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ISO6741: Enable pin

Ohad Bonneh
Intellectual 1970 points
Part Number: ISO6741
Other Parts Discussed in Thread: SN74AHC1G125, ISO6762, ISO6740, ISOM8610, SN74AHC1G126

Tool/software:

Hi Team, 

for the ISO6741 in table 6.1 it states the following: 

Output enable 1. Output pins on side 1 are enabled when EN1 is high or open and in high-impedance state when EN1 is low. EN2 10 10 10 I Output enable 2. Output pins on side 2 are enabled when EN2

If i have a PWM signal coming from my controller on the digial side of the board this controller is also generating the EN signal, 

looking at the table & the device pinout seems that the enable pins are on the wrong side and enforcing me to float the enable signal to the other side. 

1) am i reading the D.S correctly? 

2) how do you suggest overcoming this issue  

  • 0
    Guru 317390 points

    The EN pins are output enable signals (and should have been called OE).

    To control a three-state signal from the other side, use two signals through the isolator to control a three-state buffer like the SN74AHC1G125/126.

  • 0
    TI__Genius 17825 points

    Hi Ohad,

    Thanks for your questions.

    As Clemens stated, the EN pins on the device are "Output Enable" pins. This means that they only enable or disable the output pins of the device.

    EN1 enables/disables the OUTD pin on the device while EN2 enables/disables OUTA, OUTB, and OUTC.

    Ohad Bonneh said:

    If i have a PWM signal coming from my controller on the digial side of the board this controller is also generating the EN signal, 

    looking at the table & the device pinout seems that the enable pins are on the wrong side and enforcing me to float the enable signal to the other side. 

    Clemens is correct, using an extra channel (both ways, 2 total extra channels) to control the EN signal will allow you to use the EN signal across the isolation barrier. The ISO6762 is a good option for this as it has 4 forward channels and 2 reverse channels. Since the enable pin has 3 states (HIGH, LOW, HIGH-Z), a 3-state buffer can be used in conjunction with the isolator channels. 

    Regards,
    Aaditya Vittal

  • 0 in reply to Aaditya V.
    TI__Intellectual 1970 points

    If i understand correctly, I cannot send PWM & EN signal from one side of the isolator and expect the PWM signal to be turn on and off on the other side of the isolator with my enable signal. is that correct? 

    if so what you are proposing is to use additional isolator? or using a different flavor and pass the EN signal throw the isolator ?   

  • 0 in reply to Ohad Bonneh
    Guru 317390 points

    You need to use an additional channel for the enable signal. So if all channels of your isolator are already being used, then you need to use a larger one or a second one.

  • 0 in reply to Clemens Ladisch
    TI__Intellectual 1970 points

    Hi 

    I thought to use ISO6740 instead and use in/outD for the enable by short it to EN2.

    Would it be possible, or better to add a resistor between the pins?

  • 0 in reply to Ohad Bonneh
    Guru 317390 points

    Pulling EN2 low will disable all outputs on side 2, including OUTD.

    The enable signal through the isolator must stay active, so you need a separate buffer.

  • 0 in reply to Ohad Bonneh
    TI__Guru 55815 points

    Hi Ohad,

    The enable function on the digital isolator exists primarily to allow connection of outputs from multiple devices together and enable one device at a time. For example, an MCU can be connected to multiple ADCs through digital isolators over SPI and only one of these ports is enabled at a time to communicate over the same bus. This is decided by the MCU and the enable that is being controlled is on the MCU side.

    Your use-case is different. I am not sure why you want to enable/disable the PWM signals when they can be do so within the MCU. But if indeed this is a requirement there are multiple ways it can be achieved, allow me to list a few options.

    1. I am assuming that these PWMs are being connected to a gate driver. I believe most gate drives have enable input, if so, then you can use one of digital isolator channels to enable/disable the gate driver.
    2. As Clemens stated, you can use a separate signal channel digital isolator to enable/disable outputs on the other side.
    3. You can use a tri-state buffer between MCU and digital isolator so that the MCU can directly enable/disable the outputs of buffer (inputs to isolator).
      1. Make sure you use ISO6740F so that the outputs stay LOW (default state) when inputs are floated.
    4. Use a load switch or ISOM8610 to connect/disconnect power to VCC2 of isolator.
      1. In this case, we recommend using ISO64xx which can withstand input signals and not get damaged when VCC is not present.
      2. You will also have to use ISO64xxF so that the outputs stay LOW (default state) when inputs are floated.

    These are just some of the ways, I am sure this can be achieved in many other ways and I will let customer decide the one that they deem the best for their application. Let me know if you have any other questions, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Intellectual 1970 points

    Hi Koteshwar and Clemens, 

    this is very helpful and thorough explanation! 

    I talked with customer and here is the main point: 

    PWM signals are generated from the controller on the digital side and then goes throw isolation to the analog side. 

    the Enable signal derives from different circuitry and handles safety mechanism, customer wants to have redundancy is their safety mechanism hence the option to turn off the PWM signals both with enable pin and from the controller itself. 

    the issue as Clemens stated is that after disable the outputs, we cannot enable them again... do you think we can somehow overcome this requirement with a single device?     

  • 0 in reply to Ohad Bonneh
    Guru 317390 points

    There is no isolator with separate OE controls, so this is not possible with an isolator alone. An SN74AHC1G126 device is small and cheap.

    A mentioned by Koteshwar, you want to use the ISO6740FDWR so that the outputs are guaranteed to be low when side 1 is not powered.

    Do you really need the PWM signal to be disabled? If the PWM signal being constant low is safe, then you can simply force the signal low with an AND gate before the isolator. In that case, you do not need a separate channel through the isolator.

  • 0 in reply to Ohad Bonneh
    TI__Guru 55815 points

    Hi Ohad,

    Thank you for clarifying further.

    It seems like customer is looking to implement a feature called Safe Torque Off (STO) for emergency shutdown. There are many ways to achieve this and it depends on what approach customer would like to pursue. Please share the below listed resources to customer so that they understand the various ways this can be implemented.

    https://www.ti.com/tool/TIDA-01599

    https://www.ti.com/video/6359221678112

    https://www.ti.com/video/5969857680001

    As suggested earlier, the option 4 suits customer requirement best for STO implementation.

    Koteshwar Rao said:
    Use a load switch or ISOM8610 to connect/disconnect power to VCC2 of isolator.
    1. In this case, we recommend using ISO64xx which can withstand input signals and not get damaged when VCC is not present.
    2. You will also have to use ISO64xxF so that the outputs stay LOW (default state) when inputs are floated.

    If customer needs further assistance and guidance, please reach out to me over email and we can discuss this further. Thanks.


    Regards,
    Koteshwar Rao