If Vin is higher than Vout, the output voltage will follow the input voltage minus a diode drop. The diode is internal to the MAX17222. For example, for a V_OUT of 4.2V and a V_IN normally at 3V, connected to a 20mA load, if at any moment (e.g. 30secs) V_IN goes to 5V ; , the output voltage will be (5V-VDiode) which will be around 4.3V.