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Original question:

ISOW7841: Can ISO784x or ISOW784x be used as level transformer? i.e., from 3.3V LVCMOS to 5V CMOS?

ISOW7841: Pull up on ISO side of the isolator

Chetan Soni
Prodigy 70 points
Part Number: ISOW7841

Tool/software:

Can i use the Pull up as dipicted in below snap?

  • 0
    Guru 298390 points

    No; the absolute maximum ratings forbid voltage above VISO (a current would flow through the clamping diodes).

  • 0
    Prodigy 70 points

    CAN I do this...?

    From Microcontroller SPI is 3.3 V LOGIC

  • 0 in reply to Chetan Soni
    TI__Guru 55565 points

    Hi Chetan,

    Yes, pulling-up OUTA (pin 14) to VISO (pin 16) and connecting VSEL (pin 10) to VISO (pin 16) are perfectly fine. Thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 70 points

    Hi ,

    Thank you for quick response, In sleep state of my ECU, 5VCC is not availabe so 5VCC_ISO.

    then pull -up to Chip select line is absent.

     If i am using different supply to pull up(pls check my first comment) body diode in Isolator is turning ON.

    (Any solution can you suggest in sleep state to pull up Chip select line on isolated GND)?

    2nd question what is the source current i can use on 5VCC ISO (if used as shown in 2nd comment of mine)?

  • 0 in reply to Chetan Soni
    TI__Guru 55565 points

    Hi Chetan,

    Please find my inputs below.

    Chetan Soni said:
    (Any solution can you suggest in sleep state to pull up Chip select line on isolated GND)?

    Since the 5VCC_ISO is not powered-up, the Chip Select (CS) pin shouldn't be pulled-up to any other supply. If you don't want CS pin to go LOW then I recommend no powering off ISOW7841. Alternatively, you can use a small buffer IC to buffer the CS output from ISOW7841 and keep the buffer powered always even when ISOW7841 is off such that the buffered CS output is HIGH even when ISOW7841 is powered-off.

    Let me know if this is not clear.

    Chetan Soni said:
    2nd question what is the source current i can use on 5VCC ISO (if used as shown in 2nd comment of mine)?

    The output currents supported on VISO pin of ISOW7841 is listed in datasheet. I am copying below a screenshot from datasheet that indicates this current. This stands true with or without pull-up resistor on CS pin. Thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 70 points

    Thank you for reply and sugesstions,

    Please let us know the Power desipassion at 40mA load  and 100Kbos on I/O channels.

  • 0 in reply to Koteshwar Rao
    Prodigy 70 points

    What is the efficiency when VIN =3.3V and Vout or VISO is 5V? Is it a recommended mode of operation or NOT?

  • +1 in reply to Chetan Soni
    TI__Guru 55565 points

    Hi Chetan,

    Chetan Soni said:

    Please let us know the Power desipassion at 40mA load  and 100Kbos on I/O channels.

    We characterize our device for 3 different datarates - 1Mbps, 10Mbps and 100Mbps. You can use the 1Mbps data as worst-case data for 100kbps.
    The additional 40mA load at VISO will lead to an additional current at Icc of 95mA assuming as efficiency of 42% (as stated in datasheet). Please note that this additional current (95mA) at Icc is in addition to device current consumption of 28mA at 1Mbps. The total would be 123mA.

    Chetan Soni said:
    What is the efficiency when VIN =3.3V and Vout or VISO is 5V? Is it a recommended mode of operation or NOT?

    The efficiency is 42% as stated in datasheet, I am copying below a snapshot from datasheet. This is a valid input/output voltage configuration is supported by device.


    Regards,
    Koteshwar Rao