The VDS rating of the MOSFET Q1 must be minimum V_IN(max) for designs with output overvoltage cutoff where output can reach 0 V with higher loads. For designs with output overvoltage clamp, MOSFET VDS rating must be (V_IN(max) – V_{OUT_CLAMP}). The VGS rating is based on GATE-SRC maximum voltage of 15 V. TI recommends a 20-V VGS rated MOSFET. Power dissipation on MOSFET Q1 on a design where output is clamped is critical and SOA characteristics of the MOSFET must be considered with sufficient design margin for reliable operation. An additional zener diode from GATE to SRC can be needed to protect the external FET in case output is expected to drop to the level where it can exceed external FET V_GS(max) rating.

Figure 9-9 200-V Surge Stopper with Overvoltage Cutoff Using LM74502