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LM3409: PWM dimming audible ringing

Dominic Leone
Prodigy 120 points
Part Number: LM3409

When I am operating the driver under normal operation the system is quiet. On my PWM dimming circuit, i get audible ringing, I am PWM dimming about 1kHz. I read a document regarding what it is and believe it is in the capacitor, 

electronics-eetimes.com/news/improving-reliability-and-lowering-audible-noise-led-drivers/page/0/1

However, the document does not specify exactly what capacitor it is saying is doing the vibrating. I tried removing the output capacitor in my circuit, but that did not solve the ringing. Since placing a cap on the bottom of the PCB to counter the PCB vibration is not an option considering i have all components on the top side so it would not make sense to add cost just for this. I was going to drill around the caps and reduce the ability for the cap to effect the PCB and resonate. I cant seem to figure out what caps the article is referring to. 

Is it the input caps to the chip itself? there are not many caps so i have to assume it is that. Or is there some other item that could be causing the ringing from the PWM dimming function?

thanks

  • 0
    TI__Mastermind 25500 points

    Hello Dominic,

    With most buck converters including the LM3409 it is usually the input capacitors. You pull a lot of current from them at the beginning of each ON cycle and that can cause them to exhibit a piezoelectric effect and buzz. That article is helpful, but other things you can try are paralleling capacitors to reduce the current in each. Different case sizes will also buzz at different frequencies generally, so that is another good thing to try. I usually recommend doing something like replacing a single 1210 size 22uF capacitor with two 1206 size 10uF capacitors. The do also make low audible noise ceramic capacitors but they may be more costly.

    The inductor can cause audible noise as well sometimes. If that happens it is just experimentation. The first thing to try is to flip the inductor around. This changes whether the inside or outside windings of the inductor are connected to DC or a switching voltage. That can change the magnetostriction characteristics and help. If not it's a matter of trying different inductors to find out that doesn't buzz at your PWM frequency.

    Usually it is the ceramic caps though.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 120 points

    Is there anything else that may cause the ringing. I have removed all the ceramic input caps and replaced them with electrolytic for testing to try and isolate the audible noise. It is still there and is t same valume as if all the ceramic caps were still on the board.

    The inductor option you mentioned is not really a practical option for a board house and assembly. However, I do not have this audible noise when I am not PWM dimming. If I bypass the PWM and just feed 3.3V to the enable pin of the LM3409, I get no audible noise, so I can only assume it has something to do with the dimming frequency and pulsing. If this is a wrong assumption please let me know. I will also try another inductor that i have on hand to see if that may be the issue.

  • 0 in reply to Dominic Leone
    TI__Mastermind 25500 points

    Hello Dominic,

    If the input or output ceramic capacitors are not the culprit the inductor is the only other thing that can cause audible noise. I understand inductor orientation may be difficult for assembly so trying different inductors may be the way to go. You should be able to find one that doesn't make noise. I can also help to use an inductor with a higher current rating as it will be less stressed by the on/off pulses during PWM dimming. You are correct, the PWM dimming is the cause and audible noise with PWM dimming is a common problem with fast PWM dimming devices, but it can be solved.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Prodigy 120 points
    1 final confirmation question on the design. So I just happened to have another inductor, with a higher DCR, a lower Irms/Isat and the audible noise dropped immensely. Is that what you would expect? or is there some other inductor manufacturing black box magic that could also be aiding the suppression of the audible noise, since you suggested a higher Irms/Isat. The different inductor I am using has a smaller overall footprint in all dimensions LxWxH, so there has to be some manufacturing that is different, ie wire type and/or size and core design to explain the smaller footprint. Original was bourns SRR1260-470M and replacement was cooper bussman DR1040-470-R.

    thanks for all the input too.
  • 0 in reply to Dominic Leone
    TI__Mastermind 25500 points

    Hello Dominic,

    That can happen too. That is why I said it really is experimental with trying different inductors. Inductor ratings, core type, core shape, shielding/no shielding, wire size, wire type, etc... all can affect audible noise characteristics. Molded type inductors tend to do very well but may be costly. Either way if you try out different inductors you will eventually find one that does not buzz at your PWM frequency. It can be tedious sometimes but there will always be inductors out there that will not make noise in a particular application.

    Regards,

    Clint