We use active loading to measure LDO load regulation performace.
The active loading will have the fixed current sinking and is independent of Vout.
mV/mA : (Vout measure at I2 - Vout measure at I1 ) / (I2-I1)
%/mA : (Vout measure at I1 / Vout no loading - 1 ) * 100% / (I1)
ppm/mA : (Vout measure at I1 / Vout no loading - 1 ) * 1e6 ppm / (I1)
The selection of I2,I1 loading is to keep LDO in linear region to have the best result.
Since in that region the small signal transfer function work properly.
LDO line regulation is based on : ( LDO PSRR+ ) and ( Referece Voltage PSRR+ * LDO feedback factor )