When the EN pin of the REF34xx is pulled high, the device is in active mode. The device must be in active mode for normal operation. The REF34xx can be placed in a low-power mode by pulling the enable pin, EN, low. When in shutdown mode, the output of the device becomes high impedance and the quiescent current of the device reduces to 2 µA in shutdown mode. The EN pin must not be pulled higher than VIN supply voltage. See the Section 7.5 for logic high and logic low voltage levels.