Patent Publication Number: US-9847713-B2

Title: Charge pump-based split-rail supply generation

Description:
FIELD 
     An embodiment of the invention generally relates to electronic circuit techniques for generating positive and negative dc power supply voltages (or a dual power supply), from a first input dc voltage, using a charge pump circuit, and more particularly to a power supply whose dual outputs are shifted to be above and below, respectively, the midpoint of a second input voltage. Other embodiments are also described. 
     BACKGROUND 
     A split rail power supply (also referred to here as a dual, dc power supply) is needed to supply power to a signal controller for a Class D audio power amplifier, e.g., +5 Vdc, −5 Vdc. The controller produces the audio signals that are needed to drive the power stage transistors (of the power amplifier). The audio signals are typically centered about ground (zero Volts), but available power sources in many applications typically only provide a single, positive voltage relative to ground, e.g., a battery, or the output of an AC-DC wall power adapter. A power conversion circuit may be used in such cases, referred to as a split rail power supply, that converts the single, positive input voltage of the power source into positive and negative dc output voltages, or top and bottom rail voltages. The top and bottom rail voltages are produced relative to an output ground node that is at a different potential than the ground node of the power source. 
     SUMMARY 
     In some cases, the power supply voltage provided by a power source to a load, such as an audio power amplifier, sags and then recovers during operation, due to strong changes in the load that is on the power source. It is desirable in that case to maintain symmetrical headroom for the load that is being powered by the power source. For instance, this headroom may be defined in part by the available output signal swing of a signal controller that is driving the audio power amplifier, where the latter is being powered directly by the power source. To explain, consider the example of a power source whose output is 40 Vdc nominal, relative to its ground, and a split rail power supply (that is supplying power to the signal controller) whose output ground is nominally at the middle of the power source voltage, e.g., 20 Vdc. Let the output rails of the split rail supply be at 25 Vdc and 15 Vdc, just as an example. If the power source sags from 40 Vdc to 35 Vdc, then in a typical solution in which the output rails of the split rail supply remain regulated at 25 Vdc and 15 Vdc, respectively, the headroom available to the audio power amplifier that is being powered by the power source becomes asymmetrical. That is, the separation between the upper output rail, which remains at 25 Vdc, and the power source, is now 10 Vdc, which is smaller (in this case by 5 Volts) than the separation between the lower output rail, which remains at 15 Vdc, and the input ground. 
     A solution to the asymmetrical headroom problem is to design a split rail power supply circuit, that may be powered by a first input voltage, so that its output ground node follows or tracks a second input voltage (which in one case is the power source voltage for the audio power amplifier or other load), as the second input voltage sags and recovers. Thus, taking the example above, a +/−5 Vdc dual output from such an improved power supply will maintain a 10 Vdc separation between its output rails as these “float” down and up responsive to the second input voltage sagging and recovering, respectively. In other words, the output rail voltages stay at +5 Vdc above and −5 Vdc below the output ground, while the output ground changes by following the sagging and recovering second input voltage. Viewed another way, the split rail power supply output voltages may remain fixed relative to each other, or their separation remains fixed, but they “float” in accordance with the second input voltage. In one embodiment, these split rail power supply output voltages float above and below, respectively, the midpoint of the second input voltage (the power source voltage of the audio power amplifier), so that in the event of clipping by the power amplifier, the clipping is symmetrical. 
     Such a solution should be more power efficient than using merely low dropout regulators (LDOs), and it should also be less bulky than a magnetic or transformer-based power converter, in cases where the output power of the power supply is, for example, less than 3 Watts. This may enable the solution to fit within the stricter or smaller thermal envelope and physical internal volume of a portable consumer electronics device, such as a smartphone, a tablet computer, or a laptop computer. 
     An embodiment of the invention is a charge-pump based, split rail power supply circuit that can float or shift, along a second input voltage, while converting a first input voltage into a positive voltage at its top output rail and a negative voltage on its bottom output rail, relative to a new ground node (referred to here as the output ground.) In one embodiment, the voltage of the output ground moves with (stays at) the midpoint of the second input voltage, while the top and bottom rail voltages maintain a fixed separation between each other, even while the second input voltage changes during operation (e.g., the second input voltage sags and recovers due to a changing load on the second input voltage). This may be achieved using a bias voltage source that is directly connected to an intermediate node of a charge pump circuit whose outputs are the top and bottom output rails; the intermediate node is thus kept at a voltage that is between the top and bottom rail voltages of the charge pump circuit, and preferably at the midpoint of the second input voltage, even as the latter changes during operation. This helps maintain symmetrical headroom not just for a signal controller that could be powered by the top and bottom output rails but also for an audio power amplifier that could be powered directly by the second input voltage. 
     Another embodiment of the invention is a charge-pump based, split rail power supply circuit that produces balanced positive and negative output rail voltages, despite manufacturing variations that typically appear in the capacitances of the charge pump capacitors. For example, the charge pump capacitors may be designed to be equal in capacitance, but in practice exhibit a manufacturing process variation or tolerance, such as +/−10% in their capacitance. 
     Example circuit schematics of such power supply circuits are described below and shown in the drawings. 
     The above summary does not include an exhaustive list of all aspects of the present invention. It is contemplated that the invention includes all systems and methods that can be practiced from all suitable combinations of the various aspects summarized above, as well as those disclosed in the Detailed Description below and particularly pointed out in the claims filed with the application. Such combinations have particular advantages not specifically recited in the above summary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of the invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment of the invention in this disclosure are not necessarily to the same embodiment, and they mean at least one. Also, in the interest of conciseness and reducing the total number of figures, a given figure may be used to illustrate the features of more than one embodiment of the invention, and not all elements in the figure may be required for a given embodiment. 
         FIG. 1  is a combined block diagram and circuit schematic of a charge pump-based power supply circuit and an audio power amplifier. 
         FIG. 2  is a graph of several voltage waveforms that may be exhibited by the embodiment of  FIG. 1  during operation. 
         FIG. 3  is a circuit schematic of a charge pump circuit and a bias voltage source. 
         FIG. 4  depicts some example digital control signals with selected phases, to operate the switches of the switch network depicted in  FIG. 3 . 
         FIG. 5  shows a configuration of the circuit of  FIG. 3  when the phi switches are closed and the phi′ switches are open. 
         FIG. 6  shows the configuration of the circuit of  FIG. 3  when the phi switches are open and the phi′ switches are closed. 
         FIG. 7  is a circuit schematic of another charge pump circuit that may generate more balanced output rail voltages than the embodiment of  FIG. 3 . 
         FIG. 8  depicts some digital control signals having selected phases, to operate the switches of the switch network of the charge pump circuit in  FIG. 7 . 
         FIG. 9  shows a first phase of a charge pump cycle in which the charge pump capacitors are being charged in series. 
         FIG. 10  shows a second phase of the charge pump cycle, in which the capacitors are being discharged in series. 
         FIG. 11  shows a third phase of the charge pump cycle in which the capacitors are being charged in series while reversed in order as compared to  FIG. 9 . 
         FIG. 12  shows a fourth phase of the charge pump cycle in which the capacitors are being discharged in series while reversed in order as compared to  FIG. 10 . 
         FIG. 13  is a circuit schematic of an example voltage bias source. 
     
    
    
     DETAILED DESCRIPTION 
     Several embodiments are described with reference to the appended drawings. While numerous details are set forth, it is understood that some embodiments of the invention may be practiced without these details. In other instances, well-known circuits, structures, and techniques have not been shown in detail so as not to obscure the understanding of this description. 
       FIG. 1  illustrates a charge pump-based power supply circuit in accordance with an embodiment of the invention. A charge pump circuit  4  is composed of a switch network  3  that serves to couple first and second input nodes  1 ,  2  to respective nodes of first and second charge pump capacitors C 1 , C 2 . The first and second input nodes  1 ,  2  are also shown as being coupled to a first power source  8 , which produces a dc voltage Vin 1 . The first power source  8  may be a rechargeable battery, or a power converter that produces a regulated dc voltage (such as an ac to dc power adapter). The switch network  3  is also coupled to top and bottom output nodes  11 ,  9  that are respective nodes of third and fourth filter capacitors C 3 , C 4 . An intermediate node  10  is defined between the top and bottom output nodes  11 ,  9 , where the intermediate node is shared by the capacitors C 3 , C 4 . In other words, capacitors C 3 , C 4  are series coupled with each other at or through the intermediate node  10 . To explain further, the “near plate” of C 3  is directly connected (e.g., coupled by an electrical short) to the “far plate” of C 4 , while the “far plate” of C 3  is directly connected to the top output node  11  and the “near plate” C 4  is directly connected to the bottom output node  9 . Said differently, the “outer plates” of C 3 , C 4  are directly connected to the top and bottom output nodes  11 ,  9 , respectively, while the “inner plates” of C 3 , C 4  are coupled by an electrical short (at or through the intermediate node  10 .) 
     The switch network  3  serves to couple the first and second capacitors C 1 , C 2  to the third and fourth capacitors C 3 , C 4  in accordance with charge pump action, so that it produces a voltage at the top output node  11  that is higher than a voltage of the intermediate node  10 , and a voltage at the bottom output node  9  is lower than the voltage of the intermediate node  10 , and where all such voltages are deemed to be dc voltages that are produced by the charge pump circuit drawing power from the first power source  8  (input voltage Vin 1 ). To do so, the constituent switches (e.g., transistor switches) of the switch network  3  may be controlled by switch control logic circuitry  5  which produces a number of pulsed, digital control signals (e.g., periodic pulse waves, such as phi, phi′ in  FIG. 4 , or phi 1 , phi 2  . . . in  FIG. 8 ) whose phases are selected to repeatedly charge and discharge C 1 , C 2  in such a manner as to produce the desired positive and negative voltages at the top and bottom output nodes  11 ,  9 , respectively (via a charge pump action). 
     In one embodiment, the capacitances of C 1 , C 2  are designed to be equal, while the capacitances of C 3 , C 4  may also be designed to be equal, so that the voltage of the top output node  11  is higher than the voltage of the intermediate node  10  by the same amount as the voltage of the bottom output node  9  is lower than the voltage of the intermediate node  10 . This is also referred to as a balanced, dual rail power supply, for example having +5 Vdc on node  11  relative to the intermediate node  10 , and −5 Vdc on node  9  also relative to the intermediate node  10 . These numbers of course are just examples only; more generally the charge pump circuit may be designed to produce other balanced pairs of power supply output voltages. 
     Still referring to  FIG. 1 , the charge pump-based power supply circuit also has a bias voltage source  12  having an input (a pair of nodes) that is coupled to a second power source  6 . The latter has an output voltage Vin 2 , and supplies power to a load, here an audio power amplifier A 2 . In one embodiment, the second power source  6  is a power converter having voltage Vin 2  that is greater than Vin 1 . The bias voltage source  12  has an output that is to produce and maintain an output voltage Vbias regardless of its load (hence the usage of the term bias “voltage source”  12 .) The output voltage Vbias may be produced to be a predetermined proportion of the input voltage Vin 2  (e.g., 50%, or, in other words, Vbias is at the midpoint between the voltages of the output nodes of the second power source  6 ). Vbias follows Vin 2  downward (or becomes smaller) and upward (or becomes greater) as Vin 2  sags and recovers, respectively, due to its changing load (the power amplifier A 2 ). This behavior of Vbias, as being dependent upon Vin 2 , influences Vtop and Vbot (the voltages of the top and bottom output nodes  11 ,  9 , respectively). An example of such behavior and influence is depicted in  FIG. 2 , which shows waveforms over time beginning with interval  1  in which the power source  8  is in normal operating condition (or Vin 2  is at a nominal level, as are Vbias, Vbot and Vtop). Interval  1  is immediately followed by interval  2 , a high energy transient interval in which the load on the power source  6  increases; such a load is depicted by an example in  FIG. 1 , as the audio power amplifier A 2  drawing its supply current from the second power source  6  and driving a speaker  13  with elevated volume, which causes Vin 2  to sag. This is immediately followed by interval  3 , a recovery interval in which Vin 2  recovers back to its nominal level, and then interval  4 , returning to the normal operating condition. Note how, as shown in  FIG. 2 , Vbias follows Vin 2  downward and upward, and may do so in accordance with a predetermined proportion. For example, if Vin 2  starts at 40V and falls to 32V, then Vbias falls from 20V to 16V (maintaining at 50% of Vin 2 ). As a result, Vtop (the voltage at the top output node  11 ) moves in accordance with the drop in Vbias, here 4 Volts, from 25V to 21V, while Vbot (the voltage at the bottom output node) also moves by the same amount, from 15V to 11V. If Vin 2  then recovers to its nominal level, Vbias responds by also recovering to its nominal level, which influences Vtop and Vbot to recover their nominal levels as well (in the same manner, or in accordance with the same predetermined proportion). This is an example of how Vtop and Vbot “float” down and up, but in doing so maintain their fixed separation relative to each other. 
     If the nominal level of Vbias (the voltage at the intermediate node  10  during a normal operating condition of the second power source  6 ) is set to be at the midpoint, or 50%, of Vin 2 , and if Vtop and Vbot are produced in a balanced manner, that is Vtop−Vbias=|−Vbias| at all times, then symmetrical headroom is achieved for the power amplifier A 2 . This is also illustrated in  FIG. 2 , where the headroom varies over time (depending on whether Vin 2  is sagging, recovering, or remains nominal), but remains symmetrical during all four time intervals of operation. In other words, the difference between Vtop and Vin remains at all times the same as the difference between Vbot and zero (the potential of the “input ground” of the second power source  6 —see  FIG. 1 ). 
     Referring to  FIG. 1 , with symmetrical headroom, any clipping of the power amplifier A 2  also remains symmetrical, a desirable result. Such clipping may occur when the power amplifier A 2  is receiving too large of an input signal from a signal controller A 1 . The signal controller A 1  is driven by an audio source  14 , which may be an analog or digital source of an audio signal, e.g. part of a laptop computer, a desktop computer, a tablet computer, or a smartphone. The output of the power amplifier A 2  is coupled to drive a speaker  13 . In such an embodiment, the sagging of Vin 2  as depicted in  FIG. 2  may be caused by the playback of audio through the loudspeaker  13  at elevated volume levels. 
     An example charge pump circuit that can produce the behavior depicted above in  FIG. 2 , and that has the general form shown in  FIG. 1 , is illustrated in circuit schematic form in  FIG. 3 . There, the switch network  3  is composed of two sets of switches, which are controlled by the digital control signals phi and phi′. The two phi switches serve to couple the first power source  8  through the input nodes  1 ,  2 , to respective plates of C 1 , C 2 , and the two phi′ switches serve to couple those same respective plates of C 1 , C 2  to the output nodes  11 ,  9 , respectively. Respective plates of the capacitors C 3 , C 4  are coupled to the output nodes  11 ,  9 , such that C 3 , C 4  may serve to filter the voltages on the output nodes  11 ,  9  relative to the intermediate node  10  which is shared by C 3 , C 4  as shown. A third phi′ switch is shown that when closed directly connects a shared node of C 1 , C 2  to the intermediate node  10 . 
     The digital control signals phi and phi′ may be as depicted in  FIG. 4 , e.g., square waves that are essentially 180 degrees out of phase with each other (with the understanding that in practice the phases of such signals may be non-overlapping in that their adjacent clock edges are slightly shifted, to avoid unwanted cross conduction effects.) To achieve a balanced output of Vbias+½ Vin 1  and Vbias−½ Vin 1  at the top and bottom output nodes  11 ,  9 , respectively, —see  FIG. 1 , the capacitances of C 1  and C 2  may be designed to be equal, while the capacitances of C 3  and C 4  should also be designed to be equal. The capacitors C 3 , C 4  may serve to filter the output voltages, while the capacitors C 1 , C 2  may also be referred to as charge pump capacitors or flying capacitors. The capacitance values should be selected in view of the desired power output of the charge pump power supply circuit, that is the power delivered to a load that includes the signal controller A 1 , from the output nodes  11 ,  9 —see  FIG. 1 . In one embodiment, this output power is less than 3 Watts. 
     Operation of the switch network  3  as part of the charge pump power supply circuit depicted in  FIG. 3  may be illustrated using  FIG. 5  and  FIG. 6 . The charge pump circuit exhibits a repeating charge pump cycle that repeats in accordance with the repetitive or periodic waveforms of the digital control signals phi and phi′ such as those depicted in  FIG. 4 .  FIG. 5  illustrates a first phase of the cycle in which the switches that are controlled by the digital control signal phi are closed, while the switches that are controlled by the digital control signal phi′ are open, resulting in the capacitors C 1  and C 2  being charged while connected in series with the first power supply  8  (Vin 1 ). This results in one of half of Vin 1  being dropped across each of C 1 , C 2  (assuming that the capacitances of C 1  and C 2  are designed to be equal). Next, the first phase may be “immediately” followed by the second phase of the charge pump cycle, where here the use of that term is with the understanding above that it may include a slight delay to ensure no overlap between the first and second phases); the resulting configuration of the charge pump circuit is depicted in  FIG. 6 . In the second phase, the switches that are controlled by phi are now open, while the switches that are controlled by phi′ are closed, which results in the transfer of charge from C 1 , C 2  to C 3 , C 4 . In so doing however, the presence of the voltage bias source  12  forces the intermediate node  10  that is shared by the capacitors C 3 , C 4 , to Vbias, such that the charge transfer results in the top output node  11  rising to Vbias+½ Vin 1  while the bottom output node  9  (which is balanced in with the top output node) falls to Vbias−½ Vin 1 . This results in positive and negative output rail voltages on nodes  11 ,  9 , respectively, relative to the output ground on the intermediate node  10 . The phases in  FIG. 5  and  FIG. 6  repeat in each charge pump cycle, thereby producing those unregulated but balanced positive and negative voltages on the top node  11  and bottom node  9  relative to the intermediate node  10 —see  FIG. 1 . 
     In this manner, taking the example given above in the Summary section, if Vin 2 =40 Vdc, then the charge pump circuit output rails may be at 25 Vdc and 15 Vdc, a separation of 10 Volts centered at Vbias=20 Vdc. Now, if Vin 2  sags from 40 Vdc to 35 Vdc, then Vbias, being set or predetermined to be 50% of Vin 2 , responds by dropping to 17.5 Vdc. As a result, the charge pump circuit drops the top rail to 22.5 Vdc and drops the bottom rail to 12.5 Vdc, which maintains the separation between the top and bottom output rails at 10 Volts centered at the output ground which is at Vbias=17.5 Vdc. The output ground responds to and follows the sagging Vin 2 , so that the difference between the top output rail and Vin 2  and the difference between bottom output rail and zero volts (the ground of the first power source  8 ) remains the same. As a result, signal headroom remains consistent and any clipping by the power amplifier A 2 —see  FIG. 1 —remains symmetrical. 
     Operation of the charge pump based power supply circuit described above in connection with  FIG. 1  is an example of a more general method or process for electrical power conversion, where power is drawn from the first and second input nodes  1 ,  2  (first input voltage) to produce i) a voltage at the top output node  11  that is higher than a voltage of the intermediate node  10 , and ii) a voltage at the bottom output node  9  that is lower than the voltage of the intermediate node  10 . In such a process, the voltage of the intermediate node  10  is controlled, independently of the voltages at the top and bottom output nodes  11 ,  9 , so that the voltage of the intermediate node  10  i) is a predetermined proportion of a second input voltage and ii) follows the second input voltage downward and upward as the second input voltage sags and recovers, respectively. For example, the voltage of the intermediate node  10  may be controlled to always follow the second input voltage in accordance with the predetermined proportion. The voltage of the intermediate node  10  may be controlled to always be at the midpoint of the second input voltage. Lastly, drawing power from the first and second input nodes  1 ,  2  to produce the voltages at the top and bottom output nodes  11 ,  9  may be accomplished by controlling the switch network  3  of a charge pump circuit such as the one depicted in  FIG. 3 , or the one depicted in  FIG. 7  described next. 
     Turning now to  FIG. 7 , a circuit schematic of a charge pump circuit that may generate output rail voltages that are more balanced (relative to each other) than the embodiment of  FIG. 3  is shown.  FIG. 8  depicts some digital control signals having selected relative phases, to operate the switches of the switch network  3  of the charge pump circuit of  FIG. 7 . In this embodiment, the switch network  3  is designed to enable a balanced charging and discharging of the capacitors C 1 , C 2 , so that the voltages produced on the top and bottom output rails. These output rails may be the output nodes  11 ,  9 , which are directly connected to the “outer” plates of the capacitors C 3 , C 4 , respectively, while the “inner” plates of the C 3 , C 4  share the intermediate node  10 . The output rail voltage are more balanced, e.g., are more immune to the practical variation between the capacitances of C 1 , C 2  which otherwise may be specified as being equal, or closer to being exactly equal to each other in absolute value (and opposite in polarity relative to the intermediate node  10 ). In the example of  FIG. 7  and  FIG. 8 , the charge pump cycle is divided into four phases, and operation of the charge pump circuit is controlled by the four digital control signals phi 1 , phi 2 , phi 3 , and phi 4  depicted in  FIG. 8 . These control signals are pulse waves that have the same period (a complete charge pump cycle) and the same pulse width, but different phases; phi 2  is delayed a quarter cycle relative to phi 1 , phi 3  is delayed a quarter cycle relative to phi 2 , and phi  4  is delayed a quarter cycle relative to phi 3 . These control signals are applied to control the closed/open states of the switches shown in  FIG. 7 , where in this example a switch is closed when its control signal is asserted, and open when the control signal is deasserted. Here too, and as mentioned above with respect to the embodiment of  FIG. 3 , the phases of such control signals may be non-overlapping in that adjacent clock edges are slightly shifted, to avoid unwanted cross conduction effects. Note that in this particular example, one of the switches is controlled by a logical OR of phi 1  and phi 2 , while another is controlled by the logical OR of phi 3  and phi 4 . It should be noted that the switch network  3  may be implemented using other switch arrangements (and their associated digital control signals.) 
     Control of the switch network  3  that is depicted in  FIG. 7 , by the digital control signals of  FIG. 8 , may be explained with the help of the circuit schematics in  FIGS. 9-12 . These four figures show the current paths (in bold or thick lines) that are formed by the switches of the switch network  3 , during each of the four phases of the charge pump cycle, respectively, for purposes of charging or discharging of the charge pump capacitors C 1 , C 2 . 
       FIG. 9  illustrates a first phase of the charge pump cycle in which the switches that are controlled by the digital control signal phi 1  (including its logical OR) are closed, while all other switches are open, resulting in the capacitors C 1  and C 2  being charged through the current path indicated in bold, while connected in series with the first power source  8  (Vin 1 .) It is instructive to note here that the first capacitor C 1  is charged at high side while the second capacitor C 2  is charge at low side, e.g., the physical order of the series connection of C 1  and C 2  is such that the input node  1  (high side of the first power source  8 , Vin 1 ) is directly connected to C 1 , and not C 2 . This results in one of half of Vin 1  being dropped across each of C 1 , C 2  (assuming that the capacitances of C 1  and C 2  are designed to be equal). 
     Next, the first phase may be “immediately” followed by the second phase of the charge pump cycle, with the understanding given above that there may be a slight delay to ensure no overlap between the first and second phases; this results in the configuration depicted in  FIG. 10 . This discharge phase may be explained as follows. Here, C 1  and C 2 , while still coupled in series as in  FIG. 9 , are now discharged into C 3 , C 4 , respectively, through the current path shown in bold which is produced by those switches that are controlled by phi 2  (including its logical OR) being closed. Note here that the charge is transferred from C 1  at high side and C 2  at low side, to C 3 , C 4  (which are coupled in series.) 
     Next, the second phase may be immediately followed by the third phase of the charge pump cycle, which is depicted in  FIG. 11 . Here, once again C 1  and C 2  are charged in series except that now C 2  is charged at high side while C 1  is charged at low side (the reverse order of phase 1). The third phase is then immediately followed by the fourth phase depicted in  FIG. 12 . This discharge phase may be explained as follows. Here, C 1 , C 2  are again discharged, but this time the charge is transferred from C 2  at high side and C 1  at low side, to C 3 , C 4 , respectively. The reversal of the series connection of C 1 , C 2  during the charge and discharge phases, just described in connection with  FIGS. 9-12 , may result in the voltages on the output nodes  11 ,  9  being balanced, e.g., more immune to a difference between the capacitances of C 1  and C 2 . 
     Referring now to  FIG. 13 , this is a circuit schematic of an example of the bias voltage source  12 . As mentioned above, the bias voltage source  12  should produce a voltage Vbias at its output node that is independent of the resistance of R 1  (which may represent the load on the output node), but that does depend on the voltage Vin 2  at its input node. In particular, the circuit in  FIG. 13  may achieve this goal by being an “active voltage divider” that is designed so that Vbias is at all times, in this case, 50% of Vin 2 , staying at 50% of Vin 2  as Vin 2  sags and recovers. To achieve the 50% fraction, the resistance values of the voltage divider R 18  and R 17  are designed to be equal. Those of R 20  and R 21  may also be designed to be equal, as well as those of R 19  and R 2 , to provide a balanced operation, when Vin 2  drops as well as when Vin 2  rises. The resistors R 20 , R 21 , R 19  and R 2  are part of a unity gain voltage buffer amplifier circuit, implemented using bipolar junction transistors Q 1 -Q 6 , which buffers the divided voltage at the shared node of R 18  and R 17 , to yield the output voltage Vbias that is equal to the divided voltage (but with lower output impedance). The capacitors C 14 , C 13  are coupled to share the output node, and serve to filter the output voltage Vbias. Note that a voltage divider followed with a voltage buffer amplifier may alternatively be implemented using field effect transistors. 
     An embodiment of the invention is a charge-pump circuit having the switch network  3 , and the switch control logic  5 . The switch network  3  has first and second input nodes  1 ,  2  to be coupled to the first power source  8  (Vin 1 ), a first capacitor node to be coupled to a plate of the first capacitor C 1 , a second capacitor node to be coupled to a plate of the second capacitor C 2 , a first output node  11  to be coupled to a plate of the third capacitor C 3 , and a second output node  9  to be coupled to a plate of the fourth capacitor C 4 . The switch control logic is to produce two or more digital control signals that control the switch network  5 , wherein phases of the digital control signals are such that C 1  and C 2  become charged by the power source  8  and then discharged in each charge pump cycle. The charge pump cycles thus produce effectively a dc voltage at the first output node  11  that is higher than the dc voltage of an intermediate node  10  which is shared by the third and fourth capacitors C 3 , C 4 , by the same amount as a dc voltage produced at the second output node  9  is lower than the voltage of the intermediate node  10 . The addition of the bias voltage source  12  enables the output node voltages to “float” and follow any sagging and recovery by the second power source  6  (Vin 2 ), thereby contributing to symmetric headroom for an amplifier that is being supplied by the second power source  6  and whose signal controller is being supplied by the output nodes  11 ,  9 . 
     Another embodiment of the invention is a charge-pump circuit that can be used to produce more balanced, dual polarity output rails (from a single polarity power source). Referring to  FIGS. 7-12 , the first and second input nodes  1 ,  2  are to be coupled to the first power source  8 . There are first, second, third, and fourth capacitor nodes A, B, C, D, which may be coupled to the first and second capacitors C 1 , C 2  as shown. There are also the first and second output nodes  11 ,  9 . The switch network  3  couples the first and second input nodes to the capacitor nodes A, B, C, D, so as to charge the first and second capacitors C 1 , C 2 ; it also couples the capacitor nodes A, B, C, D to the first and second output nodes so as to discharge the first and second capacitors C 1 , C 2 , all under control of the switch control logic  5 , so that
         a) during a first phase of a charge pump cycle, the node A is coupled to the first input node  1  while the node B is coupled to the second input node  2 ,   b) during a second phase of the charge pump cycle, the node A is coupled to the first output node  11  while the node B is coupled to the second output node  9 ,   c) during a third phase of the charge pump cycle, the node C is coupled to the first input node  1  while the node D is coupled to the second input node  2 , and   d) during a fourth phase of the charge pump cycle, the node C is coupled to the first output node  11 , while the node D is coupled to the second output node  9 .       

     While certain embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that the invention is not limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those of ordinary skill in the art. For example, the charge pump based power supply circuits described above can be used to supply power to a variety of different loads.  FIG. 1  shows one example application where the benefits of balanced output rails and symmetric headroom are apparent, e.g., the signal controller A 1  having positive and negative power supply inputs that are coupled to be powered by the top and bottom output nodes  11 ,  9  of the charge pump circuit, and where the signal controller A 1  is driving the power stage transistors (not shown) of the audio power amplifier A 2 . The latter has upper and lower power supply inputs that are coupled to be powered by the second power source  6  as shown. Other load applications are possible. Also, while  FIG. 1  shows a direct connection between the output rails of the charge pump circuit (nodes  11 ,  9 ) and the power supply input nodes of the signal controller A 1 , in another instance a low drop out regulator may be added between the output rail and the input node (of the signal controller A 1 ), if a smaller power supply voltage is needed by the controller A 1 . The description is thus to be regarded as illustrative instead of limiting.