LED driver enables brighter flash in camera-phones

As cell phones that integrate cameras have become more commonplace, major improvements in image quality have been realized. However, the integration of new features, such as streaming video, MP3, instant messaging, and mobile email, threatens to reverse the trend in better image quality as camera-phone designers struggle to manage multiple features demanding limited battery power. Analog Devices, a global provider of signal processing technologies and high-performance semiconductors for digital cameras, is introducing a white LED driver for the camera-phone flash function that uses a new portable power management technique to maintain picture quality even when the battery is taxed by multiple phone functions operating simultaneously. An output current power management feature reduces battery-current overstress when the camera flash is activated at the same time the phone is receiving an RF (radio frequency) signal. Under normal operating conditions ADI's new ADP1653 high-efficiency, high-brightness, dual, white LED driver enables a brighter camera flash than single or low-efficiency white LED drivers, which translates into better picture quality in low-light conditions.

“As camera phones move to resolutions of two Megapixels and higher, the light level required to take quality photos rises dramatically,” said Arcadio Leon, marketing director, Portable Power Management Products, Analog Devices. “For camera phones, both efficiency and system sequencing play a critical role in minimizing battery-current overstress. Existing charge pump solutions, for example, have a power efficiency rating as low as 40 percent under low battery voltage conditions, which means that less than half of the input power is converted to output power, thereby further draining the battery. The power efficiency of the ADP1653 peaks at 92 percent and is typically greater than 80 percent over the full voltage range of a Lithium-ion battery.”

In addition, unlike standard step-up dc-dc converters, the ADP1653 features a logic-input pin that reduces the white LED current to its lowest operation level in less than 50 microseconds when the phone activates the power amplifier to send an RF transmission. This current sequencing technology further extends battery life without compromising photo image quality.

A high-efficiency flash driver solution can be developed using the ADP1653 and four small external components – a diode, microinductor and two capacitors. This bill of materials (BOM) reduces the external component cost by as much as 50 percent over existing high-efficiency solutions. The ADP1653 drives two series-connected, high-brightness white LEDs with currents up to 500 mA, as well as a separate indicator LED. The ADP1653 uses a dual-mode interface that can support an I2C interface or 2-bit logic bus, allowing designers to program the LED strings independently in hardware, with external resistors, or in software, by writing data to the device's registers. Safety features incorporated into the ADP1653 include soft-start to reduce in-rush current at start-up, flash time-out to prevent destruction of the LED caused by overheating, as well as a power NFET (N-channel field-effect transistor) current limit and thermal protection function, both of which prevent overheating of the ADP1653.

The ADP1653 is sampling now and will be available in volume production in February 2007. It is priced at $1.45 per unit in 1,000-piece quantities and is packaged in a 3 mm x 3 mm LFCSP (lead-frame chip-scale package). The device is specified over the industrial temperature range (-40 degrees C to +85 degrees C).

Analog Devices, Inc., 804 Woburn Street, Wilmington, MA 01887. Tel: +800-ANALOGD (1-800-262-5643).

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