Micropower High Side MOSFET Drivers

　　便攜式電子設備，如筆記本電腦和掌上電腦、便攜式醫(yī)療儀器電池供電工具，越來越依賴高效電源管理迎接挑戰(zhàn)從更少的電池中提取更多有用的能量（時間）體積重量。

　　One link in the power management chain which has received increasing scrutiny， as power supply efficiencies soar above 90%， is the logic controlled power switch. Large sections of a typical notebook computer system， for example， are powered via topside or “high side” MOSFET switches. These switches can become significant sources of power loss if not properly designed.

　　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

　　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.

　　Figure 1. High Efficiency Dual High Side Switch

　　The LTC1155， dual micropower MOSFET driver， generates 12V from a 5V rail to fully enhance logic-level Nchannel switches with no external components required （see Figure 1）。 Further， the supply current is typically 85μA with the switch fully enhanced and 8μA with the LTC1155 in the standby mode （both inputs off）。 This combination of low-drop N-channel MOSFET switch and micropower driver is the most efficient means of powering complex electrical loads. Switch efficiencies in the 98% to 99%+ range are easily attained with practical and economic N-channel switches.