Abstract:
A dual mode charge pump is operable in a first mode or a second mode for providing positive and negative output voltages that can be stabilized by adjusting the charging time of two terminals of a flying capacitor or by adjusting the charging/discharging time of the positive and negative voltage output terminals. The dual mode charge pump can apply to a much wider input supply voltage range with less numbers of power switches, thus requiring less die area and lower costs. Moreover, the dual mode charge pump can precisely define a common mode voltage, thus making the common voltage drift smaller and less load dependent, especially when the output supply voltages are under different load conditions.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is related generally to a dual mode charge pump and, more particularly, to a dual mode charge pump for providing positive and negative supply voltages. 
       BACKGROUND OF THE INVENTION 
       [0002]    Class H amplifiers are known to improve the efficiency of conventional Class AB amplifiers. Traditional Class G and Class H amplifiers require multiple power supplies for providing positive and negative supply voltages. Recently, combining a voltage generation method to generate a set of positive and negative supply voltages with selectable modes (to generate different bipolar supply voltages) was found to be an efficient and cost effective way to implement Class G or Class H amplifiers. 
         [0003]    A power generation method with a dual mode charge pump is first proposed by U.S. Pat. Application Publication No. 2008/0044041, which uses two flying capacitors and eight controlled switches. Later, U.S. Pat. No. 7,626,445 proposes an improved dual mode charge pump which uses only one flying capacitor and six switches to generate bipolar supply voltages, thereby reducing the die area and costs. However, the generated bipolar supply voltages are all related to the input voltage VDD of the dual mode charge pump, as being equal to either ±VDD or ±cVDD, where c is a fractional ratio number. For example, in U.S. Pat. Application Publication No. 2008/0044041, referring to  FIG. 1 , when the positive supply voltage Out+ is 1.6V, the negative supply voltage Out− must be −1.6V, and the input voltage Vbatt can only be 1.6V or a fractional times, for example ⅔, of 1.6V, and additional capacitors are needed to implement the fractional times of 1.6V. The input voltage Vbatt is fully limited by the output supply voltages Out+/Out−, thus reducing flexibility of system applications. 
         [0004]    Moreover, in the known dual mode charge pump as disclosed by U.S. Pat. Application Publication No. 2008/0044041 and shown in  FIG. 1 , the common mode voltage Vcm is loosely controlled by the switching actions, resulting in larger common voltage drift, especially when the load currents drawn from the supply voltage output terminals Out+ and Out− are not balanced. 
       SUMMARY OF THE INVENTION 
       [0005]    An objective of the present invention is to provide a dual mode charge pump circuit for generating positive and negative output voltages that are independent of its input voltage. 
         [0006]    According to the present invention, a dual mode charge pump is operable in a first mode or a second mode for providing positive and negative output voltages that can be stabilized by adjusting the charging time of two terminals of a flying capacitor or by adjusting the charging/discharging time of the positive and negative voltage output terminals. The dual mode charge pump can apply to a much wider input supply voltage range with less numbers of power switches, thus requiring less die area and lower costs. Moreover, the dual mode charge pump can precisely define a common mode voltage, thus making the common voltage drift smaller and less load dependent, especially when the output supply voltages are under different load (currents) conditions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    These and other objectives, 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  is a circuit diagram of a conventional dual mode charge pump; 
           [0009]      FIG. 2  is a block diagram of an amplifier system using a dual mode charge pump according to the present invention; 
           [0010]      FIG. 3  is a circuit diagram of an embodiment for the dual mode charge pump shown in  FIG. 2 ; 
           [0011]      FIG. 4  is a timing diagram of the switch control signals of the dual mode charge pump shown in  FIG. 3  in a first mode; 
           [0012]      FIG. 5  is a circuit diagram of the switch capacitor network shown in  FIG. 3  in a first phase of a first mode; 
           [0013]      FIG. 6  shows an equivalent circuit of the switch capacitor network shown in  FIG. 5 ; 
           [0014]      FIG. 7  is a circuit diagram of the switch capacitor network shown in  FIG. 3  in a second phase of a first mode; 
           [0015]      FIG. 8  shows an equivalent circuit of the switch capacitor network shown in  FIG. 7 ; 
           [0016]      FIG. 9  is a timing diagram of the switch control signals of the dual mode charge pump shown in  FIG. 3  in a second mode; 
           [0017]      FIG. 10  is a circuit diagram of the switch capacitor network shown in  FIG. 3  in a first phase of a second mode; 
           [0018]      FIG. 11  shows an equivalent circuit of the switch capacitor network shown in  FIG. 10 ; 
           [0019]      FIG. 12  is a circuit diagram of the switch capacitor network shown in  FIG. 3  in a second phase of a second mode; 
           [0020]      FIG. 13  shows an equivalent circuit of the switch capacitor network shown in  FIG. 12 ; 
           [0021]      FIG. 14  is a circuit diagram of the switch capacitor network shown in  FIG. 3  in a third phase of a second mode; 
           [0022]      FIG. 15  shows an equivalent circuit of the switch capacitor network shown in  FIG. 14 ; and 
           [0023]      FIG. 16  is a circuit diagram of an embodiment for the switch control circuit shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    As shown in  FIG. 2 , an amplifier system has a Class G or Class H amplifier as its audio output device  10  for amplifying an input signal audio_in to generate an output signal audio_out for a load  14 , a dual mode charge pump  12  operating in a first mode or a second mode responsive to a mode select signal to provide multiple sets of supply voltages HPVDD and HPVSS to power input terminals of the audio output device  10 , and a load amplitude sense circuit  16  for sensing the amplitude of the output signal audio_out to identify the condition of the load  14  and determining the mode select signal according to the condition of the load  14  to switch the dual mode charge pump  12  between the first and second modes. 
         [0025]      FIG. 3  is a circuit diagram of an embodiment for the dual mode charge pump  12  shown in  FIG. 2 , which includes a flying capacitor Cfly, a switch control circuit  22  and a switch capacitor network  24 . In the switch capacitor network  24 , a switch S 1  is connected between a power input terminal VIN and a first terminal  18  of the flying capacitor Cfly, a switch S 2  is connected between a ground terminal GND and a second terminal  20  of the flying capacitor Cfly, a switch S 3  is connected between the first terminal  18  of the flying capacitor Cfly and the voltage output terminal HPVDD of the dual mode charge pump  12 , a switch S 4  is connected between the first terminal  18  of the flying capacitor Cfly and the ground terminal GND, a switch S 5  is connected between the second terminal  20  of the flying capacitor Cfly and the voltage output terminal HPVSS of the dual mode charge pump  12 , a switch S 6  is connected between the second terminal  20  of the flying capacitor Cfly and the voltage output terminal HPVDD of the dual mode charge pump  12 , and bulk capacitors C 1  and C 2  are connected to the voltage output terminals HPVDD and HPVSS respectively. Control signals CS 1 -CS 6  for the switches S 1 -S 6  are provided by the switch control circuit  22 . 
         [0026]      FIG. 4  is a timing diagram of the control signals CS 1 -CS 5  shown in  FIG. 3  in a first mode of the dual mode charge pump  12 . Referring to  FIGS. 3 and 4 , the switch S 6  remains off in the first mode, and the first mode includes a first phase and a second phase. In the first phase of the first mode, as shown from time t 1  to time t 2 , the switches S 1 , S 2  and S 3  are on, and the switches S 4  and S 5  are off, as shown in  FIG. 5 , whose equivalent circuit is shown in  FIG. 6 . In this time period, the input voltage VIN charges the flying capacitor Cfly and the bulk capacitor C 1 , and the switch control circuit  22  detects the voltage V 1  at the first terminal  18  of the flying capacitor Cfly, and compares the same with a reference voltage VREF to control the on-time Ton of the switch S 1 , thereby making the voltage across the flying capacitor Cfly and the bulk capacitor C 1  equal to the reference voltage VREF, which may be any fractional times of the input voltage VIN, c×VIN of. Thus, in the first phase of the first mode, the voltage at the voltage output terminal HPVDD is equal to VREF. In the second phase of the first mode, as shown from time t 3  to time t 4 , the switches S 1 , S 2  and S 3  are off and the switches S 4  and S 5  are on, as shown in  FIG. 7 , whose equivalent circuit is shown in  FIG. 8 . In this time period, the flying capacitor Cfly discharges the voltage output terminal HPVSS to make the voltage at the voltage output terminal HPVSS equal to −VREF. In this embodiment, the absolute values of the voltages HPVDD and HPVSS are both equal to the voltage VREF between the first and second terminals  18  and  20  of the flying capacitor Cfly. However, in other embodiments, the switch control circuit  22  may make the absolute values of the voltages HPVDD and HPVSS smaller than the voltage VREF between the first and second terminals  18  and  20  of the flying capacitor Cfly by adjusting the pulse widths of the control signals CS 3  and CS 5 . 
         [0027]      FIG. 9  is a timing diagram of the control signals CS 1 -CS 6  shown in  FIG. 3  in the second mode of the dual mode charge pump  12 . Referring to  FIGS. 3 and 9 , the second mode includes a first phase, a second phase and a third phase. In the first phase of the second mode, as shown from time t 1  to time t 2 , the switches S 1  and S 6  are on and the other switches are off, as shown in  FIG. 10 , whose equivalent circuit is shown in  FIG. 11 . In this time period, the input voltage VIN charges the flying capacitor Cfly and the bulk capacitor C 1  that are connected in series between the power input terminal VIN and the voltage output terminal HPVDD, and the switch control circuit  22  detects the voltage V 1  at the first terminal  18  of the flying capacitor Cfly, and compares the same with the reference voltage VREF to control the on-time Ton of the switch S 1 , thereby making the voltage between the first terminal  18  of the flying capacitor Cfly and the ground terminal GND equal to VREF. At this time, the flying capacitor Cfly and the bulk capacitor C 1  equally share the reference voltage VREF, meaning that the voltages across the flying capacitor Cfly and across the bulk capacitor C 1  are both VREF/2. In the second phase of the second mode, as shown from time t 3  to time t 4 , the switches S 4  and S 5  are on and the other switches are off, as shown in  FIG. 12 , whose equivalent circuit is shown in  FIG. 13 , in which the flying capacitor Cfly and the bulk capacitor C 2  are connected in parallel between the power input terminal VIN and the voltage output terminal HPVSS, and the first terminal  18  of the flying capacitor Cfly is grounded, so that the charges of the flying capacitor Cfly are transferred to the bulk capacitor C 2 , making the voltage at the voltage output terminal HPVSS equal to −VREF/2. In the third phase of the second mode, as shown from time t 5  to time t 6 , the switches S 2  and S 3  are on and the other switches are off, as shown in  FIG. 14 , whose equivalent circuit is shown in  FIG. 15 , in which the flying capacitor Cfly and the bulk capacitor C 1  are connected in parallel, and the second terminal  20  of the flying capacitor Cfly is grounded, so that the charges of the flying capacitor Cfly and of the bulk capacitor C 1  are redistributed, making the voltage at the voltage output terminal HPVDD equal to VREF/2. In this embodiment, the absolute values of the output voltages HPVDD and HPVSS are both equal to VREF/2, one half of the voltage VREF between the first and second terminals  18  and  20  of the flying capacitor Cfly. However, in other embodiments, the switch control circuit  22  may make the absolute values of the voltages HPVDD and HPVSS smaller than one half of the voltage VREF between the first and second terminals  18  and  20  of the flying capacitor Cfly by adjusting the pulse widths of the control signals CS 3  and CS 5 . 
         [0028]    As compared with conventional dual mode charge pumps, the input voltage VIN of the dual mode charge pump  12  shown in  FIG. 3  is not limited by the positive and negative output voltages HPVDD and HPVSS, and the dual mode charge pump  12  allows the positive and negative output voltages HPVDD and HPVSS to be adjusted by adjusting the pulse widths of the control signals CS 1 -CS 6 , thus increasing flexibility of system applications. 
         [0029]      FIG. 16  is a circuit diagram of an embodiment for the switch control circuit  22  shown in  FIG. 3 , which includes resistors R 1  and R 2  connected in series between the reference voltage terminal VREF and the voltage output terminal HPVSS, resistors R 3  and R 4  connected in series between the voltage output terminal HPVDD and the ground terminal GND for dividing the voltage HPVDD to generate a voltage Vd, a comparator  26  for comparing the voltage Vd with the half reference voltage VREF/2 to generate a comparison signal Sc 1 , a comparator  28  for comparing the voltage HPVSS with the ground voltage to generate a comparison signal Sc 2 , and a pulse width control circuit  30  for generating the control signals CS 1 -CS 6  according to the comparison signals Sc 1  and Sc 2 . The switch control circuit  22  functions to determine the timing and the pulse widths of the control signals CS 1 -CS 6  and thereby control the absolute value of the voltage at the first terminal  18  of the flying capacitor Cfly. 
         [0030]    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.