9.2.4 Reverse Current Protection Using MOSFET and TLV1805

An N-Channel or P-Channel MOSFET may be used to protect against reverse current. Reverse current is defined as current flowing from the load (V_LOAD) to the source (V_BATT). Both the P-Channel and N-Channel circuits work on the same basic principle, where a comparator monitors the voltage across the MOSFET's Source and Drain terminals (monitoring V_DS). The described circuits also protect against reverse voltage.

Figure 68. Simplified Operational Theory

When the current is flowing from the battery (V_BATT) to the load (V_LOAD), the battery voltage will be higher than the load voltage due to voltage drop across the MOSFET caused by the R_DS(ON) or the intrinsic body diode forward voltage drop. The comparator will detect this and turn "on" the MOSFET so that the load current is now flowing through the low loss R_DS(ON) path.

In a reverse current condition, V_LOAD will be higher than V_BATT. The comparator will detect this and drive the gate to set V_GS = 0 to turn "off" the MOSFET (non-conducting). The body diode is reverse biased and will block current flow.

For a P-Channel MOSFET, the gate must be driven at least 4V or more below the battery voltage to turn "on" the MOSFET.

For a N-Channel MOSFET, the gate must be driven 4V or more above the battery voltage to turn "on" the MOSFET. If a higher voltage is not available in the system, a charge pump is usually required to generate a voltage higher than the battery voltage to provide the necessary positive gate drive voltage.