LM20133MH 5V to 4V DC-DC Converter Oscillates when starting up into Full Load

Hi Harold,

You can either post the data here or email us at SimpleSwitcherApps@ti.com. 

Best Regards,

-Juan

Bringing up the post for Harold. He is still waiting for assistance with this issue.

I have tried reducing the output inductor value and this eliminates the oscillation problem into full load.  However, the output switching noise is much higher (on the order of 400 mV p-p with Lout = 2.2 uH where the original value was <100 mV p-p with Lout = 4.7 uH).  Also, I have not seen significant reduction in the amount of transient droop when I reduce the Lout value and increase the Cout value.

I will try removing R155 to determine if the LMP8646 is contributing to both issues.

I removed R155 (steering resistor for the LMP8646) and the startup oscillation still persists with my initial values.  Increasing the output capacitance appears to kill the oscillation, but I need to re-try this experiment with all ceramic caps (used a ceramic-electrolytic-parallel combo last night).  I will perform additional experiments today.

HT

Initial values above refer to Lout = 4.7 uH and Cout = 20 uF.