Patent Publication Number: US-2005127984-A1

Title: Power supply circuit having a plurality of voltage conversion circuits

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      The entire disclosure of Japanese Application No. 2003-413708 including specification, claims, drawings and abstract is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a power supply circuit for providing as its output the output of a charge pump via a plurality of voltage conversion circuits.  
      2. Description of the Related Art  
      Voltages other than the voltage supplied from the power supply have become necessary in electrical circuits in some instances. In semiconductor integrated circuits (e.g., LSI, IC) operating on battery power, for example, a voltage higher than the battery power or a negative power supply adapted to obtain the dynamic range may become necessary. A switching regulator is employed in such a case. The switching regulator is a circuit that uses a coil to step up (or step down) the voltage, thus providing a desired voltage.  
      On the other hand, the charge pump is coming into wider use to replace the switching regulator. The charge pump is a circuit that shifts the voltage charged in a capacitor to provide a desired voltage.  
       FIG. 1  shows an example of the charge pump. In this example, two p-channel transistors Q 1  and Q 2  are connected in series between a supply voltage Vcc and the output end. One end of a capacitor C 1  is connected to the connection point between the transistors Q 1  and Q 2 . To the other end of the capacitor C 1 , the supply voltage Vcc and a ground voltage GND are alternately supplied via an inverter INV 1 . To the output end, one end of a capacitor C 2 , whose other end is connected to ground, is connected.  
      In such a circuit, with one end of the capacitor C 1  pulled to the ground voltage, the transistor Q 1  is turned on and the transistor Q 2  turned off to charge the capacitor C 1  with the Vcc voltage. Next, the transistor Q 1  is turned off and the transistor Q 2  turned on to pull the other end of the capacitor C 1  to Vcc. The one end of the capacitor C 1  becomes 2 Vcc, and this voltage is charged in the capacitor C 2 . As a result, the output end voltage becomes 2 Vcc. It is to be noted that, in  FIG. 1 , a control voltage is supplied to the transistor Q 1  and is inverted by an inverter INV 2  to be supplied to the gate of the transistor Q 2 . Diodes may be used in place of the transistors Q 1  and Q 2 . While MOSFETs are used as transistors as an example, the present invention is not limited thereto and any other types of transistors may be used.  
      Such a circuit can double the supply voltage Vcc. Besides, this circuit does not require any coil, thus offering the advantage that the circuit can be simplified.  
      The voltage obtained from the switching regulator or charge pump is regularly not supplied as is to ICs. Instead, the voltage is regulated by a series regulator to a constant voltage before being supplied. The series regulator is configured, for example, as shown in  FIG. 2 .  
      One end of a p-channel transistor Q 3  is connected to the output end of the charge pump. The other end of the p-channel transistor Q 3  is connected to ground via resistors R 1  and R 2 . The connection point between the resistors R 1  and R 2  is conected to the non-inverting input terminal of a comparator COMP 1 , whereas a reference voltage Vref 1  is fed to the inverting input terminal of the comparator COMP 1 . The output of the comparator COMP 1  is fed to the gate of the transistor Q 3 . The connection point between the transistor Q 3  and the resistor R 1  is connected to the output end of the series regulator. To this output end, one end of a capacitor C 3 , whose other end is connected to ground, is also connected.  
      The comparator COMP 1  makes the voltage at the connection point between the resistors R 1  and R 2  Vref 1 . As a result, the voltage Vref 1 ×(R 1 +R 2 )/R 1  can be stably obtained at the output end.  
      Here, the switching regulator has a large output current capacity. However, the charge pump has a smaller current capacity than the switching regulator.  FIG. 3  shows an example of the relationship between the output current (load current) and the output voltage of the charge pump. As shown in the figure, the larger the load current, the smaller the output voltage in the charge pump.  
      In the series regulator, on the other hand, the comparator operates to its full capability at startup in an attempt to raise the output voltage, thus causing a large current to flow. The large current flows through the series regulator at startup as shown by the broken line in  FIG. 4 . This current causes the output voltage to rise steeply as shown by the solid line. As a result, the predetermined voltage is obtained.  
      For this reason, if a series regulator  12  is connected to the output of a charge pump  10  as shown in  FIG. 5  and started up, the output voltage of the charge pump  10  declines considerably as shown in  FIG. 6 .  
      Therefore, if the series regulator is started up while the output of the charge pump is used in another circuit, the supply voltage to the other circuit declines considerably. As a result, the other circuit may become unable to operate properly.  
      The startup of the series regulator  12  is considered, for instance, in a circuit having two series regulators  12  and  14  connected to the output of the charge pump  10  as shown in  FIG. 7 , while the series regulator  14  is operating. In this case, the output voltage (output  1 ) of the series regulator  12  rises as the series regulator  12  starts up. At this moment, a large current flows, causing the output voltage of the charge pump  10  to decline considerably. As a result, the output voltage (output  2 ) of the series regulator  14  also declines considerably.  
      It is to be noted that the output voltage of the switching regulator does not decline much because of the high current capacity thereof as shown in  FIG. 9  even if the series regulator, connected to the succeeding stage of the switching regulator, is started up. As a result, serious problems are unlikely. The aforementioned problem has not been encountered in the conventional circuit using the switching regulator.  
     SUMMARY OF THE INVENTION  
      Thus, according to a major aspect of the present invention there is provided a power supply circuit comprising a charge pump operable to charge a capacitor to a predetermined voltage and thereafter vary the voltage at one end of the charged capacitor to produce at the other end thereof a voltage shifted in response to the variation, and a voltage conversion circuit operable to convert the output of the charge pump to a voltage based on the comparison with a predetermined reference voltage, the voltage conversion circuit outputting the converted voltage as the supply voltage of other circuit, wherein the operating current of the voltage conversion circuit is restricted so that the output voltage of the charge pump is prevented from varying over a predetermined value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  illustrates a configuration example of a charge pump according to the background technology;  
       FIG. 2  illustrates a configuration example of a series regulator according to the background technology;  
       FIG. 3  illustrates the relationship between the load current and the output voltage of the charge pump according to the background technology;  
       FIG. 4  illustrates the startup current and the output voltage of the series regulator according to the background technology;  
       FIG. 5  illustrates a system configuration consisting of the charge pump and the series regulator according to the background technology;  
       FIG. 6  illustrates the output voltage of the charge pump at startup of the series regulator according to the background technology;  
       FIG. 7  illustrates a system configuration consisting of the charge pump and a plurality of the series regulators according to the background technology;  
       FIG. 8  illustrates the output voltage of the charge pump at startup of the series regulator in the configuration of  FIG. 7 ;  
       FIG. 9  illustrates the output voltage of a switching regulator at startup of the series regulator according to the background technology;  
       FIG. 10  illustrates a system configuration according to an embodiment;  
       FIG. 11  illustrates the output voltage of the charge pump at startup of the series regulator;  
       FIG. 12  illustrates a system configuration consisting of the charge pump and a plurality of the series regulators according to the embodiment;  
       FIG. 13  illustrates a system configuration consisting of the charge pump having a plurality of outputs and the series regulator according to the embodiment;  
       FIG. 14  illustrates the plurality of the output voltages of the charge pump at startup of the series regulator;  
       FIG. 15  illustrates the output voltages of the charge pump and the series regulator under an overload condition;  
       FIG. 16  illustrates a configuration according to the embodiment adapted to control the operation of a voltage monitoring circuit with a switch;  
       FIG. 17  illustrates a configuration according to the embodiment adapted to control the operation of the voltage monitoring circuit with a timer;  
       FIG. 18  illustrates a configuration of the series regulator according to the embodiment; and  
       FIG. 19  illustrates a circuit configuration according to the embodiment adapted to control the output of the voltage monitoring circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
      An embodiment of the present invention will now be described with reference to the accompanying drawings.  
       FIG. 10  illustrates a configuration according to the embodiment. The series regulator  12  is connected to the output of the charge pump  10 . A voltage monitoring circuit  20  is provided to monitor the output of the charge pump  10 . This voltage monitoring circuit  20  controls the operation of the series regulator  12 . That is, the voltage monitoring circuit  20  controls the operation of the series regulator  12  to restrict the amount of current flowing through the series regulator  12  in the event of a decline in the output voltage of the charge pump more than a predetermined value. Therefore, the output voltage of the charge pump  10  does not vary significantly.  
      As described above, if the output voltage of the charge pump  10  declines below a set voltage as a result of increased amount of current flowing from the charge pump  10  into the series regulator  12 , the voltage monitoring circuit  20  restricts the current flowing into the series regulator  12  from the charge pump  10 , thus preventing the decline in the output voltage of the charge pump  10 . This also suppresses variations in the output voltage of the charge pump at the startup of the series regulator  12  as shown in  FIG. 11 . Consequently, the output voltage of the charge pump can be used as a stable voltage supply. This makes it possible to prevent adverse effect on another circuit operating on the output voltage of the charge pump  10 .  
       FIG. 12  is another configuration example. In this example, the series regulator  14  and a voltage monitoring circuit  22  are provided in addition to the series regulator  12  and the voltage monitoring circuit  20 . The voltage monitoring circuit  20  controls the current in the series regulator  12 , whereas the voltage monitoring circuit  22  controls the current in the series regulator  14 . This makes it possible to stabilize the output voltages of the charge pump  10  and the series regulators  12  and  14 .  
       FIG. 13  shows the charge pump  10  with a plurality of outputs. That is, when the charge pump  10  has a multi-stage configuration, the voltage can be stepped up (or down) in succession. As a result, a plurality of outputs can be retrieved. In this case, the plurality of outputs are monitored to produce the output voltages in a stable manner. That is, outputs  1  and  2  of the charge pump  10  are monitored respectively by the voltage monitoring circuits  20  and  22 . The current in the series regulator  12  is restricted in response to the results of the monitoring. In particular, when one of the series regulators starts up, the current in that series regulator is restricted. This suppresses variations in the output voltage of the other series regulator.  
      Consequently, a significant voltage drop can be prevented at both of the outputs (outputs  1  and  2 ) of the charge pump  10  as shown in  FIG. 14 .  
      As described in the above configuration examples, if the output voltage of the charge pump is monitored, the performance of the series regulator is determined by the performance of the charge pump. This also provides the advantage of eliminating the need for an output current limiting circuit that would normally be required.  
      While in the above example the output voltage of the charge pump  10  is monitored by the voltage monitoring circuits  20  and  22 , the halfway voltage of the charge pump  10  can also be monitored. The outputs of the series regulators  12  and  14 , while in operation, may be monitored as well. The reason for this is that since these voltages are relevant to and change in the same manner as the output voltage of the charge pump  10 , monitoring any of these voltages translates into monitoring of the output voltage of the charge pump  10 .  
      Here, when the monitoring is carried out, the amounts of current in the series regulators  12  and  14  are always restricted in the event of a decline in the output voltage of the charge pump  10 . Therefore, if the load of the series regulator  12  or  14  changes even momentarily, the performance of the series regulator  12  is restricted. Then, the output voltages of the series regulators  12  and  14  significantly decline as shown in  FIG. 15  even if the series regulators  12  and  14  have leeway in the load driving capacity. In particular, this operation occurs even in the event of a mustache-like short-period load current pulse and renders the output voltages of the series regulator  12  and  14  unstable.  
      For this reason, a switch  30  is provided as shown in  FIG. 16  to restrict the output of the voltage monitoring circuit  20 . The switch  30  is turned on only at startup so that the monitoring operation is carried out only at the startup of the series regulators  12  and  14 . The switching prevents the current restriction in the series regulators  12  and  14  when a mustache-like short-period load current pulse appears in the output voltage of the charge pump  10 . As a result, the output voltages of the series regulators  12  and  14  can be stabilized.  
      It is to be noted that the comparator COMP 1  of the series regulator  12  is configured with an op-amp. The comparator COMP 1  normally keeps the built-in constant current source off remaining inoperative. Here, it suffices to use a startup signal, adapted to turn on the constant current source, to turn on the switch  30  and then turn off the switch  30  in a predetermined period of time.  
      To proceed with the monitoring only at startup, a timer  32  may be provided as shown in  FIG. 17  to allow the operation of the voltage monitoring circuit  20  only for a predetermined period of time from startup. The timer  32  receives a startup signal indicating the startup of the series regulator to turn on the switch  30  only for a predetermined period of time from when the startup signal is input.  
      Here, other methods may be used in place of the timer  32 . Such methods include stopping the monitoring by the voltage monitoring circuit  20  when the output voltage thereof reaches a certain voltage based on the judgment that the series regulator  12  has started up and halting the voltage monitoring circuit in a predetermined period of time after the output voltage of the series regulator  12  reaches a certain voltage based on the judgment that the series regulator has started up.  
      Description will be given next of the voltage monitoring circuit  20  and a specific example of a circuit operable to restrict the current in the series regulator  12  by the voltage monitoring circuit  20  with reference to  FIG. 18 .  
      The transistor Q 3  and a series circuit of the resistors R 1  and R 2  are inserted between the output of the charge pump and ground. The reference voltage Vref 1  is fed to the inverting input terminal of the comparator COMP 1 . The connection point between the resistors R 1  and R 2  is connected to the non-inverting input terminal. The output end of the comparator COMP 1  is connected to the gate of the transistor Q 3 . The connection point between the transistor Q 3  and the resistor R 1  is connected to the output end. To the output end, one end of the capacitor C 3 , whose other end is connected to ground, is also connected. The series regulator  12  is thus configured.  
      The output of the charge pump  10  is connected to ground via resistors R 3  and R 4 . The connection point between the resistors R 3  and R 4  is connected to the inverting input terminal of a comparator COMP 2 . A reference voltage Vref 2  is fed to the non-inverting input terminal of the comparator COMP 2 . Therefore, the comparator COMP 2  compares the voltage obtained by dividing the output voltage of the charge pump  10  by the resistors R 1  and R 2  and the reference voltage Vref 2 . It is to be noted that if the output end voltage is fed back to the inverting input terminal of the op-amp, the output voltage becomes a reference voltage Vref. The output of the comparator COMP 2  is fed to the gate of an n-channel transistor Q 4 . The n-channel transistor Q 4  is connected to the output of the charge pump  10  at one end and to the gate of the transistor Q 3  at the other end. The voltage monitoring circuit  20  is thus configured.  
      If the output voltage of the charge pump  10  falls below the predetermined value (Vref  2 ), the output of the comparator COMP 2  goes H. Then, the transistor Q 4  turns on. A current is supplied from the transistor Q 4  to the gate of the transistor Q 3 . Consequently, the amount of drain current in the transistor Q 3  diminishes. As a result, the amount of current is restricted in the series regulator  12 .  
      It is to be noted that since the comparator COMP 1  produces a current output in this example, a current signal is also used to restrict the operating current of the series regulator  12 . However, if the operating current of the series regulator  12  can be substantially restricted, a voltage signal may also be employed. Alternatively, a resistor maybe inserted, for example, to restrict the current.  
      Alternatively, it may be chosen not to operate the voltage monitoring circuit  20  if the output voltage of the charge pump  10  drops due to a pulse current shorter than a predetermined period of time as described above. The circuit intended for this purpose is shown in  FIG. 19 .  
      The output of the COMP 2  is also fed to the data input terminal of a flip-flop  72 . The output terminal of this flip-flop  72  is connected to the data input terminal of a flip-flop  74 . The output terminal of the flip-flop  74  is connected to the data input terminal of a flip-flop  76 . The output terminal of the flip-flop  76  is connected to the data input terminal of a flip-flop  78 . A predetermined clock signal is commonly fed to the clock input terminals of the flip-flops  72  to  78 . Therefore, the flip-flop  72  is set to H by the first rising edge of the clock signal after the rising edge of the output of the comparator COMP 2 . Then, the flip-flops  74  to  78  are set to H in succession. The flip-flops  72  to  78  are all set to H during four clocks of the clock signal.  
      The data output terminals of the flip-flops  72  to  78  are input to a four-input AND gate  80 . The on/off operation of a switch SW 2 , provided between the output end of the comparator COMP 2  and the gate of the transistor Q 4 , is controlled by the output of this AND gate  80 . The switch SW 2  is turned on when the output of the AND gate  80  goes H.  
      In this configuration, the switch SW 2  turns on to restrict the current in the series regulator  12  only if the output of the comparator COMP 2  remains H until four clocks elapse after the output of the COMP 2 , i.e., the output of the voltage monitoring circuit  20 , goes H.  
      This allows the series regulator  12  to remain operational even in the presence of a short, mustache-like voltage drop in the output of the charge pump  10 .  
      In particular, it is preferred that the circuit in  FIG. 19  be provided separately from the circuit in  FIG. 17  intended for the startup so that only the circuit in  FIG. 17  is operated at startup and that the circuit in  FIG. 19  is put into operation afterwards. In this case, the time of the timer (flip-flops  72  to  78 ) is preferably shorter than that of the timer  32  in  FIG. 17 . As for the circuit in  FIG. 19 , on the other hand, it is preferred that the output be disabled because the occurrence of a large current longer than a predetermined period of time may lead to a short circuit.  
      While the illustrative and presently preferred embodiment of the present invention has been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.