1.1 N-Channel High Side Switches

Driving high side N-channel MOSFETs from ground referenced logic requires a separate power supply that is 10 to 15V higher in potential than the high side rail. This is necessary to provide a gate bias to control the MOSFETs. Alternatively, a P-channel MOSFET can be used with a voltage level translation scheme that maintains a safe 10 to 15V differential between gate to source. As shown in Figure 1-1, an N-channel MOSFET can be driven from a floating gate drive provided by the HT0440. As shown in the circuit, the input to the HT0440 is connected to the output of a microprocessor, thereby providing a convenient and safe interface. Another advantage is the lower cost of an N-channel MOSFET compared to a P-channel device, especially if low ON-resistance is necessary. This two-chip solution is cost effective, since the component count is much lower than any other previously used method.