In the block diagram shown in figure 1
, the LT8705 monitors the input voltage of the solar panel with its onboard EA3 error amplifier to make sure it does not collapse. It also charges four lead acid batteries in series with a constant current source utilizing the EA1 error amplifier. Even though the LT8705 has four servo pins this example only shows two of the servo pins for clarity purposes. The battery charger current source is in control of the LT8705 to supply a 5A charging current as shown by the red LED indicator. However, if the solar panel voltage dips below a predetermined voltage, the LT8705 will reduce the charge current to prevent the solar panel voltage from collapsing. The other two servo pins not shown in the block diagram are the input current and output voltage pins. All these servo indicators are very useful if maximum power point tracking (MPPT) is required.
A complete LT8705 schematic is shown in figure 2
that operates from a wide 28V to 72V input voltage range which is wide enough to cover the solar panel input voltage for this application. There are 4 external MOSFETs that allow this circuit to be used as a synchronous buck/boost converter and are configured as a current source to charge 4 each 12V lead acid batteries in series.
Figure 2: Complete LT8705 solar battery charger applications schematic
Click on image to enlarge