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ISO6721: How to use isolated power supply to ISO6721?

Part Number: ISO6721
Other Parts Discussed in Thread: , ISOW7721

Hello All,

I want to use ISO6721 as a Digital UART Isolator. Currently my hardware is USB Powered & I'm using a 5V to 3.3V LDO (TPS73633) to use 3.3v for MCU and other IC's on PCB.

I'm wondering how to use two different power supplies and connect VCC1 & VCC2 of ISO6721 with different Supplies. Also How do I separate both grounds ?

My MCU logic level is 3.3V . I have a ISO6721DEVM board for evaluation but I'm not sure of isolated power. I'm using a USB-UART Converter CP2102 IC for UART .

Kindly suggest.

  • Hello,

    Thank you for your question. There are a few methods used to generate an isolated power supply. In order to maintain isolation the two sides of the isolator must have a separate supply and respective ground. So VCC1/GND1 connections will be only on side 1 of the isolator and VCC2/GND2 connections will be only on side 2 of the isolator. The different VCCs and GNDs should not be shorted or connected to any components on the opposite side it is designated for.

    The most straightforward way to achieve this configuration is using an additional power supply or an additional LDO circuit. That way the power supply or additional LDO circuit will have their own respective VCC and GND. Another option is using a transformer driver like the SN6505 or the SN6501 with a transformer to generate a secondary isolated power supply from the primary power supply. Please see this FAQ for more details and an example of this method. If you have questions on either of these methods, please feel free to ask for clarification.

    Another option is using a digital isolator with integrated isolated power like the ISOW7721 instead of the ISO6721. This would however require some slight system redesign, but may be a good option for future projects.


  • Hello Darrah,

    Thank you for replying to my question.

    I have one query regarding your solution of using additional LDO Circuit. I have a single primary Power supply of 5V Coming from the USB. This Input supply will be common to both the LDO's. Also, the USB GND will be common to both the LDO's . Though the LDO output's will be two different  , but GND connection will be common in this case.

    Will it still be considered as two different GND?

  • Hello,

    Although the LDOs will have different outputs, the LDOs are not an isolated power supply solution. It would be recommended to make an isolated power supply with a transformer driver like the SN6505. The SN6505 input would come from the USB connector and the SN6505 output would go to the other side of the ISO6721. Please refer to the diagram below for an example. As you can see in the diagram, the two power supplies are entirely isolated from each other as indicated by the vertical red dashed line.

    Additionally, based on the previous description of your system, it sounds like the MCU of the hardware is being powered by USB without any isolation. You can use the ISO6721 to isolate the UART signals and then connect to the MCU, but this is only needed if you want to isolate the signals from the USB in your system. It may be beneficial to check if isolation is needed in your application. If isolation is not needed, you would not need to include the ISO6721 in your design.

  • Hello Darrah,

    Thank you for clearing my doubt. I think I will go with ISOW7721. ISOW7721 is a single chip solution for me which will save space on my PCB.

    Isolation is needed in my application because I have designed a medical application hardware and when I connect the USB Power source to mains I get Mains noise in my signal. I don't know how my customers will use my product & in which type of environment so I need to use isolator to remove Mains and ground interference from the system.

  • Hello,

    Of course, I'm glad that this explanation was helpful. The ISOW7721 is a good solution for reducing space while maintaining isolation. I will go ahead and close this thread, but if you have questions in the future related to the ISOW7721, feel free to create a new thread.