If the input voltage (VIN) is larger than the output voltage (VOUT) by a diode drop (VDIODE varies from ~0.2V at light load to ~0.7V at heavy load), then the output voltage is clamped to a diode drop below the input voltage (i.e., VOUT = VIN - VDIODE).
When the input voltage is close to the output voltage target (i.e., VOUT target + VDIODE > VIN > VOUT target), the MAX17220–MAX17225 operate like a buck converter.