Buck controller MOSFET drive voltage
From the datasheet (http://www.ti.com/lit/ds/symlink/lm3405.pdf) pg 7:

"When the LM3405 starts up, internal circuitry from VIN supplies a 20mA current to the BOOST pin, flowing out of
the BOOST pin into C3. This current charges C3 to a voltage sufficient to turn the switch on."