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POWER MANAGEMENT Some important placement considerations include available space, mechanical constraints, acceptable voltage drops along the power and GND rails (the product of load current and number of squares in traces/planes), power supply and GND currents paths, cost (PCB layer count, components), the frequency of digital or analogue signals, and the availability of a direct return path from the source. For the last example, we present a hypothetical final system with mechanical constraints. In such a system, the user interfaces and overall dimensions will control the design. Figure 3 shows a practical placement of each block. In figure 3, each power supply has been color-coded for clarity. The most important part of the image is the colouring of the GND return currents. Because multiple power supplies are in series leading to each final load, the GND currents are forced to complete the return paths in the same order they were supplied. For example, the battery powers the BUCK1.2V regulator, Fig. 3: Typical mobile tablet application blocks and placement which powers the microprocessor; therefore, the current powering the microprocessor will return directly to the BUCK1.2V regulator GND prior to returning to the battery. The failure to envision the entire current loop and the order in which the paths are completed can often create unstable operation or inadequate GND current returns because they are not properly accounted for and controlled in the layout. For example, it is easy to imagine a systems engineer placing the Bluetooth and WiFi antennas in the place of the camera and flash. The problem that would result from the reversal of the camera with the WiFi/Bluetooth blocks is that even though the +1.2V power supply would still properly split to provide power to the blocks as needed, the GND return currents of the high frequency WiFi/Bluetooth would now flow directly through and under the microprocessor/memory blocks, thereby injecting the ripple currents and voltage bounces associated with the antennas directly into the high frequency microprocessor GND and memory transactions. This will result in errors with analogto digital conversions of battery temperature, could corrupt the stereo quality to the speaker, impact the camera resolution, and cause memory errors that could lead to lost data. By comparison, as drawn, the WiFi/Bluetooth power and GND currents would remain separate and in parallel from the BUCK1.2V regulator to each independent load and back to the source (BUCK1.2V in this case), avoiding all of these issues. Note that each of the above examples assumes a single GND, and they are drawn as a copper plane that is continuous and uninterrupted on one of the PCB layers. This GND plane is shared by all blocks of the circuit instead of partitioning the GND plane or separating it into sub-sections and using components to combine GND planes and control current paths. Intentional placement of blocks has been implemented because this method uses natural current flow to shield circuits from undesired GND bounce. Any trace that carries currents or voltages (positive potential) must have a return path. The return path will flow as close as possible to the positive potential form of the signal, and it will be distributed on the GND plane under the sourcing signal/power rail. Understanding current flow and the concept of minimizing current loops leads to the obvious conclusion that the single GND method is ideal and preferred as a PCB design approach because it significantly reduces component count, layer count, and potential radiation. Every trace and block will be provided the shortest return path possible on the PCB. By following this guidance, the system designer will only have to control the PCB design from the perspective of proper trace widths as well as smart placement of components and blocks. He or she should not have to check every trace or build multiple experimental boards to obtain the correct power, signal, and GND scheme. An additional advantage offered by single, uninterrupted GND plane is that the continuity of the plane allows heat developed to spread evenly across the entire PCB surface, resulting in lower operating temperatures. Any signal (or power supply) used to drive any circuit must be given a proper path to return to its source. C circuit designers must consider the source and grounding schemes to properly implement a final system solution. Consideration of the load and the type of load is crucial during the implementation phase to keep current paths that cause voltage bounce controlled. Placing and locating those current paths in areas of the PCB that can afford GND noise without impacting performance is key to effective and efficient design. Regulator has output tracking and sequencing for FPGAs and MPUs Designed to provide a compact solution size and high frequency switching for point of load conversions, Intersil’s ISL8002B is a synchronous buck (step-down) switching regulator that delivers up to 2A of continuous output current from a 2.7V to 5.5V input supply. The device’s 2 MHz switching frequency optimises transient response, and its key features - programmable soft-start, and output tracking and sequencing of FPGAs and microprocessors - increase system reliability for point-of load conversions. The ISL8002B enables greater system reliability through features such as; the regulator’s output tracking and sequencing of FPGAs and MPUs ensures sensitive multi-rails properly start up and shut down. Its output rails are configurable for coincidental, ratiometric, or sequential settings, ensuring the FPGA or MPU’s internal ESD diodes are not biased or overstressed during rising or falling outputs. The ISL8002B’s undervoltage lockout and several other protection/stability features (overvoltage, overcurrent, undercurrent, negative current, over temperature and short-circuit) safeguard the system from damage when an unwanted electrical fault event occurs. Intersil www.intersil.com 36 Electronic Engineering Times Europe March 2015 www.electronics-eetimes.com


EETE MAR 2015
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