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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.
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.
Inner Layer 0.5oz: 6mm
Outer Layer 1oz: 3mm
0.3mm/0.5mm via: 1.1A