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TPS92691EVM-752: Noisy PWM Dimming

Norbert Nestler
Intellectual 290 points
Part Number: TPS92691EVM-752
Other Parts Discussed in Thread: TPS92691

Hello,

for a new project (LED Driver 12V Input, 28V 0.5A Output) i ordered TPS92691 Eval Board. So far it works fine. The issue is, that on PWM dimming (240Hz), the board is noisy. I know, the ceramic capacitors are producing this hearable noise because of the piezo effect (had this problem also in earlier designs). Also a little bit noise comes from the choke.

Now i would like to ask for further suggestions, to avoid this (any) noise - because more and more customers ask me for an "unhearable" LED-Driver.

What i know to make the noise smaller is:

- using no ceramic caps (using tantal.... or what else ?)

- using decoupled ceramic caps (makes only a few db's)

- using no PWM dimming (how to realize 0%...1% dimming with analog dimming?)

- using big electrolytic capacitors to make circuit voltage stable (lifetime for electrolytic capacitors?)

- using chokes out of compound materials (helps a little bit against choke noise)

Do you have any further suggestions to create an (0% to 100% dimmable), unhearable LED-Driver?

Thanks in advance,

kind regards

Norbert

  • 0
    TI__Mastermind 25500 points

    Hello Norbert,

    It sounds like you understand pretty well. You can always make ceramics work if you try enough things like replacing a 1210 cap with two or three 1206 caps to divide the current and change the acoustic characteristics. But like you mention, tantalums, aluminum electrolytics, and polymer aluminum electrolytics do not make noise. The lifetime for standard AE caps is generally lower, but that can be misleading. The lifetime spec is for a certain core temp, such as say 10,000 hours at 105C. But if you design to keep the core temp lower than that the lifetime increases very quickly. Done right they can rival ceramics.

    You also have other options. The analog dimming is only limited by the minimum on time which gives you a minimum duty cycle versus Fsw. So if you lower Fsw you can get a lower min duty cycle and a lower min current. Perhaps not all the way to 1%, but you could do a combo. You can analog dim as far as you can and then PWM dim to get the rest of the way. In this case the peak currents in the caps are much lower and you may be able to eliminate the noise as a result.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Intellectual 290 points
    Hello Clint,
    thank you for helping me on my way again.
    Now my circuit diagram is ready for layout. I did it with the help of the webench software.
    I choosed a switching frequency at 150KHz.
    Do you think it would be possible for you, to have a short look on it?
    Maybe i can send it to you by pm?
    Thanks in advance,
    kind regards
    Norbert
  • 0 in reply to Norbert Nestler
    TI__Mastermind 25500 points

    Hello Norbert,

    Sure. Send me a friend request and then you can either post a private message with attachments (I believe) or I can give you my direct contact info in a pm.

    Regards,

    Clint