Megapixel photographic capabilities are included in today’s mobile phones. Since rich lighting is crucial for yielding good photographic performance, and often, the light available for taking a photograph is insufficient, use of a flash unit as an additional light source is required in many situations. Recently introduced high power white LEDs offer a cost effective solution for flash applications in small form factor portable equipment. Besides, the same LED can be used for a torch function or a video (movie) light, by just keeping them on continuously at less current.
LED Flash Basics
Using an LED as a light source to illuminate a scene for taking a picture requires generating light pulses at the same time the image sensor is active. Typically, light pulses of 150ms or more are used (as long as the illumination is provided for not less than the module’s image framing period, it will not degrade image quality), depending on the LED light emission capability and the way the image sensor and flash are operating together. Further, it should become necessary to calculate how much light is needed for a proper picture. The minimal subject illuminance of approximately 5 lux is required, within a coverage range of up to 3 meters.
A bright white flash will call for a high current and therefore a powerful battery. Nowadays, mobile phones are using Li-Ion or Li- Polymer batteries, thanks to their high energy density and the small form factor. The voltage supplied by typical Li-Ion batteries ranges from about 3 V when discharged up to 4.2 V when fully charged. However, the required forward voltage of a flash LED can range from 3.2 V to 4.8 V under nominal conditions (during operation this range can extend in both directions), and hence to fulfil the demands, a regulator (current source to drive the LED with a defined current) must be chosen that is capable to cover both (buck and boost) conversion.
LEDs are current-driven devices in which the light output depends directly on the forward current passing through them. LEDs can be strobed faster than any other light source and have very short rise times (10 – 100ns). The resulting light quality of today’s white LEDs are comparable to that of cool white fluorescent lamps,with a color rendering index near 85. The maximum light output from a LED is often limited by the maximum average forward current it can handle, and in flash applications the LED may be operated from a pulsed current with a very short duty cycle. This allows the current and hence the light output to be increased significantly during the actual pulses, while still keeping the LED’s average current level and power dissipation within its safe ratings. There are a number of different LED assemblies that manufacturers specify for camera flash operation. Note that, majority of LED flashes today have more than one LED die in a single package, and most LED assemblies intended for camera flash applications now have wide illumination angles.
The charge pump converter and voltage boost/buck converter are two possible dc-dc converter topologies suitable for driving an LED flash. With a 4 V typical forward drop voltage, a white LED does not need extra high voltage trigger pulse, it is extremely fast to turn on-off , and all the associated electronic circuit can be housed inside a standard mobile phone. Since the white LED have electrical characteristics similar to the standard LED, it should become necessary to provide a constant forward current to control the device. Dedicated driver chips are used for this purpose, and the circuit built around the dc-dc boost converter NCP5007 (http://onsemi.com) is a good example. The demo circuit shown here supports the low beam/high power flash mode, and accepts digital pwm signal to dim the LEDs.
Another white LED camera flash driver chip is MP1517DR from Monolithic Power Systems, Inc. The MP1517 is a 4A, fixed frequency step-up converter ideal for camera flash circuits driving up to 4 white LEDs. The high 1.2MHz switching frequency allows for smaller external components producing a compact solution for size constrained devices. The MP1517 is available in a tiny 4mm x 4mm 16 lead QFN package.
Next one is the TPS6105x (TPS61058/ TPS61059) series fixed frequency, synchronous boost dc-dc converters with an integrated down conversion mode.The chips (from Texas Instruments) are optimized for driving high power single cell white LEDs up to 800 mA of forward current from a 2.7 V to 5.5 V supply input. External resistors program the forward current to different levels (ex: torch, flash). The boost converter is based on a 650-kHz fixed frequency, PWM controller with integrated power MOSFETs to obtain maximum efficiency. The maximum peak current in the boost switch is limited to 1000 mA (TPS61058) and 1500 mA (TPS61059). The TPS6105x series fulfills both attributes to be used as a flash driver. Next figure shows a typical circuit for using the LED in two current modes. In low current mode, when the enable pin and nFLASH is set to HIGH, it can be used as a torch or video light. In a high current mode, nFLASH is LOW, it drives the LED for flash illumination.
Role of Supercapacitor
A battery is an inherently poor source of bursts of high current! A burst is required to produce a flash of light from LEDs. Batteries in mobile phones are designed to supply the peak current drawn by the RF power amplifier (PA). In GSM mode, the PA can draw as much as 3A from the battery. Therefore batteries today are rated for 3A peak current, but internal protection circuitry prevents them from delivering higher currents. So in space-constrained consumer design such as a mobile phone employa an auxiliary power source dedicated to the flash light – a dual supercapacitor capable of discharging quickly at high currents. Here, the battery charges the supercapacitor when the flash is triggered, and the supercapacitor discharges the stored energy to the LEDs. So, the battery is not connected directly to the LEDs. No doubt, this solution can dramatically improve the performance of their flash LED system while maintaining their thin form factor. Since the circuit makes full use of the power storage capacity of the supercapacitor, the system designer can specify the smallest possible supercapacitor, an important benefit in consumer devices that have very thin and light-weight form factors.
Following figure shows the block diagram of the supercapacitor solution with a small, low-cost, current limited charge pump pre-charges the supercapacitor to ~5.5V. Once the supercapacitor is charged, the current switch is enabled to deliver a high current flash pulse, with the energy and power from the supercapacitor rather than battery and charge pump. During the flash pulse the charge pump can either be enabled or disabled. The charge pump is current limited in to about 300mA, and in Torch mode, the charge pump is left enabled and the battery and charge pump can deliver a constant current less than the charge pump current limit.
The AAT1282 is a versatile 2A driver chip for high-intensity LED camera flash (from Skyworks,Inc). It is an ideal power solution for LED photo flash applications in all single cell Li-Ion powered products. An industry standard I²C serial digital input (SCL-SDA) is used to enable, disable and set the movie-mode current for each flash LED with up to 16 movie-mode settings. The AAT1282 also includes a separate flash-enable (FLEN) input to initiate both the flash operation and the default timer which can be used either to terminate a flash event at the end of a user-programmed delay or as a safety feature.
The AAT1282 maintains output current regulation by switching the internal highside and lowside switch transistors. The transistor switches are pulsewidth modulated at a fixed frequency of 2MHz. The high switching frequency allows the use of a small inductor and output capacitor, making the AAT1282 ideally suited for small batterypowered applications. A high-capacity supercapacitor on the secondary side of the stepup converter provides the highpeak flash LED current, thereby reducing the peak current demand from the Li-Ion battery.