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Mixed-signal integration key to future power management in portables

Posted: 16 Oct 2003 ?? ?Print Version ?Bookmark and Share

Keywords:portable power management? cellphone? li-ion? battery? dvs?

Consumers of portable electronics are constantly demanding more functionality from smaller-sized electronic devices with longer operation times. More functionality generally implies more power. For example, 3G smart phones will offer increased functionality, but the power requirements for a 3G video call will almost double that of a 2G phone voice call.

The voltage level of a Li-ion battery begins at a nominal 3.6V when the it is fully charged, and drops as the battery is depleted. In 2G phones, the battery is typically stopped (the cellphone is turned off) at 3.3V. Consequently, linear regulators are used to step down the battery voltage to the appropriate lower voltage for each power rail when the phone is operational. The 3G phones' power requirements are larger and may cause more rapid battery drain unless a more efficient means of providing the power rails are found. Systems designers of these power rails must balance the competing goals of increased power requirements from the smallest packaged power IC, optimal efficiency for maximum battery life and acceptable power rail noise/ripple. Fortunately, the latest power ICs and power management techniques, as well as the trends in semiconductor processing and packaging, are aligned with these requirements.

Li-ion battery management consists of three components: charge control, battery monitoring and battery protection. Charge control ICs have evolved significantly from linear controllers with external pass elements to switching-based controllers with integrated switches. Battery monitoring ICs can be as simple as "Coulomb counters," from which the DSP/CPU must compute the remaining battery life, to gas gauges with integrated MCUs. Computer battery monitoring and protection ICs are typically packaged with the battery itself -and this will likely prevail for the cellphone (and other handhelds) as well.

The designer must determine the type of power converter IC for each part of the system. The choices include inductor-based switching converters with integrated FETs, inductorless switching converters (or capacitor charge pumps) or linear regulators. Each has its advantages relative to the others.

In terms of efficiency, inductor-based switchers have the highest efficiencies, followed by charge pumps and linear regulators. Conversely, linear regulators have no output ripple noise, while charge pumps have some output ripple and switchers have relatively the highest output ripple. In terms of total solution size, linear regulators are the smallest, typically only requiring an I/O capacitor. In addition to I/O capacitors, charge pumps require one or two additional "flying" capacitors, and switchers require one inductor, which can vary in package size. To maximize efficiency with switching converters, it is generally more efficient to step down (or buck) from a higher rail to a lower rail than to step up (or boost) from a lower rail to a higher rail.

Different components in the smart phone have different voltage, current and noise requirements. For example, the RF section requires a power rail with extremely low noise and high power supply rejection to ensure the highest transmit and receive performance. Therefore, although rather inefficient, a linear regulator with no output ripple is the best choice for this rail. DSP/CPU core voltages, in contrast, have fallen to around 1V. So, to improve efficiency for this rail, a high efficiency inductor-based, switching step-down converter is appropriate. White LEDs, used for backlighting the screen, can be powered from either a charge pump or inductor-based step-up/boost converter.

Various power management techniques can help optimize efficiency for each section. For example, the 3.3V I/O can be provided by a highly efficient single-ended primary inductance converter, which allows the Li-ion battery to be drained to its lowest level (approximately 2.7V). The current rails provided by the regulators are taken from the 3.3V rail to improve efficiency.

Optimizing DSP efficiency

Dynamic (or adaptive) voltage scaling (DVS) links the processor and converter in a closed loop system via a communication bus, like I2C, that dynamically adjusts the power supply voltage to the minimum level needed for proper operation. Since a processor's power dissipation is proportional to the square of its voltage times its frequency of operation, DSP/CPU efficiency can be increased dramatically if DVS and frequency scaling are used.

A power amplifier is optimized for highest efficiency at maximum transmit power. Since most handsets operate relatively close to base stations, the handset radios reduce transmit power (and thus efficiency) to the minimum required for quality communication. By employing DVS and adjusting the power amplifier's voltage optimally, efficiency can be increased by 10 percent to 20 percent.

The latest switching converter designs have very low output ripple and many have anti-ringing circuits to reduce EMI at the switching node. Shrinking technology nodes produce smaller FETs, which not only allow for smaller overall die (and thus package) size, but also lower gate capacitances, and thus faster switching speeds. For inductor-based switching power supplies, faster switching speed means smaller inductors. Simultaneously, newly developed IC fabrication processes have lower leakage currents and lower resistances (sometimes through copper overlay). This translates into FETs with lower quiescent currents and lower RDS(on), respectively, and ultimately, devices with higher efficiency.

New packages are allowing for more functionality and power dissipation in smaller packages. For example, a Li-ion linear charger with integrated FET pass element can be packaged in 3x3mm2 quad flat no-lead package that allows for up to 1.5W of power dissipation at moderate ambient temperatures. In addition, portable electronics manufacturers are requesting that new, as well as existing power ICs, be packaged in leadless chipscale packages.

While the differing voltage, current and noise requirements of cellphone sections seem to favor discrete implementations, space savings and reducing overall cost require some of all of these discrete components to be integrated.

Digital baseband sections require highly dense processes for DSP while the analog baseband and power sections need higher voltage devices. The RF section and the PLL require BiCMOS devices optimized for HF operation. Historically, digital designers oversaw process development and pushed only for high density processes, so circuits requiring high voltage devices were only possible in a different process, meaning a separate digital IC.

A concern about integration is limited flexibility. However, new manufacturing process techniques including integrated EEPROM for programming output voltage rails and post-package trimming make "spinning" simple modifications of existing ICs (e.g., an IC with a different fixed output voltage) much simpler, faster and cheaper.

Essentially, The latest process technologies make it easier to combine, quickly modify and/or leverage on existing discrete IC designs and produce various levels of integrated ICs. For example, generic dual switching converter ICs and dual high power supply rejection ratio, low noise linear regulators, application specific TFT displays and white LED supplies, cellphones, PDAs and DSC multi-rail power solutions are either available now or will be available by the end of the year. The product-specific power solutions have integrated peripherals, from ringer and buzzer controls for cellphones to GPIOs for PDAs.

The single chip smart phone is not yet available, but various levels of IC integration are simplifying the design of such portable power electronics. Specifically, system designers of portable electronics need not worry about managing the power requirements of their devices. Power management ICs at various levels of integration are available to help them maximize battery life, in the smallest board area and lowest cost.

- Jeff Falin

Applications Engineer, Portable Power Management Products

Texas Instruments Inc.

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