The SOA graph shows a relationship between the combined Ddrain-Ssource voltage, drain current, and the time duration that the MOSFET can withstand this dissipation. For example, the MOSFET in Figure 3 can withstand 10 V and 85 A (850 W) for 1 ms. If this condition persists for a greater duration, the MOSFET will be destroyed. The TIMER circuit can ensure that the length of time the MOSFET is subject to these worst-case conditions is limited by the external TIMER capacitor. For example, if the TIMER was set to 1 ms and the current exceeded the limit for more than 1 ms, the circuit would time out and shut down the MOSFET.
In the ADM1177, to provide a margin of safety, the TIMER current-sense voltage activation threshold is set to 92 mV. The hot-swap controller will thus start timing the current conservatively as the sense voltage approaches the regulated value of 100 mV.