Addressing Power Consumption Challenges in Portable Designs with Power Management ICs

Even as cell phone designers continue to pack voice, camera, GPS and other features into enclosures no thicker than half an inch, and portable media player designers combine microcontrollers, hard drives and audio circuits into pocket-sized packages, the pressure to pack multiple features into portable designs remains enormous.

Over the past few years, power semiconductor manufacturers have introduced a large number of power management ICs (PMICs) designed to integrate the main functions in portable consumer electronic systems into a single chip. These chips generally integrate a battery charger, one or more buck converters, and sometimes multiple low-dropout regulators (LDOs) to handle the various power functions in today's increasingly compact consumer electronic devices.

Given the requirements of "mobile" computing, most charging circuits in PMICs support both USB charging and AC adapter power charging. However, the highly dynamic nature of the power environment on a USB port presents some unique problems.

To ensure the integrity of the USB port, most charging circuits include port protection mechanisms. Manufacturers generally provide two charging levels, which are usually preset at the factory, although they can also be set by the designer using external reset resistors. According to the USB standard, the higher level supports a maximum load of 500 mA, and the lower second level is often set at 100 mA.

When the port source current is insufficient to meet the 500mA level requirement, the system microcontroller commands the system to use a lower charging level to ensure that the USB port is not overloaded. Although this two-level solution can protect the port, it may also unnecessarily extend the battery charging time.

Some PMIC vendors are addressing this issue by using embedded intelligence to improve the efficiency of the fast charge cycle. These circuits constantly monitor the voltage on the USB port. When power is pulled away from the port to perform other system tasks and the port is no longer providing the full 500mA current, the circuit dynamically adjusts the fast charge current downward in discrete steps until the voltage on the USB port drops below the available power. A feedback loop is typically used to compensate for the voltage drop to complete the regulation of the component.

Light load efficiency

Most new PMICs for portable consumer applications include one or more step-down DC/DC converters to drive the system microcontroller, memory, hard drive, and other I/O devices. Since the Li-Ion batteries in these systems provide an average voltage of 3.6V, and since most core microcontrollers are designed to operate at much lower voltages, designers must step down the operating voltage as efficiently as possible to maximize the run time between charges. The importance of converter efficiency under light load conditions is sometimes overlooked. Since portable devices typically spend most of their time in standby mode and bursts of active use, most systems require converters that can deliver power with high efficiency in both use cases.

Figure: Maxim's MAX8662/3 parts integrate a USB/AC charger, two step-down DC/DC converters, four LDOs, and a step-up DC/DC converter on a single chip, a typical example of power integration in portable systems.

For most DC/DC converters, the efficiency curve drops sharply when the current is less than 100mA. Many PMIC manufacturers now offer converters that can operate in both fixed and variable frequency modes to maximize efficiency under all load conditions. In addition, system designers can also solve this problem by selecting the DC/DC converter that best suits their application.

The switching frequency of the converter is also a major consideration. For most portable consumer products, board space is limited and every millimeter counts. A buck converter that operates at a higher switching frequency can work with smaller external components.

RF or audio type circuits, such as wireless transmitters, receivers or microphones, have very different power supply requirements. In these areas, high isolation and low noise power supplies are critical to prevent noise transmission from degrading performance. For example, when designing the microphone pickup circuit of a Bluetooth headset, excellent low noise performance and high levels of input and output isolation are required.

Although linear voltage regulators are much less efficient than switching DC/DC converters, their excellent low-noise characteristics make them indispensable components in RF and audio applications. The current consumption of these circuits is generally quite low, so the impact on system-wide power efficiency is usually minimal.

When selecting a PMIC with an on-chip low-dropout regulator, it is important to consider whether the LDO can provide fast transient response and support fast startup. Capacitor compatibility is also an issue: Can the regulator use lower-cost, low-ESR ceramic capacitors? If so, will it prevent the designer from using other types of capacitors if needed?

Some PMIC manufacturers integrate a large number of functions into a single chip. For example, Maxim's MAX8662 integrates a USB/AC charger and two step-down DC/DC converters, four LDOs and a step-up converter to support up to seven WLED or OLED displays. The two step-down converters can be used to support a range of loads from 1.2A to 900mA. For noise-sensitive circuits, the four LDOs support the full range of loads from 150mA to 500mA.

In the same field, the LP3910 launched by National Semiconductor integrates a dual-source lithium-ion battery charger, two buck converters with dynamically adjustable voltage, a buck-boost converter with programmable output over a wide load range, and a 4-channel 8-bit A/D converter for battery usage management to support high-performance media players and other handheld designs.

Other manufacturers are adhering to the philosophy of "rich in simplicity". Linear's philosophy is to integrate more functions without reducing performance or extending the design cycle. "Some of our customers have said that they have to waste more design time turning off things that are not needed in some PMICs just to ensure that those unused functions do not interfere with useful functions," said Tony Armstrong, power marketing manager at Linear. "We don't want to make the part too complicated and turn it into a design and layout nightmare."

Linear's highly integrated PMIC-LTC3555, in a 4mm x 5mm QFN package, integrates the company's patented PowerPath management circuit and USB/AC charger; three synchronous buck converters that can provide output currents of 1A, 400mA and 400mA respectively, and one LDO. The device targets hard disk-based MP3 players, PDAs, GPS devices and other handheld applications.

Ideally, of course, designers of consumer electronics systems want to find the best PMIC that meets their application requirements, including footprint and thermal constraints. Over the past year, power semiconductor manufacturers have been trying to simplify this task by introducing a wider range of products specifically targeted at specific applications.

AAT2554 from Advanced Analogic Technologies is a proof of this. AAT2554 is a product in AnalogicTech's SystemPower series, and its feature set is aimed at lower power applications, such as Bluetooth headsets, power meters and other ultra-portable devices. This PMIC uses a 16-pin TDFN package and integrates a 500mA battery charger, a 250mA step-down converter and a 300mA LDO. Another device in the same series, AAT2550, uses a 4x4mm QFN package and integrates a battery charger and two 600mA step-down converters to support the design of higher-performance MP3 players, smart phones and other portable devices.

Similarly, Texas Instruments' bq25015 and bq25017 combine a USB/AC charger circuit and a 300mA converter on a single chip, targeting space-constrained Bluetooth applications. And Linear recently introduced a simpler PMIC, the LTC3559, which integrates a charger and two step-down converters in a 3 x 3-mm QFN package, suitable for RAM-based MP3 or other simpler handheld designs.

Given the current market demand for smaller form factors, heat dissipation has always been a headache. Most designs cannot add a heat sink, let alone a fan. "One of the challenges of integrating three fairly high-power functional blocks is that they can generate more heat than the package can handle," said Bill Weiss, product line director at AnalogicTech.

To prevent overheating from affecting the performance of the buck converter or causing processor interruption, AnalogicTech's PMIC adds a digital thermal control loop that senses whether the IC reaches a certain temperature threshold and automatically reduces the charge current to reduce power consumption.