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SN74LVC8T245: Changing Outputs with Constant Inputs

Elizabeth Fanous
Prodigy 30 points
Part Number: SN74LVC8T245

Hello TI,

If the inputs to SN74LVC8T245 are not changing, but the outputs are changing due to components other than itself, will the part (SN74LVC8T245) get damaged?

Thanks,

Elizabeth 

  • 0
    TI__Genius 16735 points

    Hi Elizabeth,

    I am unclear on what you mean. Is the direction of the device fixed? Is the device enabled when the outputs are changing? What voltages are being applied to the outputs and what is the supply on the LVC8T245?

    A schematic would help me answer this question better for you. If the input is not changing, the direction is fixed and OE is enabled then I the only way the output is changing is by bus contention which will cause damage due to the large amount of current that will flow.

    For example if the output is driven to GND, the only way to change the output is to sink more than the recommend current into the part.

    Now if the output is disabled, then you should have no issues as long as you are applying a voltage that is within the datasheet specs.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    Prodigy 30 points

    Hi Karan,

    The device is enabled (OE* is grounded). The direction is fixed and the inputs (A4-A8) are not changing. The voltage being applied to the outputs (B5-B8) is a maximum of 5V. The supply on LVC8T245 is 3.3V on VCCA and 5V on VCCB. Will this cause damage to the part?

    Thanks again,

    Elizabeth

  • 0 in reply to Elizabeth Fanous
    TI__Genius 16735 points

    Hi Elizabeth,

    What you are describing is bus contention.This can definitely damage the device based on how much current is being driven. 

    Please read the Bus Contention section of the following app note: http://www.ti.com.cn/cn/lit/an/sdya009c/sdya009c.pdf

    The basic question you have to answer is, if the output of all 8 channels of the LVC device is high so around 4.5V and then your external driver attempts to drive all 8 channels to GND then how much power are you dissipating in the device? This is most likely by very high causing the devices involved to burn.

    You solution is to control the enable pin to allow the output to be controlled by your other devices.

    Thanks!

    -Karan

  • 0 in reply to Karan Kotadia
    Prodigy 30 points

    Hi Karan,

    I'm understanding that disabling the part (by tying the OE* pin high) will prevent damage. What if no power (VccA/VccB) is supplied to the part =? Will that prevent damage to the part, regardless of whether it is enabled (OE*) or not? 

    Thanks again,

    Elizabeth

  • 0 in reply to Elizabeth Fanous
    Guru 317370 points

    If either VCC is at GND, all outputs are disabled, and safe.

    However, when VCC rises, the outputs might be enabled, so OE should be high while the device is powering up.