Saving power in portable applications - Embedded.com

Saving power in portable applications

New generations of portable consumer devices such as wireless handsets,smart phones, PDAs and media players boast more features, higherperformance levels and often smaller solution sizes. With their latestfeatures, these devices all demand significant power.

Some examples include cameras with resolutions of 3Mpixels or more,high-power photoflash LEDs or Xenon tubes, advanced audio and speakerfunctions, and wireless phones with portable high-resolution LCD-TVdisplays. Static and dynamic power requirements challenge designers andare critical to manage. As portable devices become more feature rich,single-power requirements add up quickly. As a result, battery lifegets shorter.

In addition, analog or digital baseband IC processor units, centralprocessor hosts and graphic/ audio processors are getting more advancedand highly integrated. As these ICs become more integrated asfunctionality increases, additional power rails are needed or highersupply currents are required for the same power rails.

Most portable consumer applications use standard high performanceLi-ion batteries, typically in a one-cell configuration. Given thislimited amount of energy, manufacturers have to decide if customersprefer feature-rich applications with shorter battery lives or limitingthe number of features and power requirements of their applications.Today's customers want high-end devices without shorter batterylifetimes.

Solving the portable powerdilemma
Many techniques are available to solve these design dilemmas. Toaddress processor needs, IC manufacturers are taking the lead todecrease power consumption for given performance levels. Standarddigital IC manufacturing technologies for DSP or OMAP cores use the90nm process.

The latest generation of 65nm technology is already in volumeproduction since late 2005. With each “shrink,” the transistor densityusually usually doubles. It shrinks equivalent designs by half andboosts transistor performance by almost 40 percent. This technologydelivers much lower core supply voltages, but similar or even highercurrent requirements. On the other hand, leakage power increases,decreasing this performance advantage.

Other power-saving, non-manufacturing techniques for analog anddigital ICs include multiple low-power mode, clock gating, dynamicvoltage and frequency scaling. These techniques play an important rolein design. To solve power requirements and power-saving challenges, newmanufacturing and process technologies need to be developed. Oneexample is a new methodology called SmartReflex , used for DSP and OMAP processors.

At the silicon-IP level, static leakage power is dramaticallyreduced by a factor of 1,000. Power consumption and performance ofdifferent IC- and system-building blocks are coordinated, while a bankof power-management cells enables a granular approach to partitioning adevice's power domains.

It reduces overall power consumption, optimizes system performanceand increases battery life. With a broad range of intelligent andadaptive hardware and software techniques, it dynamically controlsvoltage, frequency and power based on device activity, operation modesand temperatures.

This includes dynamic and adaptivevoltage-scaling , dynamic-powerswitching and standby-leakagemanagement . Dynamic-voltagescaling typically involves external power-management devices andsoftware.

For example, depending on a processor's load, the core supplyvoltage can be adjusted to meet full performance or save power instandby situations. Depending on the system, discrete low-dropouts(LDOs) ,medium- orlow-power DC/DC converters, multichannel powermanagement units (PMUs) or other sources are responsible for board and processor supplies.

Power-management designs provide the necessary voltage rails, andcorrect voltage amounts and currents for any processor situation. Ifthe application is switched off or into pre-defined power “save” mode,all processors and power-management devices usually have a light loador standby mode.

Then, the current voltage levels decrease, and current consumptiongoes down to a minimum. In the best case, there's a few microamperesper IC. So far, these scenarios are static. Once the power-managementdesign is done, very little is left to influence voltage rail levels.

Recently, discrete low-power buck DC/DC converters and highly-integrated, multichannel PMUs became available with a serialI2C bus. With serialinterfaces in discrete power-management devices, new levels ofinfluence are available.

By combining software tools and processor-control functions with aserial standard I2C interface, information between digital and analogpower-management ICs can be exchanged at higher performance levels.

On time adjustments of voltage, current and power budgets becomerealistic. Moreover, software controlled power management andmonitoring can be realized, and multiple power-save modes can beimplemented between existing full-load and system standby modes.

Dynamic voltage scaling The I2C interface has two different speedoptions: standard 100Kbps and fast 400Kbps. Implemented in discretelow-power DC/DC converters or PMUs, designers can dynamically andprecisely influence output voltages of the discrete power-managementdevice and core supply voltages for any processor unit.

This design requires fast DC/DC converters. For example, converters (Figure 1, below ) with 3MHz or higherswitching frequency guarantee fast-signal transient response.

Figure1: I2C can dynamically adjust and scale the main DC/DC converter'soutput voltage.

Furthermore, low-power DC/DC converters or PMUs should featuredifferent operation modes—such as PFMor forced-PFM—to allow them to adjust themselves or via I2Ccontrol signals onto certain system-power configurations.

This design enables the system to meet exact performancerequirements without sacrificing overall performance. Thus, minimumpower is used for each operating condition or processor mode, extendingbattery life, reducing per device heat dissipation and enhancingoverall system performance.

Reducing power consumptionThe technology supported in this example is the discrete, low-powerDC/DC converter TPS62350 .Thissingle-channel buck converter provides up to 800mA output current overthe input voltage range of a single Li-ionbattery for 95 percent efficiency and comes in a tiny12-ballchipscale package. With the help of I2C interface, output voltages canbe adjusted to support processors and supply rails in “tiny steps” downto a minimum of 0.6V.

The programmable DC/DC converter helps extend battery life in 3Gsmart phones, PDAs, digital still cameras and other portableapplications.

Another technique to reduce power consumption with the help of I2Cinterface is with more complex devices like the TPS65020 (see Figure 2, below ). This is ahighly integrated PMU with six output channels, three low-power DC/ DCconverters with up to 97 percent efficiency and three LDOs.

Figure2: The TPS62350 single-channel buck converter provides up to 800mAoutput current over the input voltage range of a single Li-ion battery.

In this device, I2C can dynamically adjust and scale the main DC/DCconverter's output voltage that usually powers the processor corevoltage. The other two DC/DC converters can be used for I/O supply,memory or other functions. Different building blocks, like all threeLDOs or DC/DC converters of this IC, can be switched on/off with thehelp of I2C to reduce power consumption and heat dissipation of thecomplete PMU. Shutting down different blocks also decreases quiescentcurrent consumption.

In addition to the power saving schemes discussed, new manufacturingtechnologies will play a key role in the future. As processing stepsevolve from 90nm to 65nm and smaller, the technological implementationsdiscussed here will become more important.

Communications will increase between DSP cores and their discreteanalog power components to allow flexible on-time power adjustments andsoftware controlled power schemes. In summary, all these improvementsand methods must play together well to optimize performance andmaximize battery life for the customer's benefit.

Alexander Friebe is BusinessDevelopment Manager at Texas Instruments Inc.

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