The present mobile era may be best characterized by its unrelenting pressure to reduce power consumption. Each generation of consumer electronics must offer more functionality while maintaining or extending runtime. Initially, efficiency improvements were the Holy Grail, at least from a power management viewpoint, but the process of engineering efficiency into a device requires more than an efficient power supply. In fact, power supply efficiency improvements must, by definition, have diminishing returns (as they approach 100 percent).
A more comprehensive approach improves system efficiency by focusing on the load as much as the power supply. In a basic example, a linear regulator is added to a simple battery-powered device -- power supply efficiency actually goes down, but battery life is prolonged, and system efficiency goes up. Why? Linear regulator efficiency is essentially VOUT/VIN. (With no regulator, power supply efficiency is 100 percent.) However, using a linear regulator increases battery life. This is because system current (usually dominated by clocked CMOS logic) is less when the operating voltage is held at the minimum acceptable value by the regulator, rather than being allowed to rise with battery voltage. Total heat emitted by the device goes down, as well. This illustrates how system-level thinking provides better efficiency than focusing only on specs.
Integration can effectively enable system-level improvement in power consumption. Yes, it is possible to integrate multiple functions while doing nothing to reduce power. You can just throw two ICs on to one piece of silicon with no system optimizations and be done with it. That may help cost and size but doesn't accomplish much else. It's pretty rare for mixed-signal devices to be designed this way. Usually, more is expected for the effort.
Today efforts to save power are more intertwined with the system and require higher levels of integration. A good example is shutting off system blocks that are not used, sometimes only for a few milliseconds. This often brings constraints on sequencing and ramp rates. It's possible for an apps processor or controller to manage this, but it's not that practical. It's better if integrated power management does it, particularly when powering up from a dead state where system brains may not yet be awake.
The use of on-chip power MOSFET switches provides another counterintuitive example of using integration to improve system efficiency. The general thinking is that external FETs provide lower on-resistance and hence better performance, but a system view provides a clearer and sometimes different answer. Off-chip MOSFETs require on-chip drivers. If these are designed for a range of external devices (a common practice to allow multiple FET vendors), then the drivers must be overdesigned. These drivers will use more power than might otherwise be needed, and they can be especially wasteful at the high switching frequencies needed to keep passive components small. When driver and power MOSFETs are integrated together, the FET is precisely known, so the driver can be optimized to a degree not possible when driving an external switch.
Mobile device designs frequently include current monitoring for fault detection, thermal control, battery gauging, or other goals. Without integration, when different system blocks need current information, they often must measure it separately. If current-sense resistors are used, each of these creates a separate loss. An integrated power management device minimizes power loss by making only one current measurement and passing that to all the blocks that need it. Also, when implemented with integrated power MOSFETs, sense-FET structures can eliminate sense resistors. Then power is switched and current is monitored in a single device, further reducing losses.
Every day, new designs emerge that leverage integration to reduce heat and size while extending battery life. It's safe to say that, until some form of free portable energy is invented, a primary goal and benefit of integration will continue to be power reduction. Let us know your thoughts on this design philosophy, and share your design experiences.