Abstract:
A liquid crystal display control device, in accordance with the present invention, for supplying a voltage signal to drive a liquid crystal display unit comprises a boosting circuit, and a segment signal circuit and/or a common signal circuit. The boosting circuit includes a dc power supply first capacitor connected between a plurality of first group switches and to be connected in parallel to said dc power supply when the first group switches are operated, a plurality of second group switches, second boosting capacitor connected between the other polarity of the dc power supply and one end of the other switch of the second group switches. One of the second group switches is connected between one polarity of the power supply and one end of the first capacitor. A third electronic switching means is connected to the boosting circuit in parallel to the second capacitor for discharging the voltage charged in the second capacitor. 
     Therefore, a voltage signal having a predetermined amplitude and polarity for driving the LCD display unit is charged in the second capacitor when the first and second switches are selectively operated and it is discharged when a power interruption occurs.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a control device for a liquid crystal display (LCD) device, in particular, to an LCD device which is capable of preventing erroneous display that tends to be generated at the time of disconnecting the power supply. 
     2. Description of the Prior Art 
     A tendency exists in recent semiconductor devices to attempt to reduce the power consumption by stopping the feeding of power to the circuits that are not in operation. 
     For instance, among LCD devices for displaying desired content on the display unit, which receives the voltage necessary for driving the LCD from a boosting circuit that uses a capacitor and supplies the voltage to the LCD to be driven through operation of a plurality of switches, there are some that disconnect the power supply when there is no need to have a continued display of the content, inorder to reduce the power consumption. 
     However, in such an LCD device, discharge of a charged capacitor requires a certain length of time and the switches behave unstably due to temporary uncontrollability of the switches. Because of this, an LCD in the nonlighting condition is converted tot he lighting condition, for example, due to the residual voltage in the capacitor. Therefore, there arises an inconvenience in which there is temporarily displayed on the display unit a content which is different from what had been displayed before the power supply was disconnected. It means that there will be a problem, in particular when a display device or the like is constructed by using LCD. This is because when the power supply for the LCD is interrupted frequently, the above inconvenience will arise for each time the power supply is disconnected, giving displeasure to the use of the device. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide on LCD control device which is capable of stably and quickly erasing and displaying the display content at the time of discontinuation of the power supply to the LCD device. 
     Another object of the present invention is to provide an LCD control device which is capable of preventing erroneous display at the time of discontinuation of the power supply to the LCD device. 
     Still another object of the present invention is to provide an LCD control device which is capable of quickly erasing the display content without displaying a content which is different from the content that has been displayed on the LCD panel until the time of interruption of the power supply to the LCD device. 
     An LCD control device in accordance with the present invention is for supplying to an LCD a voltage necessary for driving the LCD unit, and comprises a boosting circuit, and a segment signal circuit and/or common signal circuit. The boosting circuit comprises a dc power supply, a first capacitor (C 1 ), which is connected between a dc power supply and a first group of switches (SW 1  and SW 3 ), that realizes a parallel connection with the dc power supply through operation of the first group of switches, a second group of switches (SW 2  and SW 4 ), and a second capacitor (C 2 ) which is connected between the other end of the dc power supply and one of the switch (SW 4 ) of the second group of switches. The other (SW 2 ) of the second group of switches is connected between one end of the dc power supply and the first capacitor. In additions, in the boosting circuit there is connected a third switch (SW 5 ) in parallel with the second capacitor for discharging the charges that are accumulated in the second capacitor. 
     These and other objects, features and advantages of the present invention will be more apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram which shows the boosting circuit of an LCD control device embodying the present invention; 
     FIG. 2 illustrates the time charts of the operation of the boosting circuit shown in FIG. 1; 
     FIG. 3 is a circuit diagram of the segment signal circuit for switching the boosting voltage that is output from the boosting circuit of the LCD control device embodying the present invention; 
     FIG. 4 is a circuit diagram of the common signal circuit for switching the boosting voltage that is output from the boosting circuit of the LCD control device embodying the present invention; 
     FIG. 5 is a diagram for explaining the operation of the segment signal circuit and the common signal circuit shown in FIGS. 3 and 4, respectively; 
     FIG. 6 is a circuit diagram which shows the boosting circuit for a second embodiment of the LCD control device in accordance with the present invention; 
     FIG. 7 is a circuit diagram which shows the boosting circuit for a third embodiment of the LCD control device in accordance with the present invention; 
     FIG. 8 is a circuit diagram which shows the boosting circuit for a fourth embodiment of the LCD control device in accordance with the present invention; 
     FIG. 9 is a circuit diagram which shows the segment and common signal circuits for a fifth embodiment of the LCD control device in accordance with the present invention; 
     FIG. 10 is a circuit diagram which shows the segment and common signal circuits for a sixth embodiment of the LCD control device in accordance with the present invention; and 
     FIG. 11 is a circuit diagram which shows the segment and common signal circuits for a seventh embodiment of the LCD control device in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, the boosting circuit for the LCD control device embodying the present invention is shown with reference numeral 1. The boosting circuit 1 is a device for supplying a voltage which is necessary for driving the LCD that forms a display unit, and outputs a voltage of -5V with negative polarity with respect to the electromotive force of a voltage source 3 by the use of the voltage source 3 with electromotive force +5V, for example. 
     The positive electrode of the voltage source 3 is connected to the V DD  terminal and the negative terminal is connected to the V SS1  terminal, and a switch SW 1  and a switch SW 2  that are connected in series are connected to the voltage source 3 in parallel. To the junction (called &#34;point A&#34; hereafter) of the switch SW 1  and the switch SW 2  is connected one end of a capacitor C 1 , and to the other end (called &#34;point B&#34; hereafter) of the capacitor C 1  is connected to one end of a switch SW 3  whose other end is connected tot he V SS1  terminal as well as one end of a switch SW 4  whose other end is connected to the V SS2  terminal. Further, a boosting capacitor C 2  is connected between the V DD  terminal and the V SS2  terminal, and a switch SW 5  for short-circuiting both ends of the boosting circuit C 2  is connected in parallel with the boosting capacitor C 2 , in order to discharge the voltage accumulated in the boosting capacitor C 2 . 
     Next, the operation of the boosting circuit 1 shown in FIG. 1 will be described by referring to FIG. 2. In FIG. 2, first at time t 1  the switch SW 1  and the switch SW 3  are in conducting state while the switch SW 2 , the switch SW 4 , and the switch SW 5  for short-circuiting are in nonconducting state. At the same time, one end, point A, of the capacitor C 1  is connected to the V DD  terminal via the switch SW 1  while the other end, point B, of the capacitor C 1  is connected to the V SS1  terminal via the switch SW 3 . In such a state, the capacitor C 1  is charged by the voltage of +5V with the point A side as positive and the point B side as negative. Further, the voltage level of the V SS2  terminal becomes indeterminate because of the nonconducting state of the switch SW 4  and the switch SW 5 . Next, at time t 2 , the switch SW 1  and the SW 3  change from the conducting state to the nonconducting state, the switch SW 2  and the switch SW 4  change from the nonconducting state to the conducting state, and one end, point A, of the capacitor C 1  is connected to the V SS1  terminal via the switch SW 2  while the other end, point B, of the capacitor C 1  is connected to the V.sub. SS2 terminal via the switch SW 4 . Therefore, in such a state, the voltage of the point A changes from +5V to 0V so that the voltage of the point B is pressed down from 0V to -5V, and a voltage of -5V is output at the V SS2  terminal. Then, a voltage of +5V is impressed to one end which is connected on the V DD  terminal side of the boosting capacitor C 2 , and a voltage of -5V is impressed on the other end which is connected on the V SS2  terminal side. Therefore, a voltage of 10V is charged on the boosting capacitor C 2 , with its one end positive and the other end negative. 
     Next, at time t 3 , the switch SW 1  and the switch SW 3  change from the nonconducting state to the conducting state while the switch SW 2  and the switch SW 4  change from the conducting state to the nonconducting state, which are the same conditions as at time t 1 . Here again, a voltage of +5V is charged with the point A side of the capacitor C 1  positive and its point B side negative. Then, although the switch SW 4  is in the nonconducting state in this condition, the voltage that was charged on the boosting capacitor C 2  at time t 2 , as represented by the dotted line in FIG. 2, is held dynamically as is so called, such that the V SS2  terminal will be held at approximately -5V. After time t 4 , the situations at times t 2  and t 3  are repeated, and a voltage of -5V is output at the V SS2  terminal, making it possible to obtain a voltage of -5V of negative polarity with respect to the voltage source 3 with an electromotive force +5V. 
     FIG. 3 illustrates the construction of the segment signal circuit that gives to the LCD segment voltage necessary for driving the LCD, by switching the voltage that is output from the boosting circuit. FIG. 4 illustrates the construction of the common signal circuit that gives to the LCD common voltage necessary for driving the LCD, by switching the voltage that is output from the boosting circuit 1. FIG. 5 is a diagram for illustrating the operation of the segment signal circuit and the common signal circuit shown in FIGS. 3 and 4, respectively. In FIG. 3, the segment signal circuit is constructed by switch SW 6  through switch SW 11 . To one end (called &#34;point C&#34; hereafter) of the switch SW 6  whose the other end is connected to the V DD  terminal of the boosting circuit 1, there are connected one end of the switch SW 7  whose the other end is connected to the V SS1  terminal of the boosting circuit, as well as one end of the swithc SW 10  whose the other end is connected to the segment terminal. Moreover, to one end (called &#34;point D&#34; hereafter) of the switch SW 9  whose the other end is connected to the V SS2  terminal of the boosting circuit 1, there are connected one end of the switch SW 8  whose the other end is connected to the V SS1  terminal, as well as one end of the switch SW 11  whose the other end is connected to the segment terminal. By appropriately closing and opening the switches SW 6  through SW 11  that are connected in the above manner, each of the output voltages +5V, 0V, and -5V of the boosting circuit 1 is arranged to be output from the segment terminal. 
     In FIG. 4, the common signal circuit is constructed by switches SW 12  through SW 17 . To one end (called &#34;point E&#34; hereafter) of the switch SW 12  whose the other end is connected to the V DD  terminal of the boosting circuit 1, there are connected one end of the switch SW 13  whose the other end is connected to the V SS1  terminal of the boosting circuit 1, as well as one end of the switch SW 16  whose the other end is connected to the common terminal. Further, to one end (called &#34;point F&#34; hereafter) of the switch SW 15  whose the other end is connected to the V SS2  terminal of the boosting circuit 1, there are connected one end of the switch SW 14  whose the other end is connected to the V SS1  terminal of the boosting circuit 1, as well as one end of the switch SW 17  whose the other end is connected to the common terminal. Through appropriate closing and opening of the switches SW 12  through SW 17  that are connected as in the above, there can be output from the common terminal each of the output voltages +5V, 0V, and -5V of the boosting circuit 1. 
     Next, referring to FIG. 5, the operation of the segment signal circuit shown in FIG. 3 and of the common signal circuit shown in FIG. 4 will be described. 
     The opening and closing at each of the times t 1  through t 7  of the switches SW 6  through SW 11  of the segment signal circuit and the switches SW 12  through SW 17  of the common signal circuit are controlled, for example, as shown by the figure, and the voltage that is output from the common terminal is varied with fixed cycle, for example, as +5V→0V→-5V→0V→+5V. By varying the segment terminal voltage in response to the common terminal voltage through charge of the voltage between the common and the segment terminals, lighting and nonlighting of the LCD can be accomplished. 
     For example, at time t 1 , the switches SW 12 , SW 15 , and SW 16  of the common signal circuit are in the conducting state while the switches SW 13 , SW 14 , and SW 17  are in the nonconducting state, so that the common terminal is connected to the V DD  terminal via the switches SW 12  and SW 16  and a voltage of +5V is output on the common terminal. On the other hand, the switches SW 6 , SW 9 , and SW 11  of the segment signal circuit are in the conducting state while the switches SW 7 , SW 8 , and SW 10  are in the nonconducting state, so that the segment terminal is connected to the V SS2  terminal via the switches SW 9  and SW 11  and a voltage of -5V is output on the segment terminal. Consequently, the voltage between the segment and the common terminals becomes 10V, which is supplied (to the LCD to light up the LCD. Next, at time t 2 ,) for example, the switches SW 12  and SW 15  of the common signal circuit are changed from the conducting state to the nonconducting state while the switches SW 13  and SW 14  of the some circuit are changed from the nonconducting state to the conducting state, so that the common terminal is connected to the V SS1  terminal via the switches SW 13  and SW 16  and the common terminal voltage becomes 0V. On the other hand, the switches SW 6  and SW 9  of the segment signal circuit are changed from the conducting state to the nonconducting state while the switches SW 7  and SW 8  of the same circuit are changed from the nonconducting state to the conducting state, so that the segment terminal is connected to the V SS1  terminal and the segment terminal voltage become 0V. Consequently, the voltage between the segment and the common terminals becomes 0V and the LCD will find itself in the nonlighting condition. 
     Analogous situations taking place for time to and thereafter, desired display can be accomplished by realizing the lighting and nonlighting conditions for the LCD according to the following manner. Namely, the LCD is brought to a lighting condition by generating a voltage of 10V between the segment and the common terminals through control of the opening and closing of each of the switches SW 6  through SW 17  of the segment signal circuit and the common signal circuit. Similarly, the LCD may be brought to a nonlighting condition by adjusting to have a voltage of 0V impressed between the segment and the common terminals through control of the opening and closing of each of the switches SW 6  through SW 17 . 
     When the power supply of an LCD control device constructed as above is disconnected, the switches SW 6  through SW 17  may become uncontrollable temporarily and behave unstably. However, even under such a condition, by changing the switch SW 5  from the nonconducting state to the conducting state by means of a control signal, such as a power supply shut-off signal or a display erasure instruction signal both ends of the boosting capacitor C 2  that is connected in parallel with the switch SW 5  can be short-circuited and the charge that was accumulated on the boosting capacitor C 2  will be discharged. Therefore, between the segment terminal and the common terminal there will not be output a residual voltage, such as the voltage of 10V which is necessary for lighting up the LCD. Therefore, when the power supply is disconnected the content which has been displayed can be erased without, for example, switching of the nonlighting condition of the LCD to the lighting condition with the temporary display of a content which is different from the content that has been displayed on the display unit until that time. 
     FIG. 6 shows the boosting circuit of the LCD control device for a second embodiment of the invention. A special feature of the circuit is to connect a MOS type P-channel transistor in parallel with the boosting capacitor C 2  of the boosting circuit 1 shown in FIG. 1. It discharges the charges that were accumulated on the boosting capacitor C 2  by short-circuiting both ends of the boosting capacitor C 2  through conversion of the P-channel transistor from the nonconducting condition to the conducting condition by the use of the same control signal that is used for controlling the switch SW 5 . Therefore, by constructing the circuit as in the above it becomes possible to obtain effects that are similar to those of the first embodiment. In the above, the component with the same symbol as in FIG. 1 signifies the same item, and its description has been omitted. 
     FIG. 7 shows the boosting circuit of the LCD control device for a third embodiment of the present invention. In contrast to the boosting circuit 1 shown in FIG. 1 which outputs a boosted voltage of negative polarity with respect to the voltage source 3, the boosting circuit 1&#39; shown in FIG. 7 outputs a boosted voltage of positive polarity with respect to the voltage source 3. The boosting circuit 1&#39; is constituted by a switch SW 3  connected between the V DD1  terminal and one end (called &#34;point B&#39;&#34; herafter) of the capacitor C 1  which is connected to one end of the switch SW 4 , a boosting capacitor C 2  &#39; which is connected between the other end of the switch SW 4  that is connected to the V DD2  terminal side and the V SS  terminal, and a switch SW 5  &#39; which is connected in parallel with the boosting capacitor C 2  &#39;. The components with the same symbols as in FIG. 1 represent the same items as in FIG. 1, and the explanation on them is omitted. 
     In a boosting circuit 1&#39; of the above construction, first, the switch SW 2  and the switch SW 3  &#39; are in the conducting state, the switch SW 1  and the switch SW 4  are in the nonconducting state, and the capacitor C 1  is charged to a voltage of +5V with negative charge on the point A side and positive charge on the point B&#39; side. Next, the switch SW 1  and the switch SW 4  are changed from the nonconducting state to the conducting state, and the switch SW 2  and the switch SW 3  &#39; are changed from the conducting state to the nonconducting state. By raising the voltage at point A from 0V to +5V, the voltage at point B&#39; is raised from +5V to +10V, which changes the boosting capacitor C 2  &#39; to a voltage of +10V and the boosted voltage of +10V is output at the V DD2  terminal. Then, when the power supply to the LCD device is disconnected the switch SW 5  &#39; of the boosting circuit 1&#39;, analogous to the switch SW 5  of the boosting circuit 1, changes from the nonconducting state to the conducting state, and the charges accumulated on the boosting capacitor C 2  &#39; are discharged by the short-circuiting of both ends of the boosting capacitor C 2  &#39;. 
     Therefore, also in the case of driving the LCD by the use of the boosting circuit 1&#39; that outputs a boosting voltage of positive polarity with respect to the voltage source 3, it becomes possible when the power supply to the LCD device is disconnected to obtain similar effects as in the first embodiment, through connection of the switch SW 5  &#39; in parallel with the boosting capacitor C 2  &#39; of the boosting circuit 1&#39;. 
     FIG. 8 shows the boosting circuit of the LCD control device relating to a fourth embodiment of the invention. A special feature of the device is to connect a MOS type N-channel transistor 11 in parallel with the boosting capacitor C2&#39; of the boosting circuit 1&#39; shown in FIG. 7. When the power supply to the LCD device is disconnected, the N-channel transistor 11 is changed from the nonconducting state to the conducting state by means of the same controlling signal that is used for controlling the switch SW 5  &#39;, to discharge the charges accumulated on the boosting capacitor C 2  &#39; by short-circuiting both ends of the boosting capacitor C 2  &#39;. By constructing the device as in the above it becomes possible to obtain the same effects as in the first embodiment. In the above, the components with the same symbols as in FIG. 7 represent the same items explanation of which has been omitted. 
     FIG. 9 shows the LCD control device relating to a fifth embodiment of the present invention. A special feature of the device consists in connecting a switch SW 18  between the segment terminal of the segment signal circuit shown in FIG. 3 and the common terminal of the common signal circuit shown in FIG. 4. When the power supply to the LCD device is disconnected, the voltage between the segment and the common terminals is made to be less than the voltage for realizing display by liquid crystal, by connecting the segment terminal and the common terminal through change of the switch SW 18  to the conducting state. With this construction, it becomes possible to erase the content that had been displayed, without displaying a content which is different from what has been displayed on the LCD control device. 
     FIG. 10 shows the LCD control device relating to a sixth embodiment of the present invention. A special feature of the device is that there is connected a MOS type P-channel transistor 13 between the segment terminal and the common terminal as means of short-circuiting the segment terminal and the common terminal at the time of disconnection of the power supply to the LCD device. When the power supply is disconnected, the voltage between the segment and the common terminals is arranged to be reduced to a value which is less than the voltage required for realizing a display by liquid crystal, by changing the P-channel transistor from the nonconducting state to the conducting state. With such a construction, effects that are similar to the fifth embodiment will become possible to be obtained. 
     FIG. 11 shows the LCD control device relating to a seventh embodiment of the present invention. A special feature of the device is that there is connected a MOS type N-channel transistor 15 between the segment terminal and the common terminal as means of short-circuiting the segment and the common terminals at the time of disconnection of the power supply to the LCD device. When the power supply is disconnected, the voltage between the segment and the common terminals is arranged to be reduced to a value which is less than the voltage required for realizing a display by liquid crystal, by changing the N-channel transistor from the nonconducting state to the conducting state. With such a construction, effects that are similar to the fifth embodiment will become possible to be obtained. 
     It should be noted that although the boosting circuit described in the first and the third embodiments is one that outputs a boosted voltage which is twice as large the voltage of the voltage source, it is of course possible according to the present invention to obtain similar effects by the use of an LCD device which uses a boosting circuit that outputs a boosting voltage that is 2+N (N≧1) times that of the power supply. 
     In summary, according to the present invention, it is arranged, when disconnecting the power supply, to discharge quickly the charges that were accumulated on the capacitor for obtaining a voltage that is necessary to drive and display liquid crystal, by carrying out charging and discharging through control of feeding. Therefore, it is possible to provide an LCD control device which is capable, at the time of disconnection of the power supply, of quickly erasing the displayed content, without displaying a content which is different from what has been displayed on the liquid display panel.