3G的到来给手机电源管理设计带来挑战(英)<系统> - 我爱研发网_52RD.com

Multi-media applications for cell phones require faster data rates which raises the question of the trade-off between mobility and data rates. 2G and 2.5G standards like GPRS (General Packet Radio Access) and EVDO introduced the idea of surfing the web on the phone or downloading ring tones but the download speeds were slow and the applications were limited and boring. With the arrival of W-CDMA and CDMA-EVDO (Evolution, Data Only) Rev. A (and EVDV (Evolution, Data and Voice) in the near future), the cell phone will have significantly faster download speeds than before. The HSPA (High Speed Packet Access) standard on W-CDMA allows for complete mobility and data speeds in the 1gigabytes per second range. Another common feature in phones is the inclusion of a Wi-Fi connection which allows Wireless LAN (Local Area Network) speeds but is restricted to wireless hotspots. There are also standards like Mobile WiMax and its Korean equivalent WiBro that expand the wireless hotspot to a metropolitan area network. Such technologies are promoting the shift from voice based communication to IP (Internet Protocol) based communication.

More features in the same form factor
The new wireless standards have dramatically and swiftly evolved the next generation handset. The trend is to try and fit as many features and applications as possible in a cell phone without changing the form factor. In fact, the popularity of devices like the Apple iPOD and the Motorola Razr have the added effect of making thinner and sleeker looking portable electronics. With 3G, consumers have the ability to download data at faster rates thereby making larger downloads practical. We already see phones with integrated MP3 players and video downloads are also gaining popularity. So, there is a requirement for larger storage capacity in cell phones leading to the integration of portable sub-1 inch hard disk drives and high storage flash memories into cell phones that have the capability of storing up to 4 gigabytes of data. Perhaps, the biggest trend will be the emergence of mini SD and micro SD cards as the new removable storage device. SD cards can be thought of as the "new floppy" disk as we will start using them to transfer data back and forth from our personal computer to our handset as well as a means of sharing data like photos among ourselves. Talking about photos, the cell phone is also set to become the primary digital camera for the majority of users. A 3 mega pixel camera is already available in the mass market in Japan and Samsung has already experimented with a 7 mega pixel albeit more of a camera than a phone. The adoption of brighter flashes on phones in addition to the higher resolutions will allow us to take good quality photos on our cell phones. Next generation handsets are in fact going to have 2 cameras on board, a high resolution one for stills and a lower resolution one to capture short videos. The buzz word for cell phone applications in 2006 is definitely digital TV. Korea is the market leader in launching digital TV services in 2005 supporting the DMB (Digital Multimedia Broadcast) standard. All the major handset manufacturers realize the potential of this feature and are investing heavily into this. Phones with the global DVB-H (Digital Video Broadcast " Handset) standard will show up in phones in Europe this summer just in time to show streaming highlights of the best goals from the soccer world cup. There are other features like Wi-Fi and gaming that are already hitting the market and new ones like e-cash that are still in their formative years. All these applications require specific processors which now need to be added to the 3G handset as well. Figure 1 shows a block diagram of what power requirements of a 3G handset will look like in late 2006 " early 2007.

Power management challenges: maintain run times
Consumers want all the new features in their handsets without compromising on run times and battery life. Portable battery technology is advancing but the year-on-year improvements in energy density are restricted to around 5% which is nowhere near enough to satisfy the extra energy overhead of the 3G phone. Therefore, the onus is on the power management supplier to define new solutions that will maintain long run times in 3G phones if not make them longer. One certain trend is the shift from using 40% efficient linear regulators to 85-90% step-down DC-DC converters. The majority of digital processors, camera modules and storage devices in 3G handsets have to be powered by switching regulators to maximize run times. Also, there are new applications like Wi-Fi and digital TV processors that will require new high efficiency power management as well. 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% average power savings. However, there are trade-offs of using more switchers in a cell phone. Switchers have a larger footprint because of the need for an inductor and larger output capacitors. This also leads to an inflated bill of materials. These are challenges that have to be overcome to enable the next generation of mobile devices.

Solution size is critical
The first step in arriving at the optimal switcher solution is to develop the smallest switcher possible. Until now, plastic packages like SOT23 have been competitive in portable electronics. Designers must start considering chip scale packaging (for example, National’s micro SMD package) that can provide the same functionality in a much smaller package. Figure 2 shows a comparison of plastic versus chip scale packages for the same device; the difference is significant.

Figure 2: Step Down DC-DC Converter Solution Size Comparison

Optimize external components
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 is 3mm x 3mm x 1.5mm in dimensions. However, new multi-layer chip inductors (chip inductors) are providing a much smaller option. Figure 2 also shows the comparison of a chip inductor versus a coil inductor. Chip inductors are expected to be 30% cheaper than coils once they are in high volume production which is critical as the handset manufacturers are trying to minimize the bill of materials. One of the "de-merits" of multi-layered chip inductors is that the inductance value rolls off to a lower value as the DC current increases as opposed to a coil inductor that has a constant inductance till the saturation current is reached. System designers need to take this into account when selecting a chip inductor. Table 1 shows a comparison of the characteristics of a chip inductor and a winding coil inductor.

Parameter Chip Inductor Winding Coil

Size 2.5mm x 2.0mm x 1.0mm 3.3mm x 3.3mm x 1.2mm

Cost (High Volumes) 30% Lower (Est.) $0.10 - $0.15

Mounting time 1/10^th of winding coil

DC Resistance <100 mΩ 200 mΩ

Electro magnetic Interference Better than coil

Inductance Roll-Off with DC Current Worse than coil

Table 1: Comparison between chip inductor and winding coil inductor

Applying switched capacitor technology
We have discussed how to reduce solution size of a magnetic buck regulator. Another effective method of achieving the optimal combination of solution size and efficiency is employing the use of charge pump technology for step down dc-dc conversion. The major benefit of using switched capacitors is that 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% smaller than a magnetic switcher and there are cost savings due to the reduced bill of materials. With improvements in charge pump technology, a high performance switched capacitor can offer better than 75% average efficiency in half the form factor of a magnetic solution and also have comparable quiescent current and accuracy specifications. The major trade-off is efficiency but for digital loads running on sub 2V voltages and low currents (less than 250mA), the run times 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 run times using a 550mAhr lithium-ion battery. The runtime for the magnetic buck was 330 minutes compared to 309 minutes for the switched capacitor buck -- a negligible difference in battery life.

Improvements in performance
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% 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 integrated circuit but since solution size and is so important, that is not a viable option.

Another critical specification for power regulators is the quiescent current, which is the current consumed by the power IC itself. This has to be extremely low for handsets in order to maximize standby times, which is where these devices spend most of their time. 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.

Conclusion
The stars are aligned for the mass adoption of 3G cell phones. There is a consumer demand for new features and faster speeds and the wireless providers are rapidly upgrading their systems. The cell phone is evolving quickly from a "phone" to the center of our multimedia experience. The handset manufacturers are packing their phones with every feature possible and it is the challenge of the power supply manufacturer to maintain the long run times. These are exciting times for the portable power industry with the future being driven by innovation and technology; the defining characteristics of high performance analog.

About the author
Vik Sangha is a product marketing manager for National Semiconductor’s Portable Power Systems Group. He is responsible for defining next generation power solutions including magnetic buck regulators, switched capacitors and linear regulators. Vik has a Bachelor of Science in computer engineering from Union College in Schenectady, NY. He has been with National Semiconductor since 2002. vik.sangha@nsc.com