SUPERCAPS Lighten the Load in LED Flash Applications

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.

POWER CHALLENGES

Low ESR presents designers with an inherent problem during the charge cycle. In any system, the capacitor is initially discharged. When the supply voltage is then applied, the supercapacitor resembles a low-value resistor. This can result in a huge inrush current if the current is not controlled or limited. Therefore, designers must implement some sort of inrush current limit to ensure the battery does not shut down. Typically, any circuit of this type also requires short-circuit, overvoltage and current-flow protection.

The challenge for designers is how to efficiently interconnect the battery, dc-dc converter and supercapacitor in a way that will limit the supercapacitor inrush charge current and continually recharge the cap between load events. Flash LEDs for digital-still cameras require 1 A to 2 A for up to 300 ms. A supercapacitor can be used to store the required current and deliver it quickly without draining the main battery. Working together with the battery, the supercapacitor discharges its power during peak loads and recharges between peaks, providing the power needed to operate systems from battery-operated hosts up to 200% longer while extending the life of the battery.

Clearly, any time designers use a supercapacitor, they must limit inrush current. In addition, the supercapacitor must be recharged when the voltage drops below the operational limit of the LEDs. Then, when the supercapacitor is fully charged, it has to be disconnected from the source. These flash-lighting systems also require short-circuit, source-overvoltage and current-flow protection.

DESIGN EXAMPLE

LED flash drivers are now available that can manage supercapacitor charging requirements and make the designer's job easier, integrating the circuitry to save space, cost and time to market. One example is AnalogicTech's AAT1282, a 2-A flash-driver IC, which contains a stepup converter used to boost the 3.2-V to 4.2-V battery input voltage up to a regulated 5.5 V. The AAT1282 also offers flash-management capabilities such as movie-mode and supercapacitor charging capabilities.

If the battery voltage is 3.5 V and the boost converter is 90% efficient, then the battery would need to supply more than 3 A for the duration of a 2-A flash pulse. This would either cause the battery-protection circuit to shut the battery down or cause a low-voltage shutdown with plenty of energy still remaining in the battery.

However, the stepup converter includes built-in circuitry that prevents excessive inrush current during startup, as well as a fixed-input current limiter of 800 mA and true-load disconnect after the supercapacitor is charged. The AAT1282's output voltage is limited by internal overvoltage protection circuitry, which prevents damage to the AAT1282 and supercapacitor from open LED (open-circuit conditions).

During an open circuit, the output voltage rises and reaches 5.5 V (typical), and the overvoltage-protection circuit disables the switching, preventing the output voltage from rising higher. Once the open-circuit condition is removed, switching resumes. At this point, the controller will return to normal operation and maintain an average output voltage. An industry-standard I2C serial digital input is used to enable and disable LEDs, and set the movie-mode current with up to 16 movie-mode settings for lower light output.

The schematic in Fig. 1 depicts the components needed to implement this flash-lighting subsystem, with some of the key components identified in the table. A 0.55-F 85-mΩ supercapacitor delivers 9-W LED power bursts using the flash LED driver IC. To achieve high light levels, the flash LEDs are driven at currents between 1 A and 2 A. The forward voltage (VF) across the LED at these high currents can range up to 4.8 V. If the 200 mV of overhead for the current-control circuitry is included, it is easy to see how the total load voltage during a flash event can range up to 5 V and require a 5.5-V stepup voltage.

Fig. 2 shows test results using two LEDs flashing at 1 A each and one LED flashing at 2 A. As the test results indicate, the supercapacitor can easily supply the necessary current for 500 ms while holding the supply voltage sufficiently above the VF of the LEDs. Between flash events, the supercapacitor is recharged at a steady rate to prepare for the next photo.

A current limit is set by the factory at 800 mA. The time to pre-charge an empty supercapacitor is about 5 seconds. The time needed to recharge the supercapacitor between two flashes is very minimal. It depends on the length of each flash. Fig. 3 shows the digital control of the flash function and movie-mode option.

The size of the supercapacitor was determined by the battery voltage, LED flash current, LED forward voltage, the efficiency of the AAT1282 and flash-pulse duration. For a 300-ms of 2-A flash, a 550 mF at 5.5-V type supercapacitor is suitable for most the application. AAT1282 has a built-in circuitry to prevent excessive inrush current to 800 mA during startup while charging the supercapacitor near ground potential. If the inrush current needs further reduction due to the size of the battery, the limit can be decreased. It also can be increased if desired.

The AAT1282 contains a thermal-management system to protect the device in the event of an output short-circuit condition. Thermal protection disables the AAT1282 when internal power dissipation becomes excessive, as it disables both MOSFETs. The junction over-temperature threshold is 140°C with 15°C of temperature hysteresis. The output voltage automatically recovers when the over-temperature fault condition is removed.

A NEW HOME IN PORTABLES

Until recently, supercapacitors have rarely been used in portable systems. Typically, they have been limited to backup or standby functions in fixed applications that use relatively low currents and offer fairly long charge times. But by combining new stepup converters with supercapacitors, designers can now create compact power designs that extend battery life. With a profile of less than 2 mm, the supercapacitor is thin enough to meet even the rigorous footprint requirements of the cell-phone market.