Comparing today's portable consumer electronics to a few 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 phased out. Today's devices feature high-performance, high-resolution, 2.5- to 3-inch diagonal color displays to support a full range of applications from Internet access and mobile TV to video playback.
Typically, these displays require four or more LEDs and drivers for backlighting. Many handheld devices (especially clamshell designs) have added a smaller secondary display to show basic information such as time, date and connectivity. These secondary displays typically require one or two more LEDs for the backlight function than the primary display.
As design engineers discover the importance of fashion lighting in product differentiation, many portable electronic devices today require additional power circuits to drive auxiliary RGB status lights and keyboard backlights.
The built-in camera feature features a more complex power circuit design. These functions typically provide a flash by driving a small number of LEDs at high current for a very short duration. But when handheld device manufacturers first introduced this feature, they used a CCD of less than 1 million pixels, which required the flash driver's current to not exceed 100 mA. Today's handheld design engineers are integrating a much higher resolution CCD that requires up to 600mA of current to be used only to drive the flash and provide enough light for high photo resolution. Newer features such as movie mode and flash function increase the design difficulty of the power circuit by requiring a lower level power supply to drive the LED for longer duration.
With these developments in mind, it's easy to understand why lighting is often the most battery-intensive source of handheld devices. Previously, design engineers used their boost converters or charge pumps to provide their lighting capabilities on a stand-alone basis. But 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. Eventually these engineers turned to system microcontrollers or dedicated controllers to address these issues with 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 higher levels of control and simplified design. Take the National Semiconductor (NS) LM27965 as an example. This white LED (WLED) driver can drive up to 9 LEDs in parallel with a total output of up to 180mA. In order to maximize the control ability of the design engineer, the output forward current can be divided into two or three independent control groups, and four to five LEDs are configured to provide backlight for the main display; two to three LEDs are configured as sub-displays. The screen is backlit; a single independently controlled drive is configured to manage the status or indicator LEDs. Each set of LEDs is controlled via a standard I2C interface.
While solutions based on inductive boost converters still offer 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 need large sensors. 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 is modulated to maintain a constant current for each LED, allowing designers to more accurately match current sources.
Similar to many competing products, Linear Technology's LTC3219, released in August, uses a multimode charge pump that turns on in 1x mode and then approaches the voltage drop when any enabled LED current source approaches Automatically switches to boost or 1.5x mode.
Subsequent voltage drops convert the device to double (2x) mode. To support primary and secondary display and RGB lighting requirements, the device drives nine individually configurable current sources.
Like NS devices, each current source in Linear Technology's devices digitally controls grayscale, brightness, flash, and grading through a two-wire I2C serial interface.
Early WLEDs required relatively high forward voltages and currents to achieve the desired luminosity. But recent technological advances have allowed manufacturers to produce WLEDs that can operate at currents below 10 mA. These devices operate at lower forward voltages than before. Recent technological developments have allowed PMIC manufacturers to begin offering linearly matched independent current sources, reducing cost and reducing footprint due to the elimination of most external components.
For another example, TI's TPS75105 provides a four-channel LDO and comes with up to four LED matched LED constant current drivers 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 for the inductive boost converter, or the switched capacitors and input/output capacitors required for stability with a mixed-mode charge pump.
The drive of the camera's flash function is a more difficult challenge. The design engineer must provide the right amount of light within the specified time frame and does not exceed the system's power consumption limits or the LED's thermal limits. Due to inadequate basic on/off control, many design engineers turned to algorithms that were run by system microcontrollers that simultaneously coordinated the flash subsystem and camera shutter. Recently introduced various intelligent LED flash controllers have been equipped with more efficient lighting control. For example, earlier this year, NXP Semiconductors announced the UBA3001, the first device in the family of LED flash drivers that supports up to 90% efficiency over the entire voltage range while providing a single high brightness LEDs provide up to 1A of current to support mobile phones with high resolution cameras. The new IC also offers full current setting control for flashlight mode and camera/video mode.
save space
Of course, the footprint of the PCB is always quite valuable for space-constrained portable applications.
In May, Micrel announced the MIC2298, a boost converter-based flash driver for embedded cameras with a resolution of more than 2 megapixels that provides 1A of current for two series LEDs. 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 component count and cost while saving board space by integrating a wider range of lighting functions into the device. For space-constrained candy bars, folding phones or slider phones with LCD and camera modules embedded in the top half of the phone, Maxim recently released a line of W LED drivers in a tiny 4 x 4mm package. Two programmable 200mA LDOs for driving the camera module are added. The Max 8645X and 8645Y use a fractional charge pump to drive up to six WLEDs at currents up to 30mA with five-bit dimming accuracy. These devices can also drive two WLEDs at up to 200mA for camera flash applications.
Similarly, AnalogicTech has released the charge pump-based AAT2846, which combines six backlight drivers, two flash drivers and two general-purpose LDOs with two separate 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 with a large number of settings. The six backlight outputs can be synthesized for a single large display or divided into 5+1 configurations for the primary and secondary displays.
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