I'm trying to make use of an Adafruit PCB with an LM3671 on it. Note on the PCB layout how the EN pin is connected to Vin via a 100Ω resistor.

On the 3.3V output, I've connected a 100Ω resistor and then a 1.8V Red LED to provide a load and show that it's working.

When I apply 5V, the LED blinks once and goes off and my power supply starts making a buzzing sound. Testing the LED after shows that it still functional.

Where have I gone wrong?

  • \$\begingroup\$ Inductors in SMPSs will "squeal" when operated with excesses ripple current, it sounds like you have an overload condition on your output \$\endgroup\$
    – sstobbe
    May 30, 2017 at 2:18

1 Answer 1


Power supplies sometimes buzz when they are run under low load. This situation can also cause the blinking you described. You can try a number of experiments:

  1. Try running the LM3761 circuit you built on batteries. (3 AA cells might be a good start. Just be sure not supply the LM3671 with more than 5.5V!) See if the blinking stops.
  2. Put a 150 Ohm, 1/4 W resistor in parallel with your circuit. (You can use a lower-valued resistor as long as it can handle the power.) Alternatively, try the power supply on another circuit or device that requires more current. If the buzzing changes in pitch or disappears, you may need to find a different power supply.
  3. Try a different power supply and see if the blinking stops. If you have a bench supply, it is likely to behave more reliably under low load than a wall wart.
  • 1
    \$\begingroup\$ Corrected the description to say that the LED blinks on once, then stays off. I'd like it to supply 3.3v continuously. I'm less concerned about the buzzing bench power supply, but offered it as a symptom. \$\endgroup\$
    – ohmantics
    May 30, 2017 at 2:13

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