PowerPath Controllers & Ideal Diodes

PowerPath and ideal diode controllers provide a simple and compact solution to seamlessly switch between multiple power sources such as AC-to-DC wall adapters, USB ports, batteries and redundant supplies. While similar to a power multiplexer, a power source selector or a power distribution switch, Linear Technology’s PowerPath and ideal diode controllers protect the supply by minimizing reverse currents back into the supply and shoot-through currents between supplies. Selection of the power source can be either by highest voltage or by highest priority.

For selection by highest voltage, ideal diodes are employed in a diode-OR configuration. An ideal diode is a controlled MOSFET emulating a diode with a very low forward voltage (< 100mV), saving both power and voltage headroom over conventional power Schottky diodes. The lower drop enables cooler operation, eliminating bulky heat sinks to save board area. External or integrated P-channel MOSFETs are utilized as ideal diodes for load currents below 5A (LTC441x devices), while N-channel MOSFETs handle currents all the way up to 100A (LTC435x devices). Linear Technology’s ideal diode controller ICs regulate the MOSFET forward voltage in a linear fashion. Compared to comparator based methods, a linear servo ensures smooth, oscillation-free supply switchover and no DC reverse current. If a power source fails or is shorted, a fast <1μs turn-off minimizes reverse current transients. Controllers for low voltage applications also provide a <1μs MOSFET turn-on to limit load voltage droop (e.g., LTC4352, LTC4353). Diode ORing controllers with current balancing (LTC4370) extend supply lifetime and reliability by sharing the load current between supplies. Ideal diode applications include redundant supply ORing, supply holdup during input brownout and reverse battery protection.

For selection by highest priority, a Prioritized PowerPath controller provides the most general purpose and flexible solution. A Prioritizer, such as the LTC4417, powers the system from the highest priority input that is within its valid voltage window. Typical applications occur in battery-equipped portable products, memory modules with capacitive holdup, and server sub-systems with main and auxiliary supplies.

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