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Manage power in next-gen handsets

Posted: 01 Mar 2006 ?? ?Print Version ?Bookmark and Share

Keywords:patrick heyer? texas instruments? 3g? IC integration? power management?

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 longer runtimes and smaller form factors.

IC integration addresses the size issue, but it adds design complexity and limits flexibility. As a result, battery management, power conversion and system management must be addressed by a combination of highly integrated power-management units and high-performance discrete components.

Today, just about every 3G phone uses a Li-ion battery, which offers the highest energy density of all electrically rechargeable battery chemistries. From a form-factor perspective, most batteries measure about 50-by-40-by-5mm and offer capacities of 900mA-hr to 1,200mA-hr.

While fuel-cell technology promises to offer higher energy density than Li-ion, its widespread 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 percent augmentation of battery capacity. Hence, system engineers are "stuck" with a power source that can supply about 1,500mA-hr to 1,800mA-hr, forcing digital and analog semiconductor technology to move to the next lower-power node and drive developments in ultra-efficient battery usage.

Integration, layout issues
With lots of functionality packed into a relatively small space, integrating the right set of high-performance analog and digital components is essential. However, in some cases, the integration of power-supply or audio capabilities may result in long traces, potentially complicating board layout or resulting in electrical-design challenges due to noise pickup. 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 functionalong with its power supplyis left off the board and costs are reduced.

To further optimize power management and maximize battery life, three key areas must be considered. First, battery management must deal with charging the battery and measuring its capacity. Second, power conversion translates the battery power as efficiently as possible to supply system components. And third, system power management, which analyzes actual power consumption in the processor domains and controls the power supplies, must optimize the battery's power use. The first and second areas are specifically addressed by the choice of power-management component, while the third involves significant software development on the processing side.

In battery management, fuel gauging is becoming increasingly popular. Traditionally, battery capacity is measured by determining the Li-ion battery's voltage, then drawing conclusions about available battery power using capacity LUTs stored in memory. These are based on a battery-specific Li-ion voltage-vs.-capacity profile.

However, given the complex power-consumption profile of 3G handsets and the behavior of Li-ion batteries over time, temperature and load conditions, this method is not precise. To accurately determine remaining capacity, which lets the processor better manage power consumption in the phone, designers deploy high-performance coulomb counting in conjunction with impedance tracking, which measures the actual charge that enters and leaves the battery. This approach lets the processor accurately deploy battery-save modes when the battery is emptied and alert the user when a new charge is required.

In the power-conversion domain, DC/DC converters play an increasing role in providing power-efficient solutions for LED drive and processor core supply. To improve digital still camera and videoconferencing features, the resolutions of CMOS and CCD sensors continue to increase. Today's 1Mpixel and 2Mpixel phones will give way to devices with higher-resolution capabilities.

To really make a difference, a high-power white LED must be driven with currents closer to 1A, a value that can't be achieved using charge-pump topology, since the resulting 2A battery current would exceed any battery power budget allowed by the system for this type of function.

Several subsystems in the phone may require an accurate core supply voltage. Linear regulators are typically perceived to be small, low-cost solutions when it comes to voltage regulation. However, above 200mA, they start to require space-consuming and expensive heat sinking due to excessive power dissipation. The dissipation is caused by the large input-to-output voltage differential, multiplied by the output current when supplying, for example, a 1.2V/500mA core voltage from a 3.6V Li-ion battery. While a linear regulator performs this type of regulation with only 33 percent efficiency and primarily "burns" battery power that produces heat, a DC/DC converter may run well in excess of 90 percent efficiency, consuming only a fraction of the power a low-dropout regulator wastes.

Switching regulators
Using one available DC/DC buck converter with integrated switching regulators, the control architecture enables the power supply to react quickly to load transients and to maintain high-voltage regulation accuracy in the 1 percent range, as required by today's high-performance processing cores.

A 3MHz switching frequency reduces the inductor size to just 1mH, permitting the use of low-profile chip inductors below 1mm in height. The device is offered in a chip-scale package to reduce IC size to 2-by-1mm. The total solution can be built to fit into a 5-by-5mm space. To further optimize power consumption, the advanced DC/DC regulator features automatic pulse-frequency-modulation (PFM)/pulse-width-modulation (PWM)-mode transition to maximize conversion efficiency over a wide load range. At light load currents, the converter enters PFM mode, while load currents above the 50mA range are supported with a PWM control scheme. A 1.8V, 500mA core supply can be delivered with power efficiencies in the 80-90 percent range.

Integration of power and other analog components is inevitable. The key to integration is choosing functions that have evolved as standards are shared across a large range of mobile-handset platforms.

Power-management devices continue to push the envelope in size, efficiency and power consumption. These components play a critical role in reducing the form factor and weight of the end system.

- Patrick Heyer
Marketing Manager, Portable Power Management
Texas Instruments Inc.

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