In a boost converter the input voltage is always less than the output voltage. Please see Figure 2-2 demonstrating the boost topology in gray. A simple way to program VINDPM in a boost charger is to user a resistor divider such as the VOC_SAMP divider shown with R_OC2 and R_OC1 in Figure 2-2. First, the converter is disabled and then the VOC of the solar panel is sampled at the input voltage. A reference voltage is measured at VOC_SAMP in the VOC condition and this is the VINDPM setting until the next sample.

The BQ25504, BQ25505, and BQ25570 are 100-mA boost chargers with MPPT via VOC_SAMP pin. In this family of devices, the input voltage is sampled once every 16 seconds (typical) by disabling the converter. The converter then regulates the input voltage to the desired percentage of the OCV. The BQ25505 and BQ25570 provide options for an 80% MPPT (or K-factor) for solar devices. For both of these devices and the BQ25504, the K-factor can be set by adjusting the VOC_SAMP divider.

Figure 2-2 Solar Boost Charger