I have designed and routed a board with the DA14580 and DA14581. I cannot get either board to work in buck mode, and I believe this is due to ringing (periodic damped oscillations with a max amplitude of +-1V from VCC lasting ~40us at ~60MHz) on VBAT_RF and VBAT_3V. I experience this ringing when the 3V supply going to the chip is coming from both a benchtop power supply, and other sources such as a battery. The ringing is not present when the power supply is not connected to the chip.
I can get the ringing to stop if I remove the external inductor connected to the SWITCH pin, and supply the regulated 1.41V from an external source, and the chip then functions properly, but the current consumption increases noticeably so this isn't reasonable long term solution.
My first idea was that the switching inductor was resonating with the switching regulator, so I made sure that the inductor had a >75MHz resonant frequency as specified in your application notes. I also verified that the decouplling capacitors on the 1.4V regulator output where the same as you specified, and I tried to add and remove capacitance from both the 1.4V and 3V nets. None of this removed the ringing.
The only thing notably different between my design and the dev kit is that the dev kit has the "7mm meandering trace" between the inductor and the capacitors, however my design does not allow for this to happen. I have attempted to follow other layout guidelines, but again I have a tight space constraint for my design.
Have you experienced this issue before? Do you know how to get it to stop? I can add damping resistors between the SWITCH pin and the inductor or the inductor and the capacitors if necessary, however since the ringing is on the 3V rail instead of the 1.4V rail I don't know if this would help. Any insight would be appreciated.
I was wondering if it is possible to completely bypass the internal boost/buck regulator inside the DA14580, and instead supply a steady 1.41V to VDCDC and VDCDC_RF. I am trying to achieve a design that is both very space and power constrained, therefore the 80-85% efficiency of the switching regulator and the size of the 2.2uH switching inductor both make the design more difficult.
I understand this is a strange request, but an informed response from an applications engineer with more knowledge of the internal power routing in the chip would be greatly appreciated.