• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 63 subscribers
  • Views 790 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

LM3429: Maximum Capacitance on VCC

Samuel Jaffe
Mastermind 25590 points
Part Number: LM3429

Hi E2E,

My customer is evaluating the LM3429 with the LM3429BKBSTEVAL.  His application has big input capacitors (4.4mF) and he'd like to fully charge them before the IC starts to drive the gate of the switching MOSFET.  One can change the startup time of the switching functionality by changing the capacitor (Cbyp) on VCC.

1. How large of a capacitor is acceptable for this pin before we start to run into issues?

2. Are there other ways to achieve this delayed start functionality?  Maybe utilizing the SR latch that drives the logic to the FET Gate (see below).

Thanks,

-Sam

  • 0
    TI__Mastermind 25500 points

    Hello Sam,

    There is no limit on VCC capacitor size. VCC can be shorted to ground and it will just current limit, so a very large capacitor will just current limit VCC until it is charged. It is a valid way of slowing the startup.

    You can also just set UVLO using the nDIM pin. The device will not start switching until the UVLO threshold is crossed. If you need more delay (like you have to wait until the caps are 100% charged and you don't want the threshold exactly at full voltage) you can use a capacitor on nDIM to form an RC delay with the nDIM UVLO resistors. Both of these options are likely better since in those cases the VCC would be fully charged at startup and you will have increased gate drive voltage.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    TI__Mastermind 25590 points
    Clint,

    Good to know! Thanks for answering all of my questions : )

    -Sam
  • 0 in reply to Samuel Jaffe
    TI__Mastermind 25500 points

    No problem Sam. Feel free to let us know if you have any further questions.

    Regards,

    Clint