At first glance, P-channel MOSFETs appear to be the natural choice for high side switching. Unfortunately, theRDS(ON) exhibited by most P-channels is prohibitively high. (Mother Nature has decreed that electron mobilityshall 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.
N-channel MOSFETs may seem less attractive because they require a gate voltage higher than the power supply voltage to become fully enhanced in high side switching applications. This limitation is eliminated by high side MOSFET drivers such as the LTC1155, which have builtin charge pumps to fully enhance N-channel switches.
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