Today's convergent devices--handhelds and smartphones--are surprisingly complex. They consist of one or more RF power amplifiers (PAs) and receivers, a digital baseband processor and microprocessor host, electronic memory, a high-resolution image sensor, at least one high-quality display, various lighting functions (white backlighting, camera flash, and even color fashion lighting), a lithium-ion battery, and a power-management system with integral battery-charging. In the last year, the demand for large amounts of memory has also given rise to handhelds with micro-hard-disk-drives (HDDs).
While scaling of IC feature sizes following Moore's Law has sustained continued miniaturization and feature integration in handhelds, battery capacity has not kept pace with power consumption. Cell-phone talk time can be reduced from 12 hours to a mere 20 minutes when operating continuously in 3G mode! Power management has therefore become crucial for maximizing battery life, the most critical function being voltage regulation.
A lithium-ion battery varies between 4.2 and 3 V during use. Because electronic components' operation and performance vary with supply voltage, this changing battery voltage must be held constant or "regulated" before it can supply power to other components and modules. Moreover, some electrical loads (such as digital ICs and memory) require a voltage lower than the battery, while others (such as LEDs used for display backlighting) need a higher voltage than the battery.
Unlike conventional 2.5G cell phones, smartphones require both step-down and step-up voltage regulators, otherwise known as dc-dc converters. 3G phones also are anticipated to require a "buck-boost" converter to power the PA, stepping the voltage up or down to maintain a supply in the middle of the lithium-ion voltage range.
Each converter must therefore be tailored specifically to maximize performance of the load it powers without unduly sacrificing battery life. Simply put, the more features a convergent handheld integrates, the more dedicated power supplies it needs.
But not all power supplies are created equal. The ubiquitous low-dropout (LDO) linear regulator, despite its low cost and noise, simplicity, and dominance in 2.5G handsets, is not efficient over a wide range of operating conditions. The switching regulator, the most versatile converter, can efficiently regulate voltages for any current, but it is notoriously noisy and requires a relatively large inductor. The charge pump, an alternative converter using only capacitors, eliminates the inductor and noise issues but cannot react to rapid changes in load current like an LDO or a switcher can. So seemingly, in the world of voltage converters, no answer is a good answer.
The only real alternative is to use a carefully conceived combination of all three voltage converter types matched specifically to each load. In this regard, the following power supplies will likely emerge for use in feature-rich handheld devices:
- a programmable output buck converter with good transient regulation and numerous sleep modes to power the baseband and host processor
- a low-noise programmable buck-boost converter with a multimegahertz clock to supply the RF power amplifier
- a programmable buck converter with high transient current capability to power a micro-HDD
- low-noise boost-type charge pumps to drive all lighting functions, including white LED backlights, camera flash, movie lighting, and color LED fashion lighting
- the replacement of conventional LDOs with a new device combining a step-down charge-pump preconverter with an integral LDO post-regulator
- substantially higher levels of integration in power-management-specific standard products (but not ASICs)
- the adoption of single- and dual-wire digital control of power converters, like Simple Serial Control (S2Cwire), Advanced Simple Serial Control (AS2Cwire), and the I2C bus.