• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 63 subscribers
  • Views 775 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.

LM3429: LM3429 Unstable

Ray Tonks
Prodigy 20 points
Part Number: LM3429

I have a new board that has two LM3429 circuits on it.

The one circuit is a buck-boost configuration running at 700 KHz, with 350mA load. That circuit is running fine. The gate drive is stable and consistent.

The second circuit is a buck configuration running at 1.8 MHz with a 2.8A load. That circuit has stability problems. I've set the Ccmp = 1uF, Cfs = 0.1uF. The inductor is 10uH. I'm seeing quite a bit of ripple on Vin, say 0.5V. I do have 20uF on the input, but that cap is placed closer to the high current components to minimize the current loop. The controller is a little further away. I've run through all of the compensation calcs a few times so I think everything is as suggested in the datasheet.

I was going to try to add some input capacitance close to the LM3429. The little 10 ohm, 0.1uF RC filter on the Vin pin doesn't seem to be cleaning up the power enough. I'm wondering if that could be affecting the controller's operation.

Also I noticed that all of the reference circuits run at lower frequency than my circuit and have larger inductors. I'm wondering if the switching frequency could be part of the problem even though the datasheet says it can run up to 2 MHz.

Other ideas?

  • 0
    TI__Mastermind 25500 points

    Hello Ray,

    It is hard to be certain without more information, but we can get to that if needed. The first thing to check is the actual FET though at such a high switching frequency. 1.8MHz is achievable but only if you are very careful about component selection and it becomes even more difficult at higher currents. I suspect you may be having a drive capability issue so that is the first thing to check as it is common with higher switching frequencies.

    The FET gate drive is derived from VCC which has a 27mA current limit typical (20mA min). The gate driver current draw will be Qg*Fsw where Qg is the total gate charge of the FET. So you can calculate that current draw with the FET you are using and if it is over 20mA you will need to lower Fsw or get a FET with a lower total gate charge. If not then we can look into other possible causes.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 20 points
    Clint,

    Ok. I think I need to look at options in this area. The Mosfet that I'm using is the FDD86102LZ and at 1.8 MHz it looks like I'm not meeting your criteria. I'll look for an alternate Mosfet or I'll need to reduce the operating frequency and bump up my inductor.
  • 0 in reply to Ray Tonks
    TI__Mastermind 25500 points

    Hello Ray,

    Sounds good. That FET definitely has too much gate charge for that switching frequency. Let me know how it goes.

    Regards,

    Clint