MPQ4470/4470A Schematic Confusion

[newbie_hs]

I am using MPQ4470/4470A in my design.In the typical application schematic you can see a capacitor C3 of 0.1uF
The datasheet does not say anything about this capacitor.
May I know the role of this capacitor

 

[KlausST]

Hi,

the capacitor ist connected to the "BST" pin.

So simply do a search on "BST" and you will find all information you need to know.

You quickly will find the pin description. Where it says "Bootstrap".
So you are free to do another search on "bootstrap".

Klaus

 

[BlackHelicopter]

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.