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SN74LVC1T45-EP: Max Load Capacitance

Yair Lior
Prodigy 60 points
Part Number: SN74LVC1T45-EP

We are using SN74LVC1T45MDCKR-EP in our circuit for driving 5V logic.

There is a question regarding the maximum allowed load capacitance.

  1. What is the maximum load capacitance and if it is a function of frequency, what is it for 1MHz.

  2. If we put a 1nF capacitor on output, how will it affect the performance and MTBF at 1MHzc (5V supply)?

  3. If we put a 10nF capacitor on output, how will it affect the performance and MTBF at 1MHz (5V supply)? 

  • 0
    TI__Mastermind 43380 points

    Hi Yair,

    There is no spec'd output capacitance and yes it technically is a function of frequency. This FAQ covers transition rate:https://e2e.ti.com/support/logic/f/151/t/718814?tisearch=e2e-sitesearch&keymatch=faq%3Atrue which plays a part in this. If your transition rate is longer than the period of the signal then it will likely not reach the desired amplitude. Downside: you will have to add a series resistor at the output to limit the current into the cap as to not damage the device output which will slow down the transition rate even further. However, I still think 1 MHz is achievable for the given cap values (signal may not look as square). 

  • 0
    Prodigy 60 points

    Hi Dylan,

    Thank you for the information.

    I now understand how to calculate theoutput resistance from the spec (thanks).

    From your unswer, I understand that the device maximum output current must not be exceeded.

    I understand that I can limit the current by an additional series resistor.

    The 3 questions refer to maximum load capacitance without any series resistor. If that is the case, are these the answers?

    1. 50pF at 5V?

    2. 1nF not allowed?

    3. 10nF not allowed?

    Regards,

    Yair

  • 0 in reply to Yair Lior
    TI__Mastermind 43380 points

    Hi Yair,

    Correct, 50 pF should be fine with no series resistor, but when you get into the nF range its likely you will need one.