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I have an LDO (TPS7A4101) which has two 10 uF, 50 V ceramic capacitors on the input. The LDO is generating 3.3 V, which is used by MCU to drive the LEDs.

When the system goes into standby mode, the LEDs turn on for 1 sec and stay off for 8 sec. During that time, the capacitors make a buzzing sound which is at 4kHz. The LEDs are driven by the MCU using PWM signal, which is running at 4kHz.

I tried using film capacitors which are too big for the PCB, but that stops the sounds for sure. I also tried different values of capacitors such as 4.7 uF, 50 V and 35 V versions, but the noise remains the same.

Does anyone know how this can be fixed in hardware?

In software I know that if, instead of PWM signal, I use high and low signal then it works, but I am looking for a hardware solution.

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    \$\begingroup\$ We cannot debug a circuit schematic we don't have. You might think the way you described your circuit makes it obvious, but what you described could be done in a hundred different ways and each way could have a hundred different problems. Not to mention you don't know what's wrong (which is why you're asking us) which means you don't know what it is we are looking for or need to know, so just give us everything. \$\endgroup\$ – DKNguyen Jan 30 '20 at 14:36
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    \$\begingroup\$ Sounds like choices are: use a different type of cap, or don't drive them at 4 kHz. Depending on the LDO requirements you may be able to substitute an electrolytic and a smaller ceramic. Outside the box : try hot melt glue or RTV to absorb the vibration. \$\endgroup\$ – Brian Drummond Jan 30 '20 at 14:49
  • \$\begingroup\$ @DKNguyen. I do not know how i can add the schematic here. \$\endgroup\$ – Pardeep Singh Jan 30 '20 at 15:11
  • \$\begingroup\$ You can use Snippet in Windows to grab anything on-screen as an image and then copy-paste the image directly into the text box. \$\endgroup\$ – DKNguyen Jan 30 '20 at 15:11
  • \$\begingroup\$ @BrianDrummond. there is no place to use an electrolyte capacitors. the only option is use to smd capacitors which are of the package 0805. i tried hot glue or rtv but they do not absorb the sound. \$\endgroup\$ – Pardeep Singh Jan 30 '20 at 15:12
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  1. Check whether during "PWM on" condition and with sound emission the LDO is stable, i.e. it does not go oscillating at output. If it oscillates, that might be the root cause.
  2. Try to use another capacitors in similar packaging, for example tantalum ones. But first check whether your LDO allows that.
  3. Which dielectric your capacitors are made of? Probably swapping X5R to X7R, or going for other capacitor manufacturers might help. Also try to select capacitor series (at the specific manufacturer) that are less susceptible to electrostriction AKA microphone effect.
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