PATENT ABSTRACT
An apparatus having a power converter circuit having a first active layer having a first set of active devices disposed on a face thereof, a first passive layer having first set of passive devices disposed on a face thereof, and interconnection to enable the active devices disposed on the face of the first active layer to be interconnected with the non-active devices disposed on the face of the first passive layer, wherein the face on which the first set of active devices on the first active layer is disposed faces the face on which the first set of passive devices on the first passive layer is disposed.

PATENT DESCRIPTION
RELATED APPLICATIONS 
     This application claims the benefit of the priority date of U.S. application 61/548,360, filed on Oct. 18, 2011, the contents of which are herein incorporated by reference. 
    
    
     FIELD OF DISCLOSURE 
     The present invention relates to energy storage elements in power converters that use capacitors to transfer energy. 
     BACKGROUND 
     Power converters generally include switches and one or more capacitors, for example, to power portable electronic devices and consumer electronics. A switch-mode power converter is a specific type of power converter that regulates its output voltage or current by switching storage elements (i.e. inductors and capacitors) into different electrical configurations using a switch network. 
     One type of switch-mode power converter is the switched capacitor converter. A switched capacitor converter uses capacitors to transfer energy. As the transformation ratio increases, the number of capacitors and switches increases. 
     A switch capacitor converter includes a switch network containing numerous switches. These switches are active devices that are usually implemented with transistors. The switch network can be integrated on a single or on multiple monolithic semiconductor substrates. 
     Typical power converters perform voltage transformation and output regulation. In many power converters, such as a buck converter, this is carried out in a single stage. However, it is also possible to split these two functions into two specialized stages. Such two-stage power converter architectures feature a transformation stage and a separate regulation stage. The transformation stage transforms one voltage into another, while the regulation stage ensures that the voltage and/or current output of the transformation stage maintains desired characteristics. 
     An example of a two-stage power converter architecture is illustrated in  FIG. 1A , where capacitors are utilized to transfer energy. The transformation stage is represented by a switched-capacitor element  12 A, which closely resembles a switched capacitor converter while the regulation stage is represented by a regulating circuit  16 A. 
     In this architecture, a switched capacitor element  12 A is electrically connected to a voltage source  14  at an input end thereof An input of a regulating circuit  16 A is electrically connected to an output of the switched capacitor element  12 A. A load  18 A is then electrically connected to an output of the regulating circuit  16 A. Such a converter is described in US Patent Publication 2009/0278520, filed on May 8, 2009, the contents of which are herein incorporated by reference. Furthermore, a modular multi-stage power converter architecture was described in PCT Application PCT/2012/36455, filed on May 4, 2012, the contents of which are also incorporated herein by reference. 
     The switched capacitor element  12 A and regulating circuit  16 A can be mixed and matched in a variety of different ways. This provides a transformative integrated power solution (TIPS™) for the assembly of such converters. As such, the configuration shown in  FIG. 1A  represents only one of multiple ways to configure one or more switched capacitor elements  12  with one or more regulating circuits  16 A. 
     Typically, the switch network of the switched capacitor element  12 A and the regulating circuit  16 A are fabricated in a semiconductor process that has passive devices. However, these passive devices are normally used in the analog circuitry to control the power converter. They are not normally used to store energy in the power converter. This is because these passive devices cannot efficiently store a significant amount of energy. 
     These passive devices are usually planar and fabricated after the active devices in a higher level of metal to reduce parasitic effects. Since these passive devices are fabricated after the active devices, and on the same wafer as the active devices, the processing steps for making these passive devices should be chosen carefully. An incorrect choice may damage the active devices that have already been fabricated. 
     To avoid possibly damaging the active devices during fabrication of the passive devices, it is preferable to only use CMOS compatible processing. Given this processing requirement, it is difficult and/or expensive to achieve high capacitance density capacitors or high Q inductors in a CMOS flow. Therefore, in power converters, it is common practice to store energy in discrete components, such as multilayer ceramic capacitors and chip inductors. However, it is possible to produce inexpensive high performance passive devices in their own wafer and process flow that can be used in specific applications. These devices will be referred to as integrated passive devices (IPDs). 
     An implementation of the power converter architecture shown in  FIG. 1A  is illustrated in  FIG. 1B-1D . 
     In the embodiment shown in  FIG. 1B , a power converter  20  draws energy from a voltage source  14  at a high input voltage VIN and delivers that energy to a load  18 A at a low output voltage VO. Without loss of generality, the load  18 A is modeled as a resistor. 
     The power converter  20  includes a switched capacitor element  12 A that features a 3:1 series-parallel switched capacitor network having power switches S 1 -S 7  and pump capacitors C 21 -C 22 . In contrast, the regulating circuit  16 A is a buck converter having first and second output power switches SL, SH, a filter inductor L 1 , and an output capacitor CO. The power switches S 1 -S 7 , the output power switches SL, SH, and the driver/control circuitry  23  are integrated in a single semiconductor die  22 . However, the pump capacitors C 21 -C 22 , the filter inductor L 1 , and a decoupling input capacitor CIN 1  are discrete components. 
     In operation, the power switches S 1 , S 3 , S 6  and the power switches S 2 , S 4 , S 5 , S 7  are always in complementary states. Thus, in a first switch state, the power switches S 1 , S 3 , S 6  are open and the power switches S 2 , S 4 , S 5 , S 7  are closed. In a second switch state, the power switches S 1 , S 3 , S 6  are closed and the power switches S 2 , S 4 , S 5 , S 7  are open. Similarly, the output power switches SL, SH are in complementary states. 
     Typically, the regulating circuit  16 A operates at higher switching frequencies than the switched capacitor element  12 A. However, there is no requirement of any particular relationship between the switching frequencies of the regulating circuit  16 A and the switching frequency of the switched capacitor element  12 A. The driver/control circuitry  23  provides the necessary power to activate the switches and controls the proper switch states to ensure a regulated output voltage VO. 
     In power converters, it is common practice to solder a semiconductor die  22  or packaged die to an electrical interface  28 , and to then horizontally mount capacitors and inductors on the electrical interface  28  around the semiconductor die  22 . Such an arrangement is shown in a top view in  FIG. 1D  and in a side view in  FIG. 1C  taken along a line  24  in  FIG. 1D . 
     An electrical interface  28  provides electrical conductivity between the power converter  20  and a load to which the power converter  20  is ultimately supplying power. Examples of electrical interfaces  28  include printed circuit boards, package lead frames, and high density laminates. 
     The discrete components in the power converter  20  include the pump capacitors C 21 -C 22 , the input capacitor CIN 1 , the output capacitor CO, and the filter inductor L 1 . These discrete components are horizontally disposed with respect to the semiconductor die  22  and electrically coupled to the die  22  by traces on the electrical interface  28 . 
     Each power switch in the power converter  20  is typically composed of numerous smaller switches connected in parallel as illustrated by the close-up  26  in  FIG. 1D . This allows the power switches to carry a large amount of current without overheating. 
     SUMMARY 
     In one aspect, the invention features an apparatus including a power converter circuit, the power converter circuit including a first active layer having a first set of switching devices disposed on a face thereof, a first passive layer having first set of passive devices disposed on a face thereof, and interconnection to enable the switching devices disposed on the face of the first active layer to be interconnected with the non-active devices disposed on the face of the first passive layer, wherein the face on which the first set of switching devices on the first active layer is disposed faces the face on which the first set of passive devices on the first passive layer is disposed. 
     In some embodiments, the face on which the first set of switching devices on the first active layer is disposed faces the face on which the first set of passive devices on the first passive layer is disposed. 
     In some embodiments, the interconnection to enable the switching devices disposed on the face of the first active layer to be interconnected with the passive devices disposed on the face of the first passive layer includes a thru via extending through at least one of the first active layer and the first passive layer. Among these embodiments are those in which the interconnection to enable the switching devices disposed on the face of the first active layer to be interconnected with the passive devices disposed on the face of the first passive layer further includes an interconnect structure connected to the thru via and to one of the first active layer and the first passive layer. 
     In other embodiments, the power converter circuit further includes one or more additional layers. Among these embodiments are those in which the one or more additional layers comprise a second passive layer containing a second set of passive devices, those in which the one or more additional layers includes a second active layer containing a second set of switching devices, and those in which the one or more additional layers comprise a second layer having a face on which a third set of devices is disposed and a third layer having a face on which a fourth set of devices is disposed, and wherein the face on which the fourth set of devices is disposed faces the face on which the third set of devices is disposed. 
     Also among the embodiments are those in which the first passive layer includes an energy-storage element. Among these are those in which the energy-storage element includes a capacitor. In some of these embodiments, the capacitor includes a planar capacitor, whereas in others, the capacitor includes a trench capacitor. 
     Some embodiments include an electrical interface, and a connection between the electrical interface and the first active layer of the circuit. Others include an electrical interface, and a connection between the electrical interface and the first non-active layer of the circuit. 
     In some embodiments, the power converter circuit further includes vias extending through the first active layer. Among these are embodiments in which the power converter circuit further includes vias extending through the first passive layer. 
     Also included among the embodiments of the invention are those in which the power converter circuit further includes additional layers, wherein the additional layers comprise a second active layer and a third active layer, the apparatus further including a thru via connected the second active layer and the third active layer. 
     In addition to all the foregoing embodiments, additional embodiments of the invention are those in which the power converter circuit further includes additional layers, wherein the additional layers comprise a second passive layer and a third passive layer, the power converter circuit further including a thru via providing an electrical connection between the second passive layer and the third passive layer. 
     The power converter circuit can implement any power converter circuit. In one embodiment, the power converter circuit implements a buck converter. In another embodiment, the power converter circuit implements a switched-mode power converter circuit. 
     In some embodiments, the first passive layer includes capacitors. Among these embodiments are those that further include an electrical interface and solder bumps connecting the power converter circuit to the electrical interface, wherein the solder bumps are disposed according to a solder bump pitch, and wherein the interconnection has an interconnection pitch, the interconnection pitch being smaller than the solder bump pitch, as well as those in which at least one of the capacitors is sized to fit at least one of above a switching device in the first active layer and below a switching device in the first active layer. 
     In some embodiments, the electrical interconnect includes a multilayer interconnect structure. 
     Other embodiments include a driver and control unit to provide power and to control the switching devices. 
     In some embodiments, the apparatus also includes a data processing unit and a touch-screen interface, both of which are configured to consume power provided by said switched mode power converter circuit. Among these are embodiments that also include a wireless transmitter and receiver, all of which are configured to consume power provided by said switched mode power converter circuit. Examples of such embodiments are smart phones, tablet computers, laptop computers, and other portable electronic devices. 
     In another aspect, the invention features an apparatus including passive layers, active layers, thru vias, and at least one interconnection layer. The interconnection layer provides electrical connection between an active layer and a passive layer. The thru vias provide electrical connection between two or more active layers, or between two or more passive layers. 
     In another aspect, the invention features an apparatus having a power converter circuit including a stack of layers, the stack including an active layer having active devices integrated on a device face thereof and a passive layer having passive devices integrated on a device face, thereof. Either an active device or a passive device is partitioned into at least two partitions. Each partition defines a current channel along a first axis, The partitioned component thus suppresses current flow along a second axis orthogonal to the first axis. 
     In some embodiments, the passive devices include a planar capacitor. 
     Other embodiments include a regulating circuit having a first regulating circuit partition and a second regulating circuit partition. The regulating circuit is connected to receive an output from the power converter circuit. The embodiment also includes a first inductor having a first terminal and a second terminal, the first terminal being connected to an output of the first regulating circuit partition, and the second terminal being connected to a load, a second inductor having a first terminal and a second terminal, the first terminal being connected to an output of the second regulating circuit partition, and the second terminal being connected to the second terminal of the first inductor, whereby in operation, the second terminal of the first inductor and the second terminal of the second inductor are at a common potential. Among these embodiments are those that include a load connected to the second terminal of the first inductor and the second terminal of the second inductor. 
     In some embodiments, the first switched capacitor unit is positioned over the first regulating circuit partition at a location that minimizes an extent to which current travels between the power converter circuit and the first regulating circuit partition. 
     These and other features of the invention will be apparent from the following description and the accompanying figures in which: 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  is a block diagram of a known power converter architecture; 
         FIG. 1B  is a particular implementation of the converter architecture shown in  FIG. 1 ; 
         FIG. 1C  is a side view of the power converter illustrated in  FIG. 1B ; 
         FIG. 1D  is a top view of the power converter illustrated in  FIG. 1B ; 
         FIGS. 2A-2C  are side views of various power converters with integrated capacitors; 
         FIG. 3A  is a circuit diagram of a power converter with integrated capacitors; 
         FIG. 3B  is a top view of one layout of the power converter whose circuit is shown in  FIG. 3A ; 
         FIG. 4  is a circuit diagram of a four-level buck converter with integrated capacitors; 
         FIG. 5  is a side view of a power converter with generic device layers; 
         FIGS. 6A-6C  are three side views of a power converter in which an active device layer is between a passive device layer and an electrical interface; 
         FIGS. 6D-6F  are three side views of a power converter in which a passive device layer is between an active layer and the electrical interface; 
         FIG. 7A  is a side view of a power converter with a planar capacitor; 
         FIG. 7B  is a side view of a power converter with a trench capacitor; 
         FIG. 8A  is a particular implementation of the power converter shown in  FIG. 6B ; 
         FIG. 8B  is a particular implementation of the power converter shown in  FIG. 6A ; 
         FIG. 9A  shows a parasitic network between the active and passive layer with one node; 
         FIG. 9B  shows a parasitic network between the active and passive layer with three nodes; 
         FIG. 10A  is a block diagram of a partitioned power converter; 
         FIG. 10B  is a top view of a particular implementation of the partitioned power converter shown in  FIG. 10A ; 
         FIG. 10C  is a close up of a switched capacitor unit from the partitioned power converter implementation illustrated in  FIG. 10B ; 
         FIG. 10D  is a close up of one switch from the switched capacitor unit illustrated in  FIG. 10C ; 
         FIG. 11  is a top view of an alternative implementation of the partitioned power converter shown in  FIG. 10A . 
     
    
    
     DETAILED DESCRIPTION 
     Power converters that use capacitors to transfer energy have certain disadvantages when packaged in the traditional way. Such power converters require a larger number of components and a larger number of pins than conventional topologies. For example, power converter  20  requires two additional capacitors and four additional pins when compared to a buck converter. 
     Furthermore, extra energy is lost due to parasitic losses in the interconnection structure between the additional capacitors and the devices in the switch network. The devices and methods described herein address these issues by vertically integrating the passive devices with the active devices within a power converter. 
     Embodiments described herein generally include three components: a passive device layer  41 A, also referred to a “passive layer”, an active device layer  42 A, also referred to as an “active layer”, and an interconnect structure  43 B. Each layer has devices that will typically be integrated on a single monolithic substrate or on multiple monolithic substrates, both of which may also be incorporated within a reconstituted wafer as in the case of fan-out wafer scale packaging. The passive layer  41 A can be fabricated by an IPD process while the active layer  42 A can be fabricated by a CMOS process. Each device layer pair is electrically connected together through a high density interconnect structure, which may also include a redistribution layer or micro bumps. 
     Additionally, thru vias  47 A can be included which allow electrical connections to additional device layers. In the case of a single monolithic substrate, the thru vias may include thru silicon vias, whereas in the case of a reconstituted wafer, the thru vias may include thru mold vias. 
     Side views of three different embodiments with thru vias  47 A are illustrated in  FIGS. 2A-2C . These are only a few of the possible permutations. Each side-view includes at least a passive layer  41 A, an active layer  42 A, thru vias  47 A, and an interconnect structure  43 B. 
     The passive layer  41 A includes passive devices such as capacitors, inductors, and resistors. The active layer  42 A includes active devices such as transistors and diodes. The interconnect structure  43 B provides electrical connections between the passive layer  41 A and the active layer  42 A. Meanwhile, thru vias  47 A allow for electrical connections to pass thru the passive layer  41 A or thru the active layer  42 A. 
     The interconnect structure  43 B can also provide electrical connection between devices on the same layer. For example, separate active devices in different locations on the active layer  42 A can be electrically connected using the interconnect structure  43 B. 
     In the particular embodiment shown in  FIG. 2A , the passive layer  41 A is between the active layer  42 A and the electrical interface  28 . An interconnect structure  43 B provides interconnections between devices on the active layer  42 A and devices on the passive layer  41 A. The interconnect structure  43 B in some cases can also provide electrical connections between two devices that are on the same passive layer  41 A or two devices on the same active layer  42 A. Each device layer  41 A,  42 A has a device face on which the devices are actually formed. The locations of these device faces are indicated by the pair of arrows. 
     In the embodiment of  FIG. 2A , the device face on the active layer  42 A faces, or is opposed to, the device face on the passive layer  41 A. Thru vias  41 A cut through the passive layer and connect to the interconnect structure  43 B. Thus, the path between devices on layers separated by intervening layers generally includes at least a portion through an interconnect structure  34 B and a portion through a via  41 A. In this way, the interconnect structure  34 B provides electrical continuity between devices in different layers, whether the layers are adjacent or otherwise. 
     In the alternative embodiment shown in  FIG. 2B , the active layer  42 A is between the passive layer  41 A and the electrical interface  28 . Thru vias  42 A in this case pass through the active layer  42 A. Once again, an interconnect structure  43 B connects the passive devices on the passive layer  41 A, the active devices on the active layer  42 A, and the thru vias  47 A. Once again, as indicated by the arrows, the device face of the passive layer  41 A and the device face of the active layer  42 A are opposite each other. 
     As shown in yet another embodiment in  FIG. 2C , it is also possible to use more than two device layers by stacking one or more passive layers and one or more active layers. In the particular embodiment shown in  FIG. 2C , such a stack includes first and second passive layers  41 A- 41 B capped by an active layer  42 A. The embodiment further includes a first interconnect structure  43 B between the first and second passive layers  41 A,  41 B and a second interconnect structure  43 C between the second passive layer  41 B and the active layer  42 A. As indicated by the arrows, the device faces of the second passive layer  41 B and the active layer  42 A face each other, but the device faces of the first and second passive layers  41 A,  41 B do not. 
     The embodiment shown in  FIG. 2A-2C  can be used to eliminate the pin count penalty in power converter  20  shown in  FIG. 1B . 
     As illustrated in  FIG. 3A , the discrete capacitors C 21 , C 22 , CIN 1  in the power converter  20  are replaced by integrated capacitors C 31 , C 32 , CIN 2  respectively that are all placed on a passive layer  41 A (not shown). Meanwhile, the active devices S 1 -S 7 , SL-SH, and control circuit  23  are all included in a separate active layer  42 A that would be stacked relative to the passive layer as suggested by  FIGS. 2A-2C . The resulting power converter  30 A has three fewer discrete capacitors and four fewer pins than the power converter  20 . 
     A top view of the power converter  30 A in  FIG. 3B  illustrates the disposition of active and passive devices on separate layers coplanar with an xy plane defined by the x and y axes shown and stacked along a z axis perpendicular to the xy plane. The capacitors C 31 , C 32 , CIN 2  are disposed on a device face of a passive layer over a device face of an active layer, on which are formed active devices S 1 -S 7 . 
     Each capacitor is arranged such that it is directly above the particular active device to which it is to be electrically connected. For example, a first capacitor C 31  is directly above switches S 1 -S 4 . This is consistent with  FIG. 3A , which shows that the positive terminal of the first capacitor C 31  is to be connected to first and second switches S 1 , S 2  while the negative terminal of the first capacitor C 31  is to be connected to third and fourth switches S 3 , S 4 . This arrangement shortens the distance current needs to flow between the active devices and the passive devices in comparison to the arrangement illustrated in  FIG. 1B-1D , thereby reducing the energy loss. 
       FIG. 3B  shows another power converter  30 B, often referred to as a four-level flying capacitor buck converter. It is a particular implementation of a multi-level buck converter. Other examples include three-level fly capacitor buck converters and five-level capacitor buck converters. Such power converters incorporate a switched-capacitor circuit and can readily be implemented using stacked layers as illustrated in  FIGS. 2A-2C . 
     If the power converter  30 B is implemented using the embodiment illustrated in  FIG. 2A , then the device stack  33 B includes a top active layer  42 A and a bottom passive layer  41 A. The active devices S 31 -S 36  are included in the active layer  42 A, while the fly capacitors C 3 A-C 3 B are included in the passive layer  41 A. The fly capacitors C 3 A-C 3 B are vertically disposed below the active devices S 31 -S 36  to reduce the energy loss in the electrical interconnection. 
     In operation, the input voltage VIN is chopped using the active devices S 31 -S 36  and the two fly capacitors C 3 A-C 3 B. This results in a pulsating voltage at an output node LX. This pulsating voltage is presented to an LC filter represented by a filter inductor L 31  and a load capacitor CL, thereby producing an output voltage VO, which is the average of the voltage at the LX node. 
     In the remaining description of  FIG. 4 , the power converter  30 B is assumed to be connected to a 12 volt source  14  and to provide 4 volts to the load  18 A. The power converter  30 B is in one of eight different states. Depending upon the state, the voltage at the output node LX is 12 volts, 8 volts, 4 volts or 0 volts, assuming that the first fly capacitor C 3 A is charged to 8 volts and that the second fly capacitor C 3 B is charged to 4 volts. 
     The power converter  30 B alternates between combinations of the states depending upon the desired output voltage VO. Additionally, the duration of time the power converter  30 B is in each state enables regulation of the output voltage VO. It is important to note that the power converter  30 B always operates such that the fly capacitors C 3 A-C 3 B are charged as much as they are discharged. This maintains a constant average voltage across the fly capacitors C 3 A-C 3 B. 
     A generalization of the embodiments illustrated in  FIGS. 2A-2C  is illustrated in  FIG. 5 , which includes four device layers  44 A- 44 D. In general, at least two device layers are required, one of which includes active devices and the other of which includes passive devices. Typically, the pitch of the interconnect structure  43 A- 43 D is finer than the pitch of the bumps  45 , such as solder balls, gold studs, and copper pillars, that couple the power converter to the electrical interface  28 . The individual capacitors in the layer with passive devices are sized and arranged so as to fit above or below one or more active devices. Furthermore, the switched capacitor elements are also partitioned and laid out in a specific way to reduce parasitic energy loss in the interconnect structures. 
     Since semiconductor processing is sequential, it is common to only process one side of a wafer. This adds one more dimension to the number of possible permutations. Assuming there is one active layer  42 A, one passive layer  41 A, one device face per layer, and thru vias  47 A, there are a total of eight different ways of arranging the two layers. 
       FIGS. 6A-6C  and  FIG. 2A  illustrate the four possible combinations in which the passive layer  41 A is on top and the active layer  42 A is on the bottom. As used herein, a “bottom” layer is the layer closest to the electrical interface and the “top” layer is the layer furthest from the electrical interface. 
     In  FIG. 6A , the interconnect structure  43 A electrically connects the active devices in layer  42 A to thru vias  47 A and bumps  45 . Similarly, the interconnect structure  43 B electrically connects the passive devices in layer  41 A to thru vias  47 A. As indicated by the arrows, the device faces of the passive and active layers  41 A,  42 A face away from each other. 
     In  FIG. 6B , the interconnect structure  43 B electrically connects the active devices in layer  42 A to thru vias  47 A and thru vias  47 B. Similarly, the interconnect structure  43 C electrically connects the passive devices in layer  41 A to thru vias  47 B. As indicated by the arrows, the device faces of the passive and active layers  41 A,  42 A face away from each other. 
     Lastly, in  FIG. 6C , the interconnect structure  43 A electrically connects the active devices in  42 A to thru vias  47 A and bumps  45 . Similarly, the interconnect structure  43 C electrically connects the passive devices in layer  41 A to thru vias  47 B. As indicated by the arrows, the device faces of the passive and active layers  41 A,  42 A, face away from each other. 
     In comparison,  FIGS. 6D-6F  and  FIG. 2B  illustrate the four possible combinations in which the active layer  42 A is on top and the passive layer  41 A is on the bottom. 
     In  FIGS. 6D-6F , the active layer  42 A and the passive layer  41 A are electrically connected together as described in connection with  FIGS. 6A-6C . The choice of configuration depends upon numerous factors, most of which relate to thru via technology and to the number of pins to the outside world. For example, if there are a larger number of electrical connections between the passive layer  41 A and active layer  42 A than to the outside world than the configurations illustrated in  FIG. 2A  &amp;  FIG. 2B  are more desirable. However, if the opposite is true than the configurations illustrated in  FIG. 6A  and  FIG. 6D  are more desirable. 
     The passive substrate and active substrate can be in any form when attached, such as singulated dice or full wafers. Two different implementations that are amenable to die-to-die attachment are shown in  FIGS. 7A-7B . Each implementation includes a different type of capacitor. 
     The capacitors can be of any structure. However, trench capacitors have a capacitance per unit area that is one to two orders of magnitude higher than that of an equivalent planar capacitor, and also have lower equivalent series resistance than equivalent planar capacitors. Both of these capacitor attributes are desirable for use in power converters that use capacitive energy transfer because they favorably affect the efficiency of the power converter. 
     In the embodiment shown in  FIG.7A , the passive layer  41 A includes a planar capacitor  71 A and the active layer  42 A includes active devices  75 . In contrast, the embodiment shown in  FIG. 7B , includes a trench capacitor  71 B in its passive layer  41 A. 
     The interconnect structure  43 B electrically connects the devices within the passive layer  41 A to the devices within the active layer  42 A. The interconnect structure  43 B can be implemented in numerous ways, one of which are illustrated in  FIGS. 7A and 7B . 
     In the case of  FIGS. 7A-7B , the interconnect structure  43 B is composed of a multilayer interconnect structure  72  on the passive substrate, a single layer of solder bumps  73 , and a multilayer interconnect structure  70  on the active substrate. 
     The bumps  45  are not visible in  FIGS. 7A-7B  because their pitch on the electrical interface  28  is typically much larger than the interconnect structure  43 B. However, to connect to the outside world, some form of connection, such as bumps  45  along with thru vias  47 A, is useful. 
     The bumps  45  can either be located above the passive layer  41 A or below the active layer  42 A. In the case in which the bumps  45  are located above the passive layer  41 A, the thru vias cut  47 A through the passive layer  41 A as illustrated in  FIG. 2B . In the case in which the bumps  45  are located below the active layer  42 A, the thru vias  47 A cut through the active layer  42 A as illustrated in  FIG. 2A . 
     Embodiments of this invention can also be implemented with wafer-to-wafer stacking as shown in  FIGS. 8A-8B . The embodiment illustrated in  FIG. 8A  is a particular implementation of  FIG. 6B , whereas, the embodiment illustrated in  FIG. 8B  is a particular implementation of  FIG. 6A . 
     The two wafers are electrically connected together using a bonding layer  83  instead of using solder bumps  73  as in the case of  FIGS. 7A-7B . There are numerous types of wafer-to-wafer bonding process. Among these are copper-copper bonding, oxide-oxide bonding, and adhesive bonding. Furthermore,  FIGS. 8A-8B  illustrate the thru vias  47 A and their respective bumps  45 , which were absent in  FIGS. 7A-7B . 
     Power converters that rely on capacitors to transfer energy generally have complex networks with many switches and capacitors. The sheer number of these components and the complexity of the resulting network make it difficult to create efficient electrical interconnections between switches and capacitors. 
     Typically, metal layers on an integrated circuit or on integrated passive device are quite thin. Because thin metal layers generally offer higher resistance, it is desirable to prevent lateral current flow. This can be accomplished by controlling the electrical paths used for current flow through the power converter. To further reduce energy loss resulting from having to traverse these electrical paths, it is desirable to minimize the distance the current has to travel. If properly done, significant reductions energy loss in the interconnect structure can be realized. This is accomplished using two techniques. 
     One way to apply the foregoing techniques to reduce interconnection losses is to partition the switched capacitor element  12 A into switched capacitor units operated in parallel, but not electrically connected in parallel. Another way is to choose the shape and location of the switches on the die to fit optimally beneath the capacitors and vice versa. 
     Partitioning the SC element  12 A is effective because it reduces the horizontal current flow that has always been seen as inevitable when routing physically large switches and capacitors to a single connection point or node as depicted in  FIG. 9A . 
     As is apparent from  FIG. 9A , current in a physically large component will tend to spread out across the component. To the extent it spreads in the lateral direction, its path through the material becomes longer. This is shown in  FIG. 9A  by noting the difference between the path length between the two nodes through the center switch and the path length between the two nodes through the lateral switches. This additional path length results in loss, represented in the equivalent circuit by RP 1 . 
     By partitioning the component into smaller sections, one can equalize the path length differences between the two nodes, thus reducing associated losses. For example, if the switch and the capacitor in  FIG. 9A  are partitioned into three sections, the equivalent circuit is approximately that shown in  FIG. 9B , in which the lumped resistances associated with the path between nodes is represented by a smaller lumped resistance RP 2 . 
       FIGS. 10A-10D  illustrate the application of both of these techniques to the implementation of a power converter. 
     As shown in  FIG. 10A , the regulating and switching components of a power converter  90  are partitioned to encourage a more direct electrical path between them, and to minimize any lateral current flow. In the particular example of  FIG. 10A , the power converter  90  includes a switched capacitor unit  92 A connected to a regulating circuit unit  94 A at a first node VX 1 , a switched capacitor unit  92 B connected to regulating circuit unit  94 B at a second node VX 2 , and a switched capacitor unit  92 C and regulating circuit unit  94 C connected at a third node VX 3 . Furthermore, first inductor L 91 , second inductor L 92 , and third inductor L 93  are located at the output of each regulating circuit units  94 A- 94 C. These inductors L 91 -L 93  are then shorted together at the load. 
     Although  FIG. 10A  shows both the regulating circuit  16 A and the switching capacitor element  12 A as both being partitioned, this is not necessary. It is permissible to partition one and not the other. For example, in the embodiment shown in  FIG. 11 , only the switching capacitor element  12 A has been partitioned. A corollary that is apparent from the embodiment shown in  FIG. 11  is that the number of partitions of regulating circuit  16 A and the number of partitions of the switched capacitor element  12 A need not be the same, as is the case in the particular example shown in  FIG. 10A . 
     A top view of the power converter  90  shown in  FIG. 10A  is illustrated in  FIG. 10B . The switched capacitor units  92 A- 92 C extend along the y direction, where the first switched capacitor unit  92 A is at the top, the second switched capacitor unit  92 B is in the middle, and the third switched capacitor unit  92 C is at the bottom. The regulating circuit units  94 A- 94 C extend along the y direction as well. 
     Like the power converter  30 A shown in  FIGS. 3A-3B , the device stack  96  includes a top passive layer  41 A and a bottom active layer  42 A. The capacitors within the switched capacitor units  92 A- 92 C are included in the passive layer  41 A, whereas the active devices within the switched capacitor units  92 A- 92 C and regulating circuit units  94 A- 94 C are include in the active layer  42 A. 
     As shown in the top view of  FIG. 10C , switched capacitor unit  92 A includes seven power switches S 1 A-S 7 A, two pump capacitors C 31 A-C 31 B, and a control/driver circuit  23 A. The exact size of the active devices need not be the same size as the passive elements for the first loss-reduction technique to be effective. They simply need to be underneath the passive devices. This arrangement allows for more uniform current distribution and reduced wire length in the interconnect structure of the power converter. 
     Furthermore, within each switched capacitor unit  92 A- 92 C, the power switches and pump capacitors can be divided up into smaller subunits. This allows for an additional reduction in lateral current flow. An example of the power switch S 1 A divided up into nine sub units S 9 A-S 9 I is illustrated in  FIG. 10D . 
     Since the single monolithic switched capacitor element  12 A is divided up into numerous smaller switched capacitor units  92 A- 92 C and placed so as to encourage current in only one direction as shown in  FIG. 10B , the equivalent circuit becomes like that in  FIG. 9B , thus reducing overall losses. 
     The technique is effective because the total capacitance increases when capacitors are placed in parallel. For example, this technique is far less effective with inductors because total inductance decreases when inductors are placed in parallel. 
     Another possible arrangement of the switched capacitor cells is shown in  FIG. 11 , in which the switched capacitor element is partitioned into small switched capacitor units  92 A- 92 F along both the x and y direction. The exact size and dimensions of the switched capacitor units  92 A- 92 F depend upon many characteristics such as metal thickness, capacitance density, step-down ratio, etc. Both of these techniques reduce the vertical and lateral distance between the switch devices and the passive devices while also providing a uniform current distribution to each individual switch and/or switched capacitor cell. Thus, the parasitic resistance and inductance of the connection between the switches and capacitors is minimized. This is important because the parasitic inductance limits the speed at which the converter can operate and hence its ultimate size while the parasitic resistance limits the efficiency of the power conversion process. 
     Among other advantages, the arrangements described above avoids the component and pin count penalty, reduces the energy loss in the parasitic interconnect structures and reduces the total solution footprint of power converters that use capacitors to transfer energy. 
     An apparatus as described herein finds numerous applications in the field of consumer electronics, particularly smart phones, tablet computers, and portable computers. In each of these cases, there are displays, typically touch screen displays, as well as data processing elements and/or radio transceivers that consume power provided by the apparatus described herein.