Partition the power management in your next-generation mobile

Third-generation (3G) mobile handsets offer a wide range of features with increased functionality. While consumers appreciate the latest, improved capabilities from their communication devices, they continue to demand more sufficient run-time from one battery charge and a smaller form factor.

While IC integration addresses the size issue, it also adds design complexity and limits design flexibility. Today’s mobile handset designer must consider several factors to effectively optimize battery power to maximize and extend battery life. As a consequence, battery management, power conversion, and system management must be addressed by a combination of highly integrated power-management units and high-performing discrete components.

Engineers face a basic dilemma when designing advanced wireless devices. They need to fit a large amount of functionality into a given form factor that’s often determined by the size of the battery and display, the complexity of the user interface, and design ergonomics. In addition, a battery’s available energy is defined by its chemistry, which determines its energy density and the battery’s physical size. These changing parameters typically force the designer to use battery power more efficiently to satisfy consumers’ expected stand-by and run times.

Today’s 3G multi-function phones support several air interfaces and offer multi-band modem connectivity, such as GSM and WCDMA. Additional connections can be made through Bluetooth, Wi-Fi, infrared, and USB interfaces. Digital photography is standard in many phones, which requires sophisticated camera engines and high-luminosity flashes to take high-quality pictures. With increased data transmission speeds, video telephony is also possible. In addition, high-speed application processors offer video and audio processing capabilities to run digital TV signals and MPEG audio encoding and decoding. Newer devices plan to add FM radio and digital TV tuners to increase the phone’s entertainment value. Increased data throughput ultimately requires high-density storage capabilities, either enabled through memory expansion slots or even micro hard-disk drives. It’s not hard to imagine that most of these wireless devices will also double as portable gaming devices.

The battery as the energy source takes a central position in the system. Today, just about every 3G phone uses a lithium-ion (Li-Ion) battery, which offers the highest energy density of all electrically rechargeable battery chemistries. From a form-factor perspective, most batteries measure around 50 by 40 by 5 mm and offer capacities of 900 to 1200 mAh. While fuel-cell technology promises to offer significantly higher energy density than Li-Ion, its wide-spread deployment is still several years away due to technical and regulatory issues. Furthermore, expected incremental improvements to Li-Ion technology may lead to a 30% augmentation of battery capacity. Hence, system engineers are "stuck" with a power source that can supply around 1500 to 1800 mAh. This dilemma ultimately forces digital and analog semiconductor technology to move to the next, lower power node and drives developments in ultra-efficient battery usage.

Integration and layout issues
It’s clear that with all the functionality being packed into a relatively small space, integrating of the right set of high-performance analog and digital components is essential (Fig. 1).

1. The principal system architecture in a 3G handset shown here highlight its complexity.

One valid question is, "what components must be integrated and how does one address the fact that the form factor impacts how components can be placed?" An obvious answer is to integrate standard supply rails for baseband processors, audio subsystems, and interface components that are shared across different mobile phone platforms and suppliers using the same basic chip set. But, there are two inherent major challenges. First, industrial design considerations allow the phone, depending on the desired functionality and ergonomics, to be designed in many different ways.

Today, the electrical design must consider that the phone can be shaped as a candy bar, clam shell, or slider, all using different display, keyboard, and speaker configurations. These design differences significantly impact where the display backlighting, the camera module, and other subsystems are situated, which also somewhat limits the integration of these components.

In some cases, the all-in-one integration of power-supply or audio capabilities may result in long traces, potentially complicating board layout or electrical design challenges due to noise pick-up. Also, one should not forget the cost-effective model portfolio management a handset maker desires. To serve a market with a palette of models, manufacturers must offer different feature and performance levels, all priced differently. To optimize margins in a fiercely competitive world, the cost of those models must vary with capability, which prohibits integrating every function into one large IC. If the feature isn’t desired for a given range of models, the specific function along with its power supply is left off the board and costs are reduced.

Furthermore, phone makers that use the same basic chip set still need to differentiate their offerings versus their competition. This drives the de-integration of major feature differentiators. Typical examples of differentiation might include a brighter camera flash, a more powerful torch light mode, class-D stereo audio performance, special display and keyboard backlight effects, MP3 audio playback capability, FM radio reception, and accurate battery fuel-gauging.

Discrete power component selection
Typical non-integrated power components that supply energy to differentiating sub-components may be battery fuel gauges as part of the mobile phone battery pack. They can also be highly efficient, but small high-frequency dc/dc core supplies or high-performance dc/dc boost drivers for white camera flash LEDs, or special white LED backlight drivers with supplies for organic LED, or OLED, sub-displays and ultra-low power supply rejection ratio (PSRR) linear regulators.

As with most integration, established features popular with consumers are getting integrated first. Leading-edge analog semiconductor technology with higher performance and efficiency, including optimized power management stand-alone components, will increasingly be integrated as shipment volumes rise and functions are standardized.