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SN74ACT244: risk when Vcc=0V, input/output with high logic

Zhou Fang67
Expert 4250 points
Part Number: SN74ACT244
Other Parts Discussed in Thread: SN74LV244A

Hi Expert,

When using ACT244 with VCC=0V, whether the voltage applied on input or output would impact VCC?

There is a thread discussed this issue for by using ALS technology. Does this also apply to ACT？

https://e2e.ti.com/support/logic/f/151/t/508746

Thanks a lot.

Zhou

  • 0
    Guru 303710 points

    Devices with Ioff protection allow voltages at the outputs when powered off. The SN74ACT244 datasheet does not mention Ioff.

    This can also be seen with the VO specification in section 6.3 of the datasheet; it is not allowed to go above VCC.

  • 0 in reply to Clemens Ladisch
    TI__Expert 4250 points
    Hi Clemens,

    So, SN74ACT244 does not have Ioff feature right?

    Do you have input and output circuit architecture for SN74ACT244? When Vcc=0, we found a high logic at input will generate a voltage at Vcc. There is a risk wrong operation in power off stage.

    Thanks a lot.
    Zhou
  • 0 in reply to Zhou Fang67
    Guru 303710 points

    Yes, it does not have Ioff.

    I don't know anything about the output architecture, but obviously it has an ESD protection diode from the output to VCC.

  • 0 in reply to Clemens Ladisch
    TI__Guru* 97026 points
    Thanks Clemens.

    Zhou,

    You can determine if there are diodes on the input/output of most devices by looking at the Absolute Maximum Ratings table. Note that on the SN74ACT244 datasheet, the input and output voltage ranges are specified as -0.5 V minimum to Vcc + 0.5 V maximum. This indicates that if the specified voltage becomes more than 0.5 V below ground or more than 0.5V above Vcc, the clamp diodes will activate. Additionally, there is a spec called I_IK - Input clamp current. This indicates how much current can be put through those clamp diodes before they will be damaged. In this case, you need to limit the clamp diode current to less than 20 mA.

    In a device that does not have clamp diodes, you will see a maximum voltage of a specified value (such as 7V) with no mention of Vcc.
  • 0 in reply to Emrys Maier
    TI__Expert 4250 points

    Hi Emrys,

    It is very good to know these information. I looked into SN74LV244A and found Ioff feature mentioned by Clemens. 

    There are 2 Vout specs at datasheet page 4,

    1. Vout -0.5V - 7V(high impedance and power off mode),

    2. Vout -0.5V - VCC + 0.5V

    Does it mean a specified circuit is used to turned internal diode on/off at power on/off?

    Then LV series is good replacement for ACT when Ioff is required.

    Thanks.

    Zhou

  • 0 in reply to Zhou Fang67
    TI__Guru* 97026 points
    When the device is powered off or the output is set to 'high impedance' mode, the output is isolated from the clamp diodes by the isolation circuitry. When the device is outputting normally, the clamp diodes are connected and will limit the output voltage during operation. Usually this is not a problem since the output should not be driven by more than one device at a time (called 'bus contention').

    In this case, I think the LV device is a good replacement.