In many mobile devices, the power supply has to be immediately switched from the battery(ies) to a DC supply. Traditionally, this is done with diodes in an ORing configuration. It is desired that only current be sunk from a battery or supply, and that batteries don’t try to do things like charge an ACDC supply input. Sometimes this is referred to as the PowerPath in datasheets or product selection material.
However, in a mobile device, efficiency can be king. We don’t want to dissipate extra heat in the device from the diode at high currents, and we don’t want to reduce battery life in the form of just heat. This is where the ideal diode comes in. Linear Technology has a series of controllers that use an external mosfet’s body diode to passively steer current in from a battery or DC supply into a power rail. It then controls the gate voltage via an internal charge pump and controller such that the current flows from source to drain in an n-channel mosfet. It attempts to keep the voltage across the diode at 25mV (compared to about a volt typically). If the target rail that it is trying to supply suddenly becomes more positive than the source on the other side of the FET, it will kill the gate-source voltage of the FET, allowing the body diode to block the current flow. Magic!
If you’re pumping 1 or 2 amps through an ORing diode, this can save you 0.7 to 1.4W in power dissipation. This is huge.
This is a critical element when it makes sense. Often, because of the expense of this circuit ($2-3 in qty 100) outweighs its use. However, in more expensive designs, this can really keep things cool.