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ISO7741: Support I2C signal?

Jason Dai
Expert 3720 points
Part Number: ISO7741
Other Parts Discussed in Thread: TPS2388, TPS23880

Dear E2E,

We would like to know if ISO7741 could support i2c signal and connection like photo below.

ISO7741 is push pull type, so we need to remove pull high resistor in the circuit below. Am i right?

BR,

Jason

  • 0
    TI__Expert 6955 points
    Jason,

    Our i2c expert is in a different timezone but we have documentation on achieving isolated i2c with our digital isolators. I have linked the document below. Any further clarifications and answers will be handled by our i2c expert.

    www.ti.com/.../slyt403a.pdf
  • 0 in reply to Lucas Schulte9
    TI__Expert 3720 points
    Hi Lucas,
    we need expert to confirm the conenction and use case based on the condition i mentioned.

    BR,
    Jason
  • 0 in reply to Jason Dai
    TI__Guru 54155 points

    Hi Jason,

    The article that Lucas has shared is the right document to refer to if customer is intending to use a digital isolator to achieve two isolated bidirectional channels (SCL & SDA).

    There are a few other devices that have two unidirectional channels SDAI & SDAO instead of a single bidirectional channel SDA. This is probably done to enable unidirectional devices also to be used to interface to such devices. One of such devices is TPS2388 which is a PoE PSE controller. Please find below its simplified schematic from datasheet showing separate SDAI and SDAO pins.

    The schematic that you have shared can be directly connected to TPS2388 and other similar devices with separate SDAI & SDAO pins. There is no change required in your schematic and can be directly used to connect to TPS2388.

    I would strongly recommend to replace ISO72xx device with ISO77xx to benefit from all the improved performance of ISO77xx devices.

    Please let me know if you have any questions, thank you.

    Regards,

    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Expert 3720 points
    Hello Rao,
    Customer want to connect tps23880 x12pcs on i2c bus and through ISO77xx to communicate between PSE and Host.
    Could you guide us how to check / calculate if ISO77xx driving capability is enough for tps23880 x12pcs?

    BR,
    Jason
  • 0 in reply to Jason Dai
    TI__Guru 54155 points
    Hi Jason,

    Sorry that I missed to come back to you earlier.

    The maximum recommended source and sink current capability of ISO77xx is ±4mA. If the total current requirement of 12 x TPS23880 devices is <4mA then yes ISO77xx can be connected to 12 such devices. I couldn't find the IIL/IIH spec of SCL and SDAI inputs in TPS23880 datasheet, this parameter indicates the current that TPS23880 SCL/SDAI inputs consume. We will be able to determine if ISO77xx will be able to drive those if we know IIL/IIH spec of TPS23880 SCL/SDAI inputs.

    IIL/IIH spec of ISO77xx is ±10uA, if TPS23880 has similar input current spec as ISO77xx then yes you can connect 12 such devices without any issues. Please do note that the rise and fall times will go higher due to higher load capacitance connected to ISO77xx outputs.

    Please do reach out to TPS23880 team or to their forum to check for IIL/IIH values. Thanks.


    Regards,
    Koteshwar Rao
  • 0 in reply to Koteshwar Rao
    TI__Expert 3720 points

    Hello Koteshwar,

    After confirmed from PoE team, the I2C interface pins are logic I/O pins, so there is significant power consumption to report or measure.

    The pin capacitance is 10pF on SCL and 6pF on SDApin which is going to determine how much load your driving circuitry will need to support.


    In 12pcs tps23880, the capacitance should be 120pF on SCL pin and 72pF on SDA pin. if so, could ISO77xx support fast I2C data rate without SI issue?

     

    RB,

    Jason 

  • 0 in reply to Jason Dai
    TI__Guru 54155 points

    Hi Jason,

    Thanks for your inputs and for confirming that RST, SCL, SDAI & SDAO are logic I/Os.
    Since digital logic inputs typically consume a few uA of current (<10uA for most TI digital isolators), it will be safe to connect such 12 inputs together as the total current only sums upto about 120uA while ISO77xx outputs support max 4mA of current.

    I understand the worst-case total cap at SCL & other pins is going to be 120pF, this is only going to affect the rise and fall times. For 3.3V supply, 120pF cap & 2mA output current drive, the typical rise/fall times are going to be about 200ns. This I think is good enough to support upto 1Mbps datarate.

    Though logic inputs can be connected together without any issues, logic outputs can't be connected together. One output driving multiple inputs is not a problem but multiple outputs driving together is going to be a problem. If one output is HIGH while one of the other outputs is LOW, then there is a potential VCC-GND short which can cause damage to device.

    Fortunately, the SDAO outputs are open drain as it is a requirement for I2C. Since these outputs can only sink current and can't source, 12 such outputs can be connected together to ISO77xx input pin without any issues. Please do note that it is important to use only one pull-up resistor per ISO77xx input, using 12 pull-up resistors coming from all 12 outputs could lead to large sink current that could violate datasheet spec of TPS23880.

    Please do check with PoE team to confirm IIL & IIH values for TPS23880 SDAI, SCL & RST inputs. This value is ±10uA for ISO77xx inputs. Thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Expert 3720 points
    Hello Rao,
    I am still checking with PSE product line to check the actual IIL and IIH values.
    I am just confused that normally we just care about load capacitance for signal integrity, but why should we still care about IIL and IIH as well?
    In addition, in customer's circuit, they want to feed 3V3 for VCC1/2 of ISO77xx. The driving capability would be +/-2mA,max.
    If so, is it ok to drive 120pF load capacitance for i2c interface in fast mode?
    Could you also guide us how to verify it by calculation or any way?

    BR,
    Jason
  • 0 in reply to Jason Dai
    TI__Guru 54155 points
    HI Jason,

    Please note that both input current and input capacitance are important for signal integrity. The typical usage of digital I/Os is that they are typically connected to only one device and hence, the input current being small is the least of the concerns. On the other hand, the input capacitance is critical from the point that it can affect the maximum datarate that the I/O can support.

    In this application, you have mentioned that the datarate is only going to be about 400kbps (fast mode) hence, 120pF of input capacitance is still acceptable and ISO77xx can support. But since ISO77xx will now be driving 12 TPS23880 inputs, I want to make sure that its output current drive is able to support the current requirement by 12 inputs which needs to be supported by one output of ISO77xx only.

    I did briefly provide some calculated values in my last reply though I made a typo by considering 4mA of output current for 3.3V instead of considering 2mA. I have made this correction in my previous post and it reflects correct values now.
    Like I mentioned earlier, that the two main things that we need to consider while connecting digital I/Os are the ability to support load current and load capacitance requirements.

    The rise and fall times of the interface can be estimated by calculating the charge and discharge times of load capacitance which is 120pF in this case. Using i = C*dv/dt, the rise and fall times are calculated to be about 200ns. The actual rise and fall times are expected to be much lower as the output channel is not current limited internally.
    The max number of inputs a digital output can drive is called Fan-Out. To know if ISO77xx output can drive 12 inputs of TPS23880, I would require IIL & IIH of TPS23880 digital inputs. For everything to work fine, Iout (2mA) of ISO77xx should be > 12*(IIL or IIH, whichever is higher).

    The above calculations indicate if this particular interface in discussion is supported. Thanks.


    Regards,
    Koteshwar Rao
  • 0 in reply to Koteshwar Rao
    TI__Expert 3720 points

    Hello Rao,

    Clear!!

    One more question that if data rate is 400kHz, the time period of clock is 2.5us and the duration of ON/OFF time is 1.25us.

    If the rising/falling time is 200ns, it will be occupid around 16% of ON/OFF time.

    How many percent as maximum is acceptable?

    or

    How could I check it?

    BR,

    Jason

  • 0 in reply to Jason Dai
    TI__Guru 54155 points
    Hi Jason,

    The acceptable amount of rise and fall times is usually defined by the user, customer or OEM. I believe rise/fall times of upto 30% are generally acceptable and 16% of rise/fall time should be just fine.

    Like I mentioned earlier, please note that the ISO77xx output channel doesn't have any internal current limit and the output current of ISO77xx to charge the cap will be much higher than 2mA till the cap is charged. Due to this you will see that ISO77xx output rise time will be much lower than 200ns.

    IOL/IOH of 2mA of is a spec listed in datasheet as the maximum continuous load that we recommend you to connect on the output pin and not to exceed it The load capacitor only demands short duration transient current and not continuous current, such operation is supported by ISO77xx. Thanks.


    Regards,
    Koteshwar Rao