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Chips address lighting challenge

Posted: 01 Nov 2007 ?? ?Print Version ?Bookmark and Share

Keywords:portable consumer electronics? power management? mobile TV? Internet?

Compare today's portable consumer electronics devices with their predecessors from just a few years ago, and you'll see why lighting has become a major power management challenge.

Handsets with a single passive LCD panel are quickly becoming relics. Today's devices feature high-performance, high-resolution 2.5- to 3-inch diagonal color displays to support applications that run the gamut from Internet access and mobile TV to video playback.

Typically, these displays require four or more LEDs and drivers for backlighting. Many handsets, particularly clamshell-style designs, add a second smaller display for delivering basic time, date and connectivity information. Usually, these subdisplays require one or two more LEDs for backlight functions.

As designers have discovered how useful fashion lighting can be to differentiate products, many portable electronic devices today require additional power circuits to drive auxiliary RGB status lights and to backlight the keypad.

Embedded camera functionality has further complicated power circuit design. These functions typically deliver flash lighting by driving several LEDs at high current for a very short duration. But when handset manufacturers first began introducing this capability, they used CCDs of less than a megapixel, which required flash drivers of little more than 100mA. Today's handset designers are integrating much higher-resolution CCDs that require up to 600mA just to drive the flash function and to deliver enough light for a high photo resolution. Newer functions such as movie-mode and flashlight capabilities complicate power circuit design by demanding lower levels of power to drive the LEDs for longer durations.

Given these developments, it's easy to see why lighting is often the largest source of battery drain in handheld devices. Early on, designers relied on discrete implementations using boost converters or charge pumps to address their lighting functions. But as the number of lighting functions grew and power requirements increased, designers needed better control to address light intensity, manage color balancing and maximize power efficiency. Eventually they turned to the system MCU or a dedicated controller to address these issues via pulse-width modulation control.

Simpler design
In recent years, power management IC (PMIC) manufacturers have developed a variety of chips to offer engineers higher levels of control and to simplify design. Consider the LM27965 from National Semiconductor Corp. The WLED driver can drive up to nine LEDs in parallel with a total output of 180mA. To maximize designers' control, the current sinks can be split into two or three independently controlled groups with four or five LEDs configured to backlight a main display; two or three for a subdisplay; and a single, independently controlled driver to manage a status or indicator LED. Each group is controlled via a standard I2C interface.

Inductive boost converter-based solutions still offer advantages in some applications. But manufacturers are in many cases turning to mixed-mode or fractional charge pumps to drive WLEDs in compact portable applications and eliminate the need for a large inductor. While the output in a fixed-boost charge pump is regulated using individual resistors, LED current matching and efficiency can suffer. With a mixed-mode charge pump, output voltage can be regulated to maintain a constant current through each LED, allowing the designer to match up the current source more accurately.

Linear's LTC3219 simplify power circuit design by driving up to nine individually configurable current sources for cellphone lighting functions at efficiency levels of up to 91 percent.

Similar to many competing offerings, the LTC3219 from Linear Technology Corp. uses a multimode charge pump that powers up in 1x mode and then automatically switches to boost or 1.5x mode when any enabled LED current source approaches dropout.

A subsequent dropout switches the device to doubler (2x) mode. Designed to support main, sub and RGB lighting requirements, the device drives nine individually configurable current sources.

As in the National Semiconductor chip, each current source in the Linear device is digitally controlled for dimming, brightness, blinking and graduation control via a two-wire I2C serial interface.

Early generations of WLEDs required relatively high forward voltages and currents to achieve desired luminosity. But advances in the technology have allowed manufacturers to produce WLEDs that can operate on currents under 10mA. These devices can be used with lower forward voltages than ever before. This recent development has allowed PMIC manufacturers to begin offering linear matched independent current sources that reduce cost and footprint by eliminating most external parts.

The TPS75105 from Texas Instruments Inc., for instance, offers a quad LDO with matching LED constant-current drives for up to four LEDs in two banks in a very compact 1.2mm x 1.2mm package. The product eliminates the inductor, output capacitor and/or feedback resistor that would be required for inductive boost converters, or the switching capacitor and I/O capacitors that would be required for stability with a mixed-mode charge pump.

The MAX8645X and MAX8645Y charge pumps from Maxim reduce footprint by up to 40 percent compared with discrete alternatives.

Powering camera flash functions poses a more difficult challenge. Designers have to deliver the right amount of light for a prescribed length of time without exceeding the power limitations of the system or the thermal limitations of the LED. Since basic on/off controls were inadequate, many designers turned to algorithms run by the system MCU that coordinated both the flash subsystem and the camera shutter. More recently, the introduction of intelligent flash LED controllers has brought more efficient control of the emitted light. Earlier this year, for example, NXP Semiconductors announced the UBA3001, the first in a family of flash LED drivers that support efficiency levels up to 90 percent across the entire voltage range while delivering up to 1A to a single high-brightness LED to support phones with high-resolution cameras. The new IC also offers full control of current settings for torch and camera/video-on modes.

Space savers
Of course, PCB space is always a precious commodity in space-constrained portable applications.

Early this year, Micrel Inc. unveiled the MIC2298, a boost converter-based flash driver for embedded cameras with capacity of 2Mpixels and higher that delivers 1A to two LEDs connected in series. The device is housed in a 3mm x 3mm MLF package and requires only a compact, 2?H inductor and small ceramic capacitors.

Other manufacturers are helping designers conserve footprint and reduce parts and costs at the same time by integrating a wider array of lighting functions into devices. For space-constrained flip-, folder- or slide-style phones that embed the LCD and a camera module in the upper half of the phone, Maxim Integrated Products announced a family of WLED drivers that add two programmable 200mA low-dropout (LDO) regulators for powering the camera module in a tiny 4mm x 4mm package. The Max 8645X and 8645Y use a fractional charge pump to drive up to six WLEDs at up to 30mA each with 5bit dimming resolution. They also drive two WLEDs at up to 200mA each for camera-flash applications.

Similarly, AnalogicTech has announced the charge pump-based AAT2846, which combines six backlight drivers, two flash drivers and two general-purpose LDOs with two independent single-wire digital interfaces on a single IC. This allows the designer to build a solution that provides independent control of backlight and flash functions, and to program those controls over a large number of settings. The six backlight outputs can be grouped for a single large display or split into a 5 + 1 configuration for a main and subpanel.

- John Mayer
EE Times

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