MPQ4470/4470A Schematic Confusion

That's a boot-strap capacitor. That capacitor is needed in order to drive the gate of the high-side MOSFET (i.e. HS-FET) to the proper voltage (Vgs>Vth) when the FET turns on. The boot-strap circuit maintains Vgs>Vth when the HS-FET turns on and it's source is pulled to the drain voltage.


When HS-FET turns on, the source of the HS-FET is pulled near the drain voltage. In order to turn and keep the FET on, the gate voltage always needs to be higher than the source voltage (Vgs > Vth). In order to do that, a boot-strap circuit is used which is essentially a capacitor charge pump feed into a regulator (BSTREG). The boot-strap capacitor is usually charged to the input voltage (V_IN) through a diode when the low side FET (i.e. LS-FET) is turn on. The voltage of the boot-strap regulator keeps the gate of the HS-FET higher than the source voltage, most importantly, when the FET turns on and the source is pulled near the drain voltage.

Have a look through the application notes. They will usually have some nice pictures of the process. Also, look at how a charge pump circuit works. Once you know the charge pump, the boot-strap circuit is pretty easy to understand.