ADP2120 operating in a discontinuous mode when synchronized.

Question

I have an ADP2120 circuit that is operating it what looks like a discontinuous mode, but is synchronized to an input clock. 
 The circuit is a 5V to 3.3 V circuit using the component values I got from ADIsimPower. The current draw on the 3.3V varies vs time. The switch waveform has an odd ring that moves around. The switch turns on (0V out) for ~200 ns, then softly releases and slews up to ~4V, then drops to 3.5V before sharply turning off (5V). The switch waveform looks a little like the DCM (discontinuous conduction mode) in the data sheet, but with only a single broad ring. The circuit is synchronized to to a 1.2MHz clock (3.3V) so it shouldn't try to go to the PFM mode. 
 I haven't found any info about the DCM in the ADI web site yet. I have another ADP2120 operating at 5V to 1.4V that behaves nearly ideally. Any ideas or suggestions as to what this is & how to make it operate in the continuous conduction mode?

LucaV · Accepted Answer

Kevin, 
 your description would be accurate if the part did not support CCM. I have the feeling your IC is not soldered well and the Sync pin is not getting the signal. How many boards have you built? As you described above the 1.4V part works as expected so there is something going on with this 3.3V design. 
 The part has internal compensation and it designed to be stable with that range of inductors. 
 LucaV)

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ADP2120 operating in a discontinuous mode when synchronized.