8.2.1.2.4 MOSFET Selection

The external MOSFET (Q1) selection should be based on the following criteria:

• The BV_DSS rating must be greater than the maximum system voltage (V_IN), plus ringing and transients which can occur at V_IN when the circuit is powered on or off.
• The maximum transient current rating should be based on the maximum worst case V_DS fault current level.
• MOSFETs with low threshold voltages offer the advantage that during turn on they are more likely to remain within their safe operating area (SOA) because the MOSFET reaches the ohmic region sooner for a given gate capacitance.
• The safe operating area (SOA) of the MOSFET device and the thermal properties should be considered relative to the maximum power dissipation possible during startup or shutdown.
• R_DS(ON) should be sufficiently low that the power dissipation at maximum load current ((I_L(MAX))² x R_DS(ON)) does not increase the junction temperature above the manufacturer’s recommendation.
• If the device chosen for Q1 has a maximum V_GS rating less than 16 V, an external zener diode must be added from gate to source to limit the applied gate voltage. The external zener diode forward current rating should be at least 80 mA to conduct the full gate pull-down current during fault conditions.