Using Low Resistance MOSFETs as Load Switches

To conserve battery power and increase operation time in portable equipment, MOSFET switches are commonly used as 'load switches' or 'power saving switches' to disable sections of a system when they are not required (Figure 1). MOSFETs are normally used due to their low resistance and ease of operation. This function is often overlooked by the designer and its implementation is frequently left until very late in the design stage, when PCB space is at a premium. The MOSFET provides a low resistance path between the battery and the load. When fully turned on, the MOSFET appears and acts like a resistor and therefore Ohms law applies; when turned off, the MOSFET appears as a diode and as such blocks current in one direction with a very high resistance (if reverse protection is required, an additional MOSFET or blocking diode is required). P-channel MOSFETs are normally used as they are very simple to turn on or off (Figure 2).