This circuit provides undervoltage and overvoltage protection for dc input supply voltages (see the figure). Its undervoltage cutoff is 6 V, and its overvoltage cutoff is 11.5 V.
When VIN is less than about 6 V, Q1 and Q2 are OFF; Q3 (a series MOSFET switch) is also OFF and VOUT = 0. When VIN goes over about 6 V, Q1 turns ON, turning ON the series MOSFET. Then, VOUT = VIN. Q2 is still OFF. Up to approximately 11 V, Q2 remains OFF, but its Ve(Q2) = VOUT R11/(R11 + R14). Thus, for VOUT = 11 V, Ve(Q2) = 1 V.
For Q2 to turn ON, Vb(Q2) has to exceed Ve(Q2) + 0.7 V. This occurs when VIN exceeds 11.5 V. Q2 then turns ON and shunts the base current of Q1. This puts Q1 into cutoff and turns Q3 OFF. During this transition, Vout drops and in turn reduces Ve(Q2) . This bootstrap action helps to turn Q2 ON faster (and turn Q1 and Q3 OFF faster). Once VOUT = 0, Ve(Q2) drops to a lower limit. Therefore, VIN has to be reduced to a smaller value (10.5 V) to turn Q3 ON again. As a result, the designer can adjust the hysteresis by adjusting the values of R11 and R14.
D3 is included for gate-to-source protection of the MOSFET. The Zener voltage can be adjusted to vary the under-and over-threshold voltages. The transistors could also be 2N3904s or equivalent.