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  • TI Thinks Resolved

ISOW7842: Latch-up?

Prodigy 115 points

Replies: 7

Views: 496

Part Number: ISOW7842

On a couple of different designs we have trouble with the ISOW7842.
The isolated voltage sometimes when starting up goes to 7V instead of the set 3.3V and the circuit get really hot.

Any idea what can cause this?

BR
// Per

  • Hi Per,

    Sorry to hear that you are facing an issue related to ISOW7842. Could you please help me with the below information?

    1. What is the input voltage and output load current?
    2. What are the input and output supply decoupling capacitor values?
    3. Will you be able to share the schematic with us so that we can review? If you do not wish to share your schematic on the E2E forum, I can write to your email ID that you have used to create E2E user account. Let me know.

    We have seen this issue when the input capacitor is relatively small compared to the output capacitor. For us to be able to comment better, please do help us with the schematic. Thanks.


    Regards,
    Koteshwar Rao
  • In reply to Koteshwar Rao:

    Hi.

    Here's a snap shot of the coupling.
    5V input, Load on 3.3V (iso side) is around 70mA.

    At some point it looked as if we got less problem when we increased the slew rate on the power supply (supplying 5V) and we thought problem was solved but then it started to arise again (even with a slow power on ramp).

    So currently we are clueless.

    BR
    // Per

  • In reply to Per Bengtzohn:

    Hi Per,

    Thanks for sharing the schematic and additional information. The schematic looks good to me, hopefully the decoupling capacitors are placed close to VCC/GND and VISO/GNDISO pins as described in ISOW datasheet.

    Will you be able to increase the 47uF capacitor at VCC/GND to 100uF and test again?
    May I know what is the current limit set on UP5V supply?
    May I know what are supply ramp rates tested? What ramp rates were causing failures?

    We expect faster supply ramp rates to be better as the device doesn't spend much at UVLO point and quickly reaches to valid supply voltage levels. We expect ramp rates of <1ms are typically good.

    Please do share the information requested above. Following our recommendation to increase input supply capacitance and using faster supply ramp rates should make the device operate in normal operating conditions. Thanks.


    Regards,
    Koteshwar Rao
  • In reply to Koteshwar Rao:

    Hi.

    We have tested with current limit set to 2A upto 20A
    We have tested with slew rate of 20us/V downto (upto?) 20ms/V
    We have also tested with more capacitance on input (5V side) upto 800µF

    We haven't been able to see any correlations with these setups and the the latch-up problem.

    When we get the problem (always from "cold-start") it usually there for a couple of restarts but when it starts to work its hard (impossible?) to get the fault back.

    BR
    // Per

  • In reply to Per Bengtzohn:

    Hi Per,

    Apologies for the delay in my response. With the experiments you have conducted, we expect the device to work normal.
    Could you please confirm if I can reach out to you on the email that you have used for your E2E profile? I would like to understand more and try to help you address the issue. Please do let me know, thanks.


    Regards,
    Koteshwar Rao
  • In reply to Koteshwar Rao:

    Hi.

    Yes you can contact me on email address used for this account.

  • In reply to Per Bengtzohn:

    Hi Per,

    Since we are continuing our discussion on this issue over email, I will go ahead and mark this post as closed for now.
    Once we have the issue resolved, I will come back and post a summary to this post. Thanks.


    Regards,
    Koteshwar Rao

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