TPS782:

Hi,

When the input voltage is less than the nominal output voltage of an LDO plus the dropout voltage (Vin < Vout + Vdo), the LDO operates in dropout. During this time, the control loop of the LDO drives the pass FET to operate in the linear region. As a result, the pass FET acts as a small resistor with a value of Rds,on. The output voltage will depend on the dropout voltage at your application's load current, which can be calculated using Ohm's Law. For the TPS782:

Rds,on =  Vdo@Iout / Iout = Vdo@150 mA / 150 mA = 250 mV / 150 mV = 1.67 Ohms (max)

Vdo@Iout(max) = Iout * 1.67 Ohms

Vout(min) = Vin - Vdo@Iout(max) = 2.5 V - Iout * 1.67 Ohms

When operating in dropout, the current is limited to the output current of your application. If the output is shorted to ground during a fault condition, the current will be limited to the specifications shown above to protect the LDO. Note that, for reliable operation, the TPS782 and other LDOs should not be operating in a current limit state for extended periods of time. 

Thanks,

Gerard