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.