LP2985-N: Output voltage rising time questions

Hi Statham, 

Thank you for sharing this information with us. 

It seems odd that the device has been used for a while without any abnormal behavior

Has there been any changes to the layout, or has the capacitor part number changed? 

Furthermore, has an ABA swap been done, and/or, has the Cbyp been replaced and the board cleaned afterwards? 

1)How to define the rising time and waveform are meet datasheet spec?

This is one of our older devices and it might take me some time to find start-up related information. Here is the startup plot shown in the DS to serve as a reference:

2. Why we use the same Cbyp but get the different delay time?

It could be that there is some leakage on this Cbyp and/or it is damaged. Another possibility is that the PCB might need some cleaning, this can be done use isopropyl alcohol. 

3. Can you provide recommendations and teach us how to sorting this abnormal behavior on our product line?

As mentioned above, one way is to do an ABA swap. 

4. How to judge the tested OK IC will be OK in every power up?

As long as the Recommended Operation Conditions are followed, all best practices have been applied, i.e soldering, cleaning, ESD protection, and the device has past all required tests, the device will behave properly in every turn on.

Best, 

Edgar Acosta

Hi Edgar:

Please refer our feedback as below.

1. please try to speed up the searching time of start-up related information, we really need your assistance.

2. we will check Cbyp value, damaged or not and ABA swap, and it's new PCBA, which no need to clean.

3. it doesn't make sense, in production line has no time and bandwidth to do ABA swap.

4. it's ok, understood.

Please try to provide more explain on question 1 and 3.

And I will try to reply the question 2 tomorrow morning.

Looking forward your feedback.

Thanks a lot.  

Hi Statham, 

I am currently working on this and will provide an answer as soon as I find anything related to startup. 

We have experienced where even when its a new board coming straight from assembly, there could be some resin left over, which can affect the performance of the device. Based on the information provided, it doesn't look like the regulator is causing the issue. If there is some sort of resin left over it could even create a small increase in resistance: 

The device does regulate and show its non-monotonic behavior as expected. But notice that there is an extra ramp. 

I understand that there might not be bandwidth, however, doing an ABA swap often times is the fastest and least expensive way to verify if the issue is on the board or the device. 

Best, 

Edgar Acosta