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ISOW7842: Can I give 3.3V signals as INA and INB if powering VCC with 5V?

William Thompson44
Prodigy 195 points
Part Number: ISOW7842

Hello,

Through testing it seems like it is working fine, but wanted to get an official answer from TI. 

Is it ok if I am only giving 3.3V level signals to INA and INB even though I am powering VCC with 5V? 

My CPU is only 3.3V tolerant so I would like to avoid any issues. Also have SPI0_CLK going to only 3.3V tolerant slave devices beside the ISOW7842 so just wanted to confirm that the signals would remain at 3.3.

Thanks,

Will 

  • 0
    TI__Mastermind 49445 points

    Hi William,

    The general recommendation is to match the input signal voltage levels to respective supply rails to avoid any violation of datasheet specs. For example, if VCC is 3.3V while input is 5V then this violates the absolute maximum rating of the input pin which should be <(VCC + 0.5V).

    I understand that your question is the opposite where you want to supply VCC with 5V while input signal is 3.3V. The VIH threshold for INA and INB is 0.7*VCC which for 3.3V supply is 2.31V. As long as you apply an input signal that meets VIH, device is going to respond and operate accurately. If this can be maintained then we do not have any concerns in using the device with this approach. But do make sure the supply voltage variation and operating temperature are not making input signal not meet VIH requirement.

    It is also important to note that OUTC and OUTD will output ~5V according its VCC being 5V. I believe this is being connected to the same 3.3V MCU. If their inputs do not tolerate 5V signals then you may need an external circuit to down convert the 5V signal to 3.3V. The best way to do this is to use a voltage level transistor between MCU and ISOW for all the I/O signals. This maintains the same noise margins as that of using MCU and ISOW with same supply voltages.

     

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 195 points

    Thank you for your prompt reply Koteshwar.  

    3.3 only being .66 of 5 does have me concerned as it is out of spec by .04 but it might be a risk I am willing to take as it seems to be working fine and my layout through  a level shifter would be cumbersome.

    Will 

  • 0 in reply to William Thompson44
    TI__Mastermind 49445 points

    Hi William,

    Thank you for your inputs. I realized I have mistakenly used 3.3V for VIH calculation instead of 5V. Like you have mentioned when VCC = 5V, VIH is 3.5V. This means that your input signal should be at least 3.5V to reliably detect the input HIGH signal.

    The typical input signal rising threshold (VITH) usually lies around 0.6*VCC (=0.6*5V=3V) and hence when you measure any device at room temperature you might see the device recognize the input signal. When multiple units are tested and at other temperatures, it is quite possible that you will find find a device that has close to 0.7*VCC thereby needing 3.5V input signal. Such device may not be register 3.3V as a valid HIGH input. Hence, we do not recommend using ISOW in such voltage configuration.

    Any specific reason why you do not want to apply 3.3V to VCC of ISOW?

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 195 points

    Koteshwar ,

    Thanks for your thoughtful response and recommendation.

    We need 5V on the ISO side to power certain ICs requiring 5V input, including your TPS65130RGER part. After reading what you said I might put in a level shifter, I was thinking the TXS0108ERGYR.

    -Will  

  • 0 in reply to William Thompson44
    TI__Mastermind 49445 points

    Hi William,

    Yes, that would be the best way to proceed. Thanks for considering to use a level shifter.

    Regards,
    Koteshwar Rao