Hello,
I am seeing some erratic behavior with the LM5118.
When I step up the load, the output voltage shifts downward by ~10%. What appears to be happening is the boost part of the circuit isn't engaging properly. When I ramp down the current, as some point, the boost circuitry starts to become active and output voltage begins to rise before it unexpected shuts down and begins the soft start process. At the end of that process, Vout is back to nominal.
The current limit appears to not be exceeded. What I found is that ringing on the VCC pin increases (to the point where it dips below 3.5V, for less than 100ns) shortly before the shutdown. My question is - 1) can ringing on the VCC pin causes a VCC UVLO condition? (Note that no external supply is driving VCCX at this point)
It appears that it does cause the shutdown, because when I drive VCCX with an external supply at 10V, the shutdown doesn't occur, but also, Vout stays at the nominal voltage (with boost circuitry working properly) after stepping up the load. Could the VCC ringing be causing that too? Note that the VCC pin has a capacitor connected to it in accordance with datasheet guidelines, and the maximum threshold voltages of the FETs are 5V.
Thanks.