Hello,
I'm having issues with a design including the LM5165 converter. The relevant portion of the schematic is shown below.
The converter is set to PFM mode. The converter is supplying a very low current (100uA) device. The input voltage is 21V and stable, however, the output voltage exhibits large "dips"
Please see oscilloscope capture below. In this image, one scope probe was attached to the 21V supply and the other to the 5V output (TP38). Both probes were ground-referenced to VSS. The image shows some very high frequency noise in the regions where the 5V signal levels off, some medium-freqeuncy "sawtooth" behavior, and some low-frequency "dips" approximately 8ms apart. The very high-frequency noise appears also in the yellow 21V trace at the same times. The medium-frequency "sawtooth" pattern I assume is normal and related to the switching frequency of the converter. The low-frequency "dips" I believe are causing lots of issues.
Please also see the image below. This shows the same signals, but over a longer duration. You can see that the dips persist for quite some time.
Please also see the image below. This shows the same traces over the same time scale, but I've let the board stay powered on for a long time prior to this capture, allowing everything to warm up. For this capture, I just quickly toggled "VDDA_EN" off and then back on. You can see in this image that the low-frequency "dips" are gone, though the high-frequency noise is still present in both the 5V and 21V signals.
I've taken a video of the 5V signal to show how it evolves after a cold start. Watch the attached video below. In this case, the dips start out separated by about 13ms. They slowly begin to spread out and eventually disappear altogether.
I am not very experienced with DC-DC converter issues. Can anyone guess at what could be causing these dips in the output voltage that seem to go away after the system has been on for a while?
