TPS798-Q1: VOUT is existed before EN enabled

Hi Lin,

How fast is the Vin rising in the scope-shot? It seems to me that the rising edge of Vin is coupling to the output of the LDO. One way to confirm this is to try to create a slower ramp on the Vin of the LDO and observe if Vout is still doing this. Do you think that is something you could try on your board?

In addition, is there any other possible charging path that could be turned on with the input of the Vin?

Do you have any type of load on the output during the startup? If so, what's the load? 

Regards, 
Jason Song

Attachment updated, thanks.

2450.ldo vout existed before enabled 0411.pdf

Hi Lin, 

Thanks for taking additional mesasurements. I have a question about the latest scope shot you provided. 

For this one, it's very different than the previous one you shared, it seems to me that Vin and EN are connected together, and the output is doing a normal startup and the output has reached the set target. Will you help me to understand the difference between this one and the previous one? 

One suggestion is to try to remove R1861 to see if it makes a difference. 

Regards, 
Jason Song

More details added, looking forward for your reply, thanks!

ldo vout existed before enabled 0413.pdf

figure0413.pdf

Hi Lin, 

I appreciate your taking the extra measurements. At this step, let's get connected through a phone call. I do speak Chinese and I will be sending you my contact via private message. 

Regards, 
Jason Song

Vout is existed even EN is grounded. See last page.

figure0414.pdf

Hi Lin, 

How many units have you found that are with the non-functional EN? If the output is starting with Vin even with EN grounded, it seems to me that the EN pin has been damaged. I would suggest you replace another fresh LDO and repeat your last experiment to perform ABA swap test. 

The A-B-A Swap Method is a simple cross check test, which can confirm the observed issue is not systemic. A-B-A Swap Method. (1) Remove the suspected component (A) from the original failing board. (2) Replace the suspected component (A) with a known good component (B) and check if the original board now works properly.

Regards, 
Jason Song

Same results found for 3 new LDOs of new boards. Looking forward for your reply.

Hi Lin, 

Please allow some time for me to investigate this on my end. Let's plan to have a quick sync at your morning time. 

Regards, 
Jason Song