Driving the Gate

ldga said:

I am wondering how the VDDA pin is setting the max gate drive voltage if VDDA is nearly at the same value as the source of M1 which is labeled as Vin.

Why do you say VDDA is nearly the same as Vin? I see VDDA is always 12v typical higher than Vin.

You didn't say but I assume your application has AC voltage at Vin. If it is a DC application then you must feed VDDA with a voltage 12v higher than Vin, directly.

ldga said:

how the proper gate drive voltage is produced for M1? And how it makes sure the voltage at the gate is higher than the source voltage to be sure that the mosfet is on?

The 4322 works as a bootstrap capacitor to generate 12v [9 to 14v spec] above Vin. Vout initially gets voltage via M1 body diode forward, and Vout supply VDDC which supplies VDDA. CvDDA is charged when Vin is lower than Vout (Vin is AC sinusoidal), and this is how VDDA voltage is higher than Vin. With VDDA higher than Vin then M1 is turned on whenever Vin is higher than Vout, and CVDDA is charged when Vin is 12v lower than Vout. 

ldga said:

Based on my understanding, in order to properly drive M1 the maximum gate drive voltage should be around 10V higher than Vin (source pin of M1) (or what the VGS min of the FET is) in order for the mosfet to be driven on.

Most NFET turned on with Vgs as little as 2v, but for higher current load Vgs should be around 10v to 18V. 

Brian