High pitch sound coming from Low-Dropout Linear Regulator (1117 3.3V) setup

Question

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?

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

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:

• Selecting ESR for linear regulator stability
• ESR, Stability, and the LDO Regulator
• Linear Regulators: Theory of Operation and Compensation

EDIT- Adding the following resources:

You can find more about Capacitor's ESR Here:

• ESR Control Multilayer Ceramic Capacitors
• Analog Engineer's Pocket Reference, which you can Download (Pages 14 ~ 17).