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Defining new solutions for 3G power challenges

Posted: 16 Oct 2006 ?? ?Print Version ?Bookmark and Share

Keywords:digital power management? digital control? 3G cellphone power management? 3G handset power? Vik Sangha?

3G wireless technology has been on the brink of becoming mainstream since the beginning of the 21st century. However, all the necessary technologies and services for its launch have not aligned, leading to inherent delays in the adoption of 3G phones by the mass market. Still, the time has come for 3G to hit the main stage.

The stars are aligned for the mass adoption of 3G cellphones. Consumers demand new features and faster speeds. Wireless providers are rapidly upgrading their systems. The handset is evolving quickly from being a "phone" to being the center of our multimedia experience. There is a requirement for larger storage capacity, leading to the integration of portable sub-1inch HDDs and high storage flash memories into handsets that have the capability of storing up to 4Gbytes of date. Next-generation handsets are going to have two cameras on boarda high resolution one for stills and a lower resolution one to capture short video. There are other features such as Wi-Fi and gaming that are already hitting the market, and new ones like e-cash that are still in their formative years. Handset manufacturers are packing their phones with every feature possible, and the challenge is for power supply manufacturers to maintain long runtimes. These are exciting times for the portable power industry, with the future being driven by innovation and technology.

Emerging trend
Consumers want new features in their handsets without compromising runtimes and battery life. Portable battery technology is advancing, but the year-on-year improvements in energy density are restricted to about 5 percent, which is nowhere near enough to satisfy the extra energy overhead of the 3G phone. Thus, the onus is on the power management supplier to define new solutions that will maintain long runtimes in 3G phones, if not make them longer.

One certain trend is the shift from using 40 percent efficient linear regulators to 85-90 percent step-down DC/DC converters. Most digital processors, camera modules and storage devices in 3G handsets have to be powered by switching regulators to maximize runtimes. New applications such as Wi-Fi and DTV processors will also require new high-efficiency power management. Switching regulators should also be used to vary the power levels of next-generation CDMA and W-CDMA power amplifiers, as this can result in more than 50 percent average power savings.

However, using more switches in a cellphone has trade-offs. Switchers have a larger footprint because of the need for an inductor and larger output capacitors, leading to an inflated BOM. These are challenges that have to be overcome to enable the next generation of mobile devices.

Designers must consider chip-scale packaging that can provide the same functionality in a smaller package.

Solution size
The first step in arriving at the optimal switcher solution is to develop the smallest switcher possible. Until now, plastic packages such as SOT23 have been competitive in portable electronics. Designers must start considering chip-scale packaging that can provide the same functionality in a much smaller package.

The next step is to optimize the external components, starting with the inductor, which is the biggest size and cost adder to the switcher. The standard winding coil inductor for handsets measures 3mm-by-3mm-by-1.5mm. However, new multilayer chip inductors are providing a much smaller option. Chip inductors are expected to be 30 percent cheaper than coils once they are in high-volume production. This is critical because handset manufacturers are trying to minimize the BOM. One of the demerits of multilayered chip inductors is that the inductance value rolls off to a lower value as the DC increases. A coil inductor, on the other hand, has a constant inductance until the saturation current is reached. System designers need to take this into account when selecting a chip inductor.

Switched capacitor
Another effective method of achieving the optimal combination of solution size and efficiency is by using charge-pump technology for step-down DC/DC conversion. With switched capacitors, there is no need for an inductor, and they need very tiny ceramic capacitors. This is a significant merit, as the total solution size is approximately 40 percent smaller than a magnetic switcher. Moreover, there are cost savings due to the reduced BOM.

With improvements in charge-pump technology, a high-performance switched capacitor can offer better than 75 percent average efficiency in half the form factor of a magnetic solution. It also has comparable quiescent current and accuracy specifications. The major trade-off is efficiency. For digital loads running on sub-2V voltages and low currents (less than 250mA), however, the runtimes for a switched capacitor and a charge pump are almost identical. A comparison was done between a magnetic DC/DC converter and a switched capacitor buck regulator to test runtimes using a 550mA-hr Li-ion battery. The runtime for the magnetic buck was 330mins compared to 309mins for the switched capacitor buck, a negligible difference in battery life.

Next-generation digital processors for 3G are moving to 90nm and 65nm process technology for size and performance benefits. This is driving the supply voltage down closer to 1V. As a result, the output voltage accuracy and the transient performance are becoming critical specifications for power supplies. A 1.2V digital processor with a tolerance of

5 percent only allows for a maximum of 60mV deviation from the nominal 1.2V supply. The challenge for the power designer is to define regulators that can achieve very tight output voltage tolerances, especially in the case of DC/DC converters. Also, these devices should have excellent transient response so that the output voltage has very small undershoots and overshoots in the case of a step in the output current or the input voltage. Such performance can be achieved at the expense of a larger IC, but since solution size is so important, that is not a viable option.

Another critical specification for power regulators is the quiescent current, the current consumed by the power IC itself. This has to be extremely low for handsets to maximize standby times. New topologies have to be implemented to achieve these low supply currents at low power levels. Also, lower quiescent current usually means lower performance for a regulator, which is not acceptable as we start moving to next-generation processors.

- Vik Sangha
Product Marketing Manager, National Semiconductor Corp.

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