In a buck converter the input voltage is always greater than the output voltage. Please see Figure 2-1 demonstrating the buck topology in grey. A simple way to program VINDPM in a buck charger is to use a resistor divider such as R3 and R4 in Figure 2-1. A reference voltage is targeted at MPPSET. The goal is to program the resistor divider such that the MPPSET reference voltage is met when the input is at the VINDPM threshold.

The BQ24650 is a 26-V 10-A buck charge controller with MPPT through the MPPSET pin. The target reference voltage for MPPSET is 1.2 V. The charger will pull only the current from the solar panel that keeps the input voltage set at the desired MPPT voltage and the MPPSET voltage at 1.2 V. The BQ24650 is standalone, but comes with two STATx pins to detail the status of the device.

Figure 2-1 Solar Buck Charger