The modern cell phone has become a pocket media center, containing at least one digital still camera; a color Internet browser with 3-D gaming capability; a digital television receiver; and an MP3 player with not just hi-fi audio playback capability, but also simulated five-channel surround sound. Newer camera phones will include higher-resolution cameras, flash attachments and personal security devices such as thumbprint sensors. How anyone gets more than 15 minutes of battery life out of such a contraption-that is, cell phone power management-is thus one of the most significant engineering challenges of the decade.

Despite the advances in lithium-ion battery technology, "there's not enough coulombs to do what you want," said Dave Heacock, Business Manager for Texas Instruments' Portable Power Management Unit (Dallas).

Current-generation rechargeable batteries pack roughly one-third the energy of a stick of dynamite, noted Peter Henry, vice president for portable power systems at National Semiconductor Corp. (Santa Clara, Calif.). Fuel cells, he believes, may be the means of increasing the energy available from a portable battery pack, but such units may not make it into pocket portables until late this decade.

The newer lithium-polymer formulations will not necessarily allow more power density but will allow the battery to take new forms and shapes, TI's Heacock said. While the proliferation of battery cells-as many as 5 billion per year-will encourage the deployment of specialized built-in chargers and fuel gauging circuits, the main technology for portable power management remains focused on techniques for increasing voltage regulator efficiency.

The popularity of camera phones has encouraged developers to increase their resolution from the 1.3-megapixel units suitable for casual use to 3.3- and 4.4-Mpixel units appealing to travelers and camera buffs. To support the higher resolutions, the camera phones need to come equipped with auto-focus mechanisms and sophisticated flash Xenon or white LED flash attachments.

For the new consumer, a handheld portable that lets the user cruise the Internet with a color LCD screen and play MP3 audio is no longer sufficient. The new clamshell design must be a media-streaming device, capable of playing not just hi-fi audio but also stereo and multichannel surround sound. And the LCD screen must vary its light intensity to portray 3-D visual effects. In Japan and Korea, the cell phone will be capable of receiving digital TV broadcasts.

And, oh yes-the device must also make phone calls and maintain voice clarity whether the talker is in a city subway tunnel or outdoors.

While such devices present a number of engineering challenges, nothing is possible without power management.

Loading a cell phone with an array of consumer features and functions is a challenge to the efficiency of voltage regulators. The regulators offer precisely controlled supply lines to the cell phone components, which, in turn, process audio, video and pictures. In some cases, the voltage regulator must step down the available 3.6-V battery voltage to the 3.3 V required by logic and I/O components, or the 1.8 V or 1.2 V required by processor cores. In other cases, the regulator must step up the voltage to the 4.5 V needed to drive LCD backlights, the 5 V required for a USB interface, the 9 V required to drive a CCD camera module or the momentary 4 kV required by a Xenon flash attachment.

In all cases, the regulator must operate as efficiently as possible, not just to avoid taxing the battery but also to avoid dissipating heat into the cluttered but minuscule space inside a cell phone. The solutions reflect the specialized talent of analog semiconductor companies.

Though switching regulators are regarded as higher-efficiency devices, their operation depends on pulse-width modulators (PWMs) that would pump current through switching MOSFETs at high frequency. Designers are required to filter the current spikes that this pumping generates at the output of the regulator. But this technology, though more efficient than LDOs for delivering higher current, would generate ripple that could interfere with the noise-sensitive RF circuitry of the cell phone. (With switchers, a higher frequency promotes smaller ancillary components, such as inductors and capacitors, but involves a higher level of switching noise.)

Some switching regulator manufacturers, such as Linear Technology Corp. (Milpitas, Calif.), developed resonant-frequency devices, which would produce a minimum of ripple on the output (or, certainly, a ripple that was easily filterable). Such devices, they argued, would enable cell phone makers to obtain switching regulator efficiencies of 90 percent or more from their voltage regulator circuits without putting switching noise onto the RF output signal.

In addition to its higher efficiencies with higher current loading, switching regulators could do something a linear regulator could not: elevate (or "boost") the voltage level of a nearly depleted battery. A number of manufacturers, among them Vishay Siliconix (Santa Clara, Calif.) and National Semiconductor, argued that buck-boost switching regulators would be necessary to step down the 3.6-V battery voltage to the 3.3 V required by logic circuits and then to step it up to 3.3 V as the depleted battery voltage dropped to 3.0 V or below.

But the argument was short-lived-at least for the no-frills cell phone. Texas Instruments' Heacock and other "coulomb counters" argued that there was very little battery life left (perhaps less than 10 percent) once the lithium-ion battery voltage dropped below 3.3 V. The efficiency of a boost regulator circuit was good, they argued, but not good enough to make it worthwhile to squeeze out a few more minutes of talk time. A better alternative: If the cell phone battery voltage drops below 3.3 V, they said, turn the phone off.

Early-generation white LEDs posed two kinds of power management problems for cell phone makers and their voltage regulator suppliers: They required a nominal 4.5-V threshold to light properly, and the quality of the first-generation LEDs had a tendency to vary. Some devices would be dim at 4.5 V; others would be exceptionally bright (and possibly subject to early burnout). Thus, makers of white LED drivers needed to evolve constant-current regulators, along with the ability to drive arrays with LEDs in parallel and LEDs in series with one another.

In National Semiconductor's concept, the cell data buses can contribute to power conservation or losses. By putting the camera image sensor, the motion video processor and display module on one side of a clamshell design, you can minimize the number of power-consuming data transfers to and from the CPU.

The enormous volumes associated with cell phones encouraged analog companies with power management capability to flood the market with white LED driver parts. The list of manufacturers included Maxim Integrated Products (Sunnyvale, Calif.), Linear Technology, National Semiconductor, Texas Instruments Inc., Intersil Corp. (Cupertino, Calif.) Vishay Siliconix, Fairchild Semiconductor International (South Portland, Maine; also a maker of LEDs) and Catalyst Semiconductor Inc. (San Jose).

The addition of digital still camera modules to cell phones required even higher voltages, as well as more power regulators. The DSP used for color correlation, JPEG compression and other image-processing functions can be powered by a low-voltage, low-current 1.8- or 1.2-V (300-mA source). But the charge-coupled device used to capture the image needs to be powered by at least 5 V. This requirement calls for additional boost regulators.

As for the power demands of this application, according to Fairchild strategic product planner Nazzareno Rossetti, the power dissipated by a palm-sized digital still camera (1.3 Mpixels) can be around 2 W during picture taking. The picture capability can be as high as 1.2 W (500 mA at 2.4 V) during viewing. In this case, two rechargeable NiMH cells with 700-mA-hour capacity in series can sustain close to one hour of picture taking and viewing, he wrote in an EE Times article (see May 5, 2003, page 57).

About two years ago, the conversation among designers of cell phones and handhelds veered sharply away from voltage regulator efficiency. A number of manufacturers-among them National Semiconductor and Analog Devices Inc. (Norwood, Mass.)-began to concentrate on turning off or slowing the central processor of the handheld unit as a means of controlling the power it consumes. The assumption is that the faster you clock a microprocessor (and its companion DSP), the more power it's going to consume. If you slow down its clock and lower its operating voltage, you can in fact lower its power consumption.

The technique, called voltage and frequency scaling, monitors the processing tasks that the central processor performs (the software code, in fact) and assigns them priorities. Streaming video decoding, say, may require processing muscle, so the device would be clocked at maximum speed and the power supply rails adjusted to provide the maximum I/O swing. But simply keeping the cell phone awake as it sits in your pocket requires little processor activity and only enough voltage to refresh the memory.

It's just not voltage regulator circuits that offer opportunities for power conservation, said Peter Henry, a marketing director for National Semiconductor's power group. The buses and interface components have a significant impact on cell phone power consumption. "The data paths represent a patchwork," Henry said. "They are not optimized for power consumption."

With voltage and frequency scaling, he said, "the processor doesn't care what the voltage is-just what the propagation delay is on the CPU clock. The reference is no longer voltage but, rather, timing."

Simply closing the loop between processor and power controller can result in a 30 percent power savings. But actually cutting the clock speed in half can provide power savings as high as 70 percent.

National Semiconductor developed power regulator circuits for the ARM processor core-circuits designed to monitor the processor's activity and scale it back (lower its frequency, even put it to sleep) in light-load situations. More recently, National developed a scaling device for Intel's PXA270 Xscale processor.

A custom PMIC would reduce the chip count for voltage regulator circuits, saving board space and manufacturing costs. But development of a custom PMIC could potentially delay time-to-market for new cell phone models. And-as with digital ASICs-the cost saving may not be entirely visible, unless you were to utilize many tens of millions of units.

"We see a trend away from that," said Tony O'Brien, marketing director for Micrel Semiconductor (San Jose). Cell phone models are turned out three or four times a year, he believes. Each phone model might offer a different configuration of features-some requiring more power control functions, others requiring less. "While there is interest in consolidating voltage regulator functions-using multiple LDOs in one package-there is no flexibility with a PMIC," O'Brien said.

To consolidate regulator functions, companies like Micrel will offer ICs with two and three linear regulators. Micrel, in fact, is working on what it calls a "baby PMIC," a device with a high-current switching regulator and two LDOs, all on the same chip.

Whether or not it makes sense to build a PMIC depends on which CPU-DSP media processor core you're tied to, suggested Fairchild's Rossetti. He sees a hegemony among three dominant cell phone architectures: the ARM CPU-based Omap camp (dominated by Nokia and Motorola), Qualcomm's CDMA partners (Samsung and Kyocera), and a third camp revolving around Intel's Xscale processor (whose users include Samsung, Motorola and others).

The Omap camp uses DSP media processors manufactured by Texas Instruments and STMicroelectronics (Phoenix) and a PMIC integrated with the analog baseband processor. In the Omap UMTS phone architecture, for example, the TBB5110 baseband processor interfaces flash memory and SRAM, the display, the camera or cameras and the feature connector (which includes a keypad and joystick). The device includes an ARM-based call processor (the man-machine interface) and a speech processor and physical-layer codec based on TI's C55x DSP. Power management is provided by STMicroelectronics' STw4200 PMIC. The transmitter and receiver are the only other big chips in this phone.

Linear Technology's power management solution for Apple's iPod used off-the-shelf building blocks to bring the music player to market quickly. Successive manufacturing iterations of the iPod used far fewer components.

The TI TCS2600 GSM/GPRS chip set reduces the cell phone to three big chips: the TRF6151 RF transceiver, the Omap730 application processor and the TWL3016 analog baseband unit, which includes the power management unit.

Qualcomm's CDMA phones use separate PMICs, such as the PM6650, designed by Qualcomm.

The Xscale architecture, used in some Hitachi, Motorola and Samsung cell phone models, relies on the DA903x PMIC, custom-made by Dialog Semiconductor (Kirchheim, Germany).

Rossetti's analysis, supported by cell phone teardown reports provided by Portelligent (Austin, Texas), concludes that there are often 12 to 18 separate voltage regulator functions, not all supported by a PMIC. The Samsung SCH-V410 CMDA-2000 camcorder phone, for example, is based on the Qualcomm chip set (including a PMIC). But it still uses nine additional regulator circuits, mostly LDOs from manufacturers as diverse as National Semiconductor, Micrel, Semtech (Newbury Park, Calif.), Maxim, Microchip Technology (Chandler, Ariz.) and Ricoh.

In addition to providing the dynamic range (a matter of power headroom) needed to drive audio headphones, the power management circuits had to drive the disk motor efficiently without instantly depleting the battery.

LTC's solution included up to five separate voltage regulator circuits-application-specific standard product building blocks that had been specially picked to serve the most efficient power management job for the task at hand. Linear Technology acknowledged that, while the use of specially selected off-the-shelf building blocks enabled Apple to bring its iPod to market quickly, far fewer LTC components were used in successive manufacturing iterations of the iPod.

A teardown report compiled by Portelligent, in fact, reveals that Apple elected to integrate separate power management functions with a custom circuit: the PCF50605 power controller and battery management unit, manufactured by Philips Semiconductors. (Though Philips makes the PMIC for the newer iPod mini, a Linear Technology LTC4055 USB power manager and lithium-ion battery charger does show up on the player's main board.)

A separate power management circuit for power amps-one that turns off the amp in idle conditions and modulates the power requirement according to the proximity to the nearest basestation-could increase efficiency to as much as 80 percent in an urban environment, Henry believes.

The clamshell design of many cell phones, putting the color display unit in the lid and the central processor in the body, can require as many 100 separate wires to be routed via flex cable through the phone's hinge, Henry said. Built on LVDS, National's Mobile Pixel Link (MPL) replaces the parallel signal lines running between the cell phone CPU and its peripherals with a simpler high-speed serial connection.

With the pixel resolution of camera phones increasing, analog manufacturers are preparing more sophisticated power management circuits. Dialog Semiconductor, for example, will partner with Carl Zeiss AG (Oberkochen, Germany) on the autofocus mechanism for new-generation camera phones. Dialog's president and CEO, Roland Pudelko, said he believes 10 Mpixels in the camera phone to be well within reach.