whats the purpose of that CBST2 capacitor?

[clar123]

what i suppose Is this: the PIN BST attached to the inductor keep switching to a value higher and lower than VDH, when he goes lower than VDH then the diode Is off, and nothing Is supplying energy to VDH pin, thats why that capacitor Is there, It acts a reservoir, i came to this explaination by imaging the circuit without that cap.

 

Is this assumption right?

 

Thanks all for the help 

 

[Peter BENSCH]

[Oops, not the answer to your question - the right one’s below]
That’s almost correct: the capacitor is what is known as a boost capacitor, which is charged when the corresponding phase (SHS1…3) is switched on and off, and then ‘boosts’ this voltage above that of the corresponding high-side driver. This allows the gate of the HS MOSFET to be driven with sufficient voltage, as its source is effectively at VDH potential.
You must bear in mind that the charge must be sufficient for one switching cycle - so at low switching frequencies, the boost capacitor must be proportionally larger.

[Right answer]
Sorry, I’d only glanced at your question and thought you were referring to the other boost capacitors.
Yes, your assumption is correct. As you’ve pointed out, the CBST2 generates sufficient potential for reverse battery protection.

Hope that answers your question?

Regards
/Peter

[clar123]

but the CBST2 Is used for the reverse protection circuitry or more for the fact that are periods where the diode Is off so VDH and drain pin may fall and its used as a kind of "tank" so It keeps for a while the voltage on these pins? until the diode conducts again?

 

Thanks!

[Peter BENSCH]

The CBST2 is indeed the output capacitor of the boost converter, and it is required to keep the bootstrap voltage stable (as for any DC/DC converter output capacitor). This boost regulator output voltage is used to supply the gate drivers (see datasheet, section 2.4), and as bootstrap voltage to turn on the reverse battery protection switch (TRB1, datasheet, fig. 8).

Regards
/Peter