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ISO7421: ISO7421 failure

Komal Shelar
Prodigy 10 points

Part Number: ISO7421

Tool/software:

Hi,

Currently we are using ISO7421 as digital isolator for UART communication.

On both sides supply pins of IC, we have given of switching regulator and IN/OUT pins are connected to controllers.

But currently this IC fails on site i.e. INA is showing UART data but OUTA is not showing data.

Please suggest any protection which can be accommodated now as this PCB is already in production.

  • 0
    Guru 325520 points

    Do both channels fail? Does this happen for all chips, or only for some of them?

  • 0
    TI__Mastermind 21565 points

    Hi Komal,

    Thanks for reaching out.

    I understand that the ISO7421 device has an issue where the OUTA signal does not follow the input signal at INA. Can you please provide the following information?

    1. When you say "OUTA not showing data", what do you mean? is the signal constantly HIGH or LOW state? or is there any glitching? Please provide waveforms of the failure shown and proper labelling of the waveforms.
    2. Have you tried replacing the device? how many devices were tested and how many failed? Have you performed ABA swap (replace issue device with new device on same PCB to check if issue still happens)?

    One suggestion from the diagram you sent: I see that you have not included the proper decoupling capacitors that is recommended on the datasheet. Please use 0.1uF capacitors  between the VCC and GND pins on both sides- (no more than 2mm away from VCC pin on actual PCB). This is recommended in order to make sure that the device gets stable supply voltage and can also possibly impact performance.

    Regards,
    Aaditya V

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Some times only INA OUTA channel fails(50%), but some times both channels are failed(50%).

    This happens to 10% of the ICs.

  • 0 in reply to Komal Shelar
    Guru 325520 points

    Typically, this is caused by overvoltage.

    Please show the complete schematic with all ESD/transient voltage protection components on the lines between the isolator and the connector(s).

  • 0 in reply to Aaditya V.
    Prodigy 10 points

    1. please refer attached video.

    2. If we replace the faulty IC by new IC then PCB works OK.

      

    3. what factors affect the 1nf and 0.1uF capacitor selection for decoupling capacitor.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Actually this ISO7421 is used for UART Isolation between two controllers within same unit, Controllers are different PCBs and PCBs are connected with each other through berg strips (taken care for loose connection issue). I/O pins of this IC doesn't interface with any outside device. 

  • 0 in reply to Komal Shelar
    TI__Mastermind 21565 points

    Hi Komal,

    Thanks for the additional information.

    There are several points I would like to ask about from the new information you have given:

    Komal Shelar said:

    2. If we replace the faulty IC by new IC then PCB works OK.

    How many devices were tested? how many failed?

    Komal Shelar said:
    what factors affect the 1nf and 0.1uF capacitor selection for decoupling capacitor.

    The 0.1uF decoupling capacitors are recommended in order to allow for stable/filtered power supply to supply the isolator. Although this may not be the direct cause of the issue you are experiencing, it could have an effect on the waveforms.

    Komal Shelar said:
    Some times only INA OUTA channel fails(50%), but some times both channels are failed(50%).

    Can you please elaborate on this? Are you saying that OUTA does not match INA and sometimes all outputs do not match all inputs? can you please provide waveforms with labels?

    Since you are saying that replacing the isolator IC with a new IC fixes the issue, this means that the issue stays with the IC. There may have been some damage due to ESD or a different event that the original isolator IC was exposed to. The above questions will help us confirm this, In which case,  the TI FAE and FQE for your region can help you initiate a quality return.

    Regards,
    Aaditya Vittal