how to understand dropout voltage in LDO datasheet?

Question

Hello, 
 Recently I have a problem about dropout voltage. I need ADP1741 to output 1.8V,the dropout voltage from ADP1741 datasheet is 280mV max,Iout=2A. Does it mean my Vin mustnot exeed 2.08V？so I cannot understand real mean about "dropout voltage".How to meet the dropout voltage in my design?Vin-Vout greater than dropout voltage or Vin-Vout less than dropout voltage,which one can make the chip ADP1741 work normally? 
 thank you very much

ADIApproved · Accepted Answer

Hi Junge, In the ADP1741 data sheet, dropout voltage is defined as " the input to output voltage differential when the input voltage is set to the nominal output voltage. This applies only to output voltages above 1.6 V." If your input voltage is greater than the nominal output voltage, then the part is not in dropout mode. To meet the specifications given on table 1 of the datasheet, a minimum headroom of 400mV must be kept between VIN to VOUT unless otherwise specified. 
 Thanks, Alex)