I have an LT8601 circuit wired such that REG1 outputs 5V. REG3 is supplied by REG1 and is configured to output 1V. However, when powered up, the output of REG3 is 0.95V and the FB3 pin has a fair amount of noise that appears to be due to burst mode operation since when I place a 500 mA load on REG1, the noise disappears and the REG3 output is 1.00 V. Any suggestions to what could be the cause? Below is the FB3 pin when REG1 is operated on a light load (~30mA). The spacing of the peaks vary (around 400 us) and the amplitude is roughly 70 mV. The switching frequency is configured for 250 kHz.
I should also note the following:
1. I see the same AC noise on the SW3 output and a similar signal on the PV_IN3 input.
2. If I remove the REG1 input of 5V from the PV_IN3 pin and replace it with a 3.3V supply (using a fly wire) from an adjacent LT8601, the problem disappears.
Same question was sent through an ADI FAE. The issue can be traced to PCB layout not good picking up much noise. Considered closed.