Abstract:
A power supply circuit includes at least one capacitor, a plurality of switching members, a power supply which outputs a plurality of voltages and a selecting section for controlling said plurality of switching sections to periodically select one of said plurality of voltages and apply the selected voltage to one terminal and the other terminal of the capacitor. The selecting section includes a member for applying the selected voltage to one terminal and the other terminal of the capacitor across a resistor, during a current limiting period immediately after the application of the selected one of said plurality of voltages to one terminal and another terminal of the capacitor is started.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-380377, filed Dec. 28, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a power supply circuit and a method of driving the same and, more particularly, to a power supply circuit which includes a charge pump circuit and generates a predetermined voltage, and a method of driving the same.  
         [0004]     2. Description of the Related Art  
         [0005]     Of various types of circuits, some circuits such as display driving circuits require a plurality of power supply voltages when they are driven. An example of a power supply circuit which supplies a plurality of power supply voltages is a power supply circuit including a charge pump circuit. This power supply circuit has one or a plurality of capacitors, and adds capacitor charge voltages to generate another voltage.  
         [0006]     This charge pump type power supply circuit used in a display driving circuit and the like is so designed that the connections between the capacitor and a plurality of power supply voltages are periodically switched. This switching of the connections between the capacitor and a plurality of voltages is controlled by on/off operation of switches.  
         [0007]      FIGS. 4A and 4B  are schematic views showing an example of a conventional charge pump type power supply circuit.  
         [0008]     As shown in  FIG. 4A , a power supply circuit  900  includes switches SW 1  to SW 4  and capacitors C 1  and C 2 . The switch SW 1  has one terminal to which a voltage VCC is applied, and the other terminal connected to a terminal C 1 M. The switch SW 2  has one terminal connected to the terminal C 1 M, and the other terminal to which a voltage VSS (GND) is applied. The switch SW 3  has one terminal to which a voltage VDC is applied, and the other terminal connected to a terminal C 1 P. The switch SW 4  has one terminal connected to the terminal C 1 P, and the other terminal connected to a terminal VOUT. The capacitor C 1  has one terminal connected to the terminal C 1 M, and the other terminal connected to the terminal C 1 P. The capacitor C 2  has one terminal connected to the terminal VOUT, and the other terminal to which the voltage VSS is applied.  
         [0009]     In the power supply circuit  900  as shown in  FIG. 4B , in a first period, the switches SW 2  and SW 3  are turned on, and the switches SW 1  and SW 4  are turned off. Since the potential of the terminal C 1 P is set at VDC and that of the terminal C 1 M is set at VSS, the capacitor C 1  is charged to the voltage VDC.  
         [0010]     Then, in a second period, the switches SW 1  and SW 4  are turned on, and the switches SW 2  and SW 3  are turned off. Accordingly, the potential of the terminal C 1 M is set at VCC, and the terminal C 1 P is connected to one terminal of the capacitor C 2 . Since the voltage VDC is held in the capacitor C 1 , the potential of the terminal C 1 P becomes (VDC+VCC), so a voltage (VDC+VCC=VGH) is applied to one terminal of the capacitor C 2  to charge it to the voltage VGH. As a result, the output terminal VOUT outputs the voltage VGH.  
         [0011]     Then, when the switches SW 1  and SW 4  are turned off and the switches SW 2  and SW 3  are turned on again in the first period, the voltage in the capacitor C 2  is held, and the output voltage from the output terminal VOUT is also maintained. By periodically charging the capacitor C 2  by repeating the above operation, the power supply circuit  900  can supply a predetermined voltage from the output terminal VOUT.  
         [0012]     In the charge pump type power supply circuit as described above, in the first and second periods, a transient current flows from the power supply to each capacitor albeit for a short time during a period immediately after each switch is turned on to apply each individual voltage to the capacitor. This transient current is a very large electric current if the line between the capacitor and power supply has a low resistance. If this transient current is generated in the charge pump type power supply circuit, latch-up occurs in transistors forming the switches or in a control circuit, thereby making the power supply circuit inoperable. Also, the large electric current flowing from the power supply circuit causes defective operation of the power supply, so an operation error of the circuit occurs.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     The present invention has the advantage that the reliability of a power supply circuit including a charge pump circuit can be improved by suppressing a transient current when the power supply circuit is in operation, thereby preventing the power supply circuit from being inoperative by latch-up or preventing a defective operation of the power supply circuit caused by an overcurrent.  
         [0014]     To obtain the above advantage, a power supply circuit according to an aspect of the present invention comprises:  
         [0015]     at least one capacitor;  
         [0016]     a plurality of switching members;  
         [0017]     a power supply which outputs a plurality of voltages; and  
         [0018]     a selecting section for controlling said plurality of switching section to periodically select one of said plurality of voltages and apply the selected voltage to one terminal and the other terminal of the capacitor,  
         [0019]     wherein the selecting section comprises a member for applying the selected voltage to one terminal and the other terminal of the capacitor across a resistor, during a current limiting period immediately after the application of the selected one of said plurality of voltages to one terminal and another terminal of the capacitor is started.  
         [0020]     The power supply circuit preferably further comprises a signal generating section for outputting a driving pulse signal which controls the plurality of switching sections, and a counting section for counting pulses of the output driving pulse signal from the signal generating section, the current limiting period is set on the basis of the count of the counting section, and the current limiting period has a time of 1 to 30 msec.  
         [0021]     The capacitor preferably comprises first and second capacitors, the plurality of voltages comprise first, second, and third voltages, and the selecting section comprises first selecting a member for alternately selecting the first and second voltages as a voltage to be applied to one terminal of the first capacitor, in accordance with the driving pulse signal, and a second selecting member for alternately selecting application of the third voltage to the other terminal of the first capacitor, and connection of one terminal of the second capacitor to the other terminal of the first capacitor, in accordance with the driving pulse signal.  
         [0022]     The first selecting member may also comprise means for inserting the resistor between one terminal of the first capacitor and the first voltage during the current limiting period, and directly connecting one terminal of the first capacitor and the first voltage after the current limiting period has passed, and the second selecting member may also comprise means for inserting the resistor between the other terminal of the first capacitor and the third voltage during the current limiting period, and directly connecting one terminal of the first capacitor and the first voltage after the current limiting period has passed.  
         [0023]     In the selecting section, after the current liming period has passed, the first selecting member may also comprise means for inserting the resistor between one terminal of the first capacitor and the first voltage, and directly connecting one terminal of the first capacitor and the first voltage, and means for inserting the resistor with respect to the other terminal of the first capacitor in the second selecting member, and directly connecting the other terminal of the first capacitor in the second switching section.  
         [0024]     To obtain the above advantage, a power supply circuit driving method according to a second aspect of the present invention, a method of driving a power supply circuit, comprising:  
         [0025]     the power supply circuit comprising a capacitor;  
         [0026]     periodically selecting one of a plurality of voltages and applying the selected voltage to one terminal and the other terminal of the capacitor;  
         [0027]     applying the selected one of said plurality of voltages to one terminal of the capacitor across a resistor, during a current liming period immediately after the application of the selected voltage to one terminal and the other terminal of the capacitor is started; and  
         [0028]     directly applying one of said plurality of voltages to one terminal and the other terminal of the capacitor after the current limiting period has passed.  
         [0029]     The above driving method preferably comprises a step of counting pulses of a driving pulse signal related to the periodic selection of the plurality of voltages, and setting the current limiting period on the basis of the count, and the current limiting period has a time of 1 to 30 msec.  
         [0030]     The capacitor comprises first and second capacitors, the plurality of voltages comprises first, second, and third voltages, and the step of periodically selecting one of the plurality of voltages and applying the selected voltage to one terminal and the other terminal of the capacitor comprises a step of periodically selecting one of the first and second voltages as a voltage to be applied to one terminal of the first capacitor, and a step of periodically selecting one of application of the third voltage to the other terminal of the first capacitor, and connection of one terminal of the second capacitor to the other terminal of the first capacitor.  
         [0031]     The step of periodically selecting one of the first and second voltages as a voltage to be applied to one terminal of the first capacitor comprises a step of inserting the resistor between one terminal of the first capacitor and the first voltage during the current limiting period, and a step of directly connecting one terminal of the first capacitor and the first voltage after the current limiting period has passed, and the step of periodically selecting one of the application of the third voltage to the other terminal of the first capacitor, and the connection of one terminal of the second capacitor to the other terminal of the first capacitor comprises a step of inserting the resistor between the other terminal of the first capacitor and the third voltage during the current limiting period, and a step of directly connecting the other terminal of the first capacitor and the third voltage after the current limiting period has passed.  
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0032]      FIGS. 1A and 1B  are views for explaining an embodiment of a power supply circuit according to the present invention, in which  FIG. 1A  shows an arrangement and  FIG. 1B  shows the states of switches;  
         [0033]      FIG. 2  is a circuit diagram showing an example of a practical arrangement of the power supply circuit according to the embodiment;  
         [0034]      FIG. 3  is a flowchart for explaining the operation of the power supply circuit according to the embodiment; and  
         [0035]      FIGS. 4A and 4B  are schematic views showing an example of a conventional power supply circuit. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]     A power supply circuit and a method of driving the power supply circuit according to the present invention will be described below on the basis of an embodiment shown in the accompanying drawing.  
         [0037]     Note that a circuit having two capacitors will be explained below as a charge pump type power supply circuit, but the present invention is not limited to this configuration. For example, the power supply circuit may also include three or more capacitors.  
         [0038]     First, an outline of the arrangement of the power supply circuit according to this embodiment will be explained.  
         [0039]      FIGS. 1A and 1B  are views for explaining the embodiment of the power supply circuit according to the present invention.  FIG. 1A  shows an arrangement, and  FIG. 1B  shows the states of switches.  
         [0040]     In the following explanation, the same reference numerals as in the power supply circuit according to the prior art denote the same parts. As shown in  FIG. 1A , a power supply circuit  100  according to this embodiment includes switches SW 1  to SW 6  and capacitors C 1  and C 2 . The switch SW 1  has one terminal to which a voltage VCC is applied, and the other terminal connected to a terminal C 1 M. The switch SW 5  has one terminal to which the voltage VCC is applied via a resistor R 1 , and the other terminal connected to the terminal C 1 M. The switch SW 2  has one terminal connected to the terminal C 1 M, and the other terminal to which a voltage VSS (GND) is applied. The switch SW 3  has one terminal to which a voltage VDC is applied, and the other terminal connected to a terminal C 1 P. The switch SW 6  has one terminal to which the voltage VDC is applied via a resistor R 2 , and the other terminal connected to the terminal C 1 P. The switch SW 4  has one terminal connected to the terminal C 1 P, and the other terminal connected to a terminal VOUT. The capacitor C 1  has one terminal connected to the terminal C 1 M, and the other terminal connected to the terminal C 1 P. The capacitor C 2  has one terminal connected to the terminal VOUT, and the other terminal to which the voltage VSS is applied. In this configuration, the switches SW 1  to SW 6  form a switching means or section according to the present invention.  
         [0041]     Next, the operation of the power supply circuit  100  will be explained. First, as shown in  FIG. 1B , in a first period during a period in which an elapsed time t from immediately after the power supply circuit  100  starts operating has not reached a preset current limiting period T 0  yet, the switches SW 2  and SW 6  are turned on, and the switches SW 1 , SW 3 , SW 4 , and SW 5  are turned off. Therefore, the voltage VDC is applied to the terminal C 1 P via the resistor R 2 , and the potential of the terminal C 1 M is set at VSS (GND). Accordingly, the potential of the terminal C 1 P becomes a potential (VDC−ΔVR 2 ) which is lower than the voltage VDC by a voltage drop ΔVR 2  across the resistor R 2 , so the capacitor C 1  is charged to the voltage (VDC−ΔVR 2 ). Then, in a second period, the switches SW 4  and SW 5  are turned on, and the switches SW 1 , SW 2 , SW 3 , and SW 6  are turned off. Therefore, the voltage VCC is applied to the terminal C 1 M via the resistor R 1 , and the potential of the terminal C 1 M becomes a potential (VCC−ΔVR 1 ) which is lower than the voltage VCC by a voltage drop ΔVR 1  across the resistor R 1 , and the terminal C 1 P is connected to one terminal of the capacitor C 2 . Since the voltage (VDC−ΔVR 2 ) is held in the capacitor C 1 , the potential of the terminal C 1 P becomes (VDC−ΔVR 2 +VCC−ΔVR 1 ). Consequently, the voltage (VDC−ΔVR 2 +VCC−ΔVR 1 ) is applied to one terminal of the capacitor C 2  to charge it to this voltage. During the period in which the elapsed time  t  has not reached the predetermined current limiting period T 0  yet, the operations in the first and second periods described above are repeated to hold the voltage of the capacitor C 2 .  
         [0042]     In a first period after the elapsed time  t  from the operation start has passed the current limiting period T 0 , as shown in  FIG. 1B , the switches SW 2 , SW 3 , and SW 6  are turned on, and the switches SW 1 , SW 4 , and SW 5  are turned off. Therefore, the voltage VDC is directly applied to the terminal C 1 P to set the potential of the terminal C 1 P at VDC, so the capacitor C 1  is charged to the voltage VDC. In a second period, the switches SW 1 , SW 4 , and SW 5  are turned on, and the switches SW 2 , SW 3 , and SW 6  are turned off, so the voltage VCC is directly applied to the terminal C 1 M to set the potential of the terminal C 1 M at VCC. Since the voltage VDC is held in the capacitor C 1 , the potential of the terminal C 1 P becomes (VDC+VCC), so a voltage (VDC+VCC=VGH) is applied to one terminal of the capacitor C 2  to charge it to the voltage VGH. The voltage in the capacitor C 2  is held by repeating the operations in the first and second periods described above, and the voltage VGH is output from the output terminal VOUT.  
         [0043]     In the power supply circuit  100  according to this embodiment as described above, after the elapsed time t from immediately after the operation start has passed the predetermined current limiting period T 0 , the operation is substantially the same as the power supply circuit  900  according to the prior art. However, during the period in which the elapsed time  t  has not reached the current limiting period T 0  yet, the voltages VCC and VDC are supplied to the individual terminals of the capacitor C 1  via the resistors R 1  and R 2 , respectively. This makes it possible to reduce the transient current flowing from the power supply of each of the voltages VCC and VDC to the capacitor C 1 . In this manner, it is possible to suppress latch-up and prevent an operation error. The current limiting period T 0  is set in accordance with, e.g., the time constants of the transient currents related to charging of the capacitors C 1  and C 2  and the upper limits of electric currents which can be supplied from the power supplies. These time constants and upper limits correspond to the resistance values of the resistors R 1  and R 2  and the capacitance values of the capacitors C 1  and C 2 . Also, when this power supply circuit is to be used as a power supply circuit of a display driving circuit, the current limiting period T 0  must be set to a time which does not interfere with the operation of the display driving circuit. In this case, the current limiting time T 0  is set to approximately 1 to 30 msec.  
         [0044]     An example of a practical configuration of the power supply circuit according to this embodiment will be described below.  
         [0045]      FIG. 2  is a circuit diagram showing an example of a practical arrangement of the power supply circuit according to this embodiment. A power supply circuit  100  includes a timing generator TG, a counter circuit  10 , inverters  11  and  12 , PMOSs  13 ,  15 , and  19 , NMOSs  14 ,  16 , and  20 , a NAND circuit  17 , an AND circuit  18 , resistors R 1  and R 2 , capacitors C 1  and C 2 , and a diode D.  
         [0046]     The timing generator TG generates and outputs a driving pulse signal CP for setting the operation periods (the first and second periods described above) of the power supply circuit  100 . The driving pulse signal CP is output to the inverters  11  and  12 , NAND circuit  17 , and counter circuit  10 . The output terminal of the inverter  11  is connected to the gate terminals of the NMOS  14  and PMOS  19 , and to one input terminal of the AND circuit  18 . An output signal from the inverter  11  is SINV 1 . The PMOS  19  has a drain terminal connected to one terminal of the resistor R 1 , and a source terminal connected to a terminal C 1 M. A voltage VCC is applied to the other terminal of the resistor R 1 . The NMOS  14  has a drain terminal connected to the terminal C 1 M, and a source terminal to which a voltage VSS is applied.  
         [0047]     The output terminal of the inverter  12  is connected to the gate terminals of the PMOS  15  and NMOS  20 . An output signal from the inverter  12  is SINV 2 . The PMOS  15  has a drain terminal connected to an output terminal VOUT, and a source terminal connected to a terminal C 1 P. The NMOS  20  has a drain terminal connected to the terminal C 1 P, and a source terminal connected to one terminal of the resistor R 2 . A voltage VDC is applied to the other terminal of the resistor R 2 . The PMOS  13 , NMOS  14 , PMOS  19 , NMOS  16 , PMOS  15 , and NMOS  20  correspond to SW 1 , SW 2 , SW 5 , SW 3 , SW 4 , and SW 6 , respectively, shown in  FIG. 1 .  
         [0048]     The counter circuit  10  counts the pulses of the driving pulse signal CP since the start of driving of the power supply circuit  100 , and outputs a low-level signal SCNT during a period in which the count is equal to or smaller than a predetermined number  n  ( n  is an integer of 1 or more). If the pulse count of the driving pulse signal CP exceeds  n , the counter circuit  10  outputs a high-level signal SCNT. The period during which the count is  n  or less corresponds to the current limiting period T 0  described above.  
         [0049]     The output signal SCNT from the counter circuit  10  is input to one terminal of the NAND circuit  17  and the other terminal of the AND circuit  18 . The output terminal of the NAND circuit  17  is connected to the gate terminal of the PMOS  13  or SW 1 . An output signal from the NAND circuit  17  is SP. The PMOS  13  has a drain terminal to which the voltage VCC is applied, and a source terminal connected to the terminal C 1 M. The output terminal of the AND circuit  18  is connected to the gate terminal of the NMOS  16  or SW 3 . The NMOS  16  has a drain terminal connected to the terminal C 1 P, and a source terminal to which the voltage VDC is applied. An output signal from the AND circuit  18  is SN.  
         [0050]     The capacitor C 1  has one terminal connected to the terminal C 1 M, and the other terminal connected to the terminal C 1 P. The capacitor C 2  has one terminal connected to the output terminal VOUT, and the other terminal to which the voltage VSS is applied. The diode D has an anode terminal to which the voltage VDC is applied, and a cathode terminal connected to the output terminal VOUT.  
         [0051]      FIG. 3  is a timing chart for explaining the operation of the power supply circuit according to this embodiment. In response to the start of driving of the power supply circuit  100 , the counter circuit  10  counts the pulses of the driving pulse signal CP. In this embodiment, the low levels (the trailing edges) of the driving pulse signal CP are counted up. During the period in which the count is  n  or less, the counter circuit  10  outputs the low-level signal SCNT. Accordingly, the signal SN changes to high level, and the signal SP changes to low level, so the PMOS  13  and NMOS  16  are turned off.  
         [0052]     At time t 1  at which the driving pulse signal CP is at low level (the first period), the signals SINV 1  and SINV 2  change to high level, so the NMOSs  14  and  20  are turned on, and the PMOSs  15  and  19  are turned off. Accordingly, the potential of the terminal C 1 P becomes a potential (VDC−ΔVR 2 ) which is lower than the voltage VDC by a voltage drop ΔVR 2  across the resistor R 2 . This voltage (VDC−ΔVR 2 ) is applied to the other terminal of the capacitor C 1 , and the electric charges are held in it.  
         [0053]     Then, when the driving pulse signal CP changes to high level (the second period) at time t 2 , the signals SINV 1  and SINV 2  change to low level, so the PMOSs  15  and  19  are turned on, and the NMOSs  14  and  20  are turned off. The potential of the terminal C 1 M becomes a potential (VCC−ΔVR 1 ) which is lower than the voltage VCC by a voltage drop ΔVR 1  across the resistor R 1 . The voltage (VCC−ΔVR 1 ) is applied to one terminal of the capacitor C 1 . Since the electric charges which are held when the voltage (VDC−ΔVR 2 ) is applied is still held in the capacitor C 1 , the potential of the terminal C 1 P becomes (VDC−ΔVR 2 )+(VCC−ΔVR 1 ). In addition, the potential of one terminal of the capacitor C 2  also becomes (VDC−ΔVR 2 )+(VCC−ΔVR 1 ), so the electric charges are held in the capacitor C 2 . Accordingly, the output terminal VOUT outputs the voltage (VDC−ΔVR 2 )+(VCC−ΔVR 1 ).  
         [0054]     The output voltage from the output terminal VOUT is maintained at (VDC−ΔVR 2 )+(VCC−ΔVR 1 ) by repeating the above operation. When the count of the counter circuit  10  reaches n+1 at time t 3 , the signal SCNT changes to high level and keeps it after that. Since the signal SN changes from low level to high level, the NMOS  16  is turned on.  
         [0055]     On the other hand, the signal SP is kept at high level, so the PMOS  13  is kept off. Also, since the driving pulse signal CP is at low level, the signals SINV 1  and SINV 2  change to high level. That is, the NMOSs  14  and  20  are turned on, and the PMOSs  15  and  19  are turned off. Although the NMOS  20  is turned on accordingly, the NMOS  16  is also turned on, so the voltage VDC is directly applied to the terminal C 1 P, instead of a voltage applied across the resistor R 2 . The voltage VDC is added to the voltage held in the capacitor C 1  to set the potential of the terminal C 1 P at {VDC+(VCC−ΔVR 1 )}.  
         [0056]     Then, when the driving pulse signal CP changes to high level at time t 4 , the signal SN changes to low level to turn off the NMOS  16 . Also, the signal SP changes to low level to turn on the PMOS  13 . Since the signals SINV 1  and SINV 2  change to low level, the PMOSs  15  and  19  are turned on, and the NMOSs  14  and  20  are turned off. Although the PMOS  19  is turned on accordingly, the PMOS  13  is also turned on, so the voltage VCC is directly applied to the terminal C 1 M, instead of a voltage applied across the resistor R 1 . Since the voltage VCC is added to the voltage held in the capacitor C 1 , the potential of the terminal C 1 P becomes (VDC+VCC), so the voltage (VDC+VCC) is applied to one terminal of the capacitor C 2 , and the electric charges are held in it. As a result, the output terminal VOUT outputs the voltage (VDC+VCC). This operation is repeated after time t 4 , and the output terminal VOUT keeps outputting the voltage (VDC+VCC).  
         [0057]     As described above, during the period in which the count of the counter circuit  10  is  n  or less, the voltages are applied to the individual terminals of the capacitors C 1  via the resistors R 1  and R 2 , so the value of the transient current can be reduced. This makes it possible to prevent latch-up and an operation error of the circuit, and improve the reliability of the power supply circuit.  
         [0058]     The power supply circuit  100  of this embodiment is an adder type circuit. However, the power supply circuit  100  may also be another form of a charge pump type power supply circuit, provided that the power supply voltages are applied to the two terminals of a capacitor across resistors during a predetermined period after the start of driving of the power supply circuit.  
         [0059]     Also, after time t 3  in the power supply circuit  100 , both the NMOSs  16  and  20  are turned on if the driving pulse signal CP is at low level, and both the PMOSs  13  and  19  are turned on if the driving pulse signal CP is at high level. However, it is also possible to turn on the NMOS  16  alone in the former case and the PMOS  13  alone in the latter case.  
         [0060]     Furthermore, in the power supply circuit  100 , the voltages VDC and VCC are applied to the terminals C 1 P and C 1 M via the resistors R 1  and R 2 , respectively. However, a resistor may also be inserted only in one of these paths. That is, it is possible to provide only the resistor R 1  in the power supply circuit  100 , and omit the resistor R 2 , NMOS  20 , and AND circuit  18 , thereby connecting the output terminal of the inverter  12  to the gate terminal of the NMOS  16 . Alternatively, it is possible to insert only the resistor R 2  in the power supply circuit  100 , and omit the resistor R 1 , PMOS  19 , and NAND circuit  17 , thereby connecting the output terminal of the inverter  11  to the gate terminal of the PMOS  13 .