SUPERCAPS Lighten the Load in LED Flash Applications

Jan 1, 2009 12:00 PM
By Thomas Delurio, Senior Applications Manager • EDDIE LEE, Applications Manager Advanced Analogic Technologies, Inc.

The use of a supercap in combination with an application-specific driver IC alleviates stress on the battery in powering an LED flash bulb in cell-phone cameras.

Cell Phones are Becoming the ultimate consumer all-in-one portable appliance, producing digital-still camera-quality pictures, supplying WiFi/Web access and delivering high-quality audio. As customers demand a wider array of new features, however, designers are struggling to ensure the phone battery provides enough peak power to drive these increasingly complex mobile applications.

Of all the functions in today's high-performance phones, the camera flash consumes the highest peak current. As a result, demand is building for circuits that can store high currents for short periods without overloading the battery to provide the power required for high-performance operation.

As designers have increased the resolution of camera phones to 3 megapixels and beyond, they also have increased the amount of light required to achieve a high-quality image. To match the photo quality of digital-still cameras, today's cell phones must either drive flash LEDs at currents as high as 2 A or xenon flash tubes charged to more than 330 V. Other applications in the phone — such as the RF power amplifier, GPS mapping, Internet access, music and video — can exceed source current availability as well.

When flash LEDs are the chosen light source, a compact power design can be created by combining a flash LED controller (a stepup converter IC) with a supercapacitor, which supplies high levels of current for short durations. This approach allows the use of smaller, lighter and less-expensive power sources while extending battery life. The advantages of this approach are illustrated by a reference design in which two flash LEDs are driven at 1 A each, delivering more light than a K800i xenon strobe. At less than 2 mm, the supercapacitor is thin enough to meet the rigorous footprint requirements of the cell-phone market; it can be used to enhance other features in the phone such as longer talk time and better audio.

COMPARING LIGHT SOURCES

Cell phones with cameras greater than 3-megapixel resolution require a high-intensity flash in medium-light to low-light conditions to produce quality pictures. Although designers can use either LED or xenon flash units, each design strategy offers challenges:

• High-current flash LEDs need up to 400% more power than a battery can provide to achieve the light intensity needed for high-resolution images. To overcome this power limitation, some camera phones have used longer flash-exposure times to compensate for the lack of light. However, that strategy often results in blurry photos.

• Xenon flash tubes deliver excellent light power. Nevertheless, their short flash exposure cannot be used for a video-capture or movie-mode function. They also require electrolytic storage capacitors that are bulky, operate at high voltages, take a long time to recharge between flashes and cannot be used for other peak-power needs in the phone.

Designers can solve this problem with flash LEDs driven at 1 A to 2 A by using a capacitor to store the current and deliver it quickly without draining the main battery. However, conventional capacitor capability would require either a very large case size or multiple devices connected in parallel. A more practical solution for space-constrained portable systems is to use very high-value supercapacitors. These devices offer high levels of capacitance in a relatively small, flat case size.

By using a supercapacitor, designers can deliver the high-current levels needed for these short-duration events, and then recharge from the battery between events. To support the battery, designers can add a thin supercapacitor to handle the phone's peak-power needs — flash photos, audio and video, wireless transmissions and GPS readings — without compromising a slim-handset design.

This approach also allows designers to reduce the system footprint by optimally sizing the battery and power circuitry to cover just the average power consumption instead of peak levels.

DEFINING A SUPERCAPACITOR

What is a supercapacitor? Like any capacitor, a supercapacitor is basically two parallel conducting plates separated by an insulating material known as a dielectric. The value of the capacitor is directly proportional to the area of the plates and inversely proportional to the thickness of the dielectric. Supercapacitors store energy in an electrostatic field rather than in a chemical state like a battery.

Manufacturers building supercapacitors achieve higher levels of capacitance, while minimizing size using a porous carbon material for the plates to maximize the surface area and a molecularly thin electrolyte as the dielectric to minimize the distance between the plates. Using this approach, they can manufacture capacitors with values from 16 mF up to 2.3 F.

The construction of these devices results in a very low internal resistance or equivalent series resistance (ESR), allowing them to deliver high peak-current pulses with minimal drop in the output voltage. These supercapacitors reduce system footprint requirements by delivering a very high capacitance in a relatively small case size. They can be manufactured in any size and shape, and recharged in seconds.

By averaging out high power demands, supercapacitors extend battery life by up to a factor of five and allow designers to specify much smaller, lighter and less-expensive batteries. Supercapacitors also offer an operating life as long as 10 to 12 years with >500,000 cycles. Their failure mode is an open circuit (high ESR) rather than a battery's destructive event. Similarly, if overvoltage is applied to the device, the only consequence will be a slight swelling and a rise in ESR, eventually progressing to an open circuit.

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