We are using an ST1S12GR device and the input current is >2.5mA even with only the feedback loop connected and ENable pin connected to Vin. Feedback resistors values are 220k resp. 59k. Quiescent current should be max 650uA when non-switching according to datasheet, so how can we reduce current consumption?
Since you have ENable pin connected to Vin, your device is already switching even if you have not connected it to any load. Therefore input current can not meet the value specified for non switching state.
Enable pin is dedicated to switching ON and OFF in application where Vin is connected to voltage source permanently. To lower the current consumption pull the ENable pin down to the ground using pull-down resistor over time you do not need the device to be operating. The device will start operating (switching) right when you pull the ENable pin to Vin.
(extracted from ST Online Support)