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Ultrasmall camera flash LED driver loads 700mA

Posted: 16 Feb 2007 ?? ?Print Version ?Bookmark and Share

Keywords:LED? flash? camera? austriamicro? LED driver?

AS3685 flash LED driver from austriamicro

austriamicrosystems has launched an ultrasmall, safe, simple and fully integrated 700mA charge-pump-based flash LED driver. The AS3685 is said to be the smallest fully integrated flash LED driver offering 700mA flash current and is targeted at camera flash applications in mobile phones.

Designing LED-based flash functions with currents in the 700mA range has a significant impact on power consumption, especially in modern mobile phones that tend to include a wide variety of fashionable but power-hungry features such as video streaming, MP3 or Wi-Fi. The LED driver needs smart power management to stay within the overall power budget, even under the most demanding conditions such as replying to RF signals while taking pictures.

Moreover, the large number of functions in a mobile phone already consumes a significant amount of PCB space, which makes adding a large flash LED and driver a challenge. The addition of these components conflicts with the ongoing trend towards ever-smaller phone designs, making actual component size a critical factor.

Wafer-thin chip
The wafer-thin AS3685 is said to address these issues. Measuring only 2-by-1.5mm and less than 600?m in height, it takes up 14mm? of PCB space, including four external components. Following 0.25mm PCB design rules, the device uses less than 50 percent of the space required by its closest competitor. The only external components that the AS3685 needs are low cost capacitors and resistors. This gives the device an excellent overall BOM and outperforms competing inductive solutions by at least 20 percent.

Safety in operation is another key requirement for flash LED applications, including low EMI and safe operation under heavy load conditions. Using switched capacitor circuitries to drive large LED currents on the output means pulling high currents from the batteries. The AS3685's smart charge-pump architecture avoids large spikes at the battery terminals, providing significantly better EMC performance than comparable products.

To avoid excessive current being drawn from the battery if the phone's power amplifier becomes active while a picture is being takena situation potentially causing the phone to shut down due to a related voltage dropthe AS3685 can reduce the flash current within a microsecond. This prevents dropped calls without compromising photo quality.

Safety features
Other safety features include a built-in flash LED time-out function that guarantees flash shutdown via a built-in factory-programmable timer, typically 800ms. This prevents the destruction of the LED or damage to the mobile phone from excessive heat produced by the LED.

The AS3685 comes with a multimode, simple 2-pin interface incorporating off, indicator, flashlight and flash operating modes. The AS3685B product variant features a single-wire interface allowing the flash current to be set in any of 18 different steps depending on actual operating and environmental conditions. This allows it to dynamically reduce the flash current during MP3 playback or at low battery conditions, thereby extending battery life.

The AS3685 is available in WL-CSP3x4 packaging. It is currently in trials and will be ready for mass production this month.

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