The device is marketed as "2.5V to 36V, 2.5MHz, PWM Boost Controller" but it does not turn on with any voltage below 4.2V
The MAX17290/MAX17292 high-efficiency, synchronous step-up DC-DC controllers operate over a 4.5V to 36V input voltage range with 42V input transient protection. The input operating range can be extended to as low as 2.5V in Bootstrapped mode.
Bootstrapped mode will not turn on the device at 2.5V however, once the device is ON and the input supply goes below 4.5V then the device will remain ON and UVLO will not be triggered even if the the voltage goes down to 2.5-3V. In bootstrap mode the device is supplied by the output of the boost regulator itself by connecting the output to the SUP pin of the device.