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SN74AVC4T245 - Controlling /OE pin

Kawai
Guru 19775 points
Other Parts Discussed in Thread: SN74AVC4T245, SN74LVC1G06, SN74AUP1G06, SN74AUC1G06, TPS74401, SN74LVC1G04, SN74AUP1G14, SN74AHC1G14, SN74AHCT1G14, SN74LVC1G14

Hi Team,

Our customer wants to control the /OEx pin of SN74AVC4T245 with DCDC converter "Power Good (PG) pin". They are designing schematic to control /OE=High when PG=Low, /OE=Low when PG=High.

[Q1]
Can you use "SN74AUC1G06", "SN74LVC1G06" or "SN74AUP1G06" to control SN74AVC4T245 in this use case ?

I am wondering that if the device would not draw any current that would make a dropout voltage by pull up resistor, when the device does not have recommended operating voltage at power up. I am asking this because SN74AVC4T245 recommends to pull /OE pin to VCCA to keep output at Hi-Z during power up and down.

As customer requires /OE control, please give us your advice.

Best Regards,

Kawai

  • 0
    TI__Mastermind 21145 points
    I am not quite sure what your questions is, I believe that when the /OE is tied to VCCA the inputs are held to high-z based on how the DIR pins are set.

    If you want to use open drain then you will have to look at the TXS family of parts.

    Can you give me more information about the power good signal. Is it open drain? what other signals, voltage levels, and data rates, are you translating? So I can help with your selection.
    -fhoude
  • 0 in reply to fhoude
    Guru 19775 points
    Hello fhoude-san,

    Sorry for the lack of information. We wanted to make sure that if these device could be used without any problem including device power up / down timing. Please let me change my question.

    [Question]
    Can you commit that the A input current of SN74AUP1G06 would never go over the datasheet specified value 0.6uA (MAX) including when device power up and power down ?

    We want to make sure that SN74AVC4T245 /OE input would have enough VIH voltage when the inverter input is Low.
    Can you also say the same thing to "SN74LVC1G06" and "SN74AUP1G06" ?


    In case, below is customer condition.

    - DC/DC part number is TPS74401 (open drain output). When DC/DC output voltage rises to the programmed voltage, power good(PG) signal will go High.
    - Requires inverter device to control /OE pin using PG signal.
    - SN74AVC4T245 is used to translate 3.2V(A side) to 1.8V(B side).

    Best Regards,
    Kawai
  • 0 in reply to Kawai
    Guru 19775 points
    Hello Team,

    Could you please help us on my question ?
    I apologize for rushing you, but, we would appreciate it if you could advise us by tomorrow(Aug 20, AM), Japan Time.

    Thanks in advance.

    Best Regards,
    Kawai
  • 0 in reply to Kawai
    TI__Expert 4535 points

    Hi TI team,

    Can you follow up this inquiry for Kawai-san, please?

    Thank you and best regards,

    Daisuke KAWASAKI

  • 0 in reply to daisuke kawasaki
    TI__Guru* 96862 points

    Konnichiwa Kawai-san!

    I saw that the A side voltage is 3.2 V, and I understand that to be the reference voltage for the control inputs on the SN74AVC4T245, so the inverter to be used should also work at 3.2 V.  I did not see a minimum edge rate for the input, but I will assume that it falls within the chosen device's parameters.

    The best device of the three for this purpose would be either the SN74AUP1G06 or the SN74LVC1G06.  The SN74AUC1G06 has a recommended maximum voltage of 2.7 V, so I would not use it for this application.

    When connected only to the OE/ pin of the SN74AVC4T245, the VOH for any of these parts will be very, very close to Vcc.  This is because the current drawn into OE/ is only 1 uA and even with a 10k pull-up, the voltage drop would only be 0.01 V.

    I can definitely guarantee that none of these devices will have larger input current than that listed on the datasheets.  In fact, the values on the datasheets are guard-banded and you will see lower values in operation.

     

    If you aren't set on the idea of an open-drain output device, you could also use a push-pull output inverter with enough drive strength to overcome the pull-up resistor, such as the SN74LVC1G04.

  • 0 in reply to Emrys Maier
    Guru 19775 points

    Konnichiwa Emrys-san,

    Thank you very much for your prompt support and detailed explanation. I apologize that I had a wrong question that I needed the inverter output sink current when it's output is High-Z.

    I have  clarified my question below. Could you please re-confirm and give us your answer ??

    Sorry for the inconvenience.

    Best Regards,

    Kawai

  • 0 in reply to Kawai
    TI__Guru* 96862 points

    From the above diagram, the inverter and switch both are powered from the same supply, which means that they will work consistently together.  The inverter will not put out anything when the power supply is too low for it to operate (ie 0 V on the output), and once the 3.3V line is high enough to turn the inverter on, it will output the appropriate value.

    In my experience, logic hardware does not un-intentionally do anything.  It will do what exactly what the inputs tell it to do -- if the power is off, the Ioff circuitry will prevent current flow in or out of any I/O ports.  If the power is on, it will output HIGH for a LOW input, and output LOW for a HIGH input.

    The 10k resistor value is fine.  That is the value I would use for this application.

     

    For the output Hi-Z leakage current, I'm afraid I do not have any data for this part.  I have measured this before in the lab on similar parts, and the values I see are generally in the nA range.  To be safe, I would use the Ioff maximum value for calculations.

  • 0 in reply to Kawai
    TI__Guru 59120 points
    Hi Kawai -San ,

    Consider SN74LVC1G14,SN74AHC1G14,SN74AHCT1G14,SN74AUP1G14 for your application instead of AUP1G06 . Whenever the PG stays low, the output of inverter would be high into OE pin keeping it in Hi-Z . When the PG signal is asserted (goes high )using the pullup ,the inverter output would go low enabling the /OE of sn74AVC4T245 device which would result in normal operation .
    Since these devices are not open drain , you wouldn't require pull up resistors before the OE input. The datasheet of TPS74401 recommends 10kohm to 10Mohm resistor to supply.
  • 0 in reply to ShreyasRao
    TI__Mastermind 21145 points

    Dear Kawai-san,

    I will first answer your first question.  The SN74AVC4T245’s /OE follows the same thresholds as the DIR pin as seen in the datasheet section I highlighted.  Sorry the datasheet is not explicit about the /OE pin but it follows DIR or Control inputs specifications.

    Your last post, see below, is answered as follows.

    The SN74AVC4T25 will stay high impedance as long the /OE pin stays within the VIH specs which is referenced to VCCA.