The function of the BQ24650 MPPSET pin is to hold the input voltage (known as the Maximum Power Point Voltage) at a set point in the event that the input power drops. It is set as a voltage divider from the input to the MPPSET pin. The desired Maximum Power Point Voltage is configured by setting the resistor divider such that the MPPSET pin is 1.2 V when the input voltage is at the Maximum Power Point Voltage. The charger will maintain the input voltage at the Maximum Power Point Voltage, reducing charge current as necessary. When this happens the STAT1 pin is LOW and the STAT2 pin is HIGH. Please note that this means if you attach an input lower than the Maximum Power Point Voltage value, the converter will not run. Please see the link to a video on Input Voltage Dynamic Power Management to better understand the MPPSET feature and how it interacts with the input power.