• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Subscribers 63 subscribers
  • Views 1356 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC27324: What is purpose using the pull down resistors on INA,INB?

Mason Noh
Expert 2490 points
Part Number: UCC27324
Other Parts Discussed in Thread: UCC28070EVM, , UCC27524, UCC28064, UCC27517

Hi, TI team

There are pull down resistors on INA pin of UCC27424D in schematic of UCC28070EVM.

What is purpose using the pull down resistors on INA,INB?

It can be removed?

  • 0
    TI__Mastermind 25570 points
    The purpose is to make sure that the INA and INB inputs are held close to zero during power up. The output of the IC supplying the INA and INB signals may be high impedance during power up when Vcc is still very low. The resistors are an insurance to guarantee that OUTA and OUTB do not turn-on when they are not supposed to. They can be removed if the source of the INA and INB signals is tied LO until Vcc is present - it depends on the controller -
    Regards
    Colin
  • 0
    TI__Genius 12990 points
    Hi Mason,

    Thanks for your question on UCC27324, Im an apps engineer supporting this device. Colin is 100% right about insurance for maintaining a defined state when the IC input pins are floating, for example on start up when uC open drain output is high impedance. The value of this pull down should be low enough so the uC can drive it more efficiently than the input stage internal pulldown. This resistor can also take away some power dissipation from the IC as the majority of current will return to the uC from this external pull down resistor.

    The effects of start up with the input signal applied can be see in figure 26 of UCC27524 datasheet (www.ti.com/.../ucc27524.pdf)
    Please let me know if you have any more questions.
    Thanks,
  • 0 in reply to Colin Gillmor
    Expert 2490 points
    Thank you for your reply.
    How about UCC27517? When I using UCC28064 (PFC controller), The pull-up resistor is necessary?
    ?
  • 0 in reply to Mason Noh
    TI__Mastermind 25570 points
    Hi Mason

    I presume you meant 'pull down' in your last post.

    The best thing to is to place the pull down resistors on the board - if they are not needed they can be removed or not populated of left in place.

    You can check the output state of the controller gate output during Vcc turn-on by placing a 10k pull up from the driver output to Vcc and see if the driver output stays close to zero or if it is pulled high momentarily as Vcc increases and then gets pulled down as the driver is enabled.

    Regards
    Colin
  • 0 in reply to Mason Noh
    TI__Genius 12990 points

    Hi Mason,

    Thanks for your followup! UCC27517 has a different input structure than UCC27324. UCC27517 has an internal pulldown to hold the output low when the input pin is floating. This accomplishes the same effort as the external pull down.

    I see you are referring to a pull up resistor, are you using the inverting configuration IN-? In this case the IN+ pin functions as an EN/DIS and needs to be externally pulled up in order to overcome the internal pulldown and enable the output.

    Thanks,