Micro-Cap Tutorial: MOSFET Default Conditioning Pulldown NMOS Lowside

How to add default conditioning to your MOSFET actuation circuit! It's pretty easy...just add a pull-down resistor from gate to source on the NMOS device. This creates a path for any built-up charge to exit back to the source through a loop. Package sizes to implement this effect/function can be very small (0603, 0402, etc), so sometimes you can fit them underneath MOSFET pins with low/no impact to your board real estate (depending on what you've chosen).

Default conditioning comes at the cost of bleeding/leaking a small current when the MOSFET signal is intended to be ON. So, for applications that have relatively fixed charge capacity (i.e., battery/cell), choose values that are appropriately high (e.g., 10.0 kOhm to 100.0 kOhm). This helps reduce the waste current and find the "goldy locks" zone where default conditioning function performs satisfactorily and energy cost function is minimal.

While it's possible to use very large resistances in the megaohm scale to squash the waste current, the pull-down resistance would start to look like the resistance of the MOSFET gate-source junction...the built up charge would then be slower to deplete through the paths available; that increases voltage potential and promotes accidental turn ON.

Remember, the point of the default conditioning function is to "waste" a small amount of current so voltage potential doesn't uncontrollably build up enough to become an issue. Increasing resistance too far will actually be an antifunction of default conditioning functionality. Also, megaohm scale resistors tend to cost more and have greater tolerances (making actual values unpredictable).

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