To ensure that the part operates down to 0.4V, EN pin should not be tied directly to Vin. The MAX17222 internal enable circuit will pull-up to the higher of the two of input and output voltage. Since MAX17222 is a boost converter, the output voltage is higher than the input voltage. Once it is powered-on and begins operating, even if the input voltage drops below the enable voltage threshold, the voltage of the enable pin can maintain a high enough voltage to keep the device operating until the battery voltage drops below 0.4V.