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PMICs pack power into small portables

Posted: 16 Aug 2007 ?? ?Print Version ?Bookmark and Share

Keywords:power management ICs? DC/DC converters? LDOs?

It's an old story: Shrinking product footprints drive functional integration. But in the power management arena, particularly for those devices supporting portable consumer electronics system designs, the story is still fresh.

With cellular handsets now squeezing voice, camera, GPS and other functions into a case less than a half-inch thick and with portable media players packing an MCU, disk drive and audio circuits into a form factor the size of a shirt pocket, the pressure to integrate basic power functions shows no signs of slowing down.

During the past few years, power IC makers have introduced a growing array of power management ICs (PMICs) targeted specifically at collapsing the primary power functions in portable CE systems into a single chip. The chips typically integrate a battery charger, one or more buck converters and in some cases, multiple low-dropout regulators (LDOs) to address the various power functions in today's shrinking portable CE devices.

Given the demands of "on-the-go" computing, most of the charging circuits in PMICs support charging from both USB and AC adapter sources. The highly dynamic nature of the power environment in a USB port presents some unique problems, however.

To ensure the integrity of the USB port, most charge circuits include some mechanism to protect it. Typically, vendors provide two charging levels, usually preset at the factory, though they may be set by the designer using external set resistors. The higher level supports the 500mA maximum load specified in the USB standard. The second, lower level is usually set to 100mA.

When available port supply current is insufficient to meet the 500mA level, the MCU instructs the system to adopt the lower charge level to ensure that the USB port is not overloaded. While that two-level approach protects the port, it can also unnecessarily extend battery charge time.

Some PMIC vendors now address the problem with circuits that embed intelligence to increase the efficiency of the fast-charge cycle. The circuits continuously monitor the voltage on the USB port. When power is pulled from the port to perform other system tasks and the port can no longer supply the full 500mA, the circuit dynamically adjusts the fast-charge current downward in discrete steps until the voltage on the USB port drops below the power available. A feedback loop typically compensates for voltage sags and keeps the part regulated.

Low load efficiency
Most new PMICs for portable consumer applications include one or more step-down DC/DC converters to drive the system MCU, memory, a hard drive and other I/Os. Since the Li-ion battery in these systems supplies 3.6V on average and since most core MCUs are designed to operate off much lower voltages, designers must step down the operating voltage as efficiently as possible to maximize runtime between recharge cycles. What is sometimes overlooked is the importance of converter efficiency in light-load conditions. Given the typical portable-device usage pattern of long periods in standby mode and bursts of active use, most systems require converters that can supply power at high efficiency under both usage conditions.

For most DC/DC converters, the efficiency curve drops dramatically when levels drop below 100mA. Many PMIC manufacturers now offer converters that operate in both fixed- and variable-frequency modes to maximize efficiency across all load conditions. But system designers can also address the problem by selecting DC/DC converters ideally sized for their application.

The switching frequency of the converter is also a key consideration. Given the space-constrained footprints of most portable consumer products, every millimeter counts. Buck converters operating at higher switching frequencies can operate with smaller and lower-profile external components.

RF- or audio-type circuits, such as wireless transmitters, receivers or microphones, place a far different set of requirements on their power source. Here, a high degree of isolation and a low-noise source are key to preventing the propagation of noise and adversely affecting performance. If you're designing the microphone pickup circuit in a Bluetooth headset, for example, you need good low-noise performance and a high level of isolation from input to output.

While linear voltage regulators offer far lower efficiency than a switching DC/DC converter, their good low-noise characteristics make them essential for RF and audio applications. Such circuits typically consume relatively low levels of current, so their impact on systemwide power efficiency levels is often minimal.

Selecting solutions
When selecting a PMIC that integrates LDOs on-chip, it is worth considering whether the LDOs will offer fast transient response and support fast startup. Capacitor compatibility may also be an issue: Will the regulator function with lower-cost, low-ESR ceramic capacitors? If so, will it prevent the designer from using other types of capacitors, if needed?

Some PMIC vendors pack a remarkable array of functions onto a single die. The MAX8662 from Maxim Integrated Products Inc. integrates a USB/AC charger with two step-down DC/DC converters, four LDOs and a step-up converter to support up to seven white LEDs or organic LED displays. The two step-down converters can be used to support an array of operations with 1.2A and 900mA loads. For noise-sensitive circuits, the four LDOs run the gamut from 150mA to 500mA loads.

Along the same lines, the LP3910 from National Semiconductor Corp. supports high-performance media players and other handheld designs by incorporating a dual-source Li-ion battery charger; two buck converters with dynamic voltage scaling; two LDOs; a wide-load-range buck-boost converter with programmable output and a four-channel, 8bit ADC for managing battery usage.

Other vendors argue that less is more. At Linear Technology Corp., the philosophy is to integrate functions only as long as they don't compromise performance or extend design time. "Some of our customers say they're spending more design time turning off what they're not using in some of their PMICs, just to make sure those functions don't interfere with the ones they are using," said Linear power-marketing manager Tony Armstrong. "We don't want to make the part so complex that it becomes a design and layout nightmare."

Linear's most highly integrated PMIC, the LTC3555, combines the company's patented PowerPath management circuit with a USB/AC battery charger; three synchronous buck converters, delivering outputs of 1A, 400mA and 400mA and an LDO in a 4mm x 5mm QFN package. The device is targeted at hard drive-based MP3 players, PDAs, GPS devices and other handheld applications.

Additional requirements
Ideally, of course, designers of consumer systems want to find the best PMIC to fit the requirements for their application, including footprint and thermal constraints. Over the past year, power IC makers have sought to simplify that task by offering a wider array of products more precisely targeted at particular applications.

AnalogicTech's AAT2550 PMIC adds a digital thermal-control loop, automatically throttles back charge current to avoid thermal shutdown.

A case in point is Advanced Analogic Technologies Inc.'s AAT2554. Part of AnalogicTech's SystemPower family, the device targets its feature set at lower-powered applications such as Bluetooth headsets, utility watches and other ultraportables. The PMIC combines a 500mA battery charger, a 250mA step-down converter and a 300mA LDO in a 16-pin TDFN package. A sister device, the AAT2550, supports the design of higher-performance MP3 players, smart phones and other portable devices by bringing together a 1A battery charger with two 600mA step-down converters in a 4mm x 4mm QFN package.

Similarly, the bq25015 and bq25017 from Texas Instruments Inc. target space-constrained Bluetooth applications with a USB/AC charge circuit and 300mA DC/DC converter on a single chip. And Linear has recently added a simpler PMIC, the LTC3559, that integrates a charger and two buck converters in a 3mm x 3mm QFN for RAM-based MP3 players or other simpler handheld designs.

Thermal constraints
Given the tight form factors in this market, thermal considerations remain a primary concern. Most designs can't afford a heat sink, let alone a fan. "One challenge of squeezing three relatively high-powered function blocks together is that they can generate more heat than the package can handle," said Bill Weiss, product line director for AnalogicTech.

To prevent excessive heat from affecting the performance of the buck converters or causing a processor interrupt, AnalogicTech's PMICs add a digital thermal control loop that senses when the IC is reaching a threshold temperature and automatically throttles back the charge current to decrease the power being dissipated.

- John Mayer

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