Power chips on the move with mobile electronic devices

The rapid popularization of portable electronic devices over the last two decades has driven incredible growth and turned the power semiconductor marketplace on its head. Twenty years ago, practically all power management integrated circuits (ICs) were various types of devices that managed, monitored and converted line power – the AC power coming from the power grid, or a DC battery plant to the requirements of semiconductors.

This has changed dramatically with the sudden rise in portable, battery-powered consumer electronic applications that includes cell phones, portable media players, digital cameras, personal digital assistants, portable medical devices, mobile video games and platforms and many others. Now, at least half of all power management semiconductors are integrated into portable devices. Another indication of this shift is the accelerated growth in lithium Ion (Li-Ion) batteries. Since 2001, the volume of Li-Ion batteries produced worldwide has grown by over 400 percent [1].

Indeed, the demand for mobile electronic systems and the resulting need for more portable power solutions is the impetus behind a thriving power management IC marketplace. While the overall annual growth rate of the semiconductor industry, in general, has been approximately seven percent in recent years, power chips have been growing at a rate of 15 percent since 2002 [2]. And the growth of power semiconductors is expected to continue at this rate through 2011 [3]. Since shipments of power devices are outstripping semiconductors in general, it’s safe to say that the power chip content of electronic systems generally, and mobile battery-operated applications specifically, is increasing considerably.

PORTABLE CHALLENGES
It’s easy to understand the basis for this trend when one examines the stringent requirements of portable power with high-power efficiency and small size, as well as the sophisticated functionality that today’s portable power devices provide. In many mobile systems, meeting all of the various power requirements that exist in a single mobile consumer electronic device takes multiple power chips.

For example, 20 years ago the number of voltage rails in an electronic system rarely exceeded three or four. Today’s cell phones often have up to 14 or more voltage rails. Laptop computers typically have 11 or more voltage rails. A voltage "rail" refers to a single voltage provided by the power supply unit (PSU). Each voltage rail typically requires conversion to a new voltage level that is either buck (decreasing) or boost (increasing), regulation and sequencing. This proliferation of voltage rails can be partly attributed to Moore’s law, which says that the density of integrated circuits doubles every 24 months.

As the fabrication processing geometries for chips have gotten smaller and smaller over the years, voltage levels have plummeted to support the new process technologies. The days when the vast majority of semiconductors were five-volt devices are long gone. The chips in a single cell phone, for example, now have voltage levels as low as 0.9 V for the digital baseband device to as high as 30 V for a series of several light emitting diodes (LED) for backlighting the keypad and display.

The lower voltage levels of chips in portable systems also challenges the sophistication and resolution of the system’s power devices. Years ago when most chips were rated at five volts, typically power was regulated to within five percent or +250 mV of 5 V. In many of today’s portable systems, a 1.5 V chip requires regulation within 1.5 percent or within +22 mV of its 1.5 V level. This represents an increase in the resolution of voltage regulation by a factor of 10.


GETTING CHARGED UP
The fact that portable electronic applications are powered by a battery and not line-powered adds complexity to the system’s power subsystem. For instance, recharging a depleted Li-Ion battery is not a simple, straightforward process. Using an incorrect recharging profile reduces the effective life of the battery.

Furthermore, a thorough charging process for a Li-Ion battery consists of three distinct phases. During the first or pre-charge phase, the battery receives a pre-charge over a short period of time. The second phase is characterized by constant current. This phase occurs over a relatively short period of time of approximately 20 to 30 percent of the entire charge cycle. The constant current phase charges the battery to approximately 70 to 80 percent of its capacity. The final charging phase features constant voltage. This is the slowest of the three charging phases, requiring 70 to 80 percent of the time to complete the entire charging process. The constant voltage phase achieves the last 20-30 percent of the battery’s power-storing capacity. The power component that oversees the charging process for a Li-Ion battery must be able to manage all of these various factors in order to maximize the charge on the battery and ensure optimum battery life.

Depending on the requirements of the portable device, several different types of battery charging devices are available. In applications where cost is more important than efficiency, linear chargers offer an excellent solution. Conversely, switching chargers are at least 90 percent power efficient, though more costly than linear chargers.

Beyond the charging process itself, the power subsystem must address other battery-related issues as well. For example, some portable systems have alternative power sources, such as a USB port which also can be used for recharging the battery. A portable media player might be recharged in the typical way by connecting to a wall socket. Or, as an alternative, it could be recharged by connecting its USB port to the USB port on another device such as a laptop computer.

Power path management is a unique feature that allows portable system operation while charging the battery. The power sub-system must have sufficient sophistication to simultaneously direct power to both operate the system and to charge the battery. Other battery features might include protection from over-voltage or over-current conditions, possibly caused by a short circuit. If neglected, these conditions could damage the system, the battery or both. Battery authentication also could be required as a way of avoiding the installation of sub-standard or poor quality aftermarket batteries in the portable system. By placing an authentication chip in the pack, the system is able to validate that a high-quality battery pack is supporting the system.

Another critical feature for a portable power subsystem is monitoring the charge level on the battery, and communicating this information to the user. This allows the user to know exactly how long the system will operate without recharging. This information is critical if the battery is going to be recharged before it is completely depleted. The discharging characteristics of Li-Ion batteries complicate this task and necessitate considerable sophistication in the battery monitoring function.

As Figure 1 illustrates, a simple voltage monitoring function is not able to accurately reflect the amount of time left on a battery pack. Misleadingly, voltage monitoring would rate the charge on a Li-Ion battery in the mid-range of its capacity for most of the discharge cycle. However, once the voltage dips to a certain level, the charge remaining on the battery would drop precipitously over a very short period of time. More sophisticated monitoring techniques, such as TI’s Impedance Track™ technology, profile the battery over a number of factors such as age of the battery, temperature, number of recharging cycles and depth of cycles. With this information, Impedance Track uses an onboard sophisticated algorithm to calculate the remaining battery capacity within one percent accuracy.

PARTITIONING THE POWER SUB-SYSTEM
Many portable systems are based on architectures typically composed of several different sections or partitions. And each partition can have its own set of power requirements. Meeting the particular needs of the various sections of the portable system often requires specialized portable power chips.

Because of the low electrical noise of the power it outputs, linear regulator chips are often implemented in a portable system’s radio frequency (RF) partition. Linear regulators provide quick turn-on, regulation, and low noise needed for RF operations. They are quite small, requiring little board space since they do not require an inductor.

Several types of DC/DC boost converters might also be needed to implement different lighting functions, such as the photographic flash in a digital camera, a continuous torch light for a camcorder, or the backlighting for the LCD display and keypad. The bright colors of an organic LED (OLED) display might also require a voltage conversion to a certain level.

Processor chips, both general purpose processors and digital signal processors (DSP), have their own set of power specifications. Most processors are high-amperage, low-voltage devices that require tightly regulated power. Voltage conversion is most often needed to bring the portable system’s 3.3 V power down to the 1.2 V level of the processor. Typically, this requires a highly efficient DC/DC converter that can supply significant amperage.

Of course, some portable systems with a high degree of complexity call for power devices to supervise or manage all or portions of the power subsystem. For example, voltage supervisory devices can monitor the power subsystem, turning on or off portions of the subsystem in response to events as they take place. A power management unit (PMU) integrates into one chip many of the power functions that might be performed by multiple discrete power devices. This integration saves cost, board space, and design time.

POWER TO GO
The sophistication and wide-ranging diversity of portable power technology reflects the myriad of mobile electronic systems that have become a staple of contemporary life. This trend toward ‘on-the-go’ electronics certainly will not abate any time soon. And, if the past is any indication of the future, the portable power industry will continue meeting and exceeding the ongoing challenges that new applications and new technologies present.

References

[1] Worldwide Market Update on NiMH, Li Ion and Polymer Batteries for Portable Applications and HEVs, March 2007, Institute of Information Technology, Inc.
[2] Databeans, October 2006, Analog Power ICs, standard linear markets WW.
[3] Databeans, October 2006, Analog Power ICs, standard linear markets WW.

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Figure 1