TPS799: Drop out voltage specification query

Hi Swaminath,
That's right. The device is not regulating... because it is in Dropout. You cannot measure the real Dropout parameter when the device is in regulation. The test condition "Vin=(Vout(nom) - 0.1V)" forces the device to enter Dropout by dropping Vin below the set Vout voltage. The idea is for the PASS FET inside the device to enter the ohmic/linear region (transistor characteristics curve) by decreasing the delta between Vin and Vout (Vds of the PASS FET) and that is achieved that by lowering the Vin.
The above method is commonly used for Fixed-output LDOs/Regulators because the testing is faster and gives similar result compared to the slower traditional way that I will describe next.
The Dropout test methodology that most users know about is this: put the device into regulation, then start ramping down Vin until Vout begins to drop by some % of the set Vout.i.e. Device cannot regulate anymore, thus in Dropout. The same idea as the first method (only slower); that is device is brought to Dropout (not regulating) before Dropout voltage is measured.
I think there is no need to discuss the other method , I hope you get the idea.

Regards,
Chito