SNOSD17G October 2017 – December 2020 LM74700-Q1
The important MOSFET electrical parameters are the maximum continuous drain current ID, the maximum drain-to-source voltage VDS(MAX), the maximum source current through body diode and the drain-to-source On resistance RDSON.
The maximum continuous drain current, ID, rating must exceed the maximum continuous load current. The maximum drain-to-source voltage, VDS(MAX), must be high enough to withstand the highest differential voltage seen in the application. This would include any anticipated fault conditions. It is recommended to use MOSFETs with voltage rating up to 60-V maximum with the LM74700-Q1 because anode-cathode maximum voltage is 65 V. The maximum VGS LM74700-Q1 can drive is 13 V, so a MOSFET with 15-V minimum VGS should be selected. If a MOSFET with < 15-V VGS rating is selected, a zener diode can be used to clamp VGS to safe level. During startup, inrush current flows through the body diode to charge the bulk hold-up capacitors at the output. The maximum source current through the body diode must be higher than the inrush current that can be seen in the application.
To reduce the MOSFET conduction losses, lowest possible RDS(ON) is preferred, but selecting a MOSFET based on low RDS(ON) may not be beneficial always. Higher RDS(ON) will provide increased voltage information to LM74700-Q1's reverse comparator at a lower reverse current. Reverse current detection is better with increased RDS(ON). It is recommended to operate the MOSFET in regulated conduction mode during nominal load conditions and select RDS(ON) such that at nominal operating current, forward voltage drop VDS is close to 20-mV regulation point and not more than 50 mV.
As a guideline, it is suggested to choose (20 mV / ILoad(Nominal)) ≤ RDS(ON) ≤ ( 50 mV / ILoad(Nominal)).
MOSFET manufacturers usually specify RDS(ON) at 4.5-V VGS and 10-V VGS. RDS(ON) increases drastically below 4.5-V VGS and RDS(ON) is highest when VGS is close to MOSFET Vth. For stable regulation at light load conditions, it is recommended to operate the MOSFET close to 4.5-V VGS, that is, much higher than MOSFET gate threshold voltage. It is recommended to choose MOSFET gate threshold voltage Vth of 2-V to 2.5-V maximum. Choosing a lower Vth MOSFET also reduces the turn ON time.
Based on the design requirements, preferred MOSFET ratings are:
DMT6007LFG MOSFET from Diodes Inc. is selected to meet this 12-V reverse battery protection design requirements and it is rated at:
Thermal resistance of the MOSFET should be considered against the expected maximum power dissipation in the MOSFET to ensure that the junction temperature (TJ) is well controlled.