Low Output from MIC5205 LDO Reg

Question

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

Answer 1

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: -

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.