Power management technology for future smartphones

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Power Management Technology for Future Smartphones
Modern cell phones have become pocket media centers that include at least a digital camera (DSC), a color Internet browser with 3D gaming capabilities, a digital TV receiver, and an MP3 player with not only high-fidelity audio playback but also simulated 5-channel surround sound. Newer camera phones will include higher-resolution cameras, flash cards, and personal security devices such as fingerprint sensors. How to maintain the battery life of such a sophisticated mobile device for more than 15 minutes through cellular phone power management technology will be one of the most significant engineering challenges for anyone in the next 10 years.
Although some progress has been made in lithium-ion battery technology, "it still can't reach the power you require," complained Dave Heacock, business manager of TI's portable device power management group. Peter Henry, vice president of portable power systems at National Semiconductor Corporation (NSC), also said that the current rechargeable battery has only about one-third the energy of a stick of dynamite.
He believes that although fuel cells may be an appropriate way to increase the power of portable battery packs, it will be difficult for the industry to install fuel cells in pocket-sized portable devices before 2010. Newer lithium-polymer batteries, while not necessarily offering greater power density, will allow for new form factors, says TI's Heacock. While the growth in the number of cells (5 billion per year) will encourage the use of dedicated embedded chargers and fuel-gauging circuits, the main focus of portable power management is on techniques to increase regulator efficiency.
Every function consumes power .
Strategy Analytics estimates that about 20% of the 670 million cell phones shipped this year will be camera phones, which have two CCD-based cameras embedded in them: one for taking pictures of friends or companions in beautiful outdoor scenery, and the other for transmitting your own picture to the other party during a phone call.
The popularity of camera phones has encouraged developers to increase resolution from 1.3 megapixels, which is suitable for casual use, to 3.3 to 4.4 megapixels that will attract tourists and photography enthusiasts. To support the higher resolution, camera phones need to be equipped with autofocus mechanisms and sophisticated xenon or white LED flashes.
For the new generation of consumers, it is no longer enough to just use a color LCD screen to browse the Internet and play MP3 music. New feature designs must be devices that stream media, not only high-fidelity music but also multi-channel surround sound. And their LCD screens must be able to vary their light intensity to create 3D visual effects. In Japan and South Korea, mobile phones will be able to receive digital TV broadcasts. Of course, such phones must also be able to make calls, whether the caller is on a city subway or outdoors, and the voice must remain clear. Such portable devices present developers with a series of engineering challenges, and none of this would be possible without power management.
Loading a mobile phone with a set of consumer features and functions is a challenge to the efficiency of voltage regulators, which provide precisely controlled supply voltages to the mobile phone components that process audio, video and graphics. In some cases, the voltage regulator must step down the 3.6V voltage of the battery to the 3.3V required by logic and I/O components, or the 1.8V or 1.2V required by the processor core.
In other cases, the regulator must step up to 4.5V to drive a backlight, 5V to drive a USB port, 9V to drive a CCD camera module, or instantly step up to the 4kV required by a xenon flash. In all cases, the regulator must not only avoid overloading the battery, but also avoid overheating in the cluttered and small space of the mobile phone. Portable Power
Solutions History
For many years, the key issue in mobile phones has been the efficiency of the regulator. The regulator needs to convert the 3.6V provided by two lithium batteries to the 3.3V required by the baseband processor. Linear regulators (now called LDOs for their "low dropout" performance) have become more popular with designers because they produce minimal noise at the voltage output, and mobile phone designers also consider that the extremely small ripple of LDOs can prevent noise from creeping into the RF carrier generated by the mobile phone's microtransmitter.
Although switching regulators are considered high-efficiency devices, their operation relies on pulse-width modulators (PWMs) that pump current through switching MOSFETs at high frequencies. Designers need to filter out the current spikes that this pumping process produces at the regulator's output, and while the technology is more efficient than LDOs in terms of delivering higher currents, it produces ripple that can interfere with noise-sensitive cell phone RF circuits.
(Using a switching device allows higher frequencies to be produced with smaller auxiliary components like inductors and capacitors, but the switching noise level will also be higher.) Some switching regulator manufacturers, such as Linear Technology, have developed resonant-frequency devices that produce minimal ripple at the output (or ripple that can be easily filtered out). The company claims that such devices allow cell phone manufacturers to get more than 90% efficiency from their regulator circuits without generating switching noise that affects the RF output signal.
In addition to the advantage of high efficiency at high current loads, a switching regulator has something that a linear regulator does not: It can boost (or "amplify") the voltage level of a nearly depleted battery. Many manufacturers, including Vishay Siliconix and NSC, argue that step-up switching regulators are needed in situations where the battery voltage must be stepped down from 3.6V to the 3.3V required by logic circuits, and then raised to 3.3V when the battery voltage drops to 3.0V or lower.
But at least for phones that provide only basic functions, their argument is short-lived. Like a "coulomb counter," lithium batteries have little charge left (less than 10%) when they drop below 3.3V, says Dave Heacock of TI. The efficiency of step-up regulators, they argue, is good but not good enough to make it worthwhile to squeeze out a few more minutes of talk time. A better solution is to turn off the phone when the battery voltage drops below 3.3V.
The white LED boom
The landscape changed completely when phone makers equipped their phones with new features and functions. Young Japanese consumers, who created demand for phones with color LCD screens, were the driving force behind the change. Phone makers discovered that falling white light-emitting diodes could be used as backlighting for small LCD screens.
Early white LEDs presented two types of power management problems. First, early white LEDs required a 4.5V nominal voltage threshold to ensure proper light emission, and the quality of the first generation of LEDs was inconsistent. Some devices glowed dimly at 4.5V, while others were extremely bright (and may have burned out prematurely). This required white LED manufacturers to develop constant current regulators and the ability to drive multiple LEDs in parallel and in series.
The huge market volume for mobile phones prompted analog device companies that produce power management devices to provide a large number of white LED drivers to the market. These manufacturers include Maxim Integrated Products, Linear Technology, NSC, TI, Intersil, Vishay Siliconix, Fairchild, and Catalyst Semiconductor.
Mobile phones equipped with digital camera modules require higher voltages, which means more voltage regulators. DSPs used for color correlation, JPEG compression, and other image processing functions can be driven with low voltage, low current 1.8V or 1.2V (300mA current source). However, the charge-coupled device (CCD) used for image capture requires a drive voltage of at least 5V, which requires additional step-up regulators.
According to Nazzareno "Reno" Rossetti, product strategist at Fairchild, a palm-sized DSC (1.3 megapixels) consumes about 2W of power when taking a picture and up to 1.2W when viewing an image (2.4V/500mA), so two 700mA-hour rechargeable NiMH batteries connected in series can provide nearly an hour of shooting and viewing time. About
two years ago, the focus of mobile phone and handheld device designers suddenly shifted from voltage regulator efficiency to other issues. Many manufacturers, including NSC and Analog Devices, began to consider shutting down or lowering the operating frequency of the handheld device's central processor to control power consumption. The assumption was that the faster the microprocessor's clock, the more power it consumed.
If the clock frequency and operating voltage were reduced, the power consumption was actually reduced. The "voltage-frequency scaling" technique monitors the processing tasks (actually software code) of the CPU and assigns them priority. Tasks such as decoding a video stream may require a lot of processing power, and the device will run at the maximum clock frequency, and the power will be adjusted to provide maximum I/O operations. But simple tasks like keeping the phone in standby mode while it's in your pocket require little processor work, just enough voltage to refresh the memory.
Peter Henry, marketing director for NSC's power group, notes that it's not just the voltage-regulating circuits that offer energy-saving opportunities; the bus and interface components also have a significant impact on the phone's power consumption. "The data path is not optimized from a power perspective," he comments. "With voltage-frequency scaling, the processor doesn't care what the voltage is, only what the propagation delay is for the CPU clock... It's no longer about voltage, it's about timing," he warns.
Simply closing the loop between the processor and the power controller can yield 30 percent power savings. But actually cutting the clock speed in half can save up to 70 percent. NSC has developed power-regulating circuits for ARM processors that monitor processor activity and reduce the frequency or even put it into sleep mode when the load is light. More recently, NSC has developed a scaling device for Intel's PXA270 X-class processor.
Using a power management SoC?
The presence of a single voltage regulator dedicated to powering the central processor and DSP in a cell phone, along with a dozen separate regulators that provide buck/boost capabilities for peripherals, has prompted analog IC makers such as TI and Fairchild to ask whether cell phone makers could benefit from a custom power management IC (PMIC). A PMIC is an ASIC that integrates a voltage regulator, voltage monitor and voltage controller on a single IC.
A custom PMIC can reduce the number of regulators, saving board space and reducing manufacturing costs. However, custom development of a PMIC can delay time-to-market for new cell phones. And unless the volume is in the tens of millions, the overall cost reduction from using a digital ASIC is not significant.
"We've noticed that PMICs are not very popular," said Tony O'Brien, director of marketing at Micrel Semiconductor. He noted that cell phone models change three or more times a year, and each model may offer different feature configurations, with some requiring more power control and others requiring less power control. "While there is some interest in integrating the regulator function (i.e., integrating multiple LDOs in one package), the shortcoming of PMICs is the lack of flexibility," he said.
To integrate the regulator function, Micrel will offer ICs that integrate two and three linear regulators. Micrel is actually developing a "low-end PMIC" that will integrate a high-current switching regulator and two LDOs. Fairchild's Reno Rossetti believes that whether it makes sense to develop a PMIC depends on your target CPU-DSP media processor core. There are currently three mainstream mobile phone architectures: the OMAP camp based on ARM CPUs (led by Nokia and Motorola), Qualcomm's CDMA partners (Samsung and Kyocera) and the third camp based on Intel's X-scale processors (whose users include Samsung, Motorola and others).
The OMAP camp uses DSP media processors made by TI and ST and PMICs that integrate analog baseband processors. For example, in the OAMP UMTS mobile phone architecture, the TBB5110 baseband processor is connected to flash memory, SRAM, display, camera and dedicated connectors (including keyboard and joystick).
The TBB5110 includes an ARM call processor (human machine interface), voice processor and physical layer codec based on TI C55x DSP. The power management device is ST's STw4200 PMIC. The transmitter and receiver are the only other large chips in the phone. The TI TCS2600 GSM/GPRS chipset reduces the number of chips in the phone to three: the TRF6151 RF receiver, the OAMP730 application processor and the TWL3016 analog baseband unit that includes the power management unit.
Qualcomm's CDMA phones use stand-alone PMICs like the Qualcomm-designed PM6650. The X-scale architecture used in some Hitachi, Motorola and Samsung phones relies on the custom DA903x PMIC from Dialog Semiconductor. According to a phone analysis report provided by Portellligent, the PMIC often does not support all 12 to 18 independent regulator functions.
For example, Samsung's SCH-V410 CMDA-2000 Camcorder phone is based on a Qualcomm chipset (including a PMIC), but still uses nine additional voltage regulators, most of which are LDOs from NSC, Micrel, Semtech, Maxim, Microchip, and Ricoh.
Apple iPod Example
Linear Technology was asked to help design the power management circuits for Apple Computer's iPod music player. The iPod player is an MP3 player that stores digital music on a microdrive, and its microdrive (a 30-gigabyte storage unit made by Panasonic) is a potential power hog. In addition to providing the dynamic range required to drive headphones (which involves power margin), the power management circuits must also drive the disk motor efficiently without quickly draining the battery.
Linear Technology's solution includes up to five independent regulators and multiple building blocks carefully selected to perform the most efficient power management job. Linear Technology admits that although the use of carefully selected standard building blocks allowed the iPod to be launched faster, almost no Linear Technology components were used in subsequent iPod production.
In fact, the analysis done by Portelligent reveals that Apple chose to integrate the standalone regulator with the PCF50605 power control and battery management chip made by Philips Semiconductor. Although Philips makes the PMIC for the newer iPod Mini, Linear Technology's LTC4055 USB power management and lithium-ion battery charger chip also appears on the iPod motherboard.
Architectural changes
The new generation of W-CDMA/UMTS phones requires the use of linear RF power amplifiers (PAs). Peter Henry of NSC explains that these components are not particularly efficient. At the maximum power output of 28dBm, their efficiency is only 45%. Most of the time, these components are idle, with power output between 0 and 12dbm, when the efficiency is only 5%.