ST Micro sees 40W LED flash in future smartphones

STMicro electronics hopes to make blurry low-light images from smartphone cameras a thing of the past

By Peter Sayer, Paris | Thursday, 02 February 2012
STMicroelectronics hopes to make blurry low-light images from smartphone cameras a thing of the past with a new chip designed to boost light output from LED-based flashes.

The ideal camera flash delivers a lot of light in a short time, freezing action and illuminating more distant objects. Professional cameras use a xenon strobe light to produce a brief burst -- or flash -- of light, but the lighting on smartphone cameras is typically provided by an LED.

The light output of a battery-powered LED is continuous, and much lower in intensity than a flash, leading to longer exposure times and darker, blurrier pictures.

But ST hopes to change that with its new STCF04 multifunction chip, which it says can control flash power up to 40W, compared to perhaps 2W for typical LED flashes today.

The key is the chip's ability to control the charging and discharging of a supercapacitor, which it uses to gradually store energy from the phone's battery and then deliver it to the LED in a short burst. That's similar to the way that xenon strobes work, but the LED-supercap combination does the job with just a few volts, making it much safer -- and the components more compact -- than the hundreds or thousands of volts needed to drive a xenon strobe.

ST's chip, just 3 millimeters square, will add US$2 or less to the cost of a cell phone, it said.

It contains a charger to store energy in a supercap, and a driver for an external transistor used to deliver 40 watts or more of peak power from the supercapacitor to a bank of LEDs. It also contains a temperature sensor to detect when the LEDs are in danger of overheating -- useful if they are being used as a torch rather than a flash; a light sensor for setting the exposure and flash intensity, and a driver for an auxiliary LED used either as a privacy indicator (you're on camera) or perhaps to help autofocus systems in low light.

The chip's built-in timer can be used to set the flash duration in steps of around one-100,000th of a second, although it takes around one-3,000th of a second for the LED to reach full power, according to ST. The controller can also discharge the supercap in stages to produce several flash pulses in a row, useful for red-eye reduction.

Samples quantities of the chip are available now, and ST said it will begin full production by the end of March.

The company has published a datasheet for the STCF04 giving full details of the chip and sample circuits.