TPS3823 Watchdog not disabled by leaving pin unconnected

Hi Frank,

We have not seen this issue to date. The device may be damaged and the device should probably be switched out. If this is does not work, please attach a schematic and waveforms of VDD, WDI, RESET, and MR. There may be transients that could be violating the operating conditions described in the datasheet.

Regards,

Darwin

It will only let me upload 1 file, how can I send you multiple files with schematic and O-Scope traces?

Trace 1 = VDD

Trace 2 = MR

Trace 3 = WDI

Trace 4 = RESET

All 3 scope traces were taken from power-off to power on triggering on VDD with different time scales.

You can see the RESET pin humming along in the 3rd trace.

Here's a portion of the schematic

Please note that this behavior was seen on 2 different boards making me believe it's not an individual part failure that would be fixed by replacing the part.

Thanks

Hi Frank,

I was not able to replicate this in lab given your application's circuit and it should not behave this way when the WDI is floating. You may need to talk to your local TI AFA and recieve new samples and possibly begin the FA process.

Regards,

Darwin

BTW, do you have a 0.1uF cap on VDD?

I tried soldering a 0.1uF cap across pins 2 and 5 and it did not help.

Did you see anything amiss in the screen shots?

I'm wondering why the WDI pin is sitting at ground potential. Doesn't it have an internal pull-up and an auto WD resetting circuit, so should we be seeing some activity on it?

Hi Frank,

I did not see any issues with the scope shots you attached other than the RESET triggering while the WDI is floating.

Checking the WDI pin through an oscope while having it float also showed no (internal) oscilliations. The datasheet's block diagram is very simplified so it can't always be interpretted exactly as shown.

Try checking the connection of ground and directly at the WDI pin with an ohm meter. Even though the pull down resistor is removed, the WDI pin may be connected somehow.

Regards,

Darwin

I've already checked that the WDI pin is is connected to ground via some sneak path.

Could we have counterfeit parts? The markings on them seem correct, PARQ

How can we get the FAE investigation process started?

Hi Frank,

Please use the link below:

http://www-k.ext.ti.com/sc/technical-support/email-tech-support.asp?AAP

Regards,

Darwin

I have confirmed that they are NOT counterfeit parts. They were purchased from TI.

I took a working one off an older board and it behaves the same erroneous way in a new board.

I have lifted the MR and WDI pins off the board to isolated it from any signals on board and it still starts up the watchdog resetting.

I have looked at and compared the VDD pin on an older working board to the new board and the vo9ltage levels and ramp time are the same.

Hi Frank,

If the IC from a known working board is not operating correctly on a new board, there may be issues with the new board. Possibly the layout or the solder/pin connections.

What is your email? Perhaps we can continue this offline.

Regards,

Darwin

How do I get you my email without posting it to the world?