ISO7742: ISO7742FDWR power on

Hello,

Thanks for reaching out.

I'm sorry you are having issue with our device. I have some follow-up questions in regards to the information you have provided to get a better idea of the issue:

1. Have you performed an ABA swap for the failed unit? (replace the failed unit with a good unit and see if the problem persists. Place the problem unit on a known good board to see if the issue occurs.) this allows us to verify that the issue is only happening with that one specific isolator device.
2. Can you please attach the schematic of the board and also the PCB layout of the board as well?
3. What is the test environment temperature? at what temperature did you see the failure occur?
4. What was the input conditions for the isolator?
   1. frequency?
   2. supply voltage ?

Regards,
Aaditya V

Hello,

Thank you for your reply.

1. I have not yet removed the part and replaced it, before I do, i want to find out much as possible.
2. See attachements below, i made a screenshot of the schematic part and the pcb layout. The isolators are used for some simple digital IO, where the same circuits are repeated six times.
3. Normal room temperature - around 22 degrees celcius.
4. Input conditions of the isolator - primary supply 3.3V, secondary supply 5V, input frequency 0Hz. The input conditions were low, high and floating. Ofcource when the input is high the output remains high, you can only see the osciilation part after around 300ms. It does not matter if the input section of the device is powered on or off, the output signal remains the same.

I do not understand where the 300ms comes from since it is a very long delay. I checked the 5V power supply startup but I could not find anything there. Also, there are six 7742F devices powered from the same 5V secondary supply.

I can't measure any signals on the inputs on powerup.

Hi Gerben,

Aaditya is Out of Office today.

Will review your inputs and respond by Thursday, May 22.

Regards
Varun

Hi Gerben,

Thank you for the information provided and sorry for the delay.

There is a possibility that this issue could be for this specific device from the symptoms you are describing. It is definitely interesting that the output signal turns on 300ms after the input signal is at a certain state. In order to confirm the failure, can you please try/clarify the below points?

1. You mentioned that the the Input would be HIGH state, but the output would stay HIGH for 300ms then start oscillating? is that correct? or does the output signal start 300ms after input signal is applied?
2. can you please attach waveforms of input, output, and VCC all in one scope shot showing the issue?
3. After confirming the above points, I would highly recommend performing an ABA swap (described in previous post). This will confirm if the device issue is only present in this specific isolator device or if it is related to another part of the system.

Depending on your results from the above, this may become a quality event, in which the customer support center would assist in the process of this. Before we get to starting that, learning about the waveforms and issue you are experiencing is beneficial.

Regards,
Aaditya Vittal

Thank you for your reply; i will share some further measurements.

Hi Gerben,

Today is a TI Holiday. 

We Will Wait for your further inputs.

Regards
Varun

Hello;

1. The output signal always goed high at startup, regardless of input. After 300ms it oscillates then goes to correct state depending on input signal.
3. 

3. I will swap the device with a new one.

Regards,

Gerben

Hello,

I noticed that on the measurements done with the Rigol scope the 300ms has halved to around 150ms - it seems that when GND1 and GND2 of the ISO7742 are connected (which was done by means of the oscilloscope) the time is halved to around 150ms. Same result was seen by the Fluke scope.

Hi Gerben,

Thanks for the information and waveforms you have provided. 

Looking forward to seeing the ABA swap results. 

Regards,
Aaditya V

Hello,

I performed the ABA swap - with a new device the PCB functions as exected. When i put the old part back the behaviour is as I decribed before. So it seems related to the part itself. Any thoughts?

Regards,

Gerben

Hello,

Thank you for your assistance. I will check the quality event.

Regards,

Gerben

Hi Gerben,

Sounds good! Please feel free to reach out if you have any additional questions.

Regards,
Aaditya V