Inductor-based switching DC-DC converters are becoming increasingly important in the expanding world of battery-powered electronics. The driving advantage for this phenomenon is extended battery life, which results from high power efficiency. Switching noise, circuit complexity (i.e., silicon and board real estate, speed, and reliability), and integration of power passive devices, however, have limited their market penetration, especially when compared to linear low dropout (LDO) regulator topologies. Circuit designers, semiconductor manufacturers, and researchers are therefore working on reversing these trends, especially within the context of a single-chip solution, which is where power inductors and capacitors have the most impact.
In tackling the integration of power inductors, both system-on-chip (SoC) and system-in-package (SiP) approaches conform well to a single-chip environment, but will they yield similar performance at comparable costs? The fact is discrete, SiP-compatible power inductor technologies are relatively more mature than their SoC counterparts. Companies are therefore starting to co-package commercially available wound inductors that are on the order of micro-Henries and no wider or longer and only slightly taller than conventional power management dies [1-2]. The issue that remains is the relative electrical-cost performance of SiP and SoC solutions, the answer of which is shaping the nature and extent of current research efforts in SiP and SoC technologies.
SiP versus SoC
Today, it is common practice to integrate every active component in a switching regulator, including the power switches, on a single controller integrated circuit (IC), and the corresponding die need not be more than four or five square millimeters to efficiently deliver one to two amps of current. For the same range of currents, discrete inductors on the order of micro-Henries, as small as two millimeters on a side and one millimeter in height, are commercially available . Co-packaging the controller and the inductor side-by-side can therefore have less than twice the dimensions of a package for the controller alone, as shown in Figure 1. This is significantly more compact than a printed-circuit board (PCB) realization of two discrete packages, where interconnect routing between the two also incurs additional overhead. The situation is further improved in smaller load-current applications, where power switches are smaller and switching at higher frequencies . For these conditions, commercially available inductors with ferrite cores in traditional surface-mount packages have even smaller footprints .
The fact that only a few products thus far feature co-packaged inductors is no indication that co-packaging costs are prohibitive. In fact, no additional fabrication step is required that is more complicated than the stacked die memory solution already in volume production for cellular handsets , only a few more bond-wires to an adjacent inductor are necessary. As it turns out, power management companies have a growing interest in stacked die integration because integrating the power switches in different process technologies can potentially decrease noise injection and the loading capacitances of the noise-sensitive controller, thereby improving its noise, bandwidth, and transient-response performance. In a die-stack environment, the height of the in-package inductor, which can be as low as one millimeter, would actually be shorter than the die stack.