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SN74LVC1T45: Power up glitch on output

Part Number: SN74LVC1T45
Other Parts Discussed in Thread: SN74LVC1G125, SN74LVC1G17

Hi, 

Good day. I hope everything is fine. 

Our customer is using the SN74LVC1T45 and they notice a glitch on output A when 1.8V VCCA rises while the 5.0V VCCB is not applied even though output is pulled down with 20K resistor. We have suggested to put a lower value resistor and they did. They use a 4.7K and then a 1K pull-down and they did mitigate the glitch, the former left a bit more than the latter as you would expect.  

With this, they want to know, will this technology always have a glitch when powering up?  They thought the outputs would be high impedance when one or both of the supplies were not present.

Kindly refer to the attached image for the said behavior of the output. SN74LVC1T45_Glitch.zip

 

I would appreciate your help on this matter. Thank you and have a great day.


Regards,

Cedrick


  • Hi Cedrick,

    Has the customer considered using two .1uF caps on the supply rails as shown in section 12?

    Thanks!

    Chad Crosby

  • Apparently, the VCC isolation feature does not work while the other supply is above GND but still below the minimum recommended voltage (1.65 V).

    AXC translators are designed to have glitch-free power up, but do not work with 5 V signals.

    For unidirectional downtranslation, you could simply use a buffer with an overvoltage-tolerant input, such as the SN74LVC1G17 or SN74LVC1G125. (Their pinout is not compatible with the SN74LVC1T45.)

  • Hi Chad, 

    Thank you for your response. 

    I've checked with the customer and you can see below the following decoupling networks on the 3.3V and 5.0V supplies:


    So, they don't think that is the issue for this case.I would appreciate your further help on this matter. Thank you. 


    Regards, 

    Cedrick

  • Hey Cedrick,

    It's possible for these dual supply devices to glitch during power up, there is different control circuitry supplied by each supply. This could lead to a short pulse on the output. Certain techniques can help mitigate this, ramping supplies in different order, adding pull-downs on the output, etc, but none of these techniques can fully guarantee no glitch.

    Clemens' made some pretty good suggestions since it looks like you are doing down translation from 5 V to 1.8 V. These device will have only one supply and you wouldn't face the same issue. They can also potentially provide you with a smaller solution size if you are space constrained.