When building an application with a microcontroller, you’ll want to control something at some point. This may be a device requiring minimal current, such as an LED, or something demanding a little more power, such as a DC motor. Most beginners quickly learn that devices, such as an Arduino or Raspberry Pi, cannot drive heavy loads directly. In such cases, a ‘driver’ is needed, a circuit that can accept the control signal from the microcontroller but has enough power to drive the desired load. MOSFETs are perfect in many cases, accepting a simple voltage at their input (gate) that allows a larger current to be controlled via their drain-source pins. However, there are times when the MOSFET itself also needs a driver. Let’s quickly review the role of MOSFETs as saturated switches before exploring how MOSFET drivers work.
Low-Side n-Channel MOSFETs for Switching
MOSFETs, specifically enhancement-mode MOSFETs, come in two types: n-channel and p-channel. N-channel MOSFETs require a higher voltage on their gate than the voltage on the source to turn on. The voltage at which this occurs is the threshold voltage, Vth. Pull out any n-channel MOSFET datasheet, and you’ll quickly find this value. For example, the Toshiba SSM3K56FS, a small high-speed switching device, gives Vth as between 0.4 V and 1.0 V when the drain-source voltage (VDS) is 3.0 V and for a drain current (ID) of 1 mA.