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UCC28730: Is 100kohm needed to insert between VBULK and UCC28730 "HV" terminal?

Part Number: UCC28730

Hello guys,

 One of my customers is considering using UCC28730 and UCC24650 for their new products which need lower stanby current feature.

 The customer has the following question about UCC28730.

 Q: Is 100kohm needed to insert between VBULK and UCC28730 "HV" terminal because UCC28730EVM-552 uses the 100kohm though Figure 23, application circuit in the device datahsheet doesn't use the 100kohm.

 Could you please give me your reply. Your reply would be much appreciated.

 Best regards,

 Kazuya Nakai. 

  • Hello Kazuya,

    Thank you for your interest in UCC28730.
    The resistor connecting Vbulk and the HV pin is to limit the current feeding into the controller to provide for the startup current to the VDD cap.
    The startup current is 250uA from the datasheet. You would be able to calculate how much of this resistance is required for your design to ensure fast startup.

  • Hello Sonal,

    Thank you very much for the prompt reply.
    Could I ask you an additional question?
    According to the device datasheet, the device has a constant current block between HV and VDD terminal. I think the current from HV to VDD is limitted by the block. So the resistor placed between VBULK and HV is not mandatory. I my understanding correct?

    Thank you very much again and best regards,
    Kazuya Nakai.
  • Hi,

    The current limiting resistor helps to drop the energy feeding into the HV pin. Since the HV pin only requires 0.1W at the peak of high line to ensure startup, it is a common practice to use an external resistor to dissipate some of the energy coming from the bulk caps.
    It is not mandatory to do so but using a current limiting resistor would lower the stress on the HV pin.


    Please hit "Answered" if your query has been resolved.