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SN74AVC8T245: SN74AVC8T245DGVR OVERSHOOT and UNDERSHOOT

xujunfeng
Prodigy 150 points
Part Number: SN74AVC8T245

Hello，

SN74AVC8T245DGVR output measurement found that the signal has serious overshoot and undershoot.

There are two cases, one is through the 6.2K resistor to another receiving chip, the measurement at the right end of R333 found the signal problem.

The other case is SN74AVC8T245DGVR open-circuit output, measurement also found problems.

Now we are worried about whether this overshoot will affect the reliability of SN74AVC8T245DGVR. Please help to check it, thank you 。

  • 0
    TI__Intellectual 1450 points

    Hi Xujunfeng,

    We can only abide by the datasheet mentioned absolute maximum conditions and cannot guarantee specs outside the abs max conditions. The TI devices are robust enough to handle overshoot/undershoot conditions. For overshoot, based on abs max conditions 4.6V is the max. Since both overshoots are slightly greater than 4.6V, exceeding the Absolute Maximum Rating, 4.6V, can result in immediate damage to the device. 

    In terms of undershoot: it looks like it is exceeding the Absolute Maximum Rating, -0.5V, can result in immediate damage to the device.

    TI does mention in the abs max footnote that I/O voltage ratings can be exceeded if the current ratings are adhered to. If there are current limiting resistors which can limit the current within the abs max limit, the voltage won't be damaging the device, as it is mentioned in the footnote. Ideally, the undershoot should be kept to less than 0.5V in magnitude unless the current is also limited. There is a diode to gnd that should not exceed -50mA. -50mA is the clamp current (I_IK in datasheet) abs max limit. 

    The undershoot seen here is acceptable only if the current through the clamp diode is limited to -50mA or less. If the input clamp current (I_IK) exceeds -50mA, catastrophic physical damage to the die can occur. Note that the negative sign only indicates current direction, so -20mA is smaller than -50mA in this context. Have you tested the option of limiting the undershoot maybe with external protective diodes or by adding a series resistor at the output?

    Best,

    Swathi