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I have a PCB that's designed to output 3.3V from a 5V source. The 5V source I'm currently using sources several amps (3A), though I only need about 100mA. I based the schematic on "Ultra-Low-Noise Fixed Voltage Application" circuit in the datasheet for MIC2505. For some reason when I plug in the power, I'm showing only 1.5V on the multimeter. Any ideas why this might be?

  • Regulator: MIC5205-3.3BM5
  • C1: 1uF / 16V ± 10% X7R Capacitor SMD-0603
  • C2: 2.2uF / 16V SMD Tantalum Capacitor ± 10%
  • C3: 470uF / 16V SMD Aluminum Capacitor

Schematic PCB Layout Actual PCB

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You have a 470uF electrolytic capacitor connected to pin 4. Pin 4 is the bypass (noise reduction) capacitor pin and should be typically 470pF i.e. a million times smaller in value.

Try removing this capacitor and see what happens. I suspect that it's the hugeness of the 470uF causing instability.

Here's what the MIC5205 looks like internally: -

enter image description here

As you can see the bypass capacitor is in parallel with the lower feedback resistor. Both feedback resistors are used to regulate to the correct voltage and with such a big cap placed here the output is liable to be unstable and what you read on a multimeter might appear to be a constant value DC but is probably the output switching up and down and a few kHz.

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