• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 240 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.

Original question:

LMQ61460: Excessive ripple and dropout spikes with load current of 3.5A or greater.

LMQ61460: Undershoots causing noise at 60MHz

Murdo McLeod
Prodigy 10 points
Part Number: LMQ61460

Hi,

I am using the LMQ61460AASRJRR for a high current DCDC convertor. Input is 14V to 28V, and output is 12.1V at 4A. It is powering either a load circuit (a resonant LRC circuit being switched on and off at 3kHz by a half-bridge, which will draw 2A rms) or two generic resistive loads (of upto 2A rms each). The schematic is literally identical, and the layout is very nearly identical to the eval board, which is also showing the same problem.

We are seeing large over- and under-shoots on the switch node. The worst is by far the negative going edge on the switch node, with a lot of settling ripple, and this is causing 60MHz noise problems.

I see from the other post on this issue (e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/980090/lmq61460-excessive-ripple-and-dropout-spikes-with-load-current-of-3-5a-or-greater?tisearch=e2e-quicksearch&keymatch=LMQ61460AASRJRR) that someone else had a similar problem but the solution wasn't shared with the community.

We have tried multiple things - increasing the turn-on time sorted the overshoots but the undershoots can't be slowed down, a Schottky diode from ground to the switch node to clamp the undershoot, changing the switching frequency, etc.

Can you advise further?

Many thanks,

Murdo.

  • 0
    TI__Guru 50120 points

    Hello Murdo,

    Generally overshoot and undershoot on switch can be controlled with a RC snubber circuit on SW. The RC value attenuates the fast dv/dt of both rising and falling edge. 

    For a guide to attenuate the ring which may pose noise EMC/EMI issues, please follow this simple step guide (https://www.ti.com/document-viewer/lit/html/SSZTBC7).

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    Prodigy 10 points

    Just as a note, we were able to fix the overshoots (rising edge) by modifying the RT-CT circuit, but we couldn't do anything with the undershoot (falling edge) as the Sw node is not easily accessible.

    We took the units to the EMC test lab yesterday and the problem is not severe enough to fail EMC.

    It is very detectable with near-field probes but obviously good layout (and some luck) means it doesn't radiate. 

    Thanks again for the help,

    Murdo.