I've made these changes:
- C1=100uF, C2=470uF
- R2(bottom one which is 2K here) is changed to 1K
- R2(top one which is 9.1K here) is changed to a 50K pot to give it a variable output
- C5 is 2x10uF, 0805 caps in parallel
- input is 5V from a USB wall charger
It oscillates too much. I tried to capture the waveform. I set the output to about 14 volts and used a 1K-3.3K resistor divider to bring it under 5V for my o'scope. The divider is considered a light load of 4.3KOhms then. Here is the waveform:
I marked one cycle which shows a frequency of about 15Hz ripple. The divider lines are 0.5V/div so the voltage shown is ~3.5V which ripples between 3V and 4V; multiplying the ripple by 4.3 (resistor divider) gives almost 4V ripple. (1V ignoring the brief spikes).
After connecting a 100 ohm resistor (the divider still exist for the sake of measuring,) the waveform changes as follows (notice the first half and the second half of waveform in the shown image):
Despite my PCB being worst case, in case of switching regulator designs, why is this ripple so high? Is there any theoretic problem with the regulator or design (e.g. the 0.6V Vref or 1.2MHz switching frequency)?
The PCB (red is top, blue is bottom and both planes are GND):
I changed the parallel 2x10uF caps back to a single 4.7uF. it got a lot more stable and now my multimeter, can show a precise voltage without much ripple under both 4K ohm and 100 ohm load. here is the waveform:
the periodic noise seems to be from input. as I measured the input from my Wall charger it shows the periodic 100Hz noise. (also, my low-end USB oscilloscope has an 8 bit, ±10V ADC inside, so maybe this tiny ripple is lower than what it seems on the waveform):