How supercapacitors solve LED flash power issues in high res camera phones -- Part 1

For example, a traditional LED Flash driver uses a boost converter in current-controlled mode as shown in Figure 1.Consider the case of driving 1A each through 2 LEDs in parallel, such as the Lumileds Luxeon PWF1, which would generate approximately 20 lux at 2m using a high uniformity optic (Lumileds Technical Datasheet DS49). The output voltage at the boost converter = LED forward voltage + voltage across current sense resistor. The maximum forward voltage = 4.8V and assuming 200mV across the current sense resistor, then the boost converter output voltage = 5V. Assuming the camera phone battery voltage = 3.3V under load, and the boost converter is 85% efficient, the battery current in this case would = 5V/3.3V/85% x 2A = 3.6A. This exceeds the typical phone battery’s capability.

Figure 1: Current controlled boost converter as LED Flash driver

The alternative solution is to use a xenon flash, but this requires
1) a storage capacitor that is very bulky for mobile phone form-factors and
2) high voltage, resulting in circuit and safety issues.

Also, in order to fit into a camera phone the smallest diameter 330V electrolytic capacitors, 5.5mm or higher, have an order of magnitude less energy than found in a typical digital camera so the total light energy is lower than a supercapacitor LED solution. Finally, with LED flash, the same LED circuit can be used at lower current for video capture/torch function.

To overcome the power limitations in current LED flash systems, some camera phone suppliers have used long flash exposure times to compensate for the lack of light, thereby increasing the total light energy, but this results in blurry photos.