This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN74ALVC245: +2V Voltage built up in the B1 line when Vcc and DIR lines is turned OFF

Part Number: SN74ALVC245
Other Parts Discussed in Thread: SN74LVC245A, SN74LV245A

Hi,

        We are using your IC : SN74ALVC245PWR for our control line isolation. When the VCC line and DIR line of the IC is not powered ON and the A1 (pin 2) is fed with +3.3V HIGH signal, a +2V voltage level is built up on the B1(Pin 18) line. Kindly provide feedback why it is happening like that? Isn't It supposed to isolate the line when external input is given when IC is Powered OFF?. Also, I have attached the circuit diagram for your reference.

Regards

Surya D S

  • Hi Surya,

    The SN74ALVC245 does not include the Ioff specification, which would indicate that it supports a supply of 0V and input signals at the I/O pins.

    In other words, if Vcc = 0V, then any I/O pin cannot be larger than 0V for the SN74ALVC245.

    -

    If you want to swap to a device that has this feature, there are two that are pin compatible and will work for you:

    SN74LVC245A

    SN74LV245A

    The LVC family device has a stronger output driver, so it will have lower delays, faster edges at the output, and can drive more current. Both devices are available in the 20 pin TSSOP (PW) package. For lighter loads (ie short traces and direct connection to an IC), I would choose the LV-A family device to reduce overshoot.

  • Hi Emrys Maier,

          The +2V voltage built up scenario is

    When Vcc=0 & Vin=3.3V (pin 2) the voltage built upon in Vout=2V (pin 3).

    Provide the feedback

    Regards

    Surya D S

  • Hi Emrys Maier,

          The +2V voltage built up scenario is

    When Vcc=0 & Vin=3.3V (pin 2) the voltage built upon in Vout=2V (pin 3).

    Provide the feedback

    Regards

    Surya D S

  • The absolute maximum ratings say that it is not allowed to apply a voltage higher than VCC to the A1 pin.

  • Hi Clemens Ladisch

            The above +2V voltage level build up in the Output (B1) pin is not happening, if the OE# line is pulled up with VCC(+3.3V) and Input (A1) pin fed with +3.3V (HIGH) in my circuit.

    Is there any 8 bit bus transceiver you people have with "OE active High" option?

    Provide your feedback

    Regards

    Surya D S

  • No; the standard for all these devices is for OE to be active low.

    The actual problem is that the A1 voltage powers up the device. Use a device with Ioff support like the SN74LVC245A.

  • Hi Surya,

    Yes, forcing OE\ high is one method to force a high-impedance state on the device.

    The other option is to select a device that has the Ioff feature, which automatically does this when the supply is at 0V.  Is there a reason you don't want to use one of these pin-to-pin compatible devices?