outputs a voltage of 1.5 V. Symbol  38  denotes a regulator whose input is connected to the positive electrode of the battery  36  and whose VSS is connected to the negative electrode of the battery  36  to keep a voltage supplied from the battery  36  constant, which outputs 3.0 V which is a voltage two times higher than the output voltage of the regulator  37 . Symbol  39  denotes a regulator whose input is connected to the positive electrode of the battery  36  and whose VSS is connected to the negative electrode of the battery  36  to keep a voltage supplied from the battery  36  constant, which outputs 4.5 V which is a voltage three times higher than the output voltage of the regulator  37 . In this case, specific output voltages are set to 1.5 V, 3.0 V, and 4.5 V. However, by changing a voltage set in accordance with an LCD driving voltage, it is possible to drive LCDs having driving voltages different from each other.  
         [0052]     Then, symbol  40  denotes a liquid-crystal display driver using the battery  36  as the power source of a circuit and outputs of the regulators  37 ,  38 , and  39  LCD driver voltage sources. The output of the regulator  37  is connected to VL 1 , that of the regulator  38  is connected to VL 2 , and that of the regulator  39  is connected to VL 3 . Symbol  41  denotes a liquid-crystal display device connected to the liquid-crystal display driver  40 , in which liquid crystal is driven in accordance with common signals COM 0 , COM 1 , and COM 2  and segment signals of SEG 0  to SEG 21 .  
         [0000]     (Regulator for Outputting a Plurality of Voltages)  
         [0053]      FIG. 6  is a block diagram showing a third configuration of the LCD driver of an embodiment of the present invention using a regulator having a plurality of output terminals.  
         [0054]     In  FIG. 6 , symbol  42  denotes a battery serving as a power source and  43  denotes a regulator whose input is connected to the positive electrode of the battery  42  and whose VSS is connected to the negative electrode of the battery  42  to keep a voltage supplied from the battery  36  as a plurality of constant voltages. In this case, the regulator  43  is constituted so as to output a plurality of voltages such as a first output of 1.5 V, second output of 3.0 V, and third output of 4.5 V.  
         [0055]     Then, symbol  44  denotes a liquid-crystal display driver using the battery  42  as the power source of a circuit and a plurality of outputs of the regulator  43  as LCD driver voltage sources, in which a first output of the regulator  43  is connected to VL 1 , second output of it is connected to VL 2 , and third output of it is connected to VL 3 . Symbol  45  denotes a liquid-crystal display device connected to the liquid-crystal display driver  44 , in which liquid crystal is driven in accordance with common signals COM 0 , COM 1 , and COM 2  and segment signals of SEG 0  to SEG 21 .  
         [0056]     It is possible to decrease the size of a power-circuit portion by the above configuration.  
         [0000]     (Description of One-Chip Configuration)  
         [0057]      FIG. 7  is a block diagram showing a fourth configuration of a LCD driver of embodiment of the present invention constituted by integrating the above regulators and LCD driver.  
         [0058]     In  FIG. 7 , symbol  46  denotes a battery serving as a power source and  47  denotes an integrated circuit including an LCD driver function for driving a regulator for generating LCD driver voltage and a liquid-crystal display device. It is allowed to constitute the integrated circuit by one chip or multichip.  
         [0059]     Symbol  48  denotes a regulator built in the integrated circuit  47  to keep a voltage supplied from a power source constant, which outputs a voltage of 1.5 V in this case. Symbol  49  denotes a regulator whose integrated-circuit input is connected to a power source and whose VSS is connected to the output of the regulator  48  to keep a voltage constant, which outputs 3.0 V two times higher than the output voltage of the regulator  48 . Symbol  50  denotes a regulator whose input is connected to a power source and whose VSS is connected to the output of the regulator  49  to keep a voltage constant, which outputs 4.5 V three times higher than the output voltage of the regulator  48 . In this case, specific output voltages are set to 1.5 V, 3.0 V, and 4.5 V. However, it is possible to drive LCDs having driving voltages different from each other by changing a voltage in accordance with the driving voltage of an LCD. Symbol  51  denotes a liquid-crystal display driver built in the integrated circuit  47  to use outputs of the regulators  48 ,  49 , and  50  as LCD driver voltage sources, in which the output of the regulator  48  is connected to VL 1 , that of the regulator  49  is connected to VL 2 , and that of the regulator  50  is connected to VL 3 . Symbol  41  denotes a liquid-crystal display device connected to the liquid-crystal display driver  40 , in which liquid crystal is driven in accordance with common signals COM 0 , COM 1 , and COM 2  and segment signals of SEG 0  to SEG 21 .  
         [0060]     Thus, a plurality of constant voltages are generated from a voltage supplied from a battery serving as a power source by using a plurality of regulators so as to turn on an LCD via a liquid-crystal display driver by using the certain voltages as LCD driver voltage sources.  
         [0061]     Moreover, a plurality of constant voltages are generated from a voltage supplied from a battery serving as a power source by using a regulator having a plurality of output terminals so as to turn on an LCD via a liquid-crystal display driver by using the constant voltages as LCD driver voltage sources.  
         [0062]     Furthermore, as shown in  FIG. 10 , because an optimum driving voltage depends on a change of environmental temperatures, it is necessary to perform temperature compensation for a driving voltage in order to perform driving so that a stable contrast can be obtained under a broad temperature environment.  
         [0063]     Therefore, as shown in  FIG. 11 , it is also possible to apply an optimum voltage to a liquid-crystal panel by providing a temperature sensor function for the inside of a regulator and thereby performing temperature compensation for a voltage output from the regulator along a driving voltage of the LCD. To realize the above mentioned, it is effective to use a method of changing power-source voltages in accordance with an output from a temperature sensor by setting the relation between temperature and output voltage in a temperature compensation circuit in the form of a reference table.  
         [0064]     Moreover, a configuration using a CMOS circuit of a small current consumption is generally used as the structure of a regulator used for the present invention. However, when a current consumption is small, it is allowed to use another type such as a bipolar type as long as it consumes a small current.  
         [0065]     As described above, a method is provided which obtains a power source for multiplex driving a display device such as a liquid-crystal display device from a voltage supplied from a power source such as a battery by using a regulator consuming a very small current constituted by a plurality of CMOS circuits. Thereby, it is possible to provide an LCD driver of a small current consumption without using a complex circuit such as a charge pump circuit.  
         [0066]     Moreover, because of a simple configuration using only a series regulator and a general-purpose liquid-crystal display driver, it is possible to always display data on a liquid-crystal display portion by a simple circuit configuration without considering the service life of a battery also in the case of a unit using a battery as a power source such as a camera. Though the present invention is described on a liquid-crystal display device, it is also effective for another display device according to a different display principle such as an organic electroluminescence device or an electrophoresis display device.

Technology Classification (CPC): 8