Abstract:
An apparatus for power conversion includes a switching network that controls interconnections between pump capacitors in a capacitor network that has a terminal coupled to a current source, and a charge-management subsystem. In operation, the switching network causes the capacitor network to execute charge-pump operating cycles during each of which the capacitor network adopts different configurations in response to different configurations of the switching network. At the start of a first charge-pump operating cycle, each pump capacitor assumes a corresponding initial state. The charge-management subsystem restores each pump capacitor to the initial state by the start of a second charge-pump operating cycle that follows the first charge-pump operating cycle.

Description:
RELATED APPLICATIONS 
       [0001]    Under 35 USC 119, this application claims the benefit of the Mar. 14, 2014 priority date of U.S. Provisional Application 61/953,303 and the Mar. 14, 2014 priority date of U.S. Provisional Application 61/953,270, the contents of which are herein incorporated by reference. 
     
    
     FIELD OF DISCLOSURE 
       [0002]    This invention relates to power converters, and in particular, to charge pumps. 
       BACKGROUND 
       [0003]    In many circuits, the power that is available to drive the circuit may not be in a form that the circuit demands. To correct this, it is useful to provide a power converter that converts the available power into a form that conforms to the circuit&#39;s requirements. 
         [0004]    One common type of power converter is a switch-mode power converter. A switch-mode power converter produces an output voltage by switching reactive circuit elements into different electrical configurations using a switch network. A switched capacitor power converter is a type of switch-mode power converter that primarily utilizes capacitors to transfer energy. Such converters are called “charge pumps.” The capacitors are called “pump capacitors.” 
         [0005]    In operation, a charge pump transitions from one pump-state to the next in a sequence of pump-states. Each pump-state is characterized by a residence time in which the charge pump remains in that pump-state, and transition times, in which the charge pump is between pump-states. The sum of the residence times for all pump-states and the intervening transition times between those pump-states is the period for one cycle of the charge pump. 
         [0006]    For correct operation, each pump capacitor should begin and end each cycle with zero change in charge. If this is not the case, charge will accumulate on the pump capacitor over the course of several cycles in the case of positive non-zero change in charge. Since the voltage across a capacitor is linearly proportional to the charge, this charge accretion/depletion will cause the voltage across the pump capacitor to drift over time. 
         [0007]    In many charge pumps, a switch connects adjacent pump capacitors. The voltage across the switch thus depends on the voltages across adjacent pump capacitors. If voltages across these capacitors drift unevenly, the voltage across the switch may exceed its rating. This may cause the switch to overheat, thus destroying the switch, and the charge pump as well. 
         [0008]    Procedures for managing charge on a pump capacitor depend in part on how the charge got there. In general, there are two ways to put charge into a capacitor: using a voltage source, or using a current source. 
         [0009]    When a voltage source is used, management of charge is relatively simple. The charge present at a capacitor is a linear function of the voltage. Thus, dropping the voltage to zero is sufficient to remove the charge from the capacitor. 
         [0010]    When a current source is used, management of charge is not so simple. This is because the charge on a pump capacitor is related to an integral of the current, and not to the instantaneous value of current. 
         [0011]    On Nov. 8, 2012, Patent Publication WO 2012/151466, which is incorporated herein by reference, made public configurations of charge pumps in which one terminal was connected to a regulator. Because of its inductor, and because of the relevant time scales associated with the switches involved, as far as these charge pump configurations are concerned, the regulator behaved like a current source. This made management of how much charge is in the pump capacitors more challenging. 
       SUMMARY 
       [0012]    The inventive subject matter described herein relates to stabilizing a charge pump coupled with a current source or load by ensuring that each pump capacitor of the charge pump begins a cycle in the same condition for every cycle. This avoids charge accretion that occurs when residual charge from the end of a first cycle is added to the beginning of a second cycle, thus causing the voltage of the capacitor to drift over time. 
         [0013]    In one aspect, the invention features an apparatus for power conversion. Such an apparatus includes a switching network, and a charge management subsystem. The switching network controls interconnections between pump capacitors in a capacitor network that has a terminal coupled to a current source. In operation, the switching network causes the capacitor network to execute charge-pump operating cycles during each of which the capacitor network adopts different configurations in response to different configurations of the switching network. At the start of a first charge-pump operating cycle, each pump capacitor assumes a corresponding initial state. The charge-management subsystem restores each pump capacitor to the initial state by the start of a second charge-pump operating cycle that follows the first charge-pump operating cycle. 
         [0014]    In some embodiments, a controller in the charge-management subsystem controls residence time. One such embodiment features a controller that controls a first residence time during which the switching network is in a first configuration. Another embodiment features a controller that controls a first residence time during which the switching network is in a first configuration, and a second residence time during which the switching network is in a second configuration. Also included are embodiments in which the controller causes the switching network to cause the capacitor network to assume a dead-time configuration. 
         [0015]    In some embodiments, a cycle includes a first configuration and a second configuration, and a controller of the charge-management subsystem controls a second configuration of the switching network based on a result of having had the switching network assume the first configuration. In other embodiments, the switching network passes through a present cycle and at least one past cycle, and the controller controls the present cycle based at least in part on performance of the switching network during at least one of the past cycles. In yet other embodiments, the switching network passes through a present cycle after having passed through past cycles, and the charge-management subsystem includes a proportional-integral-derivative controller that controls the present cycle based at least in part on performance of the switching network during at least one of the past cycles. 
         [0016]    Embodiments also include those in which the charge-management system includes a controller configured to exercise control over configurations of the switching network. Among these are embodiments in which the controller includes a feedback controller configured to control the different configurations of the switching network based on an output of the capacitor network, and those in which the controller includes a threshold-logic circuit controller configured to control the different configurations of the switching network based on an output of the capacitor network. 
         [0017]    Yet other embodiments do not rely on controlling the switching network. In one such embodiment, the charge-management system includes a controller configured to exercise control over the current source to which the capacitor network is coupled. Among these embodiments are those in which the capacitor network has two terminals, one of which is a low-voltage terminal, and the terminal that is coupled to the current source is a low-voltage terminal. Also among these embodiments is the converse case, in which the capacitor network has two terminals, one of which is a high-voltage terminal, and the terminal that is coupled to the current source is a high-voltage terminal. 
         [0018]    In some embodiments, the charge-management system includes a stabilizing capacitance connected to the current source. 
         [0019]    Other embodiments include a trim-capacitor network that includes switches that are selectively configured to define an interconnection of one or more trim capacitors, thereby defining an aggregate capacitance that reduces a mismatch between the aggregate capacitance and a desired capacitance. In some of these embodiments, the desired capacitance is a stabilizing capacitance connected to the current source. In others, the desired capacitance is a desired capacitance of a pump capacitor. 
         [0020]    Embodiments also include those in which the charge-pump operating cycle has a constant time duration, and those in which the charge-pump operating cycle has a variable time duration. 
         [0021]    In some embodiments, the second charge-pump operating cycle that follows the first charge-pump operating cycle is a charge-pump cycle that immediately follows the first charge-pump operating cycle. However, in some cases, complete restoration cannot be done in one cycle. Thus, in certain embodiments, the second charge-pump operating cycle that follows the first charge-pump operating cycle is a charge-pump cycle that is separated from the first charge-pump operating cycle by at least one intervening charge-pump operating cycle. 
         [0022]    The invention also includes embodiments that include the capacitor network that the switching network is configured to control. 
         [0023]    Embodiments of the invention also include those in which the switching network causes execution of a charge pump operating cycle that consists of no more than two configurations during which transfer of charge between capacitors occurs, as well as those in which switching network causes execution of a charge pump operating cycle that consists of at least three configurations during which transfer of charge between capacitors occurs. 
         [0024]    In some embodiments, the capacitor network and the switching network define a charge pump. Among these are embodiments in which the charge pump includes a multi-stage charge pump, embodiments in which the charge pump includes a cascade multiplier, embodiments in which the charge pump includes a multi-phase charge pump, and embodiments in which the charge pump includes a single-phase charge pump. 
         [0025]    A variety of devices function as current sources within the meaning of the invention. One such device is a regulator. Among the regulators that function as current sources are switch-mode power converters, and buck converters. 
         [0026]    In some embodiments, the charge management subsystem restores at least one of the pump capacitors to an initial state thereof at least in part by changing an amount of charge stored on the at least one pump capacitor. Among these embodiments are those in which the charge management subsystem changes an amount of charge stored one the pump capacitor by causing flow between that pump capacitor and a repository of charge. Suitable repositories include another pump capacitor or ground. 
         [0027]    In another aspect, the invention features a method for controlling a charge pump. Such a method includes causing a charge-pump switching network to cause a network of pump capacitors to which the charge pump switching network is coupled to execute charge-pump operating cycles during each of which the network of pump capacitors adopts different configurations in response to different configurations of the switching network. At the start of a first charge-pump operating cycle, each pump capacitor assumes a corresponding initial state. The method then proceeds by restoring each pump capacitor to the initial state by the start of a second charge-pump operating cycle that follows the first charge-pump operating cycle. 
         [0028]    In some practices, restoring each pump capacitor to the initial state includes restoring each pump capacitor to the initial state by the start of a second charge-pump operating cycle that immediately follows the first charge-pump operating cycle. However, there are also practices of the invention in which restoring each pump capacitor to the initial state includes restoring each pump capacitor to the initial state by the start of a second charge-pump operating cycle that is separated from the first charge-pump operating cycle by at least one charge-pump operating cycle. 
         [0029]    Some practices restore each pump capacitor to the initial state by controlling residence time. One such practice includes controlling a first residence time during which the switching network is in a first configuration. However, in another practice, restoring each pump capacitor to the initial state includes controlling a first residence time during which the switching network is in a first configuration, and also controlling a second residence time during which the switching network is in a second configuration. Yet other practices include controlling residence times for additional configurations, some of which involve charge transfer and some of which do not. For example, in one practice, restoring each pump capacitor to the initial state includes causing the capacitor network to assume a dead-time configuration. 
         [0030]    Practices also include those in which a cycle includes a first configuration and a second configuration, and restoring each pump capacitor to the initial state includes controlling a second configuration of the switching network based on a result of having had the switching network assume a first configuration. 
         [0031]    In other practices, the switching network passes through a present cycle and at least one past cycle, and restoring each pump capacitor to the initial state includes controlling the present cycle based at least in part on performance of the switching network during the at least one past cycle. 
         [0032]    In yet other practices, the switching network passes through a present cycle after having passed through past cycles, and restoring each pump capacitor to the initial state includes implementing proportional-integral-derivative control over the present cycle based at least in part on performance of the switching network during at least one of the past cycles. 
         [0033]    Some practices of the invention are those in which restoring each pump capacitor to the initial state includes exercising control over configurations of the switching network. Among these are practices in which exercising control over configurations of the switching network includes exercising feedback control of the different configurations of the switching network based on an output of the capacitor network, and those in which exercising control over configurations of the switching network includes exercising threshold-logic control over the different configurations of the switching network based on an output of the capacitor network. 
         [0034]    In yet other practices, restoration of each pump capacitor to the initial state includes exercising control over a current source to which the capacitor network is coupled. Among these practices are those that include exercising control over a current source that is coupled to a low-voltage terminal, and those that include exercising control over a current source that is coupled to a high-voltage terminal. 
         [0035]    Yet other practices are those in which restoring each pump capacitor to the initial state includes connecting a stabilizing capacitance connected to the current source. 
         [0036]    Other practices include those in which restoring each pump capacitor to the initial state includes interconnecting one or more trim capacitors to define an aggregate capacitance that minimizes an error between the aggregate capacitance and a desired capacitance. Examples of a desired capacitance include a desired capacitance of a stabilizing capacitor connected to a current source, and a desired capacitance of a pump capacitor. 
         [0037]    In other practices, causing a charge-pump switching network to cause a network of pump capacitors to which the charge pump switching network is coupled to execute charge-pump operating cycles includes causing execution of charge-pump operating cycles having a constant time duration. However, in other practices, the time duration is variable. 
         [0038]    Some practices feature starting the second charge-pump operating cycle that follows the first charge-pump operating cycle immediately after finishing the first charge-pump operating cycle. However, in other practices, starting the second charge-pump operating cycle that follows the first charge-pump operating cycle occurs only after having finished at least one intervening charge-pump operating cycle. 
         [0039]    In some practices, causing a charge-pump switching network to cause a network of pump capacitors to which the charge pump switching network is coupled to execute charge-pump operating cycles includes causing execution of a charge pump operating cycle that consists of no more than two configurations during which transfer of charge between capacitors occurs. However, in others, this is carried out instead by causing execution of a charge pump operating cycle that consists of at least three configurations during which transfer of charge between capacitors occurs. 
         [0040]    Certain practices include forming a charge pump by combining the capacitor network and the switching network. Among these are practices in which the charge pump thus formed is a multi-stage charge pump. However, other charge pumps, such as a cascade multiplier, a multi-phase charge pump, or a single-phase charge pump, can also be formed. 
         [0041]    In some practices, exercising control over a current source to which the capacitor network is coupled includes exercising control over a regulator. This can include exercising control over many different kinds of regulators, all of which effectively function as current sources. Such regulators include switch-mode power converters, and buck converters. 
         [0042]    Other practices are those in which restoring each pump capacitor to the initial state includes restoring at least one of the pump capacitors to an initial state thereof at least in part by changing an amount of charge stored on the at least one pump capacitor. Practices of this kind can include causing flow between the at least one pump capacitor and a repository of charge. Examples of a suitable repository include another pump capacitor, or ground. 
         [0043]    One effect of using a current-based load (or source) with a charge pump is that there may be situations when fixed switch timing used with current-based loads and/or sources results in charge imbalance across capacitors. Such imbalance can result in (or can co-occur with) larger than necessary ripple, extremes in voltage and/or current internal to or at terminals of the charge pump, drift in average and/or peak voltages internal or at terminals of the charge pump, and/or instability, which may be manifested by growing amplitude of voltage variation between points internal to and/or terminals of the charge pump. 
         [0044]    In another aspect, in general, an approach to avoiding and/or mitigating the effects related to charge imbalance is by adjusting the switch timing in a feedback arrangement. In some examples, a pattern of switch timing is adapted based on electrical measurements at the terminals and/or internal to the charge pump. In some examples, the instants of transition of switch states are determined by such electrical measurements. 
         [0045]    These and other features of the invention will be apparent from the following detailed description, and the accompanying drawings, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0046]      FIG. 1  shows a single-phase charge pump; 
           [0047]      FIG. 2  shows a time-line associated with the operation of the single-phase charge pump of  FIG. 1 ; 
           [0048]      FIG. 3  shows circuit configurations associated with a cycle of the single-phase charge pump of  FIG. 1 ; 
           [0049]      FIG. 4  shows a two-phase charge pump; 
           [0050]      FIG. 5  shows circuit configurations associated with a cycle of the two-phase charge pump of  FIG. 4 ; 
           [0051]      FIG. 6  shows a first controller for controlling pump-state residence times in the charge pump of  FIG. 1 ; 
           [0052]      FIG. 7  shows a second controller for controlling pump-state residence times in the charge pump of  FIG. 1 ; 
           [0053]      FIG. 8  shows an implementation of the second feedback circuit in  FIG. 7 ; 
           [0054]      FIG. 9  shows an implementation of the second timing circuit in  FIG. 7 ; 
           [0055]      FIG. 10  shows a third controller for controlling pump-state residence times in the charge pump of  FIG. 1   
           [0056]      FIG. 11  shows a fourth controller for controlling current at a load; 
           [0057]      FIG. 12  shows a fifth controller for controlling current at a regulator; 
           [0058]      FIG. 13  shows a switching network for attaining a desired capacitance for a pump capacitor in  FIG. 1 ; and 
           [0059]      FIG. 14  shows a switching network for attaining a desired stabilization capacitance. 
       
    
    
     DETAILED DESCRIPTION 
       [0060]      FIG. 1  shows a first example of a charge pump  10  coupled to a load  12  that is modeled as an ideal current source. The charge pump  10  is a multi-stage charge pump, also known as a cascade multiplier. Although the current shown is actually being drawn from the charge pump  10 , this distinction amounts to a mere sign change. The important feature of a current source is that it relentlessly drives a constant flow of current. 
         [0061]    Throughout this specification, reference will be made to a “current source.” As is well known, an ideal “current source” is an abstraction used for circuit analysis that does not in fact exist. However, for the time scales of interest, there are a variety of devices that effectively function as a current source. Examples include regulators, such as linear regulators, DC motors, depending on the load, and an IDAC, which is an active circuit that sets the current through LEDs. Thus, throughout this specification, “current source” or “current load” is understood to mean real devices, including but not limited to those enumerated herein, that effectively function as a current source. 
         [0062]    The load  12  can be viewed as a non-zero constant current, or a pulsed current that alternates between two values, one of which can be zero. Charge transfer occurs whenever the current at the load is non-zero. When the current is non-zero and constant, the charge transfer will be referred to as “soft charging,” or “adiabatic charging.” 
         [0063]    The charge pump  10  has first and second terminals  14 ,  16 . One terminal is a high voltage that carries a low current. The other terminal is a low voltage that carries a high current. In the particular example described herein, the second terminal  16  is the low voltage terminal. However, in other embodiments, the second terminal  16  is the high voltage terminal. 
         [0064]    Between the terminals  14 ,  16  are four identical pump capacitors: outer pump capacitors C 1 , C 4  and inner pump capacitors C 2 , C 3 . A first phase-node P 1  couples with the negative terminal of the first and third pump capacitors C 1 , C 3 , and a second phase-node P 2  couples with the negative terminal of the second and fourth pump capacitors C 2 , C 4 . 
         [0065]    A first switch-set  1  and a second switch-set  2  cooperate to cause the charge pump  10  to reconfigure the pump capacitors C 1 -C 4  between first and second pump-states  18 ,  20  as shown in  FIG. 2 . Through operation of the first and second switch-sets  1 ,  2 , the charge pump  10  maintains a transformation ratio M:N between the voltages at the first and second terminals  14 ,  16 . In the particular charge pump  10  shown in  FIG. 1 , the transformation ratio is 5:1. 
         [0066]    In operation, the charge pump  10  executes a series of charge pump cycles. Each charge pump cycle has a first pump-state  18  and a second pump-state  20 , as shown in  FIG. 2 . To transition from the first pump-state  18  to the second pump-state  20 , the switches in the first switch-set  1  are opened and the switches in the second switch-set  2  are closed. Conversely, to transition from the second pump-state  20  into the first pump-state  18 , the switches in the first switch-set  1  are closed and the switches in the second switch-set  2  are opened. 
         [0067]      FIG. 2  shows the configuration of the switches as “Config X/Y” where X and Y are binary variables that indicate the disposition of the switches in the first and second switch-sets  1 ,  2  respectively. A binary zero indicates that the switches in a particular switch-set are open and a binary one indicates that the switches in a particular switch-set are closed. 
         [0068]    During the first pump-state  18 , the switches in the first switch-set  1  are all closed and the switches in the second switch-set  2  are all opened. The first pump-state  18  consists of a first pump-state redistribution interval  18 A and a first pump-state steady-state interval  18 B. 
         [0069]    The first pump-state  18  begins with the opening of the switches in the second switch-set  2  and the closing of the switches in the first switch-set  1 . This begins a first pump-state redistribution interval  18 A characterized by a rapid redistribution of charge. For a brief period, the current associated with this charge distribution dwarfs that associated with the current through the load  12 . 
         [0070]    Eventually, the current associated with charge redistribution dies down and the charge pump  10  settles into a first pump-state steady-state interval  18 B. During the first pump-state steady-state interval  18 B, current through the charge pump  10  is dominated by the current through the load  12 . The sum of the time spent in the first pump-state steady-state interval  18 B and the first pump-state redistribution interval  18 A is the first residence time. 
         [0071]    During the second pump-state  20 , the switches in the first switch-set  1  are all opened and the switches in the second switch-set  2  are all closed. The second pump-state  20  consists of a second pump-state redistribution interval  20 A and a second pump-state steady-state interval  20 B. 
         [0072]    The second pump-state  20  begins with the closing of the switches in the second switch-set  2  and the opening of the switches in the first switch-set  1 . This begins a second pump-state redistribution interval  20 A characterized by a rapid redistribution of charge. For a brief period, the current associated with this charge distribution dwarfs that associated with the current through the load  12 . 
         [0073]    Eventually, the current associated with charge redistribution dies down and the charge pump  10  settles into a second pump-state steady-state interval  20 B. During the second pump-state steady-state interval  20 B, current through the charge pump  10  is once again dominated by the current through the load  12 . The sum of the time spent in the second pump-state steady-state interval  20 B and the second pump-state redistribution interval  20 A is the second residence time. 
         [0074]    In the course of transitioning between the first and second pump-states  18 ,  20  the voltage at the first phase-node P 1  alternates between ground and the voltage at the second terminal  16 . Meanwhile, the voltage at the second phase-node P 2  is 180 degrees out-of-phase with the first phase-node P 1 . 
         [0075]    Between the first pump-state  18  and the second pump-state  20  there is a dead-time interval  21  during which both the switches in the first switch-set  1  and the switches in the second switch-set  2  are open. Although not, in principle, required, this dead-time interval is a practical necessity because switches do not transition instantaneously. Thus, it is necessary to provide a margin to avoid the undesirable result of having switches in the first and second switch-sets  1 ,  2  closed at the same time. 
         [0076]    To avoid having to introduce complexity that would only obscure understanding of the principles of operation,  FIG. 3  shows currents passing through the pump capacitors C 1 -C 4  in both the first and second pump-states  18 ,  20  assuming instantaneous charge-redistribution, no dead-time, and the same non-zero current, I X , at the second terminal  16  in both pump-states. 
         [0077]    In  FIG. 3 , the time spent in the first pump-state redistribution interval  18 A is t 1   a ; the time spent in the first pump-state steady-state interval  18 B is t 1   b ; the time spent in the second pump-state redistribution interval  20 A is t 2   a ; and the time spent in the second pump-state steady-state interval  20 B is t 2   b . Lastly, the total length of one cycle is tsw. The first residence time is therefore t 1   a +t 1   b ; and the second residence time is t 2   a +t 2   b . The assumption of instantaneous charge redistribution is manifested by setting t 1   a  and t 2   a  to zero, resulting in tsw being equal to t 1   b +t 2   b.    
         [0078]    During the first pump-state steady-state interval  18 B, the outer pump capacitors C 1 , C 4  carry a current having a magnitude of 0.4I X  while the inner pump capacitors C 2 , C 3  carry a current having half of the magnitude carried by the outer pump capacitors C 1 , C 4 . This is because the inner pump capacitors C 2 , C 3  are in series and the outer pump capacitors C 1 , C 4  are by themselves. 
         [0079]    During the second pump-state steady-state interval  20 B, each outer pump capacitor C 1 , C 4  is placed in series with one of the inner pump capacitors C 2 , C 3 , respectively. As a result, each pump capacitor C 1 -C 4  carries a current with magnitude 0.5I X . Note that the inner pump capacitors C 2 , C 3  are always in series with another pump capacitor, whereas the outer pump capacitors C 1 , C 4  are only in series with another pump capacitor during one pump-state. 
         [0080]    In the limiting case where charge is redistributed instantly, the current sources can be removed during the first and second pump-state redistribution intervals  18 A,  20 A as in  FIG. 3 . The amount of charge that is redistributed depends upon the voltages across the pump capacitors C 1 -C 4  prior to a pump-state change. 
         [0081]    In general, it is desirable that the net charge change at any pump capacitor C 1 -C 4  be zero during the course of a particular cycle. Otherwise, increasing/decreasing amounts of charge will collect in the pump capacitors C 1 -C 4  over several cycles. This charge accretion/depletion over multiple cycles causes instability. 
         [0082]    Since the quantity of charge transferred is the product of current and the amount of time the current flows, it follows that one can control the quantity of charge transferred to a pump capacitor C 1 -C 4  in any portion of the cycle by controlling the amount of time that the charge pump  10  spends in that portion of the cycle. This provides a way to ensure that the net charge change at each pump capacitor C 1 -C 4  is zero during one cycle of the charge pump  10 . 
         [0083]    If the above constraint is applied to each distinct capacitor current in a charge pump  10 , it is possible to generate a system of linear equations in which the times spent in each pump-state are the unknowns. The solution to that system will be the residence times for each pump-state  18 ,  20  that avoid instability. 
         [0084]    To avoid instability in this example, assuming instantaneous charge redistribution, the first residence time should be 3/5·tsw and the second residence time should be 2/5 ·tsw. This results in an equal amount of charge being transferred from inner pump capacitors C 2 , C 3  to the first pump capacitor C 1  and to the fourth pump capacitor C 4  during the first pump-state redistribution interval  18 A; and zero redistribution charge during the second pump-state redistribution interval  20 A. 
         [0085]    Solutions for various transformation ratios M:N are shown below in tabular form: 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
               
                   
                 First residence 
                 Second residence 
               
               
                 M:N 
                 time (sec) 
                 Time (sec) 
               
               
                   
               
             
             
               
                 3:1 
                 2/3 · tsw 
                 1/3 · tsw 
               
               
                 4:1 
                 1/2 · tsw 
                 1/2 · tsw 
               
               
                 5:1 
                 3/5 · tsw 
                 2/5 · tsw 
               
               
                 6:1 
                 1/2 · tsw 
                 1/2 · tsw 
               
               
                 7:1 
                 4/7 · tsw 
                 3/7 · tsw 
               
               
                 8:1 
                 1/2 · tsw 
                 1/2 · tsw 
               
               
                 9:1 
                 5/9 · tsw 
                 4/9 · tsw 
               
               
                   
               
             
          
         
       
     
         [0086]    Although there is no guarantee that every topology will have a solution, in the case of charge pumps like that in  FIG. 1 , a solution exists. As a result of symmetry in current flow during the first and second pump-state redistribution intervals  18 A,  20 A, the solution for cases in which the transformation ratio is 2k:1 for a positive integer k, the first and second residence times will be equal. Additionally, when M is odd and N is 1, the first residence time is tsw·(M+1)/2M while the second residence time is tsw·(M−1)/2M. 
         [0087]    In the case of a two-phase charge pump  10 , such as that shown in  FIG. 4 , the currents in the first and second pump-state redistribution intervals  18 A,  20 A are inherently symmetric, as shown in  FIG. 5 . Hence, the first and second pump-state residence times are equal, unlike in the single-phase charge pump  10  shown in  FIG. 1 , even though both charge pumps have the same transformation ratio M:N. 
         [0088]    In general, the first and second pump-state residence times, in the case of charge pumps like that in  FIG. 4 , will be equal for any transformation ratio k:1, where k is a positive integer. This inherent symmetry provides two-phase charge pumps with an advantage over single-phase charge pumps when it comes to stability. 
         [0089]    However, analysis based on principles of linear circuit theory is based on an idealization of the circuit. In practice, for example, due to differences in the capacitances of the various pump capacitors C 1 -C 4  of  FIG. 1 , difference in circuit resistances, (e.g., through transistor switches and/or signal traces), or inexact timing of the pump-state durations, it can be difficult to manage charge accretion/depletion in the pump capacitors C 1 -C 4 . 
         [0090]    One method for managing charge accretion/depletion is to use feedback to control the residence times.  FIG. 6  shows an apparatus to carry out such control. 
         [0091]    For convenience in discussion,  FIG. 6  shows the charge pump  10  as divided into a capacitor array  26  and a switch circuit  28 . The capacitor array  26  includes the pump capacitors C 1 -C 4  and the switch circuit  28  includes the first and second switch-sets  1 ,  2 . 
         [0092]    A first controller  100  identifies suitable residence times for each pump-state and stores those in first and second residence-time buffers  32 ,  34 . At appropriate times, a first timing circuit  36 A, which includes a clock to keep time, reads the residence-time buffers  32 ,  34  and causes the switches in the switch circuit  28  to transition at appropriate times. 
         [0093]    To determine the correct values of the residence times, the first controller  100  includes a first feedback circuit  38 A. In general, a feedback circuit will have a measured variable, and a manipulated variable that is to be manipulated in response to the measured variable in an effort to achieve some set point. For the first feedback circuit  38 A, the manipulated variable is the pair of residence times, and the measured variable includes a voltage measured at the second terminal  16 . Optionally, the measured variable for the first feedback circuit  38 A includes measurements obtained from within the charge pump  10 , hence the dotted lines within  FIG. 6 . Examples of such measurements include voltages across the switches in the first and second switch-sets  1 ,  2  or across pump capacitors C 1 -C 4 . 
         [0094]    In one embodiment, the first feedback circuit  38 A determines residence time values based on measurements taken over a sequence of cycles. The manipulated variable of the first controller  100  is chosen based on historical values. A suitable first controller  100  is a PID (proportional-integral-derivative) controller. 
         [0095]    An advantage of the first controller  100  shown in  FIG. 6  is that the frequency of the charge pump  10  is fixed. Another embodiment, shown in  FIG. 7 , features a second controller  101  that is configured to determine residence time values based on measurements obtained during the current cycle only. This allows residence time values to be determined on a cycle-by-cycle basis. As a result, the cycle length of the charge pump  10  can vary when using the second controller  101 . 
         [0096]    The second controller  101  includes a second timing circuit  36 B that is similar to first timing circuit  36 A described in  FIG. 6 . However, the second feedback circuit  38 B is implemented as a threshold logic circuit that relies on comparing voltages. 
         [0097]    A second timing circuit  36 B provides state control signals to the switch circuit  28 . During normal operation, the second timing circuit  36 B causes transitions between the first and second pump-states  18 ,  20  using nominal first and second residence times. The nominal residence times can be based on circuit analysis assuming ideal circuit elements. 
         [0098]    The second timing circuit  36 B also includes first and second skew inputs  44 ,  46  to receive corresponding first and second skew signals  48 ,  50  from the second feedback circuit  38 B. The second feedback circuit  38 B asserts one of the first and second skew signals  48 ,  50  to prematurely force the charge pump  10  to change pump-states. The second feedback circuit  38 B makes the decision to assert one of the first and second skew signals  48 ,  50  based on feedback from one or more sources. This feedback includes measurements of electrical parameters made at one or more of: the first terminal  14 , the second terminal  16 , inside the switch circuit  28 , and inside the capacitor array  26 . 
         [0099]    If the second feedback circuit  38 B does not assert either skew signal  48 ,  50 , then the second timing circuit  36 B causes the charge pump  10  to transition between its first and second pump-states  18 ,  20  according to the nominal first and second residence times. If, while the charge pump  10  is in the first pump-state  18 , the second feedback circuit  38 B presents an asserted first skew signal  48  to the first skew input  44 , the second timing circuit  36 B immediately causes the charge pump  10  to transition from the first pump-state  18  to the second pump-state  20 . Conversely, if the second feedback circuit  38 B presents an asserted second skew signal  50  to the second skew input  46  while the charge pump  10  is in the second pump-state  20 , the second timing circuit  36 B immediately causes the charge pump  10  to transition from the second pump-state  20  to the first pump-state  18 . 
         [0100]    An advantage of the second controller  101  is that it reacts immediately on a cycle-by-cycle basis. This means that the capacitors inside the capacitor array  26  can be stabilized faster. In fact, since the second controller  101  operates by prematurely terminating charge pump-states  18 ,  20 , the notion of a frequency is not well defined. 
         [0101]    Note that shortening the first residence time while keeping the second residence time constant will generally result in an upward drift and/or a reduction in the amplitude of a lower excursion of output voltage ripple. Therefore, in one example, when the second feedback circuit  38 B detects either a downward drift in the average output or an excessive lower excursion of the output ripple, it presents an asserted first skew signal  48  to the first skew input  44 , thus truncating the first pump-state  18  and shortening the first residence time. 
         [0102]    Conversely, in another example, upon detecting an upward drift and/or an excessive upward excursion of the ripple, the second feedback circuit  38 B presents an asserted second skew signal  50  to the second skew input  46 , thereby truncating the second pump-state  20  and shortening the second residence time. 
         [0103]    As noted above, the second feedback circuit  38 B receives measurements of electrical parameters from one or more locations. However, these measurements would be meaningless without some way for the second feedback circuit  38 B to know whether the measured values are normal or not. To remedy this, it is desirable to provide expected values of these electrical parameters. 
         [0104]    The thresholds provided to the second feedback circuit  38 B can be derived in many ways. One way is through analysis of an ideal circuit corresponding to the charge pump  10 . Another way is through simulation of a physical charge pump  10 . Either of these techniques can be used to provide expected values for an average output voltage (e.g., as a multiple of the input voltage) and expected maximum and minimum values of output voltage ripple about that average. The second feedback circuit  38 B uses such pre-computed values in setting the thresholds at which the skew signals  48 ,  50  are asserted. Similar logic can be used to implement the first feedback circuit  38 A discussed in connection with  FIG. 6 .  FIG. 8  shows an implementation of the second feedback circuit  38 B shown in  FIG. 7  that limits the peaks not valleys. The illustrated feedback circuit  38 B uses first and second peak-detectors to sense the peak voltage at the second terminal  16  during the first and second pump-states  18 ,  20  respectively. The first peak-detector comprises a first voltage-buffer and a first diode D 1 . The second peak-detector comprises a second voltage-buffer and a second diode D 2 . The first peak-detector stores the peak voltage during the first pump-state  18  in a first peak-storage capacitor C 1 . The second peak-detector stores the peak voltage during the second pump-state  20  in a second peak-storage capacitor C 2 . 
         [0105]    The stored peak voltages on the first and second peak-storage capacitors C 1 , C 2  can then be connected to the inputs of corresponding first and second peak-voltage comparators by closing first and second switches S 1   a , S 2   a  simultaneously. This compares the peak voltages that were stored on the first and second peak-storage capacitors C 1 , C 2  during the preceding first and second pump-states  18 ,  20 . 
         [0106]    If the peak voltage during the first pump-state  18  exceeded that of the second pump-state  20  by a first threshold V 1 , then the first peak-voltage comparator asserts the first skew signal  48 . Conversely if the peak voltage during the second pump-state  20  exceeded that of the first pump-state  18  by a second threshold V 2 , then the second peak-voltage comparator asserts the second skew signal  50 . 
         [0107]    The first and second skew signals  48 ,  50  from the second feedback circuit  38 B make their way to the second timing circuit  36 B, an implementation of which is shown in  FIG. 9 . The second timing circuit  36 B uses these first and second skew signals  48 ,  50  to generate non-overlapping signals that control the first and second switch-sets  1 ,  2 . In the illustrated embodiment, there is no gap between the two pump-states  18 ,  20 . The first pump-state  18  starts upon a transition from the second pump-state  20 , and vice-versa. 
         [0108]    In operation, the circuit shown in  FIG. 9  begins the first pump-state  18  by closing a first switch S 4 . This resets a first timing-capacitor C 4  to be low. Meanwhile, a first SR latch U 4  is in the reset state. During the first pump-state  18 , an open second switch S 3  allows a first bias-current I 3  to charge a second timing-capacitor C 3 . Eventually, the first bias-current I 3  will have deposited enough charge in the second timing-capacitor C 3  to raise its voltage beyond a first voltage-threshold V 3  at the input of a first voltage comparator. When this happens, the first voltage comparator outputs a logical high. This, in turn, sets a second SR latch U 3 , thus terminating the first pump-state  18 . Thus, in the absence of an asserted first skew signal  48 , the residence time of the first pump-state  18  depends upon the first bias-current I 3 , the capacitance of the second timing-capacitor C 3 , and the first voltage-threshold V 3 . 
         [0109]    Upon terminating the first pump-state  18 , the second pump-state  20  begins. The operation during the second pump-state  20  is similar to that described above for the first pump-state  18 . 
         [0110]    At the start of the second pump-state  20 , the first switch S 4  opens, thus allowing a second bias-current I 4  to charge the first timing-capacitor C 4 . Eventually, the second bias-current I 4  will have deposited enough charge in the first timing-capacitor C 4  to raise its voltage past a second voltage-threshold V 4  at the input of a second voltage comparator. In response to this, the second voltage comparator outputs a logical high that sets the first SR latch U 4 , thus terminating the second pump-state  20 . During the second pump-state  20 , the second timing-capacitor C 3  is reset low when the second switch S 3  is closed, and the second SR latch U 3  is in the reset state. In the absence of an asserted second skew signal  50 , the residence time of the second pump-state  20  is set by the second bias-current I 4 , the capacitance of the first timing-capacitor C 4 , and the second voltage-threshold V 4 . 
         [0111]    The first skew signal  48  and the output of the first voltage comparator are inputs to a first OR-gate. Thus, the first pump-state  18  can be terminated in two ways. In the first way, already described above, the first pump-state  18  lasts for its nominal residence time and terminates once enough charge has accumulated in the second timing-capacitor C 3 . However, while the second timing-capacitor C 3  is still being filled with charge, the second feedback circuit  38 B may assert the first skew signal  48 , thus bringing the first pump-state  18  to a premature end. 
         [0112]    It will be apparent from the symmetry of the circuit shown in  FIG. 9  that the second pump-state  20  can be truncated in the same way by assertion of the second skew signal  50 . The second feedback circuit  38 B is thus able to shorten the first residence time relative to the second by asserting the first skew signal  48  but not the second skew signal  50 . 
         [0113]    After each comparison of the peak voltage in the first and second pump-states  18 ,  20 , the first and second peak-storage capacitors C 1 , C 2  of the second feedback circuit  38 B are reset by closing third and fourth switches S 1   b , S 2   b  and opening the first and second switches S 1   a , S 2   a . Also, the voltage buffers that sense the voltage at the second terminal  16  can be disabled or tri-stated while the first and second peak-storage capacitors C 1 , C 2  are reset. Each sample-compare-reset cycle can occur once per charge pump cycle or once per set of multiple consecutive charge pump cycles. 
         [0114]    In the methods described above, there have been only two pump-states  18 ,  20  and two residence times. However, the principles described are not limited to merely two pump-states  18 ,  20 . For example, it is possible to implement a dead time interval during which the charge pump  10  is not doing anything. This dead time interval can be used in connection with the embodiment described in  FIG. 7  to cause fixed frequency operation. To do so, the dead time interval is set to be the difference between a nominal charge pump period and the sum of the first and second pump-state intervals. 
         [0115]      FIG. 10  shows one implementation for carrying out a three-state charge pump that defines a dead time as its third state. The embodiment shown in  FIG. 10 , features a third controller  102  that uses a third feedback circuit  38 C connected to a third timing circuit  36 C to exercise control over only a second residence time in the second residence-time buffer  34 , and not the first residence time. In this embodiment, the first residence time is always set to some nominal value. The third controller  102  features an input from the switch circuit  28  that provides information on the state of the first switch-set  1 . Based on this information, if the third controller  102  determines that the switches in the first switch-set  1  are open, it has two choices. The first choice is to close the switches in the second switch-set  2 . This initiates the second residence time. The second choice is to leave the switches in the second switch-set  2  open. This initiates a dead-time interval. For proper operation, the first and second residence times must be non-zero. 
         [0116]    The dead-time interval is an example of a third pump-state in which no charge transfer occurs. However, it is also possible to operate a charge pump in three or more states, each one of which permits charge transfer between capacitors. An example of such multi-state charge pump control is given in U.S. Provisional Application 61/953,270, in particular, beginning on page  11  thereof, the contents of which are herein incorporated by reference. 
         [0117]    The rate at which charge accumulates on a capacitor depends on the current and the amount of time the current is allowed to flow. The methods disclosed thus far manage charge accumulation by controlling the second of these two parameters: the amount of time current is allowed to flow. However, it is also possible to control the first of these two parameters, namely the amount of current that flows. Embodiments that carry out this procedure are shown in  FIGS. 11 and 12 . 
         [0118]      FIG. 11  shows a fourth controller  103  similar to the second controller  101  shown in  FIG. 7  but with no connection between a fourth feedback circuit  38 D and a fourth timing circuit  36 D. Thus, unlike the second controller  101 , the fourth controller  103  does not vary the first and second residence times. Instead, the fourth feedback circuit  38 D of the fourth controller  103  adjusts the current drawn by the load  12 , such as an IDAC within a LED driver, while allowing the first and second residence intervals to be derived from a constant clock signal CLK. The fourth feedback circuit  38 D makes the decision on an extent to which to vary the current drawn by the load  12  based on feedback measurements from one or more sources. These include measurements of electrical parameters made at one or more of the first terminal  14 , the second terminal  16 , inside the switch circuit  28 , and inside the capacitor array  26 . 
         [0119]      FIG. 12  shows a fifth controller  104  that is similar to the fourth controller  103  except that instead of controlling current drawn by a load  12 , the fifth controller  104  controls current through a regulator  56 , which is modeled in the illustrated circuit as a current source. In the fifth controller  104 , a fifth timing circuit  36 E responds only to a clock signal CLK. A fifth feedback circuit  38 E decides how much to vary the current through the regulator  56  based on feedback measurements from one or more sources. These include measurements of electrical parameters made at one or more of the first terminal  14 , the second terminal  16 , inside the switch circuit  28 , and inside the capacitor array  26 . 
         [0120]    The control methods described above are not mutually exclusive. As such, it is possible to implement hybrid controllers that implement two or more of the control methods described above. 
         [0121]    One reason that charge accretion/depletion becomes a problem is that, as a practical matter, it is next to impossible to manufacture pump capacitors C 1 -C 4  that all have the same desired capacitance. Referring now to  FIG. 13 , a remedy for this is to compensate for an error in the value of a pump capacitor&#39;s capacitance by switching other capacitors in series or in parallel with that pump capacitor. These capacitors are referred to as “trim” capacitors because they trim a capacitance to a desired value. The term “trim” is not be construed as “reducing” but rather in the sense of making fine adjustments in any direction in an effort to attain a desired value. Capacitance of a pump capacitor can be raised or lowered by connecting another capacitor in parallel or in series respectively. 
         [0122]      FIG. 13  shows a trim-capacitor network  70  having two trim capacitors C 5 , C 6 , either one of which can be placed in parallel with the fourth pump capacitor C 4 . Although only two trim capacitors C 5 , C 6  are shown, a practical trim-capacitor network  70  has an assortment of capacitors with various values that can be selectively switched in series or in parallel with the fourth pump capacitor C 4 . The illustrated trim-capacitor network  70  is shown connecting one trim capacitor C 6  in parallel with the pump capacitor C 4 , thus raising the effective capacitance of the combination. Only two trim capacitors C 5 , C 6  are shown for clarity. However, it is a simple matter to add more, thus allowing greater variability in adjustment. In addition, for the sake of simplicity, the trim-capacitor network  70  shown only places trim capacitors C 5 , C 6  in parallel. However, it is a relatively simple matter to design a circuit to switch trim capacitors C 5 , C 6  in series with the fourth pump capacitor C 4 . Additionally, in  FIG. 13 , a trim-capacitor network  70  is shown only for the fourth pump capacitor C 4 . In practice, each pump capacitor C 1 -C 4  would have its own trim-capacitor network  70 . 
         [0123]    By switching in the proper combination of trim capacitors in the trim-capacitor network  70 , the overall capacitance of the pump capacitor C 4  combined with that of the trim capacitors C 5 , C 6  can be made to approach or even equal a target value. This trimming procedure may only need to be carried out once in the lifetime of the charge pump  10  or can be carried out during normal operation because the capacitance of practical capacitors normally vary with the voltage across their terminals as well as temperature. 
         [0124]    Rather than being used once to adjust for manufacturing errors, a trim-capacitor network  70  as shown can also be used during operation of the circuit as a way to control the quantity of charge on a particular pump capacitor C 4  by transferring charge between a particular capacitor, e.g. the pump capacitor C 4 , and some other charge repository, such as a trim capacitor C 5 , C 6  within the trim-capacitor network  70 , or to the ultimate repository, which is ground. This provides an alternative way to adjust the charge on each capacitor in an effort to restore all pump capacitors to their respective initial voltages at the start of a charge pump cycle. 
         [0125]    Alternately, a current sink could be coupled to each pump capacitor C 1 -C 4  allowing it to bleed any excess charge to another location or multiple locations, such as the first terminal  14 , the second terminal  16 , a terminal inside the switch circuit  28 , a terminal inside the capacitor array  26 , and even ground. 
         [0126]    Another use for the trim-capacitor network  70 , shown in  FIG. 14 , is to act as a stabilizing capacitance between the charge pump  10  and the load  12 . To reduce losses, the stabilizing capacitance is preferably just sufficient to stabilize the charge pump  10 . A larger stabilizing capacitance value than necessary may increase power loss during charge pump operation. Because of manufacturing tolerances, it will, in general, not be possible to either predict the required value of the stabilizing capacitance or, even if a prediction were available, to ensure that it has the required value over all operating conditions. Thus, one can use a technique similar to that described in connection with  FIG. 13  to switch a selected trim capacitor C 5 , C 6  from the trim-capacitor network to act as a stabilizing capacitance. 
         [0127]    The charge pump  10  can be implemented using many different charge pump topologies such as Ladder, Dickson, Series-Parallel, Fibonacci, and Doubler. Similarly, suitable converters for the regulator  56  and for the load  12  when implemented as a regulator include: Buck converters, Boost converters, Buck-Boost converters, non-inverting Buck-Boost converters, Cuk converters, SEPIC converters, resonant converters, multi-level converters, Flyback converters, Forward converters, and Full Bridge converters. 
         [0128]    Having described the invention, and a preferred embodiment thereof, what is claimed as new, and secured by letters patent is: