4 Using the BQ25638 and BQ2562x for Solar Battery Charging

As mentioned above, there are two few key things necessary to achieve solar battery charging, which are listed below:

1. Small step size for VINDPM setting
2. Ability to measure the charge current

The BQ25638 and BQ2562x both have a 40mV step size for VINDPM with a range from 3.800V to 16.800V. Since both parts are I2C controlled, the user can easily step the VINDPM setting for each charger while in search of the MPP.

The second requirement is being able to measure the charge current while the VINDPM is changing. Both devices have a charge current ADC that can be used for approximate values of charge current. A demonstration of the buck topology of the BQ25638 in gray with the attached solar application is shown in Figure 4-1.

Figure 4-1 Solar Buck Charger

The algorithm is as follows for each device. First, program the desired charge current. Second, sweep the VINDPM setting from the OCV to the lowest available. Record the charge current at each step. Third, return to the step that gave the most charge current. Fourth, periodically iterate this process as the load changes and environmental conditions change. For a simple demonstration of the algorithm using the BQ25638, see Figure 4-2.

Figure 4-2 Solar Charging Algorithm for BQ25638