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ISOW1412: Supply and output current, PROFIBUS mode

Part Number: ISOW1412
Other Parts Discussed in Thread: PROFIBUS, ISO1412,

Hello team,

When used with a termination resistor of 120 Ω, the maximum Power converter supply current is 207mA versus 180mA in the short circuit. What is the reason for this?

Also, what is PROFIBUS mode? In my understanding, we can use general 5V RS485 when MODE = VISOOUT because PROFIBUS is based on RS485. Is this correct?

Best regards,

Shotaro

  • Hello Sakai-san,

    Please allow our team another day to get back to you on this.

    Best,

    Michael

  • Hi Sakai-san,

    Thanks for reaching out.

    When used with a termination resistor of 120 Ω, the maximum Power converter supply current is 207mA versus 180mA in the short circuit. What is the reason for this?

    The typical current consumption of DC/DC converter (IDD) is 123mA with 120Ω termination resistor while the maximum current can go upto 207mA. When the RS-485 bus is short-circuited, the input current (IDD) is limited to 180mA typical. I understand your question is related to these two current and why the short-circuit 180mA is lower than the max operating input current 207mA. That's a fair question.

    The key difference between these two currents is that the 207mA is the maximum current while the 180mA is a typical current, hence, it is not a correct to compare these two currents. The 180mA (typical) short-circuit current should be compared to 123mA (typical) operating current. Now when you compare these two, you can see that the typical short-current is much higher than the typical operating current. Similarly, the max short-circuit current is also going to be much higher than the max operating current although this is not specified in the datasheet.

    what is PROFIBUS mode? In my understanding, we can use general 5V RS485 when MODE = VISOOUT because PROFIBUS is based on RS485. Is this correct?

    Profibus is a communication standard with its own protocol that runs on RS-485 communication hardware. Hence, Profibus can be implemented on any RS-485 bus. Since Profibus is defined for higher datarates, it does have a few conditions / requirements for a given RS-485 bus to be used. One of such key requirements is the on the minimum differential output voltage (VOD) which needs to be >2.1V. This is possible with ISOW14xx only when MODE = VISOOUT. Hence, we call this mode as Profibus. But this mode can equally used for non-Profibus RS-485 interface as needed without any issues.

    Let me know if this answers your question, thanks.


    Regards,
    Koteshwar Rao

  • Hi Rao-san,

    Thank you for supporting!

    Do you know why how to calculate Idd? My customer consider 123mA (typ) when driver and receiver=EN, Vdd=5V is a little bit big because ISO1412 Idd is  only 3.3mA(typ). If you have the Idd value for each block, it would be helpful.

    Best regards,

    Shotaro

  • Hi Shotaro-san,

    Thanks for following up.

    Please note that ISO1412 has two currents ICC1 and ICC2 where ICC1 is logic side current which is typically going to be small while ISOW1412 has only one IDD pin and one IIO pin where IIO is similar to ICC1 of ISO1412 and IDD is similar to "ICC2 divided by the efficiency".

    Please consider the below example data comparison between ISO1412 and ISOW1412.

    ISO1412 (typical):
    ICC1 = 3.3mA; ICC2 = 113mA

    ISOW1412 (typical):
    IIO = 4.6mA; IDD = 123mA; Calculated current into VISOIN pin = IDD * Efficiency = 123mA * 0.46 = 57mA

    From above data, the ISOW1412 is optimized to consume much lower current than ISO1412. I hope this answers your question, thanks.


    Regards,
    Koteshwar Rao

  • Hi Rao-san,

    I understood. The below is an additional question related to IDD.

    • The datasheet says IDD is 175mA (typ) and 292mA (max) when VDD=5V and bus load=54Ω. --> 5V/54Ω≒93mA
    • If we set efficiency to 46%, calculated IDD current is 202mA (93mA/0.46).
    • Why do we set 292mA as the max spec of IDD?

    Best regards,

    Shotaro

  • Hi Shotaro-san,

    Thanks for following up with additional question.

    Please note that the currents specified in supply current characteristics tables are based of actual characterization measurement data. The typical and maximum currents observed are also based on actual measured data and not calculated values.

    I would also like to mention that the typical IDD value of 175mA is measured under typical operating conditions along with typical efficiency of ~46%. While the max IDD value of 292mA is measured on a worst-case sample, worst-case supply voltage (~4.5V), worst-case temperature and worst-case efficiency (and not at 46%). Hence, the IDD value is going to be much higher than what you see at typical values. We do not expect any sample to hit this max value understand stated operating conditions.

    I hope this clarifies and address your questions, thanks.


    Regards,
    Koteshwar Rao