Low Side vs High Side Drivers

Background

• Low-side switch = load is between the power rail and N-channel MOSFET
• Can be used as a high-side switch using a MOSFET gate driver for high amperage switching scenarios
• Lower resistance than high-side
• High-side switch = load is between ground and P-channel MOSFET
• Generally driven by open-drain output pin due to pull-up resistor
• Logic is backwards, to turn MOSFET on you ground the gate (send 0 to pin). To turn it off, you leave the pin as floating (for open-drain pins, this is done by sending it a logic 1)
• Can use N-type transistor if the control voltage is higher than the load voltage (usually requires some circuitry to boost this voltage)

Advantages of Low-Side Switch

• Can generally drive load from a microcontroller output as long as the logic level is higher than gate voltage of MOSFET
• If a short-to-GnD fault occurs, the load is permanently on (no immediate danger)

Disadvantages of Low-Side Switch

• Not good at supplying power to other circuits because GnD ref for driver circuit is above the real GnD by the amount of the voltage drop across the MOSFET
• If a short-to-supply fault happens, load is shorted to supply (requires protection)

Advantages of High-Side Switch

• If a short-to-supply fault happens, load is permanently on (no immediate danger)
• Obtain a true ground reference
• Requires only 1 fuse for protection, low side requires 2

Disadvantages of High-Side Switch

• If Vdd is more than 0.6V higher than supply voltage for microcontroller, this can damage the microcontroller (e.g. microcontroller uses 5V supply and Vdd is 12V)
• To avoid this, use a small N-channel MOSFET, whose outputs feeds the gate of the High-Side P-channel MOSFET
• If a short-to-GnD fault occurs, the load is shorted to GnD which can be dangerous (requires protection circuitry)
• Design is more complex than low-side, usually need another transistor driving the main P-channel MOSFET

Usage Scenarios

Helpful Links

MOSFET Basics || n & p Channel MOSFETs || MOSFET Drivers