App note: Using High-Side NMOS drivers

Here is an app note describing how and why to use NMOS transistors for high-side switching operations, and how to implement it using associated drivers.

At first glance, P-channel MOSFETs appear to be the natural choice for high side switching. Unfortunately, the RDS(ON) exhibited by most P-channels is prohibitively high.(Mother Nature has decreed that electron mobility shall exceed hole mobility in silicon by about 2.5 times, so that a P-channel MOSFET with the same RDS(ON) and voltage rating as its N-channel counterpart is roughly 2 to 3 times larger and more expensive.) Also, the gate drive for a P-channel switch is limited to the supply voltage which may not fully enhance the switch as the supply voltage drops.