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DC12 Safe Power for Protection Class II - For home medical equipment - Available in white or black - Designed for high inrush current - Suitable for V-Lock cord retaining schurter.com/emc_news does not interfere with the audio driver and only the small ripple is present. In the previous example there were only two current loops for the application. Now, we will move on to a much more complicated example. The next system considered is a complex system, such as a tablet. In this example, the tablet will consist of items such as a backlight, touch screen, camera, charging system (USB and Wireless), Bluetooth, Wi-Fi, audio outputs (speaker, headphones), and memory for storing data. Of course, most of these applications require various voltage supply rails to operate well. As can be seen in figure 2, this system has five power supply rails and two methods of charging the battery, which means that there will be at least five current loops. But there will be more to consider than just the DC power supplies and the current paths related to each. There are multiple switching regulators and broadcasting and receiving antenna systems that all need to be coordinated and controlled using the microprocessor. In order to visualize the power and GND paths that are associated with each power supply and the block they power, it helps to tabulate each supply and load current estimate in order to consider component power ratings and tolerances; determine trace widths; determine sensitivity to voltage drops, noise injection or generation; and limit current loop areas to reduce EMI emissions. In figure 2, the main power supply rails have been color-coded, and the current flowing in the respective GND symbols has been matched to the rail that supplies the current. For example, every component not related to charging the battery (RED) has an end current return to the battery, but the USB to audio IC is powered by the 3.3V BUCK regulator, which is powered by the 5V Boost, then the battery. Therefore, the GND current returns from the audio IC to each regulator in series and then to the battery; the audio IC current does not return straight to the battery. The system shown in figure 2 runs off of a Lithium-ion battery that is charged by a USB charger or a wireless power transmitter and receiver. The battery is boosted up to +5V (for the camera zoom motor, +3.3V step-down regulator for microprocessor, audio, and touchscreen), stepped down to +1.2V (for the microprocessor, Fig. 2: Typical mobile tablet schematic blocks memory, Bluetooth, and Wi-Fi), and boosted to +7V for camera flash. Obviously, voltage regulators should be placed near their respective loads, but the form factor of the final product often forces designers to place loads far from power supplies or intermingled around the board. Each power supply supports multiple loads, so strategic routing and placement has to be used to control current paths and unintended EMI. PCB Prototypes & Small Series PCB Manufacturing from 8 hours FREE SMT stencil with EVERY Prototype order! sales@pcb-pool.com www.pcb-pool.com PCB-POOL® is a registered trademark of Beta LAYOUT GmbH www.electronics-eetimes.com Electronic Engineering Times Europe March 2015 35


EETE MAR 2015
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