TPS3851-Q1: Watchdog WDO pin times out faster than it should

Our team will respond to you shortly.

Hi Bokar, 

Thanks for your questions. 

Can you please provide the full OPN (orderable part number first) ? I would like to check you have the extended version. To get extended timing you need to have a OPN something similar to "TPS3851Xyy(y)E-Q1".

Best,

Sila Atalar  

Hi Sila,

the full part number is TPS3851H30EQDRBRQ1. I believe this is the extended version(E Vs S). I also confirmed the part number with the device marking on the chip that's probed. 

Hi Bokar,

Thanks for verifying the top mark and the OPN.

You have the extended version and as the datasheet recommends if you have 1 uF capacitor at CWD pin, the timeout you are getting should be 77.7 s. So can you please verify/ confirm on your board if the capacitor correctly placed.

I'm trying to eliminate the case CWD is open. From the waveform, the timeout duration is very similar when CWD is open.

Please let me know if this helps. If not, can you please share your schematic. 

Thanks,

Sila Atalar 

Yes, there's a 1uF capacitor connected to CWD. We also removed the capacitor to see what the "factory program" looks like. 

Here's the wave capture for that. Blue is the wdo pin for the board in question. Violet is the wdo pin for a different board. The board in question resets every 200ms while the other board resets in about 1.8s.  

1uF capacitor was placed back to CWD and got the same result shown in the first post.

Hi Bokar, 

Thanks for sharing the oscilloscope picture.

Every time you change the settings ( change or remove cap) can you please power cycle the device? The CWD pin evaluation is controlled by an internal state machine that determines which option is connected to the CWD pin.

Can you please share your schematic? I would like to set up here and test it on our side. 

Best,

Sila 

Sila,

The board is completely disconnected from power and any external components before work is done on it.

The symptom that I see is the wdo reset interval gets wider and wider as it runs. After some minutes, the wdo reset interval gets wide enough that allows the wdi to kick in and stops the reset. 

Here's a snippet of the schematic: Please, note resistor R50 is no longer there. R42 has been replaced with a 10K pull-up resistor. This enables the watchdog on power.

Also please note, this is not the behavior for every board. Some of the boards work just fine. Please, see the capture below for a working board. 

Hi Bokar, 

I'm not sure fully sure how many board you have and how many of them having the timeout period (WDO) issue. Can you please clarify this for me to better assist you?

And have you ever tried to swap them (ABA Test) to see if you getting a different result? If not, can you please test the chip that is having timing issue on the board that you are not experiencing any issue to see if the chip still acting off.

Best,

Sila Atalar 

Sila,

All of the working and non-working boards are the same exact design. The failure rate is about 50%. We are trying to understand and fix the issue so we can deliver these boards. 

Unfortunately, it's not possible to swap or remove the chip on our end. 

We will follow up on 06/24. Thanks.

Hi Bokar,

It's very unfortunate that you are not able to swap the chips. That would help us to find the root cause. \
Without specifying where the issue is repeating it's harder to diagnose. Currently we don't know when the issue is repeating. We don't know if it is on the board or on the chip in question. 

But I will meet with our design team tomorrow to understand internal circuitry and get their review on this issue.  

Could you please provide more information on your comment. What do you mean by wdo reset interval gets wider? Is it mean the expected twd reaches 70 seconds  after some time later? 

Bokar Khalid said:

The symptom that I see is the wdo reset interval gets wider and wider as it runs. After some minutes, the wdo reset interval gets wide enough that allows the wdi to kick in and stops the reset. 

Best Regars, 

Sila

Please also find my bench result in below.

CWD = OPEN, twd = 1.56 seconds.

CWD = 1uF , twd = 77.4 seconds

The device should work with 1 uF capacitor at the CWD pin but I'm not sure what is causing your 50% failure rate. I'll try to provide feedback tomorrow  after checking your inqury with our design team. 

Best,

Sila,

Please, see the wave capture. The highlighted sections for wdo (violet Vs green) show that the interval gets larger. 

Here's another wave capture that shows the wdo interval getting wider and wider as the chip runs