What's the use of the LODRV pin in the 2BQ24650 mppt batery charger circuit
That's for the flyback currents.  In simple converters, this would be a diode.  However with a little more complexity inside the chip, they are able to use a FET instead, which reduces losses and improves efficiency.
The name of the topology is &quot;synchronous converter&quot;.  There's lots of info about them on the internet.
Here is an example of both topologies side by side:

Image from Maxim