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# UCC27211A: UCC27211 Bootcap value

Part Number: UCC27211A
Other Parts Discussed in Thread: CSD19536KTT, UCC27211, UCC27288, , UCC27289

Dear,

I am designing a BLDC Motor stage with UCC27211 as gate driver and CSD19536KTT mosfet. Power supply of motor is 48 Volts.

My schematic is as follow :

My questions :

1) Gate charge total for this mosfet is 118 nC so C2,C5,C8 are 100 nF capacitor , does this value is enough to drive Mosfet correctly ?

2) On the HI, LI of each gate does any filter is mandatory or just a resistor in serie is enough ?

3) Is there any advantage to use the new UCC27288 instead of the UCC27211 IC ?

Best Regards

• Hi,

We have been suffering from power outages here in Dallas which is delaying some of our team members' responses.

We will reply to your questions early next week.

• Hello,

Thank you for the interest in the UCC27211A.

For the 1st question I would refer to the UCC27288 section 8.2 which provides guidance for bootstrap capacitor values. Since this is motor drive, I am assuming that the switching frequency might be low, so you should include the effect of the IC quiescent current discharge.

Use the UCC27211A datasheet parameters to determine the boot cap value.

Qtotal is determined from the MOSFET Qg, switching frequency and quiescent current.

Qtotal= Qg + IHBS x (Dmax/fsw)+(IHB/Fsw), I will assume 20kHz for this example.

Qtotal=118nC+ 1uA x (.9/20kHz)+(100uA/20kHz) = 123nC

If you target 1V rippled on the bootstrap bias, then Cboot=Qtotal/deltaVHB = 123nC/1V=123nF.

I suggest increasing the boot strap capacitors to 220nF for margin with 20kHz operation.

For the LI and HI filter we recommend making provisions for an R/C filter on the driver inputs in case there is a noise issue in the application board. The filter can be depopulated if not needed.

The UCC27288 driver is a good alternative for the UCC27211A as well as the UCC27289 which has the internal boot diode. I would recommend considering these drivers for your design as they are newer devices with improved timing specifications and narrow pulse response.

Confirm if this addresses your questions or you can post additional questions on this thread.

Regards,