The dropout voltage is the minimum voltage that must exist between input and output for proper functioning of the regulator.
For IC 7805, the dropout voltage is VDO=2.5V
∴Vin=5+2.5=7.5V
In car batteries the input can drop to a very low value in cold conditions; but its efficient operation is very much essential. In such cases low dropout voltage regulators are used.
The following figure shows low dropout regulator using op-amp.
The voltage at inverting terminal of op-amp is the zener voltage Vz. Due to virtual ground concept the non-inverting terminal is also appeared to be at same potential i.e. V+. Thus the voltage Vz is given as
V^+=Vz=[Vo/(R1+R2 )] R2
∴Vo=[(R1+R2)/R2 ] V^+
∴Vo=[1+R1/R2 ] V^+