In a buck-boost converter the input voltage can be greater than, less than, or equal to the output voltage. Please see Figure 2-3 demonstrating the buck-boost topology in grey. A host-controlled charger presents more options to program MPPT. A robust way to implement MPPT is to program the desired K-factor via I²C as shown by SDA and SCL pins in Figure 2-3. The charger will periodically disable charging and measure the input voltage, also known as the OCV of the solar panel. Next, the charger multiplies the OCV by the K-factor and will hold the input to this value as VINDPM if the panel is overloaded.

The BQ25798 is a 18.8-V 5-A I²C buck-boost charger with MPPT. The BQ25798 is well suited for environments that change temperature because as the panel cools or heats the BQ25798 will change the input voltage regulation accordingly without having to set a fixed OCV. The I²C capability also gives flexibility to change the K-factor on the fly.

Figure 2-3 Solar Buck-Boost Charger