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Original question:

LM5023: LM5023

LM5023: VSD connection in DCDC mode, do not have the High Voltage Start-Up FET

Jason Shen
Intellectual 1945 points
Part Number: LM5023
Other Parts Discussed in Thread: UCC28C43

Dears

we are using the LM5023 in DCDC mode with 12V input, so we do not have the High Voltage Start-Up FET, we left the VSD floating, facing a protection issue. so want to know how to connect the VSD in this mode, thanks

Jun Shen

  • 0
    TI__Guru 70515 points

    Hello Jason,

    The LM5023 has a VCC turn-on threshold of 12.8V, I don't believe it is the correct device for your application.

    With this low input voltage I am not sure how much value valley switching would bring you on the primary with such a low input voltage.

    You might consider using a UCC28C43 which is peak current mode controller with on board error amplifier for feedback.  It has a VDD turn-on of 8.4 V and will work for a 12 V application.

    If you device and have questions, please repost with UCC28C43 in the thread title so the appropriate applications engineer can answer your question.

    Regards,

  • 0 in reply to Mike O'
    TI__Intellectual 1945 points

    Mike

    Thanks for the support, and sorry for the error of supply voltage, after confirmed with customer, they are using 14V as the supply. and now the board working, but want to confirm with you how to connect the VSD when do not have the High Voltage Start-Up FET?

    Thanks

    Jun Shen

  • 0 in reply to Jason Shen
    TI__Guru 70515 points

    Hello,

    Theoretically you should not have to connect the pin.  However, leaving open pins sometimes makes devices susceptible to noise. 

    To reduce the noise seen at this pin you could terminate this pin to ground through a 1k ohm resistor.

    Regards,