Comparing today's portable consumer electronics devices with years ago, you will understand why lighting has become a major power management challenge. Handheld devices with a single passive LCD panel are rapidly being eliminated. Today's devices have high-performance, high-resolution, 2.5- to 3-inch diagonal color displays to support applications ranging from Internet access and mobile TV to video playback.

Typically, these displays require 4 or more LEDs and drivers for backlighting. Many handheld devices (especially clamshell designs) have added a smaller sub-display to display basic information such as time, date, and connectivity. These sub-displays usually require one to two more LEDs than the main display for backlighting.

As design engineers discover that fashion lighting plays an important role in product differentiation, many portable electronic devices today require additional power circuits to drive the auxiliary RGB status lights and keyboard backlight.

The built-in camera features a more sophisticated power supply circuit design. These functions typically provide a flash by driving a small number of LEDs with high current for a very short duration. But when handheld device manufacturers first introduced this feature, they used a CCD with less than 1 million pixels, which required the flash driver's current to not exceed 100 mA. Today's handheld device design engineers are integrating much higher resolution. The CCD, this CCD requires up to 600mA of current only to drive the flash function, and provides enough light to achieve high photo resolution. Newer features, such as movie mode and flash functions, increase the difficulty of designing the power circuit due to the need for a lower level of power to drive the LEDs for longer durations.

Taking these development factors into consideration, it is easy to understand why lighting is usually the source of the most drained battery in handheld devices. Previously, design engineers provided their lighting functions by using boost converters or charge pumps on their own. However, as the number of lighting functions increases and power requirements increase, design engineers need better control to handle light intensity, manage color balance, and maximize power. Ultimately these engineers turned to system microcontrollers or dedicated controllers to solve these problems through pulse width modulation (PWM) control.

Simpler design In recent years, power management integrated circuit (PMIC) manufacturers have developed various ICs designed to provide engineers with a higher level of control and simplified design. Take National Semiconductor's LM27965 as an example. White LED (WLED) drivers can drive up to 9 LEDs in parallel with a total output of up to 180mA. To maximize the design engineer's control capabilities, the output forward current can be split into two or three separate control groups and configured Four to five LEDs provide backlighting for the primary display; two to three LEDs are configured to provide backlight for the secondary display; individual independently controlled drivers are configured to manage status or indicator LEDs. Each set of LEDs is controlled through a standard I2C interface.

Although inductive boost converter based solutions still show advantages in many applications. But in many cases, manufacturers are turning to hybrid mode or fractional charge pumps to drive WLEDs in compact portable applications, and no longer require large inductors. Although the output in a fixed boost charge pump is modulated with a separate resistor, LED current matching and efficiency may be affected. With a mixed-mode charge pump, the output voltage can be modulated to maintain a constant current per LED, allowing design engineers to more accurately match the current source.

Similar to many competing products, Linear Technology's LTC3219, released in August, uses a multi-mode charge pump that can be turned on in 1x mode and then approach any voltage drop when the LED current source is enabled. Automatically switch to boost or 1.5 times mode.

Subsequent voltage drops convert the device to double (2x) mode. To support the primary and secondary displays and RGB lighting requirements, the device drives nine individually configurable current sources.

As with the NS devices, each current source in Linear Technology's devices uses a two-wire I2C serial interface to digitally control grayscale, brightness, flash, and grading adjustments.

Early WLEDs required relatively high forward voltages and currents to achieve the desired luminosity. However, recent technological advances have allowed manufacturers to produce WLEDs that can operate at currents below 10 mA. These devices can operate at lower forward voltages than before. Recent technological developments have allowed PMIC manufacturers to start providing linearly matched independent current sources, which reduces cost and footprint by eliminating the need for most external components.

For another example, TI's TPS75105 offers a four-channel LDO and matched LED constant-current drivers with up to four LEDs in two slots in an extremely compact 1.2 x 1.2mm package. This product eliminates the need for inductors, output capacitors, and/or feedback resistors required to sense the boost converter, or the switching capacitance and input/output capacitance required for stability with a hybrid-mode charge pump.

The driving of the camera flash function is a more difficult challenge. The design engineer must provide the appropriate amount of light within the specified time frame, and does not exceed the system's power consumption limit or LED's thermal limit. Due to inadequate basic on/off control, many design engineers have turned to algorithms that run on a system microcontroller that coordinates the flash subsystem and camera shutter simultaneously. Recently introduced smart LED flash controllers have more efficient light control. For example, earlier this year, NXP Semiconductors announced the UBA3001, the first device in the LED flash driver family that can support up to 90% efficiency over the entire voltage range while providing a single high brightness The LED provides up to 1A of current to support mobile phones with high-resolution cameras. The new IC also offers full current setting control for torch mode and camera/record mode.

Save Space Of course, PCB footprint is always precious for space-constrained portable applications. In May, Micrel released the MIC2298, a boost converter-based flash driver for embedded cameras, with a resolution of more than 2 million pixels and providing 1A of current for two LEDs in series. The device is available in a 3 to 3mm MLF package and requires only a compact 2uH inductor and a small ceramic capacitor.

Other manufacturers are helping design engineers reduce board space while reducing component count and cost by integrating a wider range of lighting functions into the device. For space-constrained candy bar, folding or slider phones with LCD and camera modules embedded in the top half of the phone, Maxim recently released a family of WLED drivers in a miniature 4 x 4mm package. Two programmable 200m ALDO.Max8645X and 8645Y used to drive the camera module incorporates a fractional charge pump to drive up to six WLEDs at a current of up to 30mA with five-position dimming accuracy. These devices can also drive two WLEDs up to 200mA for camera flash applications.

Similarly, AnalogicTech has also released the charge pump-based AAT2846, which integrates six backlight drivers, two flash drivers, and two general-purpose LDOs with two independent single-wire digital interfaces on a single IC. This allows design engineers to build a solution that provides independent backlight and flash function control, and allows them to program these controls through a large number of settings. The 6 backlight outputs can be synthesized for a single large display or split into 5+1 configurations for the main and sub displays.