We have several designs using LMR16006 as a switching pre-regulator from 7-40V input (nominal 12V) down to 5V.
During EMC qualification testing we have seen some issues with RF Immunity susceptibility in the product between 200-400MHz which is leading us to suspect the LMR16006 5V supply to the rest of the PCB (top of L4 in schematic below). While checking the design on the test bench we are seeing an unusual waveform on the SW pin - by unusual, I mean depending on the output current/inpuut voltage it is not pure square wave but instead it is a combination of sine and square wave as shown below:
At very low output current the SW node is part sinewave/part squarewave as in (1) above; as the current increases the sine portion reduces (2) above, until at a certain load current (3) above the signal becomes pure squarewave.
This effect is a combination of input voltage AND output current, ie, if the input voltage is reduced then the same effect happens as increasing the output current, ie the sinewave reduces until the signal becomes only squarewave.
Question: is this normal behaviour for this regulator?
The 5V rail itself appears stable under all conditions, the question is only regarding the SW node waveform.
The schematic and PCB layout is shown below. Ceramic capacitors C1, C5, C6 are X7R dielectric with recommended values and the layout guidelines have been followed where possible to keep SW node small:
Perhaps this sinewave portion of the switching node signal is a normal part of the ECO low-current efficiency regime?
Our concern is, if this is not normal, this could indicate some instability in the regulator implementation which is contributing to the EMC susceptibility that we are seeing.
If this is not normal behaviour for the SW node, what could be causing it, particularly in light of the fact that it appears to only happen at lower current/higher input voltage (ie lower effective duty cycle)?
Thank you in advance for any guidance or assistance that can be provided. We are keen to rule out the regulator asap in our EMC investigations if possible.