CAP-XX supercapacitors power super-bright LED flash

As phone manufacturers continue to introduce handsets with high-resolution cameras (2 megapixels and above), they are also seeking to enhance the performance of LED flash modules to allow users to take decent photos in low light conditions.

Although high-power LEDs are being used in handsets, it is difficult to supply sufficient current from the battery to drive the LEDs to the level required for adequate light output during the flash "event".

Fig 1. Nokia retrofitCAP-XX Inc, a manufacturer of supercapacitors based in Sydney, Australia, has developed a drive architecture which combines high-power white LEDs with a supercapacitor and associated driver circuitry.

The CAP-XX BriteFlash power architecture uses a low-current charge pump (boost converter) to charge the supercapacitor to 5.5 volts, then the supercapacitor drives the LED at very high current for the flash pulse.

The supercapacitor approach makes it possible to drive four LEDs at close to 1 A each, while only drawing a maximum of 300 mA from the battery. The supercapacitor is charged with a current of 250 mA to support a recovery time between flashes of approximately 2 seconds. CAP-XX says that this is less time than the LED needs for thermal recovery between flashes.

Most solutions for driving high-power LEDs use special-purpose boost converter ICs that are capable of delivering around 1 A to a single LED. Without a supercapacitor, the boost converter has to be sized for peak flash current, while in the CAP-XX case a lower-cost, smaller boost converter or charge pump is needed to supply the supercapacitor with its low charging current of 250 mA.

Figure 2a. Unmodified phone Anthony Kongats, CEO of CAP-XX, told LEDs Magazine that the supercapacitor route is the only way to drive several power LEDs at close to 1A. "The battery simply can't provide enough current without using a supercapacitor," he said. "If it tried to provide 4 A of current it would probably cause the phone to shut down."

Figure 2b. Modified phone To demonstrate the increased flash power and ease of design-in, CAP-XX engineers retrofitted a Nokia 6680 handset (figure 1). The original 4-die LED was replaced by 4 Lumileds PWF1 devices (rated at 1 A), and a supercapacitor, charge pump and associated circuitry were added. The supercapacitor is 1.2 mm thick, and the phone was reassembled with almost no difference in appearance.

While the original LEDs drew 70 mA per die (or 1 W in total), the supercapacitor drove the new LEDs at 0.9 A each, or 15 W in total, with a flash duration of 160 ms.

The photos in Figure 2 from the unmodified phone (dark) and the modified phone with supercapacitor, at a distance of 2 m with no ambient light, clearly show the difference in flash performance.

Figure 3 Figure 3 shows that while the supercapacitor provides a peak current of 4 A during the flash, the maximum current draw from the battery is only 300 mA which goes to recharge the supercapacitor.

“Greater than 2-megapixel camera phones require a high-intensity flash in medium to low light conditions to ensure good pictures,” said Anthony Kongats. “Some solutions are available but lack adequate power to produce quality photos in all light conditions. Our BriteFlash power architecture completes the equation with the power to drive today’s LEDs.”

The total cost of the CAP-XX BriteFlash solution is US $4 to $5 including the LEDs, supercapacitors and circuitry. The supercapacitor alone costs $1.50 for the single-cell solution and $2.50 for the dual-cell one in quantities of 500,000 to 1 million.

Kongats adds that CAP-XX is working with flash/LED driver suppliers to develop supercapacitor-optimized charge pump LED drivers to further increase camera phone power subsystem integration and reduce costs.

CAP-XX, which develops and manufactures supercapacitors, is a private company based in Sydney, Australia, with additional production facilities in Malaysia.