<And time is of the essence. Consider that cell-phone maker Qualcomm Inc., San Diego, has already rolled out a chipset whose core
voltage goes as low as 1.8 V. >
2/25/00 - Shifting Gears To Power A Wireless World -- Digital wireless handsets are moving away from low-dropout
regulators to switching DC/DC controllers.
Feb. 25, 2000 (Electronic Buyers News - CMP via COMTEX) -- Like the storm-swept ship that must battle high winds and waves, digital
wireless devices such as cellular phones, pagers, global positioning systems, personal digital assistants, ultra-subnotebook
computers, emerging Bluetooth devices, and Web pads must successfully manage twin forces.
On one hand are changing source voltages as systems shift from alkaline, nickel-cadmium, and nickel-metal/hydride battery
chemistries to lightweight lithium-ion technology. On the other are the multitude of operating voltages, many also in flux, needed to
power logic and baseband circuits, processors, power amplifiers, and displays.
Against this backdrop is the high priority placed on long battery-driven operating time. That priority, in turn, speaks to the need for
voltage converters that remain highly efficient from low-power standby mode to full-load conditions. Toward that end, handset makers
are being forced away from low-dropout (LDO) voltage regulators to more power-efficient inductor- and capacitor-based switching DC/DC
controllers.
As a result, worldwide shipments of these controllers are expected to grow from more than $1 billion last year to $2 billion by 2003,
according to analyst Mark Gaboriault at Venture Development Corp., Natick, Mass.
For DC/DC-controller vendors looking to ride this wave, the path to success is clear enough: fine-tune efficiency to the highest-possible
degree while carving component size and cost to a minimum.
Voltage variations
The move away from LDOs to switching DC/DC controllers stems from the need for highly efficient voltage conversion in the face of
disparate source and load voltages. On the source side, much of that disparity stems from a switch to lithium-ion (Li-ion) battery cells,
which have a higher voltage than alkaline, nickel cadmium (NiCd), or nickel-metal/hydride (NiMH) cells.
At the same time, the operating voltage of many logic and baseband chipsets has dropped due to shrinking feature size and the quest
to cut power consumption. The effect has been to scatter source voltages and operating requirements, with the specifics depending on
other system factors like features, cost, and size.
Mickey McClure, a senior product manager at Texas Instruments Inc., Dallas, sees the requirements of handheld products falling into
two categories-those powered from one or two alkaline cells and 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," McClure
said. "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 cells (like NiCd and NiMH batteries) develop about 1.2 V, a Li-ion cell generates a nominal 3.6 V.
Moreover, Li-ion swings from about 2.5 to 4.2 V, a range that falls above and below the 3.3 V need to power a typical cell-phone
chipset. That means the main power controller for a cell phone must perform both boost and buck (step-down) functions.
Even that requirement is quickly changing, however, as logic, baseband, and power-amplifier voltages continue to drop. Soon enough,
with logic voltages headed toward 1.8 V, that buck/boost capability will give way to the need for buck only.
And time is of the essence. Consider that cell-phone maker Qualcomm Inc., San Diego, has already rolled out a chipset whose core
voltage goes as low as 1.8 V.
"By pushing geometries and operating voltages much lower than anyone expected, the industry has beaten its CMOS technology
roadmap by several years," said Peter Henry, director of power-management integration at National Semiconductor Corp., Santa Clara,
Calif. "The net result now is that digital circuits operate at 1.8 V and 1.5 V, and we are starting to look at 1.2 V."
Power play
Moreover, the power amplifier, typically a GaAs FET, has followed its own evolutionary path, bringing with it changing voltage and
therefore DC/DC-controller requirements.
"It used to be that the power amplifier required a pretty high voltage, say 5 or 6 V, to operate, which meant that no matter what battery
chemistry you used, you needed a boost converter," McClure said. "Unfortunately for us, the power amplifier voltages are dropping."
It's unfortunate because power amplifiers don't need a regulated main voltage, and "more and more, you hear about people running the
power amplifier directly off the battery," McClure said.
Yet, even with power amplifiers taking 3.6 V, which some designers supply directly from the Li-ion cell, other designs still call for a
boost converter to stretch the battery's run time.
"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 at Vishay Intertechnology Inc.'s Siliconix subsidiary, Santa Clara. Moreover, with the
third-generation wideband phones "on the short-term horizon, manufacturers are getting much more clever at how they drive the power
ampli-fier," he said.
Specifically, DC/DC controllers are seen as a way to vary the amplifier's power level to the minimum needed to communicate with a
base station.
"They're 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," Armstrong said.
The choice of a synchronous over a nonsynchronous converter is guided by the underlying theme of efficiency, said Vishay
business-development manager Kin Shum. Unlike nonsynchronous converters, which use a MOSFET and a diode, synchronous
converters wield two MOSFETs, eliminating the diode's inherent voltage drop. As a result, systems designers tend to move to
synchronous converters for their low-voltage applications, according to Shum.
Emerging display technology is another potential source of new voltage requirements in wireless handsets. The introduction of color
displays is affecting power controllers in two ways, said David Bell, general manager of the power business unit at Linear Technology
Corp., Milpitas, Calif.
First, growing demand for color displays is forcing a move away from using green or amber LEDs for backlights and toward white LEDs,
Bell said. Because white LEDs have a much higher forward voltage, 4 V compared with 1.8 or 2 V for conventional LEDs, they need a
step-up converter where none was needed before.
Additionally, color LCDs run off a higher contrast-bias voltage, "up in the 9-V range, and once again requiring a boost converter," he
said.
A wild card in the cell-phone-display game is the emerging technology of organic electroluminescent (EL) displays, which produce
color, operate at a lower voltage than the 9 V for color LCDs, and preclude the need for a backlight, thus saving power.
"Everyone is waiting to see if organic EL displays take off," said Bill Edmiston, semiconductor product manager at Toko America Inc.,
Mt. Prospect, Ill. If so, cell-phone power requirements will change once again.
(Where conventional monochrome EL displays are used as display or keypad back- lights in cellular phones and PDAs, they command
their own type of high-voltage AC-driver chips, a specialty of companies such as IMP Inc., San Jose.)
In 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 of power products at Micrel Semiconductor Inc., San Jose. Not only do the larger
displays in PDAs take higher voltages (around 20 to 30 V), but the processors have higher performance levels, especially those that run
Windows CE.
Product parade
The broad, shifting range of source and load voltages has spurred a crop of products that combine flexibility and small size.
To power systems carrying two 1.2-V battery cells, which today calls for boosting battery voltages, TI in November introduced its
TPS6012x charge-pump family. The TPS6012x drives up to 200 mA at 3.3 V plus/minus 4% from the 1.8 to 3.6 V generated by two
alkaline, NiCd, or NiMH cells. Efficiency is as high as 90%, and standby mode draws just 60 microamps.
For driving 5-V circuits, a companion TPS6013x family develops 5 V plus/minus 4% from a 2.7- to 5.4-V input, typically derived from
three alkaline, NiCd, or NiMH cells or one Li-ion cell. Available driving current is 150 mA or 300 mA, depending on the model.
As switched-capacitor charge pumps, these chips store energy in four low-cost capacitors, eschewing an inductor that can radiate
hard-to-suppress EMI. In addition to a power-saving "pulse skip" mode that keeps efficiency high under low loads, both chips have a
logic-shutdown function that cuts supply current to 0.5 microamps.
In September, Linear Technology announced its LTC1754-5, a 5-V charge pump in an SOT-23 package. Delivering 50 mA from one
Li-ion cell, the chip draws 13 microamps of operating current and only 1 microamps when shut down. With an input range of 2.7 to 5 V,
the LTC1754-5 has greater than 82% efficiency at 3-V input, while delivering an output current from 1 to 50 mA.
Another SOT-23 boost controller from Linear, the inductor-based LT1615, generates up to 34 V from inputs of 1.2 to 15 V, making it
well suited for LCDs in PDAs and digital cameras. For high efficiency, the chip's quiescent current is only 20 microamps and its
shut-down current an impressively small 0.5 microamps. A 400-ns off time allows the use of small, low-cost inductors and capacitors.
Dual-output boost controllers from Maxim Integrated Products Inc., Sunnyvale, Calif., address logic and display-voltage requirements in
PDAs and save space in the process. The MAX1677 includes on-chip MOSFETs to supply 350 mA to logic circuits and up to
plus/minus 28V at 20 mA for an LCD from two or three alkaline, NiCd, or NiMH cells.
For PDAs that carry four such cells or two Li-ion cells, the MAX1774 delivers a main output of 2 to 5.5 V at up to 2 A for logic and 1 V
at up to 1 A for the processor core. Included are flags that indicate when the PDA is being powered from an AC line as well as from
either main or backup batteries.
GaAs power amplifiers require a negative, low-noise, regulated voltage supply, and these attributers are available from Maxim's
MAX881R, which produces -0.5 V to -Vin from an input of 2.5 to 5.5 V. The part features a package, called microMAX, that's half the
size of an SO-8.
National Semiconductor also fills this bias voltage need with its LM2687, which inverts a positive voltage in the range of 2.7 to 5.5 V and
then regulates the adjustable output to deliver -1.5 to -4.5 V at 10 mA.
National also carries a dual-output DC/DC controller, the LM2685, which packs a switched-capacitor doubler, LDO regulator, and
switched-capacitor inverter in a TSSOP-14. Accepting 2.85 to 5.5 V, the LM2685 develops a regulated 5-V output for components that
need it as well as a -5-V unregulated supply for LCD contrast bias voltage.
A switched-capacitor controller from Toko America, the TK75018, also serves as a positive-voltage inverter, as well as a noninverting
step-up converter or a dual-output voltage doubler. It operates from a supply of 3.5 to 7 V. Typical output current is 35 mA, typical
quiescent current is 2.5 mA, and standby current is less than 200 microamps.
Systems powered by one Li-ion cell needing a converter that delivers both buck and boost functions can take advantage of the
UCC39421 from TI's Unitrode operation. Introduced in June, the chip operates at a greater than 95% efficiency, accepts voltages down
to 1.8 V, and drives an external p-channel or n-channel MOSFET.
A switching frequency of up to 2 MHz keeps the required size of the inductor and capacitors small. But the high switching frequency
can also be cranked down for high efficiency at low power, or synchronized to an external clock.
Other combined buck/boost-controller ICs are available in the Si916x family from Vishay Siliconix. Among the most recent, the
Si9168BQ, announced in February, touts typical efficiencies of 95% under full load and 80% under light load. Configured as a buck
converter, the chip produces output as low as 1.3 V from input voltages between 5 and 10 V.
As a boost converter, it produces up to 12.6 V from input voltages as low as 5 V. Output current depends on the designer's choice of
an external MOSFET. The chip operates at frequencies up to 2 MHz and comes in a 16-pin TSSOP.
A second family member, the Si9169BQ, generates voltages between 1.5 and 6 V from inputs of 2.7 to 6 V.
A novel buck/boost design from National Semiconductor, the LM3352, relies strictly on switched-capacitor voltage regulators. Where
other switched-capacitor controllers work as voltage inverters or doublers, the LM3352 achieves a fractional voltage gain. Offered as a
low-cost alternative to inductive switchers and a high-efficiency alternative to LDO regulators, the LM3352 delivers, depending on the
model, a constant 2.5, 3, or 3.3 V at up to 200 mA over a Li-ion cell's full voltage range. Typical operating current is only 400
microamps, while shut-down current is just 2.5 microamps.
Another novel switched-capacitor product is the SC1460 "capless" charge pump from Semtech Corp., Newbury Park, Calif. Switching
at 8 MHz makes it possible for the SC1460 to bring its charge-pump capacitors on-chip, leaving only input- and output-filter capacitors
to be added. The family's two models are the SC1460-3.3, which produces 3.3 V from a 2.5-V input, and the SC1460-5.0, which
generates 5 V from a 3.3-V input.
Two other charge-pump boost controllers from Semtech were announced in January. The 60-mA SC1464 doubles any input voltage
from 2.5 to 3.5 V, offers four selectable switching frequencies between 8 kHz and 1 MHz, takes three external surface-mount
capacitors, and comes in an MSOP-8.
Semtech's 20-mA SC1517-5 develops a regulated 5-V plus/minus 4% output from an input between 2.7 and 5 V. It switches at a fixed
800-kHz (typical) rate, takes three surface-mount capacitors, and draws only 10 microamps of quiescent current.
As the operating voltage for logic and DSP circuits drops to 2.5- and 1.8 V, handsets powered by Li-ion cells will forgo-at least for those
circuits-the need to boost voltage. Instead, attention will shift to highly efficient buck controllers, like the LTC1701 that Linear
Technology introduced in December. Billed as the industry's first step-down current-mode DC/DC converter in a five-lead SOT-23
package, the chip accepts input voltages of 2.5 to 5.5 V to produce an output down to 1.25 V. Complementing its small size is a
1-MHz operating frequency and an internal MOSFET switch having a 0.28-ohm RDS(on) and 0.5-A output current.
Linear also takes the unusual step of building a step-down controller from a charge-pump topology in its two-member LTC1503 family.
The aim is to achieve greater efficiency than an LDO while avoiding the need for an inductor. Both family members source 10 mA, one
at 1.8 V, the other at 2 V, from input voltages between 2.4 and 6 V.
But it's still a synchronous, inductor-based topology that digital-handset designers turn to for highest efficiency at low voltage, drawing
still other vendors such as Micrel, with its MIC2177/78 and MIC2179; Toko, with its TK654xx; and Vishay Siliconix, with its
Si9167BQ-each touting efficiencies of up to 95%.
Gil Bassak is a freelance technical writer based in Ossining, N.Y.
ebnonline.com
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By: Gil Bassak
Copyright 2000 CMP Media Inc. |
