There are various design considerations for powering digital image sensors in automotive camera applications. Image quality is the key money-making specification for this application. Spot-on image quality in different environmental conditions such as rain, snow, fog, and other difficult driving conditions is essential. When available, automobile drivers rely on this technology much more than their usual visual checks when backing up. A solution that gives good performance in good weather conditions may not be acceptable in bad weather conditions if key design considerations like the low noise requirement of analog power rails, efficiency, and size are not optimized.
Advanced driver assistance system
For example, a popular 1.2 megapixel sensor for automotive advanced driver assistance systems (ADAS) and surround view/parking assist camera applications with a high dynamic range and 720p60 capability is the Aptina AR0132. It requires 1.8V for the core (VDD) and I/O section (VDD_IO), and 2.8V for the analog section (VAA, VAA_PIX, and VDD_PLL). It is intuitive that the chip’s analog section is more susceptible to noise on its supply line. Many designs utilize low-dropout regulators (LDOs) for all power rails down-converted from a 5V input rail.
The overall objective is to provide a practical power system implementation for the AR0132AT image sensor that allows successful integration into an automotive application and form factor. And while the details of other sensors and systems may differ, the principles discussed here will be generally applicable.
Size is important because it must fit into a 25mm cube, which automatically places a heavy emphasis on overall efficiency and good thermal management. Although LDOs have low noise output, they are not ideal to work in a 25mm cube due to high power losses and resultant unwanted heating of the image sensor.
A switching power supply solution has much lower power dissipation, but intrinsically higher output ripple and noise. Image sensor noise (ripple) sensitivity on its analog input power supply rails requires additional care when implementing a switching power supply solution.