Off-state FETs act like diodes by blocking current in one direction (first quadrant) and allowing current in the other direction (third quadrant) with a corresponding diode like voltage drop. FETs, though, can also conduct third-quadrant current in the on-state at a significantly lower voltage drop. Ideal-diode mode (IDM) is when an FET is controlled to block first-quadrant current by going to the off-state and conduct third-quadrant current by going to the on-state, thus achieving an ideal lower voltage drop.

FET off-state third-quadrant current flow is commonly seen in power converters, both in normal and fault situations. As explained in GaN FET Operation Definitions, GaN FETs do not have an intrinsic p-n junction body diode to conduct off-state third-quadrant current. Instead, the off-state third-quadrant voltage drop for the LMG3522R050 is several times higher than a p-n junction voltage drop, which can impact efficiency in normal operation and device ruggedness in fault conditions.

To improve device ruggedness in a GaN FET overtemperature fault situation, LMG3522R050 devices implement a GaN FET overtemperature-shutdown ideal-diode mode (OTSD-IDM) function as referenced in Overtemperature Shutdown. The OTSD-IDM function is described in more detail in the following section.