P-FET based reverse polarity protection is a very commonly used scheme in industrial and automotive applications to achieve low insertion loss protection solution. A low loss reverse polarity protection solution can be realised using LM74500-Q1 with an external N-FET to replace P-FET based solution. LM74500-Q1 based reverse polarity protection solution offers better cold crank performance (low V_IN operation) and smaller solution size compared to P-FET based solution. Figure 9-3 compares the performance benefits of LM74500-Q1 +N-FET over traditional P-FET based reverse polarity protection solution. As shown in Figure 9-3, for a given power level LM74500-Q1+N-FET solution can be three times smaller than a similar power rated P-FET solution. Also as P-FET is self biased by simply pulling it's gate pin low and thus P-FET shows poorer cold crank performance (low V_IN operation) compared to LM74500-Q1. During severe cold crank where battery voltage falls below 4 V, P-FET series resistance increase drastically as shown in Figure 9-3. This leads to higher voltage drop across the P-FET. Also with higher gate to source threshold (V_T) this can sometimes lead to system reset due to turning off of the P-FET. On the other side LM74500-Q1 has excellent severe cold crank performance. LM74500-Q1 keeps external FET completely enhanced even when input voltage falls to 3.2 V during severe cold crank operation.

Figure 9-3 Performance Comparison of P-FET and LM74500-Q1 Based Reverse Polarity Protection Solution.