Mixed-signal integration key to future power management in portables

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

The voltage level of a Lithium-Ion begins at a nominal 3.6V when the battery is full charged, and drops as the battery is depleted. In 2G phones, the battery is typically stopped (the cell phone is turned off) at 3.3 V. Consequently, linear regulators are be 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 IC's 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 IC's have evolved significantly from linear controllers with external pass elements to switching based controllers with integrated switches. Battery monitoring IC's can be as simple as "columb counters," from which the DSP/CPU must compute the remaining battery life, to gas gauges with integrated micro-controllers that provide remaining capacity, time to empty, voltage, temperature, and average current measurements via a simple communication interface directly to the DSP/CPU. Computer battery monitoring and protection IC's are typically packaged with the battery itself — and this will likely prevailed for the cell phone (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 input and output capacitor. In addition to input and output 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 1-V. So, to improve efficiency for this rail, a high efficiency inductor-based, switching step-down converter is appropriate. White LED's, used for backlighting the screen, can be powered from either a charge pump or inductor based step-up/boost converter.

Various power management techniques that help to optimize efficiency doe each section. For example, the 3.3V I/O can be provided by a highly efficient SEPIC (single-ended primary inductance converter, buck/boost) converter, which allows the Li-Ion battery to be drained to its lowest level (approximately 2.7 V). The current rails provided by the regulators are taken from the 3.3-V 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 (PA) 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 dynamic voltage scaling and adjusting the power amplifier's voltage optimally, efficiency can be increased by 10 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 FET's, 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 FET's with lower quiescent currents and lower RDSon's, 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 (like the TI bq24010) can be packaged in 3X3 mm2 QFN package that allows for up to 1.5 W of power dissipation at moderate ambient temperatures. In addition, portable electronics manufacturers are requesting that new as well as existing power IC's be packaged in leadless chipscale packages.

While the differing voltage, current and noise requirements of cell phone sections seem to favor discrete implementations, space savings and reducing overall cost require some of all of these discrete components to be integrated. And many of the hindrances to integration - particularly, the ability to combine high-voltage and high-density processes on the same chip — have been removed.

Digital baseband sections require highly dense processes for digital signal processing while the analog baseband and power sections need higher voltage devices. The RF section and specifically the PLL require BiCMOS devices optimized for high frequency 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.

Recently, semiconductor manufacturers like TI have been developing not only single BiCMOS processes with lower minimum gate lengths, for high density and high speed, but also drain extended devices capable of higher voltages for more analog and power applications. Another 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 IC's (e.g., an IC with a different fixed output voltage) much simpler, faster and cheaper.

Essentially, The latest process technologies make it much easier to combine, quickly modify and/or leverage on existing discrete IC's designs and produce various levels of integrated IC's. For example, generic dual switching converter IC's and dual high PSRR, low noise linear regulators, application specific TFT display and white LED supplies, and cell phone, PDA, and digital still camera 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 cell phones to general purpose I/O's (i.e. GPIO's) for PDA's.

The single chip smart phone is not here yet, 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 IC's at various levels of integration are available to help them maximize battery life, in the smallest board area and lowest cost.