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LM3402: High-frequency ringing on output

Richard Wright
Prodigy 70 points
Part Number: LM3402

We are using the LM3402 in a public transportation  application where there are several (up to 40) LED strips, for internal lighting.  We have been using them since 2012.  However, recently the customer has decided to add tire inflation indicators, which work with RF at about 500Mhz, and send a signal per tire to a main receiver.  They are having problems, and have determined our lighting strips to be causing RF interference.  The LM3402 is running at about 500Khz in our application.  However, we are seeing a ringing in the outputs at about 100Mhz on each switch cycle.  Is there a know means of reducing this ringing without interfering with the operation of the LM3402 itself?  Do we need to put a snubber on the switching diode?  I have attached a screen capture from the scope to show the ringing.  Thanks.

TEK00034.TIF

  • 0
    TI__Mastermind 25500 points

    Hello Richard,

    Yeah, this sounds like an EMI issue and your assumption is pretty good. It's usually the reverse recovery of the diode in buck converters coupled with all of the other parasitics. It may be as simple as switching the schottky diode you are using depending on other EMI concerns. But I have seen that ringing frequency change from about 160MHz to 60MHz just by changing the diode before. It all depends on the diode parasitic capacitance as well as all of the other parasitics. But changing the diode may do the trick, or an RC snubber across the diode can damp it as well. Another option that has been successful for some is using a ferrite bead in series with the diode to dampen the higher frequencies. I hope this helps.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 70 points

    Thanks for the info, much appreciated.  So if I am using an ultra-fast recovery diode, maybe just going to a fast recovery might do it?  I've had to use snubbers in other topologies,

    but was hoping to avoid adding to many additional parts, as there are space restrictions.  I'll see what other diodes we stock here to give it a try.  Hope this works, would be a fast fix.

  • 0 in reply to Richard Wright
    TI__Mastermind 25500 points

    Hello Richard,

    It really depends on exactly what you need to accomplish, basically where you need the frequency to go where it doesn't interfere and/or where you can filter it. But different diodes will change things, it may be that you need to go the other way. In either case it's a good experiment and EMI filtering could help as well.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 70 points

    Hi Clint,

    I worked out a snubber that has reduced the ringing from 2.5V pk to pk, to 1.25V pk to pk, and reduced the frequency from 2.5Mhz to 100Mhz.  I opted against the diode, as it was already a fast-recovery, and not an ultra-fast recovery.  

  • 0 in reply to Richard Wright
    TI__Mastermind 25500 points

    Hi Richard,

    That is good news if it solves your problem. A snubber usually isn't a huge cost adder. If you are good please let me know and I will close this thread. It will still be visible and you can still reply to it if you need further help and it will re-open. It just helps us with sorting what needs attention. Thanks.

    Regards,

    Clint