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The growing demand for color displays in cell phones, digital cameras and PDAs has created a huge market for white LEDs as backlights in these applications. Because white LEDs require low dc voltage between 3V and 4V, and provide the miniaturization needed, they are becoming the light source of choice in color LCD displays employed in myriad small handheld electronic devices. Unlike their CCFL counterparts, which require a very high (200 to 500Vrms) ac waveform and an expensive and bulky transformer-based circuit for power, white LEDs' relatively low dc voltage means that simple inductor-based or capacitor-based circuits can be used to drive them.
As more features are added to these handheld gadgets, associated color displays also are getting more complex with all kinds of LEDs lighting the screen. As a result, the chips driving and controlling white and other LEDs in a single display system have evolved over the last few years. In fact, these simple LED drivers are now complete lighting management solutions on a single chip. Several suppliers have emerged to grab a piece of this growing pie
Suppliers offering integrated solutions for lighting management include National Semiconductor, Linear Technology and Fairchild Semiconductor. Besides driving multiple white LEDs in series or parallel in complex color displays, these integrated lighting management solutions also deliver the right voltage and current to drive other types, including red, green and blue (RGB) indicator LEDS in the same system.
Envisioning such a trend, early this year National Semiconductor unveiled a monolithic system-level solution to provide complete lighting and analog power management for mobile phone displays. National's lighting management units (LMUs) LP3933 and LP3936 are designed to provide high-efficiency, fully featured display lighting control and drive for clam-shell or flip-phone mobile phones that typically have a camera, one or two RGB indicator LEDs, a main display and a sub/CallerID display.
Integrating a boost regulator and pulse-width modulation (PWM) lighting control functions, the LP3933 is able to drive up to six white LEDs and two sets of RGB LEDs, as shown in Fig. 1. Because the RGB LEDs are PWM driven with programmable color, intensity and blinking patterns, they also combine to function as flash in camera-enabled flip-phones. All functions are software controllable via an SPI serial interface and 19 internal registers. It is housed in a 32-pin laminate CSP plastic package. The LP3936 has all the functions of the LP3933 with only one RGB indicator LED driver function.
Likewise, Linear Technology has readied an integrated driver chip, labeled LTC3205, that is capable of powering multiple LEDs used in cell phone displays. It is crafted to drive one to four white LEDs for main display, one to two for sub-display, as well as RGB LEDs normally used as flash in cell phone cameras. All the drivers are encased in a single 24-lead QFN package. The LTC3205 is an 800kHz, low-noise, high-efficiency step-up/step-down fractional charge pump converter. Each display has digital control with independent dimming and programming via a 3-wire serial interface. With an input voltage range of 2.7V to 4.5V, it is optimized for single-cell Li-ion cellular applications and can furnish efficiencies as high as 92%, while quiescent current is only 50µA. The LTC3205 requires only four small capacitors and two resistors to create a tiny (<25mm2, <0.8mm high) solution footprint capable of powering the LEDs used in three separate displays, as shown in Fig. 2.
The LTC3205 charge pump optimizes efficiency based on VIN and LED forward-voltage conditions. The device powers up in step-down mode and only switches to step-up mode when any enabled LED current source begins to enter dropout. Maximum currents for the main/sub/RGB displays are set with a single resistor. LED currents are controlled with internal current sources, while dimming and on/off control for all displays is achieved via a 3-wire serial interface. Four dimming states are available for the main and sub-displays, while 16 dimming states are available through internal PWM for the RGB LEDs, resulting in 4096 color combinations.
A single-chip lighting management solution is also in the works at Fairchild Semiconductor. Planning to extend its reach, developers at Fairchild are prepping integrated drivers that can address the power needs of white, flash and RGB LED fun lights in a single display. According to Fairchild, its integrated lighting management controller is expected to be released toward the year-end. Meanwhile, the manufacturer has also expanded the LED driver portfolio with several new members. The latest to join this line is FMS2000, a cell phone camera flash controller. It has built-in processing support for all flash functions, such as red-eye reduction, programmable flash, movie mode and white balance adjust. Additionally, the maker also has expanded the white LED driver portfolio. Early in the year, the company added FAN5608 to give designers the flexibility to connect a series of white LEDs in two parallel channels. Combined with an adaptive inductor-based dc-dc boost converter, the FAN5608 features all three brightness controls — digital, analog and PWM.
Additional Sources
While National and Linear are leading the charge with LMUs, and Fairchild is moving aggressivlely to narrow the gap, several suppliers have emerged to go after white LEDs alone. With proliferation of white LEDs, makers like Intersil, Maxim Integrated Products, Texas Instruments and Catalyst Semiconductor have readied drivers for powering multiple white LEDs in series/parallel configurations.
Intersil, for instance, has developed a high-frequency PWM step-up regulator capable of driving up to 12 LEDs, ensuring uniform brightness in white LED backlighting in small color LCD displays for cell phones and other battery-powered electronics. Offering improved current accuracy for uniform brightness, the EL7513 is designed to generate output voltage required to drive up to four LEDs in series or 12 LEDs in series/parallel configuration.
In essence, the EL7513 is a constant current boost regulator specifically designed for driving white LEDs. Claiming to achieve up to 91% efficiency, brightness of the LEDs is adjusted through a voltage level on the device's control pin. When the level falls below 0.1V, the EL7513 enters shutdown mode and consumes less than 1µA of current for input voltage less than 5.5V. It also features soft-start and dimming control. In production, it comes in 8-pin TSOP and MSOP packages.
Maxim Integrated Products is another player in this race with its newest MAX1553/1554 step-up dc-dc converters, which are tailored for driving two to 10 white LEDs in series. They regulate with a constant current, providing efficient display backlighting in PDAs, cell phones and other handheld devices. The step-up dc-dc converter includes an internal 40V, low RDS(on), n-channel MOSFET switch for high efficiency and maximum battery life. The MAX1553/MAX1554 are available in a 3mm × 3mm, 8-pin TDFN package, making them the smallest devices for powering 10 white LEDs.
While the MAX1553's current limit is 480mA to drive two to six white LEDs, the MAX1554 can deliver 970mA to drive up to 10 white LEDs with 82% efficiency at 20mA from a 3.6V input voltage. At 20mA from a 3.6V input voltage, the MAX1553 drives six white LEDs with 88% efficiency. A single dual-mode input adjusts brightness through a PWM signal or a direct dc voltage. A separate enable input provides on/off control, and soft-start minimizes inrush current during startup.
TI's TPS61042 high-frequency white LED driver has a 1.8V to 6V input range, supplies output voltages up to 28V and can drive up to eight white LEDs with a constant current. The driver allows the LED current to be set with an external sense resistor. To control LED brightness, the LED current can be pulsed by applying a PWM signal with a frequency range of 100Hz to 50kHz to a control pin. It comes in a tiny 8-pin QFN package.
Leveraging its E2PROM process technology, Catalyst Semiconductor has developed its first driver for this application, the CAT32. Designed to drive up to four series connected white LEDs efficiently, the CAT32 is a constant-current boost regulator operating at 1.2MHz fixed switching frequency. To improve efficiency, the supplier claims that its quiescent current has been lowered by a factor of two over competitive solutions. Plus, it features open load protection and is housed in a 6-lead SOT23.