Dear Team,
Do you have any comments for the below Vout abnormal?
What is the root cause and how to solve it?
BR
Kevin
Hi Kevin,
Based on what I can see in those scope plots, something is dragging the output capacitors of the LDO up before the LDO is enabled.
There is pretty clear evidence that this is what is happening. The input power is being applied but the enable is low and the soft start capacitor is discharged during the anomaly. The output voltage is also somewhat exponential in nature, which is not what you typically see when the linear regulator powers up on its own. During normal operation you will see a linear ramp, as you eventually do see in your plots. An exponential rise is more evidence that a leakage path exists somewhere in the circuit card assembly.
The concerns you would have is making sure you are meeting the maximum voltage rating of the pins on the LDO under these type of circumstances. I don't see anything in the Absolute Maximum Ratings table that you should be worried about, based on the oscilloscope plots. You would also be concerned that the linear regulator can start up in a prebias condition (nonzero output voltage). You've demonstrated the linear regulator can do this in the oscilloscope plots. Still, you might want to discover where the leakage path is coming from in your circuit so you can bound the maximum voltage the output caps will charge to.
Thanks,
- Stephen
