Abstract:
A low noise multiphase charge pump comprises a plurality of capacitors and a plurality of switches configured as a network, the switches are so switched that the charge pump operates in at least three phases by turns, and the operational durations and the operational currents of the phases are preferably balanced, so as to reduce the noise of the charge pump.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is related generally to switching power supplies, and more particularly, to a charge pump switching power supply with smaller current and voltage ripples. 
       BACKGROUND OF THE INVENTION 
       [0002]    Current electronic circuits often need more than one direct-current (DC) power supply, and therefore various systems are designed for power conversion, for example charge pumps. A conventional charge pump is switched between a charging phase and a discharging phase so as to operate in two phases, i.e., the charging phase and the discharging phase by turns. For example, in a 1.5x mode charge pump as shown in  FIG. 1 , two capacitors C 1  and C 2  have a same capacitance C, and the difference between the maximum voltage and the minimum voltage on the capacitors C 1  and C 2  is ΔV 1 . This charge pump operates in a charging phase when switches  10 ,  16  and  22  turn on and the others turn off, in which a charging path  11  is established to charge the serially coupled capacitors C 1  and C 2 . Let the charging time to be T 1 , therefore the charging current Iin 1  is C×ΔV 1 /T 1 . Contrarily, the charge pump operates in a discharging phase when the switches  12 ,  14 ,  18  and  20  turn on and the others turn off, in which a discharging path  21  is established to discharge the parallel coupled capacitors C 1  and C 2 . Let the discharging time to be T 2 , therefore the discharging current Iin 2  is 2×C×ΔV 1 /T 2 . The input current Iin is equal to (Iin 1 ×T 1 +Iin 2 ×T 2 )/(T 1 +T 2 ), and the output current lout will be 2×C×ΔV 1 /(T 1 +T 2 ).  FIG. 2  shows a relationship between the input current Iin and time. If T 1 =T 2 , the charging current Iin 1  will be equal to the output current lout, and the discharging current Iin 2  is two times of the output current lout. In the charging phase T 1 , Iin=Iin 1 =Iout; while in the discharging phase T 2 , Iin=Iin 2 = 2 Iout. Hence, this charge pump has huge current ripple. Since the voltage ripple is proportional to the current ripple, the charge pump also has huge voltage ripple, which causes high noise. To improve this problem, U.S. Pat. No. 6,504,422 to Rader et al. proposes a charge pump which is switched by a switching circuit to charge a capacitor and discharge another in two phases, in order to reduce the voltage ripple. However, it still cannot improve the current ripple. 
         [0003]    Therefore, it is desired a charge pump and a control method thereof which can improve the current ripple and the voltage ripple to reduce noise. 
       SUMMARY OF THE INVENTION 
       [0004]    An object of the present invention is to provide a control method to reduce the current ripple and the voltage ripple of a charge pump. 
         [0005]    Another object of the present invention is to provide a low noise multiphase charge pump. 
         [0006]    In a charge pump having a plurality of capacitors and a plurality of switches configured as a network, according to the present invention, the plurality of switches are so switched that the charge pump operates in at least three phases by turns, and preferably, the operational durations of the phases are balanced with each other, and/or the operational currents of the phases are balanced with each other. Therefore, the current ripple and the voltage ripple of the charge pump are reduced, and further, the noise is also reduced. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0007]    These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
           [0008]      FIG. 1  shows a conventional 1.5x mode charge pump; 
           [0009]      FIG. 2  shows a relationship between the input current of the charge pump shown in  FIG. 1  and time; 
           [0010]      FIG. 3  shows an embodiment according to the prevent invention; 
           [0011]      FIG. 4  shows a relationship between the input current of the charge pump shown in  FIG. 3  and time; 
           [0012]      FIG. 5  shows two simulated current-to-time diagrams of the three phase charge pump and the two phase charge pump; and 
           [0013]      FIG. 6  shows two simulated voltage-to-time diagrams of the three-phase charge pump and the two-phase charge pump. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]      FIG. 3  shows a 1.5x mode multiphase charge pump according to the present invention, in which two capacitors C 1  and C 2  have a same capacitance C, and the difference between the maximum voltage and the minimum voltage on the capacitors C 1  and C 2  is ΔV 2 . When switches  30 ,  36  and  42  turn on and the others turn off, a charging path  31  is established and the charge pump operates in a charging phase, by which the capacitors C 1  and C 2  are coupled in series and charged. Let the charging time to be T 3 , therefore the charging current Iin 3  will be C×ΔV 2 /T 3 . When switches  32  and  34  turn on and the others turn off, a discharging path  41  is established and the charge pump operates in a first discharging phase, by which the capacitor C 1  is discharged. Let the discharging time of this phase to be T 4 , therefore the discharging current Iin 4  of this phase will be C×ΔV 2 /T 4 . Then, the charge pump is switched to another discharging phase by turning on the switches  38  and  40  and turning off the others, in which a discharging path  51  is established and the capacitor C 2  is discharged. Let the second discharging time is T 5 , therefore the second discharging current Iin 5  will be C×ΔV 2 /T 5 . After the second discharging phase, the charge pump is switched to the charging phase again. The input current Iin=(Iin 3 ×T 3 +Iin 4 ×T 4 +Iin 5 ×T 5 )/(T 3 +T 4 +T 5 ), and the output current Iout=2×C×ΔV 2 /(T 1 +T 2 +T 3 ).  FIG. 4  shows a relationship between the input current Iin and time. If T 3 =T 4 =T 5 , the operational currents Iin 3 , Iin 4  and Iin  5  are all 1.5 times of the output current lout. Since the operational currents Iin 3 , Iin 4  and Iin  5  are all the same, i.e., the operational currents of the charge pump in the all phases are balanced with each other, the ripple in the input current Iin is reduced. By comparing the waveforms of  FIGS. 2 and 4 , it can be seen that the ripple in the input current Iin of  FIG. 4  is much smaller than that of  FIG. 2 , so the voltage ripple is also reduced, and the noise is reduced accordingly. 
         [0015]      FIG. 5  shows two simulated current-to-time diagrams of the three-phase charge pump according to the present invention and the conventional two-phase charge pump of  FIG. 1 . S 1  is the operational current of the three phase charge pump according to the present invention, and has a current ripple of 130 mA. S 2  is the operational current of the conventional two phase charge pump of  FIG. 1 , and has a current ripple of 210 mA. It is thus shown that the current ripple of the three-phase charge pump according to the present invention is much smaller than that of the conventional two-phase charge pump of  FIG. 1 .  FIG. 6  shows two simulated voltage-to-time diagrams of the three-phase charge pump according to the present invention and the conventional two-phase charge pump of  FIG. 1 . S 3  is the output voltage of the three-phase charge pump according to the present invention, and has an output voltage ripple of 14 mV. S 4  is the output voltage of the conventional two phase charge pump of  FIG. 1 , and has an output voltage ripple of 20 mV. It is thus shown that the output voltage ripple is improved because of the reduction of the current ripple. 
         [0016]    Since the charge pump of the present invention suffers smaller current ripple, it can use smaller switches. Additionally, comparing with the charge pump of U.S. Pat. No. 6,504,422, the circuit of the present invention is simpler and easier to implement, and requires less switches, thus the resistance of the serially coupled switches is reduced, and hence less energy is consumed, thereby reducing the cost. 
         [0017]    In other embodiments, the numbers of the capacitors and the switches can be changed, depending on the specific applications, and the number of the operational phases can be also changed, depending on the requirement of the applications, preferably under balanced operational durations and balanced operational currents between the operational phases of the charge pump. 
         [0018]    While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.