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UCC21225A: UCC21225 question.

Masazumi Ishii
Genius 3945 points
Part Number: UCC21225A
Other Parts Discussed in Thread: UCC27211

Hello.

I am evaluating UCC21225 and I have a question.

Please check the attached document

thank you.

Microsoft PowerPoint - UCC21225.pdf

  • 0
    TI__Mastermind 23215 points
    Hello Masazumi,
    I am an applications engineer with TI and will work to answer your questions and concerns. In the circuit diagram you show in the power point, I have observed similar behavior with half bridge MOSFET configurations. When both driver outputs are low, the high side and low side MOSFETs are off, and the SW node is basically open, or high impedance. If there is no load connected, or power train to the SW node this node will be high resistance with both MOSFETs off. There is a path from VDD through the boot diode to charge CBOOT which provides a current path to the switch node. The voltage on CBOOT is charged when the SW node is pulled low.
    IF the SW node is open, this path from VDD through the boot diode can cause the SW node to rise, if the SW node is high resistance.
  • 0 in reply to Richard Herring
    Genius 3945 points
    hello Richard san.

    Thank you for your answer.
    We removed the CBOOT and applied the voltage to the VDD, the same voltage occurred at the SW node.
    Apart from the path that charges CBOOT, we think that there is a path through which the current flows to the SW node inside the IC.

    Is this as per the specification at IC development?
    We planned to use it with the driver of the secondary side rectifier circuit. However,
    at this rate, a voltage of about 5.8 V is generated at both ends of CIN at the time of the stop of the driving pulse,
    so it can not be used.

    This phenomenon does not occur when UCC27211 is used.

    thank you.
  • 0 in reply to Masazumi Ishii
    TI__Mastermind 23215 points

    Hello Masazumi-san,

    On the above experiment, you mentioned removing CBOOT, and applying the voltage to VDD and still observe that the SW node can still rise. There is still a path from HB to the SW node with the floating high side driver.

    Can you try the same with removing the boot diode? In this case there should not be any path for the current to flow from VDD to the SW node.

    In your design, is there expected to be a high resistance, or impedance on the SW node? This is not usually the case.

    One difference I see in the UCC27211, and the UCC21225 is the UCC27211 has lower HB quiescent current of 0.1mA Vs 1.0mA for the UCC21225.

    Regards,

    Richard Herring