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LTC4418 - Dual Channel Prioritized PowerPath Controller (ciiva.com)

Enables dual powering of loads, where if the primary supply is no longer available it will switch automatically to a secondary supply.

Inrush

LTC4418 minimized startup inrush by soft starting the gate drive of the first input to connect the output. However, during supply switchover excessive inrush due to the voltage difference between the 2 supplies can lead to undesirable UV fault.

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Given the large amount of inductance that the long power cables would have, it is conceivable that during input switching, there can be large inductive voltage spikes on the power rail which might exceed 20V. Thus, the Vds of the Pmos was chosen to be at least 30V, and the lowest cost option with a Rdson of < 10mV was chosen.

Load Switch

Current Sensing

The IS pin outputs a current proportional to the load current during nominal operation. Sensing voltage to load current calculation can be found in output table above.

Clamping Diodes

During a fault or open load scenario, the ISense voltage can exceed the 4V maximum rating on the MCU ADC pins. No discrete clamp diode is added since the mux clamps inputs to Vcc + 0.5V, where Vcc is 3.3V.

Layout

Given the max steady state current is anticipated to be 3.5A, minimum conductor width will be sized for 4A with a temp rise of 25C. Conductors will be enlarged beyond the minimum where possible to reduce power loss.

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Outer Layer 1oz: 3mm

0.3mm/0.5mm via: 1.1A

Thermal Reliefs

Where pads connect directly to very large planes (typically ground) a thermal relief is added. Due to the high density of parts many smaller polygons do not feature thermal reliefs to pads, since they would take up a significant portion of the polygon area.