In automotive systems, loads like body motors, and so on can potentially deliver energy back to the input supply and can require a protection against reverse over current flow. Figure 2-2 shows the TPS12111-Q1 typical application circuit based on back to back MOSFETs (Q1 and Q2) to design a safety disconnect switch for body motor load.

The precharge resistor R₃ and MOSFET Q3 forms a path for output capacitor C_(BULK) charging. The precharge path is commonly used in the system designs with parallel connection of the main FETs Q1 and Q2 in high current designs. Q1 MOSFET serves the purpose of load disconnection during system faults such as over current, short circuit and under voltage.

In the absence of MOSFET Q2, during input reverse battery condition there will be very high reverse currents due to the closed circuit formed from PGND, the motor bridge MOSFETs and MOSFET Q1. This high current is limited by the circuit parasitics and can damage the motor bridge, MOSFET Q1 and the PCB traces. Use the MOSFET Q2 to block the reverse current during this condition as the TPS12111-Q1 pulls PD to SRC keeping the Q2 in OFF state.

Diode D1 and resistor R₄ are needed to protect the TPS12111-Q1 from reverse current injection during reverse battery condition. The diode D1 adds an offset voltage to the control input signals and also to the threshold setting for under voltage fault detection.

This offset voltage can be eliminated by replacing the D1 and R4 circuit with a disconnect switch Q4 as shown in Figure 2-3. D1 is a gate clamp Zener with a Vz below the VGS abs max rating of Q4.