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CLOUDSPLITTER Connector System • Single cable solution for devices requiring data and power. • Combines Cat5e performance and 250 W of power. Available now - search for CLOUDSPLITTER at www.rs-components.com halving their thermal stress and exponentially raising power system reliability. However, many load sharing schemes that adjust the supply are beset with oscillating loops. The complications arise from the load sharing control loop interacting with the supply dynamics. The ideal diode concept provides the solution here. By adjusting the ideal diode voltage drop to compensate for the supply voltage difference, the output voltage of the two ideal diodes is equalized. Using sense resistors from these two equal points to the common load ensures an equal or ratiometric flow of current from the two supplies. The LTC4370 diode-OR current balancing controller implements this method of current sharing between two supplies - see figure 3. It compensates for supply voltage differences up to 600mV, translating to a ±2.5% tolerance on two 12V supplies or ±6% on two 5V supplies. Non-identical supplies The diode-OR and load sharing methods are well-suited when the two supplies are identical as in the server example. It does not apply to battery-powered systems, where the input comes either from the battery, a wall adapter, or a 5V USB source, i.e., sources with very different nominal voltages. In some instances super-capacitor backups are also involved. A more general-purpose solution is needed instead of one that relies on the simple metric of supply voltage. This solution is called a prioritizer. It is based on the reality that a battery-powered system has its supplies ranked by preference. Typically, the wall adapter is at the top of this list, i.e., the system draws power from the wall adapter whenever it is present. Each supply needs to have a defined valid voltage range (to detect presence) and a priority. If a supply is present, it is considered for powering the system based on its priority. The LTC4417 prioritizer connects only one of three input supplies to the output based on their valid voltage windows and priorities – see figure 4. Careful switching never connects two supplies together, connecting a supply to the output only when the output is below the input voltage. This minimizes or eliminates any reverse current flow back in to the supply. Also, it implements controlled fast switching to limit output droop and inrush current. Conclusion Depending on the kind of supplies housed in the system, the appropriate solution for supply multiplexing requires selection Fig. 3: The LTC4370 balancing a 10A load current between two diode-ORed 12V supplies. Sharing is achieved by modulating the MOSFET voltage drops to offset the mismatch in the supply voltages. first. The choices are a diode-OR (with or without load sharing) and a prioritizer. Irrespective of the method, selecting the correct supply to power the load needs careful design to not bring down the entire system. Reverse currents back in to the supply and the output voltage droop need to be minimized without triggering back-andforth oscillatory switching between supplies. The solutions described here take care of these pitfalls in a simple and elegant manner. Fig. 4: LTC4417 three-supply prioritizer. www.electronics-eetimes.com Electronic Engineering Times Europe January 2014 25


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