How to deal with the latest lighting requirements in portable system design

As designers began to use higher-performance color displays and began embedding value-added features like cameras in portable systems, the power requirements were significantly improved. Today, typical cellular handsets or handheld devices use a high resolution, primary display with four or more LEDs that produce backlights. Many of them also use a small display that is used outside the flip phone to provide additional information. Usually these sub-panels are backlit with two other LEDs. Power circuits are also often required to drive auxiliary RGB status lights and provide backlighting for the device's keyboard.

At the same time, the integrated camera function brings a whole new set of requirements for the power supply design of portable systems. Embedded cameras require a flash that typically requires one or several LEDs to be driven at high power for a short duration. In the early camera implementation, a CCD with less than 1M pixel was used, and the required flash drive current was less than 100mA. But as portable system designers began to adopt high-resolution CCDs, the need for power for flashing has also increased. Nowadays, for many portable devices, only the flash function of the camera and the highest resolution of the photo and the red-eye correction function are required, and the current of up to 600 mA is required, which is even higher for the xenon flash lamp (see Fig. A typical of the AAT3171). application). In addition, adding other new flash functions or movie modes will give designers more complex power management requirements and system design.

Independent implementation

Typically, portable system designers primarily use separate implementations to manage the power of the lighting device. For backlight applications, designers typically use a separate charge pump to drive the parallel LEDs, and a boost converter to drive the connected LEDs and provide basic illumination. Color displays have increased the need for power, but in early designs, designers were rarely able to flexibly adjust the intensity of the illumination. Currently, color balances such as white balance to control illumination intensity and different colors that produce status lights are becoming more and more important, requiring system controllers or transaction management microcontrollers to provide more sophisticated control. In the early days, most designers used analog PWM control signals with relatively large noise to control LED brightness and backlighting.

Initially, the flash LED function was implemented as a separate part, providing only simple switching control. Later, the designer added some basic algorithms to the system processor to control the flash subsystem. Since few systems provide flash intensity control, designers typically control the amount of light reaching the CCD by managing the camera shutter. But with the improved performance of embedded cameras, designers have to upgrade the performance of the flash power management system. These improved performance requirements require a fixed photographic shutter and better control of flash intensity. To reduce the overhead of the system controller, designers began to adopt smarter flash LED controllers. But with increasing market pressures to reduce system size and cost, and the continued increase in the number, variety, and complexity of lighting functions, portable system designers are looking for newer alternatives to these separate lighting subsystems.