UCC21551-Q1: UCC21551-Q1 driving coolMos

Hi Niya,

Niya Tojo said:

1)What is your recommendation value for the capacitor Cboot &Cvdd?Will circuit be affected if capacitor value is increased or decreased?

These can be calculated to ensure proper function of these capacitors.

Refer to 3.1 Bootstrap Capacitor and 3.2 VDD Bypass Capacitor for guidance of the selection of these components.

Link to app note: Bootstrap Circuitry Selection for Half Bridge Configurations (Rev. A) 

Niya Tojo said:

2)What is the recommended value for bootstrap Resistor?

The app note also contains a section for sizing the bootstrap resistor in 3.4 Bootstrap Resistor.

Just as a side note:

• In the layout, ensure that the smaller capacitor be placed closest to the gate driver pin on the same side as the gate driver.
  • This will help minimize the VDD-VSS decoupling loop stray inductances from the bond wires which cause ringing on the supply
• Ensure the bootstrap diode is rated higher than the bus voltage (added margin), as failure of the diode could lead to shoot-through 

If you have any further questions, please feel free to ask below.

Regards,

Hiroki

Hi Niya,

That is around the typical value that is used.

A 10uF VDD capacitor would be sufficient for your application.

Regards,

Hiroki

Hi Niya,

Yes, that should be sufficient enough for the bootstrap capacitor as well.

Let me know if you have further questions.

Regards,

Hiroki

Hi Niya,

This will not affect the circuit.

As long as the bootstrap capacitor is greater than 10 times the total gate capacitance, it will be able to function as intended.

Regards,

Hiroki

Hi Niya,

Ideally, these capacitors should be as close as possible to the gate driver pins to have maximum effectiveness.

We want to minimize the amount of stray trace inductances in the VDD-VSS decoupling loop as much as possible.

For this reason, it will be best to isolate the VDD capacitor calculations on the gate driver.

Let me know if you have any further questions.

Regards,

Hiroki