Intelligent DC-DC Converter Drives High-Power White LED Flash

Mobile phones, PDAs, and portable devices in general must meet increasing functional requirements within the constraints of small size. Cameras that integrate images and videos are becoming the most popular functional requirements for portable products. Supporting the flash function of the camera is the key to determining performance and power consumption. This article will explore how to improve the flash function and show a complete design solution to illustrate how to drive high-power white light LEDs.

Flash The integration of functions in portable devices, such as combining the functions of PDAs and mobile phones, is the latest trend. Until now, integration has been difficult to achieve without adding new functions to mobile phones and some "prehistoric" terminal devices that were originally only used for receiving and speaking. This includes MP3/MP4 players; wired (USB and USB-OTG); wireless connections such as infrared (IrDA), Bluetooth, Wi-Fi, GPS; and the recent popularity of digital video broadcasting, which are the main and attractive features built into the latest mobile phones. However, almost every latest mobile phone has built-in most common functions, such as high-quality camera modules for still images or video phone calls. In the past, camera modules were often regarded as simple toys because of their very low image resolution, but now the precision of camera modules has been greatly improved, and the image resolution is close to that of the most advanced digital cameras. Since camera modules require a light source that can operate in low-light environments, and the latest high-resolution cameras require higher lumens and brightness, built-in flash functionality provides an important function and promotes the application of overall camera modules. Early flash built into mobile phones only provided entertainment flash functions because they provided almost the same shooting results as without flash. In order to support the latest high-resolution cameras and make the final product attractive and meet the needs of end users, its lighting function must achieve better and decisive photographic performance. This article will then explore two different methods of implementing flash.

Implementing the Flash Function There are currently two options for flash lighting: (1) flashlights; (2) LEDs. (Figure 1) is a model of a flashlight. The bulb consists of a glass shell filled with xenon gas. When the electrodes are triggered, both the positive and negative electrodes are filled with gas instead of being connected to the surface of the bulb. When the electrical impedance of the xenon gas drops to a very low value, visible light generated by a large amount of current flows from the positive electrode to the negative electrode. This function can be achieved by triggering the electrode. This function provides a high voltage peak of up to kilovolts, which ionizes the xenon gas and puts it into a low impedance state.

This flash has the outstanding characteristics required for high-quality image performance. The light output is very dense and can be easily diffused over a wide area. In addition, since the color temperature of the flash is about 5500-6000°K, which is very close to the temperature of natural light, no color correction is required.

On the other hand, since the output light involves very high energy levels, usually hundreds of volts at the positive terminal, it takes some time to raise the battery voltage to meet the power supply requirements of the mobile phone.

The standard charging time required for two consecutive flashes in the range of 1 to 5 seconds depends on the input power, capacitor value, charging circuit characteristics and required energy. The illumination of the flash may be just a pulse that makes the flash a good solution for still image flash, but not suitable for video applications.

In addition, the xenon flash tube and its additional driving circuit are quite space-consuming for mobile phones with space constraints, and the higher and more dangerous voltage required to trigger the xenon to provide enough energy to output light requires a precise but expensive design, which limits the application of flash in mobile phones.

For these reasons, mobile phone manufacturers are turning to white LEDs as flash sources.

LEDs provide continuous light, making them ideal for video phone flashes in low-light conditions. In addition, the driver circuits required for LEDs are relatively simple and much cheaper than flash bulbs.

Entry-level camera phones typically have resolutions in the sub-mega pixel range, and these phones typically have standard white LEDs with relatively low illumination capabilities built in. Because of this, multiple standard white LEDs, either in series or in parallel, must provide a minimum amount of light to capture an image, otherwise the captured image will be almost invisible and unusable. Standard white LEDs

have a standard forward voltage of 3.2V to 4.5V, and the single power source that drives them typically ranges from 4.2V when fully charged to a lithium-ion battery that drops to 2.8V to 2.7V when discharged.

Standard white LEDs are typically driven using a step-up (boost) DC-DC converter or a charge pump. Both solutions require controlling the forward current through the LED, which is proportional to the illumination intensity. This article will then explore both approaches simultaneously.