Handheld devices, such as mobile phones and personal digital assistants (PDAs), incorporate several types of circuits, components, technologies, and interfaces that each requires a power supply. Because of the diversity of these applications, there is a need to select the best component for each power management application and type of load.

Power management components such as low-dropout linear-voltage regulators (LDOs) perform a relatively simple function — regulate power to a load. But now designers can choose from at least three categories of LDOs, each of which is optimized for a specific load or circuit application. At first glance, LDOs perform a simple function of voltage regulation. Choosing the optimal LDO, however, can improve system performance and extend battery life.

In the past, system designers were forced to use general-purpose LDOs for all power regulation functions. This practice was acceptable for most applications, particularly for systems that plug into a wall socket with ground currents (quiescent current consumption) of greater than 1 milliamp (mA), making power savings relatively unimportant. Using one type of LDO had the added advantage of simplifying the bill of materials.

Mobile-system designers found the general-type LDO had several limitations. Although conventional bipolar and CMOS LDOs were cost-effective, they were not optimized for low-ground current. This problem becomes more important as handheld devices add functions, which boost power consumption. For example, PDAs now use color displays and run multimedia functions, such as MPEG video and MP3 audio playback, which reduces battery life between charges.

In addition to suffering from high-ground current, low-cost, general-purpose LDOs add further problems. These LDOs can introduce a great deal of noise and have low power-supply rejection ratios (PSRR), both of which are significant hurdles for cellular handsets. Even systems that have access to wall plugs, such as set-top boxes, can suffer detrimental effects from a linear regulator that introduces noise and suffers from low PSRR. In such a system, the linear regulator follows a switch-mode power supply. If the linear-voltage regulator does not offer adequate PSRR, the noise from the supply may couple into sensitive VHF/UHF receiver circuitry, compromising the design.

Additionally, portable systems need regulators that can respond quickly to load transients. The system may not be able to extend battery life through the use of power management schemes because of the inability to put circuitry in and out of standby mode in real time. This means that loads must be left on when they are not required.

With the introduction of LDOs for specific applications, designers can select regulators based on the requirements of the power source, system, and load. These categories of devices can be selected on a continuum of ground current from 1 mA down to 1 microamp.

The first category can be called OmniPower. These are general-purpose devices suitable for many applications. In most cases, OmniPower is the right choice when price is the most important concern and higher ground current is acceptable, such as for systems running on wall sockets with ground current that exceeds 1 mA. This type of LDO is a good choice as a post-regulator in systems running from an offline switch-mode power supply, such as a silver box in a computer. This is not a good choice for portable system with loads that operate in standby modes when regulators must be designed to consume little ground current.

The next category is MicroPower devices that are optimized for low ground current ranges — from 100A down to 10A — as well as other "load-specific" requirements like low noise, high PSRR, and fast transient response. RF circuitry in particular is sensitive to noise since noise affects the phase margin of sensitive transmit and receive voltage-controlled oscillator (VCO) circuits. An LDO's noise performance can make the difference between complying and not complying with specifications. CDMA and TDMA-based communication protocols allow very little room for error when it comes to carrier signal deviation, which makes low-noise RF power-supply circuits critical.

Low power-supply rejection ratios can have similar effects. Many portable RF systems, such as smart phones and PDAs, use dc/dc converters, which can inject switching noise back into the battery. If the LDO does not attenuate the noise that comes out of the battery, it can negatively affect other circuits that share the same battery source.

More battery life

Fast transient response LDOs can also extend battery life. TDMA-based cell phone protocols such as Global System for Mobile Communications (GSM) have a transmit/receive duty factor of only 12.5 percent, enabling power savings by putting much of the baseband circuitry into standby mode in between transmit cycles. In baseband circuits, the load often transitions virtually instantaneously from 100 A to 100 mA. To meet this load requirement, the LDO must react very quickly without a large voltage droop or overshoot — a requirement that can not be met with conventional, general-purpose LDOs.

The ability to turn on circuits in as little as tens of microseconds and still meet a GSM transmit window provides a mobile phone with significant gains in battery life. This type of fast turn-on, or fast transient response, requires an LDO that is optimized for this task, without sacrificing low ground current across the load range.

There is another LDO selection issue related to fast transient response. A system designer will want to know which types of capacitors the LDO can support to optimize stability. Historically, general-purpose LDOs required expensive tantalum or aluminum electrolytic capacitors that provide high equivalent series resistance (ESR), preventing LDOs from oscillating. However, the new MicroPower types of LDOs offer fast transient response with low-cost ESR ceramic capacitors, which inherently respond to transient events faster than their more expensive high-ESR counterparts. Using proper capacitors with a fast transient response LDO allows advanced power management schemes that weren't possible with previous generations of LDOs.

The third category of LDOs, called NanoPower, operate with ground current of less than 10A. These LDOs are fine-tuned for applications that require the lowest possible ground current since they need the absolute minimum of current to operate the LDO's internal control circuit. With ground current as low as 1A, these LDOs are the best choice for loads that need to be "powered on" 100 percent of the time, even when the system is in standby.

Of course, extremely low ground current comes with certain tradeoffs. Typically this type of LDO is not optimized for the lowest noise performance, conceivably making it inappropriate for RF circuitry in mobile phones or wireless PDAs. However, NanoPower LDOs are an ideal choice for digital signal processing or logic chips, which often have to remain on at all times.

Power management circuits will continue to become more specialized. Along with some ac-powered systems, portable systems will face more stringent power management requirements for a variety of factors — low noise, low ground current, low PSRR, fast transient response, and other criteria related to specific types of circuits and loads. The growing diversity of LDOs and other power management devices provides designers with new categories of devices that meet the more stringent requirements of new function-rich portable devices.