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TPS92641 EVM face noise issue when PWM dimming

Wentao Zeng
Prodigy 590 points
Other Parts Discussed in Thread: TPS92641, TPS92512

My customer use TPS92641 EVM and they found there has noise when input PWM from in PDIM pin. Only input 100% duty cycle there is no noise. Have you meet this issue in EVM before. Because I don't have EVM at my hand, Could you please check our EVM will meet this issue Or give some suggestion for this issue.

Best and regards

Wentao Zeng

  • 0
    TI__Mastermind 25500 points
    Audible noise is common with any LED driver under certain conditions when PWM dimming at an audible frequency. It is either the inductor, the ceramic capacitor(s), or both. It all depends on the application and the di/dts and dv/dts involved as well as the components themselves. So you need to isolate which component(s) are making the sound and then it is trial and error. If it is the output capacitor it can be removed or you can try using a different case size or multiple smaller caps to split up the current. The input capacitors it is the same thing. You cannot remove them but using more capacitors/different case sizes, etc... can work. You can also use tantalum or aluminum electrolytic or something other than ceramics that do not have piezoelectric properties. Some manufacturers (I know Murata is one) also produce low audible noise ceramics. If it is the inductor you generally need to just try different manufacturers and/or case sizes and/or technologies (shielded, unshielded, toroid, flat wire, etc...) until you find one that does not make noise at your PWM frequency. Of course you can always PWM dim at >20kHz but that will limit the maximum linear dimming range.
  • 0 in reply to Clinton Jensen
    TI__Prodigy 590 points

    Hi Clinton

    Thanks for your reply.

    And I have tested it in TPS92512 EVM and found the following issue.  There still have the audible noise in EVM.  The first waveform(PDIM pin ,input 1KHZ 50% PWM) has the audible noise, PWM will stop for a long time. the other two waveforms(PDIM no input)  is normal.  Have you face this issue in EVM. If you don't meet this issue, I will try to find this issue in EVM.

  • 0 in reply to Wentao Zeng
    TI__Prodigy 590 points
    There is no audible noise when using Analog dimming , But there has audible noise using PWM dimming. And I found there has a big difference in two input paths.
  • 0 in reply to Wentao Zeng
    TI__Mastermind 25500 points

    That is normal operation during PWM dimming. The part stops switching when the PDIM pin is pulled low, that is how PWM dimming is achieved with a switching regulator and many of them will make noise during PWM dimming due to the caps and/or inductor. It will need to be remedied like I mentioned in my first reply.

  • 0 in reply to Clinton Jensen
    TI__Prodigy 590 points
    Thanks, I got it.
    But I still don't understand what is the PDIM the function. From datasheet, There have two ways to dimming, In my opinion , there should have the same result, Even they have different input. But Customer don't understand why our PWM dimming function donesn't work like analog input. There has the audible noise when using PWM dimming(I know We can change Cap and inductance). Because Analog dimming is very good. Maybe I need suggest customer to use RC filter to get analog voltage to dimming LED.
  • 0 in reply to Wentao Zeng
    TI__Mastermind 25500 points

    Analog dimming is fine for a lot of applications, but PWM dimming is needed for some as well. It all depends on the requirements. Here are some differences:

    1. Analog dimming - this works well for a lot of applications. However there are two things to consider. The first is that the light spectrum changes with current, so if you have an application that cannot have any light shift it may not be the best approach. The range and accuracy is another factor. The error amplifier offset voltage is about +/- 10mV. When the current sense voltage (ISENSE pin) used is 200mV your current accuracy will be +/- 5% as a result. If you analog dim down to 50mV current sense voltage the accuracy is now +/- 20%. So if you require very high accuracy at low dimming levels it may not work well.

    2. PWM dimming - this allows you to keep the LED current high during the ON times for better accuracy all the way down to a much lower light level. It also will not have the light spectrum shift that analog dimming has. But as you know you can have some noise issues that need to be resolved.

    If analog dimming is fine it is the much easier way to do it. If only a PWM signal is available then you can RC filter it and feed it into the IADJ pin.