Squeezing out longer battery life in increasingly smaller, more sophisticated, feature-laden wireless products can be a challenge. The list of portable wireless devices grows-cell phones, pagers, global positioning systems (GPS), personal digital assistants (PDAs), laptop and palm computers with Bluetooth devices, and Web pads are available now, with other appliances on the way. With these products, the need increases for high efficiency across a wide range of supply and operating voltages."Consumer demand has stretched the capabilities of portable devices, in terms of power and functionality while pressuring for smaller size and significantly longer battery life. Now they want it all: months-long battery life, 2 W RF transmission and Web surfing capability from a product small enough to comfortably fit in a shirt pocket," writes Darryl Phillips, a senior applications engineer at National Semiconductor Corp. (Santa Clara, Calif.) and a contributor to this week's focus on power-management strategies for portable products. Among the issues at stake: regulation schemes for different system requirements, emerging battery chemistries and the trade-offs in cost, efficiency and space among low-voltage dropout and switched regulation schemes.
High efficiency is forcing designers of wireless digital handsets away from using LDO regulators and toward increased use of inductive and capacitive switching-type voltage converters. Behind the move is that designers must successfully juggle two dynamics if they are to succeed. One is a shift in source voltages as systems move from alkaline, nickel cadmium, and nickel-metal hydride battery chemistries to lightweight lithium-ion. The other dynamic is the need to tailor these source supplies to the broad range of operating voltages needed to power logic and baseband circuits, processors, power amplifiers and displays. And like the shift in source voltages, many of these circuit operating voltages are also in flux.
Driving the need to efficiently convert source voltages to operating voltages is the high priority placed on battery-driven operating time. Efficient conversion means minimal waste of precious battery power. What's more, designs need to maintain their high efficiency across operating states that range from low-power standby mode to full load. To achieve the high efficiency they seek, portable equipment makers are largely drifting away from low-dropout voltage regulators to more power-efficient inductor- and capacitor-based switching dc/dc converters designed to minimize loss while keeping down the size and cost.
True, LDOs have their place, and can often work well with switching converters. Indeed, in his article, Phillips offered some examples of where LDOs work best for portable power management. In particular, he wrote, "providing that the input voltage range is close to the desired output voltage, LDOs provide good efficiency."
But often those input and output voltages differ significantly. Then it is the drive to achieve the highest efficiency voltage conversion in the face of those disparate source and load voltages that fuels a move from LDOs toward switching dc/dc converters. The shift is not insignificant given the appeal that LDOs have historically held for handset designers, who appreciate the fact that, unlike switch-mode supplies, they generate no EMI. "LDOs are cheap, easy to use, and do the job. Customers love them. The problem is that efficiency is not their strong point," said Peter Henry, National Semiconductor's director of power management integration.
Mickey McClure, a senior product manager at Texas Instruments Inc. (Dallas) puts the requirements of handheld products into two categories. One category includes those products powered from one or two alkaline cells; the other, those powered from Li-ion cells. "For devices powered off one or two alkaline cells, you need to boost the voltage, primarily through a charge pump or conventional inductor-based boost converter," said McClure. "But with new cell phones almost exclusively going to lithium ion, you're not just interested in simply boosting voltage."
The reason is that, where alkaline, NiCd and NiMH cells develop about 1.2 V each, a Li-ion cell produces a nominal 3.6 V. Moreover, the voltage swing of a Li-ion as it discharges goes from about 4.2 V to 2.5 V, a range that falls both above and below the 3.3 V needed to power a typical cell phone chip set. As a result, the main power supply for a cell phone, if it is to make the most of the battery's energy, must be able to switch between boost (step-up) and buck (step-down) functions.
With logic voltages dipping to 1.8 V, that buck/boost capability needed with Li-ion cells reverts to a buck-only requirement. Moreover, not only has that low logic voltage already been reached with a chip set from cell phone maker Qualcomm Inc. (San Diego, Calif.) that takes a core voltage of 1.8 V, but additional voltage drops loom.
Beyond the baseband chips, another key power-consuming element in cell phones, the GaAs FET power amplifier, has blazed its own trail of change. And here too, changing voltages have affected power management and therefore regulator requirements.
"As the lithium-ion battery discharges down to 3 or 2.7 V, some designers boost that voltage up to a nominal 3.6 V," said Tony Armstrong, power ICs marketing director for Vishay Siliconix Inc., (Santa Clara, Calif.). Dc/dc converters are being harnessed to vary the amplifier's power level to the minimum needed to communicate with a basestation. "They are talking about synchronous buck converters that can adjust the output voltage from a nominal low of 0.3 V all the way up to 3.4 V and thereby optimizing battery life," said Armstrong. Vishay business development manager Kin Shum pointed out that synchronous converters wield two MOSFETs, eliminating the diode's inherent voltage drop. As a result, noted Shum, "People tend to move to a synchronous converter for all their low-voltage applications."
New display technology is another source of changing voltage requirements in portable handsets. David Bell, general manager for the power business unit of Linear Technology Corp., (Milpitas, Calif.)said that with color displays, designers are replacing the green or amber LEDs used for backlights with white LEDs. These have a forward voltage of 4 V compared to 1.8 V or 2 V for conventional LEDs and need a step-up converter where none was called for before. Also, said Bell, the color LCDs themselves run off a higher contrast-bias voltage, "up in the 9-V range, and once again requiring a boost converter."
As for PDAs, which have more complex power-management requirements than cell phones, boost converters are already part of the display subsystem, said Andrew Cowell, business manager for power products at Micrel Inc. (San Jose, Calif.). Not only do the larger displays in PDAs take higher voltages, around 20 V to 30 V, but the processors have higher performance levels, especially those that run Windows CE.
And on the subject of processors, Intel's power-saving SpeedStep specification for its latest mobile Pentium III processors is yet another curve ball that designers must field. That specification switches both the clock frequency and the core operating voltage when going from ac to battery power. Specifically, the switch is from 1.6 V and 600 MHz when ac-powered, to 1.35 V at 500 MHz when drawing power from the battery.
The bottom line is that power management in portable and wireless designs has changed the field of power from the static and staid field that it used to be, to one that is technically demanding, fast changing, and one which depends on that key product feature: battery-powered run-time.
Gil Bassak is a technical journalist and former practicing engineer who lives in Ossining, N. Y.