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SN74AVCH4T245 pull up resistor

Berlauber
Intellectual 260 points
Other Parts Discussed in Thread: SN74AVCH4T245

Hello!

I am using SN74AVCH4T245 to translate LVCMOS SPI signals to/from FPGA and DACs. I want the device always enabled so I would like to know the appropiate value of the OE pull-up resistor when it is fixed (I mean that OE pin is not driven externally, it is attached to a fixed voltage) and connected to VCC.  1KOhm would be ok??

Thank you so much!

  • 0
    Intellectual 260 points

    Sorry, I made a mistake. The OE pin is active low! so If I want the device always enabled, is it correct to tie the OE pin directly to ground? This sentence in the datasheet makes me doubt: 

    "To ensure the high-impedance state during power up or power down, OE should be tied to VCC through a pullup
    resistor; the minimum value of the resistor is determined by the current-sinking capability of the driver".

    I would appreciate any help with this issue,

    Thank you in advance

    Regards

  • 0 in reply to Berlauber
    TI__Guru* 97046 points

    Moving to voltage translation.

    You are correct that the OE (Output Enable) pin is active low -- a low signal at the 1OE\ input will enable the output of the first device, and 2OE\ works the same for the second.  The statement you quoted from the datasheet is also correct, but is talking about power-on behavior of the device.  It is saying that the output will remain turned off (ie in their high-impedance state) during power-up of the device if you tie the OE pins to Vcc through a pull-up resistor.

    Personally, I would use a 10k resistor to ground the OE pins for that device if I wanted them to be always active.  The resistor isn't really necessary, but provides a failsafe if anything were to short out.

  • 0 in reply to Emrys Maier
    Intellectual 260 points
    Dear Emrys,

    Thank you so much for your answer. Maybe you can help me with some more doubts with this chip:

    - If I need one channel always disabled, what is the most suitable value for the pull-up resistor at the OE pin?
    - Could you recommend me pull-up/down resistor values for the DIR pins?
    - Datasheet says that unused inputs should be tied to GND (or VCC), what should I do with the unused outputs?

    Thank you so much for your help!

    Regards
  • 0 in reply to Berlauber
    TI__Guru* 97046 points
    Just like with the pull-down, a resistor is not necessary but I would use a 10k just in case.

    The same is true for the pull-up/down resistor values on the DIR, I would use 10k.

    Just as a side note, if you have another 1k to 10k resistor value in your design already, I would recommend using that. I have been known to use a 2.2k or 4.7k resistor for pull-up/down resistors just because I needed a bunch of them in other places in my circuit.

    Unused outputs should be left floating. With the inputs forced to a known voltage, the output will remain at the appropriate voltage. Connecting it to something could cause unwanted current draw.