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ISO7741: Intrinsic Safety Applications

Part Number: ISO7741
Other Parts Discussed in Thread: ISO7041,

Hello,

We're working on a design incorporating the ISO7741DBQ for an IS application that needs to be the IS circuit to IS circuit dielectric strength requirement of 500V.  The DQB package exceeds this and test results confirm this.  However, the IEC 60079-11 doesn't directly recognize IC's as isolation devices where it has definitions around devices such as opto couplers and capacitors.

In discussions with the IS approval body, they have pointed out a part from TI - the ISO7041 - that seems to carry an IS approval - see Section 8.7 Safety-Related Certifications.

The ISO7741 is a UART to RS-485 chip and the ISO7041 is a digital isolator (UART to UART).  Both seem to be based on the same SiO2 galvanic barriers so I don't see why the ISO7741 wouldn't be suited for an IS application.  What further testing, or specifications, allow the ISO7041 to be IS certified.

What other parts from TI, with this SiO2 barrier, are IS certified?

Thanks.

Dave

  • Hi David,

    Thanks for reaching out to us with your questions.

    Like you mentioned it right, ISO7041 is ATEX certified and suitable for Intrinsic Safety (IS) applications while ISO7741 is not ATEX certified. ISO7041 is developed targeting IS applications and hence we have them ATEX certified.

    Please do note that both ISO7741 and ISO7041 are digital isolators and are meant for UART or SPI applications where both inputs & outputs on both sides support only CMOS logic levels. Hence, you can use ISO7041 in place of ISO7741 for your application if you only need a digital isolator. If you need an isolated RS-485 solution, then you can use ISO7041 along with a non-isolated RS-485 transceiver to meet your requirements.

    Currently, ISO70xx family is the only digital isolator family that has ATEX certification and suitable for IS applications. Let me know if you have any other questions, thank you.

    Regards,
    Koteshwar Rao

  • Koteshwar,

    Thanks for the reply.  I'm very familiar with IS standards and I've seen very few IC's carry an actual IS certification.  When I look at the datasheets of those two parts, I don't see any technical reason why the ISO7741 can't be used in an IS application - I'm having that discussion with the IS certifying body.

    From a technical point of view, is there any difference between the isolation barriers and performance between these two parts?  From the datasheets, they seem identical?

    Both parts have sections called Safey Limiting Values - when we read this section, limiting the power available to the device seems to be the key parameter.  There are 3 data points based on voltage but the power level remains consistent over those 3 points.  Does voltage matter?  In our design, from an IS point of view, we may have 8.4V available  With 8.4V, and the power limited within those limits, will we satisfy the barrier requirements?

    I'm aware we can use this part and re-configure our design to find a solution, however, we'd like to avoid this re-design effort at this stage.

    From a barrier point of view, what steps did TI take to approve this ISO70xx family?  Was there specific testing done or documentation provided? 

    Thanks.

    Dave

  • Koteshwar,

    Further to my follow up questions above, I've heard back from the certifying body with some further clarification on what makes the ISO7041 IS.  Here is the entity parameters that they've provided to me - see below.  The Pi entity parameter is based on the ambient temperature range and this data would have come from the datasheets - the Safety Limiting Values along with some derating based on temperature.  This portion is pretty straightforward when you read the datasheet.

    The Ui = 50V and the Ii = 300mA would have come from TI - these are the maximum voltage and current (individually) that cannot be exceeded to maintain the barrier integrity - these are not in the datasheet - but that's all that's needed from an IS point of view.  In essence, the ISO7041 doesn't have an approval, TI has just published these 2 numbers.  The product that this IC is designed into must guarantee these numbers.

    I'm assuming TI would have these numbers for the ISO7741 as these would have most likely come from internal testing TI has done as part of the barrier integrity testing.  With that, would it be possible to get these numbers for the ISO7741? 

    Thanks.

    Dave

  • Hi Dave,

    Thank you very much for sharing your viewpoints on the topic and the inputs from certifying body.

    You are right to say that the ISO7741 is also a suitable device that can be certified for IS applications and I agree with you on that. We have not yet considered getting this device ATEX certified as the ISO7741 is a general purpose digital isolator that can be used in wide range of applications. For this reason, the ISO77xx device family has many options from 1 to 6 channels, available in 4 different packages and most of them are also available in Q1 automotive grades.

    While ISO70xx family is an ultra-low power digital isolator (~3.5µA Iq/channel as against ~0.5mA Iq/channel for ISO77xx) specifically developed for the applications where current consumption is very critical, i.e., Field Transmitters. Since many of these applications require IS certification, we have these devices certified for IS. Please also note that only two isolator part numbers, ISO7041 & ISO7041F, are currently certified for IS while the ISO77xx family has over 100 device part numbers and it may not make sense to certify part of or all of such big family.

    I hope the above explanation clarifies your confusion on why ISO7041 was chosen for IS certification while ISO7741 isn't chosen yet.

    Regarding the entity parameters of ISO7741, please allow me to check back on the Ui and Ii values for ISO7741 and come back to you. Meanwhile, do check the possibility of using ISO7041 in your application. If the EN pins of ISO7741 were pulled-up to VCC then ISO7041 can be used in place of ISO7741 without needing any other changes. Let me know if you find any other concerns in using ISO7041, thanks.

    Regards,
    Koteshwar Rao

  • Koteshwar,

    We do believe the ISO7041 will work in our design with some subtle design changes and we are looking into those changes along with this discussion.

    I've had further thoughts on the entity parameters provided by TI and I believe they can be derived from the datasheets.

    Ui (the maximum input voltage) is given as 50V.  In Section 11.1.1 Insulation Lifetime, the insulation life time is specified to be greater than 100 years with a working voltage of 400Vrms.  Figure 15 on page 23 shows that the insulation lifetime can be greater than 4000 years with 100Vrms as a working voltage.  With that, my sense is that 50V was chosen to be at a very conservative point within this data yet high enough to avoid any special voltage protection around this device from an IS point of view.  Does this accurate and can you confirm?  I would assume that a Ui =50V on the ISO7741 would be an adequate entity parameter as well.

    Ii and Pi - this is the max current and the max power.  When I look at the entity parameter numbers provided and compare then with derated current and power curves provided in Section 8.16 Insulation Characteristics Curves, the datasheet has more conservative numbers when is comes to both current and power.  Is that correct?  For the ISO7741, if we we're to use the datasheet numbers for 60°C, would those be sufficient to ensure barrier integrity?

    My main purpose at this point in digging deeper into these parameters is to frame the discussion for a project we'll be starting shortly - we need a part with higher data rates than the 2Mbps that the ISO7041 is capable of.

    The certifying body is open to us using the ISO7741 currently and we're weighing those options.  However, confirmation of my questions above would help.  However, we do have a path forward on this project with the ISO7041.

    Would TI be open to helping certify some components for the future project I've mentioned?  I believe the parameters that TI has provided to be a paperwork exercise as I think they are derived from existing data.  Rocky Mountain Embedded Design Inc. is an engineering services company and this would be for our end client. 

    Thanks.

    Dave

  • Hi Dave,

    Thanks for your inputs again. I am glad to know that your current project can proceed further with ISO7041.

    I would like to clarify the relevance of entity parameters for isolators. The safety limiting values specified in section 7.8 of ISO7741 datasheet (please refer to latest version, revised in May 2019) and the associated insulation characteristics curves define the limits, which if exceeded, could cause damage to the isolation barrier due to overheating. The limits will enable one to design their system with necessary limits so that the isolation barrier is not damaged.

    I believe the IS certification requirement is something similar but not the same. The IS test is not related specifically to isolation and rather is about overall device heating under fault conditions. Since TI high voltage isolation barrier is much stronger and supports much higher voltages than rest of the electrical parameters of device, IS certification conducts tests on the weakest paths of device. I am not very familiar with the IS testing requirement but I believe the test pass requirements are for supply or I/O pins with respect to their local GND (and not the isolated GND). This is the reason the entity parameters are given in two sets, one for Side1 and another for Side2.

    From what I know, IS body requires the device temperature rise under specific predefined fault conditions to be controlled and limited to a particular value irrespective of whether device and/or isolation barrier is damaged or not. This requires some testing to be done at our end to get entity parameters for a particular device to submit to IS certification body. We currently do not have this data for ISO7741 though we expect it to be around similar to what ISO7041 has.

    Regarding your question on getting ISO7741 certified for IS applications, can I reach out to you on your email that you have used to create your E2E profile? Thanks.

    Regards,
    Koteshwar Rao

  • Koteshwar,

    We'll work with the certifying body to move our current project forward as we do have a path there.

    In regards to future applications, please reach out to me via the e-mail on my E2E profile such that we can discuss this further - this is also great news that TI is open to this.

    We're fairly early on in regards to the new project and the ISO7741 may not be the device we chose, however, we can go over all that in private correspondence.

    Thanks again.

    Dave

  • Dave,

    I'm closing this thread for now. Please feel free to start a new thread if you have any further questions.

    Respectfully,

    Lucas