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.
I have also included the schematic of the 1.8V Voltage Regulator.
Edit: I have added the schematic for the Buck Converter and the 3.3V Regulator
Best Answer
The 3.3V regulator(1) (NCP1117 as commented) is missing its output capacitor.
For the output capacitor of the NCP1117 applies:
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.