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ISOW7841: VISO output problem when connected with TPS60403

toshi-e2e
Genius 4850 points
Part Number: ISOW7841
Other Parts Discussed in Thread: TPS60403, ISO7841

Hi, all

My customer is facing the following problem.
 When the power is on, the ISOE7841 output voltage of Viso is 5 V in normal state, but in case of abnormality it rises to 8 V with a probability of about once every tens of times.
As a result, TPS 60403 will be broken.

The customer suspected the noise from TPS 60403 and tried putting FB after Viso, but it did not improve.

Would you give me any advice on how to solve it?

Best regards,
Toshi

[Unlocked 3/28/2018 by _Nic]

  • 0
    TI__Intellectual 2900 points
    Hi Toshi,

    This behavior is not expected from the device. Could you please clarify a few points that can help us with this debug?

    1) I am not able to see any decoupling caps on the VISO pin. Could you please confirm that there is sufficient decoupling capacitor on the VISO pin? Recommended value is 0.1uF + 1uF + 10uF. If sufficient decoupling cap is not present on VISO, the device may not function properly.

    2) Could you please explain a bit more on the type of failure seen on ISOW7841 device. I understand the voltage goes upto 8V. Is the ISOW7841 device damaged after this happens or does the device resume normal operation (returns to 5V) once it is power cycled?

    Regards,
    Anand Reghunathan
  • 0 in reply to Anand Reghunathan
    Genius 4850 points
    Hi Anand-san, thanks for your quick response.

    Below is the answer for your questions.
    1) There are decoupling capacitors ( 0.1uF + 1uF + 10uF) on VISO pin. I am sorry I forgot to draw these.

    2) Only the TPS60403 was damaged. And the voltage of VISO is dropped to about 1.5V. It seems that the internal circuit of TPS60403 is shorted.

    After replacing TPS60403 with new one, ISOW7841 can operate normally.

    Thanks and regards,
    Toshi
  • 0 in reply to toshi-e2e
    Genius 4850 points

    Hi all,

    I would like to update the test results as follows.

     

    Customer tried to add the FET switch between VISO pin and IN pin for making some delay time. 
    And they conducted 5000 power-on tests using eight prototypes and as a result, no problem occurred at all.

    Why is this method effective?

    Why does the voltage of VISO rise to 8V if it is the original method?

    Is the control of the ISO7841 DC/DC converter open loop or closed loop?

    When the ISOW7841 is in the transient state of SOFT START, is there a possibility of malfunction of overvoltage when the load with the charge current of TPS60403 is the load?

    Is there a possibility that VISO will exceed VISO_max when VCC is around 2V?

    Is there overvoltage protection in the output of VISO of ISO 7841?

    Regards,
    Toshi

  • 0 in reply to toshi-e2e
    Genius 4850 points
    Hi all,
    Would you please give me a reply?

    If you have any question, please let me know.

    Best regards,
    Toshi
  • 0
    TI__Guru**** 1225732 points
    [Unlocked 3/28/2018 by _Nic]
  • 0 in reply to _nic
    TI__Intellectual 2325 points

    Nic : Thanks!

    Toshi-san,

    As we discussed over email, I will post a short summary of the solution here. The intent here is to help others also who may be searching for this online.

    After further testing and analysis by the team in our labs, we have found that adding 100uF extra decoupling cap on Vcc1 side (compared to Vcc2 side) makes the issue go away. We have tried this on 10000 power up cycles  and found no issues. This solution has also been working smoothly on a couple of different system/setups.As an example, we could have 100uF + 10uF + 1uF + 0.1uF = 111.1uF in input side (vcc1 side)  and  10uF + 1uF + 0.1uF = 11.1uF on output side (Viso side).

    Again, this is beneficial for that may have tight current limits in input or large cap load requirements at output.  Otherwise the regular cap configuration is ok. There are other potential solutions possible, but it involves extra components and is not as simple as the  capacitor solution proposed above. 

    Please don't hesitate to reach out to us for any additional support or questions. Thank you!

    Best Regards,

    Abhi