Lighting requirements in handsets and portable devices have evolveddramatically over the past few years. Not very long ago, a typicalcellphone featured a simple passive LCD with little or no backlighting.The greatest challenge designers faced was how to most efficientlydrive a number of LEDs in parallel or in series without draining thesystem's battery.
As designers moved to higher performance color displays and began toembed various valueadded features such as cameras into their portablesystems, power requirements have escalated.
Today, the typical handset or handheld device features a relativelyhigh-resolution main display, which is backlit by four or more LEDs.Many now also add a second smaller display – such as those used on theoutside of a clamshell-style phone – to deliver additional information.
These subpanels usually require two additional LEDs forbacklighting. Power circuits are also often needed to drive auxiliaryRGB status lights and to backlight the keypad on the device.
At the same time, the integration of a camera functionality hasbrought an entirely new set of power requirements to portable systemdesign. Embedded cameras require a flash typically implemented bydriving one or a few LEDs at high power for a very short duration.Early camera implementations that used CCDs of less than 1Mpixeldemanded flash drivers of little more than 100mA.
|Figure1: Many portable devices require as much as 600mA of power just todrive the camera flash function and achieve maximum photo resolution.|
Portable device lighting needs
But as portable system designers have moved to higher-resolution CCDs,power requirements for flash functions have risen as well.Today, manyportable devices require as much as 600mA (or more for Xenon flashlamps) of power just to drive the camera flash function, and achievemaximum photo resolution and red-eye correction (Figure 1 above ). Moreover, theaddition of new flashlight or movie-mode functions promises to furthercomplicate power management requirements and system design.
Traditionally, portable system designers have relied primarily ondiscrete implementations to manage power for lighting applications. Forbacklight applications, designers have typically used discrete chargepumps to drive a number of LEDs in parallel or boost converters todrive a number of LEDs in series to provide basic illumination. Colordisplays increased power requirements, but few early implementationsoffered designers any flexibility in terms of setting light intensity.
Gradually, issues such as white balancing to control light intensityand color balancing to create different colors for status lightingbecame increasingly important, and required more sophisticated controlby the system controller or a housekeeping microcontroller.
Early on, most designers relied on relatively noisy analogpulse-width modulation control signals to control LED brightness andbacklight functions. Flash LED functions were first implemented assingle units with simple on/off control.
Designers later added rudimentary algorithms residing in the systemprocessor to control the flash subsystem. Since few systems offered anyflash intensity control, designers controlled the amount of lightreaching the CCD by managing the camera shutter.
As embedded camera performance improved, however, designers wereforced to also upgrade the performance of their flash power managementsystem. These advances demanded a fixed camera shutter and bettercontrol of the amount of light emitted by the flash. To reduce theoverhead on the system controller, designers gradually migrated to moreintelligent flash LED controllers.
As the market has been pressured to further reduce system size andcost and as the number, variety and complexity of lighting functionshas continued to expand, portable system designers have begun to lookfor new ways to replace these discrete lighting subsystems.
|Figure2: Designers can use highly integrated chips that combine all functionssuch as camera flash driver and LDOs.|
Recently, power management semiconductor manufacturers have announcedmajor advances in the vertical integration of these functions.
By combining many of these circuits into a single IC, these newdevices reduce component count, shrink PCB space requirements, minimizethe overhead requirements of the system processor and help portablesystem designers drive down cost.
As a result, instead of using a dedicated white LED driver fordisplay backlight applications, a dedicated camera flash driver IC anddiscrete generalpurpose LDOs or a dual LDO IC, designers can now usehighly integrated chips that combine all functions.
One of the risks with integrating multiple functions in a single ICis the limitation potentially placed on the designer's ability toconfigure the system. To maximize design flexibility, these devices addtwo separate serial interfaces that allow the designer to drive theLEDs for backlight or keypad functions and the LEDs for flashapplications independently.
With these independent interfaces, designers can enable, disable orset current up to 16 different levels for backlight/keypad and flashfunctions. Moreover, by allowing the designer to tune the output of thebacklight or flash LEDs to their respective applications, thiscapability also helps save power. Alternately, the backlight/keypad andflash LEDs can be controlled via external resistors.
Other highly-integrated power management ICs help design- ersdevelop more efficient and cost-effective solutions when they have tobacklight multiple displays or add auxiliary lighting.
These devices combine a high efficiency tri-mode charge pump withdrivers for backlight LEDs, camera flash and auxiliary lighting.Instead of just driving four LEDs for backlight functions, however,these ICs drive up to six. By supplying individual control for eachchannel, these new devices allow designers to drive up to four LEDs tobacklight a main display and two for a subpanel or use all six tobacklight a single display.
Some of these new ICs supply up to 600mA to drive up to four flashLEDs and support increasingly common requirements for flashlight andmovie-mode applications. Moreover, they add three additional driversthat can handle up to 60mA each for RGB, keypad or other auxiliarylighting applications.
The tri-mode charge pump sitting at the heart of these new devicescan typically supply enough power to operate all three types of LEDfunctions – backlight, flash and auxiliary – simultaneously. To helpreduce overhead on the system controller, many of these new displaypower management ICs also add an integrated flash timer.
Traditionally, portable system designers have relied on ahigh-to-low transition on the flash enable pin to control the durationof a flash event. If the system controller software hangs up or thecontrol line becomes disconnected, the designer using this architectureruns the risk of the flash LEDs staying on at full power.
Since flash LEDs are only designed to work in very short bursts, afailure of this type could quickly damage the LEDs and rapidly depletebattery power.
The recent addition of integrated flash timers into this new generationof display power management ICs not only reduces overhead at the systemcontroller, but also provides a safety feature to protect against thesetypes of failures.
This timer function enables the flash current sinks for a programmedamount of time. Length of on-time is set by loading the timing registerin the device with a value and then selecting a value for an externaltiming capacitor. Once data is latched into the timing register, theflash current sinks are automatically enabled for the duration of theprogrammed time and then disabled.
By letting the controller “set and forget” the on-time duration,this integrated timer eliminates the need for the controller to trackthe duration of the flash event. It also ensures that the flash LEDswill only be illuminated for the prescribed amount of time limited bythe value of the capacitor.
The evolution of lighting functions in portable system designs showsno signs of slowing down. Next-generation systems will likely add newlighting requirements as designers bring an ever-widening array offunctionality to users. Inevitably, these innovations will pose newpower management challenges for designers.
To address these power management issues, semiconductormanufacturers are already developing various devices that bringtogether new combinations of charge pumps, boost converters, backlight,flash, RGB and LDO functional blocks.
As these lighting features proliferate and the size of portableelectronic products continue to shrink, this new generation ofvertically-integrated, system-level devices will play a pivotal role indesigners' quest for lower-cost, higher-performance systems.