On my circuit board, I have three different voltage regulators. I have a 5V Buck converter that goes into a 3.3V LDO and then finally 1.8V LDO. When I power the board with a USB and do not use external power, I get a good 1.8 Volts on the output of the 1.8V voltage regulator.

When I use external power, I get 3.26 Volts output on the 1.8V voltage regulator. My design is already extremely noisy, which I will be fixing on the next design. My questions is, can excessive noise on the 3.3V rail cause a voltage regulator to output 3.3 volts? Or is there something else going on here?

I am using the TLV702 Voltage Regulator. DataSheet

I have shown the waveforms on my oscilloscope of both the 1.8V and 3.3V voltages when powered with USB and External Power. I also included the measurements with the probe at 1x and 10x. enter image description here enter image description here

I have also included the schematic of the 1.8V Voltage Regulator. enter image description here

Edit: I have added the schematic for the Buck Converter and the 3.3V Regulator enter image description here

enter image description here

  • 1
    \$\begingroup\$ It's probably dead and failed short. \$\endgroup\$
    – DKNguyen
    Commented Oct 1, 2019 at 0:02
  • \$\begingroup\$ @DKNguyen I have replaced the device twice with new voltage regulators to make sure that was not the case. When I go back to USB power from external power the device still works at 1.8 Volts. \$\endgroup\$ Commented Oct 1, 2019 at 0:14
  • \$\begingroup\$ show us the circuit where the power enters up to what you have shown us \$\endgroup\$
    – DKNguyen
    Commented Oct 1, 2019 at 1:38
  • \$\begingroup\$ @DKNguyen I have added it to the question \$\endgroup\$ Commented Oct 1, 2019 at 1:57
  • \$\begingroup\$ where is the usb entering and where is the external power entering? what is "external power" anyways? \$\endgroup\$
    – DKNguyen
    Commented Oct 1, 2019 at 2:20

1 Answer 1


The 3.3V regulator(1) (NCP1117 as commented) is missing its output capacitor.

For the output capacitor of the NCP1117 applies:

Frequency compensation for the regulator is provided by capacitor Cout and its use is mandatory to ensure output stability. A minimum capacitance value of 4.7μF with an equivalent series resistance (ESR) that is within the limits of 33 mΩ (typ) to 2.2Ω is required. See Figures 12 and 13. The capacitor type can be ceramic, tantalum, or aluminum electrolytic as long as it meets the minimum capacitance value and ESR limits over the circuit’s entire operating temperature range.

NCP1117 of On Semi

Maybe you could put a SMD capacitor on top of the legs of the NCP1117. Or you could replace C14 and put the 100nF cap on top of the legs of the TLV702.

(1)Please use reference designator for all components.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.