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I am using the below schematic to drop voltage from 5V to 3.3V (1117 3.3V - http://www.farnell.com/datasheets/2100276.pdf?_ga=2.141255921.210515873.1532314816-790306462.1508240417) for a Wifi Micro-controller and after few weeks of usage it started producing some high pitch sound.

Upon evaluation I noticed that after few minutes of powering the circuit, the output voltage increases from 3.3V to 4.8V and keeps varying drastically. I have checked all component connections (dry solder, short circuit etc) and everything seems to be fine. The power supply is rated 5V 600mA and the MCU takes 300 ~ 350 mA max during peak operations.

I finally had to change the LDO 1117 and its output capacitor (changed C4, 22uF to 100uF) though I am not very sure if its the LDO or the capacitor which failed (during the LDO replacement, the output capacitor got spoiled and hence I couldn't test them separately). Now the noise is absent and output voltage seems to be stable (3 days continuous operation).

As shown in the schematic I am using multiple MLCC output capacitors (as required by MCU). Would this have caused any issue?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ The 1117 wasn't designed to work with MLCC. I suspect it was oscillating at a low enough frequency that you heard it through the piezoelectric effect of the caps. \$\endgroup\$
    – Mike
    Jul 24, 2018 at 3:41
  • \$\begingroup\$ If you look at the datasheet Page 6, AP1117 is suitable for use with MLCC. \$\endgroup\$
    – Zac
    Jul 24, 2018 at 3:47
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    \$\begingroup\$ You are right, usually other 1117 LDOs don't mention the usage of ceramics, are you sure you have exactly this device (by Diodes Inc)? Also note the contradiction that the datasheet requires ESR of 0.15 to 0.5 ohm. MLCCs often have much lower ESR. \$\endgroup\$
    – Mike
    Jul 24, 2018 at 3:57
  • \$\begingroup\$ Yes I have used the same LDO. Can we assume that the output MLCC would have caused the issue? \$\endgroup\$
    – Zac
    Jul 24, 2018 at 4:23
  • \$\begingroup\$ Most likely the ESR of your 100uF and 22uF caps are general Purpose and if so are the most likely cause of instability. The datasheet specifies what is recommended for output caps which is not demonstrated in your question. Do these have ESR< 0.5? <0.1? Applying your load pulse to output impedance and Cap ESR may give you some insight \$\endgroup\$ Jul 24, 2018 at 4:38

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It is possible that your LDO has become unstable and it is oscillating at a frequency that is audible and you hear it through the piezoelectric effect of the capacitors as Mike mentioned.

Keep in mind that the reason an 'Aluminum' or 'Tantalum' if often recommended for some LDOs Output Capacitor (such as TLV1117 or LM1117) is that these tend to have higher ESR (Equivalent Series Resistance) than Ceramic or MLCC Capacitors.

Although the AP1117 does mention it works with MLCC capacitors it still mentions it needs an ESR between 150 mΩ and 500 mΩ, while most Ceramic Capacitors today can have an ESR ranging from low as ~5 mΩ to around ~500 mΩ.

You have a few of options if you want to fix the issue:

1. You can use an LDO that doesn't require a minimum ESR, such as TLV1117LV: "Unlike traditional linear regulators that need a minimum ESR for stability, the TLV1117LV series are ensured to be stable with no ESR.".

2. You could try finding MLCC Capacitors with Higher ESR.

3. You could put a small resistor in series with C4 to ensure you meet with ESR requirements.

You can find more information on the topic of ESR and its importance on LDO stability here:

EDIT- Adding the following resources:

You can find more about Capacitor's ESR Here:

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  • \$\begingroup\$ Most of the capacitor datasheet does not have ESR specified. Is there any standard reference for ESR in ceramic capacitors? \$\endgroup\$
    – Zac
    Jul 24, 2018 at 6:35
  • \$\begingroup\$ Some MLCC Datasheets do have it, for example: yageo.com/exep/pages/download/literatures/…, where they show the ESR of a 'X5R 0603 10uF 6.3V' Capacitor versus Frequency and it is around ~20 mΩ - I edited my response to add some more links about Capacitors. \$\endgroup\$
    – Victor S
    Jul 24, 2018 at 6:45
  • \$\begingroup\$ Would you be able to recommend few other LDO's which does not require minimum ESR ? \$\endgroup\$
    – Zac
    Sep 10, 2018 at 6:25

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