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TPS63031 Noise/Regulation Issues

Other Parts Discussed in Thread: TPS63031, TPS63051

Hello everyone,

I am using the fixed 3.3V TPS63031 Buck-boost converter to get a stable 3.3V out of a 1-cell Li-ion battery. The output of the TPS63031 however, is less than ideal and has a waveform as shown below. There is a 150 mVpkpk ramp wave riding on top of the 3.3V DC. The frequency of the ramp wave is 8 Khz in the image below, but I have seen it as high as ~12 Khz, so it seems to change.

The circuit was designed as recommended by the datasheet. There is a total of 4.7 uF input capacitance, and 22 uF + 10 uF + 0.1 uF output capacitance, The inductor used is 2.2 uF, and the total current drawn is always less than 25 mA. I should also note that I enabled PS mode by grounding the PS pin. The layout on the PCB is as shown below:

The regulator is on the bottom layer, and the ground plane is located on the top layer. The circuit reaches ground through the two big vias I have very near to the chip. 

There is a 1 Ohm resistor in series, in the path of the current coming from the battery into the regulator. I originally thought that the 1 Ohm series resistance caused this, but I have a jumper shorting the resistor, which effectively "disables" the resistor. The scope shot shown above was with the 1 Ohm resistor shorted across, so it has no effect. I should also note that this circuit makes audible noise! I believe it is my capacitors making the noise, and it is quite high pitched!

I would appreciate any suggestions you have to increase regulation. I was hoping to achieve a somewhat low noise 3.3V bus with variations no more than 50 mV.

Thanks!