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TPS92691EVM-752: Audible Noise From TPS92691

Part Number: TPS92691EVM-752

I have the TPS92691EVM-752 evaluation kit setup to run a string of 16 LEDs.  There is a very noticeable audible frequency generated from the board and I am wondering what I can do to significantly reduce this noise.

Setup:
- Input Voltage 12V from bench power supply connected to POS and NEG on J2
- LED String connected to LED+ and LED- on J3
- J4 Jumper position moved to pins 1 and 2 for external IADJ
- 0.45V from bench power supply feeding IADJ TP4 input for 200mA drive current (negative terminal of power supply connected to NEG on J2)
- PWM connected to function generator (negative terminal of power supply connected to NEG on J2)

Function Generator PWM:
- 200 Hertz square wave
- 100 uS pulse width
- 3.3 V amplitude

LEDs:
- V_f 2.3V at 200mA

Scope Trace:
Channel 1 Yellow - PWM Input - TP8
Channel 2 Light Blue - Voltage on LED+ on J3
Channel 3 Purple - IMON - TP2
Channel 4 Blue - Current Clamp on LED String

  • Hi,

    Someone will answer the question soon.

    Thanks

    Sumeet

  • Hi Matt,

    Your are PWM'ing at 200Hz which is audible range.  The audible noise that your are hearing is most likely from the inductor.  You can use molded inductor or different type of inductor to reduce this.

    Thanks Tuan

  • Tuan - Thanks for the information.

    When I change the PWM frequency to 10Hz (leaving pulse width unchanged), I still hear a clicking noise coming from the board (each time the LEDs turn on).  I do believe the sound is coming from the inductor.  I have also tried changing the switching frequency of the regulator by changing the value of R18.  At higher switching frequencies, the noise is reduced.

    What other type of inductors should I look at?

    Do you have any suggestions on inductors to try?

    For my end application, I will be running from a 12V battery with fluctuating voltage approximately 5-18V. 

  • Hi Matt,

    As I mentioned before you should use some molded inductors or inductors that are designed for lower audible noise.  This is common with magnetic when PWM'ing in the audible range.  Most automotive application there is shielding and even potting of the module so that helps with the PWM'ing noise or you can change the PWM dimming frequency.

    Thanks Tuan