Hello, the image below is extracted of the datasheet of your device “TPS63021”
So is easy, the problem is the selected regulator, the regulator is working well, that noise that you see is the standard output ripple for low power mode, in PWM you can reduce a little bit the noise, but is still noisy. also you are choosing a buck boost regulator (stepUp/StepDown) not the best kind of regulator for this application.
So the recommendations are:
- For Ultra Low Noise in anchors Follow the next topology e.g. 24V-> DC-DC(5V) → Linear (3.3V), use a step down regulator to down the high input voltage, and then a LDO with a high PSRR (Power Supply Ripple Rejection) for example you can use the TPS7A4700 it has a super low noise ouput Output Voltage Noise:4 µVRMS (10 Hz, 100 kHz) + a super High Ripple Rejection (PSRR): 82 dB (100 Hz)
*For tags always focus on three factors
“Low Quiescent Current”
“High Efficiency”
“LowNoise/HighPSRR”
*For Step Down in tags (LiPo or Similar) I recommend the “TPS62842”, Iq=60nA
*For Step Up in tags (AA/AAA or similar) I recommend the “TPS613221” , Iq=6.5uA