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WLED drivers allow brightness modification

Posted: 28 Aug 2008     Print Version  Bookmark and Share

Keywords:white LED  flash drivers  WLED brightness  boost converters 

AAT1271, AAT1272white LED (WLED) flash drivers. Supporting either two WLEDs at up to 750mA each or one WLED at up to 1.5A and coming in a compact 3x3-mm TDFN package, the AAT1271 and AAT1272 dual-channel boost converters are the first devices of their type in AnalogicTech's product line to feature the company's proprietary AS2Cwire and industry-standard I2C interface respectively. AnalogicTech says that the additional addressing capability of these two buses offers designers flexibility to modify drive current, and therefore WLED brightness. At the same time, the boost converters' high drive current and small 3 x 3mm footprint delivers a highly attractive alternative to bulky xenon lamp solutions.

"Many digital still cameras and phone cameras today maximise photo quality by compensating for ambient light deviations," explains Phil Dewsbury, AnalogicTech product line director for lighting products. "By taking advantage of the additional control capabilities in the AS2Cwire and I2C bus, the AAT1271 and AAT1272 allow designers to modify flash brightness or movie mode brightness on-the-fly across multiple levels and thereby improve picture quality."

Increased current control

Both the AAT1271 and AAT1272 are high efficiency, high current boost converters designed for LED photo flash applications. Operating off a 2.7V to 5.5V input voltage range, the two devices are targeted at single-cell Lithium-ion battery powered mobile systems. Both converters use a high 2MHz switching frequency, which helps reduce system footprint by allowing the use of smaller external inductors and capacitors. The two new converters can achieve up to 85% efficiency with 1µH inductors.

The AAT1271 and AAT1272 feature two channels capable of driving two high current WLEDs at up to 750mA each. Alternately, the two channels can be combined to drive a single WLED at up to 1.5A. Both converters supply separate flash enable and movie-mode enable inputs to control flash and movie-mode operation. Designers use a single, external resistor to set full-scale flash LED current. A default 7.3:1 flash-to-movie-mode current ratio is provided. Designers can adjust the movie-mode current level from the default setting to any of 15 other levels using either the AS2Cwire or I2C interface. The AAT1271 adds a flash inhibit input that can be used in GSM mobile applications to reduce the flash current during PA transmission to avoid system reset due to excessive battery droop in high battery current conditions.

Given their high drive current capability, both converters also feature input current limit capability and a true load disconnect switch that reduces supply current in shutdown to < 1µA.

The new converters also include a user-programmable flash safety timer. This function can be used to protect the flash LEDs from thermal damage in the event of a software failure by terminating the flash pulse regardless of the FL enable signal condition. Using the AS2C or I2C bus, the safety timer can be programmed across 16 different levels.

Both devices are qualified across the -40� C to +85� C temperature range. The AAT1271 and the AAT1272 sell for Rs.63.86 ($1.49) in 1K quantities.

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