By Vincent Biancomano, EE Times
Power-managing ICs in wireless products are now playing a major role in system-level solutions. Cell phones, which are being combined with PDAs and cameras, are becoming increasingly small, dense and power-hungry, offsetting the size and energy density advances made by the lithium-ion battery sources powering them.
IC makers under the gun to redress the net loss in overall power margins are focusing on reducing the quiescent operating current of chips in the system to as little as one-tenth that of the previous generation. They're also adding more connectivity to allow wireless systems to sample the power environment and dynamically adjust to changing conditions.
"Voice communication devices have become multimedia devices," said Laurent Jenck, director of the Portable Power Unit at ON Semiconductor (Phoenix), noting the replacement market exceeded 50 percent of total units for the first time last year. That suggests that there's a more sophisticated audience of second- and third-generation users looking for more features, and thus a greater need to watch every microwatt. "There's huge pressure to lower quiescent current. And there's a proliferation of different voltages and the need for dynamic programmability."
With design targets changing very quickly, developers are taking a subsystems, or non-ASIC, approach toward power management. Such an approach establishes monitoring at the local level, yet keeps the blocks in close contact globally to maintain a watch on battery drain. The architecture is supported by a collection of chips, largely dc/dc regulator devices often running in the 1-MHz region. These devices have suitable control/supervisory circuitry that provides more efficient charging and have the flexibility to work from multiple sources like the universal serial bus.
Fuel-gauging devices keep more accurate tabs on a battery's remaining run-time. More wireless devices carry two displays, and usually both are in color, pointing out the desirability of more efficient white-LED drivers for double-duty backlighting tasks.
Standalone ICs have also arrived to perform photoflash charging for higher-resolution cameras that use high-intensity sources, which deliver performance well beyond that of traditional LEDs. Apart from the multitude of general-purpose, multivoltage baseband dc/dc converters for the current crop of wireless devices, specialized and dedicated dc/dc converters are coming into their own to distribute power efficiently to subsystems ranging from cameras to RF power amplifiers.
With the overall on-time of wireless devices tending to increase and the need for frequent and easily accessible battery charging becoming the rule rather than the exception, vendors have come up with a spate of new chargers that strike the best balance for a given application. The offerings include generally noisy but efficient switching-type chargers; pulse-type chargers, which may be more efficient but tend to complicate accurate fuel gauging and degrade the life of a cell; and low-cost but inefficient linear chargers.
The Linear Technology Corp. (Milpitas, Calif.) LTC4054 is an 800-milliamp charger IC for lithium-ion cells. It's suitable for charging batteries from a USB port.
Texas Instruments Inc. (Dallas) says its bqTinyII series represents the first 3 x 3-mm charger IC for single-cell lithium-ion cells. Features include the ability to charge a cell from a USB port or ac wall adapter. The series includes an internal FET rated at 1 amp.
The MAX1501 charger IC from Maxim Integrated Products (Sunnyvale, Calif.) is billed as the first to integrate constant-temperature regulation circuitry to optimize battery charge time and eliminate excessive power dissipation. The chip, which works as a constant-current, constant-voltage charger from an unregulated 4.5- to 13-volt input, can deliver up to 1.4 A. It accommodates both lithium-ion and nickel metal hydride batteries. Other chips from Maxim include the MAX1925/26, for use in digital still cameras and PDAs, and the MAX1908, which runs off an ac adapter and delivers up to 4 A to charge two to four Li-ion cells in series.
At the other end of the charging process, fuel gauge ICs reckon remaining battery run-time with increasing accuracy. "Battery advances haven't increased proportionally to new cell phone functions; also, fuel gauge accuracy has to be higher," said Madhu Rayabhari, director of strategic marketing for the Integrated Circuits Group at Fairchild Semiconductor (San Jose, Calif.). The more advanced ones, in so-called smart batteries, which have at least some part of the fuel gauge embedded in the battery pack, are evolving toward complete units that can store algorithms specific to a given cell as well as track operating temperature and the number of charge cycles.
"The lithium-ion's nominal voltage is 4.2, but as it depletes it flattens out to about 3.6 V for 80 percent of the operating cycle. Then it drops drastically," said Mark Richey, product-marketing managerat Intersil Corp. (Milpitas). "The area between points two and three is steep, and simple A/D analog-to-digital conversion techniques to measure charge as tradi-tionally indicated by bar displays isn't good enough. It's more important to know the exact remaining run-time, with direct readout in minutes and seconds." The readout function, which is likely to be microcontroller-based, is expected to be a standard function within a year.
Maxim/Dallas bills the DS2740 high-precision coulomb counter as the most affordable fuel gauge on the market, unit-priced at $1 in 100,000-piece quantities. The part performs a 15-bit current measurement (resolution of 78 microamps), has an A/D offset error of less than 2 microvolts and has a gain error of less than 1 percent to accurately report battery capacity. It comes with a one-wire interface to give the host system read/write access to status and current-measurement registers. The company's DS2751 multichemistry gauge, in an eight-pin TSSOP, integrates precise current, voltage and temperature measurements. It can be mounted on lithium-polymer cells.
TI's bq270x0 bqJunior fuel gauge is intended to work in 2.5- and third-generation mobile handsets and PDAs. It measures the battery's charging and discharge current to within 1 percent using a voltage-to-frequency converter. A separate A/D converter measures battery voltage and temperature. The bq270x0 then calculates remaining battery capacity and system run-time using its on-board processor.
When it comes to the cell phone's display devices-especially color displays-and the white-LED driver products required for the most efficient backlighting, the need for power management becomes acute. LED drivers commonly provide a constant current to achieve their backlighting goal of powering four to six LEDs for the handheld devices' primary and secondary displays, with ICs available for driving as many as a dozen LEDs. Most of the chips provide smaller size, lower operating currents and greater flexibility, and more allow white LEDs to be driven in parallel for maximum efficiency. Series-driven LEDs usually require a boost converter when the wireless device is powered by a single lithium-ion battery; alternatively, charge pumps are used when there are issues with electromagnetic interference.
Maxim calls its MAX1582 the first dual-output (i.e., for both main and subdisplays) white-LED driver. It powers up to six LEDs with a constant current at 84 percent efficiency, and, according to the company, it reduces power requirements by about 25 percent. It comes in a 12-pin QFN.
Also new from Maxim is the MAX1573, said to be the smallest, most efficient charge pump for driving up to four white LEDs. The chip is packaged in a 14-ball UCSP measuring 2.1 x 2.1 x 0.6 mm.
The company also released the MAX-1561, a 26-V step-up dc/dc converter for powering six LEDs with up to 84 percent efficiency. It features a 30-V power MOSFET switch that can support up to 12 LEDs in multiple chains. Dimming can be achieved via either an analog signal or digital pulse-width-modulation input. It comes in a 3 x 3-mm QFN.
Maxim's MAX1984/85/86 have individual regulators for driving eight, six and four LEDs, respectively. The devices, in 4 x 4-mm QFNs, use synchronous rectification to achieve efficiency that's said to exceed 90 percent. An LED select pin allows one or two subsets or all LEDs to be illuminated.
Fairchild's FAN56xx driver series (outputs matched to plus/minus 3 percent for up to four LEDs) and its collection of low-forward-voltage LEDs in its QTLP6XX series are part of its effort to offer the broadest available range of backlighting options.
National Semiconductor's LP3933 Lighting Management System contains four white-LED drivers for a wireless device's main display, two white-LED drivers for the second display and RGB LED drivers for driving a device's so-called "fun lights." The fun lights, which can be activated on an incoming call or on initiation of a given function, can be programmed for color, brightness and flashing sequence. The RGB drivers also feature a flash function to support picture taking for cellular phones that have cameras.
Sipex Corp. (Milpitas) has just released its SP6690, which drives up to six white LEDs in series. It's billed as a low-current (20-mA, typical) device and has a standby current of 10 nA.
Answering the call for small and thin, Linear Technology's LT3465 dc/dc step-up converter delivers inherently matched currents for each of six LEDs driven in series from a lithium-ion cell. Maximizing its functionality as appropriate for its ThinSOT package, the device includes an integrated Schottky diode, output voltage limiter (if LEDs become disconnected) and internal soft-start circuitry, and uses one pin for handling the dual functions of dimming and device shutdown. The LT-3465 operates at 1.2 MHz. It requires a 0.22-microhenry inductor and 1-microfarad capacitor at the input and a 0.22-microfarads capacitor at the output.
ON Semiconductor's NCP5009 white-LED driver has an input for a photodetector that senses ambient light. The NCP5007 from ON is due for a July release.
Also on the market is Intersil's EL7513, which will drive up to four LEDs in series or 12 LEDs in series/parallel.
Advanced Analogic Technologies (Sunnyvale) earlier released its AAT3110 charge pump device, a voltage doubler with a quiescent operating current of 13 microamps. The company's most recent charge pump is the AAT3123/4 for driving four or six LEDs, respectively. It uses a dual-mode switch and fractional (1.5 x) dc/dc conversion.
Converging on chargers
White LEDs are often linked to the photoflash function in cameras, but that's changing, and with the change comes a different class of charger. "The convergence of camera and phone proved to be popular. Now, there's demand for higher-resolution CCD charge-coupled device imagers, and that calls for a brighter flash than required previously by low-resolution devices," said Tony Armstrong, product-marketing manager at Linear Technology. "It requires a bit more power, but the light coming from, say, a xenon device is five to 10 times as much."
The company's LT3420, for photoflash duties, is a standalone flyback converter with automatic refresh capability to charge high-voltage (typically 320-V) capacitors. The company touts it as four times better than competing devices. Having a peak current output of 1.4 A, it can charge any size photoflash capacitor at an efficiency of greater than 75 percent. Typically, it charges a 220-microfarads capacitor in less than 3.7 seconds from a 5-V input. The part maintains current-limited transformer operation through the entire charge cycle to eliminate the high inrush current normally encountered in flash modules. This device, in a 10-lead MSOP, eliminates the zener diode or resistor voltage divider normally required for regulating output voltage.
Specialized dc/dc power blocks, for cameras through powers amplifiers, are also making contributions of their own. Maxim's MAX1565 is a five-output IC that integrates one step-up and one step-down converter with three step-up controllers for most digital camera requirements. Maxim's MAX1958/59 800-mA power-management ICs have adaptive bias control aimed at heterostructure bipolar transistor power amps.
National's LM2614 dc/dc converter is optimized for RF power amps running from a single lithium-ion cell. It can deliver up to 400 mA at 1 to 3.6 V.