simple zener, pnp and pmos cut off high side power to a sensitive load above zeners reverse voltage rating. protects a low voltage load from high supply voltages up to the pmos's voltage rating. note this is a simplified circuit, in practice if the lockout voltage threshold is higher than the pmos vgs rating, there should be another zener to protect the pmos gate from exceeding vgs, although this is not an issue if the cutoff voltage is less than vgs, the existing zener does double duty in that case. next time: sending voltages other than the zener threshold with a divider and a differential long tailed pair?
