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ISO1540: about spec

Part Number: ISO1540

Hi team,

Please let me know some question about spec?
*Capacitive of C1&2
Why are the C1&2 setting the value 40pF/400pF?
In the case of these capacitive over / low, how is the circuit affected?
*Output current side1&2
What is the reason the setting the current value of VCC1/2 are 3.5 mA / 35 mA due to the setting of the pull UP resistance?
And, what is the problem when it gets bigger?
best regards
Hayashi
  • Hi Hayashi,

    Thank you for using E2E forum, I will be glad to help you with answers for your queries.

    Side1 of ISO154x is MCU/Host side where only one device is expected to be interfaced to ISO154x whereas Side2 is the I2C bus side where it is allowed to connect more than one I2C devices. For this reason, the maximum allowed load current and capacitance on Side2 is much higher than that on Side1. I hope that answers your question, thanks.


    Regards,
    Koteshwar Rao
  • Rao san

    Thank you for your reply.

    I know the comparison of side 1 and side 2.
    I want to know the reason about each capacitance will set to under the 40pF/400pF.
    What is the impact if you set more than recommended capacity?

    What is the upper limit of the output current set with anything? What is expected to be the impact of exceeding this value?

    Best regards
    Hayashi
  • Hi Hayashi-san,

    Thank you for clarifying your questions.

    If the load capacitance and load current are less than the datasheet, there wouldn't be any problem and the I2C interface will be working fine. But if the load capacitance on either side of device exceeds, then the device will not be able to support the max datarate that is guaranteed in the datasheet.
    If the load connected is more than what the device can support on either side of the device and the exceeded current passes through the device, then there is a possibility of the device getting stressed and not supporting full lifetime. The I/Os could get damaged gradually.

    Do you expect the load current and capacitance to exceed in your application?


    Regards,
    Koteshwar Rao
  • Koteshwar Rao san

    Thank you for your reply.
    But, your reply is different a little bit from something I want to know .

    *Why it must be below this capacity value?
    *What kind of concern do you have, if it is larger than this capacity value?

    Best regard
    Hayashi
  • Koteshwar Rao san

    I talk about the 9.2.2 Detailed Design Procedure in datasheet.
    I want to know how to setting of capacitance and current.
    Why does it setting the value and, how have it gone if it is over value?

    Best regards
    Hayashi
  • Hi Hayashi-san,

    I am sorry if my answers weren't clear enough. I think I have already answered your question in brief but let me explain my answer with some more details.

    The input capacitance of ISO1540 Ci is about 7pF, similarly all other I2C devices will have their own input capacitance. Let's assume the capacitance of the I2C slave devices connected on Side2 of ISO1540 have 40pF each. Since the slave devices have 40pF of Ci each, I can connect a maximum of 10 devices on Side2 of ISO1540 so that the 400pF of capacitance driving capability of ISO1540 Side2 is not violated. If I try to connect more devices, ISO1540 will have much higher rise and fall times leading to slower datarate. Hence you must limit the number of devices connected to ISO1540 Side2 based on the input capacitance of each device.
    For Side1, since there is going to be only one device please choose the device such that its input capacitance is lower than 40pF. If this is violated, it will again lead to higher rise and fall times thereby limiting the maximum datarate.

    All I2C devices need pull-up resistors for them to operate normal. The pull-up resistor connected to a device decides the maximum current that will sink into device I2C pins. For example, VCC2 = 5V & Rup = 1.5kΩ then current sink into I2C lines by this one pull-up resistor is about 3.33mA. When such 10 slaves are connected to Side2 of ISO1540 then the total current sink into Side2 of ISO1540 I2C lines is sum of 10*3.33mA = 33.3mA (by slaves) & 3.33mA (by ISO1540 pull-ups). This current of 36.6mA violates 35mA of ISO1540 spec for Side2. Since this is violating current spec, the life of device may get reduced.
    The same rule applies to Side1 if the pull-ups are not chosen correctly.

    I hope the above two points answer your questions in detail. Let me know if you have any questions, thanks.


    Regards,
    Koteshwar Rao
  • Hi Hayashi-san,

    Let me know if the above explanation clarifies your questions, if so I can close this thread. If you still need clarifications, please let me know. Thanks.


    Regards,
    Koteshwar Rao