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LVC to LVC overshoot

Sebastian Wue
Prodigy 215 points
Other Parts Discussed in Thread: SN74AUP2G17

Hi

I have a direct connectuib if a LVC schmitt Trigger (74LVC2G17) to a LVC OR-gate (74LVC1G32).

Both are supplied with 5V (same supply), both have a 10nF buffer cap at its supply Pins (very Close) and they are places very close together on a pcb (Output to Input <7mm)

When i measure the Signal at the OR-Input, i see a overshoot of up to 7V.

I measure with an 1.5GHz active probe with very short wires (<10mm) between the OR Input and the buffer cap GND.

The Absolute Maximum of the Input is 6.5V. With the overshoot, i exceed this Limit every time (it is a CLK-Signal).

Is the Input capable of withstanding the overshoot?

Is it mandatory to implement a line resistor in the Output (on costs of space...)?

Or are there any other possibilities to prevent the overshoot?

 

Regards, Sebastian

  • 0
    TI__Mastermind 46480 points

    Hi Sabastian

    It is allowed to have 1V above or below absolute max for less than 10ns. So you are OK

    I would recomend getting rid of the spkie if possible but if not then it is OK.

    LVC devices are very strong drivers at 5V and the inputs ar very light loads and the combination of strong drive an light load will cause overshoot and undershoot. To reduce the spike you could use a series resistor or use a capacitve or resistive load to gnd.

  • 0
    TI__Genius 9860 points

    Hi Sebastian,

    The LVC logic devices have high current drive. There are a few options here:

    1. The most practical option would be to try a 50-ohm series resistor on the output of the first LVC device to slow down the output signal and dampen the overshoot.

    2. You can try replacing the LVC2G17 with a SN74AUP2G17, which has a much lower drive strength of 4 mA.

    It's possible that the device may be able to withstand small periods of voltage higher than absolute maximum, but we strongly advise against this, as behavior across multiple devices may be unpredictable and it would be operating outside of safe datasheet limits.

    -Ryan

  • 0 in reply to C. Ryan Land
    Prodigy 215 points

    Hi Ryan,

    Thx for the replacement suggestion. The part you suggest is a 3.3V part and i would use it in a 5V environment. And also for the temperature range, i would need >85°C.

    But i may check for another part with less drive strength which fits to my other design constraints.

    Sebastian

  • 0 in reply to Chris Cockrill
    Prodigy 215 points

    Hi Chris

    Thank you for your answer.

    I wanted to avoid modifying a series resistor into my short line or put an additional (capacitive or resistive) load to it. Its also a question of space and cost because i would need to change the layout...

    I will search if i find another part with less drive strength in our system with less drive strength and check the Signal again.

    My overshoot is much less than 10ns so if i dont find another solution i may try to live with it.

     

    Sebastian