I am working with the LM5122 on the two phase evaluation board (LM5122EVM-2PH) and am seeing some odd behavior. I have separated the 2 phases and am only using one at the moment. The design has been modified for 48V output. The inductor and power switches were changed along with the input and output caps. The compensation loop was also redone and simulated in webench.
The issue I am seeing is a droop in the output voltage, but the output current is what is requested by the load. (Supply folding back). I have adjusted the slope compensation, and the current sense resistor and have seen no change in output, which indicates it is not hitting the cycle current limit. Bottom gate is on the entirety of its part of the conduction cycle. I have also seen that the top gate drive is not pushing much (if at all) above the source voltage. At lower loads, I do see the gate drive go the expected Vsw+Vcc (around 7Vgs). It was also noted that once the converter starts folding back, the duty cycle does not decrease further (I would expect this as it should take more charge through the inductor). Input supply is good quality and is capable of much more than the load requirements.
Any suggestions on trouble shooting this are appreciated. A few scope traces below:
Top side timing, 48V, 1A output (just before foldback). Yellow is gate, blue is source
Here is the top and bottom gate waveforms.
..and the large ripple at the output with 1A load. Cin (bulk) = 2 X 47uF, Cout(bulk) = 4 X 47uF
